HK1030509A - A method of and apparatus for selecting base stations to communicate with a remote station - Google Patents

Description

Method and apparatus for selecting base stations to communicate with remote stations

Background

I. Field of the invention

The present invention relates to a method and apparatus for selecting a base station to communicate with a remote station. The present invention may be used to perform handoff in a wireless communication system.

II. background of the invention

Code Division Multiple Access (CDMA) modulation techniques are but one of several techniques that facilitate the presence of many system users in communication. While other techniques such as Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) are known, as well as AM modulation schemes such as amplitude companded single sideband, CDMA has significant advantages over these modulation techniques. The use of CDMA techniques in multiple access communication systems is disclosed in U.S. patent nos. 4,901,307 and 5,103,459, both assigned to the assignee of the present invention and the contents of which are incorporated herein by reference, entitled "spread spectrum multiple access communication system using satellite or terrestrial repeaters" and systems and methods for generating signal waveforms in a CDMA cellular telephone system. The telecommunications industry association has standardized the CDMA mobile communications method in TIA/EIA/IS-95-a entitled "mobile station-base station compatibility standard for dual mode wideband spread spectrum cellular systems".

The above-mentioned patent discloses a multiple access technique in which each mobile telephone user has a transceiver and a large number of users communicate via a satellite repeater or a terrestrial base station (also referred to as a cellular base station or cell site) using Code Division Multiple Access (CDMA) spread spectrum communication signals. When using CDMA communications, the spectrum may be reused multiple times, thereby increasing the capacity of system users. Higher spectral efficiency can be achieved using CDMA techniques than using other multiple access techniques.

Us patent No. 5,109,390 (' 390 patent) discloses a method of simultaneously demodulating data transmitted along different propagation paths from one base station, and simultaneously demodulating redundant data provided by more than one base station. The patent entitled "diversity receiver in a CDMA cellular communication system," assigned to the assignee of the present invention, is incorporated herein by reference. In the' 390 patent, the separately demodulated signals are combined to estimate the transmitted data with a higher reliability than data demodulated using either path or data from either base station.

Handover is generally divided into two categories: hard handover and soft handover. In hard handoff, when a mobile station leaves an origination zone and enters a destination zone, the mobile station interrupts its communication link with the origination zone and then establishes a new communication with the destination zone. In soft handoff, the mobile station completes its communication link with the destination area before interrupting its communication link with the origination area. Therefore, in soft handoff, the mobile station may have redundant communication with the origination area and the destination area for some time.

Soft handoffs are much less likely to drop a call than hard handoffs. In addition, as the mobile station moves closer to the cell boundary, it may repeatedly make handover requests in response to minor changes in the environment. This problem is known as ping-pong and soft handoff can make it much more relaxed. U.S. Pat. No. 5,101,501 describes in detail the execution of a soft handover. The patent entitled "method and system for providing soft handoff in communications in a CDMA cellular telephone system," is assigned to the assignee of the present invention and is incorporated herein by reference.

U.S. patent No. 5,267,261 discloses an improved soft handoff technique. The patent entitled "mobile station assisted soft handoff in a CDMA cellular communication system," is assigned to the assignee of the present invention and is incorporated herein by reference. In the system of the' 261 patent, the soft handoff process is improved by measuring, at the mobile station, the strength of the "pilot" signal transmitted by each base station in the system. These pilot strength measurements facilitate soft handoff procedures by facilitating identification of candidates suitable for base station handoff.

The applicable base station candidates can be divided into four groups. The first group, referred to as the active group, includes the base stations that are currently communicating with the mobile station. The second group, referred to as the candidate group, includes base stations that have been identified as having sufficient strength to be useful to the mobile station. When the measured pilot energy for base stations exceeds a predetermined threshold TADD, the base stations are added to the candidate set. The base stations of the third group are located in the vicinity of the mobile station and are not included in the active or candidate groups. And the fourth group is the remaining group, which consists of all other base stations.

In an IS-95-a communication system, a mobile station sends a pilot strength measurement message when a pilot with sufficient strength IS found but not associated with any of the forward traffic channels currently being demodulated, or when the strength of the pilot associated with one of the forward traffic channels being demodulated IS below a threshold for a predetermined period of time. The mobile station sends out a pilot strength measurement message after detecting a pilot strength change in the following three states:

1. the pilot strength of the neighbor set or remaining sets is found to be above the threshold TADD.

2. The pilot strength of the candidate set exceeds the pilot strength of the active set by an amount greater than a Threshold (TCOMP).

3. The time when the pilot strength in the active set or the candidate set is below the Threshold (TDROP) is greater than a predetermined time.

The pilot strength measurement message may identify the base station and the measured pilot energy (in decibels).

The negative impact of soft handoff is that it consumes available communication resources because it transmits information redundantly. However, soft handoff can greatly improve communication quality. Therefore, there is a need in the art to provide a method that minimizes the number of base stations transmitting redundant data to a user of a mobile station and that provides a sufficiently good transmission quality.

Disclosure of Invention

According to one aspect of the invention, there is provided a method of selecting a base station to communicate with a remote station, the method comprising the steps of: measuring, at the remote station, a combination of energy signaled by base stations capable of communicating with the remote station for a predetermined time; at the remote station, responsive to the measurement, calculating a first threshold; at the remote station, comparing the signal energy of the first base station to the first threshold; and at the remote station, selecting the first base station when the signal energy of the first base station is greater than the first threshold.

In accordance with another aspect of the present invention, there is provided a mobile station for use in a communication system including a plurality of base stations controlled by a base station controller, the mobile station comprising: a receiver for receiving signals from a plurality of base stations that together form a set of candidate base stations; means for determining a received signal power; first means for comparing the received signal power to a first threshold; means for identifying a base station, a received signal power of the identified base station being greater than a first threshold; and means for sending a signal to the controller, the signal indicating that the identified base station is eligible for inclusion in the active set.

The present invention is applicable to a novel and improved method and apparatus for providing soft handoff in a mobile communication system. It should first be noted that one of the biggest problems with current systems is to determine the membership of the active set based on the comparison of the measured pilot energy to a fixed threshold. However, the value of providing redundant communication links to a mobile station is highly dependent on the energy of other signals provided to the mobile station. For example, if the mobile station has received a transmission with signal energy corresponding to a-5 db pilot strength, then it is not worth much to redundantly transmit signals with received energy corresponding to a-15 db pilot strength to the mobile station. However, if the mobile station is receiving a transmission whose signal energy corresponds to only-13 db pilot energy, then it is valuable to redundantly transmit to the mobile station a signal whose received energy corresponds to-15 db pilot strength.

At the mobile station, when determining whether to send a message indicating that a PILOT should be moved from the candidate set to the modified active set, the measured PILOT energy for each PILOT in the candidate set is iteratively compared to a threshold generated based on a variable, COMBINED _ PILOT, where COMBINED _ PILOT is the energy of the PILOT in the active set (i.e., E_C/I₀) And (4) summing. In the preferred embodiment, the optimum value of the threshold is determined by the mobile station itself, without the threshold having to be broadcast or otherwise determined at the base stationAcknowledging the request of the mobile station. If the strongest PILOT in the candidate set satisfies the threshold condition, it is added to the revised active set and combimed _ PILOT is recalculated to include the newly added PILOT signal.

The candidate set members are iteratively processed a second iteration to determine if pilots should be removed from the modified active set. In this process, pilots are tested from the weakest member to the strongest member of the active set after correction. A COMBINED _ PILOT energy value is calculated, which is the sum of the energies of all PILOTs belonging to the active set. As previously described, a threshold is generated based on the COMBINED _ PILOT value and the PILOT signal under test is compared to the threshold. Again, to avoid additional signaling, the threshold is determined at the mobile station. If the pilot is below the threshold for a predetermined period of time, a message is sent to the base station indicating that such pilot should be deleted.

The revised active group list is transmitted to the base station controller by a base station in communication with the remote station. The base station controller establishes a communication link with the base stations in the modified active set list generated in the mobile station and acknowledges to the mobile station when the link is established. Then, the mobile station carries out communication by correcting the active station.

In a preferred embodiment, the mobile station monitors the pilot signal and compiles members of the candidate set in response to the monitored pilot signal. In addition, in view of the above criteria, the mobile station determines whether it is desired to change the current active set by measuring the pilot energy in the active set and the candidate set, and dynamically adjusting the necessary thresholds based on its own estimates of the communication environment. When a determination is made that there is a change in membership of the active set, the mobile station generates a pilot strength measurement message that includes an identification of the pilots in the candidate set and the active set, their corresponding energy measurements, and a corresponding indication of whether the pilots should remain in the set or move into a neighbor set, as described above.

Brief description of the drawings

The features, objects, and advantages of the present invention will become more apparent from the following detailed description of embodiments of the invention when taken in conjunction with the accompanying drawings. In the drawings, like reference numerals designate corresponding parts throughout the several views, and wherein:

FIG. 1 illustrates a cellular communication network;

FIG. 2 is the cellular communications network of FIG. 1 and includes a base station controller;

FIG. 3 is a block diagram illustrating a mobile station that implements the present invention;

FIG. 4 is a block diagram illustrating a base station embodying the present invention;

FIG. 5 is a graph showing dynamic threshold versus combined pilot energy in the active set, representing a linear operation on soft handoff parameters;

FIG. 6 is a flow chart illustrating a method of generating a corrected active set in the mobile station; and

fig. 7 is a state diagram illustrating the operation of the present invention.

Detailed description of the preferred embodiments

Fig. 1 illustrates a wireless communication network in which a geographic area is divided into a number of coverage areas, referred to as cells, which are represented by a set of adjacent hexagons. Each cell is served by a respective base station 4. Each base station 4 transmits a pilot signal for uniquely identifying the base station. In the illustrated embodiment, the base station 4 is a CDMA base station. The above-mentioned U.S. Pat. nos. 5,101,501 and 5,267,261 describe in detail a soft handover in a CDMA wireless communication system.

Mobile station 2 is located within a cell serviced by base station 4A. Since the mobile station 2 is close to the cell border it may be in a soft handover state, i.e. communicating with more than one base station at the same time. For example, it may communicate with base stations 4A and 4B. Thus, canSo to say that the base stations 4A and 4B constitute the active set. In addition, the mobile station 2 may have concluded that there are other base stations in its vicinity with more than a predetermined threshold T_ADDBut these base stations are not currently communicating with the mobile station. These pilots constitute a candidate set. The candidate set may consist of base stations 4C and 4G.

Referring to fig. 2, a typical communication network is shown. Data addressed to the mobile station 2 is provided to the base station controller 6 by a public switched telephone network or other wireless system (not shown). The base station controller 6 provides data to the base stations in the active set list of the mobile station 2. In this example, base station controller 6 redundantly provides and receives data to base stations 4A and 4B.

The invention is equally applicable to the case where each cell is sectorized. Mobile station 2 may receive and demodulate communications to and from each sector separately. For simplicity, the base stations are uniquely positioned in each base point of the base station 4. However, those skilled in the art will readily recognize that the present invention is equally applicable to sectorized cells, considering the possibility of co-configuring base stations and transmitting to discrete sectors within the cell. A mobile station communicating with more than one sector in a cell at the same time is called a softer handoff. Co-pending U.S. patent application No. 08/144,903 describes in detail a method and apparatus for performing softer handoffs. The filing date of this patent application, entitled "method and apparatus for performing handoff between sectors of a public base station," 1993 at 10/30, is assigned to the assignee of the present invention and is incorporated herein by reference.

Within the mobile station 2, each copy of the data packet is received, demodulated and decoded separately. The decoded data is then combined to make an estimate of the data that is more reliable than the estimate of any one of the demodulated data.

Fig. 3 depicts the mobile station 2 in more detail. The mobile station 2 continuously or intermittently measures the pilot signal strength of the base station 4. The signal received by the antenna 50 of the mobile station 2 is provided through a duplexer 52 to a receiver (RCVR) 54. the receiver 54 amplifies, frequency downconverts, and filters the received signal and provides it to a pilot demodulator 58 in a search subsystem 55.

In addition, the received signals are provided to traffic demodulators 64A-64N. The traffic demodulators 64A-64N, or a subset thereof, individually demodulate the signals received by the mobile station 2. The signals demodulated by traffic demodulators 64A-64N are provided to combiner 66, and combiner 66 combines the demodulated data and then provides an improved estimate of the transmitted data.

The mobile station 2 measures the strength of the pilot channel. The control processor 62 provides acquisition parameters to the search processor 56. In the illustrated embodiment of a CDMA communication system, the control processor 62 provides a PN offset (PN offset) to the search processor 56. The search processor 56 generates a PN sequence that is used by the pilot demodulator 58 to demodulate the received signal. The demodulated pilot signal is supplied to an energy accumulator 60, and the accumulator 60 accumulates energy for a predetermined time, thereby measuring the energy of the demodulated pilot signal.

The measured pilot energy value is provided to the control processor 62. In the illustrated embodiment, the control processor 62 compares this energy value to a threshold T_ADDAnd T_DROPAnd (6) comparing. When at the threshold value T_ADDIn the above, the received signal has sufficient strength to effectively communicate with the mobile station 2. When at the threshold value T_DROPIn the following, the energy of the received signal is not sufficient to effectively communicate with the mobile station 2.

The mobile station 2 sends out a pilot strength measurement message comprising an energy greater than T_ADDAnd its measured pilot energy in the currently active set is below T_DROPHas not exceeded a predetermined time. In the illustrated embodiment, the pilot strength measurement message is generated and sent out after the mobile station 2 detects a change in pilot strength in three states:

1. finding the pilot strength of the neighbor set or the remaining set at a threshold T_ADDThe above.

2. The pilot strength of the candidate set exceeds that of the active setThe amount of pilot strength is greater than a threshold (T)_COMP)。

3. The strength of the pilots in the active set is below a threshold (T)_DROP) Is greater than a predetermined time.

In the illustrated embodiment, the pilot strength measurement message can identify the pilot and provide a corresponding measured pilot energy. In the illustrated embodiment, the pilot offset is used to identify the base station in the pilot strength measurement message and provide its corresponding measured pilot energy in decibels. Can be combined with T_ADDAnd T_DROPIs preprogrammed into the mobile station 2 or is provided to the mobile station 2 by the base station 4 (see fig. 4). In addition, the mobile station 2 itself can calculate these values.

The control processor 62 provides the identification of the pilot and its corresponding measured pilot energy to the message generator 70. Message generator 70 generates a pilot strength measurement message containing this information. The pilot strength measurement message is provided to a transmitter (TMTR)68, and the transmitter 68 decodes, modulates, upconverts, and amplifies the message. The message is then transmitted through duplexer 52 and antenna 50.

Referring to fig. 4, an antenna 30 of base station 4 receives the pilot strength measurement message and provides it to a receiver (RCVR)28, and receiver 28 amplifies, downconverts, demodulates and decodes the received signal and provides the message to a Base Station Controller (BSC) interface 26. The Base Station Controller (BSC) interface 26 sends messages to the Base Station Controller (BSC) 6. The message is provided to the selector 22 and the selector 22 may also receive redundant messages from other base stations in communication with the mobile station 2. The selector 22 combines the message estimates received from the base stations in communication with the mobile station 2 to provide an improved packet estimate.

Preferably, the mobile station 2 monitors the pilot signal and compiles (compile) each of the above-mentioned sets of members (active set, candidate set and neighbor set). In addition, the mobile station 2 preferably determines whether it is desired to change the current active set based on the following linear relationship:

Y1＝SOFT_SLOPE*COMBINED_PILOT+ADD_INTERCEPT(1)

y2 (SOFT SLOPE common PILOT + DROP interval (2)) where Y1 is the dynamic threshold above which the measured energy of the candidate set PILOTs must be greater before the mobile station requests a base station to be added to the modified active set, and Y2 is the dynamic threshold below which the energy of the active set PILOTs must be reduced before the mobile station requests a base station to be removed from the active set. To create retardation, Y1 is preferably greater than Y2.

As can be seen from equations (1) and (2), if the measured energy of a particular active set pilot is below Y2, it is moved into the candidate set. In order to return the same pilot to the corrected active set, one of two things must occur: decrease in value of COMBINED _ PILOT by Δ₁Or the measured energy of the pilot itself is increased by Δ₂. Thus, it can be seen that₁And Δ₂Are the COMBINED PILOT and hysteresis values of the individual PILOT energies, which are used to prevent a given PILOT from repeatedly entering and exiting the active set, respectively.

Thus, when the value of COMBINED _ PILOT is less than or equal to x₁When a PILOT should be added to the modified active set, and when the value of comblned _ PILOT is greater than or equal to x₂Pilots should be removed from the active set. As can be seen from equations (1) and (2),

SOFT_SLOPE＝Δ₂/Δ₁； (3)

DROP_INTERCEPT＝TDROP-x₂*Δ₂/Δ₁； (4)

ADD_INTERCEPT＝DROP_INTERCEPT+Δ₂。 (5)

fig. 5 further illustrates this relationship. The dynamic thresholds Y1 and Y2 are in decibels, and are the combined pilot energy (i.e., E) also in decibels_c/I₀) As a function of (c). As can be seen, they are all linear functions with a SLOPE of SOFT _ SLOPE (i.e., Δ in equation (3))₂/Δ₁) The y-INTERCEPTs are ADD _ INTERCEP and DROP _ INTERCEPT, respectively. Note thatThat is, the y-INTERCEPT value may be negative, and DROP interval in fig. 5 is a negative value.

The value of SOFT _ SLOPE is, for example, 2. In the preferred embodiment, the mobile station 2 itself can calculate the value of SOFT SLOPE by monitoring the fluctuation of all pilots in the active and candidate sets as described above with reference to fig. 3, estimating the desired values of Δ 2 and Δ 1, and then applying the relationship of equation (3). The mobile station 2, and in particular the control processor 62, may estimate the value of Δ 1 by measuring the change in comblned _ PILOT over a predetermined time. For example, Δ 1 in the preferred embodiment is equal to the standard deviation of COMBINED _ PILOT over a predetermined time to prevent the natural variation of COMBINED _ PILOT from triggering a handoff request. In addition, Δ in the preferred embodiment may be₂Is set to T_ADDAnd T_DROPDifference between them due to T_ADDAnd T_DROPDifference and delta₁Is of the same order of magnitude.

As previously mentioned, x₁Is a value of COMBINED PILOT _ PILOT sufficient to add the PILOT to the corrected active set (in this case, Y1 and T)_ADDAn intersection). Likewise, x₂Is a value of comboned PILOT sufficient to cause the PILOT to be deleted from the active set (in this case, Y2 and T)_DROPAn intersection). X is to be₂Is preprogrammed into the mobile station or is provided to the mobile station with a signaling message from the base station. In the preferred embodiment, this value is large enough to provide a robust forward link while avoiding unnecessary redundancy. x is the number of₂For example, a value of-7.11 decibels. In the preferred embodiment, the mobile station itself may be based on its pair Δ₁And Δ₂And known x₂And T_DROPTo determine x₁The value of (c). Thus if x₁＝1.5，Δ₂＝3，x₂-7.11 decibel, and T_DROP12.44 db, then get SOFT SLOPE 2, ADD interval 1.22 db, DROP interval-1.78 db, and x according to equations (1) - (5) above₁-7.61 db.

The handover parameters are generated at the mobile station 2. These switching parameters are used to generate a modified active set, as described below. Since the handoff parameters are generated at the mobile station 2 rather than at the base station 4 or base station controller 6, the generation is much faster and excessive signaling is not required. In addition, this may avoid any acknowledgement calculations at the base station 4 or the base station controller 6. As described above for fig. 3, the mobile station 2 measures the received pilot energy. The pilot energy value is provided to the control processor 62. In response, the control processor 62 generates a handover parameter. If a pilot is required to be added to or removed from the current active set based on handoff parameters generated by the mobile station, the mobile station 2 sends a message through the base station informing the base station controller 6 of the modified active set members. The base station controller 6 establishes communication with the mobile station 2. Mobile station 2 reconfigures the traffic channel demodulators 64A-64N to demodulate the received signals based on the modified active set generated by the mobile station.

In the illustrated embodiment, the control processor 62 in the mobile station 2 generates a modified active set in accordance with the method illustrated in fig. 6. In block 200, the measured energy is passed a threshold T_ADDAdding the pilot of (2) to the candidate list and measuring the energy below T_DROPPilots with a time exceeding a predetermined time are deleted from the candidate list. In the illustrated embodiment, a timer is used in the control processor 62 to track pilot below T_DROPIs called T, the timer is called T_TDROPA timer. T is_TDROPThe timer tracks when the pilot is below the drop threshold. T is_TDROPThe purpose of the timer is to avoid erroneously discarding a stronger pilot when it has lower energy (e.g., fast fading) due to short time variations in the propagation environment.

In block 202, the pilots in the candidate list are ordered from strong to weak. Thus, P_C1Ratio P_C2Strong, etc. P as defined in EIA/TIA IS-95A, section 6.6.6.2.2_CiE preferably of candidate pilots I_C/I₀. In block 204, the variable comblned PILOT is set to the energy of all PILOTs in the active set. Also, in block 204, the loop variable (i) is initialized to 1. At block 206, test waitMembers of the group P_CiTo determine whether it should be part of the corrected active set. Will P_CiCompared to a threshold generated based on the current value of comblned _ PILOT. In the illustrated embodiment, the threshold value (Y1) is generated according to equation (1) above.

If P is_CiExceeds the threshold Y1, flow proceeds to block 208. In block 208, mobile station 2 sends a Pilot Strength Measurement Message (PSMM) to base station 4 requesting that pilot P be transmitted_CiAdded to the active set. The base station 4 then issues a response message instructing the mobile station 2 to direct the pilot P_CiAdded to the active set. At block 210, a new combination _ PILOT is calculated that is equal to the old combination _ PILOT plus PILOT P_CiThe energy of (a). At block 212, the loop variable (i) is incremented.

At block 213, a determination is made whether all pilots in the candidate set have been tested. If all pilots in the candidate set have not been tested, flow proceeds to block 200 and continues as described above. If all pilots of the candidate set have been tested, or if block 206 is returned, P_CiDoes not exceed the threshold Y1, flow proceeds to block 214. In block 214, the modified active sets are sorted from minimum energy to maximum energy. Thus, in the corrected active set, P_A1With minimum measured energy, P_A2With the second smallest measured energy, and so on, P_ANIs the last member of the modified active set.

At block 218, a loop variable i is set to 1. At block 220, calculate for test P_AiCOMBINED _ PILOT. Comblned PILOT is assigned as the sum of the measured energies of all PILOTs currently in the active set, and its energy is greater than the PILOT currently being tested. Thus, COMBINED _ PILOT is determined using the following equation:

where N is the number of pilots in the active set.

In block 222, the PILOT currently being tested is compared to a threshold (Y2) determined from the calculated value of combimed _ PILOT. In the illustrated embodiment, the threshold Y2 is determined according to equation (2) above. If the pilot energy P is measured_AiGreater than threshold Y2, flow proceeds to block 224 and P is incremented_Ai-P_ANT of_TDROPThe drop timer is reset to zero and the determination of the modified active set is ended at block 234.

If the measured pilot energy P_AiIf the threshold Y2 is not exceeded, then flow proceeds to block 226. At block 226, P is judged_AiT of_TDROPWhether the timer has expired. If T is_TDROPThe timer has expired, then at block 228 the mobile station 2 sends a PSMM to the base station 4 requesting removal of pilot P from the active set_AiAnd put into the candidate set. The base station 4 issues a positive response message and flow proceeds to block 230. If at block 226, a decision P is made_AiT of_TDROPThe timer has not expired, then flow proceeds directly to block 230. At block 230, the loop variable (i) is incremented. Then, at block 232, a determination is made as to whether the active set P has been tested_AiAll pilots in (1). If all pilots in the active set have been tested, flow proceeds to block 234 where generation of the corrected active set is completed. If all pilots in the active set have not been tested, flow proceeds to block 220 and continues as described above.

Fig. 7 shows an operating state diagram of the invention. Given pilot P_NiStarting in the neighbor set 700. If pilot P_NiE of (A)_C/I₀Greater than a threshold value T_ADDThen the mobile station 2 adds it to the candidate set 702. If pilot P_CiIs located in the candidate group 702, and E thereof_C/I₀Down to threshold T_DROPThe following, and its T_TDROPThe timer expires, and mobile station 2 moves it from candidate set 702 to neighbor set 700. Two just describedThe step transition corresponds to block 200 of fig. 6, i.e., adding and removing pilots to the candidate set.

If pilot P in the candidate set_CiE of (A)_C/I₀Greater than the dynamic threshold Y1 determined by equation (1) above, then the mobile station 2 sends a PSMM706 to the base station 4 requesting P_CiAdding the active set 708. In response, the base station 4 issues an Extended Handoff Direction Message (EHDM) instructing the mobile station 2 to place P_CiAdding the active set 708. The two-step transition just described corresponds to block 202 and 213 of fig. 6.

If pilot P in active set_AiE of (A)_C/I₀Less than the dynamic threshold Y2, and T thereof_TDROPUpon expiration of the timer, mobile station 2 transmits PSMM710 to base station 4 requesting removal of pilot P from the active set_Ai. In response, the base station 4 issues an EHDM instructing the mobile station 2 to place P_AiMove from the active set to the candidate set 702. The two-step transition just described corresponds to block 214 and 228 of figure 6.

If pilot P in active set_AiE of (A)_C/I₀Less than threshold T_DROPAnd T thereof_TDROPUpon expiration of the timer, mobile station 2 transmits PSMM704 to base station 4 requesting removal of pilot P from the active set_Ai. In response, the base station 4 issues an EHDM instructing the mobile station 2 to place P_AiMoving from the active set to the neighbor set 702. The flowchart does not have the corresponding parts of the two-step conversion.

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

Claims (18)

1. A method of selecting a base station to communicate with a remote station, the method comprising the steps of:

measuring, at the remote station, a combination of energy signaled by base stations capable of communicating with the remote station for a predetermined time;

at the remote station, responsive to the measurement, calculating a first threshold;

at the remote station, comparing the signal energy of the first base station to the first threshold; and

at the remote station, selecting the first base station when the signal energy of the first base station is greater than the first threshold.

2. The method of claim 1, wherein the signal energy of the first base station is an energy of a first base station pilot signal measured at the remote station.

3. The method of claim 2 wherein said combining of signal energies transmitted by base stations capable of communicating with said remote station comprises receiving a sum of pilot energy values having energies greater than a pilot signal of said first base station.

4. The method of claim 3, wherein said step of calculating a threshold comprises performing a linear operation on said combination of signal energies emitted by base stations capable of communicating with said remote station.

5. The method of claim 4 wherein the slope of said linear operation is calculated at said remote station in response to a change in said combination of signal energies over a predetermined time.

6. The method of claim 5, wherein an intercept of the linear operation is calculated at the remote station in response to the stored system parameters.

7. The method of claim 6, wherein the linear operation comprises:

multiplying said combination of signal energies emitted by base stations capable of communicating with said remote station by a first variable; and

adding a second variable to the product.

8. The method of claim 7, further comprising the steps of:

the remote station transmits a message indicating the measured pilot signal.

9. A remote station for use in a communication system including a plurality of base stations controlled by a base station controller, the mobile station comprising:

a receiver for receiving signals from a plurality of base stations that together form a set of candidate base stations;

means for determining a received signal power;

first means for comparing the received signal power to a first threshold;

means for identifying a base station, a received signal power of the identified base station being greater than a first threshold; and

means for sending a signal to the controller indicating that the identified base station is suitable for inclusion in the active set.

10. The mobile station of claim 9 wherein said means for determining power comprises means for accumulating energy values of received signals for a predetermined time.

11. The mobile station of claim 9 or 10, wherein said first means for comparing comprises means for dynamically calculating a first threshold.

12. The mobile station of claim 11, further comprising means for receiving one or more predetermined parameters in a signal transmitted by the base station.

13. The mobile station of claim 11, further comprising means for storing one or more pre-programmed predetermined parameters.

14. The mobile station of any of claims 9 to 13, further comprising:

second means for comparing the received signal power of the base stations in the active set with a second threshold;

means for identifying base stations in an active set, the identified base stations having received signal power less than a second threshold for a predetermined period of time; and

means for sending a signal to the controller indicating that the identified base station is suitable for deletion from the active set.

15. The mobile station of claim 14, wherein said second means for comparing comprises means for dynamically calculating a second threshold.

16. The mobile station of claim 15, further comprising means for receiving one or more predetermined parameters in a signal transmitted by the base station.

17. The mobile station of claim 16, further comprising means for storing one or more pre-programmed predetermined parameters.

18. A mobile station as claimed in any one of claims 14 to 17, wherein base stations identified as being suitable for deletion from the active set are transferred to a set of neighbouring base stations.