MXPA99006557A - Method and apparatus for performing mobile assisted hard handoff between communication systems - Google Patents

Abstract

A method and apparatus for performing inter-system hard handoff between communication systems (S1, S2) or inter-frequency hard handoff within a CDMA communication system is disclosed. The purpose of this invention is to reduce the probability of dropped calls during intersystem hard handoff. In the event that a hard handoff attempt is unsuccessful, the mobile station (18) will return to the original system (S1) with information which the communication system of the present invention uses to assist in the performance of future handoff attempts. Alternatively, with nohandoff attempt made, the mobile station (18) monitors the destination system (S2), then returns to the original system (S1) with information used to assist in subsequent handoff attempts. The information returned from monitoring a CDMA system consists of results of a search for one or more pilots given at offsets in a specific list provided to the mobile station (18) by the base station (B1-B5) or a set of offsets based upon a predetermined search algorithm.

Description

METHOD AND APPARATUS FOR REALIZING HARD ASSISTED MOBILE TRANSFER BETWEEN COMMUNICATION SYSTEMS BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method for hard transfer between different wireless communication systems. II. Description of the Related Art In a broadcast multiple code distributed broadcast (CDMA) spectrum communication system, a common frequency band is used for communication with all base stations within that system. An example of such a system is described in Internal Standard TIA / EIA IS-95-A entitled "Mobile Station Compatibility Standard-Base Station for a Dual-Mode Broadband Broadcast Spectrum Cell System" ("Mobile Station-Base" Station Compatibility Standard for Dual-Mode ideband Spread Spectrum Cellular System "), incorporated herein by reference. The generation and reception of CDMA signals are described in the U.S. Patent. No. 4,401,307 entitled "BROADCAST SPECTRUM MULTI ACCESS COMMUNICATION SYSTEMS USING SATELLITE OR TERRESTRIAL REPEATERERS" ("SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITE OR TERRESTRIAL REPEATERS") and in the US Patent. No. 5,103,459 entitled "SYSTEM AND METHOD FOR GENERATING WAVE FORMS IN A CDMA CELLULAR TELEPHONE SYSTEM" ("SYSTEM AND METHOD FOR GENERATING AVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM") both of which were assigned to the assignee of the present invention and incorporated herein for reference. The signals occupying the common frequency bands are discriminated at the receiving station through the waveform properties of the spread spectrum CDMA, based on the use of a high speed pseudo-noise (PN) code. A PN code is used to modulate signals transmitted from base stations and remote stations. The signals from different base stations can be received separately at the reception station by discrimination of the unique time offset that is entered into the PN codes assigned to each base station. The high-speed PN modulation also allows the receiving station to receive a signal from a single transmission station where the signal has traveled on different propagation paths. The demodulation of multiple signals is described in the U.S. Patent. No. 5,490,165 entitled "ALLOCATION OF THE DEMODULATION ELEMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS" ("DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS") and in the US Patent. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM" ("DIVERSITY RECEIVER 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 common frequency band allows simultaneous communication between a remote station and more than one base station, a condition known as flexible transfer, described in the U.S. Patent. No. 5,101,501 entitled "FLEXIBLE TRANSFER IN A CDMA CELLULAR TELEPHONE SYSTEM" ("SOFT HANDFF IN A CDMA CELLULAR TELEPHONE SYSTEM") and the US Patent. No. 5,267,261 entitled "FLEXIBLE TRANSFER ASSISTED BY A MOBILE STATION IN A CDMA CELLULAR COMMUNICATIONS SYSTEM" ("MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM"), both assigned to the assignee of the present invention and incorporated herein for reference. Similarly, a remote station may be simultaneously communicating with two sectors of the same base station, known as more flexible transfer, as described in the U.S. Patent Application. entitled "METHOD AND APPARATUS TO MAKE TRANSFER BETWEEN SECTORS OF A COMMON BASE STATION" ("METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION"), Serial No. 08 / 405,611, filed March 13, 1995, assigned to the assignee of the present invention and incorporated herein by reference. The transfers are described as flexible and more flexible because they make the new connection before breaking the existing one. If a mobile station travels outside the boundary of the system with which it is currently communicating, it is desirable to maintain the communication link by transferring the call to a nearby system, if one exists. The nearby system can use any wireless technology, examples of which are CDMA, NAMPS, AMPS, TDMA or FDMA. If the nearby system uses CDMA in the same frequency band as the current system, a flexible inter-system transfer can be performed. In situations where flexible inter-system transfer is not available the communication link is transferred through a hard transfer where the current connection is broken before the new one is made. Examples of hard transfers are those from a CDMA system to a system that uses an alternate technology or a transferred call between two CDMA systems that use different frequency bands (hard inter-frequency transfer). Hard inter-frequency transfers can also occur within a CDMA system. For example, a region of high demand such as an area of the city center may require a greater number of frequencies to serve the demand than the suburban region that surrounds it. It may not be profitable to deploy all available frequencies throughout the system. A call originating from a frequency displayed only in the area of high congestion must be transferred as the user moves to a less congested area. Another example is that of a microwave or other service that operates on a frequency within the limits of the system. When users move within an area that suffers from interference from the other service, their call may need to be transferred to a different frequency. Transfers can be initiated using a variety of techniques. Transfer techniques, including those that use signal quality measurements to initiate the transfer, are found in the U.S. Patent Application. copending issue 08 / 322,817 entitled "METHOD AND APPARATUS FOR TRANSFER BETWEEN DIFFERENT CELLULAR COMMUNICATIONS SYSTEMS" ("METHOD AND APPARATUS FOR HANDOFF BETWEEN DIFFERENT CELLULAR COMMUNICATIONS SYSTEMS"), filed on October 16, 1994, assigned to the assignee of the present invention and incorporated herein by reference. An additional disclosure about the transfers, including the measurement of the round trip signal delay to initiate the transfer is described in the U.S. Patent Application. No. 08 / 652,742 entitled "METHOD AND APPARATUS FOR HARD TRANSFER IN A CDMA SYSTEM" ("METHOD AND APPARATUS FOR HARD HAND IN A CDMA SYSTEM"), filed May 22, 1996, assigned to the assignee of the present invention and incorporated herein by reference. Transfers from CDMA systems to alternative technology systems are described in the U.S. Patent Application. Copending issue 08 / 413,306 (application * 306) entitled "METHOD AND APPARATUS FOR CDMA ASSISTED BY MOBILE UNIT FOR HARD TRANSFER OF A ALTERNATIVE SYSTEM "(" METHOD AND APPARATUS FOR MOBILE UNIT ASSITED CDMA TO ALTERNATIVE SYSTEM HARD HANDOFF "), filed March 30, 1995, assigned to the assignee of the present invention and incorporated herein by reference.In the application? 306, pilot beacons are placed at the boundaries of the system. a mobile station reports these pilots to the base station, the base station knows that the mobile station is approaching the limit.

When a system has determined that a call must be transferred to another system via hard transfer, a message is sent to the mobile station, directing it to do so together with parameters that allow the mobile station to connect to the destination system. The system has only estimates of the location and current environment of the mobile station, so that parameters sent to the mobile station are not guaranteed to be accurate. For example, with beacon assisted transfer, the measurement of the resistance of the pilot beacon signal may be a valid criterion for activating the transfer. However, the appropriate cell or cells in the destination system that are to be assigned to the mobile station (known as the Active Set) are not necessarily known. In addition, including all likely possibilities may exceed the maximum allowable in the Active Set. In order for the mobile station to communicate with the destination system, it must lose contact with the old system. If the parameters given to the mobile station are not valid for some reason, for example changes in the environment of the mobile station or lack of information about the precise location in the base station the new communication link will not be formed and the call may be interrupt After an unsuccessful transfer attempt, the mobile station can return it to the previous system if it is still possible to do so. Without additional information and without significant change in the mobile station environment, repeated transfer attempts will also fail. Thus there is a felt need in the art for a method to perform additional hard transfer attempts with greater probability of success. SUMMARY OF THE INVENTION The purpose of this invention is to reduce the probability of interrupted calls during hard inter-system transfer. In the event that a hard transfer attempt is unsuccessful, the mobile station will return to the original system with information that the communication system of the present invention uses to assist in the performance of future transfer attempts. Prior to the transfer, the original base station will have an approximate estimate of the most likely base stations of a destination system to provide service to a mobile station when it travels to the destination system. In the implicit mode, a message from the base station will be sent to the mobile station containing this list of nearby base stations in the destination system, a minimum received total energy threshold and a minimum pilot energy threshold. When the base station in the original system has determined that a hard transfer is appropriate, it signals the nearby base stations in the destination system to begin transmitting forward link traffic to the mobile station entering the system. A first hard transfer is attempted after a message is received by the mobile station from the base station initiating the hard inter-system transfer. The mobile station switches to the frequency of the destination system and attempts to acquire the base stations of the destination system according to the acquisition parameters provided (ie, the travels of the pilot PN). If the minimum pilot energy threshold is exceeded, the transfer is considered to be successful and the mobile station remains in the destination system. If the minimum pilot energy threshold is not exceeded, recovery techniques are initiated. The mobile station measures the total in-band energy of the target system and compares it to the total energy threshold received. If the minimum received total energy threshold is not exceeded, the transfer is immediately abandoned. The mobile station returns to the original system and reports that no significant energy was detected at the new frequency. If the total received minimum energy is exceeded it is probable that the destination system is available, but that the nearby base stations provided by the original system (referred to as the new Active Set) is not acceptable for communication. The mobile station then performs a search to locate viable pilot signals in the destination system. In general, a list of scrolls for search provided to the mobile station will be sufficient to locate available pilots, although other search algorithms may be employed. At the termination of the search, the mobile station returns to the original system and reports the failure and any pilot signal found in the search that exceeds a third threshold. If no significant received power was detected or no pilots were found in the search, the system controller may choose to delay a second transfer attempt pending a beneficial change in the mobile station environment. In the alternative, the mobile station may abandon the hard transfer attempt completely, which would likely result in the eventual interruption of the call. However, in those cases where the destination system is present, the system controller can update the Active Set based on the returned search information, and the destination system can modify the base stations transmitting to the mobile station according to the above. Then a second hard transfer attempt message may be sent to the mobile station. Unless the environment has changed, the second attempt is likely to be successful. BRIEF DESCRIPTION OF THE DRAWINGS The features, objects and advantages of the present invention will be more apparent from the detailed description set forth below when taken in conjunction with the drawings in which similar reference characters are identified accordingly by all and in which: Figure 1 is a schematic overview of an exemplary spread spectrum CDMA communication system in accordance with the present invention; Figure 2 is a representation of exemplary scenarios by which the various situations to which this invention responds can be described; Figure 3 is an illustration of an exemplary base station; Figure 4 is an illustration of an exemplary mobile station; and Figure 5 is a flow diagram illustrating the operation of the present invention. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Figure 1 represents a modality of a communication system employing the present invention. A typical CDMA communication system consists of a system controller and a switch 10 in communication with one or more base stations examples of which are 12, 14 and 16. The system controller and the switch 10 also connect to the Public Switched Telephone Network (PSTN) (not shown) and with other communication systems (not shown). Mobile station 18 is an example of suscpptor with forward links 20B, 22B and 24B and reverse links 20A, 22A and 24A. The system controller and the switch 10 control the flexible transfers and the hard inter-frequency transfers within the system and in conjunction with the nearby systems it controls the inter-system flexible transfer as well as the inter-system hard transfers. The method and method of the present invention deals with hard transfers of the frequencies of the CDMA system to the CDMA system. It will be understood by one skilled in the art that the teachings of the present invention can be applied to transfers using multiple access schemes and for transfers between systems using different modulation schemes.

Figure 2 represents three different possible scenarios in the use of the present invention. Three mobile stations, MI, M2 and M3 move from the system where their respective calls originated, Sl, to a nearby system of different frequency, S2. Initially, all mobile stations MI-M3 are in communication with one or more base stations (not shown) in the system Sl. As each mobile station moves across the limit of Sl to S2, a hard transfer attempt will be made. The target system, S2, contains the base stations Bl-B5, each of which covers a cell area Cl-C5, respectively. System S2 may have other base stations (not shown) that do not affect the given scenarios. As shown, some cells intersect with other cells. In this overlap region, a mobile station may be in communication with any base station or both simultaneously if the mobile station is in flexible transfer. Obstructions 01-03 are also shown. These obstructions distort coverage areas that would otherwise be circular cells. Cell C5 is shaded to clearly indicate its unusual shape. Consider the first mobile station MI. This is an example of a case that would result in a successful hard transfer both in the state of the art and in the current invention. When MI approaches the edge of Sl - S2, the origin system Sl predicts the probable neighbors in the destination system S2, based on its best estimate of the location of MI. Sl, through a base station in contact with MI (not shown) then notifies MI of the PN offsets of the cells in the target system S2, for example Cl, C2, C3, C4 and C5. In the exemplary embodiment, S2 also sends parameters for the minimum received total pilot, MIN_TOT_PILOT and the minimum received energy, MIN_RX_PWR. In an alternative mode, MI may store values of MIN_T0T_PIL0T and MIN_RX_PWR or may be able to generate the values based on the system data. Sl then initiates forward traffic to system S2 with instructions to establish the appropriate forward link for data directed to mobile station MI at base stations B2 and B3. Base stations B2 and B3 are the most likely target base stations and are found in the new Active Set. Then Sl sends a start message to the mobile station MI to initiate the hard transfer process. Due to the benign propagation environment in the vicinity of the mobile station MI, when MI switches to the new frequency it will find the pilots and successfully demodulate the forward link traffic from the new Active Set, the base stations B2 and B3, as predicted for the system Sl. MI determines that the hard transfer is successful because the total received pilot exceeds the MIN_TOT_PILOT threshold. The system Sl will deallocate previously allocated system resources to communicate with the mobile station MI after it is determined that the hard transfer was successful. This determination can be made upon receipt of a message from the S2 system or based on the pre-established time duration, in which no further communication takes place between the system Sl and the mobile station MI. Next, consider the mobile station M2, which is in an inadequate coverage area by S2, often referred to as a gap. When the mobile station M2 approaches the edge of Sl-S2 the system Sl predicts that the coverage in the system S2 is provided in the cell Cl. The transfer starts in the same way as described above. However, after switching to the frequency of the target system S2, no significant signal energy is received by the mobile station M2 due to the interference caused by the obstruction 03. That is, the total received pilot is less than the threshold MIN_TOT_PILOT . In current systems this call would be interrupted. In the present invention, the mobile station starts the recovery techniques. Once the mobile station determines that the pilot or pilots predicted by Sl are not available, M2 measures the total energy received in the new frequency band and compares it to the threshold MIN_RX_PWR. In this example, the only transmitter near M2 is the base station Bl and its signal is blocked by obstruction 03, so that no significant energy is found in the frequency band of the target system. The mobile station M2 then abandons the transfer and returns to the system Sl, notifying it that the S2 system was not found. Assuming that the mobile station M2 continues to move away from the system Sl. Since the call was not interrupted, as would have been the case using the current methods, there are a number of options. At a minimum, the call can continue in the system Sl until it is eventually interrupted, because the distance has become too large. Since the environment of the mobile station is susceptible to changes, a second transfer attempt after a delay may be successful. Finally, consider the mobile station M3. In a similar manner to the mobile stations MI and M2, the transfer procedures with the Cl and C2 cells are initiated, the new Active Set being predicted. Due to the obstructions 01 and 02, no predicted cell is available for the mobile station M3, hence MIN_T0T-PIL0T is not exceeded. Again the recovery procedures begin. This time base station B5 is within the range, however its displacement is not in the new Active Set, nor is it transmitting forward link data to M3. As such, although the predicted cells are not available, the minimum received energy threshold, MIN_RX_PWR is exceeded. In the exemplary embodiment of the present invention, since the system appears to be available, a search is made for available pilots. When the search is completed, the mobile station M3 returns to the system Sl and notifies of the failed transfer attempt, as well as of the pilots available in this case the pilot for the cell C5. In the present invention, Sl sends a message to the destination system S2 to establish a forward link on the base station B5 and then a second attempt can be made in the transfer. If the environment has not changed substantially, the second time M3 switches to the new frequency, the call will be successfully transferred to the base station B5 of the target system S2.

Figure 3 represents an exemplary base station. The base station 300 communicates with other systems (not shown) and with the system controller and the switch 10, shown in Figure 1, through the interface 310 of the system. The inter-frequency transfer is a distributed process, with the system controller and the switch 10 transmitting signals with the other switch and the base station 300 handling some of the details of the transfer. The controller 10 of the system determines in conjunction with the base station 300, that a hard inter-system transfer is necessary. There are many alternatives for determining the transfer as described above, including the location of the mobile station or the reception of pilot beacons. The destination system (not shown) is instructed by the originating system to initiate forward link traffic transmission over the destination system frequency from a select set of base stations. A database (not shown) in the control processor 360 may contain the candidate base stations. Alternatively, an appropriate list of base station transfer candidates can be returned from the target system to the control processor 360 through the system infer 310. In situations where the destination system is not a CDMA system, other useful parameters may be provided to acquire the destination system to the control processor 360 through the interface 310 of the system. The parameters and instructions of the control processor 360 are formed in messages in the message generator 320. These messages are modulated in the modulator 330 and sent to the mobile station through the transmitter 340 and the antenna 350. In the exemplary embodiment the modulator 330 is a CDMA modulator described in the US Patents. Nos. 4,901,307 and 5,103,459 mentioned above. In the exemplary embodiment, the list of adjoining base stations, MIN_T0T_PIL0T and MIN_RX_PWR are combined in a single message, referred to herein as the Message of List of Other Frequency Collantes (OFNLM). The message from the base station to the mobile station that sends the signal to the mobile station to initiate the attempt to acquire the destination system contains the Active Set of the destination system and the Extended Transfer Address Message (EHDM) is named. Additional parameters that could be sent to the mobile station are provided to facilitate the improved hard transfer in the event of a failure in the transfer attempt. For example, a specific list of displacements to investigate, a range of displacements to be investigated or a specific search algorithm such as search displacements in increments of 64 chips, away from those movements attempted from those of the base stations listed in the OFNLM. After a failed hard transfer attempt, the mobile station will follow the instructions as they are given, then return to the original system to communicate their discoveries. The reverse link signals from the mobile station to the base station 300 are received through the antenna 390, subverted in the receiver 380 and demodulated in the demodulator 370 under the control of the control processor 360. Figure 4 depicts an exemplary mobile mobile station 500. The messages arrive at the control processor 520 from the base station 300 through the antenna 610, the duplexer 600, the receiver 590 and the demodulator 570. In the exemplary embodiment the receiver 590 is a CDMA modulator as described in US patents Nos. 4,901,307 and 5,103,459 mentioned above. Upon receiving the EHDM message from the base station 300, the control processor 520 directs the receiver 590 and the transmitter 560 to tune the destination frequency. At this point the communication link with the original system has been broken. The control processor 520 directs the demodulator 570 to try to demodulate the pilots in the displacements in the Active Set as given by the base station 300 in the EHDM. The energy in the signals demodulated with these displacements accumulate in the accumulator 530 of the pilot energy. The control processor 520 uses the accumulation results to compare them to MIN_TOT_PILOT. If MIN_TOT_PILOT is exceeded, the transfer is considered successful. If MIN__TOT_PILOT is not exceeded, recovery operations are initiated. Alternatively, a requirement to receive some number N of good structures (without CRC errors) within a specific time T may be used to determine if the transfer attempt is successful. The first stage that follows an unsuccessful hard transfer attempt is to determine if the destination system is available. The received energy accumulator 540 accumulates the total energy received in the frequency band of the target system and provides the result to the control processor 520. The control processor 520 compares these accumulation results with the threshold MIN_RX_PWR. If MIN_RX_PWR is not exceeded, the transfer attempt is suspended. The receiver 590 and the transmitter 560 are returned to the original frequency and the control processor 520 generates a message notifying the base station 300 that the transfer attempt failed and that the destination system was not significantly present. The message is provided to the modulator 550, which modulates the message and provides the modulated signal through the transmitter 560, the duplexer 600 and the antenna 610 for transmission. The mobile station 500 contains system privileged information, stored in table 510 of system privileges. If the destination system is not present, the mobile station 500 can send information from the alternate system to the base station 300, so that the mobile station 500 may try to acquire a different system in the next hard transfer attempt. For example, a nearby region may be covered by multiple systems, which may include a combination of CDMA systems as well as alternative technology systems. The system privilege table 510 can be programmed in such a way that if a first preferred system is not available, the acquisition of a second system is attempted. There may be additional systems with which to attempt the transfer, if the second system is not available. Transfer attempts can be made in a priority ranking order until the acquisition has been attempted in all candidate systems. If MIN_RX_PWR is exceeded, indicating that the destination system is available, the mobile station 500 proceeds as previously instructed. In the exemplary embodiment, the explorer 580 conducts a search to locate pilot journeys where base stations are available in the destination system. To perform a search, the 580 scanner generates the PN sequence with a specific offset. The demodulator 570 correlates the data that it enters with the PN sequence of displacement. The pilot energy accumulator 530 measures the pilot energy for that displacement, by accumulating samples for a predetermined time interval. The control processor 520 compares this result to a threshold called T_ADD, to determine if a pilot is available for that displacement. The explorer 580 then moves to the next displacement candidate. The process is repeated until there are no more candidate displacements to measure. The search operation process is described in detail in the U.S. Patent Apption. co-pending with Serial No. 08/509/721 entitled "METHOD AND APPARATUS FOR PERFORMING SEARCH IN A CDMA COMMUNICATION SYSTEM" ("METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM"), presented on July 1996, which is assigned to the assignee of the present invention and incorporated by reference herein. Alternate search algorithms may be substituted in scanner 580 without modification for the present invention. The search subsequent to the hard transfer failure can be made on all possible displacements or a subset thereof. For example, a range of displacements can be explored. In the ejlificativa ej modality the OFNLM contains the subset of displacements to be explored. In the exemplary system, the nearby base stations are separated by integers multiples of 64 chips. If a base station offset is known in the system (even if it is not currently available), only the displacements that are multiple integers of 64 of that known offset need to be scanned, they need to be scanned in order to try the acquisition over the complete set of nearby base stations. A combination of shifts spaced in a specific range or number of ranges can also be explored. When the target system is an alternate technology, there may be different procedures to perform it, which will provide information that will improve subsequent hard transfer attempts. For example, when the destination system is TDMA, the mobile station can measure the in-band energy in a plurality of frequency sub-bands and report this information to the originating system. 0 in the case of a nearby AMPS system, the base station can send an OFNLM specifying frequencies for the analog control channels. However, it may not be necessary to send the frequencies of the control channels if they are already known. In this case, if the mobile station finds that the voice channel to which it was transferred is too weak, the mobile station can proceed to measure the received energy over the analogous control channels. The digital color code (DCC) can also be determined for the control channel. The DCCs provide a better determination of the cell in case the mobile station may be able to receive multiple cells in one area. The frequencies and DCCs of the strongest analog base stations can be returned as information to assist with a subsequent transfer attempt. An additional discussion of the use of DCCs can be found in the chapter 3 of "Mobile Cellular Telecommunications Systems" ("Mobile Cellular Telecommunications Systems") by William C.Y. Read. After the mobile station 500 completes the requisite tasks, the receiver 590 and the transmitter 560 are returned to the original frequency and the control processor 520 notifies the base station 300 through the modulator 550, the transmitter 560, the duplexer 600 and the antenna 610 that the transfer attempt failed and supplies any information that has been discovered during subsequent system search procedures. The flow chart in Figure 5 illustrates the operation of the preferred embodiment of this invention. After determining that a transfer is imminent the originating system predicts the list of nearby base stations on the frequency of the near system in box 50. Proceeding to 52, a base station in the originating system sends the mobile message to the mobile station. List of Other Frequency Adjacents (OFNLM) described above. In block 53, the Active Set is determined for the new frequency. In block 54 the destination system establishes the forward link as specified in the Extended Transfer Address Message (EHDM). In block 56, the base station in the originating system sends the Extended Transfer Address Message (EHDM) to the mobile station to initiate hard inter-frequency transfer. Following this message at 58, the mobile station tunes to the new frequency and tries to acquire the destination system according to the information of the Active Set in the EHDM message. In block 60, the mobile station measures the pilot energy, the sum of the energy of all the pilot in the active set, and if the total pilot energy received exceeds that of the MIN_TOT_PILOT parameter, it proceeds to 62, where a hard transfer has occurred successful The exemplary mode foresees that a mobile station is capable of transferring directly to a flexible transfer condition in the destination system, although this is not a requirement. A single pilot in the new Active Set whose received pilot power exceeds that of the MIN_TOT_PILOT parameter is sufficient for a successful transfer. Starting at 60, if MIN_T0T_PIL0T is not exceeded, proceed to 68. At 68, if the total energy received in the frequency band exceeds the MIN_RX_PWR parameter indicating the general presence of the target system, proceed to 66, otherwise it goes to 69. An alternative mode would be to verify the total energy received before the pilot energy. If the threshold of MIN_RX_PWR is not exceeded, the transfer is suspended. This may be faster in some implementations. At 66, look for possible offsets for available pilot signals. Any alternative search strategy can be done here as well. When the search is completed, proceed to 65. The mobile station returns to the original system at 65, then proceeds to 64.

At 64, the necessary changes are made to OFNLM and it is returned to 52 where the operation proceeds as described above. At 69, the mobile station returns to the original system, then proceeds to 72. From 72, the decision can be made to continue trying the transfer when proceeding to 70 or the transfer procedure can be suspended proceeding to 74. An optional delay is entered at 70 , then proceeds to 64. In an alternative embodiment of the present invention, the base station sends to the mobile station an expanded list of base stations that may be available at the point at which the mobile station is entering the destination system . In this alternative mode, no forward links are established in the destination system. Rather, the mobile station simply determines whether the resistance of any of the signals provided by any of the extended list of candidate systems is adequate to support a communication link. The mobile station monitors the forward link signals of each of the base stations in the expanded list of candidate base stations. After monitoring the signal strength of each of the base stations in the expanded list of candidate base stations, the mobile station necessarily returns to the original system and sends a message indicating the resistance of the forward link signal of the candidate base stations. In the exemplary embodiment, the mobile station compares the resistance of the signals received by each of the base stations in the extended list to a predetermined threshold T_ADD and reports only if the energy of the measured signal is above or below the threshold . The base station of the original system receives the information regarding the resistance of the signal from each of the base stations in the destination system and from this information the base station of the original system generates an Active Set list. This list is provided to the destination system that establishes forward links for the mobile station according to the Active Set list provided by the original system. The base station of the original system transmits the active list to the mobile station, which attempts to acquire the base stations in the active list and, if the acquisition is successful, the transmission to the mobile station is available without interruption. Referring to Figure 2, the alternative mode will be described in terms of the acquisition of the M3 mobile. When the original system Sl determines that the mobile M3 must initiate hard transfer operations to the destination system S2, the base station in the original system Sl that is currently in communication with the mobile station M3 generates an expanded list of base stations in S2 that the mobile station may be able to acquire. In the exemplary embodiment, the extended list of candidates probably consists of the parameters necessary to perform a search on all base stations Bl, B2, B3, B4 and B5, as well as the additional base stations on the target systems S2 (not shown) ). Note in the alternative mode, that no information has been provided with respect to M3, hitherto to the destination system S2. The mobile station M3 tunes the frequency of the target system S2 and measures the energy on each of the pilot channels of the base stations in the expanded candidate list. In the example of the mobile station M3, the mobile station would transmit back a message to the base station on the original system Sl, a message indicating that the acquisition on the base station B5 was possible. In response to this message, the base station in the original system would generate a list of Active Sets consisting only of the base station B5.

The base station in the original system would send a message to the destination system S2, indicating that a forward link for the mobile station M3 should be provided in the base station B5. In response to this message, the destination system S2 establishes a forward link for the mobile station M3 at the base station B5. The Active Set list is sent to the mobile station M3. In response to the message of the Active Set, the mobile station M3 attempts to acquire the base station B5. Referring to Figure 3, the base station 300 of the original system generates an expanded candidate list in the message generator 320 and provides the message to the modulator 330. The message is modulated by the modulator 330 and is provided to the transmitter 340 which overconverts and amplifies the signal and transmits the resulting signal through the antenna 350. Referring to Figure 4, the transmitted signal is received by the mobile station 500, by the antenna 610 and is subconverted, filtered and amplified by the 590 receiver. The received signal is then demodulated by the demodulator 570 and is provided to the control processor 520. The control processor 520 then generates a set of commands directing a search to be performed by the explorer 580. The explorer 580 provides a set of demodulation parameters to the demodulator 570. The demodulated signals are provided to the pilot energy accumulator 530 which measures the resistance of the pilot stations of the base stations of the expanded candidate list. The energy of each of these candidates is provided to the control processor 520 which compares the measured energy with a threshold T_ADD. The control processor 520 generates a message which means which, if any, of the signals from the candidate base stations exceed the threshold. The message is provided to the modulator 550 where it is modulated. The modulated signal is then provided to the transmitter 560 where it is overconverted, amplified and transmitted through the antenna 610. Referring again to Figure 3 the message indicating the resistances of the candidate base stations is received by the antenna 390 of the base station 300 of the original system. The signal is downconverted and amplified by the receiver 380 and is provided to the demodulator 370. The demodulator 370 demodulates the signal and provides the result to the control processor 360. The control processor 360 generates a list of Active Sets for the destination system of according to the information contained in the message transmitted by the mobile station 500, indicating the results of its search. In the exemplary embodiment, the list of Active Sets will consist of all base stations whose signals, when monitored by mobile station 500, exceed the energy threshold D_ADD. The 360 control processor sends the list of Active Sets to interface 310 of the system, which sends a message indicating the list of the Active Set to the target S2 system. If the capacity issues allow it, the destination system S2 provides link channels in advances on each of the systems in the list of Active Sets. The control processor 360 also provides the list of Active Sets to the message generator 320. The resulting message is modulated by the modulator 330 and transmitted as described above. The mobile station 500 receives the message by the antenna 610, demodulates the signal as described above and provides the message to the control processor 520. The control processor 520 then provides information regarding the list of Active Sets to the demodulator 570 and the receiver 590 and a hard transfer is attempted to the destination system S2 using the parameters of the base stations in the list of Active Sets. It should be noted that because in this example, the active list was determined by the mobile station 500, the mobile station does not need to receive the list of Active Sets, since he knows a priori the station in the list. Thus in an alternative mode, the mobile station may be delayed for a predetermined period of time and perform the transfer to base stations whose signal exceeded the threshold. If, on the other hand, the Active Set is not simply a copy of the base stations that exceeded the threshold, but also takes into account unknown parameters for the mobile station, such as the capacity parameters of S2, then transmission of the message would be valuable. . In a variation of the alternative embodiment described above, the mobile station periodically tunes to the new frequency and measures the offsets provided in the OFNLM without address of the base station. The period can be specified in the OFNLM. After the search is completed, the mobile station returns to the originating system and reports its discoveries. This information gained by polling the nearby system can be used to determine the Active Set for a subsequent transfer attempt, as well as to help determine if a transfer to that system is initiated. The prior description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The 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, it is not intended that the present invention be limited to the embodiments shown herein, but be in accordance with the broadest scope consistent with the principles and novel features described herein.

Claims (7)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and therefore the property described in the following claims is claimed as property. 1. In a wireless communication system in which a mobile station moves from the coverage area of a source system to the coverage area of a destination system and where a first attempt to acquire said destination system failed, a method for preventing loss of communication with said mobile station comprising the steps of: transmitting from said mobile station to said source system a set of parameter data; receiving said parameter data in said source system; generating in said source system a search list according to said parameter data; and attempting by said mobile station to acquire said destination system in accordance with said second search list. The method according to claim 1, characterized in that it further comprises the step of measuring said parameter data in said mobile station. The method according to claim 2, characterized in that said step of measuring said parameter data comprises measuring the signal energy of the pilot signals of said destination system. 4. The method according to claim 2, characterized in that said step of measuring said parameter data is performed in accordance with a set of predetermined search parameters. The method according to claim 4, characterized in that said set of search parameters is transmitted to said mobile station by said origin system. The method according to claim 1, characterized in that it further comprises transmitting said search list from said origin system to said mobile station. 7. A wireless communication system wherein a mobile station moves from the coverage area of a source system to the coverage area of a destination system, a method for providing a transfer from said origin system to said system of origin. destination, comprising the steps of: transmitting from said source system to said mobile station a set of search parameter data; determining in said mobile station the availability of said destination system according to said set of parameters; transmitting from said mobile station to said origin system a message indicative of the availability of said destination system; generating in said origin system a set of acquisition parameters according to said message from said mobile station; attempting in said mobile station to acquire said destination system in accordance with said set of acquisition parameters.