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

ES 2 245 018 T3

DESCRIPTION

Procedure and apparatus to carry out transfers with continuity in a wireless communication system.

Background of the invention

I. Field of the invention

The present invention relates to communication systems. More particularly, the present invention relates to a new and improved method and system for effecting handoffs in a wireless communication system.

II. Description of Related Art

The use of code division multiple access (CDMA) modulation techniques is only one of several existing techniques that provide communications in which a large number of users of the system are present. Although other techniques are known, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation systems, such as amplitude compacted single sideband modulation system. (ACSSB), CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in US Patent No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", and US Patent No. 5,103. 459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which have been assigned to the assignee of the present invention and incorporated herein by reference. The procedure for providing CDMA mobile communications was standardized by the Telecommunications Industry Association in the TIA / EIA / IS-95-A standard, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”.

In the above-mentioned patents, a multiple access technique is disclosed in which a large number of mobile phone users, each of whom has a transceiver, communicate via satellite repeaters or terrestrial base stations ( also known as cellular base stations or cell sites) using code division multiple access (CDMA) spread spectrum communication signals. When using CDMA communications, the frequency spectrum can be reused multiple times, thereby increasing the user capacity of the system. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved by other multiple access techniques.

In US Patent No. 5,109,390 (the '390 patent), entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR COMMUNICATION SYSTEM", assigned to the assignee of the present invention and incorporated herein by reference there is disclosed a process for demodulating simultaneously data that has been transmitted on different propagation paths from one base station, and to simultaneously demodulate data provided redundantly from more than one base station. In US Patent No. 5,109,390, the separately demodulated signals are combined to provide an estimate of transmitted data that has higher reliability than data demodulated by any other path or from any other base station.

Transfers can be broadly divided into two categories: non-continuous transfers and continuous transfers. In a seamless handoff, when a mobile station leaves a source cell and enters a destination cell, the mobile station breaks its communication link with the source cell and then establishes a new communication link with the cell. of destiny. In soft handoff, the mobile station terminates a communication link with the destination cell before breaking its communication link with the originating cell. Therefore, in soft handoff, the mobile station is in redundant communication with both the source cell and the destination cell for a certain period of time.

In seamless transfers, call drop is much less likely than in seamless transfers. Furthermore, when a mobile station approaches the edge of a cell, it can make repeated handover requests in response to small changes in the environment. This problem, called the pingpong effect, is also greatly alleviated in seamless handoff. The procedure to carry out a continuous transfer is described in detail in US Patent No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee hereof. invention and incorporated herein by reference.

An improved continuity transfer technique is disclosed in US Patent No. 5,267,261, entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM", assigned to the assignee of the present invention and incorporated herein by reference title. In the system of the '261 patent, the soft handoff process is enhanced by measuring, at the mobile station, the intensity of the pilot signals transmitted by each base station in the system. These pilot signal strength measurements are useful in the soft handoff procedure, because they allow identification of viable candidate base stations for handoff.

ES 2 245 018 T3

Viable candidate base stations can be divided into four groups: the first group, called the active group, comprising the base stations that are currently communicating with the mobile station, the second group, called the candidate group, comprising the stations that have an intensity sufficient to be useful to the mobile station (base stations are added to the candidate group when the measured pilot energy exceeds a threshold T_ADD default), the third group, called the neighbor group, which is the group of base stations located in the vicinity of the mobile station (and which are not included neither the active group nor the candidate group), and the fourth group, called the group remaining, consisting of the rest of the base stations.

In an IS-95-A communication system, the mobile station sends a pilot intensity measurement message when it encounters a pilot signal of sufficient intensity that is not associated with any of the forward traffic channels that is currently being demodulated, or when the intensity of the pilot signal that is associated with one of the forward traffic channels being demodulated falls below a threshold for a predetermined period of time. The mobile station sends a pilot intensity measurement message upon detection of a change in intensity of a pilot signal, under the following three conditions:

1. it is determined that the intensity of a pilot signal of the neighboring group or of the remaining group is above the threshold TADD,

two. the intensity of a pilot signal of the candidate group exceeds the intensity of a pilot signal of the active group by a value that exceeds a threshold (T_COMp),

3. the intensity of a pilot signal from the active group or the candidate group has fallen below a threshold (T_DROP) for longer than a predetermined period of time.

The pilot intensity measurement message identifies the base station and the pilot energy measured in decibels.

A negative issue with soft handoff is that, because it includes redundant transmission of information, it consumes the available communication resource. However, seamless handover can greatly improve the quality of communication. Accordingly, there is a need in the art for a method of minimizing the number of base stations transmitting redundant data to the user of a mobile station, and providing a transmission of sufficient quality.

In addition, it is worth mentioning document WO 95 12297, in which a plurality of procedures is disclosed to carry out the procedure of continuous transfer or intracellular transfer with continuity (soft and softer hand-off), which improve the performance of a system. A first procedure consists in delaying the intracellular transfer procedure with continuity. A second method is to reduce the power of the transmissions in the sector with the weakest signal strength. A third procedure is to remove the transmissions from the sector with the weakest signal strength. A fourth procedure is to add a new base station or sector only when the mobile unit needs more power to function properly. In all four procedures, the reverse link demodulation of each sector can continue with or without forward link transmission. In all four procedures, operation can be based on the signal strength of the reverse link or the forward link.

Another document worth mentioning herein is document US-A-5 577 022, which discloses a method for carrying out a pilot signal search operation prior to transferring a communication from a mobile station between base stations. The mobile station maintains a list of an active group of pilot signals transmitted from the base stations with which the mobile station is to communicate, and enables a group of pilot signals from the base stations that are at a predetermined distance from the mobile station. . Apart from the neighboring and active group of pilot signals, the mobile station maintains a list of candidate and pre-candidate pilots. Based on the analysis of the intensity of the pilot signals received at the mobile station, the base station inputs of the neighboring group can be assigned to the pre-candidate and candidate groups and finally to the active groups. The search operation takes into account the possibility of comparing the pilot signal intensity measurements corresponding to each base station input of the neighboring group with a first predetermined level. The entry (s) of a neighboring group that has a base station signal strength measurement above the first predetermined level may be placed in the pre-candidate group. Next, the intensity of the pilot signals associated with the inputs of the pre-candidate group is evaluated to determine eligibility within the candidate group, from which the selection of the inputs that comprise the active group is made.

According to the present invention, there is provided a method for selecting the base stations to communicate with a remote station, according to claim 1. Preferred embodiments of the present invention are defined in the dependent claims.

Summary of the invention

The present invention relates to a new and improved method and apparatus for providing seamless handoffs in a mobile communication system. It should be noted, first, that one of the major problems with current systems is that the elements of the active group are determined based on comparisons of the pilot energy measured with fixed thresholds. However, the value of providing a communication link

ES 2 245 018 T3 redundant with a mobile station is highly dependent on the power of the other signals arriving at the mobile station. For example, the value of redundant transmission to a mobile station of a signal with a receive power of -15 dB will not be of great value, if the mobile station is already receiving a transmission with a signal of -5 dB of power. However, redundant transmission to a mobile station of a signal with a receive power of -15 dB can be of significant value, if the mobile station is receiving transmissions with a signal of only -13 dB of power.

In a first embodiment of the present invention, the mobile station transmits, under the conditions described above, a pilot intensity measurement message indicating the base stations of the active and candidate group and their corresponding measured pilot energies. The pilot energy measurement message is received by the base stations communicating with the mobile station. Base stations provide this information to a central control unit, called a base station controller.

In the base station controller, the active group is determined according to the combined intensity of other pilots in the active group. The base station controller classifies the pilot signals of the pilot intensity measurement message according to their pilot intensity measured at the mobile station. Therefore, after sorting, the base station list consists of P₁, P₂, ..., P_N, where P₁ the strongest pilot signal and P_N the weakest. Next, an iterative procedure is undertaken to determine which of the pilot signals P₁, P₂, ..., P_N must be part of the active group reviewed.

Initially, the revised active group comprises only the strongest pilot signals P1 and P2. When deciding whether or not a pilot signal P, should be part of the active group, the COMBINED_PILOT value is calculated. The COMBINED_PILOT value consists of the sum of the energies of the pilot signals that are currently in the revised active group (P₁, P₂, ..., P_i_₁). Next, a threshold is generated based on the COMBINED_PILOT value. In the exemplary embodiment, the threshold is generated by performing a linear operation with the value COMBINED_PILOT. If the pilot energy value, P, exceeds the threshold, the pilot signal is added to the revised active group and the procedure is repeated for the next pilot signal P_{i + 1}. If the pilot energy value P, does not exceed the threshold, the revised active group will comprise P₁, P₂, ..., P_i_₁. This revised active list is transmitted to the mobile station and the base station controller then establishes communications with the mobile station according to the revised active group.

In an alternative embodiment, the revised active group is generated at the mobile station. The mobile station continuously measures the pilot intensities received from the base stations. To determine whether or not to send a message indicating that a pilot signal of the candidate group should be switched to the active group, the measured pilot energy of the pilot signal of the candidate group is compared with a threshold generated according to the COMBINED_PILOT value, of the way described above. If the strongest pilot signal in the candidate group satisfies the standard, then a message is sent containing all the pilots in the active group and the candidate group.

After the iterative procedure carried out with the elements of the candidate group, a second iterative procedure is carried out to determine whether a pilot signal should be removed from the revised active group. In this operation, the pilot signals, from the weakest to the strongest, of the reviewed active group are checked. The COMBINED_PILOT energy value is calculated, which is the sum of the energies of all the pilot signals belonging to the active group. A threshold value is generated according to the COMBINED_PILOT value, in the manner described above, and the pilot signal being tested is compared to the threshold. If a pilot signal has remained below the threshold value for a predetermined period of time, a message is sent to the base station indicating that the pilot signal should be excluded.

The revised active list is transmitted to the base station controller through the base stations with which the mobile station is communicating. The base station establishes the communication links with the base stations of the revised active list generated in the mobile station and transmits an acknowledgment to the mobile station once the links are established. The mobile station then routes the communications through the base stations of the reviewed active group.

In the preferred embodiment, the mobile station monitors the pilot signals and, in response to the monitored pilot signals, the mobile station collects the elements of the candidate group. On the other hand, the mobile station determines whether a change to the current active group is advisable in view of the criteria described above. Upon detecting any change in the desired elements of the active group, the mobile station generates a pilot intensity measurement message which, as indicated, includes the identities of all the pilot signals of the candidate group and of the active group, the corresponding measured energy values ​​and a corresponding indication that specifies whether the pilot signal should remain in the groups or be switched to the neighboring group (a fact that is indicated by introducing the variable KEEP described above). In the exemplary embodiment, the base station determines the elements of the revised active group according to the procedure described with reference to Figure 5.

Brief description of the drawings

The characteristics, objectives and advantages of the present invention will become apparent from the detailed description provided below, taken in conjunction with the drawings, in which equivalent reference characters are used for equivalent identifications and in which:

ES 2 245 018 T3 Figure 1 is an illustration of a cellular communication network;

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 procedure for generating the revised active group in the base station controller;

Figure 6 is a flow chart of the procedure for generating the revised active group at the mobile station;

Figure 7 is a flow chart illustrating the preferred procedure for generating the candidate group at the mobile station and Figure 8 is a flow chart illustrating the preferred procedure of the present invention, in which a change in the preferred elements of the active group, and a pilot intensity measurement message is transmitted to the base station in response to the detected change.

Detailed description of the preferred embodiments

Figure 1 illustrates a wireless communication network, in which the geographic area has been divided into coverage areas, called cells, which 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 embodiment, the base stations 4 are CDMA base stations. In the aforementioned US Patent Nos. 5,101,501 and 5,267,261, soft handoff in a CDMA wireless communication system is disclosed in detail.

Mobile station 2 is within the cell served by base station 4A. Since mobile station 2 is located near the edge of the cell, it is likely to be in a soft handoff condition, in which it remains in simultaneous communication with more than one base station. Mobile station 2 can communicate, for example, with base stations 4A and 4B. Therefore, base stations 4A and 4B are considered to compose the active group. On the other hand, it is possible for mobile station 2 to determine that other nearby base stations have a measured pilot energy higher than a predetermined threshold T_ADD, but that these base stations are not currently communicating with the mobile station. These pilot signals are considered to make up the candidate group. The candidate group could be made up of base stations 4C and 4G.

In Figure 2, a common communication network is illustrated. Data destined for mobile station 2 is provided by a public switched telephone network or other wireless system (not shown) to base station controller 6. Base station controller 6 provides data to base stations on the active list of the mobile station 2. In the example, the base station controller 6 redundantly provides data to base stations 4A and 4B and receives data redundantly from them.

The present invention is equally applicable to conditions where cells are divided into sectors. Communications through each sector can be received and demodulated separately by mobile station 2. For simplicity of description, each of the base stations 4 will be considered to be uniquely located base stations. However, as will become apparent to those skilled in the art, the present invention is equally applicable to cells divided into sectors, simply taking into account the possibility that the base stations may be located in the same location and transmit to separate sectors. of a cell. When a mobile station communicates simultaneously with more than one sector of a cell, what is called seamless intracellular handover takes place. The procedure and apparatus for performing intracellular transfers with continuity are described in detail in pending US patent application No. 08 / 144,903, entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION," filed on October 30, 1993, assigned to the assignee of the present invention and incorporated herein by reference.

At mobile station 2, each copy of the data packet is received, demodulated, and decoded separately. The decoded data is then combined to provide an estimate of the data with greater reliability than any of the demodulated estimates of the data.

Figure 3 illustrates mobile station 2 of the present invention. The mobile station 2 measures continuously or at intermittent intervals the strength of the pilot signals from the base stations 4. The signals received by the antenna 50 of the mobile station 2 are provided, through the duplexer 52, to the receiver (RCVR) 54 that effects the amplification, the frequency reduction and the filtering of the received signals and provides them to the pilot demodulator 58 Search subsystem 55.

ES 2 245 018 T3

In addition, the received signals are provided to traffic demodulators 64A to 64N. Traffic demodulators 64A to 64N, or a subset of these, separately demodulate the signals received by mobile station 2. The demodulated signals from traffic demodulators 64A to 64N are provided to combiner 66 which combines the demodulated data which, as in turn, they provide an improved estimate of the transmitted data.

Mobile station 2 measures the intensity of the pilot channels. Control processor 62 provides acquisition parameters to search processor 56. In the exemplary CDMA communication system embodiment, control processor 62 provides a PN offset to search processor 56. Search processor 56 generates a PN sequence that is used by pilot demodulator 58 to demodulate the received signal. The demodulated pilot signal is provided to energy accumulator 60 which measures the energy of the demodulated pilot signal, accumulating the 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 ​​to the thresholds T._ADD and T_DROP. T_ADD is the threshold above which the received signal is strong enough to allow effective communications with mobile station 2. TDROP is the threshold value below which the energy of the received signal is insufficient to allow effective communications with mobile station 2.

Mobile station 2 transmits a pilot intensity measurement message that includes all pilot signals with energy greater than TADD and all elements of the current active group whose measured pilot energy has not remained below TDROP for longer than a period of time predetermined. In the exemplary embodiment, mobile station 2 generates and transmits a pilot intensity measurement message after detecting a change in intensity of a pilot signal, under the following three conditions:

1. the detected intensity of a pilot signal from the neighboring group or the remaining group is greater than the TADD threshold,

two. the intensity of a pilot signal of the candidate group exceeds the intensity of a pilot signal of the active group by a value greater than a threshold (t_comp),

3. the intensity of a pilot signal of the active group has remained below a threshold (T_DROP) for longer than a predetermined period of time.

In the exemplary embodiment, the pilot intensity measurement message identifies the pilot signal and provides the corresponding measured pilot energy. In the exemplary embodiment, the base stations of the pilot intensity measurement message are identified by their pilot offsets, and the corresponding measured pilot energies are given in units of decibels.

Control processor 62 provides the identities of the pilot signals and the corresponding measured pilot energies to message generator 70. Message generator 70 generates a pilot intensity measurement message containing the information. The pilot intensity measurement message is provided to transmitter (TMTR) 68, which performs encoding, modulation, frequency raising, and amplification of the message. The message is then transmitted through duplexer 52 and antenna 50.

Referring to Figure 4, the pilot intensity measurement message is received by the antenna 30 of the base station 4 and provided to the receiver (RCVR) 28, which performs amplification, frequency reduction, demodulation and decoding the received signal and providing the message to the base station controller (BSC) interface 26. The base station controller (BSC) interface 26 sends the message to the base station controller (BSC) 6. The message is provided to selector 22, which may also redundantly receive the message from other base stations communicating with mobile station 2. Selector 22 combines estimates of messages received from base stations communicating with the mobile station. mobile station 2 to provide improved packet estimates.

The selector 22 provides the pilot intensity measurement message to the handoff control processor 20. In the first embodiment example, the handoff control processor 20 selects the base stations to communicate with the mobile station 2, ie that is, the elements of the revised active group, according to the procedure illustrated in Figure 5.

In block 100, the handoff control processor 20 classifies the pilot signals of the pilot intensity measurement message according to their intensities. Thus, for example, P₁ will be the strongest pilot signal received, P₂ it will be the second strongest pilot signal received and so on. In block 102, it is determined that the revised active group (ACTIVE_SET) must include P₁ And p₂. In block 104, the COMBINED_PILOT variable is set as the sum of the energies of P₁ And p₂. In block 106, the loop variable i is set to 3.

At block 108, the pilot signal energy of the received ith strongest signal (Pi) 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 according to the following equation (1):

T = SOFT_SLOPE * COMBINED: PILOT + SOFT: INTERCEPT (1)

ES 2 245 018 T3

In the example embodiment, SOFT_SLOPE is set to 2.25 and SOFT_INTERCEPT is set to 3.0. The SOFT_SLOPE and SOFT_INTERCEPT values ​​may be parameters that are sent over the air to the mobile station, or they may be selected values ​​that are programmed into the mobile station. The values ​​of SOFT_SLOPE and SOFT_INTERCEPT can be determined according to certain factors, such as the amount of smooth transfers that is acceptable to a network manager and empirical studies on the quality of transmission links. If the energy value P₁ is less than the threshold value, then the flow continues through block 110 and the revised active group includes the signals corresponding to the pilot signals {P₁ ... P_i-1}.

If the energy value P_i is higher than the threshold value in block 108, the flow continues through block 112. In block 112, a new COMBINED_PILOT value is calculated by adding the energy value of the ith strongest signal of the pilot intensity measurement message (Pi) with the current value of COMBINED_PILOT. Since the energy of the pilot signals is given in decibels in the exemplary embodiment, the energies must be converted to linear representations before being added and converted back to decibels. In block 114, P is added_i to the revised active group.

At block 116, the loop variable (i) is incremented. At block 118, handoff control processor 20 determines whether all base stations in the pilot intensity measurement message have been checked. If there is no pilot signal left to check, flow continues through block 120 and the reviewed active group comprises all base stations in the pilot intensity measurement message. If, at block 118, there is any base station from the pilot intensity measurement message to check, flow returns to block 108 and continues in the manner described above.

After generating the revised active group, the base station controller 6 determines if the base stations in the revised active list can support communications with the mobile station 2. If any of the base stations in the reviewed active group does not support communications with the mobile station 2, the base station is removed from the reviewed active group. After generating the revised active group, the transfer control processor 20 provides the selector 22 with the information indicating the elements of the revised active group. In response to the reviewed active group provided by handoff control processor 20, selector 22 allocates traffic channels to establish communications with the mobile station via the base stations of the reviewed active group.

Handover control processor 20 provides a message indicating the revised active group to message generator 24. Message generator 24 generates a message, called a handover address message, for transmission to mobile station 2. The address message Transfer indicates the base stations of the reviewed active group and the corresponding channels that these base stations will use to communicate with mobile station 2. The message is provided through selector 22 and provided to base stations that were communicating with mobile station 2 prior to the generation of the revised active group. Base stations communicating with mobile station 2 transmit the handover address message to mobile station 2.

Referring again to Figure 3, antenna 50 of mobile station 2 receives the handover address message. This message is provided to receiver 54, which performs amplification, frequency reduction, demodulation, and decoding of the message and provides it to control processor 62. Next, control processor 62 configures traffic channel demodulators 64A through 64N to demodulate traffic channels according to the revised active group indicated in the transfer direction message.

In an alternative embodiment of the present invention, the revised active group is generated in mobile station 2. This alternative embodiment provides a more timely generation of the revised active group. Because the pilot intensity measurement message is only transmitted under the three conditions described above, the update of the active group may experience an unwanted delay. However, the alternative embodiment results in a more timely transmission of the pilot intensity measurement message.

In the alternative embodiment, mobile station 2 measures received pilot energy in the manner described above. Pilot energy values ​​are provided to control processor 62. In response, control processor 62 generates a revised active group. If the revised active group differs from the current active group, mobile station 2 transmits a message indicating the elements of the revised active group to base station controller 6 via base stations 4. Base station controller 6 establishes communications with mobile station

two. Mobile station 2 reconfigures traffic channel demodulators 64A to 64N to demodulate received signals according to the revised active group generated at the mobile station.

In the exemplary embodiment, the control processor 62 of the mobile station 2 generates the revised active group according to the procedure shown in Figure 6. In block 200, the pilot signals whose measured energy exceeds the threshold T_ADD are added to the list of candidates and pilot signals whose measured pilot energy has remained below T_DROP for longer than a predetermined period of time are removed from the candidate list. In the exemplary embodiment, the time that a pilot signal remains below T_DROPis recorded by a control processor timer 62, referred to herein as the "timer ^TDROP.

In block 202, the pilot signals in the candidate list are ranked from highest to lowest intensity. Thus, P_C1 will be more intense than P_C2 and so on. In block 204, the COMBINED_PILOT variable equals

ES 2 245 018 T3 the energy of all the pilot signals of the active group. Also, in block 204, the loop variable (i) is initialized with the value 1. In block 206, element P is checked_Ci of the candidate group to determine if they should be part of the reviewed active group. P_Ci it is compared to a threshold generated based on the current value of COMBINED_PILOT. In the exemplary embodiment, the threshold (T) is generated according to equation (1) provided above.

If the energy of the pilot signal P_Ciexceeds the threshold T, then the flow goes to block 208. In block 208, the pilot signal P_Ci is added to the revised active group. In block 210, a new value of COMBINED_PILOT is calculated that is equal to the previous value of COMBINED_PILOT plus the energy of the pilot signal P_Ci. At block 212, the loop variable (i) is incremented.

At block 213, it is determined whether all the pilot signals in the candidate group have been checked. If not all pilot signals in the candidate group have been checked, flow proceeds to block 206 and continues as described above. If all the pilot signals of the candidate group have been checked or if, again in block 206, the energy of the pilot signal P_C1 has not exceeded the threshold T, the flow goes to block 214. In block 214, the revised active group is classified from lowest to highest energy. Thus, P_A1 will have the minimum energy measured in the revised active group, P_A2 will have the second lowest energy and so on until the last element of the revised active group P_AN.

In block 216, it is determined whether PA is an element of the candidate group. If PA is an element of the candidate group, the flow goes to block 234 and the review of the active group is complete. In block 218, the loop variable i is set to 1. In block 220, the COMBINED_PILOT value is calculated to check PAi. This COMBINED_PILOT value is equal to the sum of the measured energy of all the pilot signals that present an energy greater than the pilot signal that is currently being tested. Therefore, the COMBiNed_PILOT value is determined by the following equation:

COMBINED_PILOT = ^F P_Aj j = i + i ⁽²⁾

In block 222, the currently tested pilot signal is compared to a threshold (T) determined based on the calculated value of COMBINED_PILOT. In the exemplary embodiment, the threshold T is determined according to equation (1) indicated above. If the energy of the measured pilot signal PA exceeds the threshold T, the flow proceeds to block 224 and the exclusion timers for the pilot signals PAi to PAN are reset to zero and the determination of the revised active group ends at block 234.

If the measured energy of the pilot signal PA does not exceed the threshold T, the flow proceeds to block 226. In block 226, it is determined whether the timer T_DROP for PA, it has expired. If the timer T_DROP then, at block 228, the pilot signal PA has expired, is removed from the reviewed active group and placed in the candidate group, and flow continues through block 230. Yes, at block 226, it is determined that the timer T_DROP for PA, it has not expired, flow continues directly through block 230. At block 230, the loop variable (i) is incremented. Next, at block 232, it is determined whether all the pilots for the revised active group PA, have been checked. If all the pilots in the reviewed active group have been checked, the flow continues through block 234 and the generation of the reviewed active group is complete. If not all pilots in the reviewed active group have been checked, flow goes to block 220 and continues as described above.

In Figures 7 and 8, a preferred method of implementing the present invention is illustrated. In the preferred embodiment, the mobile station monitors the pilot signals and, in response to the monitored pilot signals, the mobile station collects the elements of the candidate group. On the other hand, the mobile station determines whether a change to the current active group is desirable in view of the criteria described above. Upon detecting any change in the desired elements of the active group, the mobile station generates a pilot intensity measurement message which, as indicated, includes the identities of all the pilot signals of the candidate group and the active group, the corresponding measured energy values ​​and a corresponding indication that specifies whether the pilot signal should remain in the groups or be switched to the neighboring group (a fact that is indicated by introducing the variable KEEP described above). In the exemplary embodiment, the base station determines the elements of the revised active group according to the procedure described with reference to Figure 5.

The preferred embodiment allows the timely modification of the active group elements and allows the determination of the revised active group elements at the base station. In this way it is possible to reduce the calculations carried out in the mobile station and to include, in the selection procedure, capacity restrictions of the base stations. Base station capacity constraints can be taken into account by the base station controller simply by removing or weighting the pilot signals transmitted by the base stations under high load capacity conditions.

Figure 7 is a flow chart illustrating the procedure for updating the candidate group which, in the exemplary embodiment, is carried out at the mobile station. In block 300, the loop variable (i) is initialized to the value 1. In block 302, the pilot signals of the neighboring group (P_N) are classified in such a way that P_N1 > P_N2> P_N3 and so on. At block 306, the neighboring group pilot signal currently being tested (P_NEITHER) is compared with the threshold TA_DD. If the pilot signal energy (P_N,) exceeds the threshold, then, in the block

ES 2 245 018 T3

310, the pilot signal is added to the candidate group and flow continues through block 308. If the pilot signal energy (P_Neither) does not exceed the threshold, then, at block 306, the flow goes directly to block 312.

At block 308, the index number of the neighboring group pilot signal being tested is incremented. Next, at block 304, it is determined whether all items in the neighboring group have been checked. If not all items in the neighboring group have been checked, the flow proceeds to block 306 and continues in the manner described above. If all the elements in the neighboring group have been checked, then the flow continues through block 312.

In block 312, the index variable (i) is reset to 1. Next, in block 314, the pilot signals of the candidate group (P_C) are classified from least to greatest intensity, such that P_to <P_C2 <P_C3 and so on. At block 318, the pilot signal energy of the candidate list being tested (P_Ci) is compared with the TDROP exclusion threshold. If the energy is below the exclusion threshold, the flow continues through block 324. If the energy is above the exclusion threshold, the flow continues through block 320. Since the list of pilot signals is ordered, the rest of the elements that remain to be checked will necessarily have more energy than T_DROP. Therefore, in block 320, the timers T_DROP to P_Ci and all pilot signals of greater intensity than (P_Ci) are restored, the update of the candidate group having then finished.

As described above, the TDROP timer is a timer that records how long a pilot signal remains below the exclusion threshold. The purpose of the TDROP timer is to avoid the mistaken exclusion of a strong pilot signal that may have low measured energy, due to a short duration change in the propagation environment, such as rapid fading. At block 324, the TDROP timer is activated if the timer for P_Ci it's not working yet, or it goes ahead otherwise.

At block 326, it is checked whether the timer T_DROP for the pilot signal (P_Ci) has expired. If the timer has expired, the flow goes to block 328 and the pilot signal (P_Ci) is removed from the candidate group. Next, the flow goes to block 322. On the contrary, if the timer has not expired in block 326, the flow goes directly to block 322. In block 322, the index variable (i) of the candidate group is incremented . Next, at block 316, it is determined whether all the pilot signals in the candidate group have been tested. If all the elements of the candidate group have been checked, the update of the candidate group is complete. If not all elements of the candidate group have been checked, the flow proceeds to block 314 and continues in the manner described above.

In the preferred embodiment, the selection of the candidate group elements is carried out at the mobile station. This is because, in the selection of the candidate group, it is usually not necessary to know the capacity restrictions of the base stations in the network. However, in an alternative embodiment, the procedure for excluding items from the candidate group and including them in the neighboring group can be carried out in the base station controller. On the other hand, the addition of elements to the candidate group can be carried out in the base station controller, provided that the base station controller knows the elements of the neighboring group of the mobile station or can have such information.

Figure 8 illustrates the procedure for detecting the need to review the active group which, in the preferred embodiment, is carried out at the mobile station. In block 400, the strongest pilot signal is selected from the candidate group (P '_Ci) (the symbol "prime" is used to differentiate the pilot signal from P_Ci which, in Figure 7, represented the weakest candidate pilot signal). At block 402, the energy of (P_Ci) with a threshold (T) based on the accumulated energy of the active group pilot signals, represented in equation 3 below.

T = f (LPAi) = SOFT_SLOPE * ZPAi + SOFT_ADD_INTERCEPT (3)

Yep'_Ci) exceeds the threshold (T), the mobile station transmits the pilot intensity measurement message to the base station, in block 404.

If (P'cí) does not exceed the threshold (T), the flow continues through block 406. In block 406, the active group is classified in increasing order of intensity of the pilot signals. In block 408, the index variable (i) of the active group is set to 1. Next, in block 410, the active group pilot signal (PA,), which is being checked to determine whether or not it should remain in the active group, is compared with a threshold (T) generated according to a sum of the energies of the strongest pilot signals that is represented in equation (4) below:

T = f ΣP_Ai = SOFT_SLOPE * 7 PA + SOFT_DROP_INTERCEPT

V> i 7 j> ¡(4)

If the pilot signal being tested (PAi) exceeds the threshold (T), then it and all pilots of higher intensity should remain in the active group. Therefore, at block 412, the TDROP timers for all pilot signals of intensity greater than PAi are reset, and the current search to determine the need for review of the active group will have been completed, the mobile station having not detected the need for any revision. In the preferred embodiment, the intercept value (SOFT_ADD_INTERCEPT) used to generate the add threshold is allowed to take a different value than the intercept value.

ES 2 245 018 T3 SOFT_DROP_INTERCEPT function used to generate the exclusion threshold. This provides greater flexibility and allows the network to introduce additional hysteresis into the signal levels.

If the pilot signal (P_ai) is less than the threshold (T), the flow goes to block 422. In block 422, the timer T_DROP for the pilot signal (P_ai) is activated if it is not working yet and is advanced otherwise. At block 424, it is checked whether the timer T_DROP for the pilot signal (P_ai) has expired. If the timer T_DROPhas expired, the mobile station transmits a pilot intensity measurement message to the base station in block 430. If the TDROP timer has not expired, the flow proceeds to block 426, where the index of the active group pilot signal is incremented (i). The flow then proceeds to block 420, where it is determined whether all items in the active group have been checked. If all elements of the active group have been checked, the search is interrupted, without detecting the need to check the active group. If not all items in the active group have been checked, the flow proceeds to block 410 and continues as described above.

The preceding description of the preferred embodiments is provided to enable those skilled in the art to create 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 need for inventiveness. Therefore, it is not intended to limit the present invention to the embodiments depicted herein, but to give it the broadest scope consistent with the new principles and features disclosed herein.