WO2001003357A1

WO2001003357A1 - Method and apparatus for determining a reverse link transmission rate in a wireless communication system

Info

Publication number: WO2001003357A1
Application number: PCT/US2000/018322
Authority: WO
Inventors: Paul E. Bender; Matthew Stuart Grob; Gadi Karmi; Roberto Padovani
Original assignee: Qualcomm Incorporated
Priority date: 1999-07-02
Filing date: 2000-06-30
Publication date: 2001-01-11
Also published as: NO333454B1; ATE430420T1; PT1192749E; JP4685302B2; EP1192749A1; CN100367694C; DE60034338T2; US20030112774A1; EP2077638A2; TWI226163B; AU5911500A; US6807161B2; KR20020026482A; JP4886065B2; JP2011091816A; CA2377060C; US6556549B1; ES2275524T3; NO20016409L; ES2325182T3

Abstract

Reverse link busy bits are independently generated by each base station (102, 104 and 106) and indicative of whether the transmitting base station (102, 104 and 106) has reached a reserve link capacity limit. In a first exemplary embodiment, the remote station (122) combines the multipath components of the reverse link busy bits from each of the transmitting base stations (102, 104 and 106) in its Active Set and in response transmits a reverse link signal only when all of the reverse link busy bits indicate that the base stations (102, 104 and 106) in the remote stations Active Set have reverse link capacity. In a first alternative embodiment, the remote station weights, the reverse link busy signals in accordance with the signal strength of the base station (102, 104 or 106) transmitting the busy signal and determines whether to transmit based on the weighted sum of the busy signals.

Description

METHOD AND APPARATUS FOR DETERMINING A REVERSE LINK TRANSMISSION RATE IN WIRELESS COMMUNICATION SYSTEM

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for performing signal combining during soft handoff in a wireless communication system.

II. Description of the Related Art The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Patent No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Patent No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.

CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Patent No. 5,101,501 entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Patent No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.

A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the mobile station at a base station. The base station in response to the monitored power level transmits power control bits to the mobile station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Patent No. 5,056,109, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.

There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a remote station to a central base station is to allow the remote station to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the remote station to transmit its information using a small set of orthogonal channels, this method is described in detail in copending U.S. Patent Application Serial No. 08/886,604, entitled "HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and apparatus describing the combining of signals in a high rate wireless communication system. In the exemplary embodiment, each base station in communication with a remote station transmits forward link data including traffic data, pilot symbols and overhead data. In the exemplary embodiment, the overhead data includes a reverse link busy bit, reverse link power control (RPC) commands and a forward link activity (FAC) bit. The reverse link busy bit indicates when the base station has reached its reverse link capacity limit. The RPC bit indicate to each mobile station in communication with the base station whether their transmission energy should be increased or decreased. The FAC bit is a message that indicates when a base station will have no forward link data to transmit a predetermined number of slots in the future.

In the exemplary embodiment of the present invention, the forward link traffic is only transmitted from one base station to a given remote station. Thus, there is no soft handoff of the forward link traffic data. The multipath components of the forward link traffic data are combined using a traditional RAKE receiver to provide an improved estimate of the forward link traffic data. In the exemplary embodiment of the present invention, the reverse link busy bits are independently generated by each base station and indicative of whether the transmitting base station has reached a reverse link capacity limit. In a first exemplary embodiment, the remote station combines the multipath components of the reverse link busy bits from each of the transmitting base stations in its Active Set and in response transmits a reverse link signal only when all of the reverse link busy bits indicate that the base stations in the remote stations Active Set have reverse link capacity. In a first alternative embodiment, the remote station weights the reverse link busy signals in accordance with the signal strength of the base station transmitting the busy signal and determines whether to transmit based on the weighted sum of the busy signals. In a second alternative embodiment, the remote station weights the reverse link busy signals in accordance with the signal strength of the base station transmitting the busy signal and determines a maximum reverse link data rate based on the weighted sum of the busy signals.

In the exemplary embodiment, the FAC signals are independently generated. The FAC signals from common base stations, multipath components, are soft combined and decoded. Each of the FAC signals are provided to a corresponding SNR calculator for each base station. The calculated SNR for each base station is used to determine which base station should transmit forward link data to the remote station and at what data rate.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 1 is a diagram illustrating the components and signals of a soft handoff environment; FIG. 2 is an illustration of the forward link slot format of the exemplary embodiment;

FIG. 3 is a flowchart illustrating the method of combining signals in the exemplary embodiment; FIG. 4 is a block diagram illustrating the base station transmission system of the exemplary embodiment;

FIG. 5 is a block diagram of the remote station of the present invention;

FIG. 6 is a block diagram of the traffic demodulator of the exemplary embodiment; FIG. 7 is a block diagram of the reverse link busy bit demodulator of the exemplary embodiment;

FIG. 8 is a block diagram power control demodulator of the exemplary embodiment;

FIG. 9 is a block diagram of the forward link activity (FAC) demodulator of the exemplary embodiment; and

FIG. 10 is a block diagram of the remote station transmission subsystem.

DETAILED DESCRIPTION OF THE PREFERRED

EMBODIMENTS

FIG. 1 illustrates the elements of a wireless communication system during a soft handoff operation. In the soft handoff condition illustrated in FIG. 1, mobile station 122 is in simultaneous communication with base stations 102, 104 and 106. A method and apparatus for performing soft handoff in a wireless communication system is disclosed in the aforementioned U.S. Patent No. 5,101,501. Base station controller 100 sends information to be transmitted to remote station 122 as to base stations 102, 104 and 106.

In the exemplary embodiment, the forward link traffic data is transmitted to remote station 122 by the selected base station (102, 104 or 106) with the best propagation path to remote station 122. Base stations 102, 104 and 106 transmit forward link signals, including forward link traffic, pilot symbols and overhead data on forward link signals 110, 114, and 118, respectively. In the exemplary embodiment, forward link signals 110, 114 and 118, as well multipath component signal 108, are code division multiple access (CDMA) communications signals.

Signal 108 illustrates the condition referred to as multipath, whereby the signal transmitted by base station 102 traverses two different propagation paths to remote station 122. The first signal 110 traverses a line of sight propagation path, while a second signal is reflected from an obstacle 124 as forward link signal 108. In a CDMA communications system, multipath components can be combined at the receiver to provide an improved estimate of the transmitted data as is disclosed in the aforementioned U.S. Patent No. 5,109,390. Remote station 122 transmits data to base stations 102, 104 and 106 on reverse link signals 112, 116 and 120, respectively. In the exemplary embodiment, reverse link signals 112, 116 and 120 are CDMA communications signals. The reverse link signals received by base stations 102, 104 and 106 are soft combined in base station controller (BSC) 100 to provide a better estimate of the information transmitted by remote station 122. It should be noted that reverse link signals 102, 104 and 106 are actually the same signal traversing different propagation paths.

FIG. 2 illustrates a forward link slot of the exemplary embodiment. In the exemplary embodiment, a slot is 1.66 ms in duration. The slot includes two pilot bursts 206 and 214. The second pilot burst 214 has overhead data 212 and 216 included on both sides of it. The overhead data of the exemplary embodiment includes forward link activity (FAC) information, reverse link busy bits and reverse link power control commands. The different overhead data are distinguished from one another by means of an orthogonal covering. Orthogonal coverings are well known in the art and are disclosed in the aforementioned U.S. Patent No. 5,103,459. Forward link activity information is a bit that when set indicates that a predetermined number of slots in the future, there will be no forward link traffic data to be transmitted by the base station. The reverse link busy bits indicate that the reverse link capacity limit of the base station has been reached. The power control commands are covered with unique Walsh coverings and request that a particular remote station increase or decrease its transmission energy. Forward link data is transmitted in the remainder of the frame in sections 202, 210 and 218.

FIG. 3 is a flowchart describing the received signal combining operations performed by remote station 122 when in soft handoff with a plurality of base stations. In block 250, the multipath components of the forward link signal carrying traffic data to remote station 122 are combined. In the exemplary embodiment, only the base station with the best propagation path between it and remote station 122 transmits forward link traffic data to remote station 122. If for example base station 102 has the best propagation path to remote station 122, then base station 102 transmits forward link traffic data to remote station 122. In this example, remote station 122 soft combines multipath signals 108 and 110 to provide an improved estimate of the forward link traffic data. In the exemplary embodiment, the soft combining is performed as a weighted sum wherein the weight of demodulated symbols is determined in proportion to the received signal strength of the signal carrying the symbols. The act of soft combining of multipath signals is described in detail in aforementioned U.S. Patent No. 5,109,390.

In block 252, remote station 122 soft combines multipath components of the reverse link busy bits transmitted by each base station in the Active Set of remote station 122 to provide an estimate of the reverse link busy bit transmitted by each base station. It should be noted that the power control commands from different base stations may have different values and so cannot be combined meaningfully. That is base station 102 may have exhausted its reverse link capacity while base station 104 may still have remaining reverse link capacity, and as such would transmit reverse link busy bits having different values. In block 254, The reverse link busy bits from each of base stations 102,

104 and 106 are combined to determine a maximum data rate for the next reverse link transmission by remote station 122. In a first exemplary embodiment, the remote station transmits a reverse link signal only when all of the reverse link busy bits indicate that the base stations in the Active Set have additional reverse link capacity. In a first alternative embodiment, the remote station 122 weights the reverse link busy bits in accordance with the signal strength of the base station transmitting the busy bit and determines whether to inhibit it reverse link transmissions based on the weighted sum of the busy bits. In a second alternative embodiment, the remote station weights the reverse link busy bits in accordance with the signal strength of the base station transmitting the busy bit and determines a maximum reverse link data rate at which to transmit based on the weighted sum of the busy bits.

In block 256, remote station 122 soft combines the multipath components of the reverse power control bits transmitted by each base station to provide an estimate of the reverse power control bits transmitted by each base station. It should be noted that the power control commands from different base stations may not be the same value and so cannot be combined meaningfully. For example, reverse link signal travelling 114 may exceed the energy necessary for reliable transmission of signals to base station 104, while simultaneously the energy of the reverse link signal 112 may be inadequate for reliable reception by base station 102. In this case, base station 104 would transmit an "Up" command, while base station 104 would transmit a "Down" command. Thus, soft combining of power control commands from different base stations should not be performed. In the exemplary embodiment, for each base station, a hard decision regarding the value of its power control command is determined. Proceeding to block 258, in the exemplary embodiment, remote station 122 increases its transmission energy only when all of the power control commands transmitted by the base stations in its Active Set request remote station 122 to increase its transmission energy.

In block 260, the forward link activity bits (FAC) received on multiple paths from common base stations are soft combined. In block 262, each of the combined forward activity bits are then provided to a corresponding SNR calculator which uses the information in its computation of the signal to noise ratio energy for a corresponding base station in the Active Set of remote station 122. Referring back to FIG. 2, if the slot does not include data then the estimated signal to noise ratio computation for the slot must be adjusted to account for this gated portion of the frame during which no signal energy is present.

FIG. 4 is a block diagram illustrating the elements of base stations 102, 104 and 106. Forward link traffic data is provided to Walsh spreading element 300 and is covered in accordance with Walsh code (W_τ). The covered traffic data is then provided to multiplexer 312. It will be understood by one skilled in the art that processing of the signal prior to its provision to Walsh spreading element 300 is within the scope of the present invention. In particular, it is anticipated that the forward link traffic data will be forward error correction coded using a convolutional encoder, turbo coder or other forward error correction coder that is known in the art. In the exemplary embodiment, thirty two Walsh sequences of length thirty two are used to cover the forward link transmissions. Generation of and spreading in accordance with Walsh codes is disclosed in aforementioned U.S. Patent No. 5,103,459.

A predetermined set of pilot symbols, typically all ones, is provided to Walsh spreading element 302 and, in the exemplary embodiment, covered in accordance with Walsh code zero (W₀). Covering by Walsh zero is a no op and may operationally be omitted but is provided for illustrative purposes. The covered pilot symbols are then provided to multiplexer 312.

The forward activity (FAC) bit is provided to spreading element 304 and covered in accordance with Walsh code one W_r The reverse link busy bit is provided to Walsh spreading element 306 and covered using Walsh code seventeen (W,₇). In addition, up to twenty eight power control commands (PC,- PC₂₉) are provided to Walsh spreading elements 308a-308n and are covered using Walsh sequences (W₂-W_]5 and W₁₈-W₃₁). The Walsh spread overhead bits including the FAC, the reverse link busy bit and the power control commands are summed in summer 310 and provided to multiplexer 312.

Multiplexer 312 inserts into the slot the forward link traffic data and two pilot bursts with the second pilot burst having the overhead bits on either side of it. In the exemplary embodiment, the overhead information on both sides of the second pilot burst are replicas of one another and each are 64 Walsh chips in duration spread using thirty two bit Walsh codes providing four redundant versions of each piece of overhead information.

The slot, including the forward link traffic, the pilot bursts and overhead bits, as illustrated in FIG. 2 are provided to PN spreader 314. In the exemplary embodiment, each base station spreads the data for transmission using a different PN sequence. In the preferred embodiment, each base station generates its PN sequence using different phase offsets of generate using a common PN generator polynomial as is described in the aforementioned U.S. Patent 5,103,459. In the preferred embodiment, the data is transmitted in accordance with a QPSK modulation wherein the in-phase and quadrature phase components are spread using to different pseudonoise sequences (PN, and PN_Q). The PN spread signal is provided to transmitter (TMTR) 316 which up converts, amplifies and filters the signal for transmission through antenna 318.

FIG. 5 illustrates remote station 122 of the present invention. The forward link signal is received at antenna 500 and provided through duplexer 502 to receiver (RCVR) 504. The received signal is provided to traffic demodulator 506, which demodulates the received signal to provide the forward link traffic data to the user of the remote station.

The received signal is provided to reverse link busy demodulator 508 which demodulates the signal to provide an estimate of the reverse link busy bits transmitted by each of the base stations in communication with remote station 122. The reverse link busy bits are provided to rate determination element 510. In the exemplary embodiment, rate determination element 510, inhibits the transmission of the reverse link signal when any of the busy bits from a base station in the Active Set indicate that the reverse link capacity limit for that base station has been reached. In an alternative embodiment, rate determination element 510 selectively inhibits the reverse link transmissions based on a weighted sum of the received busy bits from the base stations in the Active Set of remote station 122. In the first alternative embodiment, the received busy bits are weighted in accordance with the energy of the received signals. In a second alternative embodiment, rate determination element 510 selects a maximum reverse link data rate based on the received busy bits. For example, if the signal from a base station indicating that it has reached reverse link capacity is very weak, rate determination element 510 may select a non zero reverse link data rate that it estimates will not cause undue interference to the base station due to its poor propagation path to that base station. A signal indicative of either the maximum data rate or an inhibition of the reverse link signal is provided to transmit control processor 520, which determines a set of parameters for transmitting the reverse link signal.

In the preferred embodiment, the mobile station is aware of a transmission rate profile for the base stations in its Active Set in which each of its potential reverse links transmission rates has a known probability of successful transmission under the condition that the base stations in the Active Set are not in a capacity limit condition. In the preferred embodiment, remote station 122 computes a metric referred to herein as a Derating Metric (DM) in accordance with the equation:

DM = \- l ∑SNR RLB_; (1)

where SNR, is the estimated signal to noise ratio of the ith base station, Max SNR, is the maximum signal to noise ratio of the base stations in the Active Set of remote station i, RLB_ι is the value of the reverse link busy bit for the ith base station in the Active set which takes a value of 0 or 1. Using equation 1, the stronger the forward link signal from a base station transmitting a reverse link busy bit indicating a reverse link capacity limit condition, the greater will be the derating. This derating metric assumes a value of between 0 and 1 which is used to scale the transmission rate profile such that rates are reduced for a given probability of successful transmission.

The reverse link signal is also provided to reverse link power control demodulator 512. Reverse link power control demodulator 512 demodulates the received signal and combines the multipath components from common base stations to generate improved estimates of the reverse link power control command transmitted by each of the base stations in the Active Set of remote station 122. In the exemplary embodiment, each remote station in communication with a given base station demodulates its reverse link power control commands in accordance with a unique Walsh code assigned to that mobile station. It should be noted that the reverse link power control Walsh codes assigned to the remote station may be different for different base stations in communication with remote station 122. The improved estimates of the power control commands from each base station are provided to power control combiner 518. In the exemplary embodiment, remote station 122 increases its transmission energy only when all base stations in the Active Set of remote station 122 transmit power control commands requesting remote station 122 to increase its transmission energy. Otherwise, remote station 122 decreases its transmission energy. In addition, the present invention is equally applicable to multi-bit power control systems wherein the base station specifies the amount of the transmission energy adjustment requested. In the simplest implementation of power control combiner 514 for use in a multi-bit power control system, power control combiner 514 selects the smallest requested increase or largest requested decrease in transmission energy.

FAC combiner 518 combines the FAC bits from multipath components of the forward link signal of a common base station to provide an improved estimate of the FAC bit transmitted by each of the base stations. Transmit control processor 520 receives each of the FAC bit estimates and adjusts the computation of the signal to noise ratio for each base station based on the estimate of the FAC bit transmitted by that base station. Transmit control processor 520 uses the calculated signal to noise ratio of each of the base stations to select the base station with the best propagation path and to determine the maximum data rate of the transmission.

Based on the estimates of the reverse link busy bits, the reverse link power control commands, and the forward activity bits, transmit control processor 520 determines the rate of its next reverse link transmission, an adjustment to its reverse link transmission energy and selects the base station with the best propagation path and the maximum forward link data rate that can be reliably transmitted upon that propagation path. These parameters are provided to transmit subsystem 522 which generates the reverse link signal in accordance therewith. The reverse link signal from transmit subsystem 522 is provided through duplexer 502 for transmission through antenna 500.

FIG. 6 illustrates the elements of traffic demodulator 506. Searcher 600 searches potential PN offsets for strong forward link signals. Searcher 600 assigns PN despreaders 602 PN offsets to demodulate. In the exemplary embodiment, each of PN despreaders 602 despreads the received signal in accordance with a different PN offset and provides the result to a corresponding demultiplexer 604. In the exemplary embodiment, PN despreader 602 despreads the received signal in accordance with a single PN sequence used to spread a BPSK signal. However, the present invention is equally applicable to complex PN despreaders that use two distinct PN code sequences (PN, and PN_Q) to complex despread a QPSK signal. The implementation of PN despreader 602 is well known in the art for both PN despreading of a BPSK signal and complex PN despreading of a QPSK signal. Demultiplexer 604 separates the pilot burst portion of the received signal and provides the demodulated pilot symbols to synchronization (SYNC) element 606. Synchronization element 606 determines adjustments to the frequency and phase of a corresponding Walsh demodulator 608. A signal indicative of the adjustments to the phase and frequency are provided to Walsh demodulators 608.

Demultiplexer 604 separates out the portions of the slot carrying forward link traffic data and provides those portions to Walsh demodulator 608. Walsh demodulator 608 demodulates the receive signal in accordance with the Walsh sequence W_τ. The implementation of Walsh demodulator 608 is well known in the art and described in detail in U.S. Patent No. 5,103,459.

The demodulated forward link symbols are provided to soft combiner 610 which accumulates the multipath components of the base station transmitting the forward link traffic data to remote station 122. The accumulated demodulated symbol energies are then provided to decoder 612 which decodes the forward traffic data and provides the decoded symbols to the user of remote station 122. In the exemplary embodiment, decoder 612 is either a trellis decoder, such as a Viterbi decoder, or a turbo decoder.

FIG. 7 illustrates the elements of reverse link busy bit demodulator 508. As described with respect to FIG. 6, searcher 600 searches potential PN offsets for strong forward link signals. Searcher 600 assigns PN offsets to each PN despreaders 602. As described above, each of PN despreaders 602 despreads the received signal in accordance with a different PN offset and provides the result to a corresponding demultiplexer 704.

Demultiplexer 704 separates the pilot burst portion of the slot and provides the pilot symbols to synchronization (SYNC) element 706. Synchronization element 706 determines adjustments to the frequency and phase of a corresponding Walsh demodulator 708. A signal indicative of the adjustments to the phase and frequency are provided to Walsh demodulators 708. It will be understood by one skilled in the art that synchronization elements 706 and synchronization elements 606 perform identical operations and are shown as distinct elements for illustrative purposes only.

Demultiplexer 704 separates out the portions of overhead data from the received slot and provides those portions to Walsh demodulator 708. In the exemplary embodiment, Walsh demodulator 708 demodulates the received signal in accordance with the Walsh code W₁₇.

The demodulated forward link symbols are provided to soft combiner 710 which accumulates the multipath symbols from each of the base stations. The accumulated symbol energies are then provided to rate determination logic 510 which operates as described previously.

FIG. 8 illustrates the elements of reverse link power control demodulator 512. As described with respect to FIG. 6, searcher 600 searches potential PN offsets for strong forward link signals. Searcher 600 assigns PN offsets to each PN despreaders 602. As described above, in the exemplary embodiment, each of PN despreaders 602 despreads the received signal in accordance with a different PN offset and provides the result to a corresponding demultiplexer 804.

Demultiplexer 804 separates the pilot burst portion of the slot and provides the pilot symbols to synchronization (SYNC) element 806. Synchronization element 806 determines adjustments to the frequency and phase of a corresponding Walsh demodulator 808. A signal indicative of the adjustments to the timing phase and frequency are provided to Walsh demodulators 808. It will be understood by one skilled in the art that synchronization elements 806 and synchronization elements 606 perform identical operations and are shown as distinct elements for illustrative purposes only.

Demultiplexer 804 separates out the portions of overhead data from the received slot and provides those portions to Walsh demodulator 808. In the exemplary embodiment, Walsh demodulator 808 demodulates the received signal in accordance with a Walsh code that is specific for transmission of the power control signals for a corresponding base station. For example, base station 102 may cover its power control commands to remote station 122 using Walsh code five, while base station 104 may cover its power control commands to remotes station 122 using Walsh code thirteen. Thus, the multipath components of forward link transmitted from a common base station are demodulated using a common Walsh code to extract the power control commands from that base station. Whereas, power control commands from different base stations are demodulated using different Walsh codes. The demodulated power control commands from each base station are provided to soft combiners 810 which accumulate the multipath symbols for a corresponding one of the base stations in its Active Set. The accumulated symbol energies are then provided to power control combiner 514 which operates as described previously.

FIG. 9 illustrates the elements of FAC demodulator 516. As described with respect to FIG. 6, searcher 600 searches potential PN offsets for strong forward link signals. Searcher 600 assigns PN offsets to each PN despreaders 602. As described above, in the exemplary embodiment, each of PN despreaders 602 despreads the received signal in accordance with a different PN offset and provides the result to a corresponding demultiplexer 904.

Demultiplexer 904 separates the pilot burst portion of the slot and provides that to synchronization (SYNC) element 906. Synchronization element 906 determines adjustments to the frequency and phase of a corresponding Walsh demodulator 908. A signal indicative of the adjustments to the phase and frequency are provided to Walsh demodulators 908. It will be understood by one skilled in the art that synchronization elements 906 and synchronization elements 606 perform identical operations and are shown as distinct elements for illustrative purposes only.

Demultiplexer 904 separates out the portions of overhead data from the received slot and provides those portions to Walsh demodulator 908. In the exemplary embodiment, Walsh demodulator 908 demodulates the received signal in accordance with a Walsh code one (W,). The demodulated FAC symbols from common base stations are provided to a combiner 910. Combiners 910 combine the energies of the FAC symbols to provide an improved estimate of the FAC bits for each base station in the Active Set of remote station 122. The maximum data rate from rate determination element 510, the combined power control command from power control combiner 514 and the estimated forward activity bits for each of the base stations in the Active set of remote station 122 are provided to transmit control processor 520. In accordance therewith, transmit control processor 520 determines the data rate of the next reverse link transmission, from remote station 122 generates a signal to adjust the transmission energy of the reverse link signal, selects the base station to send forward link traffic data to remote station 122 and the determines the maximum rate at which the forward link data can be reliably transmitted. FIG. 10 illustrates the elements of transmit control processor 520 and transmit subsystem 522. In transmit control processor 520, the combined power control command (PC) is provided to gain adjustment element 1000. The power control command in the exemplary embodiment is a single bit up /down command in response to which gain adjustment element 1000 generates a control signal increasing or decreasing the transmission energy of the reverse link signal by adjusting the gain of a variable gain amplifier (not shown) within transmitter (TMTR) 1010. The FAC estimates for each base station are provided to a corresponding signal to noise computers 1002. In response to the FAC bits, signal to noise computers 1002 calculate the signal to noise ratio of the forward link signals from a base station in the Active Set of remote station 122. Slots received without forward link traffic data are incorporated into the signal to noise ratio computation differently from those frames that include forward link traffic data. If the occurrence of frames without forward link traffic data is sufficiently rare, these frames may be excluded from the computation entirely. In a preferred embodiment, the signal to noise energy of frames without forward link traffic data are scaled prior to being accumulated into the signal to noise ratio computation.

The estimates of the signal to noise ratio for the forward link signal from each base station is provided from signal to noise computers 1002 to DRC control processor 1004. DRC control processor 1004 selects the base station that has the highest signal to nose ratio and determines a maximum transmission rate in accordance with the signal to noise ratio of the selected base station. A signal indicative of the identity of the selected base station and the maximum data rate is generated by DRC control processor 1004 and provided to multiplexer (MUX) 1016.

The reverse link data rate derated by the method described with respect to equation (1) is determined by rate determination element 510 and provided to reverse link controller 1006. Reverse link controller 1006 determines the rate at which to transmit its reverse link signal in accordance with this maximum data rate. In the exemplary embodiment, reverse link controller 1006 determines the reverse link data rate in accordance with the maximum data rate, the amount of data queued to be transmitted by remote station 122, and the amount of battery power remaining in remote station 122.

A signal indicative of the selected reverse link data rate is provided to message generator 1008. In response message generator 1008 generates a signal indicative of the selected reverse link data rate and provides the reverse rate indicator (RRI) message to multiplexer 1016. In addition, reverse link controller 1006 provides a signal indicative of the selected reverse link data rate to reverse link traffic processing element 1018. In response to the reverse link data rate signal, memory element 1020 in reverse link traffic processing element 1018 provides an amount of data for transmission. The data is encoded by encoder 1022. The encoding rate and encoding algorithm used by encoder 1022 may also be selected in response to the selected reverse link data rate. The encoded symbols are provided to interleaver (INT) 1024 which reorders the symbols in accordance with a predetermined interleaving format. The interleaved symbols are provided to Walsh modulator 1026.

In the exemplary embodiment, the Walsh modulation is performed using variable length Walsh sequences in which the length of the Walsh sequence (and accordingly the spreading gain) is varied inversely with rate of the reverse link transmission. The use of variable length Walsh sequences is described in detail in U.S. Patent No. 5,571,761, entitled "SYSTEM AND METHOD FOR ORTHOGONAL SPREAD SPECTRUM SEQUENCE GENERATION IN VARIABLE DATA RATE SYSTEMS", which is assigned to the assignee of the present invention and incorporated by reference herein.

The Walsh spread reverse link traffic data is provided to complex PN spreader 1012. Multiplexer 1016 multiplexes the data rate control message and the reverse rate indicator message with pilot symbols and provides the multiplexed data to Walsh modulator 1014. Walsh modulator 1014 spreads the multiplexed data in accordance with the Walsh code zero and provides the spread data to complex PN spreader 1012.

In the exemplary embodiment, the PN spreading of the reverse link signal is performed in accordance with two distinct PN sequences (PN, and PN_Q) in order to evenly distribute the load the in-phase and quadrature-phase components of the transmitted QPSK signal. The implementation of complex PN spreader 1012 is disclosed in the aforementioned copending U.S. Patent Application Serial No. 08/886,604.

The complex PN spread data is provided to transmitter 1010 which amplifies, filters and upconverts the complex PN spread signal for transmission.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, 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.

I (WE) CLAIM:

Claims

1. In a communication system in which each base station in communication with a remote station transmits a reverse link busy bit indicating whether its reverse link capacity has been exhausted, a method of determining the reverse link transmission rate of said remote station comprising the steps of: determining a reverse link transmission rate in accordance with a combined reverse link busy signal generated in accordance with reverse link busy bits transmitted by each of said base stations; and transmitting said reverse link data in accordance with said reverse link transmission rate.

2. The method of Claim 1 further comprising the step of soft combining multipath components of the reverse link busy bits from each of said base stations to provide said estimate of the reverse link busy bit transmitted by each of said base stations.

3. The method of Claim 1 wherein said step of determining said reverse link transmission rate, comprises inhibiting the transmission of said reverse link data when any of said reverse link busy bits indicate the base station is in a reverse link capacity condition.

4. The method of Claim 1 wherein said step of determining said reverse link transmission rate is performed in accordance with the values of the reverse link busy bits transmitted by each base station and the strength of the forward link signals from each base station as received by said remote station.

5. The method of Claim 1 wherein said step of determining said reverse link transmission rate, comprises the steps of: computing a derating metric in accordance with the values of the reverse link busy bits transmitted by each base station and the strength of the forward link signals from each base station as received by said remote station; and adjusting a rate transmission profile indicative of the probability of successful transmission for each potential reverse link transmission rate in accordance with said derating metric; and selecting said reverse link transmission rate in accordance with said adjusted rate transmission profile.

6. The method of Claim 5 wherein said step of computing said derating metric (DM) is performed in accordance with the equation:

where SNR, is the estimated signal to noise ratio of the ith base station, Max SNR, is the maximum signal to noise ratio of the base stations in the Active Set of remote station i, RLB_; is the value of the reverse link busy bit for the ith base station in the Active set which takes a value of 0 or 1.