HK1100861B - Base station based methods and apparatus for supporting break before make handoffs in a multi-carrier system - Google Patents

Description

Base station based method and apparatus for supporting break-before-make handover in multi-carrier systems

Technical Field

The present invention relates to multicarrier communication systems, and more particularly to methods and apparatus for performing inter-sector and/or inter-cell handovers in such systems.

Background

A cell may include one or more sectors. A cell without multiple sectors is a single-sector cell, i.e. it comprises one single sector. Typically, signals are transmitted by sector transmitters using a carrier frequency and corresponding bandwidth, such as one or more tones (tones) around the carrier frequency. Different cells and/or sectors of a cell typically use different frequency bands centered around the carrier frequency used by the sector or cell. The carrier frequencies of adjacent cells and/or sectors are typically different. In order to receive signals corresponding to a carrier frequency, a wireless terminal typically has to adjust its receiver (e.g., receiver filter) to correspond to the frequency band associated with the carrier frequency to be used. Switching the receiver between carrier frequencies may take time. Thus, in a receiver with a single filter chain, switching between different carriers may cause the receiver to experience a time interval during which information cannot be received due to the switching process.

A wireless terminal (e.g., mobile node) communicating with a base station on a given carrier frequency and moving through a multi-carrier system needs to determine when to switch and switch to a new carrier frequency (e.g., corresponding to a new cell and/or sector). As noted above, adjacent sectors and/or cells may use different carrier frequencies, and when sector or cell boundaries cross, the wireless terminal typically has to identify and switch to the new carrier frequency.

Due to hardware and cost constraints associated with the receiver, it is often the case that a mobile node includes a single receiver chain and listens to one carrier band at a given time. This is because multiple parallel receiver filter chains are generally too expensive for practical applications in terms of cost. In some known systems, the mobile node waits until communication is lost or severe fading occurs on the operating carrier frequency band used before switching to another carrier. In some systems, a wireless terminal periodically switches its receiver to a different carrier frequency band to detect the presence and/or strength of a signal. Unfortunately, when switching for searching for another carrier, the receiver is unable to receive signals from the currently used receiver. Known methods for determining what carriers are available for switching and when to switch to a new carrier may result in communication interruptions during the handoff process and may occur at intervals and/or waste resources in monitoring and determining the appropriate carrier frequency band.

In addition to the problems in determining which carrier/band is active and which carrier/band is used at any given time, there are also problems associated with switching between sectors and/or cells using different carriers with respect to adjusting receiver and/or transmitter circuitry to switch between carrier frequencies. The problem associated with switching between carrier frequencies occurs when switching between carriers occurs, i.e. there is a change in position, and is typically encountered when switching between carrier frequencies occurs. For cost reasons, it is often desirable to implement a communication device having a single receiver and transmitter.

When switching between carrier frequencies, the analog filter used by the receiver and the analog filter used by the transmitter typically have to be changed to match the new frequency band. This typically involves adjusting the filter according to the carrier frequency of the new sector or cell. The conventional periodicity required to implement such a filter may vary in cases where a device with a single receiver/transmitter results in a time interval during which the communication device is unable to receive information from and/or transmit information to the base station.

In a system where each cell/sector uses the same frequency, for example in a system with a frequency reuse rate of 1, such a filter change operation is not required for a handover between sectors and/or cells because the frequency bands used in the respective sectors/cells are the same. In such systems, "make before break" switching is fairly easy to implement. In a make-before-break handoff operation, the communication device communicates directly with the new sector and/or cell prior to the break, e.g., terminating the connection with the old base station. Given that in such systems the carrier frequency is the same before and after handoff, there is typically no need to change the filters in the receiver and/or transmitter circuitry, so that the time required to switch between two sectors and/or cells is relatively small.

Regardless of whether the handoff operation involves a change in carrier frequency, in many systems, the mobile node performs timing and/or power control synchronization operations when handing off from one base station or sector to another before allowing the mobile device to transmit user data (e.g., application layer data such as audio, text, etc.). Registration in a cell or sector is also typically required before allowing user data to be sent to a new base station or sector. Such signal level synchronization operations may be important to prevent transmissions entering a cell and/or sector from mobile devices that interfere with transmissions from other mobile devices that have entered the cell and/or sector. In some systems, certain time periods are periodically cancelled for a mobile device entering the system to send a signal to register and/or perform initial timing and/or power control synchronization operations. During such time periods, devices entering a cell/sector can contact the base station to perform timing and/or power control synchronization operations without interfering with devices already present in the system, e.g., because the registered devices know that no signal needs to be transmitted during that particular time period. The signaling within the dedicated time period is typically contention-based (contention), e.g., one or more new devices may attempt to register using the same communication resource (e.g., tone set). In this case, the signals may be conflicting and registrations by devices attempting to use the same tone set may not require them to be attempted again, for example during a subsequent dedicated registration using another tone set. As part of the registration process, the physical layer signaling problem may be solved by implementing physical signal timing for control symbol transmission and/or transmission power control, e.g., based on control signals received from the new base station. In addition, one or more device identifiers used to identify devices in the new cell may be assigned to devices that wish to register in the new cell/sector. Once the synchronization and ID assignment issues are resolved for the new cell/sector, higher level signaling, such as IP packet transmission and reception, can begin between mobile devices entering the new sector and/or cell and the base station in that sector/cell.

In this case, the frequency bands of the new and old sectors and/or cells are the same, and it is generally possible to maintain communication with the old base station while simultaneously communicating with the new base station in the same frequency band, in order to perform the registration operations discussed above, such as timing control, power control, and cell/sector ID specific assignment operations. When communicating with a base station with the same old and new carrier frequencies, it is possible to avoid having to change the frequency of the filter used in the receiver and/or transmitter. Therefore, in a system where the new and old frequency bands are the same, the mobile device can complete the physical layer signaling operations that need to be completed before receiving/transmitting IP packets in the new cell, while also being able to receive IP packets from the old base station. Once physical layer operations (e.g., timing synchronization, etc.) and other registration operations are completed with the new base station in the new sector/cell, a signal can be sent by way of the new sector/cell to trigger rerouting of IP packets to the mobile device and to stop routing of packets intended for the mobile device to the old sector/cell. In this manner, in various known systems, after a connection is established that is sufficient to communicate IP packets with the new cell or sector, the connection with the old cell is broken.

There is also a disadvantage when using the same single carrier as the carrier in each sector and cell of the system to simplify the handover operation due to the rather high degree of interference at the sector and cell boundaries. At these boundaries, the mobile node may experience a significant 0dB degradation in signal conditions for extended periods of time, assuming the signal is fading.

When different frequency sets are used in adjacent sectors/cells, e.g., using a frequency reuse pattern greater than 1, the signal conditions at the sector and cell boundaries will typically be significantly better than if the entire frequency were completely reused. Thus, signal interference at the cell/sector boundary provides a reason to avoid frequency reuse scheme 1, although it provides the benefit of handover.

The delays associated with adjusting the filters of the transmitter and/or receiver to operate on the new frequency band make it difficult to switch the receiver and transmitter circuits between the old and new carrier frequencies at a rate fast enough to support the make-before-break switching process discussed above. Therefore, in handover between sectors and/or cells using different frequency bands, a break-before-make handover operation is typically used, wherein radio signaling with the old base station is terminated before a link is established with the new base station. Unfortunately, mobile devices are typically rendered incapable of receiving IP packets not only during the time they switch their filter circuits to a new carrier frequency, but also during other time periods: the mobile device needs to register with the new cell/sector and perform the required timing and/or power synchronization operations as well as any IP packet redirection operations that may be required.

In some systems, there is a need to wait for a periodically occurring time period in which registration is allowed in a sector or cell, which can be accompanied by an uncertainty that for a mobile device in a cell or sector, these resources are valid for registration in a particular registration period, which may result in unpredictable and sometimes excessive delays after terminating a connection with an old base station and before receiving IP packets in a new cell or sector.

In view of the foregoing discussion, it should be appreciated that there is a need to provide a method and apparatus for reducing the amount of time required to complete a handover in a system using different frequency bands. It would be desirable to provide at least one or more methods for problems related to handover (e.g., registration signaling, allocation of resources associated with the air link, such as allocation of a local identifier, etc.) that avoid terminating a connection with a current base station and/or cell before a mobile device can begin communicating with a new base station or cell. It is further desirable that, in at least some embodiments, the mobile device be able to anticipate with a reasonably high degree of certainty: the communication resources required to complete the registration process are available at or near the time the mobile device terminates communication with the previous base station.

Disclosure of Invention

The present invention is directed to a method and apparatus for switching between communication links implemented using different carrier frequencies, for example, as part of a handoff between sectors and/or cells, or as part of an intra-sector handoff between two different carrier frequencies used in one sector. The method of the present invention may be particularly well suited for use in situations where the system supports the use of different frequencies for different communication purposes, e.g. in different sectors, cells or sectors.

In a system employing the present invention, base station transmitters in different sectors and/or cells periodically transmit high power signals (sometimes referred to as beacon signals) into the frequency bands used in adjacent sectors or cells. Beacon signals are signals that include one or more narrow (frequency-dependent) signal components (e.g., signal tones) that are transmitted at a relatively high power compared to other signals, such as user data signals. In some embodiments, each beacon signal includes one or more signal components, where each signal component corresponds to a different tone. The beacon signal component in some embodiments includes a tone signal energy that is 10, 20, 30 or more times the average of the tone signal energy of each signal tone used to transmit user data and/or non-beacon control signals.

Although in many embodiments a single beacon signal is transmitted by the transmitter in most cases during any given transmission time period (e.g., symbol transmission period), multiple tones, such as multiple high power tones, may also be transmitted simultaneously. The single beacon signal may include a single high power signal tone or, in some embodiments, not very high power tones.

According to the present invention, a handoff operation is initiated via a current base station sector by a wireless terminal, such as a mobile communication device, having a wireless communication connection with the current base station sector, such as a first communication link implemented using a first carrier frequency. A mobile device communicates its desire to complete a handoff via a first communication link and a current base station sector to a different base station, sector, or carrier in the sector in which the mobile device is located. A new communication link is established using a new carrier frequency, typically different from the first carrier frequency. The base station sector with which a new communication link is to be established (referred to as the new base station sector) allocates to the mobile device, via the current base station sector and the first communication link, one or more air links associated with resources to be used when the mobile device enters the new base station sector or switches to a new carrier frequency in the current sector, in the case where the new base station sector is the same as the current base station sector. The resources associated with the airlink may be one or more device identifiers (such as MAC state identifiers, e.g., ON state identifier, ACTIVE state identifier) to be used when communicating using the new carrier frequency in the new base station sector. As part of the handoff process, the new base station sector may dedicate and thereby reserve physical signaling resources associated with the new carrier frequency, e.g., dedicated communication bandwidth such as a tone set, to mobile devices initiating handoff operations used to complete the registration process upon entry into the cell by using the new carrier frequency. For power control and/or timing control operations when entering a new base station sector, a dedicated tone set may be used. Such dedicated resources may be allocated during periodically occurring access or registration time periods. In some embodiments, the new base station sector may communicate information identifying the particular registration period in which the mobile device is allocated dedicated resources. In various embodiments, this information is used to determine when to terminate the current communication link with the current base station sector and when to establish a connection with a new base station sector using a new carrier, so that service disruption due to termination of the first communication link can be minimized.

After determining to initiate a handoff operation, the mobile node and/or the current base station sector sends an IP routing update message to a mobility agent, such as a Mobile IP home agent, for redirecting IP packets intended for the mobile device to the base station sector for attaching the mobile device to the network. The IP routing message causes the mobility agent to begin redirecting IP packets intended for the mobile device to the new base station sector to which it is ready to be handed off. In some embodiments, the sending of the IP routing update message occurs after receiving the device identifier to be used in the new base station sector and/or the dedicated resources to be used in the new base station sector, e.g., to complete the registration process. This ensures that the new base station has resources available to serve mobile devices that wish to handoff to the new base station sector.

In the manner described above, the mobile node is able to initiate a handoff via its existing communication link to a new base station, sector, or to a carrier in a sector that includes a change to a different carrier frequency. In this manner, the need to tune to a new carrier frequency to begin establishing a connection using the new carrier frequency can be avoided, and the mobile node can receive resources allocated corresponding to the new base station, sector, and/or carrier frequency without having to first change to the new carrier frequency. The resources allocated by the new base station or sector may include, for example, sector-specific and/or sector-carrier-specific device identifiers used when communicating in the new sector and/or when using the new carrier frequency. Dedicated communication segments (segments) for establishing communication signaling, e.g., power, timing control and/or registration signaling, may also be allocated by the new base station and/or sector with an allocation communicated to the mobile node based on the first communication link before signaling is established using the new carrier frequency based on the new communication link.

In accordance with a feature of the present invention, in some embodiments, an IP routing message is sent generally after a handoff to a carrier frequency within a new base station, sector, or sector is initiated, but before the mobile node completes registration, power control, and/or timing control based on the communication link being established with the new base station, sector, or carrier frequency. In this case, an IP route update process is typically initiated to redirect IP packets to cells, sectors, or circuits in a sector corresponding to the new carrier frequency before the mobile node can transmit user data based on the new communication link being established. Thus, in many cases, to facilitate a communication link established as part of the handover process, an IP routing message is typically sent prior to completing the handover, e.g., prior to terminating the current communication link. In such embodiments, the IP route update delay will at least partially overlap with a period during which the mobile node may be unable to communicate with the old base station sector or the new base station sector as a result of the process for changing the receiver and/or transmitter circuitry (e.g., filter circuitry) to correspond to the new carrier frequency to be used in communicating with the new communication link (established as part of the handoff process).

In the case of a single sector cell, the handover between the old and new base stations corresponds to the handover between the base stations of different cells, due to the one-to-one correspondence between cells and base station sectors. However, in the case of a multi-sector cell embodiment, an intra-cell inter-sector handover of new and old sectors within the same cell is possible. In some embodiments, timing synchronization is maintained between base station sectors during intra-cell inter-sector handovers, and timing synchronization steps typically performed in the handover process are omitted. In such cases, a handoff to a new sector of the same cell may be accomplished without having to perform timing synchronization. Thus, upon entering a new sector, the mobile device can begin transmitting user data before receiving a timing synchronization signal from the base station or performing a timing synchronization operation after the old communication link is terminated. This is because in some embodiments timing synchronization between sectors of a cell is maintained and reliance on timing synchronization initially achieved in one sector of a cell is unlikely to cause problems of interference in other synchronized sectors of the same cell. Ignoring the initial timing synchronization step that is typically required when entering a new cell reduces the delay associated with performing an intra-cell handover (compared to an inter-cell handover) when performing an intra-cell handover.

Although the methods and apparatus of the present invention may still involve breaking the communication link over the existing communication link implemented using the first carrier frequency before establishing radio communication using the second (e.g., different) carrier frequency, the signaling exchanged by way of the existing communication link using the first carrier frequency prior to this operation may also allow the mobile device to obtain certain benefits of make-before-break handover, such as ID allocation and allocation of air link resources prior to actually breaking communication based on the existing link, thereby reducing the delay and uncertainty associated with many make-before-break handover operations.

The method and apparatus of the present invention thus represents an improvement over the old break-before-make handoff method in which the mobile device first breaks the existing link before being able to receive a resource allocation in the case of a new communication link implemented using a different carrier frequency.

Various additional features and benefits of the methods and apparatus of the present invention are discussed in greater detail below.

Drawings

Fig. 1 is a diagram of an exemplary three sector cell including a sectorized base station and wireless terminals at sector boundaries, both implemented in accordance with the present invention.

Fig. 2 is a diagram of an exemplary multi-cell multi-sector wireless communication system including three sectorized base stations and wireless terminals located at cell boundaries, which communication system is implemented in accordance with the present invention.

Fig. 3 is a diagram illustrating exemplary downlink signaling from various sectors of an exemplary three sector base station, according to an exemplary embodiment of the present invention.

Fig. 4 is a diagram illustrating exemplary downlink signaling from sectors with two same type assignments from different neighboring cells in accordance with the present invention.

Fig. 5 is a diagram of an exemplary communication system implemented in accordance with the present invention and using methods of the present invention.

Fig. 6 is a diagram of an exemplary access node (base station) implemented in accordance with the present invention and using methods of the present invention.

Fig. 7 is a diagram of an exemplary wireless terminal (end node) implemented in accordance with the present invention and using methods of the present invention.

Fig. 8 is a diagram of an exemplary downlink beacon signal, exemplary uplink dedicated segments and contention-based uplink segments that may be used for access purposes, and an exemplary timing relationship, which may be used to explain various features of the present invention.

Fig. 9 is a diagram of an exemplary system implemented according to an exemplary embodiment of the invention that may be used to explain various features and signal flows associated with a switching operation in accordance with the invention.

Fig. 10 is a diagram illustrating exemplary switching signal transmission according to the present invention.

Fig. 11 is a flow diagram of an exemplary method for operating a wireless communication system to perform handoff of a wireless terminal from one base station sector attachment point (attachment point) to another base station sector attachment point.

Fig. 12 is a flow chart of an exemplary method for operating a mobile communication device having a first wireless communication link with a first base station when initiating a handover to perform the handover of the mobile communication device between the first base station and a second base station in accordance with the present invention.

Fig. 13 is a flow chart of an exemplary method for operating a mobile node to perform a handoff between a first link with a first base station sector and a second link with a second base station sector, where the first link uses a first carrier and the second link uses a second carrier, at least the first sector being different from the second sector or the first carrier being different from the second carrier, in accordance with the present invention.

Fig. 14 is a flow chart of an exemplary method of performing a handoff between base station sectors in accordance with the present invention.

Fig. 15 is a diagram containing an exemplary uplink dedicated access segment and an exemplary contention-based uplink access segment in accordance with the present invention.

Fig. 16 is a diagram illustrating an exemplary cell in an exemplary multi-sector multi-carrier system in which multiple carriers are used in the same sector at different power levels.

Fig. 17 shows the use of sectors supporting multiple carriers, with beacon signals being transmitted into the frequency bands of the respective carriers by respective sector transmitters.

Detailed Description

The present invention aims to provide a method and apparatus for implementing a handover involving a change in carrier frequency. These handovers may be between different cells, e.g., an inter-cell handover; inter-sector within the same cell, e.g., intra-cell inter-sector handover; or between different carriers in a sector, e.g., intra-sector inter-carrier handoff. Inter-cell handover and intra-cell inter-sector handover typically involve a change of carrier.

The handover implemented according to the present invention generally comprises: the first communication link is terminated before the handover is completed and the second communication link is successfully established, e.g., using a different carrier frequency. Although discussed in the context of a handoff involving a change in carrier frequency, certain aspects of the present invention can be used to perform a handoff in which the old and new carrier frequencies used are the same but the network point of attachment has changed. For example, in the case of a cell having a timing synchronization sector using the same carrier frequency among a plurality of sectors, even if the mobile node changes the sector among the cells to which it is attached to the network via a wireless connection, timing synchronization remains valid again, so that a handover from one sector to another sector of the cell can be performed without having to perform timing synchronization in a new sector before transmitting user data.

In the exemplary system, each cell includes a base station for transmitting different signals to various sectors of the cell. These cells may include one or more sectors. In various embodiments, a single carrier frequency is used in each sector of the cell. However, in some embodiments, multiple carrier frequencies are used in each sector. In these embodiments, intra-sector inter-carrier handoff is possible with a mobile device that uses handoff from a transmitter/receiver or other signal processing circuitry associated with one carrier frequency to a transmitter/receiver or other signal processing circuitry associated with other carrier frequencies.

Separate antennas and/or transmitters may be provided for each sector of the cell. In some, but not all embodiments, symbol timing and carrier frequency are synchronized by multiple sectors of a cell. In addition, the frame structure is also synchronized by the sectors of the cell so that the slots or super slots (superslots) of signals in one sector start with a fixed time offset, which may be zero in some embodiments, from which the slots or super slots of signals in other sectors start. However, symbol timing or carrier frequency is not typically synchronized by the cell. A base station according to various embodiments of the present invention transmits multiple beacon signals, e.g., at different times, from various sectors of a cell. One or more beacon signals are typically transmitted in the frequency bands used by the various sectors or, for example, in the case of multiple carriers in a sector, to convey information to wireless terminals within the sector. A beacon signal is a narrowband signal that is transmitted using a relatively high power (e.g., a power level that is higher than the average power level used to transmit user data). In most cases, the power level of the beacon signal is several times higher than the power level of the average user data. These beacon signals can be used to communicate information such as a sector identifier, a slope (slope) as a cell identifier, and/or information about the carrier frequency/band used in the sector transmitting the beacon.

In some embodiments of the invention, the base station uses a sector transmitter to periodically transmit beacon signals at a predetermined frequency within the frequency band used by neighboring sectors or cells. As a result, multiple sectors may transmit beacon signals to the same frequency band, e.g., at different times. In this manner, a receiver in one sector can identify the presence and signal strength of an adjacent sector and obtain information about the sector without having to change to a different frequency band used in the adjacent sector. To easily distinguish sectors that are the source of the beacon signal in a particular frequency band, each sector transmits beacons at a different predetermined frequency in any given frequency band used by the sector. Carrier frequency information may be associated with beacons in addition to sector information. The predetermined frequency with a given frequency band may vary on a time basis according to a pre-selected order. This sequence repeats, for example, at some point after a fixed number of premium slots.

The strength of a beacon signal received from an adjacent sector and/or cell, or from the same sector corresponding to a different carrier frequency, may be compared to the strength of a beacon signal corresponding to the sector and carrier frequency with which the mobile station has a communication link to determine when a handoff should be performed. According to the present invention, the monitoring and evaluation of beacon signals from the same sector/cell or different carriers of the same sector allows a wireless terminal to perform relatively seamless handover in many cases, while avoiding relatively long interruptions in service that may exist in the system, where handover to a different carrier requires a determination of the carrier frequency to be used after handover.

In one exemplary OFDM (orthogonal frequency division multiplexing) embodiment, the beacon signal is implemented as a relatively high power signal transmitted on a single tone (e.g., frequency or a few tones). The power used to transmit the beacon signal is typically twice the highest power signal tone used to communicate data or pilot signals in the sector. When transmitting a beacon signal in this exemplary OFDM embodiment, most of the transmit power is concentrated on one or a small number of tones, e.g., a single tone that includes the beacon signal. Most tones not used for beacon signals are not necessary and are not typically used. Thus, most or all of the tones used in the sector frequency band used to transmit the beacon signal need not be used by the sector's transmitter when transmitting the beacon signal into the frequency band used by the adjacent sector.

Fig. 1 illustrates an exemplary 3 sector cell 100 corresponding to a Base Station (BS)102, implemented according to an exemplary embodiment of the invention. Base station 102 is a sectorized base station. Base Station (BS)102 uses carrier frequency f₁General signals (ordinarys)Signal) to sector 1106. Base Station (BS)102 uses carrier frequency f₂Ordinary signaling to sector 2108 and use carrier frequency f₃Normal signaling is to sector 3110. Wireless Terminal (WT)104, implemented in accordance with the present invention, is shown at the boundary area between sector 1106 and sector 2108. In accordance with the method of the present invention, handoff of WT104 may be performed between different base station sectors of the same cell.

Fig. 2 shows three exemplary cells (cell 1202, cell 2204, cell 3206) in an exemplary wireless communication system 200 according to the present invention. Each cell includes a base station and 3 sectors, each of the 3 sectors using a different carrier frequency (f)₁、f₂、f₃) And a corresponding frequency band for ordinary communications with wireless terminals in a particular sector. The same 3 carrier frequencies f₁、f₂、f₃And the bandwidth associated with each carrier is reused in each cell. The cell 1202 includes a base station 1(BS1)208 and uses carrier frequencies (f) respectively₁、f₂、f₃) And 3 sectors (sector 1214, sector 2216, sector 3218). Cell 2204 includes base station 2(BS2)210 and uses carrier frequencies (f) respectively₁、f₂、f₃)3 sectors (sector 1220, sector 2222, sector 3224). Cell 3206 includes base station 3(BS3)212 and uses carrier frequencies (f), respectively₁、f₂、f₃)3 sectors (sector 1226, sector 2228, sector 3230). Figure 2 also includes an exemplary Wireless Terminal (WT)232 implemented in accordance with the present invention. The WT is located at the boundary between sector 1214 of cell 1202 and sector 2222 of cell 2204. Handoff of WT232 may be performed between different base station sectors of different cells or between different base station sectors of the same cell in accordance with the methods of the present invention.

The total frequency band of the example of fig. 2 is subdivided into 3 frequency bands (time slots) arranged adjacent to each other and is the same in each sector. In general, the total frequency band in each sector need not be the same, and the frequency bands (time slots) in each sector may be disjoint and need not be the same. In some embodiments, the BSs 208, 210, 212 transmit beacon signals. In various embodiments, the beacon signal is implemented as one or more narrow-band high-power broadcast signals. In some embodiments, transmission of beacon signals in various sectors may alternate between 3 frequency ranges (bands) based on time, according to a prearrangement. In other embodiments, the base stations in each sector can transmit beacon signals in more than one carrier frequency bandwidth range (frequency band), where beacons are transmitted simultaneously in multiple frequency bands from the sector transmitter.

Fig. 3 shows 3 graphs 302, 304, 306 representing exemplary base station sector transmission signaling versus frequency. The exemplary signaling may be sent in any one of the cells, such as the exemplary cell 100 shown in fig. 1 or the exemplary cells (202, 204, 206) shown in fig. 2.

The upper graph 302 of fig. 3 shows signaling from base station sector 1. Graph 302 is a combination of signals that may be transmitted at different times (e.g., during different symbol transmission periods). At carrier frequency f₁A first frequency band 310 around the center is used to transmit signals and information to wireless terminals in sector 1, represented by tag ordinary signaling 319. For example, when not transmitting data such as a normal signal, a transmitter located in sector 1 will periodically transmit a beacon signal S1F1 (sector 1 carrier frequency 1)320 in the first frequency band. The frequency may be a fixed offset value relative to the first carrier frequency and can be used by the wireless terminal to identify and synchronize with the carrier frequency used in the first sector. In order to direct to an adjacent sector (where carrier frequency f₂Periodically used), the first sector transmitter to transmit information on a frequency corresponding to the second carrier frequency f₂Transmits a beacon signal S1F 2322 at a predetermined frequency within the second frequency band 312. The signal may be detected by WTs in adjacent sectors where there are no terminals that have to adjust their receiver frequency to another frequency band, such as the first frequency band 310 used in sector 1. In addition, toTo use the frequency f periodically₃The first sector transmitter to transmit at a frequency corresponding to the third carrier f₃Transmits a beacon signal S1F 3324 at predetermined frequencies within third frequency band 314. The signal may be detected by WTs in an adjacent sector using a third frequency band in which there are no terminals that have to adjust their receiver frequency to another frequency band, such as the first frequency band 310 used in sector 1.

The middle graph 304 of fig. 3 represents signaling from base station sector 2. Graph 304 is a combination of signals that may be transmitted at different times (e.g., during different symbol transmission periods). At carrier frequency f₂A second frequency band 312 around the center is used to transmit signals and information to wireless terminals in sector 2 as represented by tag ordinary signaling 331. A transmitter located in sector 2 will periodically transmit a beacon signal S2F2 (sector 2 carrier frequency 2)332 in the second frequency band 312, for example when not transmitting data such as normal signals. The frequency may be a fixed offset value relative to the second carrier frequency and can be used by wireless terminals in sector 2 to identify and synchronize with the carrier frequency used in the second sector. In order to direct to an adjacent sector (where carrier frequency f₁Periodically used), second sector transmitter to transmit information in a frequency corresponding to the first carrier frequency f₁Transmits a beacon signal S2F1330 at a predetermined frequency within the first frequency band 310. The signal may be detected by WTs in adjacent sectors using the first carrier frequency where there are no terminals in the adjacent sector that have to adjust their receiver frequency to another frequency band, e.g., the second frequency band 312 used in sector 2. In addition, in order to periodically use the frequency f₃Provide information to WTs in adjacent sectors, the second sector transmitter to transmit at a frequency corresponding to the third carrier f₃Transmits a beacon signal S2F 3334 at predetermined frequencies within third frequency band 314. The signal may be detected by WTs in an adjacent sector using a third frequency band, where there is no need to tune their receiver frequency to another frequency band (e.g., in a sector)2, second frequency band 312) used in the second communication channel.

The lower graph 306 of fig. 3 represents signaling from base station sector 3. Graph 306 is a combination of signals that may be transmitted at different times (e.g., during different symbol transmission periods). At carrier frequency f₃The third frequency band 314 around the center is used to transmit signals and information to wireless terminals in sector 3 as represented by tag ordinary signaling 343. A transmitter located in sector 3 will periodically transmit a beacon signal S3F3 (sector 3 carrier frequency 3)344 in the third frequency band, for example, when not transmitting data such as ordinary signals. The frequency of the beacon signal may be a fixed offset value relative to the frequency of the third carrier and can be used by wireless terminals in sector 3 to identify and synchronize with the carrier frequency used in the third sector. In order to direct to an adjacent sector (where carrier frequency f₁Periodically used), third sector transmitter to transmit information in a manner corresponding to the first carrier frequency f₁Transmits a beacon signal S3F 1340 at the predetermined frequency within the first frequency band 310. The signal may be detected by WTs in adjacent sectors using the first carrier frequency, in which there are no terminals that have to tune their receiver frequency to another frequency band, e.g., third frequency band 31 used in sector 3. In addition, in order to periodically use the frequency f₂Provide information to WTs in adjacent sectors, and a third sector transmitter to transmit signals on a carrier frequency corresponding to the second carrier frequency f₂Transmits a beacon signal S3F 2342 at the predetermined frequency within the second frequency band 312. The signal may be detected by using WTs in an adjacent sector of the second frequency band in which there are no terminals that have to adjust their receiver frequency to another frequency band, such as third frequency band 314 used in sector 3.

Each beacon signal is capable of uniquely identifying the carrier associated with the sector from which the beacon signal originated, and in various embodiments may provide additional information. In fig. 3, the nine exemplary beacon signals shown differ in frequency. Thus, it is possible to match the frequency of the beacon signal to a set of known beacon frequencies to determine which sector transmitter is the source of the particular detected beacon signal.

In accordance with the present invention, a wireless terminal, such as a mobile node, can receive beacon signals from both itself and from different (e.g., neighboring) sector base station transmitters. The beacon signal is received within the same frequency band as that used by the wireless terminal for ordinary signaling, such as data and/or control signaling. In addition to frequency measurements, beacon signal strength (e.g., power) is also measured. The results of the comparisons of different beacon signal strengths received from different base station sector transmitters are used by the WT to determine when to handoff to different base station sectors. The beacon signal comparison result also indicates to the wireless terminal: after the handover, the wireless terminal will use which carrier frequency for ordinary signaling. In some embodiments, the carrier frequency is determined to be the carrier frequency used for ordinary signaling by the base station sector transmitter that transmitted the stronger of the received beacon signals.

By way of example, wireless terminal 104 shown in fig. 1 operating in sector 1 uses carrier frequency f₁And its associated frequency band 310 for ordinary signaling (e.g., receiving and transmitting information to a base station). However, wireless terminal 104 also pairs signals within frequency band 310 that correspond to carrier frequency f₁The beacon signal of (a) is monitored. Referring to the left hand side of fig. 3, it is shown that at a frequency corresponding to the carrier frequency f₁In each of the three sectors of the first frequency band 310, by the BS. Wireless terminal 104 compares the strength of beacon signal 320 received from sector 1 with the strength of received adjacent sector beacon signals 330 and 340, also detected in first frequency band 310. As the wireless terminal approaches the boundary of sector 1 and sector 2, the received strength of the beacon signal S2F1330 within the first frequency band transmitted through BS sector 2 increases in strength relative to the received signal strength from the beacon signal S1F1320 of sector 1. At some point, based on this comparison of received beacon signal strength and criteria in the wireless terminal, the wireless terminal may be launched to carrier frequency f₂Is cutAlternatively, the frequency is used for ordinary signaling in sector 2. The wireless terminal knows to switch to the carrier frequency f, e.g. based on the position in the frequency domain of the more strongly received beacon signal₂Instead of the carrier frequency f₃。

The signaling from the various sectors of the same cell may be synchronously timed with respect to each other. Thus, in accordance with the present invention, it is not necessary to perform certain timing synchronization-related operations in an intra-cell inter-sector and/or an intra-cell inter-sector handoff operation, which are typically performed as soon as a wireless terminal enters a cell or sector before being able to transmit user data, before the user data, such as voice or text, can be transmitted to a receiver corresponding to a new sector or carrier frequency.

Similar methods of the present invention, described with reference to handing off at a sector boundary, may also be used for handing off at a cell boundary, with the wireless terminal 232 shown in fig. 2 being located at a cell boundary. In this case, a handover is made from a sector of one cell to a sector of another cell. For cells, the location of the beacon may also be used to convey cell information, such as a slope value used as a cell identifier in some embodiments. Different cells, sectors, and carriers in a sector may use different predetermined frequencies for beacon signals. In some embodiments, periodic changes in beacon signal frequency that are predetermined based on time may be used to convey slope information. In one embodiment, the change in the beacon signal is altered in the beacon location by a hopping pattern (hopping pattern) on tones that may indicate the slope of the corresponding cell.

Figure 4 shows an example where there is a slight change in the beacon frequency location designation of two different neighboring cells in the same sector (e.g., sector 1) to provide the beacon signal identity and cell rank to the sector. For example, graph 402 may correspond to a signal transmitted from a transmitter of BS1208 of sector 1214 of cell 1202 of fig. 2, while graph 404 may correspond to a signal transmitted from BS 2210 of sector 1220 of cell 2204 of fig. 2A signal. Graph 402 includes a correlation with carrier frequency f₁Associated bandwidth 406 with carrier frequency f₂Associated bandwidth 408 and associated carrier frequency f₃An associated bandwidth 410. At a frequency f corresponding to the carrier frequency₁Sector 1 transmitter of BS1 transmits beacon signals 412 and ordinary signaling 414, e.g., user data and control signals. At a frequency f corresponding to the carrier frequency₂Sector 1 transmitter of BS1 transmits beacon signal 416 in bandwidth 408. At a frequency f corresponding to the carrier frequency₃Sector 1 transmitter of BS1 transmits beacon signal 418 in bandwidth 410. The various signals 412, 414, 416 and 418 may be transmitted at different times, such as normal signaling 414 being transmitted most of the time, and beacon signals from the beacon groups comprising 412, 416, 418 being transmitted from time to time in a predetermined order based on the location of normal signaling 414 periodically. Graph 404 includes a correlation with carrier frequency f₁Associated bandwidth 406 with carrier frequency f₂Associated bandwidth 408 and associated carrier frequency f₃An associated bandwidth 410. At a frequency f corresponding to the carrier frequency₁Sector 1 transmitter of BS2 transmits beacon signals 420 and ordinary signaling 422, e.g., user data and control signals, in bandwidth 406. At a frequency f corresponding to the carrier frequency₂Sector 1 transmitter of BS2 transmits beacon signal 424 in bandwidth 408. At a frequency f corresponding to the carrier frequency₃Sector 1 transmitter of BS2 transmits beacon signal 426 in bandwidth 410. The various signals 420, 422, 424 and 426 may be transmitted at different times, for example ordinary signaling 422 is transmitted most of the time, and beacon signals from the beacon groups comprising 420, 424, 426 are transmitted from time to time in a predetermined order based on the location of the ordinary signaling 422 on a periodic basis. Beacon signals 412 and 420 in the same frequency band 406 are at different frequency locations that allow wireless terminals receiving the beacon signals to distinguish between the two cells. Beacon signals 416 and 424 in the same frequency band 408 are at different frequency locations that allow wireless terminals receiving the beacon signals to distinguish between the two cells. Beacon signals 418 and 426 in the same frequency band 410 are at different frequency locations that allow for reception of the beacon signalsThe wireless terminal of (1) distinguishes between two cells.

There is no need, and often no synchronization timing between the various cells. Therefore, in an inter-cell handover operation, a wireless terminal typically needs to perform a timing synchronization operation before transmitting user data, so that symbols, e.g., symbols carrying user data transmitted by a mobile terminal, are synchronized at a BS with symbols transmitted by other terminals. Beacon signals or other broadcast signals may be used in achieving coarse timing synchronization and minimizing off-times during handoff operations in accordance with the present invention.

Fig. 5 illustrates an exemplary communication system 500 implemented in accordance with the present invention that employs the methods of the present invention. The exemplary system includes a plurality of cells (cell 1502, cell M504). Each cell represents a radio coverage area for an access node, e.g., a base station. Cell 1502 corresponds to base station 1506 and cell M504 corresponds to base station M508. Each cell is in turn subdivided into a plurality of sectors. The exemplary system shows a 3 sector embodiment; however, cells with less or more than 3 sectors are also possible according to the invention. The exemplary system uses different carrier frequencies in each sector of the cell. In other embodiments, the frequencies may be reused by multiple sectors in a cell, for example, by sectors that are not adjacent to each other. Alternatively, in some embodiments, multiple carriers are used in respective sectors having different power levels for a particular carrier in adjacent sectors using the same carrier frequency. In the illustrative example of fig. 5, sector 1 uses a carrier frequency f₁(ii) a Sector 1 uses carrier frequency f₂(ii) a Sector 1 uses carrier frequency f₃. The same carrier frequency is used in the same sector (e.g., sectors 1, 2, and 3) of other cells of the exemplary system.

In some embodiments, the carrier frequencies used in different cells of the system may vary slightly. In other embodiments, the carrier frequencies used in different cells may actually be different. Cell 1502 includes sector 1510, sector 2512, and sector 3514. Cell M504 includes sector 1516, sector 2518, and sector 3520. The area where sector 1510 of cell 1 overlaps sector 2518 of cell M illustrates an exemplary boundary region 522 where a handoff operation between cells is likely to occur in accordance with the method of the present invention. According to the method of the present invention, handover operations may also exist in boundary areas between different sectors of the same cell.

The exemplary system of fig. 5 also includes a plurality of end nodes EN1, EN N, e.g., wireless terminals such as mobile nodes in respective sectors of each cell. These wireless terminals are connected via a wireless link domain base station. If the end nodes are mobile devices, they can move around in all sectors and cells of the system. In accordance with the method of the present invention, these end nodes can initiate and perform handoff operations from one base station sector attachment point to another base station sector attachment point. These mobile devices are sometimes referred to herein as mobile communication devices or mobile nodes. Sector 1510 of cell 1502 includes multiple ENs (EN 1524, EN N526); sector 2512 of cell 1502 includes multiple ENs (EN 1528, EN N530); cell 1502 sector 3514 includes multiple ENs (EN1532, EN N534). Sector 1516 of cell M504 includes a plurality of ENs (EN 1536, EN N538); sector 2518 of cell M504 includes multiple ENs (EN 1540, EN N542); sector 3520 of cell 1504 includes multiple ENs (EN 1544, EN N546).

The access nodes (base stations) (506, 508) are coupled to a network node 548, such as a router, via network links (550, 552), respectively. Network node 548 is coupled to other network nodes and the internet via network link 554. The network links (550, 552, 554) may be, for example, optical fibers.

The sector boundary regions are identified by boundaries within the respective cells that separate the three sectors (510, 512, 514) or (516, 518, 520), and the overlapping area between cell 1 and cell M illustrates an exemplary cell boundary region 522. Wireless terminals move throughout the system and may perform handoff operations, including changes in carrier frequency, near and/or across sector and/or cell boundaries in accordance with the present invention.

According to the invention, the base stations (506, 508) are arranged in three frequency bands (with three carrier frequencies f)₁、f₂、f₃Associated) periodically transmits beacon signals into respective sectors of respective cells. In accordance with the present invention, end nodes (524, 526, 528, 530, 532, 534, 536, 538, 540, 542, 544, 546) monitor beacon signals in the frequency band of current operation in order to make decisions regarding inter-sector, intra-sector (if multiple carrier frequencies are used in a sector), and/or inter-cell handovers.

Fig. 6 illustrates an exemplary access node (base station) 600 implemented in accordance with the invention. The base station of fig. 6 may be a more detailed representation of any of the base stations in the systems of fig. 1, 2 or 5. The base station 600 comprises a processor 602, e.g. a CPU; a plurality of receivers, e.g., receivers for each sector of base station 600 (receiver 604 of sector 1, receiver 606 of sector 2.... receiver 608 of sector N); a plurality of transmitters, e.g., transmitters for respective sectors of a base station (transmitter 610 for sector 1, transmitter 612 for sector 2.... transmitter 614 for sector N); I/O interface 616; a clock module 618; a memory 620; and in some embodiments a plurality of beacon transmitters, such as beacon transmitters for each sector of the base station (beacon transmitter 622 for sector 1, beacon transmitter 624 for sector 2,... beacon transmitter 626 for sector N), which are connected together via a bus 628, the various components being capable of exchanging data and information with each other based on bus 628. In the case of a sector supporting the use of multiple carrier frequencies, different transmitter circuits can or typically are included for each carrier frequency used in the sector. Each base station sector receiver (604, 606, 608) is coupled to a sector antenna (receive antenna 630 for sector 1, receive antenna 632 for sector 2, receive antenna 634 for sector N), respectively, and is capable of receiving signals from wireless terminals covered by the sector, e.g., uplink signals including handoff requests, timing control signals, power control signals, and user data. In the case where a plurality of carrier frequencies are used in one sector, different receiving circuits can be or are generally included for the respective carrier frequencies used in the sector. Each receiver (604, 606, 608) includes a decoder (636, 638, 640), respectively, that decodes the received uplink encoded signal to extract the information being conveyed. According to the present invention, each sector receiver (610, 612, 614) is connected to a sector antenna (sector 1 transmit antenna 642, sector 2 transmit antenna 644, sector N transmit antenna 646), respectively, and is capable of transmitting to the covered sector signals including downlink broadcast signals such as beacon signals, and user-specific downlink signals such as signals containing information used in handoff operations to identify dedicated resources. Each sector transmitter (610, 612, 614) includes an encoder (648, 650, 652), respectively, for encoding downlink information prior to transmission. In some embodiments, base station 600 includes and uses a separate receiver, transmitter, and/or antenna for each sector, and optionally, carrier frequency in one sector of the cell. In some embodiments, the base station uses a single receiver that is functionally sectorized to receive signals from various sectors covered by the base station, a single transmitter that is functionally sectorized to transmit signals to various sectors covered by the base station, and/or sectorized antennas, e.g., antennas having different elements corresponding to different sectors. In some embodiments, sector beacon transmitters (622, 624, 626) are also included and coupled to the transmit antennas (642, 644, 646), respectively, and the sector beacon transmitters (622, 624, 626) are configured to transmit some or all of the beacon signaling to allow for the transmission of multiple beacon signals simultaneously, and in some embodiments, to limit interruptions in ordinary signaling by offloading some or all of the beacon transmission functions.

Base station I/O interface 616 connects base station 600 with other network nodes, such as other access nodes (base stations), routers, AAA servers, home agent nodes, and the internet. According to some embodiments of the invention, the handover signaling is communicated over the I/O interface 616 between the base stations before the current radio link is terminated and a new radio link is established.

The clock module 618 is used to maintain timing synchronization between the various sectors covered by the base station. Synchronization between different sectors of the same cell allows intra-cell inter-sector and intra-cell intra-sector inter-carrier handoff operations to be performed in a more efficient manner, such as: reduced or eliminated wireless terminal timing synchronization steps are employed as compared to inter-cell handoff operations where the WT needs to perform timing synchronization steps with a new attachment point before communicating power control information and/or user data.

Memory 620 includes routines (routine)654 and data/information 656. The processor 602 executes the routines 654 and uses the data/information 656 in memory 620 to control the operation of the base station 600, including ordinary scheduling functions, base station power control, base station timing control, communications, signaling, and also including the novel features of the present invention including beacon signaling and handoff operations.

Data/information 656 in memory 620 includes multiple data/information sets, e.g., data/information sets for various sectors covered by a base station (sector 1 data/information set 658, sector N data/information set 660, sector 1 data/information set 658 including data 661, base station to base station information 662, sector information 664, beacon information 666, and Wireless Terminal (WT) data/information 668, data 661 including user data to be transmitted to and received from a wireless terminal, base station to base station information 662 including information communicated between BSs pertaining to handoff signaling and stored security information, e.g., security keys used to establish a secure link between base stations prior to communicating handoff information between base stations, sector information 664 including carrier information 670, e.g., carrier frequency and bandwidth associated with the sector, sector information 664 further including resource information 672, e.g., information identifying dedicated resources that may be allocated to WTs for use in handoff operations, e.g., WT identifiers assigned to base stations, uplink dedicated segments such as timing control channel segments, power control channel segments, and traffic channel segments.

Beacon information 666 includes tone information 674, such as information associated with beacon signals in various sectors having particular frequencies; timing information 676, e.g., information identifying beacon signal transmission timing and information identifying a timing relationship between the beacon signal and dedicated uplink resources that may be allocated for handover operations; and tone hopping information 678, e.g., information used to generate hopping sequences used, e.g., for beacon signals used to convey cell identification information (e.g., slope).

WT data/information 668 includes a plurality of WT data/information sets for respective WTs: WT1 data/information 680, WT N data/information 682. WT1 data/information 680 includes user data 684 in the route to/from WT 1; terminal ID686 associating the WT with the base station; and sector ID information 688, which includes information identifying the sector in which WT1 is currently located by sub-ID and associating WT1 with a particular carrier frequency for ordinary signaling. Sector ID information 688 also includes information identifying the sector where WT1 requested as a new attachment point in a handoff request. WT1 data/information 680 also includes dedicated resource information 690, e.g., information from the set of sector-specific resource information 672 assigned to WT1 for use in a set of handoff operations. In different types of handoff operations, different resources may be allocated to WT1, which may also be included in dedicated resource information 690. For example, an inter-cell handoff to sector 1 of BS600 may include assigning a dedicated device identifier, a dedicated uplink timing channel segment, and/or a dedicated uplink power control channel segment for use in a particular sector communicating on a particular carrier to WT1, while an inter-cell sector or intra-cell intra-sector inter-carrier handoff to sector 1 of BS600, or a handoff within sector 1 of BS600 may ignore assigning uplink timing control channel segments to WT1 and include assignment of uplink power control channel segments to WT 1. Handoff messages 692 include handoff messages belonging to WT1, e.g., handoff request messages received directly or indirectly from WT1, WT1 requesting initiation of a different attachment point; a dedicated resource allocation message sent to WT1 identifying resources, e.g., identifiers and/or uplink segments, which may be used to establish a new wireless communication link with a new attachment point; and the base station establishes a message for the base station secure communication link. Mode information 694 includes information identifying the operational status (e.g., ON, hold, access, etc.) of WT1, as well as information identifying whether a wireless link has been established, is being established, or is in the process of terminating between WT1 and sector 1 of base station 600. Mode information 694 also includes information identifying new wireless links that are established between WT1 and other base stations and/or other sector attachment points.

Routines 654 include a plurality of routine sets such as routines for each sector covered by the base station (sector 1 routines 651.... sector N routines 653). Routines 651 include communications routines 655 and base station control routines 657. The communications routines 655 implement the various communications protocols used by the base station. Base station control routines 657 use data/information 658 to control the operation of base station sector 1 including the receiver 604, transmitter 610, optional beacon transmitter 622, I/O interface 616, scheduling, normal control and data signaling, beacon signaling, and handoff operations in accordance with the present invention. Base station control routines 657 include scheduler module 659, signaling routines 661, handoff routines 663, WT timing control module 665, and WT power control module 667. A scheduler module 659, e.g., scheduler, schedules air link resources, e.g., time-based bandwidths in segments, to wireless terminals for uplink and downlink communications.

Signaling routines 661 control one or more of: receiver, decoder, transmitter, encoder, ordinary signal generation, beacon signal generation, data and control tone hopping, signaling, signal repetition, and handover signaling. Signaling routines 661 includes a beacon module 669 and a handoff signaling module 671. Beacon module 669 uses beacon information (e.g., beacon information 666 for sector 1) to control the generation and transmission of beacon signals in accordance with the present invention. According to the present invention, a beacon signal can be transmitted in each sector in each carrier band used in the sector. In some embodiments, the beacon signal is transmitted by a sector transmitter (610, 612, 614). In other embodiments, some or all of the beacon signals may be transmitted by a beacon transmitter (622, 624, 626). The handover signaling module 671 controls handover signaling, such as handover message 692 sent from sector 1 of base station 600 or received through sector 1 of base station 600.

The handover routine 663 includes a request processing module 673, a secure base station to base station link establishment module 675, a dedicated resource allocation module 677, a registration module 679, and a radio link establishment/termination module 681. Request processing module 673 receives and processes WT requests to establish a new wireless communication link with a base station sector attachment point. The base station-to-base station link establishment module 675 uses data/information 656 containing BS-BS information 662 to establish a secure communication link between sector 1 of BS600 and other base stations that can be used to communicate handover information via I/O interface 616. Dedicated resource allocation module 677 allocates dedicated resources, such as resources identified in resource information 672, to WTs that have requested a handoff to sector 1 of base station 600. Module 677 may generate information such as dedicated resource information 690 and form such information into a handoff message 692 for specifying an identifier, an uplink timing control channel segment, an uplink power control channel segment, and/or an uplink traffic channel segment, which may be communicated to WTs, either directly or indirectly, via handoff signaling module 671, e.g., depending on whether an inter-cell or intra-cell handoff operation is involved. Registration module 679 may control the performance of registration operations when the WT requests initiation and establishment of a new wireless link with a sector 1 attachment point of base station 600. For example, different registration operation sequences may be used depending on whether the handover is inter-cell or intra-cell, as to whether the timing synchronization step is performed or not. The radio link establishment/termination module 681 controls operations in establishment and termination of a radio link for sector 1 of the BS 600. For example, in the case of establishing a new wireless link, module 681 confirms: a new link may be established when the earliest assigned dedicated uplink segment has been assigned to the WT requesting the handoff, looking for uplink signaling from the WT at the appropriate time. In the case of a radio link termination between sector 1 of BS600 and a WT, for example, the termination may be based on the BS not receiving any signaling from the WT at a predetermined time, and after a timeout determination, module 681 performs timeout measurements and relinquishes resources, such as identifiers and associated dedicated segments. Alternative termination methods are possible, such as sector 1 of the BS being able to monitor the signaling of the handoff corresponding to a new attachment point, such as handoff messaging I/O interface 616, and determine when to establish and terminate a new wireless link based on the determined time. Alternatively, the WT may communicate a termination message to sector 1 of BS 600.

WT timing control module 665 operates to control the timing of WTs, e.g., to synchronize WTs with sector 1 of BS600 so that signals can be processed and decoded. Module 665 processes received timing control information received on a dedicated uplink timing control segment assigned by sector 1 of BS600 to WTs seeking to establish a new wireless link. In addition, timing control module 665 generates timing correction signals that are transmitted over the established wireless link via the BS sector transmitter, which the WT uses for transmission of timing adjustments.

WT power control module 667 operates to control the power of WTs, e.g., their uplink transmission power. WT power control module 667 processes received power control information received on dedicated uplink power control segments allocated by sector 1 of BS600 to WTs seeking to establish a new radio link.

Fig. 7 illustrates an exemplary wireless terminal (end node) 700, such as a mobile node, implemented in accordance with the present invention. The wireless terminal 700 of fig. 7 may be shown in more detail as any one of the end nodes in the system of fig. 1, 2 or 5. Wireless terminal 700 includes a receiver 702, a transmitter 704, a processor 706 (e.g., CPU), a user interface (I/O) device 708, and a memory 710 coupled together via a bus 712 over which the various elements can exchange data and information with one another. The receiver 702, including the decoder 714, is coupled to an antenna 716, and the wireless terminal 700 may receive downlink signaling including beacon signaling and a handover message including information identifying the dedicated resources transmitted from the base station 600 through the antenna 716, according to the present invention. A decoder 714 in receiver 702 may decode the normal signaling and user error correction coding processes intended for WT700 in an attempt to recover information written thereon or interfered with by other signals, including beacon signals. Transmitter 704, which includes encoder 718, is coupled to antenna 720 and can transmit signals to base station 600 including a request to initiate handoff of WT700 to another base station sector attachment point, timing synchronization information based on dedicated uplink timing channel segments, power synchronization information based on dedicated uplink power control channel segments, and user data based on dedicated uplink traffic channel segments. According to the invention, different types of handovers are possible, said handovers containing one or more of the following features: inter-cell, inter-sector, and/or inter-carrier.

User I/O devices 708, e.g., speakers, microphone, keyboard, display screen, mouse, video camera, etc., provide the user of WT700 with the ability to input user data/information intended for a peer node, as well as the ability to access user data/information received from a peer node. Wireless terminal memory 710 includes routines 722 and data/information 724. Processor 706 executes routines 722 and uses data/information 724 in memory 710 to control the operation of wireless terminal 700, including performing the beacon functions and handoff operations of the present invention.

Wireless terminal data/information 724 includes user data 726, such as voice, text, or other types of data, and/or files that are intended to be transmitted to and/or received from a peer node in a communication session with the wireless terminal 700, for example. Data/information 724 also includes current base station sector user information 728, new base station sector user information 730, and system information 732.

Current base station sector user information 728 includes terminal ID information 734, base station ID information 736, sector ID information 738, mode information 740, identified beacon information 742, received timing correction signal information 744, and a determined time to terminate wireless link 746. Terminal ID information 734 may be an identifier assigned by a base station sector to WT700, or a plurality of identifiers, to which WT700 is currently connected via a wireless link for identifying wireless terminal 700. The base station ID information 736 can be, for example, a base station identifier, such as a slope value associated with the base station and used in a frequency hopping sequence. Sector ID information 738 includes information for identifying the ID of the transmitter/receiver of the base station currently sectorized through which ordinary signaling is communicated, and corresponds to the sector of the cell in which the wireless terminal is located. Carrier frequency information (CF)735 indicating the carrier frequency used for the current communication link is also sometimes stored in information 728 of data/information 724 in memory 710. The mode information 740 identifies whether the wireless terminal is in an active (ON)/hold/sleep state. The identified beacon information 742 may include: information received and measured about each beacon signal, such as cell/sector ID, signal strength level, filtered signal strength level, and carrier frequency associated with ordinary signaling in the sector from which the beacon signal is transmitted. Identified beacon information 742 may also include information identifying the current attachment point sector beacon, information comparing neighboring sector beacons to the current WT sector beacon, and information comparing measured beacon signals and/or information derived from measured beacon signals to handoff criteria. Received timing correction signal information 744 includes timing correction signals received over the established radio link and transmission timing adjustment information used to correct the timing of signals transmitted by WT700 over the established radio link. The determined time for terminating wireless link 746 is, for example, signaling received over the air link (over the air) from a new base station sector attachment point, a time determined by WT700 to terminate its established wireless link during handoff, such as beacon signaling and assigned dedicated uplink segments, and/or communications received based on an existing link with the current base station.

New BS sector user information 730 includes terminal ID information 748, base station ID information 750, sector ID information 752, mode information 754, identified beacon information 756, dedicated resource information 758, handoff message 760 and handoff type information 762, and carrier frequency information (CF) 759. Terminal ID information 748 can be an identifier or identifiers assigned by a base station sector to WT700 that WT700 has requested handoff to a base station sector. The base station ID information 750 may be, for example, a slope value associated with the new base station and used in the frequency hopping sequence. Sector ID information 752 includes a sector ID identifying the transmitter/receiver of the new sectorized base station through which ordinary signaling is communicated via the new wireless link. Mode information 754 identifies the WT's operational state with respect to the new BS sector attachment point, e.g., sending a handoff request, waiting for dedicated resource assignments, receiving and processing dedicated resources such as assigned identifiers and/or assigned dedicated uplink segments, performing handoff operations such as sending timing control and/or power control signaling on dedicated uplink segments, completing handoff, sending user data, holding state, active state, dormant state. Identified beacon information 756 includes information such as timing information for beacons received from the new BS sector attachment point. A timing relationship exists between the new BS sector attachment point beacon signal and a dedicated uplink segment that may be allocated to WT700 as a resource, e.g., in a handoff operation, allowing WT700 to determine in time the point for terminating a currently established wireless link and initiate uplink signaling for the new BS sector attachment point for establishing the new wireless link to enable minimization of interruption time intervals during the handoff process.

Dedicated resource information 758 includes information such as a WT identifier assigned to a BS sector and/or information from a new BS sector attachment point identifying dedicated uplink channel segments that have been assigned to WT700 for use in handoff operations. In different types of handoff operations, different resources may be dedicated to WT700 and may be included in dedicated resource information 758. For example, information identifying dedicated uplink timing channel segments and uplink power control channel segments for WT700 may be included in inter-cell handover information 758, whereas the location of uplink timing control channel segments with respect to WT700 may be ignored in intra-cell inter-sector handover information 758 and the location of uplink power control channel segments with respect to WT700 is included. Handoff message 760 comprises a handoff message for WT700, e.g., a handoff request initiation message sent via the currently established wireless link and BS sector, then over the backhaul link to the newly requested BS sector attachment point. Handover message 760 can also include dedicated resource allocation messages that are originally sourced from the new base station sector attachment point, sent from the base station to the base station via the backhaul link, and received from the current base station sector attachment point via the current burst period. The handover type information 762 includes information for identifying a type of a requested handover, such as an inter-cell handover operation, an intra-cell inter-sector handover operation, or an intra-cell inter-carrier handover operation. In some embodiments, inter-cell and inter-sector handovers are also distinguished by determining whether the handover operation is an inter-carrier handover operation.

System information 732 includes beacon ID information 764, handover criteria 766, cell/sector ID information 768, beacon/dedicated segment timing information 770, and handover type/operation information 772. System information 732 includes structure information of the wireless communication system, such as the use of base station frequencies, timing structure, and reception intervals. Beacon ID information 764 includes information, e.g., look-up tables, equations, etc., that are used to associate a particular sector/cell beacon in the communication system with a particular frequency at a particular time, allowing WT700 to identify the received beacon signal. Handoff criteria 766 may include threshold values used by wireless terminal 700 to trigger handoff requests for neighboring sectors/cells, such as a minimum threshold value relating to the strength level of a beacon signal from a neighboring sector and/or a threshold level relating to the strength of a neighboring sector received beacon compared to the WT's own current sector received beacon signal strength. Cell/sector ID information 768 may include information for constructing frequency hopping sequences for use in the processing, transmission, and reception of data, information, control signals, and beacon signals. Cell/sector ID information 768 may also include carrier information 774. Carrier information 774 includes information associating individual sectors/cells of a base station in the communication system with particular carrier frequencies, bandwidths, and tone sets. In some embodiments, the base station sector uses different sets of uncovered tones for uplink and downlink signaling. Beacon/dedicated segment timing information 770 includes information defining the timing relationship between beacon signals transmitted by BS sectors and dedicated uplink segments that may be assigned by BS sectors to WT700 for handoff. The handover type/operation information 772 includes information indicating a step or a sequence of steps performed as a function of the handover type. For example, an inter-cell handover may include a timing synchronization step that is ignored for intra-cell handovers.

Routines 722 include a communications routine 776 and wireless terminal control routines 778. Wireless terminal control routines 778 include signaling routines 780 (including beacon routines 782, handoff routines 784), user data signaling module 786 and ongoing wireless terminal timing control module 788. Wireless terminal communications routines 776 implement the various communications protocols used by the wireless terminal.

Wireless terminal control routines 778 perform basic control functions for the wireless terminal including power control, timing control, signaling control, data processing, I/O control of beacon-related functions, and control of handover signaling and operations in accordance with the present invention. Signaling routines 780 use the data/information 724 in memory 710 to control the operation of receiver 702 and transmitter 704 to perform operations including beacon signal reception and processing, handover signaling and processing, and user data signaling and processing.

The beacon routine 782 includes a beacon processing and ID module 790, a beacon strength measurement module 792, a beacon comparison module 794, and a handoff decision module 796. The beacon processing and ID module 790 uses system information 732, including beacon ID information 764 and cell/sector ID information 768, to identify received beacon signals and stores this information in user-identified beacon information 742. The beacon signal strength measurement module 792 measures the signal strength of the received beacon signal and stores this information into the user identified beacon information 742. The beacon comparison module 794 compares the identified beacon information 742 to obtain information that can be used to determine when to initiate a handoff to a neighboring sector/cell. The beacon comparison module 794 may compare the individual beacon signal strength levels to a minimum threshold level in the handoff criterion 766. The beacon comparison module 794 may also compare the relative signal strength levels between the WT's own beacon strength and the neighboring sector/cell beacon signals. The beacon comparison module 794 may also compare the relative strength level difference measurements to thresholds in the handoff criterion 766. The handoff decision module 796 receives the output information from the beacon comparison module 794 and determines whether to initiate a handoff request, which carrier frequency the base station sector will use to initiate the handoff request. The handoff decision module 796 may consider other information such as in processing the user data session to minimize disruption when considering the time for initiating the request.

When triggered by an output from handover decision module 796, handover routine 784 generates signaling to initiate an inter-sector, inter-cell, and/or inter-carrier handover and performs operations to complete the handover. In various embodiments, the beacon signals discussed above are typically used to identify the carrier frequency and base station sector attachment point to be used for the new wireless link after handoff. Handoff routines 784 include a request module 701, a dedicated resources module 703, a registration module 705, a radio link establishment/termination module 707, a wireless terminal timing control module 709, and a wireless terminal power control module 711.

Request module 701 generates a request by WT700 to initiate and establish a new wireless communication link with a different base station sector attachment point. Dedicated resource module 703 receives and processes received signals containing signals identifying dedicated resources, e.g., identifiers and/or dedicated uplink segments, that have been assigned by the new BS sector attachment point to WT700 for handoff operations. Module 703 may receive handover message 760, and may extract and store dedicated resource information 758 from the handover message 760. Such information in handover message 760 specifies the identifier, uplink control channel segment, uplink power control channel segment, and/or uplink traffic channel segment. Registration module 705 controls the performance of registration operations by WT700 requesting initiation and establishment of a new wireless link with a base station sector attachment point using data/information 724 including handoff type information 762 and handoff type/operation information 772. Different registration operation sequences may be used depending on whether the handover is inter-cell or intra-cell, e.g. whether a timing synchronization step is performed. The registration module 705 may also include signaling for a home agent associated with the WT to identify a new point of attachment at the appropriate time. The radio link establishment/termination module 707 controls operations in the process of new radio link establishment and old radio link termination for handover. For example, in the case of a new wireless link setup, module 707 acknowledges that the new link can be established by the new base station sector attachment point when the earliest assigned dedicated uplink segment has been assigned to the WT requesting the handoff, and thus by performing uplink signaling at the assigned time. Termination may be performed, for example, by WT700 as part of a handoff operation, i.e., in the case of terminating an established wireless link, to stop transmission on the established wireless link at an appropriate time, e.g., just prior to the occurrence of the earliest dedicated uplink segment that has been assigned to the WT by a new BS sector attachment point. The timing of received beacon signals stored in information 756, which information 758 has been assigned to WT700 by the new BS sector, and its known relationship to dedicated resources identified in information 758, may be used in conjunction with a beacon for dedicated segment timing information 770 indicating an offset between the dedicated resources and the beacon to determine a termination time, e.g., such that the termination occurs shortly before the time that resources dedicated to the WT to establish a new link can be used. Alternative approaches are possible, e.g., WT700 may communicate a termination message to the base station sector attachment point over the original wireless link before communicating to the new BS sector on the earliest dedicated uplink segment. In another embodiment, upon successfully receiving uplink signaling from the WT during the assigned dedicated segment, the new BS sector may communicate a termination message to the original BS sector WT attachment point based on the backhaul BS-to-BS link.

WT timing control module 709 operates to control the timing of WT700, e.g., to synchronize WT700 with a new BS sector attachment point so that signals can be processed and decoded. As part of the timing synchronization operation, module 709 generates and transmits timing control information on a dedicated uplink timing control segment allocated by the new BS sector attachment point. In response to the timing signals received from the BS, the WT timing control module 709 will modify the symbol transmission timing, e.g., a clock used to control the symbol transmission timing, so that symbols are received at the BS from different WTs in a synchronized manner. As part of the WT power control operation, WT power control module 711 generates and transmits power control information on a dedicated uplink power control segment allocated by the new BS sector attachment point. Thus, module 711 adjusts the WT transmit power level in response to power control signals received from the BS, e.g., as part of power control operations. In some embodiments, modules 709 and 711 receive and process control signals from a new BS sector attachment point in addition to generating and transmitting control signals as part of WT timing and/or power control operations, e.g., adjusting WT transmit timing and/or WT transmit power as part of a handoff operation.

User data signaling module 786 performs operations that include the use of dedicated resources, e.g., dedicated uplink traffic channel segments allocated by the new BS sector attachment point to WT700 for the new wireless link, to control the transmission of user data based on the new wireless link. An operating wireless terminal timing control module 788, which receives and processes timing control signals that have been communicated over the established wireless link, is used by the established wireless communication link to maintain timing control between the current BS sector attachment point and WT 700. Processing of block 788 includes, for example, operating WT700 to make transmit timing adjustments, i.e., to adjust timing of signals, e.g., symbols, transmitted by WT700 over an established wireless link. In certain embodiments, intra-cell inter-sector and/or inter-sector carrier handoff operations performed by WTs 700 can use timing synchronization performed with a fixed offset such that dedicated resources for timing adjustment are not necessary and allocated to new BS sector attachment points and are not necessary and not used by WTs 700 prior to allocation and use of at least one uplink segment for communicating non-timing control data. In such embodiments, in the case of an intra-cell handoff, the WT can terminate an existing link, establish a new link with a new carrier or sector, and transmit power control signals and/or user data, before changing its transmitter timing in response to timing control signals transmitted over the air link from a new BS sector attachment point.

According to this particular embodiment, the base station may not have to transmit beacon signals corresponding to the respective system bands to a given sector. In some embodiments, a base station may limit beacon signals transmitted to a given sector to correspond to a subset of frequency bands used by its own sector and neighboring sectors. In some embodiments, for each individual sector, the base station may limit the beacon signals transmitted to a given sector to a subset corresponding to the frequency bands used in adjacent sectors.

Although a communication system having a bandwidth divided between 3 carrier slots (frequency bands) is shown, the present invention is applicable to other communication systems that do not use the same frequency band in the system.

In some embodiments various features or elements of the invention may be implemented in one part of a communication system and not in other parts of the system. In such embodiments, wireless terminals implemented in accordance with the present invention may utilize the beacon signaling features and methods of the present invention in determining that inter-sector and/or inter-cell handover is available.

Various features of the switching method and apparatus of the present invention will now be described with reference to fig. 6-11.

In the case of cells without sectorization, each cell is typically served by a single base station. In the case of sectorized cells, each sector may be served by a different base station or sectorized base stations may be used. Fig. 6 illustrates an exemplary sectorized base station (access node) 600, where each sector is served by a separate receiver (sector 1 receiver 604, sector 2 receiver 606.... receiver 608, sector N receiver) and transmitter (sector 1 transmitter 610, sector 2 transmitter 612.... transmitter 614, sector N transmitter 614) connected to different antennas used in each sector. Alternatively, each sector receiver may be coupled to a different portion (e.g., element) of an antenna that performs similar sectorization, where each portion corresponds to a sector. Similarly, each sector transmitter may be coupled to a different portion (e.g., element) of a sectorized antenna, where each portion corresponds to a sector. In some embodiments, for example, where uplink and downlink signals use different sets of non-cross tones for a given sector, the same antenna or antenna portion may be used by the receiver and transmitter for the given sector.

Thus, in the case of the sectorized base station embodiment 600, the cell base station 600 includes one receiver and transmitter in each sector, each of which includes analog filters along with associated routines, modules and data/information that operate on a sector basis to handle mobile node registration and other operations in individual sectors. Thus, each sector of the base station 600 includes multiple routines sets (the routines 651.... for sector 1, routine 653) and multiple data/information sets (the data/information set 658.. for sector 1, data/information set 660). An intra-cell inter-sector handoff from one sector to another can be considered as: handover from a base station sector or module included therein corresponding to a first sector to a base station module corresponding to a second sector in the same cell.

In some embodiments, the use of a single base station 600 in a sectorized cell facilitates timing synchronization between the sectors of the cell. A common clock circuit included in clock module 618 may be shared among the base station modules that make up a multi-sector cell so that symbol timing and other operations in a single sector of the cell are synchronized. In the case of an intra-cell handover, it is possible to reduce or eliminate the need to perform an initial timing synchronization operation when performing a handover from a timing synchronization hold reliable start while maintaining symbol timing by different sectors of the cell. Thus, in at least some embodiments, the time required to effect an intra-cell handover is reduced by avoiding timing synchronization operations that are used when non-synchronized mobile devices enter the system. The intra-cell handover may be an inter-sector handover. Thus, intra-cell handovers may be implemented using less time and/or resources than inter-cell handovers.

For purposes of explaining the present invention, it should be understood that each cell includes at least one sector and one base station. A multi-sector cell and base station 600 as shown in fig. 6 is used in some embodiments. In some embodiments a sector can support multiple carrier frequencies. Handoff exists between sectors or between carriers in a sector. In the case of a multi-sector cell, there may also be intra-cell as well as inter-cell handovers. Handoff includes the transfer of information, signaling of the physical layer (e.g., including device ID assignments for a sector and/or carriers in a sector), and other signaling layer operations, such as through power and/or timing control performed by modules of a sector involved in the handoff. For example, data may be transferred from one sector to another sector via communication links, such as wireless links or wired links over optical fiber, that exist between one or more base stations and/or between modules corresponding to multiple sectors of a single base station.

For purposes of discussion, it is assumed that neighboring cells use different frequencies. However, the handoff method of the present invention may also be used in systems having frequency reuse factors, such as in implementations of the same frequency used in different sectors (e.g., adjacent sectors), with steps related to adapting the filter/receiver to different frequencies omitted from the handoff process.

Figure 9 is a diagram of an exemplary system 900 implemented in accordance with the present invention including a first base station (BS1)901, a second base station (BS2)903, a WT902, and a mobile Internet Protocol (IP) Home Agent (HA) node 914. BSs 901, 903 may be similar to exemplary BS600, while WT900 may be similar to exemplary WT 700.

Using a variety of approaches, a mobile node such as the Wireless Terminal (WT)902 shown in fig. 9 for use in an existing communication session with a first base station sector 904, via the first base station (BS1)901, may identify a cell and/or sector 906 in a second base station (BS2)903 (and/or sector carrier if multiple carriers are supported in one sector) for handoff, e.g., because there are better signal conditions between the same cell or sector 906 than the current cell or sector 904 has. For purposes of explaining the present invention, the discussion will be limited to only examples where a single carrier is used in each sector. For purposes of discussion, communications using the current wireless link 950 via wireless signaling (e.g., radio signaling) with base station sector 904, a mobile device, WT902, will now be described with "current base station sector". The mobile device, WT902, has network connectivity and the wireless connection 950 is connected to the current base station sector 904 through the network and to other base station sectors in the same or other cells via links 920, 924, 922. The base station sector to which the mobile device, WT902, wishes to handoff is referred to as the "new base station sector," which in this example is base station sector 906. In the case where there is one sector per base station, such as in the case of a single sector cell, the new base station sector will be the new base station for which the handover operation is completed. In the case of a multi-sector cell, the new base station sector may be part of a new base station or a different base station sector in the same cell as the current base station sector.

According to various embodiments of the invention, each sector of a base station periodically transmits a beacon signal, e.g., to f₂Frequency band or f₃Frequency band-the frequency band used by the current sector and the physically adjacent sector. Graph 802 of fig. 8 shows an exemplary downlink beacon signal (beacon 1808, beacon 2810.. beacon N812) from a base station sector transmitter on vertical axis 804 versus horizontal axis 806. In this example, there is at least one transmission of a beacon signal to the frequency band for a given base station sector transmitter during a first plurality of symbol timings (sometimes referred to as beacon slots). In one exemplary embodiment, each base station sector transmitter transmits a beacon signal during a beacon slot. The beacon signal sequence of beacon signals transmitted during one beacon slot uses different tones than beacon signals transmitted in another beacon slot where the sequence may be used. The beacon signal sequence transmitted by the sector transmitter may comprise beacon signals of different types, e.g. of carrier frequency f₁Associated beacon signal, associated carrier frequency f₂Associated beacon signal and associated carrier frequency f₃An associated beacon signal. Other types of beacon signals are also possible in accordance with the present invention, such as beacon signals used to convey cell and/or sector information. The beacon signal sequence is repeated for each superslot (ultraslot), each superslot comprising N beacon slots, where N is a positive integer. In this example, each beacon slot includes 8 super slots; each super slot comprising a fixed positive number of OFDM symbol times, e.g. 113An OFDM symbol time. Row 814 shows the premium slots, with 8 premium slots included in one beacon slot, and the beacon is transmitted at a fixed predetermined time within each 8 th premium slot. Row 816 shows a beacon slot that includes multiple beacon slots and row 818 shows a superslot. The beacon signal in the beacon slot at a particular index value of the super slot repeats from super slot to consecutive super slots. The physically adjacent sector used to transmit its own set of beacon signals may be the current (current attachment point) cell or an immediately adjacent cell.

Graph 820 shows the uplink frequency (tone) for the access segment on the vertical axis 822 versus the horizontal axis time 806. It should be noted that there is a time offset 824 between the start of the premium time slot on the downlink and the start of the corresponding time interval on the uplink. In this example, there is a set of 12 access segments, corresponding to respective super slots, that may be allocated as dedicated resources by a base station sector attachment point to a wireless terminal that has requested a handoff operation to the base station sector attachment point. Exemplary sets of access segments (826, 828, 830, 832, 834, 836, 838, 840, and 842) are shown in diagram 820. The transmission interference of WTs already registered in the cell is minimized by using a dedicated time period corresponding to the access slot. An access segment is a segment that allows a WT entering a sector to begin transmitting, e.g., to perform initial timing control operations for purposes of registration in the sector, and/or initial power control operations in the sector.

Groups of access segments exist during the access slot, e.g., group 826 exists during access slot 868. The combination of 12 access segments 826 includes access segments (844, 846, 848, 850, 852, 854, 856, 858, 860, 862, 864, and 866). The access segments corresponding to the base station sector attachment points have a fixed timing relationship with respect to the beacon signals transmitted by the base station sector transmitters. In some embodiments, the access segments corresponding to a base station sector attachment point have a fixed timing relationship with respect to other beacon signals transmitted by the same base station. It should be noted that because the beacon signals of different carriers transmitted by the same base station are synchronized to a fixed timing relationship, the access segment of one carrier has a fixed timing relationship with respect to the beacon signals transmitted by the same base station to the other carrier frequency bands used by the base station, and not just to the beacon signals of the same frequency band to which the access segment corresponds. This known relationship may be used by a wireless terminal used in a handover operation for determining a point in time to terminate a wireless link with a currently connected base station sector attachment point and for determining a point in time to begin communicating on an uplink using an assigned uplink access segment based on a new wireless link. Timing offset 870 illustrates an exemplary offset between the beacon signal 1808 and the earliest group of access segments 830. Each access segment includes one or more symbol timings and uses one or more tones. In the exemplary embodiment, each access field includes the same number of tone-symbols, which is one basic unit representing air link resources for one tone of one OFDM symbol interval time. In other embodiments, different numbers of access segments may be useful, and different types of access segments (e.g., access segments for different purposes) may include different numbers of tone-symbols. For example, an access segment used for timing control operations may have different characteristics than an access segment used for power control operations. Each access segment is a dedicated segment dedicated to mobile device access to uplink signals, e.g., registration, operations, e.g., device ID assignment, timing control and/or power control operations, wherein devices entering a sector are capable of performing such operations, e.g., using one or more of a plurality of fields (844, 846, 848, 850, 852, 854, 856, 858, 860, 862, 864, 866) dedicated to that purpose that have been scheduled by the base station sector for assignment to WTs.

In some embodiments, the access segment allocated for timing control operations will precede the segment allocated for power control operations. For example, in the case of an inter-cell handover operation, a WT may be assigned a segment from the combination of segments (844, 846, 848, 850, 852, 854) used to transmit timing control signals and a segment from the combination of segments (856, 858, 860, 862, 864, 866) used to transmit power control signals. The allocation of these dedicated resources has been communicated to the wireless terminal via the original wireless link, e.g., via current wireless link 950, after being communicated from BS 2903 to sector 904 of BS 1901 over network link 924. Different segments may use different tone-symbol combinations. In some embodiments, different types of access segments use different tone combinations. As shown in fig. 8, in some embodiments, the tone-symbols of the segments are adjacent; however, in other embodiments, the tone-symbols included in a segment may be discontinuous.

In some embodiments of the invention, the beacon signals are transmitted by the sectors to adjacent sectors during a set of N consecutive beacon slots using different frequencies. The N consecutive beacon slots are referred to as superslots. The exact pattern of beacon signaling in the superslot will not repeat in this exemplary embodiment, e.g., different beacon slots may use a slightly different frequency for the beacon tones, but the next superslot will not repeat. However, the beacon signaling pattern will repeat from one super slot to the next.

Based on the beacon signals received from the neighboring base station sectors, mobile device 902 can, and in various embodiments does, perform one or more of the following: the quality of the communication channel between the mobile device and the base station sector transmitting the beacon signal is determined and a decision to handoff is made by selecting between multiple sectors based on beacon signal measurements and other information such as traffic loading, determining a cell and/or sector identifier, e.g., slope of the cell including the transmitting sector, determining the frequency band of the sector (e.g., sector type) and/or sector carrier corresponding to the transmitted beacon signal, determining the relative timing in the premium time slot between the timing in the current base station sector and the timing in the premium time slot of the base station sector selected by the mobile node as the new base station sector, the handoff operation is completed for the new base station sector.

In accordance with the present invention, upon determining that a handoff is complete by the mobile device, for example, based on the relative strengths of beacon signals received from different network attachment points, the mobile device initiates the handoff through the current base station sector with which it is communicating. In this manner, a handoff can be initiated by the current base station sector without having to switch its transmitter/receiver circuits from the frequency band of the current base station sector to the frequency band of the new base station sector for the mobile device. Fig. 10 illustrates various exemplary handoffs involving signaling that occur in some embodiments. The mobile device 902 can transmit a signal 1002 to a current base station sector 904, e.g., a cell identifier and/or an identifier of a selector type corresponding to a neighboring sector 906 to which a handoff is to be effectuated. The current network attachment point, i.e., base station selector 904 in this example, uses this information to enable communication between mobile node 902 and the new network attachment point (e.g., base station sector 906 in this example). In some embodiments, the current BS sector acts as a router and simply relays handoff messages between the mobile node and the new point of network attachment. However, for communication purposes and to reduce the amount of signaling required on the current airlink for the mobile device, the current base station sector 904 can, and often does, act as a proxy for relaying communications between the mobile device 902 and the credit base station sector 906 or negotiating a handoff with a new base station sector 906 on behalf of the mobile device 902. Thus, handoff information for a mobile node is communicated to a new base station sector over a link within a base station connecting the base stations and/or sectors, where the link is often wired (e.g., copper wire or fiber optic line). For the case of links between base stations, such links may include backhaul links. Handoff communications between base station sectors may, and typically do, undergo authentication and/or other security procedures, including encryption, before being allowed to proceed. In these embodiments, a secure communication link is established between a current network attachment point and a new network attachment point having a handover message that includes a resource allocation communicated over the secure link.

In fig. 10, signal 1004 represents transmission of a signal from BS1 to new BS sector 906 to initiate a handoff on behalf of mobile node 902. The signaling may include mobile node identification information as well as a base station identifier, and a sector identifier provided by the mobile node 902 and/or other information indicating an intention to initiate a handoff. The new BS sector 906 responds to the current BS sector 904 by sending a security query 1006. BS sector 904 responds with a correct reply 1008 to establish a secure communication link to facilitate other handoff-related signaling. In an alternative embodiment, at least some of the steps 1004, 1006, and 1008 described above are omitted. The WT sends information (see step 1010 below) to the new base station sector via the current base station sector.

Once a sufficient level of security is established between the current base station sector 904 and the new base station sector 906, the mobile node 902 can communicate information containing its intent to complete the handoff to the new base station sector 906 via the current base station sector 904 and/or receive information from the new base station sector 906. After the current base station sector 904 signals to the new base station sector 906 that a mobile device handoff is to begin, the new base station sector 906 allocates a dedicated communication resource, e.g., at least one device identifier to be used by the mobile device 902 with respect to air link signaling upon entering the new sector 906, to the mobile device 902. In some systems, multiple device identifiers are assigned to mobile node 902 for use in sector 906, e.g., one of the device identifiers is used by the mobile node when operating in an "active state" while another identifier identifies the mobile node in a set that includes a substantial number of mobile nodes that are capable of operating in a sector in a "hold" state in a cell. Signal 1010 indicates the delivery of a device identifier and resource allocation information to WT902 via current BS sector 904. Thus, the mobile node 902 is assigned a device identifier by the new base station sector 906 via the current base station sector 904, the device identifier being used for physical layer signaling such as over-the-air signaling. In addition to assigning a device identifier to be used in the new base station sector, the new base station sector 906 can, and typically does, reserve dedicated resources, e.g., uplink and/or downlink channel segments, for the purpose of a mobile device performing access, e.g., including initial closed loop power control and/or timing control signaling upon entering a cell as part of a registration process. In various embodiments, a set of tone-symbols dedicated to timing control (e.g., segment 844) and/or a set of tone-symbols dedicated for power control signaling purposes during registration (e.g., segment 856) is assigned to mobile device 902 by new base station sector 906. Each set of dedicated tone symbols (e.g., segment 844) may be one of a plurality of tone-symbol sets (e.g., segments) available in a particular portion of the air link resources (e.g., available for handover but not available for a new initial entry into the cell). These tone-symbol sets (e.g., segment 844) are used based on the allocation given by the base station. Thus, although these tone-symbol sets, e.g., segments, are used for access, there is no contention among these tone-symbol sets, e.g., segments, because they are dedicated to a particular WT. Also, since the tone-symbol sets are used based on allocations, the base station knows which mobile device should use which tone-symbol set, e.g., segment, in contrast to the contention-based random access case, where the base station does not need to know the identity of the mobile device even after the base station detects one or more access signals. In signal 1010, which may include multiple IP packets and/or separate messages, the allocation of dedicated resources to be used to accomplish timing and/or power control upon entry into a new base station sector, as well as information identifying the time period in the superslot in which the resources are dedicated to the mobile device (e.g., the timing of dedicated uplink segments in the superslot) is communicated to the mobile device. In some embodiments, the time period is specified in the superslot to account for the fact that a mobile node 902 in a current base station sector 904 takes longer to communicate via between the current base station sector 904 and a new base station sector 906 than a premium time slot period for communication due to communication delays associated with the link between the base station sectors 904, 906. The mobile device (e.g., WT in some embodiments) interprets the assignment information using stored information regarding the frame structure of the communication channel used by the new network access point. This information may be, and in some embodiments is, accessed and retrieved using beacon information. For example, the WT may retrieve channel information related to the network access point corresponding to the beacon signal from memory communications, which results in the network access point being selected as a new network access point. This information can be used to interpret resource allocation information received from the new network access point and/or to determine the time of the dedicated segment relative to the time of receiving the beacon signal.

In addition to dedicated resources, e.g., tone-symbol sets (e.g., segments) are cancelled in a particular access slot for a pre-allocated mobile node that has signaled an intention that a new base station sector wants to transmit; other tone-symbol sets (e.g., other segments) may be effective in some embodiments based on contention usage, e.g., for mobile nodes that newly enter a cell, without prior performance of timing and power control operations via other base station sector notifications. Fig. 8 shows such contention-based segments cancelled during each access slot. During the access slot 858, the 4 exemplary contention-based segments are cancelled, as represented by segment 872. Similarly, in subsequent access slots, the combination of contention-based segments 874, 876, 878, 880, 882, 884, 886, and 888 is active. Each 4 segment combination, e.g., segment combination 872, can accommodate 2 WTs, with each WT using one segment for timing synchronization operations and one segment for power control operations. In some embodiments, the WT utilizes the allocated dedicated access segment to use the contention-based access segment after the handover attempt fails.

By using dedicated pre-allocated resources, e.g., registration slots, during the access (registration) time interval, as opposed to attempting to use resources that may conflict, e.g., due to competing devices attempting to use the same set of tones at the same time, the mobile node has the opportunity to enter the base station sector and be able to complete registration, time control, and/or power control operations at predictable times, e.g., at a particular time within a superslot, in a much greater increase than is the case with contention-based resource allocation.

Upon entry into a base station sector from another cell, the mobile node may request that timing synchronization and/or power control signaling be completed before allowing reception/transmission of IP packets corresponding to the communication session via the new base station sector. The handoff method of the present invention increases the predictability of when such IP signaling exists while reducing the time required to complete the physical layer power control and timing synchronization operations once a new base station sector is entered.

In accordance with one feature of the invention, an IP route update signal 1012 is sent via the current base station sector 904 to cause IP packets intended for mobile node 902 to be redirected to the new base station sector 906 upon initiation of a handoff operation. This typically occurs before handover signaling has been completed in the new sector, e.g., before registration, power control signaling, and/or timing control signaling have been completed, which is necessary for the WT to receive/transmit packets via the new link. The signaling 1012 may be a mobile IP home agent 914 that is responsible for redirecting packets addressed to the mobile node 902 to the mobile node's current network point of attachment. A given delay associated with communication of such route update signals, by initiating redirection of IP packets from the current base station before physical layer signaling setup operations are completed in the new base station, may correspond to a time period during which the mobile device 902 is temporarily unreachable due to delays associated with band switching, timing synchronization operations, and/or power control signaling. Thus, the IP route update operation can be completed by the time the mobile device 902 can receive IP packets in the new base station sector 906, or shortly thereafter.

In some embodiments, an IP route update request 1012 is sent in response to the mobile device 902, the mobile device 902 having been assigned resources, e.g., an identifier, to be used in the new base station sector 906 and/or dedicated communication resources needed to complete any timing control and/or power control operations that need to be completed before the mobile device 902 can receive IP packets in the new base station sector 906. In such an embodiment, the IP routing update 1012 is sent via the current base station sector 904 after it has been certain that the handover operation will be successfully completed. A route update message may be triggered at BS1 by receiving a resource allocation message from BS2 directed to the WTs seeking to complete the handoff. In these cases, if new base station sector 906 is unable to allocate the resources needed to accept mobile device 902, e.g., because the maximum supportable number of devices already exist and are valid in the cell excluding the device ID allocation, then an IP routing update will not be triggered. If packets are received at the new BS before the WT establishes a communication link with the new BS, the BS stores the received packets in a buffer and provides packets addressed to the WT based on the newly established communication link once handoff and establishment of the communication link are successfully completed.

A handoff initiated via the current base station sector 904 may not be successfully completed, e.g., due to interference with the new base station sector 906 by the dedicated tone-symbol set allocated for initial timing and/or power control operations. In some cases, the handoff process described above is repeated, but this requires re-established connectivity through the old BS sector 904. However, in other cases, rather than attempting to initiate a handoff again via the current base station sector 904, having transitioned to the sub-band of the new base station sector 906 to terminate the communication link via the old sector, the mobile node 902 registers in the cell in the same manner as other mobile devices entering the cell (which do not have pre-existing communication sessions with neighboring sectors). In such embodiments, as part of a handoff operation, if registration is not successfully completed using the dedicated resource set assigned to the device, base station sector 906 releases air link resources dedicated to WTs seeking to complete the handoff, e.g., releases the assigned mobile device identifier for use by other devices.

After successful registration with a new sector by the mobile node, the new BS sector 906 becomes the current BS sector through which IP packets are communicated between the mobile device 902 and other devices. Signaling 1014 represents the transmission of radio signals to new BS sector 906 via wireless link 952 to convey IP packets after successful registration.

Fig. 11 is a flow chart 1100 of an exemplary method of operating a wireless communication system, such as an OFDM wireless communication system using beacon signals, to perform handoff of a wireless terminal from one base station sector Attachment Point (AP) to another base station sector assistance point in accordance with the present invention. In the case where a single carrier is used in each sector, the sector serves as a base station sector attachment point. The steps in fig. 11 refer to the BS sector. These reference signs will be interpreted as referring to the BS sector attachment point, which in the case of a single carrier BS sector is actually the same as the BS sector. However, when multiple carriers are used in a sector, the sector can include multiple BS sector attachment points, one of which is supported in the sector. In embodiments that support multiple BS sector attachment points per sector, each BS attachment point corresponds to a different carrier, and the receiver components corresponding to the respective carriers serve as independent base station attachment points. In such an embodiment, there is a handoff from one carrier to another carrier frequency in one sector when the attachment point changes from a BS attachment point associated with one carrier to a BS attachment point in the same sector corresponding to another carrier. Operation begins in step 1102, where an exemplary WT is currently attached to a base station sector attachment point.

For purposes of explaining the method of the present invention, it is assumed that the base stations in the system periodically transmit beacon signals based on each possible BS sector attachment point and that the WT has strength information regarding the last received beacon signal corresponding to the current attachment point. Fig. 3 and 4 are examples of the type of signaling that may exist in a multi-sector cell with one carrier per sector, where each sector serves as a single point of network attachment.

Operation proceeds from step 1102 to step 1104. In step 1104, the WT monitors for beacon signals. The detected beacon signals are identified as their transmission resources (e.g., corresponding base station sectors and corresponding carrier frequencies) and their received signal strength levels and the obtained information are stored.

Then, at step 1106, a comparison is made against each detected beacon to determine if the beacon signal corresponding to the potential carrier of the different sector and/or carriers in the current sector is stronger than the beacon signal of the current BS sector attachment point. If the potential carrier BS sector beacon signal is not as strong as the current BS beacon signal, operation returns to step 1104 where the WT continues to monitor for additional beacon signals. However, if the detected neighbor BS sector beacon signal is stronger than the current BS sector beacon signal, operation proceeds to step 1108. In step 1108, the WT is operated to detect if the handoff criteria are met. For example, the satisfied handoff criteria may include a potential carrier BS beacon signal that is one predetermined margin (margin) stronger than the current BS beacon signal, a potential carrier BS beacon signal that satisfies a minimum signal strength threshold level, and/or a potential carrier BS beacon signal that has an excess over the current BS beacon signal for a predetermined amount of time or for a number of consecutive iterations. If the handoff criteria of step 1108 are not met, operation proceeds from step 1108 to step 1104 where the WT continues to monitor for additional beacon signals. If the handover criteria of step 1108 are met, operation proceeds to step 1112.

In step 1112, the WT determines the cell ID, sector ID and other identifying information, e.g., carrier frequency, of the new BS sector attachment point (e.g., new sector and/or current sector) other than the new carrier frequency selected for handoff. Then, in step 1114, the WT is operable to signal to the current BS sector to initiate a handoff to the new BS sector. The new BS sector requesting the attachment point may be, for example, in a different cell, in a different sector of the same cell, or in the same sector of the same cell using a different carrier frequency. Operation proceeds from step 1114 to step 1116. In step 1116, the current BS sector attachment point initiates secure communications over the network to the new BS sector attachment point. In the case of an inter-cell handover, a secure communication link is established between the two base stations, e.g., over a backhaul network link, and a request from the WT is communicated from the secure base station to the new BS sector based on the secure link. In the case of intra-cell or intra-sector handovers, the signaling is internal to the BS, and therefore the limitation on the physical characteristics of the passing link is safe. Operation proceeds from step 1116 to step 1118. In step 1118, the new BS sector assigns air link resources to the WT, e.g., an active state identifier and/or a hold state identifier to the WT, reserves other resources for the WT such as uplink transmit segments, and transmits information of access state contained in a dedicated segment or in some fixed time to the new attachment point prior to uplink signaling. In some embodiments, the reserved resources include dedicated uplink timing control channel segments, dedicated uplink power control channel segments, and/or dedicated uplink traffic channel segments. In some embodiments, each type of channel uses a different set of tones. In an intra-cell handoff, the initial dedicated uplink timing control channel segment may not be needed and may not be reserved and allocated because the new BS sector attachment point, which is configured with the current base station sector and shares common clock circuitry, may be operated to be timing synchronized with respect to the current base station sector, allowing the WT to skip the initial timing re-synchronization step in the registration process for the handoff. Operation proceeds from step 1118 to step 1120. In step 1120, the WT terminates the wireless signaling with the old BS sector attachment point, e.g., by discontinuing the transmission of additional signals on the uplink on the original wireless link. The point in time selected for terminating the original wireless link is determined by the WT to be before the earliest uplink signaling is sent to the new BS sector attachment point using the allocated dedicated resources, e.g., just before uplink timing control signaling to the new BS sector using the allocated dedicated segments, or some fixed time before uplink signaling to the new attachment point. At this point in time, or shortly after terminating the connection at that point, the current BS sector in step 1121 may send a route update message signaling to the IP routing system to initiate routing of packets containing an address corresponding to the WT to send IP packets to the new BS, even if registration with the new BS is not complete. Operation proceeds from step 1121 to step 1122. The dedicated segments assigned by the new BS sector have a fixed timing relationship with the beacon signal corresponding to the new BS sector and this known relationship can be used by the WT in determining the original link termination time. Operation proceeds from step 1120 to step 1122. In step 1122, the WT tunes its receiver to the frequency band of the new BS sector attachment point. The WT then registers with the new BS sector attachment point using dedicated resources (e.g., assigned identifiers, dedicated uplink channel segments including dedicated tone sets in particular access slots) in step 1124. In the case of an inter-cell handover, further comprising transmitting a timing control and/or power control signal to the new BS sector before transmitting user data to the new BS sector. In some but not all embodiments, the timing control signal for the BS is used for multiple purposes and can be used, for example, as a registration signal in addition to being used as a timing control signal. In the case of an intra-cell handover, the operation of timing control signaling is omitted in some embodiments, since timing synchronization is maintained by the sectors of the cell in some embodiments. The power control signal is optional and need not be performed in all intra-cell and inter-cell handovers before the WT can receive and transmit user data. The BS sectors are responsive to timing and/or power control signals by transmitting corresponding control signals to the WTs when used as part of the registration process. In response to the received timing control signal, a timing synchronization (control) signal is transmitted to the WTs. The timing synchronization signal can indicate to the WT that it will advance, delay, or truncate its unchanged transmit timing. In the case of power control signaling, a power control signal is transmitted to direct the WT to, for example, increase, decrease, or maintain a constant transmit power.

In the case of an inter-cell handoff, operation proceeds from step 1124 to step 1125, where the WT adjusts its transmit timing in response to a timing synchronization signal received from the new BS. In the case of an intra-cell handoff, when symbol timing synchronization is maintained across the sectors of the entire cell and the WT already has one or more previous symbol transmission timing adjustments as a result, the initial timing control performed in step 1125 as part of the handoff can be ignored, the adjustments being made based on one or more timing control signals received from the BS sector to which the WT was attached prior to the handoff. Operation proceeds from step 1125 to step 1126, where the WT adjusts its transmit power in response to a transmit power control signal received from the new BS, assuming that transmit power control is performed as part of the registration process. In various embodiments, transmit power control is optional in the registration process. Thus, in some embodiments, step 1126 is omitted. Operation proceeds from step 1126 (or step 1125 if step 1126 is ignored) to step 1127 where the new BS sector checks to determine if registration is successful. For example, the new BS sector attachment point checks whether it successfully received registration information from the WT during the assigned access slot based on the dedicated assigned uplink segment, e.g., receiving appropriate identifiers and signaling to implement WT timing synchronization and WT power control signaling when executed. If the registration was successful, operation proceeds from step 1126 to step 1132. In step 1132, the new BS sector attachment point becomes the WT's network attachment point at which point the WT may begin transmitting user data, e.g., text, voice, and/or image data included in IP packets, to the BS. The new BS sector attachment point may also begin transmitting IP packets toward the WT based on the established communication link. As a result of the route update process (which in some embodiments of the invention begins before the registration process with the new BS sector attachment point is completed), reception of packets addressed to the WT may begin through the new BS sector attachment point (e.g., the new BS sector in the case of a single carrier sector) before the registration process is completed. Once the registration process with the new BS sector attachment point (the network attachment point that is serving as the WT) is completed in step 1132, such packets are temporarily stored and forwarded to the WT based on the new communication link. Operation proceeds from step 1132 to step 1104 where the WT monitors for additional beacon signals. Returning to step 1126, if registration is not successful, e.g., the new BS sector attachment point is unable to obtain the appropriate registration information and signals (e.g., due to interference), operation proceeds to step 1128. In step 1128, the new base station sector attachment point releases the assigned ID and the assigned dedicated resources, e.g., dedicated uplink segments. Operation proceeds from step 1128 to step 1130. In step 1130, the WT registers with the new BS sector attachment point as a new WT entering the new BS sector, e.g., will use contention-based uplink resources to request registration with the BS sector. Operation proceeds from step 1130 to step 1132 where the new BS sector attachment point becomes the WT's network attachment point.

Fig. 12 is a flow chart 1200 of an exemplary method of operating a mobile communication device (e.g., a mobile wireless terminal such as a mobile node) to perform a handover of the mobile communication device between a first base station and a second base station, the mobile communication device having a first wireless communication link with the first base station when the handover is initiated. The method of performing a handover begins at step 1202 and proceeds to step 1204. In step 1204, the mobile communication device is operated to receive a signal, e.g., a beacon signal, from the second base station, the signal having a known timing offset relative to an uplink channel segment dedicated to the mobile communication device. The first and second base stations need not be mutually synchronised with each other and a mobile communications device operating with respect to the timing of the first base station can and does preferentially use the second base station beacon signal to determine the timing associated with the second base station dedicated to the uplink segment. Operation proceeds from step 1204 to step 1206. In step 1206, the mobile communication device is operated to signal to the second base station via a first signal communicated over the first link that it intends to initiate a handover to the second base station. For example, the mobile communication device can send a request for handover based on the first wireless communication link, and the first base station can forward the request to the second base station via the backhaul network. Operation proceeds from step 1206 to step 1208. In step 1208, operating the mobile communication device to receive from the second base station, via a second signal communicated via the first link, information indicative of resources dedicated by the second base station to the mobile communication device to be used during communication with the second base station. For example, the second base station may transmit information indicating the dedicated resources to convey an acknowledgement corresponding to the handover request. This information may be communicated from the second base station to the first base station via a backhaul link, and the first base station may forward information, such as the second signal, over the first wireless link. The dedicated resource may be, for example, an uplink timing control segment, an uplink power control segment, an uplink traffic channel segment, and/or a base station specific wireless terminal identifier to be used during communication with the second base station, dedicated to the mobile communication device by the second base station. Operation proceeds from step 1208 to step 1210.

At step 1210, the mobile communication device is operated to terminate the first communication link. Termination may be performed, for example, by a mobile communication device for aborting communications based on the first communication link. Step 1210 includes sub-steps 1212 and 1214. In sub-step 1212, the mobile communication device is operated to determine a time to terminate said first link based on said received signal (e.g. received beacon signal) from the second base station and the indicated dedicated uplink channel segment. For example, the mobile communication device may determine the time of termination at that point in time just prior to the time of the earliest indicated dedicated uplink segment (e.g., the time of the assigned dedicated timed air uplink segment) to be used by the mobile communication device to transmit signals to the second base station based on the second wireless link. Operation proceeds from sub-step 1212 to sub-step 1214. In sub-step 1214, the mobile communication device is operated to terminate the first link at the time determined in sub-step 1212 based on the signal received from the new BS. In some embodiments, the terminating may include transmitting a termination signal from the mobile communication device to the first base station based on the first wireless link. In some embodiments, the mobile communication device terminates the first wireless communication link by discontinuing the transmission of additional signaling over the link. Operation proceeds from step 1210 to step 1216.

In step 1216, the mobile communication device is operated to communicate via a second wireless communication link with said second base station using said dedicated communication resource. For example, the mobile communication device may be assigned an identifier by the second base station to be used in a wireless communication link established with the second base station based on the second communication link. In some embodiments, certain specific dedicated uplink segments may be associated with a specific identifier and reserved for use by the mobile communication device assigned by the base station that is to use that specific identifier. In some embodiments, certain dedicated uplink segments are assigned to the mobile communication device by the base station. Step 1216 includes sub-steps 1218, 1220 and 1222. In sub-step 1218, the mobile communication device performs a timing control synchronization operation using a dedicated resource (e.g., an allocated uplink timing control segment). For example, the mobile communication device transmits uplink signaling during the assigned uplink timing control segment and receives the signaling through the second base station. The signal received from the BS is then used for synchronizing timing between the mobile communication device and the second base station. Timing synchronization operations typically include: the symbol transmission timing of the WT is adjusted based on the signal received from the BS. Operation proceeds from sub-step 1218 to sub-step 1220. In sub-step 1220, the mobile communication device is operated to perform power control operations using dedicated resources (e.g., allocated uplink power control segments). For example, the mobile communication device uses the allocated uplink power control or other segment to transmit a signal at a particular power level to be received and measured by the second base station. The base station in effect communicates a power adjustment signal to the mobile communication device to which the mobile communication device responds by adjusting its transmit power level. Operation proceeds from sub-step 1220 to sub-step 1222. In sub-step 1222, the mobile communication device is operated to transmit user data, such as voice, text or other information, over a second communication link that has been established with a second base station. User data can be communicated using one or more dedicated uplink traffic segments that can be allocated to the mobile communication device by the second base station and that the mobile communication device has performed timing synchronization and power control in advance based on signals from the new base station that can communicate user data to the second base station on the uplink in a reliable manner.

Fig. 13 is a flow diagram 1300 of an exemplary method of operating a mobile node to perform a handoff between a first link with a first base station sector and a second link with a second base station sector, where the first link uses a first carrier and the second link uses a second carrier. At least the first sector is different from the second sector or the second carrier is different from the first carrier. For example, the exemplary method may be used for inter-cell sector handover of a mobile node, where the carriers used by the mobile node are the same or different. The exemplary method may also be used for inter-sector inter-carrier handover of a mobile node within a cell. The exemplary method of performing a handoff begins at step 1302 and proceeds to step 1304. At step 1304, the mobile node is operated to receive a timing correction signal on said first communication link, e.g. as part of a normal timing control process performed when operating in a cell. Operation proceeds from step 1304 to step 1306. In step 1306 the mobile node is operated to perform transmit timing control to adjust the timing of signals, e.g. symbols, transmitted by the mobile node on the first link. The mobile node is then operated to signal the intent of the handover to the second link at step 1308. For example, the mobile node may send a handoff request signal to a first base station sector attachment point based on the first link and may forward the handoff request to a second base station sector attachment point, where the first and second base station sectors may be different sectors of the same base station. Alternatively, where intra-sector inter-carrier handoff is performed, signals may be sent via a module corresponding to a first carrier in a first sector to a module corresponding to a second carrier in the first sector, in which case the first and second sectors are the same but use different carriers. The second base station sector attachment point may authorize the handoff request and respond by allocating certain dedicated resources to the mobile node and communicating the dedicated resources identifying these allocations to the mobile node via the first base station sector attachment point and its wireless link (i.e., the first link). Operation proceeds from step 1308 to step 1310. In step 1310, the mobile node is operated to receive information on the first communication link indicating dedicated resources to be used by the mobile node when communicating on the second communication link. The dedicated resources may include, for example, identifiers specific to the second sector and the second carrier, dedicated uplink power control segments, and/or dedicated uplink traffic channel segments. Operation proceeds from step 1310 to step 1312. In step 1312, the mobile node is operated to terminate the first communication link. In some embodiments, the mobile node terminates the first communication link by transmitting termination information to the first base station sector. In some embodiments, the mobile node terminates the first communication link, which ceases to transmit additional signaling based on the first communication link. The mobile node can preferentially terminate the first communication link in time, e.g., just prior to utilizing the earliest dedicated uplink segment, e.g., a dedicated uplink power control segment or a dedicated uplink traffic channel segment, assigned to the mobile node by the second base station sector. Operation proceeds from step 1312 to step 1314. In step 1314, the mobile node is operated to transmit at least one of user data and non-timing control signals, e.g., power control signals, on the second communication link prior to receiving timing control signals on the second communication link. In some embodiments, this is done after the first link is terminated before changing the transmit timing based on signals received from the new BS attachment point. For example, the first and second base station sectors are part of the same base station, allowing synchronization between sectors, thus allowing the mobile node to maintain timing synchronization when performing handoffs between sectors; this also allows the timing synchronization steps normally required for inter-cell handover operations to be omitted in some embodiments, minimizing overhead control signaling involved in intra-cell handovers and enabling faster intra-cell handovers with shorter interruptions in operation to be provided.

In some embodiments, i.e., intra-sector and inter-sector handoff embodiments, a mobile node uses dedicated air link resources for communications during a time interval extending from said time when said mobile node terminates said first communication link in time, wherein the mobile node transmits user data based on said second communication link, said mobile node refrains from using shared communication resources, wherein other mobile nodes are capable of concurrent access with said mobile node during said time interval. By utilizing dedicated resources for control signaling (e.g., power control) during the time interval in the handover operation rather than utilizing shared resources, collisions between users that result in interruptions in smooth handover and associated loss of time and repetition of steps may be avoided, which may result in more consistent and efficient handover operations than using shared resources.

Fig. 14 is a flow chart 1400 illustrating an exemplary method of operating base station sectors to perform handoff between base station sectors and/or between carriers in sectors corresponding to different attachment points in accordance with the methods of the present invention. As will be appreciated, control circuits or modules associated with different carriers in a sector or different sectors can operate as different attachment points. Operation of the exemplary method begins in step 1402 and proceeds to steps 1404, 1406, and 1408.

In step 1404, a base station sector is operated to generate and periodically broadcast signals, e.g., beacon signals, having a fixed timing relationship with dedicated uplink channel segments used in handoffs associated with a BS sector from which the beacon signals originate.

In step 1406, the BS sector is operated to receive signals on other wireless interfaces, such as sector receive antennas and sector receivers. Operation proceeds from step 1406 to step 1410. In step 1410, BS sector operation is determined based on the type of received signal. As shown in block 1412, if the signal received in step 1406 is a handoff request for another BS sector, operation proceeds to step 1414. As shown in block 1426, if the signal received in step 1406 is a timing control signal that uses dedicated resources, then operation proceeds to step 1428. In step 1428, the BS sector processes the received timing control information, e.g., in establishing a new radio link, to establish timing synchronization as part of a handover operation. As shown in block 1430, if the received signal in step 1406 is a power control signal using dedicated resources, then operation proceeds to step 1432. In step 1432, the BS sector processes the received power control information, e.g., performs WT power control signaling as part of a handoff operation in the process of establishing a new radio link. In some embodiments, if timing control processing in step 1428 is required and was not performed successfully in advance, then the BS sector will not process the WT power control signal in step 1432. As shown in block 1434, if the signal received in step 1406 is user data communicated using dedicated resources, then operation proceeds to step 1436. In step 1436, the BS sector processes the received user data, e.g., forwards the user data to other WTs. In some embodiments, if timing control processing in step 1428 is required and has not been successfully performed in advance, then the BS sector will not process the user data signal in step 1436.

Returning to step 1414, which involves a handover between different attachment points, the BS is operated based on whether the handover request is an inter-cell or an intra-cell handover request to determine the type of handover and direct operational control. If the request is an intra-cell request, e.g., an intra-cell inter-sector or intra-cell inter-carrier handover request, operation proceeds to step 1416; however, if the request is an inter-cell handover request, operation proceeds to step 1418. In step 1416, the BS sector attachment point is operated to forward the request to the requested BS sector attachment point, e.g., a neighboring sector in the same BS or a circuit corresponding to a different carrier in the same sector. From step 1416, operation proceeds to step 1420. In step 1420, the BS sector is operated to receive information from the new (requested) BS sector indicating dedicated resources, e.g., identifiers and/or dedicated segments for handover operations, e.g., dedicated uplink power control channel segments in a particular access slot. In some embodiments, the dedicated resources of step 1420 do not include uplink timing control channel segments in the access slot, and in some embodiments the base station sectors in a given BS are timing synchronized with each other. Operation proceeds from step 1420 to step 1422. In step 1422, the BS sector is operated to forward dedicated resource information to the requesting WT based on the originally established wireless link.

Returning to step 1418, in step 1418, the BS sector attachment point is operated to forward certain information to the newly requested BS sector attachment point indicating a handoff request. Operation proceeds from step 1418 to step 1424. In step 1424, the BS sector is operated to establish a secure BS-BS link. Once such a secure link is established, detailed information regarding the handoff can be communicated between the newly requested BS sector attachment point and the WT via the originally established BS sector using the backhaul network and the existing established wireless link.

Returning to step 1408, in step 1408, involving an inter-cell handover, the BS sector is operated to receive signals via its network interface. Operation proceeds from step 1408 to step 1411, where the BS sector is operated based on the received signal type. If the signal received in step 1408 is information indicating a handoff request to BS sector 1438, operation proceeds to step 1440 where the BS sector is operated to establish a secure BS-BS link. Operation proceeds from step 1440 to step 1442. In step 1442, the BS sector is operated to dedicate resources, e.g., identifiers and/or dedicated segments, e.g., uplink timing control channel dedicated segments and uplink power control channel dedicated segments, to the handoff operation by the WT during a particular access slot. In some embodiments, the dedicated segments in step 1442 comprise dedicated uplink traffic channel segments used by the WT after timing and power control has been established in step 1442. In step 1442, the BS sector is also operated to signal information identifying these dedicated resources to other BSs based on the secure BS-BS links.

If the information received in step 1408 is information indicating dedicated resources (e.g., an identifier and/or dedicated segments used by the WT for a handoff operation), operation proceeds from step 1411 to step 1446. In step 1446, the BS sector conveys the received dedicated resource identification information to the requesting WT based on the originally established wireless link.

It should be noted that in some embodiments, the intra-cell and inter-cell handover methods described above may be used sequentially. For example, the intra-cell handover method of the present invention can be used for handover from one sector to another sector in a cell, using the inter-cell handover method of the present invention one or more times before the WT performs handover from one cell to another cell. In the case of an intra-cell handover, user data is transmitted upon entering a new sector or using a new carrier in the cell, prior to timing adjustments, in response to signals received over the air link after the old communication link is terminated. However, when there is an inter-cell handover, the WT typically performs a timing synchronization operation, e.g., adjusting its symbol transmission timing, based on one or more signals received over the air link from a transmitter in the new cell before transmitting user data in the new cell.

It is also possible to implement a number of different systems and handover methods using the methods and apparatus of the present invention.

For example, in one exemplary system having multiple frequency bands, the mobile node listens to one frequency band at a time, transforms the received signal from the time domain to the frequency domain (e.g., by performing an FFT or DFT), measures the energy (e.g., signal energy per tone) in the frequency domain on the various signal components produced by the frequency transform operation; and detecting the presence of a beacon signal component based on the received energy of each tone signal. In this particular exemplary embodiment, the mobile node determines the cell/sector and/or carrier information (e.g., cell ID, sector ID, and/or carrier frequency) of the base station transmitter transmitting the beacon based on the location of the beacon tone. The mobile node then determines, based on the energy of the beacon signal component (e.g., detected beacon signal tone), the relative signal strengths of the different base station transmitters that transmit the beacon signals into the frequency band used by the mobile node. Based on the relative energies of the beacon signal components received from the different transmitters, and various handover criterion information stored in the mobile node, the mobile node determines whether a handover should be performed and, in the event that it is determined that a handover is to be performed, switches to a new base station attachment point corresponding to the transmitter from which the beacon signal was received. In some embodiments of such exemplary systems, the beacon signal component is identified based on the result of comparing the signal component energy level to the average of each tone energy level. In some embodiments, where the beacon power is greater than 20 times the average of the power of each tone signal based on a time period such as 1 or 2 seconds, the beacon detection threshold is set at an average of 20 times the detected or expected energy of each tone, for example at 15 times the expected energy of each tone.

Once the system determines to perform a handover, various handover methods of the present invention may be used. The handover techniques described herein do not depend on a particular determination method when performing a handover. However, the syntax of one or more beacon signals is used in various handoff embodiments to determine timing and/or other network attachment point related information, such as the sector for which the mobile node wishes to complete the handoff.

When performing handovers according to various embodiments of the present invention, both inter-cell and intra-cell handovers have many common steps and characteristics, and an inter-cell handover may include timing synchronization and/or power control steps that may need to be performed before the mobile node is allowed to transmit user data (e.g., text, video, or audio data) to a new network attachment point. This is because in an intra-cell handover situation, where portions such as sectors are synchronized, the mobile node can rely on previously established timing synchronization with the access points in the cell, wherein a reasonably reliable given timing synchronization in the cell will be maintained even in the event that the mobile node changes the network attachment point being used in the cell.

Some of the characteristics that can be used to support intra-cell and inter-cell handovers are observed from the base station to be some of the following: such as the use of multiple frequency bands, to transmit beacon signals over a network attachment point to the frequency band used by the network attachment point to communicate data to the frequency band being used by an adjacent sector, cell, or network attachment point for communicating user data. Therefore, the network attachment point transmitter will typically transmit beacon signals to multiple frequency bands. For convenience in explaining the allocation of air link resources as part of a handover and the process of determining when to terminate an existing communication link, a fixed frame structure for the uplink and/or downlink supported by the various network attachment points is used. Traffic, access and other types of segments used to communicate specific types of data and/or for specific purposes repeat the fixed communication channel frame structure in a given predictable, well-known manner. As a result, once the time position at which the beacon signal has been received is known, the communication channel structure (e.g., the super slot/beacon slot structure of the frequency band in which the transmitter transmits its user data (traffic channel, etc.)) can be uniquely derived from the beacon signal position according to the frequency or beacon frequency that is repeatedly generated in a known periodic manner due to the communication channel structure. In some embodiments, the communication channel frame structure defines content such as frequency hopping, traffic channel segment definition, which may be stored in the mobile node in advance and accessed based on information derived from stored beacon signals. Thus, beacon signals are transmitted in a periodic, predictable manner, and a fixed communication channel structure is used that can be stored and associated with beacon information that can be used to determine which channel structure is being used and which channel structure facilitates interpretation of resource allocation and when a particular segment will occur at a new network access point before the mobile node achieves symbol timing synchronization with the new network access point (e.g., in the case of an inter-cell handover).

In various embodiments, for the implementation of a handoff from a mobile node, the handoff trigger mechanism has been explained above in terms of the use of beacon signals to determine when a handoff occurs.

In some exemplary handoff embodiments, the mobile node uses the beacon signal received from the handoff target to determine one or more of the following: cell ID, sector ID and/or carrier frequency used by the destination network attachment point transmitting the detected beacon signal. The mobile node may also determine the frame structure of the communication channel used by the network attachment point (e.g., the second base station sector) to which to handoff based on the detected beacon signal and, for example, stored communication channel structure information associated with different cells and/or sectors. Based on the determined frame structure and information regarding when the beacon signal was received, the mobile node can determine when to drop an existing wireless communication link and when to begin establishing a new communication link. The timing may be based on information about dedicated resources (e.g., specific access segments) present at the destination network attachment point, in addition to the time at which the detected beacon signal was received.

In various embodiments, the mobile node's intent to prepare it for handoff to a new network attachment point is communicated to a network attachment point, e.g., a base station sector, which serves as the mobile node's current network attachment point, by transmitting one or more signals based on an existing communication link. Thus, according to the present invention, a mobile node will typically transmit its intention to perform a handover by transmitting a signal based on an existing wireless communication link. The current network connection forwards the handover signal from the mobile node to the second network connection point (handover destination) and/or acts as a proxy for the mobile node and exchanges handover signals with the second network connection point on behalf of the mobile node. The destination network node allocates (e.g., is dedicated to) one or more air link resources to the mobile node and communicates information about the dedicated resources to the mobile node via the current network attachment point. The dedicated resources may comprise one or more tone sets, e.g., part of an access segment, used for transmitting, e.g., registration signals, timing control signals, and/or power control signals, to the base station as part of a registration operation. The dedicated resources may also include one or more device identifiers that are used when communicating with the new network attachment point and are, for example, active state identifiers used when communicating in active state operation and hold state identifiers used when communicating in hold state operation. The resource information is communicated to the mobile node over a current wireless communication link with a current network attachment point. The mobile node terminates (e.g., drops) the existing communication link before establishing a new communication link with the network attachment point that is the handover destination. The time to terminate the link may be based on the time of the received beacon signal and based on an expected time offset from the received beacon signal to a set of tones in a communications segment or segment dedicated to the mobile node for registering with a new network attachment point. The mobile node accesses the new point of network attachment in a contention-free manner using resources dedicated to the new point of network attachment and thereby establishes a communication connection with the new point of network attachment. If something happens and the mobile node cannot use the dedicated resources to complete the establishment of the new communication link, the mobile node will, in various embodiments, wait and attempt to register with the new network attachment point using contention-based signaling.

It is contemplated that various embodiments using the general approach described above have minor variations in order to be particularly suited for inter-cell or intra-cell applications. In one particular inter-cell handover embodiment, the link resources allocated by the network access point to which the handover is directed (e.g., the second base station) include a dedicated access segment in the uplink of the second base station. In a particular illustrative example of such an embodiment, the mobile node determines a definition of the allocated access segment, e.g., a definition of the tone set and OFDM symbol times it is allocated for registration, from the detected beacon signal and information returned by the first base station over the existing communication link through which the mobile node is connected to the network at the time of initiating the handover. In such an exemplary embodiment, the mobile node drops the first link before beginning to transmit to the second base station using the assigned access segment. Using the allocated access segment, the mobile node transmits one or more signals, such as registration, power control and/or timing control signals, to the base station. In response to signals transmitted on the uplink, the mobile node receives timing and/or power control signals from the second base station and makes timing and/or power adjustments in response to the control signals. In such an embodiment, as part of the registration process, the mobile node may obtain more dedicated resources, such as an ON identifier, a dedicated control channel to continue communicating with the second base station (if these resources have not been previously allocated). In various embodiments of this type, the dedicated access segment is a non-contention based access segment, and other mobile nodes are not allowed to use the access segment dedicated to that mobile node. If the use of the access segment is unsuccessful and a communication link is not established with the second base station, the mobile node attempts to register again with the contention-based contention resource signal in the following manner: a mobile node entering the system will register without the benefit of previously allocating dedicated registration resources.

Some exemplary intra-cell handover embodiments are discussed below. In some intra-cell handoff embodiments, such as in an inter-sector cell and/or an inter-carrier sector embodiment, current and new network access points are assigned to the same cell and synchronized with symbol timing (selectable carrier frequency). In such a handoff embodiment, the mobile node is synchronized at the beginning of the handoff as a result of timing control of the current link using the symbol timing of the current network attachment point (e.g., sector). In some intra-cell handover embodiments, the dedicated resources allocated by the new network access point include resources such as a dedicated access segment, an ON identifier used in communicating with the new network attachment point, a dedicated control channel segment for performing timing and/or power control communications with the new network access point. In some embodiments of intra-cell handover, the mobile node does not have to send any access, timing control and/or power control signals in order to establish the second link and skip some of all of this type of signalling steps (which may occur in the case of inter-sector handovers). In many embodiments, assuming a handover to an intra-cell handover, messages associated with the handover are located between network attachment points within the cell without having to use a backhaul link between cells to complete the handover. In some intra-cell handovers, the mobile node drops the current communication link for a very short period of time, e.g., less than the time for completing the inter-cell handover, and in some cases begins using dedicated channel resources allocated by the new network attachment point in less than 5 milliseconds, and may also actually begin transmitting user data almost immediately, e.g., without having to wait for timing and/or power control signals to first be received from the network attachment point over the air link.

Fig. 15 shows a diagram 1500 of one exemplary embodiment of using access segments in uplink and downlink channels in accordance with some embodiments of the invention. Although the access and traffic segments are shown in the downlink of fig. 15 as separate elements in terms of time, it should be understood that different sets of tones may be used during the same time period to implement the access and traffic segments. That is, although the access and traffic segments may be time multiplexed in the downlink, multiplexing is not required by the present invention.

Diagram 1500 includes a vertical axis 1502 representing uplink tones and a horizontal axis 1504 representing time. Each small partition of vertical axis 1502 represents a tone and each small partition of horizontal axis 1504 represents an OFDM symbol transmission time interval. Row 1513 corresponds to an uplink channel and row 1543 corresponds to a downlink channel. In the downlink channel, beacons are periodically transmitted within beacon slots 1541, 1541'. In the uplink channel, one or more beacon signals in the beacon slots are transmitted with a known fixed offset relative to the access slots 1514, 1414'. For example, at least one beacon signal transmitted in slot 1541 will occur after a fixed number of symbol transmission time periods from the beginning of the uplink access slot 1514'.

In addition to the beacon slots, the downlink also includes access slots 1544, 1544 'that can be used to convey registration acknowledgement signals, WT timing control signals and/or WT power control signals to one or more wireless interrupts, e.g., terminals desiring to establish a communications link with a network attachment point with which uplink and downlink channels 1513, 1543' are associated.

The uplink tones are shown and grouped into four exemplary sets of tones (first set of tones 1506, second set of tones 1508, third set of tones 1510, fourth set of tones 1512). Downlink tones are not shown, but may also be grouped into sets used by different wireless terminals. In the uplink channel 1513, the OFDM symbol transmission time intervals are grouped into exemplary time intervals, e.g., slots, including access time interval 1514, traffic interval 1516, traffic interval 1518, and traffic interval 1520. The order of the time intervals repeats based on time interval 1514 ', traffic interval 1516', traffic interval 1518 'and traffic interval 1520' as shown, and has a known timing relationship with these time intervals in downlink channel 1543.

During the access interval, the WT can use the access segment, e.g., for registration with a BS sector attachment point establishing a new wireless link. Two types of access segments are shown, namely a contention-based access segment and a dedicated access segment. Contention access segment 11522 uses first set of tones 1506 during access interval 1514; contention access segment 21524 uses second set of tones 1508 during access interval 1514; the dedicated access segment 11526 uses the third tone set 1510 during the access interval 1514; dedicated access segment 21528 uses fourth set of tones 1512 during access interval 1514. Similarly, contention access segment 11522 'uses first set of tones 1506 during access interval 1514'; contention access segment 21524 'uses second set of tones 1508 during access interval 1514'; the dedicated access segment 11526 'uses the third tone set 1510 during the access interval 1514'; dedicated access segment 21528 'uses fourth set of tones 1512 during access interval 1514'.

During a traffic interval (1516, 1518, 1520, 1516 ', 1518 ', 1520 '), there are multiple segments comprising uplink traffic channel segments, wherein the WT can transmit user data to the base station sector attachment point via the established wireless link.

In some exemplary systems, the access segment may be used to transmit uplink signals including registration information, time control information, and/or power control information when an exemplary WT is in an access operating state. While indicating an intent for registration, a registration signal transmitted in the uplink may be used for space-time and/or power control purposes. Thus, a single uplink signal can serve multiple purposes. Alternatively, different signals can be used for each function. As part of the access operation, the WT transmits certain uplink signals to the BS using access segments from which the base station performs measurements. The WT will use a dedicated non-contention slot, assuming for uplink signal transmission that a dedicated segment is allocated for this purpose. The uplink signal may be, for example, a signal transmitted at a predetermined WT power level (e.g., at a predetermined time) within an access segment set relative to the WT's current timing. The base station receives the uplink signal, performs measurements and sends one or more signals back to the wireless terminal on the downlink segment or downlink segments, e.g., in the corresponding downlink access slot 1546 or 1546'. In addition to the timing and/or power control signals, the BS may also send an acknowledgement of the uplink registration signal. The WT performs any controlled adjustments and is also able to transmit uplink signals containing user data on the assigned uplink traffic channel segments (e.g., traffic segments 1516, 1518, 1520, etc.).

According to some embodiments of the present invention, a dedicated access segment, such as dedicated access segment 11526, is assigned to the wireless terminal that has requested a handoff. Assignment information for such dedicated access segment is communicated to WTs via the current radio link.

Generally, the use of dedicated access segments in handover operations provides for more efficient handover. Using dedicated air link resources allocated via existing communication links and timing information obtained from broadcast signals transmitted over the air link may be beneficial for the frequency band used by the WT, such benefits may include: less time loss between termination of the original radio link and establishment of a new radio link, higher handover success rates due to the use of dedicated access segments as opposed to general contention-based (contention-prone) access segments, and/or cancellation of certain operations such as certain timing control adjustments (e.g., in intra-cell handovers).

A contention access segment, e.g., contention segment 1522', is used by WTs that do not currently have an established communication link, e.g., power-only WTs, to register with the BS sector attachment point and establish a wireless link. In some embodiments, if a handoff using the dedicated access segment fails, the WT expects to register using the contention-based access segment. The use of contention access segments may result in contention with other WTs desiring to establish a wireless link, possibly resulting in unsuccessful registration attempts. When using the contention-based access segment, the WT generates and transmits uplink signals to the BS sector attachment point, which are received, measured and used by the BS to calculate WT timing control and power control adjustment information, which is transmitted to the WT via the downlink segment. Thus, when performing contention-based access operations, timing control is typically performed before the WT transmits user data in the uplink.

In some cases, where the handoff request is an intra-cell handoff request, such as an inter-sector or inter-intra-sector carrier handoff, the registration signal sent via the access segment in the phase segment is not used for timing control operations and/or registrations, but may be omitted entirely. Assuming that the sectors are timing synchronized and that the WT has been assigned the dedicated resources needed to transmit user data via the existing link, the WT can begin transmitting user data and/or other signals to the new network assistance point, which can utilize the new attachment point via the air link to assign the necessary resources without first having to perform registration, timing control, and/or power control. This is possible because in some embodiments, timing synchronization is maintained by the various sectors of the cell.

Fig. 16 illustrates an exemplary wireless communication system 1600 that includes three exemplary cells (cell 11602, cell 21604, cell 31606), each indicated by a solid circle. Each cell (1602, 1604, 1606) represents a wireless coverage area covered by a base station (1608, 1610, 1612) located at the center of the cell (1602, 1604, 1606), respectively. Each cell (1602, 1604, 1606) is divided into three sectors A, B and C. Cell 11602 includes sector a1614, sector B1616, and sector C1618. Cell 21604 includes sector a1620, sector B1622, and sector C1624. Cell 31606 includes sector a1626, sector B1628, and sector C1630. Carrier frequency f₁Represented by dotted lines shown by legend 1632; carrier frequency f₂Indicated by the dotted/dashed line shown by legend 1634; carrier frequency f₃Indicated by the dashed line shown in legend 1636. In the exemplary embodiment, each carrier frequency f₁、f₂、f₃5MHz 1.25MH with effective Total BWThe z-bandwidth segments are associated and the BW segments are non-overlapping. The radius of each line (dotted, dashed, or dashed) represents the transmit power associated with the carrier in a given sector. In fig. 16, there is a frequency reuse factor of 1, i.e. the same set of frequencies is already used in each cell in each sector.

In each of the three cells (1602, 1604, 1606), the transmitter of base station sector a uses the carrier frequency (f) at a power level (high, medium, low), respectively₁、f₂、f₃) For communicating, e.g., traffic and control channel signals, from the base stations (1608, 1610, 1612) to the wireless terminals. In each cell (1602, 1604, 1606), the transmitter of base station sector B uses the carrier frequency (f) at a power level of (high, medium, low), respectively₁、f₂、f₃) To communicate, e.g., downlink traffic and control channel signals from a base station (1008, 1010, 1012) to a wireless terminal; the transmitter of base station sector C uses the carrier frequency (f) at power levels (high, medium, low), respectively₁、f₂、f₃) For conveying, e.g., downlink traffic and control channel signals from the base stations (1608, 1610, 1612) to the wireless terminals. For carrier frequencies, the following notation is used to illustrate base station transmitter power levels in system 600: (cell, sector, high power carrier/medium power carrier/low power carrier): (cell reference number, sector reference number, arc reference number for high power carrier/arc reference number for medium power carrier/arc reference number for low power carrier). The system 1600 includes: (cell 1, sector A, f₁/f₂/_f3): (1602, 1614, 1638/1640/1642): (cell 1, sector B, f)₂/f₃/f₁): (1602, 1616, 1644/1646/1648); (cell 1, sector C, f)₃/f₁/f₂): (1602, 1618, 1650/1652/1654); (cell 2, sector A, f₁/f₂/f₃): (1604, 1620, 1656/1658/1660); (cell 2, sector B, f)₂/f₃/f₁): (1604, 1622, 1662/1664/1666); (cell 2, sector C, f)₃/f₁/f₂): (1604, 1624, 1668/1670/1672); (cell 3, sector A, f₁/f₂/f₃): (1606, 1626, 1674/1676/1678); (cell 3, sector B, f)₂/f₃/f₁): (1606, 1628, 1680/1682/1684); (cell 3, sector C, f)₃/f₁/f₂)：(1606，1630，1686/1688/1690)。

Fig. 16 shows the same frequency reuse levels in various sectors of the overall system and may represent the system in an advanced scheduling level, e.g., where the scheduler has been completed and/or the service provider has large customers with high demand for being able to adjust the scheduling level. However, in each sector, at different power levels P₁，P₂，P₃To transmit the three carriers.

Although in each sector the three carriers are at different power levels P₁，P₂，P₃Is transmitted, however, in various embodiments, at the three power levels P described above₁，P₂，P₃There is a fixed relationship between them, which is used in each sector. In one such embodiment, P is in each sector₁＞P₂＞P₃And P is₁To P₂And P₂To P₃Is the same, independent of the sector. An uplink carrier may be associated with each downlink carrier.

Inter-cell, inter-sector and inter-sector carrier handoffs may exist in the system of fig. 16 in accordance with the methods of the present invention.

In implementations such as that shown in fig. 16, each carrier and sector is associated with one or more modules that can be used by the mobile node as a network attachment point. Switching between individual carriers within a cell may result in switching between network attachment points, thereby resulting in handover between network attachment points in the cell. In the case of a sector supporting multiple carriers, this may include a handoff from a network attachment point corresponding to a first carrier to a network attachment point corresponding to a second carrier, the first and second carriers being different carriers in the same sector.

Fig. 17 illustrates a base station sector 1701 with two exemplary network attachment points 1801, 1807, the network attachment points 1801, 1807 corresponding to different carriers f, respectively₁And f₂. In some embodiments illustrated in fig. 16, each sector will include three network attachment point modules, e.g., sector 1701 will include a sector corresponding to carrier f₁And a third network connection point. Thus, in addition to the network point of attachment modules 1801, 1807 shown in fig. 17, there is a third network point of attachment module.

Each network attachment point 1801, 1807 can serve as an attachment point for wireless terminals to connect via a wireless connection to a network connected to a base station containing sector 1701. Although shown in sectors, it is to be understood that the network attachment points 1801, 1807 may be different sectors in the same cell or multiple sectors in different cells, rather than one sector in the same cell. Each network attachment point 1801, 1807 uses a different frequency band 1718, 1722 for communicating user data. The network attachment point module 11801 includes a first BS transmitter 1702, a first BS sector receiver 1703 and a first control module 1713 coupled together by a bus. The control module 1713 causes the first network attachment point to operate in accordance with the invention, for example, to interact with other network attachment points in the manner described above to coordinate handover and allocate radio link resources. The second network attachment point module 1807 includes a second base station sector transmitter 1704, a corresponding BS sector receiver 1705 and a second control module 1715 coupled together by a bus. The control module 1715 causes the second network attachment point to operate in accordance with the invention, e.g., to interact with other network attachment points in the manner described above to coordinate handover and allocate air link resources. The control modules of the different network access links are connected to control the modules of the other sectors by links in the BS, where they are included and connected to the network connection point control modules of the other cells by implemented backhaul links (e.g. with optical fibre or wired connections).

The use of the first and second transmitters 1702, 1704 corresponding to different network attachment points will now be described. The transmitters 1702, 1704 transmit downlink signals including, for example, ordinary traffic channel signals, e.g., user data, optionally pilot signals, and beacon signals. The relative timing of the various signals may be as shown in fig. 15. The transmitters 1702, 1704 may use different antennas directed to different sectors or cells. The signaling from each sector transmitter includes ordinary signaling, such as allocation signals, optionally pilot signals, and/or optionally beacon signals, in its own designated carrier band, as well as beacon signals in one or more (e.g., the remaining two) of the carrier bands used in the cell. First transmitter 1702 transmits downlink signal 1706 including, for example, downlink traffic signals for transmitter 1, assignment signals for transmitter 1, WT control signals for transmitter 1, optionally pilot signals for transmitter 1, and/or beacon signals for transmitter 1 to corresponding carrier frequency f₀1724, transmitting beacon signal 1708 of transmitter 1 to frequency band 1718 corresponding to carrier frequency f₁1726, and transmits beacon signal 1710 of transmitter 1 to frequency band 1720 corresponding to carrier frequency f₂1728, and band 1722. Transmitter 21704 transmits downlink signals 1712 including, for example, downlink traffic signals for transmitter 2, assignment signals for transmitter 2, optionally pilot signals for transmitter 2, WT control signals for transmitter 2, and/or beacon signals for transmitter 2 to corresponding carrier frequency f₂1728, and band 1722. Transmitter 21704 also transmits the beacon signal of transmitter 2 to correspond to carrier frequency f₀1724, and transmits beacon signal 1716 of transmitter 2 to frequency band 1718 corresponding to carrier frequency f₁1726, band 1720.

Assume that WT1730 is tuned to have carrier frequency f₀1724, carrier band 1718. The receiver in WT1730 receives two signal components 1732, 1734, the first signal component 1732 comprising, for exampleOrdinary signaling, assignment signals, pilot signals, and/or beacon signals from the transmitter 1602 to be processed. At the same or a different time, a second signal component 1734 is received and processed, including, for example, a beacon signal from second transmitter 21704. Based on respective slave signals corresponding to different carrier frequencies f₀And f₂The WT may initiate a handoff from the first network attachment point 1801 to the second network attachment point 1807 using the existing communication link with the network attachment point 1801 using the energy in the received beacon signal in the transmitter (1702, 1704). Thus, in accordance with the present invention, WT1730 can request and receive dedicated air link resources from network attachment point 1807 via an existing communication link and then terminate the link and establish a new link with attachment point 1807, e.g., prior to link termination, at a time determined from a beacon signal received from transmitter 1704 and assignment information communicated over the link with the first network attachment point 1801.

Although the present invention has been described in the context of an OFDM system, the methods and apparatus of the present invention are applicable to a wide range of communication systems, including a variety of non-OFDM and/or non-cellular systems.

In various embodiments nodes described herein are implemented using one or more modules for performing steps corresponding to one or more methods of the present invention, such as signal processing, beacon generation, beacon ID, beacon measurement, beacon comparison, handover, message generation and/or transmission steps. In certain embodiments, various modules are used to perform various features of the present invention. Such modules may be implemented using software, hardware, or a combination of software and hardware. Many of the above described methods or method steps can be implemented using machine executable instructions, such as software, included in a machine readable medium such as a memory, e.g., RAM, floppy disk, etc. which control a machine, e.g., general purpose computer with or without additional software, in one or more nodes to perform all or a portion of the above described methods. Accordingly, the present invention is directed, among other things, to a machine-readable medium including executable instructions for causing a machine, such as a processor and associated hardware, to perform one or more of the steps of the above-described method(s).

Numerous other variations on the methods and apparatus of the present invention described above will be apparent to those of ordinary skill in the art in view of the above description of the invention. Such variations are to be considered within the scope of the invention. The methods and apparatus of the present invention may, and do in various embodiments are, used with CDMA, Orthogonal Frequency Division Multiplexing (OFDM), and/or various other types of communications techniques which may be used to provide wireless communications links between access nodes and mobile nodes. In some embodiments the access nodes are implemented as base stations which establish communications links with mobile nodes using OFDM and/or CDMA. In various embodiments the mobile nodes are implemented as notebook computers, Personal Digital Assistants (PDAs), or other portable devices including receiver/transmitter circuits and logic and/or routines, for implementing the methods of the present invention.

Claims (34)

1. A method of operating a second network attachment point in a communications system including a first network attachment point which communicates with a mobile node using signals in a first frequency band, the mobile node having a communications link with the first network attachment point over which data can be communicated, the method comprising the steps of:

periodically transmitting a beacon signal having a predetermined frequency to the first frequency band, the first frequency band being a frequency band into which the second network attachment point does not transmit user data; and is

Establishing and maintaining a communication link with a mobile node over which user data can be communicated using a second frequency band different from the first frequency band, the step of using the second frequency band comprising allocating segments of an uplink communication channel to the mobile node, the uplink communication channel having a structure that includes a periodically repeating pattern of access segments and traffic segments, an access segment being a segment over the air to which a mobile node that does not have an existing wireless communication link established with the second network attachment point can transmit at least one signal to establish a wireless communication link with the second network attachment point, each access segment occurring at a fixed time offset from a point at which one of the beacon signals is transmitted to the first frequency band by the second network attachment point,

wherein the first network attachment point is a sector of a first base station and wherein the second network attachment point is a sector of a second base station, the method further comprising:

allocating dedicated resources to be used by the mobile node in establishing a second communication link with the second network attachment point;

transmitting information about dedicated resources to the first network attachment point over a communication link between the first and second network attachment points;

receiving, by the second network attachment point, a timing control signal from the mobile node in an uplink communications segment dedicated to the mobile node over an air link; and

after receiving the timing control signal, transmitting a timing synchronization signal to the mobile node before receiving user data from the mobile node.

2. The method of claim 1, further comprising:

receiving a signal from the first network attachment point indicating that the mobile node is seeking to perform a handover from the first network attachment point to the second network attachment point.

3. The method of claim 2, wherein each access segment comprises: a plurality of tones capable of being used for uplink transmissions during at least one symbol transmission time period, said symbol transmission time period occurring during a period of said access segment, said plurality of tones including at least a first set of tones, the method further comprising:

dedicating the first set of tones to a first mobile node coupled to the first network attachment point; and is

Information indicating that the first set of tones is dedicated to the first mobile node is communicated to the first mobile node via the first network attachment point.

4. The method of claim 3, further comprising:

prior to the access segment, communicating information to the first mobile node via a first network attachment point indicating a time at which the access segment will occur relative to one of the beacon signals transmitted to the first frequency band.

5. The method of claim 4, further comprising:

monitoring a first set of tones in a first access segment for registration signals from a mobile node, wherein the first access segment is assigned to the mobile node; and is

Receiving a registration signal from the mobile node that communicates using the dedicated first set of tones.

6. The method of claim 5, further comprising:

at least one additional set of tones is monitored for registration signals from a mobile node in a first access segment, wherein the additional set of tones is not dedicated to the mobile node and is used based on contention.

7. The method of claim 5, further comprising:

establishing a secure communication link with a first network connection point before communicating information to the first mobile node via the first network connection point indicating that the first set of tones is dedicated to the first mobile node.

8. The method of claim 7, further comprising:

receiving at least one internet protocol, IP, packet having an internet protocol, IP, address corresponding to the first mobile node prior to the time of the first access segment;

storing the internet protocol, IP, packets; and is

Forwarding the Internet Protocol (IP) packet to the mobile node over a wireless communication link established using a set of tones dedicated to the first mobile node in the first access segment.

9. The method of claim 1, wherein the information regarding dedicated resources provides information sufficient for identifying, by a mobile node, an uplink communications segment dedicated to the mobile node.

10. The method of claim 1 wherein the information regarding dedicated resources comprises a device identifier used by the mobile node when communicating over an air link with the second network attachment point.

11. A method of operating a second base station for performing a mobile node handover operation, the mobile node handover operation comprising a handover by a mobile node from a first base station having a first communications link with the mobile node, the method comprising:

allocating dedicated resources for use by the mobile node in establishing a second communication link with the second base station;

transmitting information on dedicated resources to the first base station over a communication link between the first and second base stations;

receiving, by the second base station, a timing control signal from the mobile node in an uplink communication segment dedicated to the mobile node over an air link; and

after receiving the timing control signal, transmitting a timing synchronization signal to the mobile node before receiving user data from the mobile node.

12. The method of claim 11, wherein the information regarding dedicated resources provides information sufficient for identifying, by a mobile node, an uplink communications segment dedicated to the mobile node.

13. The method of claim 11, wherein the information regarding dedicated resources provides information sufficient for identifying, by a mobile node, an uplink communications segment dedicated to the mobile node.

14. The method of claim 11, wherein the information regarding dedicated resources comprises a device identifier used by the mobile node when communicating with the second base station over an air link.

15. The method of claim 11, further comprising: transmitting a transmit power control signal to the mobile node after receiving the timing control signal and before receiving user data from the mobile node.

16. The method of claim 13, further comprising:

receiving user data from the mobile node after the transmission of the timing synchronization signal.

17. The method of claim 15, wherein the timing synchronization signal is a control signal for instructing the mobile node to perform one of the following operations: advancing, delaying or leaving unchanged the transmission timing of symbols transmitted by the mobile node.

18. The method of claim 17, further comprising:

establishing a secure communication link with the first base station before transmitting the information regarding resources dedicated to the mobile node to the first base station.

19. The method of claim 18, further comprising:

periodically transmitting a high power narrowband signal; and is

Communicating information to the first base station indicating a point in time at which the mobile node should register with the second base station over an air link, the point in time having a fixed relationship to a time at which the base station transmits at least one of the high power narrowband signals.

20. A method according to claim 19, wherein said high power narrowband signal is a beacon signal having a known frequency relationship with a frequency band used by said mobile node to transmit signals to said second base station.

21. The method of claim 15, wherein the second base station starts receiving internet protocol, IP, packets directed to the mobile node from other network nodes after sending the dedicated resource information to the first mobile node and before sending the timing synchronization signal to the mobile node.

22. A base station capable of performing a handover operation of a mobile node, the handover operation comprising a handover of the mobile node from a further base station having a first communication link with the mobile node to the base station, the base station comprising:

means for allocating dedicated resources for use by the mobile node in establishing a second communication link with a second base station;

means for transmitting information on dedicated resources to a first base station over a communication link between the first and second base stations;

means for receiving a registration signal from the mobile node over an air link; and

means for transmitting a timing synchronisation signal to the mobile node after receipt of the registration signal, prior to receipt of user data from the mobile node.

23. The base station of claim 22, further comprising:

means for transmitting a transmit power control signal to the mobile node after receiving the registration signal and before receiving user data from the mobile node.

24. The base station of claim 23, further comprising:

means for receiving user data from the mobile node after transmitting the power control signal and the timing synchronization signal.

25. The base station of claim 23, wherein said timing control signal is a signal for instructing a mobile node to advance or delay a transmission timing of a symbol transmitted by said mobile node.

26. The base station of claim 25, further comprising:

means for establishing a secure communication link with the first base station prior to transmitting the information regarding resources dedicated to the mobile node to the first base station using the secure communication link.

27. The base station of claim 26, further comprising:

means for periodically transmitting a high power narrowband signal; and

means for communicating to the first base station information indicating a point in time at which the mobile node should register with the second base station over an air link, the point in time having a fixed relationship to a time at which the base station transmits at least one of the high power narrowband signals.

28. The base station of claim 27 wherein said high power narrowband signal is a beacon signal having a known frequency relationship with a frequency band used by said mobile node to transmit signals to said second base station.

29. The base station of claim 23, wherein the second base station starts receiving internet protocol, IP, packets directed to the mobile node from other network nodes after sending the dedicated resource information to the mobile node and before sending the timing synchronization signal to the mobile node.

30. A method of operating a base station to perform a handover of a mobile node between a first link with a first base station sector and a second link with a second base station sector, the first link using a first carrier and the second link using a second carrier, the first and second sectors being located within the same base station, at least the first sector being different from the second sector or the first carrier being different from the second carrier, the method comprising the steps of:

transmitting a timing correction signal to the mobile node over the first communication link;

receiving a signal communicated over said first link from a mobile node indicating an intention to switch to said second link;

transmitting information on the first link indicating dedicated resources used by the mobile node when communicating on the second communication link;

terminating the first communication link with the mobile node; and

receiving at least one of user data and a non-timing control signal from the mobile node prior to transmission of a transmit timing control adjustment signal over the second communication link, the transmit timing control adjustment signal being generated based on any timing control signal received from the mobile node over the second communication link.

31. The method of claim 30, wherein the transmit timing adjustment signal instructs the mobile node to adjust the time at which symbols are transmitted such that the symbols arrive at the base station in synchronization with symbols transmitted by other mobile nodes.

32. The method of claim 30, wherein the dedicated resource is an identifier specific to the second sector and the second carrier.

33. The method of claim 32, wherein the identifier is a device identifier used by the mobile node when communicating on the second communication link.

34. The method of claim 30, wherein said dedicated resource is a dedicated airlink communications segment for use in establishing communications with said second base station sector when transmitting signals over an airlink to said second base station sector using said second carrier frequency to establish said second communications link.