CN1123268C - Method and apparatus for performing idle handoff in a multiple access communication system - Google Patents

Abstract

In a mobile communication system (14) having a first communication device (2) for communicating with a plurality of second communication devices, a method for performing handoff within the mobile communication system (14) includes transmitting from at least one of the second communication devices to the first communication device (2) a handoff list (60) of second communication devices to which the first communication device is permitted to handoff. A channel assignment message is transmitted from each of the second communications devices in the handoff list (60). The handoff list (60) includes base stations (26 a-o) controlled by a single controller (32) that determines which of the second communication devices transmits the channel assignment message. The base stations (26 a-o) on the handoff list (60) have pilot signals and the energy levels of the pilot signals are measured by the first communication device. The measured energy levels are transmitted to the controller (32) and compared by the controller (32) to provide an energy level list of base stations (26 a-o) according to the comparing. The energy level list is transmitted to the first communication device and the handoff is performed according to the energy level list.

Description

Method and device for performing idle handover in multiple access communication system

field of invention

The present invention relates to communication systems. In particular, the present invention relates to novel methods and apparatus for performing idle handover in a multiple access communication system. Furthermore, the present invention relates to a novel method for allocating traffic channels in a multiple access communication system. The present invention also relates to a method for reducing the amount of handover required to occur when queuing a mobile station and waiting for a traffic channel in a multiple access communication system.

Description of related technologies

Generally, the communication system prohibits handover while the mobile station is in the system access state. The system access state is the state in which communication is initiated by the mobile station by transmitting on the access channel or by the base station by transmitting on the paging channel. In an example embodiment, messages are sent according to the Code Division Multiple Access (CDMA) communication format, which is described in U.S. Patent No. 4,901,307 (titled "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Relay Stations") and U.S. Patent No. 5,103,459 (invention titled "System and Method for Generating Waveforms in a CDMA Cellular Telephone System"), both of which are assigned to the assignee of the present invention and are incorporated herein by reference. The use of paging and access channels for call initiation is known in the prior art and is described in the TIA/EIA interim standard IS-95-A (invention title "Used for Dual Mode Wideband Spread Spectrum Cellular System's Mobile Station-Base Station Compatibility Standard"). A characteristic of CDMA systems is the re-use of the same frequency in each cell. Diversity combining is the method by which a receiver receiving signals carrying the same information combines signals propagated by different paths to provide an improved estimate of the transmitted signal. A receiver for taking advantage of diversity signals carrying the same information but traveling over different propagation paths or sent by different transmitters is described in U.S. Patent No. 5,109,390 (invention title "Diversity Receiver in a CDMA Cellular Telephone System") design, the aforementioned patents are assigned to the assignee of the present invention and are incorporated herein by reference.

Soft handover is a method for a mobile station moving from one cell to another to receive information at a border (as long as the mobile station is near the border) from a base station serving two or more cells. By the diversity combining method described above, the signals transmitted by the base stations are combined in the receiver of the mobile station. In U.S. Patent No. 5,101,501 (titled "Method and System for Providing Soft Handover in Communications in a CDMA Cellular Telephone System") and U.S. Patent No. 5,267,261 (titled "Invention in a CDMA Cellular Communications System") A method and system for providing soft handoff in a CDMA communication system in which a plurality of base stations are communicating with a mobile station at or near a cell boundary is disclosed in "Mobile Station Assisted Soft Handoff", both of which have been assigned to assignee of the present invention and is incorporated herein by reference. Hard handover differs from soft handover in that a mobile station is lost from one cell to another by the leaving cell before being picked up by the entering cell.

The use of the same frequency in each cell and the use of soft handover results in high CDMA system capacity. Reuse of the same frequency in neighboring cells results in faster changes in the forward link signal-to-noise ratio near the cell border. This is because the cells received by the mobile station may fade, and neighboring cells may increase in strength (anti-fade). In general, when a mobile station receives two cells, the ratio of the received traffic channel energy per spreading chip to the total spreading noise density for the signal transmitted by cell 1 is given by the following equation (1): $\frac{{\mathrm{E.}}_c}{I_o}1=\frac{\frac{{\mathrm{E.}}_c}{I_{\mathrm{or}}}1}{\frac{I_{\mathrm{oc}}}{{\hat{I}}_{\mathrm{or}}1}+\frac{{\hat{I}}_{\mathrm{or}}2}{{\hat{I}}_{\mathrm{or}}1}+1}----\left(1\right)$

In addition, the following equation (2) gives the ratio of the received traffic channel energy per spreading chip to the total spreading noise density of the signal transmitted by cell 2: $\frac{{\mathrm{E.}}_c}{I_o}2=\frac{\frac{{\mathrm{E.}}_c}{I_{\mathrm{or}}}2}{\frac{I_{\mathrm{oc}}}{{\hat{I}}_{\mathrm{or}}2}+\frac{{\hat{I}}_{\mathrm{or}}1}{{\hat{I}}_{\mathrm{or}}2}+1}----\left(2\right)$

where, in equations (1) and (2),

I _oc is the total thermal noise,

E _c /I _or1 , E _c /I _or2 are the fractional parts of the traffic channel power transmitted from cell 1 and cell 2 respectively, and

 _or1 ,  _or2 are the fractional parts of the traffic channel power received at the mobile station from cell 1 and cell 2, respectively.

Considering that relative to  _or1 ,  _or2 , I _oc is very small. When cell 1 fades relative to cell 2,  _or1 becomes smaller relative to  _or2 , so the ratio  _or2 / _or1 becomes larger. Then, E _c /I _o1 becomes small. This change in signal-to-noise ratio can cause problems if the mobile station is not in a soft handoff state. However, if the mobile station is in a soft handover state with a neighboring cell, the change in signal-to-noise ratio is not a problem since the mobile station is performing diversity combining on the forward traffic channels from both cells. While the first path given by E _c /I _o1 becomes smaller, the second path given by E _c /I _o2 becomes larger. Thus, fading in one cell increases the signal-to-noise ratio of other cells.

Paging is a method of sending information to a mobile station indicating the initiation of a mobile terminated service or ordering the mobile station to receive new overhead information. In U.S. Patent No. 5,392,387 (titled "Apparatus and Method for Reducing Power Consumption in a Mobile Communication Receiver") and pending U.S. Patent Application No. 08/206,701 (filed March 7, 1994, It is a continuation of US Patent No. 5,392,287, which is the method for initiating a base station initiated call described in US Patent No. 5,392,287), both of which are assigned to the assignee of the present invention and incorporated herein by reference. The present invention may also be used for mobile initiated calls as described in co-pending U.S. Patent Application Serial No. 08/219,867 (filed January 18, 1996, assigned to the assignee of the present invention and incorporated herein by reference) describe.

In a slotted paging system, the mobile station monitors the paging channel for a short predetermined time interval and does not monitor the paging channel again until the next predetermined time interval. In IS-95-A, the method of periodically monitoring the paging channel is called a slotted mode, and the mobile station can monitor the paging channel for 80 milliseconds in every 1.28 seconds. The period between monitoring intervals can be made longer by the user as desired. Before each pre-assigned time slot in which the mobile station can be paged, the mobile station wakes up (becomes active) and resynchronizes or improves its synchronization with the base station. The mobile station then monitors the time slot for pages or other messages. After the interval, the mobile station may become inactive and not monitor the paging channel until before the next assigned time slot. The mobile station is in an idle state before the mobile station is actively disseminating traffic information with the mobile communication system and after the mobile station has acquired timing synchronization with the communication system. In the idle state, a mobile station may receive messages, receive an incoming call, initiate a call, initiate registration, or initiate message transmission. While in the mobile station idle state, IS-95-A allows the mobile station to perform an idle handover at any time except at intervals that require the mobile station to monitor its assigned time slot.

However, when a mobile station originates a call or receives a page, it enters the system access state to send an origination message or a page response message. When in the system access state, an IS-95-A mobile station does not operate in slotted mode. This is so-called non-slotted operation. Specifically, the mobile station continuously monitors the paging channel until a different state is indicated by the base station or an error condition occurs, allowing the mobile station to exit the system access state. The example embodiments are described with respect to origination operations and origination messages, but the principles apply directly to mobile terminating call processing and page response messages. After the mobile station sends the origination message and receives an acknowledgment, the mobile station waits for a channel assignment message indicating on which channel traffic communication from the base station to the mobile station should be performed.

Upon receiving the channel assignment message, the mobile station returns to the assigned traffic channel, receives information on the forward traffic channel and begins transmitting on the reverse traffic channel. A forward traffic channel is a channel on which information is sent from a base station to a mobile station, and a reverse traffic channel is a traffic channel on which information is sent from a mobile station to a base station.

The interval between the time the mobile station sends the origination message and the time the mobile station receives the channel assignment message depends on the vendor implementing the respective infrastructure. It can range from less than 1/2 second to several seconds. The mobile station is in the system access state until the mobile station receives a channel assignment message.

The paging channel generally does not support soft handover. Then, the aforementioned fading problem occurs. Generally, these phenomena are counteracted by making the transmit power of the paging channel higher than that of the traffic channel, since one paging channel can handle call origination and termination of multiple traffic channels, the capacity loss is minimized by this higher power . To support soft handover on the paging channel, the system sends the same information on the paging channel in all cells, thus dynamically reducing the capacity of the entire paging channel.

While in the idle state, the mobile station is allowed to perform handover. Typically, a mobile station performs a handoff whenever the level of received paging from one cell is significantly higher than another. Typically, this idle handoff occurs before the mobile station starts monitoring the time slot. However, there are cases where the mobile station cannot select the correct cell before the time slot starts, and the mobile station must continue to monitor the existing cells. While in the system access state, the mobile station is not allowed to perform idle handover.

However, when the mobile station is in the system access state, there are cases where the SNR E _c /I _o1 changes so rapidly that the message bit error rate is high and the mobile station cannot correctly receive Signaling messages sent above. As a result, the mobile station cannot receive channel assignment messages. This causes call origination to be unsuccessful. IS-95-A allows the mobile station to exit the system access state and return to the mobile station idle state if it does not receive any paging channel messages within one second. Thus, the mobile station does not receive the channel assignment message, and call origination is unsuccessful.

A similar problem exists when a mobile station is first assigned to a traffic channel. IS-95-A only allows a single base station to be assigned to a mobile station. If the other cell is or becomes more powerful, the mobile station cannot successfully receive the forward traffic channel. As a result, calls are lost. The problem is to assign a mobile station to a traffic channel with a single active group member and not in soft handoff.

Under IS-95-A, in order for a mobile station to enter the soft handoff state, the following steps must occur. First, the mobile station must determine whether the pilot of another base station is above a predetermined energy threshold. Second, the mobile station must send a pilot strength measurement message. Third, the infrastructure must establish the handoff and send a handoff direction message to the mobile station. Depending on the environment and implementation, this can take anywhere from a few hundred milliseconds to well over a second.

Therefore, although soft handover is generally supported in the IS-95-A system, soft handover is not supported when the mobile station is in the system access state. Therefore, there is a need for a system that allows soft handoff while the mobile station is in the system access state to provide increased reliability and other benefits in the system access process.

Summary of the invention

In a mobile communication system having a first communication device communicating with a plurality of second communication devices, a method for performing handover in the mobile communication system includes switching the second communication device switching table from at least one of the second communication devices to the first telecommunications device, wherein the facility is allowed to switch from the first telecommunications device to the second telecommunications device. A channel assignment message is sent from each second communication device in the switching table. The switching table includes base stations controlled by a single controller which determines which second communication device sends the channel assignment message.

The base stations on the switching table have pilot signals, and the energy level of the pilot signals is measured by the first communication device. The measured energy levels are sent to the controller, and the controller comparates them to provide an energy level meter for the base station based on the comparison. The energy level meter is sent to the first communication device, and switching is performed based on the energy level meter.

The present invention also describes other improvements that may improve operation on the paging and access channels. The invention allows handover while the mobile station is in the system access state. This allows a mobile station to receive a base station with a high signal-to-noise ratio and thus a low message error rate. This avoids lost call setup due to inability to receive the paging channel.

Another feature of the present invention is that it allows the infrastructure to determine which base stations should send assignment messages to the mobile station. This ensures that the mobile station can handover to a different base station and have its traffic channel assigned to it on the new base station without delay.

Yet another feature of the present invention is that it allows the infrastructure to determine which base stations should be in a mobile station's active set before assigning the mobile station to a traffic channel. The active set is the set of base stations that assign the strongest signal to the mobile station. This allows the infrastructure to determine whether sufficient resources exist to place the mobile station in a soft handoff state before assigning the mobile station to a traffic channel. This is useful because a mobile station located near a cell border can request to be placed in a soft handoff state immediately after being assigned a traffic channel. Furthermore, this minimizes call drops due to the above-mentioned rapid changes in the signal-to-noise ratio.

Finally, the features set forth above provide a special implementation of Priority Access and Channel Allocation (PACA) operation, which allows users to access limited communication resources according to established user priorities. Although the invention is described in terms of a CDMA system, the invention can be used with any cellular or satellite communication system.

Description of drawings

The characteristics, objects and advantages of the present invention will be apparent from the following detailed description in conjunction with the accompanying drawings, wherein the same reference numerals are correspondingly represented in the accompanying drawings:

1 is a block diagram of the communication system of the present invention including a mobile station communicating with a base station in a group of base stations;

Figure 2 shows a cell layout corresponding to the base station of Figure 1;

3A-D illustrate base station tables sent to mobile stations in the communication system of FIG. 1; and

Figure 4 shows the pilot E _c /I _o of a mobile station moving between two base stations.

Detailed description of the preferred embodiment

Referring now to FIG. 1 , a communication system 14 is shown. The communication system 14 includes a mobile station 2 having a control processor 18 and base stations 26a-o. In an exemplary embodiment, base stations 26a respectively correspond to cells 36a in FIG. 2 . Thus, for example, base station 26a provides coverage for cell 36a. The mobile station 2 enters the system determination substate after power-up. A system determination processor (not shown) selects the system that performs the acquisition attempt and provides the receiver 8 with the required frequency information. Although shown separately, the system determination processor may be implemented in the control processor 18 . Control processor 18 may be implemented in a microprocessor or microcontroller operating under the control of a program stored in memory.

In an example embodiment, the mobile station 2 moves into a pilot acquisition substate in which it attempts to demodulate the pilot signal according to the acquisition parameters received in the system determination substate to acquire the CDMA pilot signal according to the acquisition parameters. The signal is received at antenna 4 (if present) and passes through duplexer 6 to receiver 8 . The receiver 8 down converts, amplifies the received signal, converts the analog signal to a digital representation and sends the signal to the searcher 10 . The searcher 10 attempts to acquire the pilot signal by testing the pseudo-noise (PN) offset. The PN bias is tested by demodulating the signal based on the PN bias assumption and measuring the signal energy of the demodulated signal. Designing and implementing searcher hardware for CDMA acquisition is known in the art and is detailed in the aforementioned US Patent No. 5,109,390.

When the searcher 10 detects a pilot signal having an energy higher than a predetermined threshold, the mobile station 2 enters the synchronization channel acquisition substate and attempts to acquire the synchronization channel. Typically, the synchronization channel, as propagated by the base station, includes basic system information, such as a system identification (SID) and a network identification (NID), but most importantly provides timing information to the mobile station 2 . The mobile station 2 adjusts its timing according to the synchronization information, and enters the mobile station idle state.

Once the synchronization channel has been successfully acquired, the mobile station 2 starts monitoring the paging channel according to a predetermined paging format. The mobile station 2 demodulates the signal according to the Walsh sequence reserved for paging channel transmission. For example, if the acquisition pilot signal is transmitted by base station 26a, then mobile station 2 monitors the paging channel using predetermined Walsh paging based on the timing information provided by the synchronization channel. Base station 26a intermittently transmits overhead information on the paging channel.

Referring now to Figures 3A-D, there are shown base station lists 50, 60, 70, 80 sent to the mobile station 2 to allow idle handover within the communication system 9 of the present invention. In IS-95-A, the neighbor list 50 is provided to the mobile station 2 by the base stations 26a-n in a neighbor list message. Here, the neighbor list 50 is also referred to as NGHBR_LIST_BASE50. NGHBR_LIST_BASE 50 is a list of base stations 26b-k in the geographic vicinity of base station 26a that can provide a strong signal to mobile station 2 and are candidates for mobile station 2's idle handoff.

The base station controller 32 is responsible for providing information between the base stations 26a-o, for selectively providing the base stations 26a-o with information from a primary telephone exchange (not shown), and for providing internally generated messages to the base stations 26a-o . It should be noted that the present invention is equally applicable to cases where some of the base stations within the neighbor list 50 are not under the control of the same base station controller 32 .

If the pilot signal acquired by the mobile station 2 is sent by the base station 26a after receiving the overhead information, the mobile station 2 can register with the base station 26a by sending its mobile identification number to the base station 26a. Thus, after successfully registering with the base station 26a, the mobile station 2 enters the idle state and monitors its assigned paging channel in the slotted paging mode. If no registration is performed, the mobile station 2 also enters the idle state and monitors its assigned paging channel transmitted by the base station 26a in the slotted paging mode.

In the slotted paging mode, the base station 26a sends any paging or signaling messages directed to the mobile station 2 during predetermined time intervals called slots. In an example embodiment, the time slot and paging channel are determined from a hash function of the mobile identification number whose registration is known to the base station 26a and mobile station 2 .

In the present invention, the base station 26a transmits to the mobile station 2 the handover table 60 of the base station in which the mobile station 2 is allowed to perform idle handover in the system access state. The switching table 60 is also referred to herein as LIST_BASE 60 . The base stations in LIST_BASE 60 are generally a subset of the base stations in NGHBR_LIST_BASE 50 and generally utilize the same base station controller 32 . For example, NGHBR_LIST_BASE50 may include all base stations 26b-k, but LIST_BASE60 may only include base stations 26b, 26c, 26g, and 26h.

When the mobile station 2 originates a call, the message generator 20 generates the origination message and sends it on the access channel. Message generator 20 may be implemented in a microprocessor programmed to perform the functions described above. Although shown as a discrete component, message generator 20 may be implemented in control processor 18 . The origination message is received and demodulated by the base station 26a that the mobile station is currently monitoring. In response to receiving the origination message, each base station in LIST_BASE 60 sends a channel assignment message, which indicates the traffic channel on which to perform communications. In general, the Walsh channel used to communicate with a first base station in LIST_BASE 60 is not necessarily the same Walsh channel used to communicate with a second base station in LIST_BASE 60 . Since multiple base stations send channel assignment messages, mobile station 2 can perform an idle handover while in the system access state and after sending an origination message to any base station in the system access state, and still be able to receive channel assignment messages .

In another embodiment, mobile station 2 sends the origination message to base station 26a and then waits for an acknowledgment of the origination message. Mobile station 2 is not allowed to perform handover until mobile station 2 receives an acknowledgment. However, after mobile station 2 receives the acknowledgment, it can perform an idle handoff to any base station in LIST_BASE 60 .

In another embodiment, mobile station 2 sends the origination message using the procedure described in IS-95-A, which is described in co-pending U.S. patent application Ser. filed on March 12, 2010, and assigned to the assignee of the present invention, and is incorporated herein by reference) as described in detail. If no acknowledgment is received from base station 26a within a predetermined timeout period, the mobile station increases its transmit power and attempts to transmit the message again. If mobile station 2 cannot receive an acknowledgment from base station 26a after a certain number of attempts, and another base station, such as base station 26b, is more powerful, then mobile station 2 is allowed to perform an idle handover to base station 26b, and restarts the original transmission. send a message.

In one embodiment, each base station in LIST_BASE 60 sends a channel assignment message, which indicates only the traffic channel used to communicate with itself. In another embodiment, each base station 26a-i in LIST_BASE 60 sends the same channel assignment message, which not only indicates the use of the traffic channel for communication with that particular base station, but also indicates the use of the traffic channel for communications with the base station in LIST_BASE 60. All base stations in the communication. This requires the base stations in LIST_BASE 60 to propagate available traffic channels through base station controller 32 . By providing channel assignment messages from multiple base stations, the success rate of the channel assignment process can be greatly enhanced.

The above process allows the infrastructure to establish soft handoffs and include more than one member of the active set in the channel assignment message. Instead of first communicating with one base station and then moving into soft handoff, mobile station 2 can immediately enter soft handoff and immediately receive traffic from two or more base stations. This speeds up the process of bringing mobile station 2 into soft handoff, which improves the performance of communication system 9 and minimizes call drops due to low forward traffic channel signal-to-noise ratio.

In one embodiment of this process, the base station establishes soft handoffs with all base stations in LIST_BASE 60 . In another embodiment of this process, the base station establishes a soft handover with the subset of base stations in LIST_BASE 60 and sends the information in the channel assignment message required by the mobile station 2 to enter the soft handover. This information includes the identity of the subset of base stations. In IS-95-A, the pilot PN offset identifies the base station.

At the antenna 4 of the mobile station 2, paging messages transmitted by the base stations 26a-i are received. The received message is then provided via the duplexer 6 to a receiver 8 in which the received signal is downconverted and amplified. The downconverted message is provided to a demodulator 12a-j which demodulates the received message. Based on the information from the searcher 10, the control processor 18 selects the incoming paging channel or the channel that the mobile station 2 demodulates. In one embodiment, demodulators 12a-j monitor only one base station.

From the searcher 10 cooperating with the control processor 18 it can be determined that another base station is preferred. Then, the demodulator demodulates the received signal from another base station. Since the mobile station 2 receives allocation messages from more than one base station, the mobile station 2 can perform idle handover in the system access state. In another embodiment, mobile station 2 monitors all base stations in LIST_BASE 60 and demodulates the signal identified in LIST_BASE 60 .

In a preferred embodiment, the LIST_BASE 60 cannot be provided separately from the neighbor list 50 in the neighbor list message. Instead, a composite table 70 including all members of the neighbor table 50 is provided. Flags 72 are also included in combination table 70 to indicate which members of neighbor table 50 are also members of LIST_BASE 60 . In the example embodiment, a reserved value 72 is used in the overhead message to indicate which base stations specified in the neighbor list message are in the LIST_BASE 60 . In IS-95-A, a base station reserved value 72 is provided in the overhead message to specify which members of the neighbor table 50 are in the LIST_BASE 60 .

In an example embodiment, the IS-95-A neighbor list message includes pilot PN offsets for base stations in NGHBR_LIST_BASE 50 and indicates which base stations in the neighbor list message are in LIST_BASE 60 . The pilot PN sequence for the current base station is sent to provide the mobile station 2 with a reference to identify the PN offsets of other base stations.

All base stations in LIST_BASE 60 are required to send channel assignment messages to mobile station 2 as described above. While this allows mobile stations to handover and then increases the success rate of the channel assignment process, it requires additional paging channel capacity for all call setups.

A variation of this procedure to reduce the impact on paging channel capacity is to send the base station pilot list to mobile station 2 above a predetermined power threshold. The table is LIST_MOBILE80, which is also called energy table 80. In one embodiment, the searcher 10 demodulates the pilot signal while favoring the PN offset of the base station in LIST_BASE60 followed by the PN offset of the base station in NGHBR_LIST_BASE50, and then demodulates according to the remaining PN offset. A method for providing optimal search prioritization is described in the aforementioned US Patent No. 5,267,261.

In an example embodiment, the searcher 10 demodulates the received signal according to the pilot PN offset and measures the energy of the demodulated pilot. The energy value is provided to the control processor 18 . The control processor 18 compares the energy of the demodulated signal to threshold values and compiles an energy table 80 . Energy table 80 includes a table of PN biases above the energy threshold used by control processor 18 . Energy table 80 may be designated as LIST_MOBILE80. Once the LIST_MOBILE 80 has been compiled, it is sent on the access channel and received by the base station 26a that the mobile station 2 is monitoring. In an example embodiment, LIST_MOBILE80 is included in the origination message.

In another embodiment, LIST_MOBILE 80 is received by more than one base station 26a-o. LIST_MOBILE 80 is provided to base station controller 32 . In the preferred embodiment, the thresholds used by the mobile station 2 to determine whether to include base stations in the LIST_MOBILE 80 are sent by the base stations 26a-o as part of the overhead message. In a preferred embodiment, the threshold may be the T_ADD value sent in the IS-95-A System Parameters message. This T_ADD value is currently used by the IS-95-A mobile station to determine whether to send an IS-95-A Pilot Strength Measurement message to the base station on the traffic channel, which indicates that the mobile station has detected a pilot exceeding T_ADD.

Referring now to FIG. 4, there is shown a graphical representation of Ec _{/ Io} for the IS-95-A pilot channel propagated by base stations 26a,b as mobile station 2 moves from base station 26a to base station 26b. When mobile station 2 is fully within the coverage area of base station 26a, as indicated by region 38, the pilot channel of base station 26b is below the T_ADD level. Similarly, when mobile station 2 is completely within the coverage area of base station 26b, as indicated by area 41, the level channel of base station 26a is below the T_ADD level. When mobile station 2 is within area 38, it does not report base station 26b in the origination message. Similarly, when mobile station 2 is in area 41, it does not report base station 26a in the origination message.

When mobile station 2 is in area 39, the pilot _Ec / _Io for base station 26b is higher than T_ADD, and the mobile station reports base station 26b in the origination message. Similarly, when mobile station 2 is in area 40, the pilot for base station 26a is above T_ADD, and base station 2 reports base station 26a in the origination message. The preferred embodiment uses the _Ec / _Io disclosed in IS-95-A for these measurements. However, another measure of signal strength or signal-to-noise ratio known in the prior art may likewise be used.

In the preferred embodiment, the mobile station 2 is allowed to perform idle handoffs only to base stations in LIST_MOBILE and LIST_BASE 60. A group of base stations in the above table may be designated as LIST_BOTH. It has two advantages. First, the infrastructure need only send channel assignment messages in those base stations identified by the mobile station as viable candidates for idle handover, and the mobile station is allowed to handover. This is the group of base stations given in LIST_BOTH. It greatly reduces the additional message sending required. Second, LIST_MOBILE provides the base station controller 32 with a pilot list higher than T_ADD. This allows the infrastructure to identify which base stations should be active set members of the mobile station. Thus, in order for the base station controller 32 to establish a soft handoff when assigning a mobile station to a traffic channel, it may only use the base stations in the LIST_MOBILE to establish the soft handoff.

In another embodiment, mobile station 2 sends the base stations in LIST_BOTH to the base stations in its origination message. This reduces the amount of information that has to be sent from the mobile station 2 . Furthermore, it allows the infrastructure to establish soft handoffs and include more than one member of the active set in the channel assignment message. The channel assignment message can include the pilot PN offsets of the base stations in the active set, and the mobile station 2 can enter the soft handover state immediately and receive traffic from two or more base stations immediately, instead of first proceeding with one base station communication, and then enter soft handover, which may not be feasible due to capacity or other constraints.

For example, if mobile station 2 is located in cell 36a at location 37 near the border with cell 36b, LIST_MOBILE identifies the pilot PN offset of base station 26b. The channel assignment messages sent by the base stations 26a, b identify a traffic channel for communication between the base stations 26a, b and the mobile station 2 for use by the mobile station 2 . At least one demodulator 12a-j is tuned to receive traffic channel information from base station 26a, and the other demodulator 12a-j is tuned to receive traffic channel information from base station 26b. A plurality of demodulators 12a-j start demodulating the traffic channel signals transmitted by the base stations 26a,b. The demodulated signals are used in diversity combining 34, which combines the received signals to provide an improved estimate of the transmitted data.

In practicing the method of the invention, the mobile station 2 does not perform an idle handover until it receives an acknowledgment for sending the message or a timeout indicating that the acknowledgment period has expired. This allows mobile station 2 to receive acknowledgment of its access channel probe. It also allows the base station 26a to generate acknowledgments other than those generated by the base station controller 32 to which the mobile station sends its access channel inquiry. This reduces latency and makes the call setup process faster. Furthermore, if the mobile station 2 is in the system access state and performs an idle handover after the confirmation timeout expires, then the mobile station 2 must restart the access channel inquiry transmission procedure. This is the same as the case where the mobile station 2 sends a new initial message.

In another embodiment, the mobile station 2 performs an idle handover to the base stations in LIST_BASE 60 before receiving the acknowledgment. Thus, all base stations in LIST_BASE 60 must send acknowledgments, and base station controller 32 must help generate acknowledgments. In a modification of another embodiment, the mobile station 2 may perform an idle handover to a base station in LIST_MOBILE before receiving an acknowledgment. Similarly, all base stations in LIST_MOBILE must send an acknowledgment, and then the base station controller 32 must be included in the process of generating the acknowledgment.

In a preferred embodiment, the present invention provides for the occasional channel assignment message to be sent by base station 26a, but not received by mobile station 2. Base station 26a can receive the origination message from mobile station 2, but mobile station 2 cannot receive a channel assignment message acknowledging receipt of the origination message from base station 26a. Even if no acknowledgment message is received, the mobile station 2 can perform, for example, an idle handover to the base station 26b.

Base station 26b sends the channel assignment message to mobile station 2, while mobile station 2 retransmits the origination message. In an example embodiment, when an acknowledgment message is sent, it is accompanied by an indication of which message was acknowledged. The mobile station 2 ignores the channel assignment messages unless the indicator corresponds to the most recently sent origination message. The present invention includes several methods to correct for this problem. One method is the same acknowledgment indicator that base station 26b uses in the origination message received by base station 26a. This may be done by sending an acknowledgment indicator value from base station 26a to base station 26b. In another embodiment, mobile station 2 may stop sending access probes if it receives a channel assignment message and tunes to the channel specified by the channel assignment message.

In a modified embodiment, the paging channel structure is the same for all base stations (base stations in LIST_BASE) to which the mobile station 2 is allowed to handover. Base stations that do not support these capabilities are not included in LIST_BASE60.

This approach is also used to support PACA. Such PACA characteristics are known in the art and are described in detail in "TIA/EIA/IS-53-A Cellular Characterization". When PACA is invoked, mobile station 2 is given priority over other mobile stations in obtaining a traffic channel if the traffic channel is not available. Specifically, the mobile station sends an origination message including a PACA feature code and a dialed number. If a traffic channel is immediately available, mobile station 2 is assigned to the traffic channel. If a traffic channel is not immediately available and mobile station 2 is authorized to employ PACA, the base station monitoring mobile station 2 places mobile station 2's request in the PACA queue. For example, base station 26a may place the request in a queue. Alternatively, the PACA queues may be managed by the base station controller 32 . The position in the queue depends on the priority of the PACA request and the age of the request. When a traffic channel is available, the request at the head of the PACA queue is assigned to the traffic channel.

When mobile station 2's request is in the PACA queue, mobile station 2 may receive periodic messages informing mobile station 2 of the user queue status. One problem with PACA is that the infrastructure needs to know the cells currently available to the mobile station 2 in order to determine if the channel is free. For most systems, this requires the mobile station 2 to register or resend the origination message each time an idle handoff is performed. Due to the abrupt nature of the transition between CDMA base stations, mobile station 2 may register or retransmit the origination message several times while passing the boundary between base stations. A second consideration for CDMA is that mobile station 2 can enter a soft handoff state immediately after it is assigned to a traffic channel. Allocation cannot be successful unless resources are available in multiple base stations to support the call.

Through the above-mentioned modified origination message, the mobile station 2 indicates other base stations which should be in the mobile station's active set from which the mobile station 2 has been monitoring for strong pilot signals. In one embodiment, the mobile station 2 sends LIST_MOBILE and the base station determines LIST_BOTH. In another embodiment, mobile station 2 sends LIST_BOTH. This allows the infrastructure to determine if resources are free among all the base stations needed for the PACA call. In order to reduce the rate at which origination messages are sent, the base stations in LIST_BOTH are the base stations where the mobile station moves without having to resend origination messages. When this feature is invoked, the infrastructure needs to send queue status information in all base stations in LIST_BOTH. If mobile station 2 moves out of the coverage area of the base stations in LIST_BOTH, then mobile station 2 resends the origination message.

The above description of the preferred embodiment is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein can be applied to other embodiments without inventive effort. Thus, the present invention is not limited to the embodiments shown here, but is to be accorded the widest scope consistent with the principles and novelty disclosed herein.

Claims (15)

1. one kind is used for carrying out the method for switching in mobile communication system, and this mobile communication system has first communicator that communicates with a plurality of second communication devices, it is characterized in that, described method comprises the following steps:

(a) make described first communicator and at least one described second communication device synchronous;

(b) described at least one second communication device sends to described first communicator with second communication device switching table, wherein allows described first communicator to switch to described second communication device in the described switching table;

(c) described first communicator sends originating information and sends to described second communication device; With

(d) in response to originating information, a plurality of described second communication device channel assignment message in the described switching table.

2. the method for claim 1 is characterized in that, the described second communication device of each in described switching table channel assignment message.

3. the method for claim 1 is characterized in that, described switching table comprises the base station that is subjected to single controller control.

4. method as claimed in claim 3 is characterized in that, described controller determines which described second communication device sends described channel assignment message.

5. method as claimed in claim 3 is characterized in that, the described base station in described switching table has pilot signal, and described method also comprises the following steps:

(e) measure the described energy level of described pilot signal by described first communicator;

(f) described mensuration energy level is sent to described controller;

(g) by the more described transmission energy level of described controller; With

(h), provide the energy level table of base station according to described comparative result.

6. method as claimed in claim 5 is characterized in that, comprises the step that described energy level table is sent to described first communicator.

7. method as claimed in claim 6 is characterized in that, comprises according to described energy level table carrying out the step of switching.

8. method as claimed in claim 7 is characterized in that, comprises the step of the switching of only execution and the base station in described switching table and described energy level table.

9. the method for claim 1 is characterized in that, comprises the step that the contiguous table of second communication device is sent to described first communicator, and the second communication device in the wherein said contiguous table is positioned at the geographic vicinity of described first communicator.

10. method as claimed in claim 9 is characterized in that, described switching table and described contiguous table send simultaneously.

11. method as claimed in claim 9 is characterized in that, described contiguous table comprises the base station by a plurality of controller controls.

12. method as claimed in claim 9 is characterized in that, is included in the described second communication device subclass that described vicinity is shown at the described second communication device on the described switching table.

13. the method for claim 1 is characterized in that, step (d) is afterwards followed by carrying out the switching of described first communicator from a described second communication device to another described second communication device.

14. method as claimed in claim 13 is characterized in that, when described first communicator is in idle pulley, the switching of described first communicator takes place.

15. the method for claim 1 is characterized in that, step (d) comprises the channel assignment message of transmission for each base station in described switching table.

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