HK1140352A - Method and apparatus for performing mobile assisted hard handoff between communication systems - Google Patents

Description

Method and apparatus for mobile assisted hard handoff between communication systems

The present application is a divisional application of the invention patent application having application number 97181396.5, international filing date 1997 of 12/19, entitled "method and apparatus for mobile assisted hard handoff between communication systems".

Technical Field

The present invention relates to communication systems. In more detail, the present invention relates to a novel and improved method for hard handoffs between different radio communication systems.

Background

In a Code Division Multiple Access (CDMA) spread spectrum communication system, a common frequency band is used to communicate with all base stations in the system. An example of such a system IS described in the TIA/EIA interim standard IS-95-a entitled "mobile station-base station compatibility standard for dual mode wideband spread spectrum cellular systems," which IS incorporated herein by reference. The generation and reception of CDMA signals is disclosed in U.S. patent No. 4,401,307 entitled "spread spectrum multiple access communication system using satellite or terrestrial repeaters" and U.S. patent No. 5,103,459 entitled "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.

Signals occupying the common frequency band are discriminated at the receiving station by spread spectrum CDMA waveform properties based on high rate pseudo-noise (PN) codes. The signals transmitted by the base station and the remote station are modulated with a PN code. By discriminating the unique time offset introduced to each base station, signals from different base stations can be received at the receiving station separately. High rate PN modulation also allows a receiving station to receive a signal from a single transmitting station where the signal travels through different propagation paths. Demodulation of multiple signals is disclosed in U.S. patent No. 5,490,165 entitled "demodulation unit assignment in a system capable of receiving multiple signals" and U.S. patent No. 5,109,390 entitled "diversity receiver in a CDMA cellular telephone system," both assigned to the assignee of the present invention and incorporated herein by reference.

The shared frequency band allows simultaneous communication between a remote station and more than one base station, a situation known as soft handoff as disclosed in U.S. patent No. 5,101,501 entitled "soft handoff in a CDMA cellular telephone system" and U.S. patent No. 5,267,261 entitled "mobile assisted soft handoff in a CDMA cellular telephone system," both of which are assigned to the assignee of the present invention and are incorporated herein by reference. Similarly, a remote station is able to communicate simultaneously with two sectors in the same base station, a situation referred to as softer handoff disclosed in copending U.S. patent application serial No. 08/405,611 entitled "method and apparatus for handoff between sectors of the same base station," assigned to the assignee of the present invention and incorporated herein by reference. Handovers are described as soft and softer handovers because they establish a new connection before breaking an existing connection.

If the mobile station moves outside the boundaries of the system with which it is communicating, it is desirable to maintain the communication link by transferring the call to a nearby system (if it exists). The neighboring systems may employ any radio technology, CDMA, NAMPS, AMPS, TDMA, or FDMA are some examples. If the neighboring system uses CDMA of the same frequency band as the current system, a soft handoff between systems is enabled. In the case where an intersystem software handoff is not available, the communication link is transferred by a hard handoff in which the existing connection is broken before the new connection is established. Examples of hard handoffs are handoffs from a CDMA system to a system using another technology, or call transfers within two CDMA systems using different frequency bands (inter-frequency hard handoffs).

Inter-frequency hard handoffs can also occur within a CDMA system. For example, a high demand area such as a downtown requires a greater number of frequencies for service requirements than the suburban areas surrounding it. It may not be economical to use all available frequencies within the overall system. Calls initiated on frequencies used only in highly congested areas must be handed off as the user moves to an uncongested area. Another example is a microwave call or other service operation on a frequency within the boundaries of the system. As users move into areas that are subject to interference from other services, their calls need to be handed off to a different frequency.

Various techniques can be used to initiate the handoff. Handover techniques, including those that use signal quality measurements to initiate a handover, may be found in co-pending U.S. patent application No. 08/322,817 entitled "method and apparatus for handover between different cellular communication systems", filed on 10/16 of 1994, assigned to the assignee of the present invention, and used herein by reference. Further disclosure of handoffs (including round-trip signal delay measurements to initiate handoffs) is disclosed in copending U.S. patent application serial No. 08/652,742 entitled "method and apparatus for hard handoff in a CDMA system," filed on 1996, 5/22, assigned to the assignee of the present invention, and incorporated herein by reference. Handoff from a CDMA system to a system using other techniques is disclosed in co-pending U.S. patent application serial No. 08/413,306 (the' 306 application), entitled "method and apparatus for mobile-assisted CDMA to other system hard handoff," filed on 3/30 of 1995, assigned to the assignee of the present invention, and incorporated herein by reference. In the' 306 application, pilot beacons are set at the boundaries of the system. When the mobile station reports these pilots to the base station, the base station knows that the mobile station is moving towards the boundary.

When one system determines that a call should be transferred to another system through a hard handoff, a message is sent to the mobile station directing it to make such a handoff along with its parameters to enable the mobile station to continue with the destination system. The system simply estimates the actual location and environment of the mobile station and therefore the messages sent to the mobile station cannot be guaranteed to be accurate. For example, for beacon-assisted handoff, the signal strength of the pilot beacon may be a valid criterion for triggering handoff. However, the appropriate cell or cells (called Active Set) to be assigned to the mobile station in the destination system are not necessarily known. Furthermore, in the active set, all possibilities included will exceed the maximum allowed value.

In order for the mobile station to communicate with the destination system, contact with the old system must be abandoned. If the parameters given to the mobile station are invalid for any reason, i.e. a change in the mobile station environment or lack of accurate location information at the base station, a new communication link will not be formed and the call may be lost. After the handover attempt fails, the mobile station can revert to the previous system if it is still possible to do so. Repeated efforts to handover will also fail due to the lack of further information and no significant changes in the mobile station environment. Thus, it is recognized in the art that there is a need for a method to perform additional hard handoffs with a greater likelihood of success.

Disclosure of Invention

It is an object of the present invention to reduce the likelihood of dropped calls during inter-system hard handoffs. In the event that the hard handoff efforts were due to failure, the mobile station returns information used by the communication system using the present invention to the original system to assist in future handoffs.

Prior to handover, the original base station will have a rough estimate of the most likely base station for the destination system to serve the mobile station moving into the destination system. In the illustrated embodiment, a message is sent from the base station to the mobile station containing a list of neighboring base stations in the destination system, a minimum value for the total received power threshold, and a minimum value for the pilot energy threshold. When a base station in the legacy system determines that a hard handoff is appropriate, it signals neighboring base stations in the destination system to begin transmitting forward link traffic to the mobile station entering the system. A first hard handoff attempt is made after the mobile station receives a message from the base station that initiated the inter-system hard handoff. The mobile station switches to the frequency of the destination system and attempts to acquire the base station of the destination system based on the provided acquisition parameter (i.e., pilot PN offset). If the minimum value of the pilot energy threshold is exceeded, the handoff is deemed successful and the mobile station remains on the destination system.

If the minimum of the pilot energy threshold is not exceeded, the recovery technique is initiated. The mobile station measures the total in-band energy of the destination system and compares it to a total received power threshold. If the minimum of the total received power threshold is not exceeded, the handover is immediately discarded. The mobile station returns to the original system and reports that no significant power was detected at the new frequency. If the minimum value of the total received power is exceeded, it is possible to acquire the destination system but the neighboring base stations provided by the original system (referred to as the new active set) are not suitable for communication. The mobile station then searches to locate a useful pilot signal in the destination system. In general, a table of search offsets provided to the mobile station will suffice for useful pilot positioning, although other search algorithms can be employed. After the search is completed, the mobile station returns to the original system and reports the failure and any pilot signals found in the search that exceed the third threshold.

If no significant received power is detected or no pilots are found in the search, the system controller may choose to delay the second handoff effort in anticipation of a favorable change in the mobile station's environment. Alternatively, the mobile station may abandon the hard handoff efforts altogether, and thus it may result in the eventual loss of the call. However, in those situations where a destination system is present, the system controller can update the active set based on the returned search information, and thus the destination system can change the base station transmitting to the mobile station. A message can then be sent to the mobile station for a second hard handoff attempt. Unless the environment changes, the second effort may be successful.

Drawings

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

fig. 1 is a schematic diagram of an exemplary spread spectrum CDMA communication system in accordance with the present invention;

FIG. 2 is a diagram showing an exemplary scenario in which various scenarios corresponding to the present invention may be described;

FIG. 3 is a diagram of an exemplary base station;

FIG. 4 is a diagram of an exemplary mobile station; and

FIG. 5 is a flow chart describing the operation of the present invention.

Detailed Description

Fig. 1 depicts one embodiment of a communication system using the present invention. A typical CDMA communications system includes a system controller and switch 10 that communicates with one or more base stations, examples of which are 12, 14 and 16. The system controller and switch 10 is also connected to a Public Switched Telephone Network (PSTN) (not shown) and other communication systems (not shown). Mobile station 18 is a user with forward links 20B, 22B, and 24B and reverse links 20A, 22A, and 24A. System controller and switch 10 controls intra-system soft and inter-frequency hard handoffs, and in conjunction with neighboring systems controls inter-system soft and inter-system hard handoffs. The exemplary embodiments of the present invention handle inter-frequency hard handoffs from a CDMA system to a CDMA system. It will be appreciated by those skilled in the art that the teachings of the present invention can be applied to switching using multiple access schemes and switching between systems using different modulation schemes.

Fig. 2 depicts three different scenarios for using the present invention. Three mobile stations M1, M2 and M3 move from the system S1 originating their respective calls to a nearby system S2 of different frequencies. Initially, mobile stations M1-M3 are in communication with one or more base stations (not shown) in system S1. As each mobile station moves across the boundary of S1 into S2, it will be counted as a hard handoff. The destination system S2 includes base stations B1-B5, which cover cells C1-C5, respectively. System S2 may have other base stations (not shown) that do not affect a given scenario. As shown, some cells intersect other cells. In the overlapping area, the mobile station is able to communicate with either base station, or both if the mobile station is in soft handoff. Obstacles O1-O3 are also shown. These obstacles deform the coverage area, which would otherwise be a rounded cell. The illustrated cell 5 is hatched to clearly indicate its unusual shape.

Consider first mobile station M1. This is an example of a situation that may lead to successful hard handoff in the prior art and the present invention. As M1 approaches the S1-S2 boundary, the legacy system S1 points out possible neighbors in the destination system S2 based on its best guess of the M1 location. S1 then informs M1 of the PN offsets of the cells (e.g., C1, C2, C3, C4, and C5) in the destination system through the base station (not shown) in contact with M1. In the illustrated embodiment, S2 also sends out parameters for a total received PILOT minimum MIN _ TOT _ PILOT and a received power minimum MIN _ RX _ PWR. In another embodiment, M1 may store the values of MIN _ TOT _ PILOT and MIN _ RX _ PWR, or may generate values based on system data. S1 then begins delivering traffic along with instructions to system S2 for establishing appropriate forward links with base stations B2 and B3 for data to mobile station M1. Base stations B2 and B3 are the most likely target base stations and are in the new active set. S1 then sends an initiation message to the mobile station M1 to initiate the hard handoff process. Due to the good propagation environment near mobile station M1, when M1 switches to a new frequency, pilots will be discovered and forward link traffic from the new active set (base stations B2 and B3) will be successfully demodulated, as predicted by system S1. M1 determines that a hard handoff will be successful because the total received PILOTs exceed the threshold MIN TOT PILOT. System S1 will reassign the originally assigned resources to communicate with mobile station M1 upon determining that the hard handoff was successful. The decision is made by receiving a message from system S2 or on the basis of a pre-arranged duration during which no new communication takes place between system S1 and mobile station M1.

Second, consider mobile station M2, which is in an area that cannot be covered by S2 (often referred to as a hole). As mobile station M2 approaches the S1-S2 boundary, system S1 indicates that coverage in system S2 is provided in cell C1. The handover is started in the same manner as described above. However, after switching to the frequency of the destination system S2, the mobile station M2 does not receive significant signal energy due to interference from barrier O3. That is, the total received PILOTs are below the threshold MIN _ TOT _ PILOT. In current systems, this call will be lost. In the present invention, the mobile station begins a recovery technique.

Once the mobile station determines that the pilot or pilots expected by S1 are not available, M2 measures the total received power in the new band and compares it to the threshold MIN RX PWR. In this example, the only transmitter near M2 is base station B1, and its signal is blocked by barrier O3, so no significant energy is found in the frequency band of the destination system. The mobile station M2 then drops the handover and returns to system S1, informing it that system S2 is not found. Assume that mobile station M2 continues to move away from system S1. There are a number of options because the call is not lost as with the existing methods. At a minimum, the call can continue to exist on system S1 until eventually lost as the distance becomes too great. Assuming that the mobile station environment is subject to change, a second handoff attempt after a delay period may be successful.

Finally, consider mobile station M3. The handover procedure is started in the same way as mobile stations M1 and M2, where cells C1 and C2 are the intended new active set. Due to the presence of obstructions O1 and O2, mobile station M3 does not get any of the anticipated cells and MIN _ TOT _ PILOT is not exceeded. The recovery process is started again. At this point base station B5 is within range, however its offset is not within the new active set, nor does it transmit forward link data directed to M3. Thus, the minimum value of the received power threshold, MIN RX PWR, is exceeded, although the expected cell is not obtained. In the exemplary embodiment of the invention, available pilots are searched for as if a system were available. When the search is complete, the mobile station M3 returns to system S1 and informs it that the handoff effort was due to the failure and the available pilot, in this case the pilot of cell C5. In the present invention, S1 sends a message to the destination system S2 to establish the forward link of base station B5, and then a second handover attempt can be made. If the environment has not changed significantly, M3 again switches to the new frequency, handing the call over successfully to base station B5 of the destination system S2.

Fig. 3 depicts an example base station. The base station 300 communicates with other systems (not shown) and with the system controllers and switches shown in fig. 1 through a system interface. Inter-frequency handover is a distributed process, signaled by the system controller and switch 10 and other switches, while the base station 300 handles the details of certain handovers. The system controller 10 in conjunction with the base station 300 determines whether an inter-system hard handoff is required. As mentioned above, there are many alternatives for handoff decision making, including mobile station location or pilot beacon reception. The destination system (not shown) is instructed by the legacy system to begin transmitting forward link traffic from a selected set of base stations on the destination system frequency. A database (not shown) in control processor 360 may contain the candidate base stations. Alternatively, a list of candidate handoff base stations may be returned from the destination system to the control processor 360 via the system interface 310. In the case where the destination system is not a CDMA system, other parameters useful for acquiring the destination system may be passed to control processor 360 through system interface 310.

Parameters and instructions from control processor 360 form a message in message generator 320. These messages are modulated in modulator 330 and sent to the mobile station via transmitter 340 and antenna 350. In the illustrated embodiment, the modulator 330 is a CDMA modulator as described in the above-mentioned U.S. Pat. nos. 4,901,307 and 5,103,459. In the illustrated embodiment, the neighbor base station's table, MIN _ TOT _ PILOT, and MIN _ RX _ PWR are combined into a single message, referred to herein as the other frequency neighbor table message (OFNLM). The message that signals the mobile station to begin attempting to acquire the base station of the destination system to the mobile station contains the destination system active set and is referred to as an Extended Handoff Direction Message (EHDM). Additional parameters that can be sent to the mobile station can be envisioned to help improve hard handoff in the event of a failed handoff attempt. For example, a specific table of offsets to search, a range of offsets to search, or a specific search algorithm (such as searching in increments of 64 subcodes from those offsets that are desired by those base stations listed in the OFNLM).

After a hard handoff attempt fails, the mobile station will follow the instructions given back to the original system to inform it of the discovery. Reverse link signals from the mobile station to base station 300 are received by antenna 390, downconverted in receiver 380, and demodulated in demodulator 370 under the control of control processor 360.

Fig. 4 depicts an exemplary mobile station 500. Messages from base station 300 arrive at control processor 520 through antenna 610, duplexer 600, receiver 590, and demodulator 570. In the illustrated embodiment, the receiver 590 is a CDMA modulator as described in the above-mentioned U.S. Pat. nos. 4,901,307 and 5,103,459. Upon receiving the EHDM message from the base station 300, the control processor 520 directs the receiver 590 and the transmitter 560 to tune to the frequency of the destination. At this point, the communication link with the original system has been broken. Control processor 520 directs demodulator 570 to demodulate the pilot with the offset in the active set as given by base station 300 in EHDM. The energy in the signal demodulated with those offsets is accumulated in a pilot energy accumulator 530. The control processor 520 uses the accumulated results for comparison to the MIN _ TOT _ PILOT. If MIN _ TOT _ PILOT is exceeded, the handover is deemed successful. If MIN _ TOT _ PILOT is not exceeded, recovery operations begin. Alternatively, the success of the switching effort can be determined using a requirement to receive N good frames (no CRC errors) within a predetermined time T.

The first step following an unsuccessful hard handoff attempt is to determine whether the destination system is available. The accumulator 540 of received energy accumulates the total power received in the frequency band of the destination system and provides the result to the control processor 520. The control processor 520 compares those accumulated results with MIN _ RXPWR. If MIN _ RX _ PWR is not exceeded, the handover effort fails. Receiver 590 and transmitter 560 return to the original frequency and control processor 520 generates a message informing base station 300 of the failed handover attempt and the absence of the apparent presence of the destination system. This message is provided to modulator 550, which modulator 550 modulates the message and provides the modulated message for transmission through transmitter 560, duplexer 600, and antenna 610.

The mobile station 500 contains system priority information, which is stored in a system priority table 510. If no destination system exists, mobile station 500 may send additional system information to base station 300 so that mobile station 500 intends to acquire a different system in the next hard handoff attempt. For example, the neighborhood may be covered by multiple systems, including CDMA systems and combinations of systems of other technologies. The system priority table 510 may be programmed to intend to acquire a second system if the first preferred system is not available. If the second system is not available, there may be additional systems for which a handoff is intended. The handover can be performed in order of priority until all candidate systems have been subjected to acquisition efforts.

If MIN RX PWR is exceeded indicating that the destination system is available, mobile station 500 proceeds as instructed previously. In the preferred embodiment, searcher 580 performs a search to determine the pilot offsets available to base stations in the destination system. To perform the search, searcher 580 generates a PN sequence at a specified offset. Demodulator 570 correlates the input data with the shifted PN sequence. Pilot energy accumulator 530 measures the pilot energy with accumulated samples for the offset over a predetermined time interval. Control processor 520 compares the result to a threshold value, referred to as T _ ADD, to determine whether a pilot is available for the offset. Searcher 580 then moves to the next candidate offset. This process is repeated until there are no candidate offsets to measure. The search operation is described in detail in co-pending U.S. patent application No. 08/509,721 entitled "method and apparatus for search acquisition in a CDMA communication system," filed 7/26 1996, assigned to the assignee of the present invention, and incorporated herein by reference. Other search algorithms can be substituted in searcher 580 without altering the present invention.

The search after a hard handover failure may be performed for all possible offsets or a subset thereof. For example, a range of offsets may be searched. In the illustrated embodiment, the OFNLM contains a subset of the offsets to be searched. In the illustrated system, adjacent base stations are separated by integer multiples of 64 sub-codes. To acquire the complete set of neighboring base stations, if the offset of one base station within the system is known (even if it is not currently available), then only the offsets need to be searched for, which are integer multiples of the 64 subcodes of the known offset. Combinations of spaced offsets within a specified range or ranges can also be searched.

When the destination system is a system of other technology, a different procedure may be performed which will yield information that will facilitate a subsequent handover effort. For example, when the destination system is a TDMA system, the mobile station may measure in-band energy at multiple sub-bands and report this information to the legacy system. Alternatively, in the case of a neighboring AMPS system, the base station can transmit an OFNLM specified frequency for the analog control channel. However, if the frequencies of the control channel are already known, it is not necessary to transmit these frequencies. In this case, if the mobile station finds that the voice channel it has switched to is too weak, the mobile station can proceed to measure the received power on the analog control channel. It may also determine a digital color class code (DCC) for the control channel. DCC may provide a better determination of a cell if the mobile station may receive multiple cells within an area. The frequency and DCC of the strongest analog base station can be returned as information to assist in subsequent handover efforts. Further discussion of the use of DCC can be found in chapter 3 of "mobile cellular telecommunications system" by William c.y.lee.

After mobile station 500 has completed the necessary tasks, receiver 590 and transmitter 560 return to the original frequency and control processor 520 informs base station 300, via modulator 550, transmitter 560, duplexer 600 and antenna 610, of the failure of the handover attempt and passes any information found during subsequent system searches.

The flow chart of fig. 5 describes the operation of the preferred embodiment of the present invention. After determining that a handover is imminent, the legacy system forecasts a list of neighboring base stations on frequencies of neighboring systems, block 50. Proceeding to 52, a base station in the legacy system sends the Other Frequency Neighborhood List Message (OFNLM) described above to the mobile station. In block 53, a valid set of new frequencies is determined. In block 54, the destination system establishes the forward link as specified in the Extended Handoff Direction Message (EHDM). In block 56, the base station in the legacy system sends an Extended Handoff Direction Message (EHDM) to the mobile station to initiate an inter-frequency hard handoff. In 58 the mobile station tunes to the new frequency in accordance with the message and intends to obtain the destination system in accordance with the active set information in the EHDM.

In block 60, the mobile station measures the PILOT energy (the sum of the energies of all PILOTs in the active set) and if the received PILOT energy exceeds the parameter MIN _ TOT _ PILOT, then proceeding to 62, a successful hard handoff occurs. The illustrated embodiment envisions that the mobile station can be directly handed off to a soft handoff condition in the destination system, although that is not a requirement. A single PILOT in the new active set (whose received PILOT energy exceeds the parameter MIN TOT PILOT) is sufficient for a successful handoff.

From 60, if MIN _ TOT _ PILOT is not exceeded, proceed to 68. At 68, if the total received power in the band exceeds the parameter MIN RX PWR (indicating that a destination system is generally present), proceed to 66, otherwise to 69.

Another embodiment would check the total received power before the pilot energy. If the MIN _ RX _ PWR threshold is not exceeded, the handover fails. This may be faster in some implementations.

At 66, the available pilot signals are searched for possible offsets. Any other search strategy can be performed here as well. When the search is completed, proceed to 65. The mobile station returns to the original system at 65 and then proceeds to 64. At 64, a change must be made to the OFNLM, and a return is made to 52 where the operation described above is performed.

At 69, the mobile station returns to the original system and then proceeds to 72. From 72, a determination can be made to proceed with the effort to continue the handover by proceeding to 70, or to stop the handover process by proceeding to 74. An optional delay is introduced at 70 and then to 64.

In another embodiment of the invention, the base station sends the mobile station a list of extended base stations that are available at the mobile station's entry destination system. In this embodiment, the forward link is not established immediately in the destination system. The mobile station simply determines whether the strength of any signal provided by any of the extended candidate system lists is appropriate to support the communication link. The mobile station monitors the forward link signal of each base station in the extended list of candidate base stations.

After monitoring the forward link signal of each base station in the extended list of candidate base stations, the mobile station must return to the original system and send a message indicating the forward link signal strength of the candidate base stations. In the illustrated embodiment, the mobile station compares the strength received by each base station in the augmented table to a predetermined threshold T _ ADD and only reports whether the measured signal power is above or below the threshold.

The base stations of the legacy system receive information about the signal strength of each base station in the destination system and the base stations of the legacy system generate an active set table based on this information. This table is provided to the destination system which establishes the forward link for the mobile station in accordance with the active set table provided by the original system. The base station of the legacy system sends the active list to the mobile station that intends to acquire the base station in the active list and, if the acquisition is successful, can obtain the transmission to the mobile station without interruption.

Referring to fig. 2, another embodiment will be described with the capture of mobile station M3. When the legacy system S1 determines that mobile station M3 should initiate a hard handoff operation to the destination system S2, the base stations in legacy system S1 currently communicating with mobile station M3 generate an augmented list of base stations in S2 that may be acquired by the mobile station. In the illustrated embodiment, the augmented candidate table will likely include the parameters necessary to search all base stations B1, B2, B3, B4, and B5, as well as additional base stations (not shown) in the destination system S2. Note that in another embodiment, so far, the destination system S2 has not been provided with information about M3.

The mobile station M3 tunes to the frequency of the destination system S2 and measures the energy of each pilot channel of the base stations in the augmented candidate list. In the example of mobile M3, the mobile would transmit a message back to the base station in legacy system S1 indicating that base station B5 may be acquired. In response to this message, the base station in the legacy system generates an active set table consisting solely of base station B5.

The base station in the legacy system will send a message to the destination system S2 indicating that the forward link for mobile station M3 should be provided to base station B5. In response to this message, the destination system S2 establishes a forward link for mobile station M3 to base station B5. The active set list is sent to mobile station M3. In response to the active group list message, mobile station M3 intends to acquire B5.

Referring to fig. 3, the base station 300 of the legacy system generates an extended candidate list in the message generator 320 and provides the message to the modulator 330. Modulator 330 modulates the message and provides it to transmitter 340, which upconverts and amplifies the signal and transmits the resulting signal via antenna 350.

Referring to fig. 4, the transmitted signal is received by the mobile station 500 through an antenna 610 and down-converted, filtered and amplified by a receiver 590. The received signal is then demodulated by demodulator 570 and provided to control processor 520. Control processor 520 then generates a set of commands directing the search to be conducted by searcher 580. Searcher 580 provides a set of searched demodulation parameters to demodulator 570. The demodulated signal is provided to a pilot energy accumulator 530, which measures the strength of the pilots of the base stations of the extended candidate list. The energy of each of these candidate base stations is provided to a control processor 520 which compares the measured energy to a threshold T _ ADD. Control processor 520 generates a message indicating which, if any, of the signals of the candidate base stations exceed the threshold.

The message is provided to a modulator 550 where it is modulated. The modulated signal is then provided to a transmitter 560 where it is upconverted, amplified and transmitted via an antenna 610.

Referring back to fig. 3, a message indicating the strength of the candidate base station is received by the antenna 390 of the base station 300 of the legacy system. The signal is downconverted and amplified by a receiver 380 and provided to a demodulator 370. Demodulator 370 demodulates the signal and provides the result to control processor 360. Control processor 360 generates an active set table for the destination system based on information in a message transmitted by mobile station 500 indicating the search results. In the illustrated embodiment, the active set table will include all base stations that, when monitored by the mobile station 500, exceed the energy threshold T _ ADD for the signal.

The control processor 360 sends the active set table to the system interface 310, which sends a message indicating the active set table to the destination system S2. The destination system S2 provides the forward link channel for each system in the active set table if capacity permits.

Control processor 360 also provides the active set table to message generator 320. The resulting message is modulated by modulator 330 and transmitted, as described above.

As described above, mobile station 500 receives messages from antenna 610, demodulates the signal, and provides the messages to control processor 520. Control processor 520 then provides information regarding the active set table to demodulator 570 and receiver 590 and uses the parameters of the base stations in the active set table in an effort to switch to the destination system S2. It should be noted that since the active list is determined by the mobile station 500 in this example, the mobile station does not have to receive the active group list because it knows the base stations in the list a priori. Thus, in another embodiment, the mobile station may delay for a predetermined time interval and perform a handoff to a base station whose signal exceeds a threshold. On the other hand, if the active set is not just a simple copy of the base station that exceeds the threshold, but rather takes into account parameters that are unknown to the mobile station (such as the capacity parameter of S2), then the transmission of the message will prove valuable.

In a variation of the above-described further embodiment, the mobile station periodically tunes to a new frequency and measures the offset provided in the OFNLM without direction from the base station. The time interval can be specified in the OFNLM. After the search is complete, the mobile station returns to the original system and reports its findings. Information obtained by polling neighboring systems can be used to determine the active set for subsequent handoffs, as well as to assist in determining whether to initiate a handoff to that system.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (29)

1. A method of providing a handoff from an origination system to a destination system in a wireless communication system in which a mobile station is moving from a coverage area of the origination system to a coverage area of the destination system, wherein the origination system operates at a first frequency and the destination system operates at a second frequency, the method comprising the steps of:

predicting at least one PN offset associated with the destination system;

transmitting a set of search parameter data related to said at least one PN offset from said originating system to said mobile station at a first frequency;

determining, at the mobile station, availability of the destination system based on the set of search parameter data;

transmitting a message from said mobile station to said originating system, said message indicating a measured received strength of a signal associated with said at least one PN offset at said mobile station;

generating, at the initiating system, a set of acquisition parameters based on the message from the mobile station; and

attempting, at the mobile station, to acquire the destination system based on the set of acquisition parameters.

2. The method of claim 1, wherein the generating step further comprises: at the originating system, a new set of at least one PN offset associated with the destination system is selected.

3. The method of claim 1, wherein the attempting step further comprises: at the mobile station, a new set of at least one PN offset associated with the destination system is selected.

4. A method of providing a handover for a mobile station from an originating system operating at a first frequency to a destination system operating at a second frequency, the method comprising the steps of:

receiving from the originating system at least one PN offset and a minimum total received pilot value associated with the destination system;

at the mobile station, tuning to a second frequency;

determining a pilot energy associated with the at least one PN offset received at the mobile station;

comparing the determined pilot energy to the minimum total received pilot value; and

sending a message from the mobile station to the originating system based on the comparison.

5. The method of claim 4, wherein the message comprises the determined pilot energy.

6. The method of claim 4, wherein the receiving step further comprises: receiving an additional set of parameters from the initiating system, the additional parameters including instructions to the mobile station after a hard handoff attempt has failed.

7. The method of claim 6, wherein the sending step is based on the instruction.

8. The method of claim 4, further comprising: and searching at a second frequency according to the comparison result.

9. The method of claim 8, wherein the searching step is based on the instruction.

10. A method for providing a handoff of a mobile station from an originating system operating at a first frequency to a destination system operating at a second frequency, the method comprising the steps of:

receiving at least one PN offset and a received power threshold associated with the destination system from the originating system;

at the mobile station, tuning to a second frequency;

determining a pilot energy associated with the at least one PN offset received at the mobile station;

measuring an in-band energy received by the mobile station at a second frequency;

comparing the measured in-band energy to the received power threshold; and

sending a message from the mobile station to the originating system based on the comparison.

11. The method of claim 10, wherein the message comprises the measured in-band energy.

12. The method of claim 10, wherein the receiving step further comprises: receiving an additional set of parameters from the initiating system, the additional parameters including instructions to the mobile station after a hard handoff attempt has failed.

13. The method of claim 12, wherein the sending step is based on the instruction.

14. The method of claim 10, further comprising: and searching at a second frequency according to the comparison result.

15. The method of claim 14, wherein the searching step is based on the instruction.

16. A method for providing a handoff of a mobile station from an originating system operating at a first frequency to a destination system operating at a second frequency, the method comprising the steps of:

transmitting from the originating system to the mobile station at least one PN offset associated with the destination system and a minimum total received pilot value;

receiving a message from the mobile station, the message based on a total received pilot value measured by the mobile station at a second frequency; and

transmitting at least one adjusted PN offset to the mobile station in accordance with the message, the adjusted PN offset having a different value than the at least one PN offset.

17. The method of claim 16, wherein said step of transmitting to said mobile station further comprises: sending a set of additional parameters to the mobile station, the additional parameters including instructions to the mobile station after a hard handoff attempt has failed.

18. The method of claim 16, wherein the message further comprises measurements made by searching at a second frequency.

19. A method for providing a handoff of a mobile station from an originating system operating at a first frequency to a destination system operating at a second frequency, the method comprising the steps of:

transmitting from the origination system to the mobile station at least one PN offset and a received power threshold associated with the destination system;

receiving a message from the mobile station, the message based on a received power measured by the mobile station at a second frequency; and

transmitting at least one adjusted PN offset to the mobile station in accordance with the message, the adjusted PN offset having a different value than the at least one PN offset.

20. The method of claim 19, wherein said step of transmitting to said mobile station further comprises: sending a set of additional parameters to the mobile station, the additional parameters including instructions to the mobile station after a hard handoff attempt has failed.

21. The method of claim 19, wherein the message further includes measurement results obtained by searching at a second frequency.

22. A mobile station apparatus, comprising:

a receiver configured to tune to one of at least two system frequencies in accordance with a frequency control signal; and

a control processor configured to generate the frequency control signal instructing the receiver to tune to a first frequency associated with an originating system and to receive at least one PN offset associated with a destination system and a minimum total received pilot value from the originating system at the first frequency; instructing the receiver to tune to a second frequency associated with the destination system and comparing a pilot energy measured at the second frequency to the minimum total received pilot value to produce a comparison; and commanding the receiver to call back to a first frequency according to the comparison result, and generating a message for the initial system according to the comparison result.

23. The mobile station apparatus of claim 22, wherein said message includes said pilot energy measured at a second frequency.

24. The mobile station apparatus of claim 22, wherein the control processor is further configured for receiving an additional set of parameters from the initiating system, the additional parameters including instructions to the mobile station after a hard handoff attempt has failed.

25. The mobile station apparatus of claim 24, further comprising a searcher configured to search at a second frequency in accordance with commands from the control processor, and wherein the control processor commands the searcher to search in accordance with the instructions.

26. A mobile station apparatus, comprising:

a receiver configured to tune to one of at least two system frequencies in accordance with a frequency control signal; and

a control processor configured to generate the frequency control signal instructing the receiver to tune to a first frequency associated with an originating system and to receive at least one PN offset and a received power threshold associated with a destination system from the originating system at the first frequency; instructing the receiver to tune to a second frequency associated with the destination system and comparing the received power measured at the second frequency to the received power threshold to produce a comparison; and commanding the receiver to call back to a first frequency according to the comparison result, and generating a message for the initial system according to the comparison result.

27. The mobile station apparatus of claim 26, wherein said message includes said received power measured at a second frequency.

28. The mobile station apparatus of claim 26 wherein the control processor is further configured for receiving an additional set of parameters from the initiating system, the additional parameters including instructions to the mobile station after a hard handoff attempt has failed.

29. The mobile station apparatus of claim 28, further comprising a searcher configured to search at a second frequency in accordance with commands from the control processor, and wherein the control processor commands the searcher to search in accordance with the instructions.