MXPA99007003A - Method and apparatus for performing soft hand-off in a wireless communication system - Google Patents

Abstract

A method and apparatus for providing soft hand-off in a mobile communication system. In current systems, members of an active set of base stations (4, 4A, 4B, 4C) are determined by comparing measured pilot energy with fixed thresholds. The value of providing a redundant communication link to a mobile station (2) primarily depends on the energy of other signals being provided to the mobile station (2). In the present invention, the signal strength of each signal transmitted by other base stations (4, 4A, 4B, 4C) in communication with a mobile station (2) is considered when determining whether to add a base station to the set of base stations (4, 4A, 4B, 4C) in communication with the remote station. A base station is added only if the signal received from that base station provides sufficient added value to justify the impact on system capacity.

Description

METHOD AND APPARATUS FOR CARRYING OUT A TRANSFER OF SOFT TRANSMISSION IN A WIRELESS COMMUNICATIONS SYSTEM I. FIELD OF THE INVENTION The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and system for effecting a transfer in a wireless communication system.

II. DESCRIPTION OF THE RELATED TECHNIQUE The use of access modulation techniques multiple per division is code (CDMA - code multiple division access) is one of several techniques to facilitate communications where a large number of users are present. Although other techniques are known, for example multiple access by division of time (TDMA-time multiple access division), multiple access by frequency division (FDMA-frequency multiple division access) and AM modulation schemes as single sideband of compacted amplitude (ACSSB- amplitude co panded single sideband), the CDMA has important advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Patent No. 4,901,307, entitled "SYSTEM P1453 / 99MX OF COMMUNICATION OF MULTIPLE ACCESS OF SCALED SPECTRUM USING TERRESTRIAL OR SATELLITE REPEATER "and U.S. Patent No. 5,103,459, entitled" SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS "-" SYSTEM AND METHOD FOR GENERATE FORMS OF SIGNAL WAVE IN A CDMA CELLULAR TELEPHONE SYSTEM ", both patents assigned to the assignee of the present invention and incorporated as reference The method to provide CDMA mobile communications was regulated by the TELECOMMUNICATIONS INDUSTRY ASSOCIATION in TIA / EIA / IS- 95-A entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System." In the aforementioned patents, a multiple access technique is exposed in which a large number of mobile phone users, each having a transceiver, communicate through satellite or ground-based relay stations (known also as cellular base stations or cellular sites), using code division multiple access (CDMA) stepped spectrum communication signals. When using CDMA communications, the frequency spectrum can be reused several times, thus allowing an increase in the capacity of the system user. The use of CDMA techniques results in much greater spectrum efficiency than can be achieved using other access techniques P1453 / 99MX multiple. A method for simultaneously demodulating data that has traveled through various propagation paths from a base station and for simultaneously demodulating data redundantly provided from more than one base station is disclosed in U.S. Patent No. 5,109,390 (the patent 390), entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR COMMUNICATION SYSTEM" - "RECEIVER OF DIVERSITY IN A CDMA CELLULAR COMMUNICATION SYSTEM", assigned to the assignee of the present invention and which is incorporated herein by reference. In the patent? 390, the demodulated signals are combined separately to provide an estimate of the transmitted data that has higher reliability than the data demodulated by any other path or from any base station. Transfers can generally be divided into two categories: strong transfers and soft transfers. In a strong transfer, when a mobile station leaves a source cell and enters a destination cell, the mobile station interrupts its communication link with the originating cell and establishes a new communication link with the target cell. In the soft transfer, the mobile station completes a communication link with the target cell before interrupting its communication link with the cell P1453 / 99MX origin. In this way, in the soft transfer, the mobile station is in redundant communication with both the source cell and the target cell for some period of time. It is far less likely that the call will be lost in soft transfers than in strong transfers. In addition, when a mobile station travels near a cell border, the mobile station can make repeated transfer requests in response to small changes in the environment. This problem, known as pin-pon effect is greatly diminished by the smooth transfer. The process for carrying out the soft transfer is described in detail in U.S. Patent No. 5,101,501 entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT TRANSFER IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM" - "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM ", assigned to the assignee of the present invention and incorporated herein as a reference. An improved soft transfer technique is disclosed in U.S. Patent No. 5,267,261 entitled "MOBILE ASSISTED SOFT TRANSFER IN A CDMA COMMUNICATIONS CELLULAR SYSTEM", which is assigned to the assignee of the present invention which is incorporated herein by reference. reference. In the x261 patent system, the process of P1453 / 99MX _ _ soft transfer is improved by measuring the power of "pilot" signals transmitted by each base station within the system in the mobile station. These pilot power measurements are of assistance in the smooth transfer process, facilitating the identification of viable base station transfer candidates. Viable base station candidates can be divided into four groups: The first group, known as the Active Group, comprises base stations that are currently in communication with the mobile station. The second group, known as the Candidate Group, comprises base stations that have been determined to be of sufficient power to be in use in the mobile station. Base stations are added to the candidate group when their measured pilot power exceeds a predetermined threshold DD. The third group is the group of base stations that are in the vicinity of the mobile station (and which are not included in the Active Group or in the Candidate Group). And the fourth group is the Remnant Group that consists of all the other base stations. In an IS-95-A communication system, the mobile station sends a Pilot Power Measurement Message when it finds a pilot signal of sufficient power that is not associated with any of the Send Traffic Channels that are currently being demodulated or when the power of a pilot signal P1453 / 99MX number of base stations that transmit redundant data to a mobile station user that provides sufficient transmission quality.

SUMMARY OF THE INVENTION The present invention consists of a novel and improved method and apparatus for providing smooth transfer in a mobile communication system. It should be noted in the beginning, that one of the biggest problems with current systems is that the members of the active group are determined in accordance with comparisons of measured pilot power with fixed thresholds. However, the value of providing a redundant communication link to a mobile station depends to a large extent on the power of other signals that are provided to the mobile station. For example, the value of redundantly transmitting to a mobile station, a signal with received energy of -15 dB will not be of much value if the mobile station is already receiving a transmission with signal power of -5 dB. However, redundantly transmitting to a mobile station, a received power signal of -15 dB can be of a substantial value, if the mobile station is receiving transmissions with signal power of only -13 dB. In a first embodiment of the present invention, the mobile station under the conditions P1453 / 99MX that is associated with one of the Send Traffic Channels that is demodulated falls below a threshold for a predetermined period of time. The mobile station sends a Pilot Power Measurement Message after the detection of a change in the power of a pilot signal under the following three conditions: 1. The power of a Neighbor Group or Remnant Group pilot signal is above the TADD threshold. 2. The power of a Candidate Group pilot signal exceeds the power of an Active Group pilot signal by more than one threshold (TCOMP) • 3. The power of a pilot signal in the Group Active Candidate Group has fallen below a threshold (TDROp) for more than a predetermined period of time. The Pilot Power Measurement Message identifies the base station and the pilot energy measured in decibels. A negative aspect of soft transfer is that because it involves transmitting information in a redundant manner, it consumes the available source of communication. However, the smooth transfer can provide a great improvement in the quality of communication. Therefore, there is a need in the art for a method to decrease the minimum P1453 / 99MX analyzed in the above, transmits a Pilot Power Measurement Message, which identifies each base station in the active and candidate groups and the corresponding measured pilot power. The Pilot Power Measurement Message is received by the base stations in communication with the mobile station. The base stations provide this information to a central control center, known as the base station controller. In the base station controller, the active group is determined in accordance with the combined power of other pilot signals in the active group. The base station controller classifies the pilot signals of the Pilot Power Measuring Message in accordance with its measured power in the mobile station. In this way, after classification, the list of base stations consists of i, P2. -. N where Pi is the strongest pilot signal and PN is the weakest. An iterative process is then undertaken to determine which of the pilot signals Pi, P2 ... PN must be part of the revised active group. Initially, the revised active group comprises only the strongest pilot signals Pi and P2 - When determining either a Pi pilot signal, it must become part of the active group, a COMBINED_PILOT value is calculated. The COMBINED_PILOT value consists of the sum of the energies of P1453 / 99MX the pilot signals currently in the revised active group (P?, P2 / • - -Pi-i) • A threshold in accordance with the * COMBINED_PILOT value is then general. In the exemplary embodiment, the threshold is generated by developing a linear operation on the COMBINED_PILOT value. If the pilot energy value, P? exceeds the threshold, is added to the revised active group and the process is repeated for the next pilot P? +? . If the pilot energy value, Pi, does not exceed the threshold, the revised active group comprises Pi, P2, ... Pi_ ?. The revised active list is transmitted to the mobile station and the base station controller then establishes communications with the mobile station in accordance with the revised active group. In an alternative embodiment, the revised active group is generated in the mobile station. The mobile station continuously measures the pilot powers received from the base stations. When determining whether or not to send a message indicating that a pilot signal from the candidate group must be moved to the active group, the measured pilot energy of a pilot signal in the candidate group is compared against a threshold generated in accordance with the COMBINED_PILOT value as described above. If the strongest pilot signal in the candidate group satisfies the rule, then a message containing all pilot signals from active group and candidate P1453 / 99MX will be sent. Following the iterative process carried out on the members of the candidate group, a second iterative process is carried out to determine if a pilot signal should be removed from the revised active group. In this operation, the pilot signals are tested from the weakest member of the active group reviewed to the most powerful. An energy value C0MBINED_PIL0T is recorded, which is the sum of the energies of all the pilot signals belonging to the active group. A threshold value is generated in accordance with the COMBINED_PILOT value as described above, and the pilot signal being tested is compared to the threshold. If a pilot signal has been below the threshold value for a predetermined period of time, a message will be sent to the base station, which indicates that the pilot signal must be lost. The revised active list is transmitted to the base station controller through the base stations with which the mobile station is in communication. The base station establishes the communication links with the base stations in the generated revised active mobile list and transmits an acknowledgment to the mobile station when the links are established. The mobile station then conducts communications through the base stations of the revised active group. P1453 / 99MX In the preferred embodiment, the mobile station reviews the pilot signals and, in response to the revised pilot signals, the mobile station compiles the members of the candidate group. Moreover, the mobile station determines whether a change in the current active group is desirable in view of the criteria set forth above. Upon detecting any change in the desired membership of the active group, the mobile station generates a pilot power measurement message which, as written above, includes the identities of all the pilot signals in the active and candidate groups corresponding to energy values measured and a corresponding indication of whether the pilot signal should remain in the groups or should be derived to the neighboring group (indicated by establishing the KEEP variable described above). In the exemplary embodiment, the base station determines the members of the revised active group in accordance with the method ^ described with respect to Figure 5.

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, taken in conjunction with the drawings, through which, the reference numbers are used consistently and where: Figure 1 is an illustration of a network of P1453 / 99MX cellular communication; Figure 2 is an illustration of the cellular communication network of Figure 1 including the base station controller; Figure 3 is a block diagram of the mobile station of the present invention; Figure 4 is a block diagram of the base station of the present invention; Figure 5 is a flow chart of the method for generating the revised active group in the base station controller; Figure 6 is a flow chart of the method for generating the revised active group in the mobile station; Figure 7 is a flow chart of the preferred method for generating the candidate group in the mobile station; and Figure 8 is a flow diagram illustrating the preferred method of the present invention, wherein a change in the preferred members of the active group is detected and a pilot power measurement message is transmitted to the base station, in response to the change detected.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Figure 1 illustrates a wireless communication network in which the geographical area has been divided into known coverage areas such as cells P1453 / 99MX and that are illustrated by means of a group of adjacent hexagons. Each cell is served by a corresponding base station 4. Each base station transmits a pilot signal that uniquely identifies that base station. In the exemplary mode, base stations 4 with CDMA base stations. A detailed description of soft handoff in a CDMA wireless communication system is described in detail in the aforementioned US Patents No. 5,101,501 and 5,267,261. The mobile station 2 is located within the cell served by the base station 4A. Since the mobile station 2 is located near the cell border, it will likely be in a soft transfer condition, in which it is simultaneously in communication with more than one base station. The mobile station 2 can, for example, be in communication with the base stations 4A and 4B. In this way, it is said that the base stations 4A and 4B make up the active group. Moreover, it may be that the mobile station 2 has determined other base stations in its vicinity to have a pilot power measured above a predetermined TADD threshold, but that those base stations are not currently in communication with the mobile station. It is said that these pilot signals make up the candidate group. The candidate group could be made up of 4C and 4G base stations. P1453 / 99MX Referring to Figure 2, a typical communication network is illustrated. The data driven mobile station 2 is provided from a public switched telephone network or from another wireless system (not shown) to the base station controller 6. The base station controller 6 provides the data to the base stations in the active list of the mobile station 2. In the example, the base station controller 6 redundantly provides data to the base stations 4A and 4B and receives data from from the same. The present invention also applies in conditions where each cell is divided into sectors. Communications to and from each sector can be received and demodulated separately by the mobile station 2. For simplicity, the analysis will be described wherein only base stations are located on each base of the base station 4. However, it will be readily appreciated by one skilled in the art, that the present invention can equally be applied to sector-shaped cells, simply by considering the possibility that base stations can be placed and transmitted to separate sectors within a cell. The condition in which a mobile station is in simultaneous communication with more than one sector of a cell, is known as a smoother transfer. The method and apparatus to effect P1453 / 99MX smoother transfer are described in detail in copending United States Patent Application No. 08 / 144,903, entitled "METHOD AND APPARATUS FOR TRANSFER BETWEEN SECTORS OF A COMMON BASE STATION" - "METHOD AND APPARATUS FOR PERFORMING HANDOFF" BETWEEN SECTORS OF A COMMON BASE STATION ", filed on October 30, 1993, which is assigned to the assignee of the present invention and incorporated herein by reference. Within mobile station 22, each copy of the data packet is received, demodulated and decoded separately. The decoded data is then combined to give an estimate of the most reliable data of any of the states of data odulada. Figure 3 illustrates the mobile station 2 of the present invention. The mobile station 2 measures, continuously or at intermittent intervals, the power of the pilot signals of base stations 4. The signals received by the antenna 50 of the mobile station 2 are provided through the duplexer 52 to the receiver 54 (RCVR) which amplifies , subconverts and filters the received signal and provides it to the pilot demodulator 58 of the search subsystem 55. In addition, the received signal is provided to the 64A-64N traffic demodulators. The 64A-64N traffic demodulators, or a subset of P1453 / 99MX thereof, separately demodulate signals received by the mobile station 2. The signals demodulated from the traffic demodulators 64A-64N are provided to the combiner 66 which combines the demodulated data which, in turn, provides an improved estimate of the data Dear. The mobile station 2 measures the power of the pilot channels. The control processor 62 provides acquisition parameters to the search processor 56. In the exemplary embodiment of a CDMA communication system, the control processor 62 provides a PN offset to the search processor 56. The search processor 56 generates a sequence PN that is used by the pilot demodulator 58 to demodulate the received signal. The demodulated pilot signal is provided to the energy accumulator 60 which measures the energy of the demodulated pilot signal, accumulating energy for predetermined periods of time. The measured pilot energy values are provided to the control processor 62. In the exemplary embodiment, the control processor 62 compares the energy values in thresholds TADD and TDROP-TADD is the threshold above which the received signal is of sufficient power to efficiently provide communications with the mobile station 2. TDROP is a threshold value below which the received signal energy is insufficient for P1453 / 99MX effectively provide communications with mobile station 2. Mobile station 2 transmits a Pilot Power Measurement Message that includes all pilot signals with power greater than TADD Y to all members of the current active group whose measured pilot power has not fallen below TDROP for more than a predetermined period of time. In the exemplary embodiment, the mobile station 2 generates and transmits a Pilot Power Measurement Message after detection of a change in the power of a pilot signal under the following three conditions: 1. The power of a Neighbor Group pilot signal or Remaining Group is above the TADD threshold. 2. The power of a Candidate Group pilot signal exceeds the power of an active Group pilot signal by more than one threshold (TCO P) • 3. The power of a pilot signal in the Candidate Group Active Group has fallen below a threshold (TDROp) for more than a predetermined period of time. In the exemplary embodiment, the Pilot Power Measurement Message identifies the pilot signal and provides corresponding measured pilot power. In the exemplary modality, the P1453 / 99MX base stations in the Pilot Power Measurement Message are identified by their pilot deviations and their corresponding measured pilot power is provided in units of decibels. The control processor 62 provides the identities of the pilot signals and their corresponding measured pilot energies to the message generator 70. The message generator 70 generates a Pilot Power Measurement Message containing the information. The Pilot Power Measurement Message is provided to the transmitter (TMTR) 68, which encodes, modulates, overconverts and amplifies the message. The message is then transmitted through the duplexer 52 and the antenna 50. Referring to Figure 4, the Pilot Power Measurement Message is received by the antenna 30 of the base station 4 and is provided to the receiver (RCVR) 28 , which amplifies, subverts, demodulates and decodes the received signal and provides the message to the base station controller interface (BSC - base station controller). The base station controller interface (BSC) 26 sends the message to the base station controller (BSC) 6. The message is provided to the selector 22, which may also receive the message redundantly from other base stations that are in communication with the mobile station 2. The selector 22 combines the message estimates received from the base stations in P1453 / 99MX communication with mobile station 2 to provide improved packet estimates. The selector 22 provides the power measurement message to the transfer control processor 20. In the first exemplary embodiment, the transfer control processor 20 selects the base stations that will communicate with the mobile station 2, i.e., the members of the revised active group, in accordance with the method provided in Figure 5. In the block 100, the transfer control processor 20 classifies the pilot signals in the Pilot Power Measurement Message in accordance with its powers. Thus, for example, P would be the most powerful pilot received signal, P2 would be the second strongest pilot and so on. In block 102, the revised active group (ACTIVE_SET) is set to include Pi and P2. In block 104, the variable C0MBINED_PIL0T is set for the sum of the energies of Pi and P. In block 106 the circuit or variable cycle i is set to 3. In block 108, the energy of the pilot signal of the received i-th most powerful signal (? ±) is compared to a threshold value to determine whether it should be added to the revised active group. In the exemplary embodiment, the threshold (T) is determined in accordance with equation (1) below: T = SOFT_SLOPE * COMBINED_PILOT + SOFT_INTERCEPT (1) In the exemplary embodiment, S0FT_SL0PE is set to P1453 / 99MX 2.25 and SOFT_INTERCEPT is set to 3.0. The values of SOFT_SLOPE and SOFT_INTERCEPT can be parameters that are sent by air to the mobile station or the selected values could be programmed within the mobile station. The values of SOFT-SLOPE and SOFT_INTERCEPT can be determined in accordance with factors such as the amount of soft transfer that is acceptable to a network administrator and for empirical studies on the quality of transmission links. If the energy value Pi is less than the threshold value, then the flow continues to block 110 and the revised active group includes the signals corresponding to the pilot signals. { P? ... Pi_? > . If the energy value Pi is greater than the threshold value in block 108, then the flow continues to block 112. In block 112, a new value COMBINED_PILOT is recorded by adding the value of the energy of the ith signal plus Powerful in the pilot power measurement message (Pi) with the current value of COMBINED_PILOT. Since in the exemplary mode, the energy of the pilot signals are provided in decibels, the energies must be converted to linear representations before being added and returned to decibel form. In block 114, i is added to the revised active group. In block 116, the loop variable (i) is increased. In block 118, the transfer control processor 20 checks to determine if they have been P1453 / 99MX tested all base stations in the pilot power measurement message. If there are no remaining pilots to be tested, then the flow proceeds to block 120 and the active group comprises all base stations in the pilot power measurement message. If, in block 118, there are base stations in the pilot power measurement message that remain to be tested, the flow returns to block 108 and proceeds as described above. After generating the revised active group, the base station controller 6 determines whether the base stations in the revised active list can adjust the communications with the mobile station 2. If any of the base stations in the reviewed active group can not adjust the communications with mobile station 2, the base stations are removed from the revised active group. After generating the revised active group, the transfer control processor 20 provides the information to the selector 22 indicating the members in the revised active group. In response to the revised active group provided by the transfer control processor 20, the selector 22 assigns traffic channels to perform communications to the mobile station using the base stations in the reviewed active group. The transfer control processor 20 provides a message indicating the revised active group to the message generator 24. The generator 24 Message P1453 / 99MX generates a message for transmission to the mobile station 2, known as the transfer address message. The transfer address message indicates the base stations in the reviewed active group and the channels corresponding to those base stations that they will use to communicate with the mobile station 2. The message is provided through the selector 22 and is provided to the base stations that they were in communication with mobile station 2 before the generation of the revised active group. The base stations in communication with the mobile station 2 transmit the transfer address message to the mobile station 2. Referring again to Figure 3, the transfer address message is received by the antenna 50 of the mobile station 2. The message is provided to the receiver 54, which amplifies, sub-conveys, demodulates and decodes the message and provides it to the control processor 62. The control processor 62 then configures the traffic channel demodulators 64A-64N, in accordance with the active group. reviewed in the transfer address message. In an alternative embodiment of the present invention, the revised active group is generated in mobile station 2. This alternative mode provides more timely generation of the active group P1453 / 99MX revised. Since the Pilot Power Measurement Message is transmitted only under the three conditions described above, updating the active group may be undesirably delayed. However, the alternative mode results in the transmission of the pilot power measurement message, in a more timely manner. In the alternative mode, the mobile station 2 measures the received pilot power as described above. The pilot energy values are provided to the control process 62. In response, the control processor 62 generates a revised active group. If the revised active group differs from the current active group, the mobile station 2 transmits a message indicating the members of the revised active group to the base station controller 6 through the base stations 4. The base station controller 6 establishes the communications with the mobile station 2. The mobile station 2 reconfigures the traffic channel demodulators 64A-64N to demodulate the received signals, in accordance with the generated mobile reviewed active group. In the exemplary embodiment, the control processor 62 in the mobile station 62 generally revises the active group in accordance with the method shown in Figure 6. In block 200, pilot signals with energy measured in excess of threshold T? DD are add to the list of candidates and signs P1453 / 99MX pilot whose measured energy has fallen below TDROP for more than a predetermined period of time are removed from the candidate list. In the exemplary embodiment, the time that a pilot signal is below TDROP is tracked by a timer within the control processor 62, known herein as the timer TDROP- In block 202, the pilot signals in the candidate list are They rank from most powerful to weakest. In this way, PC? it is more powerful than Pc2, and so on. In block 204, the variable C0MBINED_PIL0T is set equal to the energy of all pilot signals in the active group. Also, in block 204, the variable (i) of the cycle is initialized to the value 1. In block 206, the member Pci of the candidate group is tested to determine whether it should be part of the revised active group. PCi is compared to a threshold generated in accordance with the current value of C0MBINED_PIL0T. In the emplificative mode, the threshold (T) is generated in accordance with equation (1) above. If the PCi pilot power exceeds the threshold T, then the flow moves to block 208. In block 208, the pilot signal PCi is added to the revised active group. In block 210, a new value of COMBINED_PILOT is registered which is equal to the old value of C0MBINED_PIL0T plus pilot power PCi. In block 212, the variable (i) of the cycle is increased.

P1453 / 99MX In block 213 it is determined if all pilot signals in the candidate group have been tested. If all the pilot signals in the candidate group have not been tested, then the flow moves to block 200 and proceeds as described above. If all the pilot signals in the candidate group have been tested or if, returning to block 206, the PCi pilot energy did not exceed the threshold T, then the flow moves to block 214. In block 214, the active group reviewed It is classified from lower energy to higher energy. In this way, PAi has the minimum energy measured in the active group reviewed, PA2 has the second lowest and so on, until the last member of the active group reviewed PAN. In block 216, it is determined if PA? is a member of the candidate group. If PAi is a member of the candidate group then the flow moves to block 34 and the revision of the active group is completed. In block 218, the loop variable i is set to 1. In block 220, COMBINED PILOT to test PAi is recorded. The value of C0MBINED_PIL0T is set equal to the sum of the measured energy of all pilot signals that have higher energy than the pilot signal that is currently being tested. Thus, C0MBINED_PIL0T is determined by the equation: N C0MBINED_PIL0T =? PAj (2) In block 222, the current pilot signal that P1453 / 99MX is being tested, it is compared against a determined threshold (T), in accordance with the registered value of C0MBINED_PIL0T. In the exemplary embodiment, the threshold T is determined in accordance with equation (1) above. If the pilot energy PAi measured exceeds the threshold T, then the flow moves to block 224 and the derivation timers for pilots PAi to PAN are reset to zero and the determination of the revised active group ends in block 234. If the energy PA pilot? When the measurement does not exceed the threshold T, then the flow moves to block 226. In block 226, it is determined whether the timer TDROP for PAi has expired. If the timer TDROp has expired, then in block 228, the pilot signal PA? it is removed from the revised active group and placed in the candidate group and the flow continues to block 230. If in block 226 it is determined that timer TDROp for PAi has not expired, then the flow proceeds directly to block 230. In block 230, the variable (i) of the cycle is increased. Then, in block 232, it is determined whether all the pilot signals in the revised active group PAi have been tested. If all the pilot signals in the revised active group have been tested, then the flow continues to block 234 and the generation of the revised active group is completed. If all the pilot signals in the revised active group have not been tested, then the flow proceeds to the block P1453 / 99MX 220 and proceeds as described above. Referring now to Figures 7 and 8, a preferred method for implementing the present invention is illustrated. In the preferred embodiment, the mobile station reviews the pilot signals and in response to the revised pilot signals, the mobile station compiles members of the candidate group. Moreover, the mobile station determines whether a change in the current active group is desirable, in view of the criteria set forth above. Upon detecting any change in the desired membership of the active group, the mobile station generates a pilot power measurement message, which, as described above, includes the identities of all pilot signals in the candidate and active groups corresponding to measured energy values and a corresponding indication of whether the pilot signal should remain in the groups or should be derived to the neighboring group (indicated by the establishment of the KEEP variable described above). In the exemplary embodiment, the base station determines the members of the revised active group, in accordance with the method described with respect to Figure 5. The preferred embodiment provides timely modification to the members of the active group and provides determination of the members of the active group. revised at the base station, which reduces the records at the mobile station and allows the selection process to include restrictions on the capacity of P1453 / 99MX base stations. The capacity restrictions of the base stations can be taken into account by the base station controller by simply removing or weighting the pilot signals that are transmitted by the base stations under high load capacity conditions. Figure 7 is a flow diagram illustrating the method for updating the candidate group, which in the exemplary embodiment is performed within the mobile station. In block 300, the variable (i) of the cycle is initialized to the value 1. In block 302, the pilot signals of the neighbor group (PN) are sorted so that PNi > P2 > PN3 And so on. In block 306, the neighboring group pilot signal that is currently tested (PNi) is compared to the TADD threshold • If the pilot signal energy (PNI) does not exceed the threshold, then, in block 310, the flow proceeds directly to block 312. If the pilot signal energy (PNI) exceeds the threshold, then, in block 310, the pilot signal is added to the candidate group and the flow proceeds to block 308. If the pilot signal power (PNi) does not exceed the threshold, then in block 310, the flow proceeds directly to block 312. In block 308, the index number of the neighboring group pilot signal increases. Then, in block 304, it is determined whether all the members of the neighboring group have been tested. If all the members P1453 / 99MX of the neighboring group have not been tested, then the flow is moved to block 306 and proceeds as described above. If all the members of the neighbor group have been tested, then the flow moves to block 312. In block 312, the index variable (i) is reset to 1. Then, in block 314, the pilot signals in the group candidate (Pc) are classified from the weakest to the most powerful, so that Pc? < Pc2 < Pc3 and so on. In block 318, the energy of the candidate list being tested (PCI) is compared to the derivation threshold TDROp. If the energy is below the derivation threshold, then the flow proceeds to block 324. If the energy is above the derivation threshold, then the flow proceeds to block 320. Since the list of pilots is classified, all the remaining members to be tested are necessarily greater than TDROP- ASÍ, in block 320, the TDROP timers for PCi and all the more powerful pilot signals that (PCÍ) are restarted and the updating of the candidate group is completed. As described in the above, the TDROP timer is a timer that keeps track of the time that a pilot signal has been below the derivation threshold. The purpose of the TDROp timer is to avoid the erroneous derivation of a powerful pilot signal that may have an energy P1453 / 99MX weakly measured due to a short-term change in the propagation environment, such as rapid fading. In block 324, timer TDROp is started if the timer for PCi is no longer running, or is advanced if the timer is running. In block 326, a test is made to determine if the TDROP timer for the pilot signal (PCi) has expired. If the timer has expired, then the flow moves to block 328 and the pilot signal (Pei) is removed from the candidate group. Then the flow moves to block 322. Also, if the timer has not expired in block 326, the flow moves directly to block 322. In block 322, the candidate group index variable (i) is increased. . Then, in block 316, it is determined whether all the pilot signals in the candidate group have been tested. If all the members of the candidate group have been tested, the candidate group's update is complete. If less than all the members of the candidate group have been tested, the flow moves to block 314 and proceeds as described above. In the preferred embodiment, the selection of the members of the candidate group takes place in the mobile station. This is because the selection of the candidate group usually does not require knowledge of capacity restrictions of the base stations in the P1453 / 99MX network. However, in an alternative embodiment, the method for deriving the members of the candidate group to the neighboring member may be carried out in the controller of the base station. Moreover, the addition of the members to the candidate group could be done in the base station controller unless the base station controller has or is provided with the knowledge of the members of the neighboring group of the mobile station. Figure 8 illustrates the method for detecting the need to review the active group which, in the preferred embodiment, is carried out in the mobile station. In block 400, the most powerful pilot in the candidate group is selected (P'ci) • Note that the premium is for differential PC pilot signal? which is mentioned in Figure 7, which represented the weakest pilot of the candidate group. In block 402, the energy of (Pei) is compared to a threshold (T) that is based on the cumulative energy of the pilot signals in the active group, as shown in Equation 3 below: T = f (? PAi ) = SOFT_SLOPE *? PAi + SOFT_ADD_INTERCEPT (3) If (P'ci) exceeds the threshold (T), then the mobile station transmits the pilot power measurement message to the base station, in block 404. If (P'ci) n ° exceeds the threshold (T), then the flow proceeds to block 406. In block 406, the active group is classified as the pilot signal P1453 / 99MX weaker to the more powerful. In block 408, the index variable (i) of the active group is set to 1. Next, in block 410, the pilot signal of the active group (PA?), Which is being tested to determine whether it should remain in the group active, it is tested against a threshold (T) generated in accordance with a sum of energies of all the most powerful pilot signals as shown in equation (4) below: T = f (PAJ) = SOFT_SLOPE * ^ P ^. + SOFT_DROP_INTERCEPT / > / J > i (4) If the pilot signal being tested (PAi) exceeds the threshold (T), then this pilot and all pilot signals of power greater than that of the pilot signal being tested must remain in the active group . In this way, in block 412 the timers TD OP for all pilot signals with power greater than A? they are reinitialized and the current search for a revision of the active group is completed without the need for revision detected by the mobile station. In the preferred embodiment, the intercept value (SOFT_ADD_INTERCEPT) used to generate the addition threshold is allowed to be of a value different from the intercept value SOFT_DROP_INTERCEPT used for the derivation threshold. This provides greater flexibility and allows the network to introduce additional hysteresis at the signal levels. If the pilot signal (PAÍ) is smaller than the P1453 / 99MX threshold (T), then the flow proceeds to block 422. In block 422, timer TDROp for the pilot signal (PAi) is started if it is not running and is advanced if it is already running. In block 424, if the timer TDROP for the pilot (PAi) has expired, it is tested. If the timer TDR0P has expired, then the mobile station transmits a pilot power measurement message to the base station in block 430. If the TDROP timer has not expired, then the flow moves to block 426 where the pilot index (i) the active group is advanced. Then, the flow moves to block 420, where it is determined whether all members of the active group have been tested. If all the members of the active group have been tested, then the search ceases without the need to review the detected active group. If less than all the members of the active group have been tested, the flow moves to block 410 and proceeds as previously described. The above description of the preferred embodiments is provided to allow anyone to experience the art, make use of the present invention. The various modifications to these modalities will be readily apparent to those experienced in the field, and the generic principles defined herein can be applied to other modalities without the use of the inventive faculty. In this way, the present invention is not intended for P1453 / 99MX is limited to the modalities shown therein, but to be in accordance with the broadest scope consistent with the principles and novel features set forth herein.

P1453 / 99MX

Claims (10)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS-1 is claimed as property.
2. A method for selecting base stations to communicate with a remote station, comprising: a threshold value in accordance with a combination of the base stations, capable of communicating with the remote station; compare a threshold; and select the first base station when the threshold.
3. The method according to claim 1, wherein the signal energy of the first base station is the energy of a first base station pilot signal measured at the remote station. wherein the combination of signal energy from base stations capable of communicating with the remote station comprises the sum of pilot energy values of the pilot signal with higher received power than the first base station.
4. The method according to claim 1, wherein the step of computing a threshold value comprises performing a linear operation by combining the signal P1453 / 99MX of the base stations, able to communicate with the remote station.
5. The method according to claim 3, wherein the step of computing a threshold value comprises performing a linear operation by combining the signal from the base stations, capable of communicating with the remote station.
6. The method according to claim 4, wherein the linear operation comprises: multiplying the combination of the signal coming from the base stations capable of communicating with the remote station by means of a first variable; and add a second variable with the product of multiplication.
7. The method according to claim 6, wherein the first variable has a value of 2.25.
8. The method according to claim 6, wherein the variable has a value of 3.0.
9. The method according to claim 1, further comprising measuring at the remote station, the power of pilot signals transmitted by a predetermined group of base stations to provide the signals from base stations capable of communicating with the mobile station.
10. 10. The method according to claim 9, which P1453 / 99MX further comprises the step of transmitting a message indicative of the measured pilot signals coming from the remote station. The method according to claim 1, further comprising the step of removing the first base station from a group of base stations in communication with the remote station, when the signal energy of the first base station is below the threshold. P1453 / 99MX