Abstract:
Apparatus for a transmitter and a receiver which enhance the performance of a system coherent demodulation by utilizing non-pilot sub-channels to enhance the accuracy of estimates of amplitude and phase noise inherent in the transmission channel is described. This enhancement is accomplished by utilizing the corrected received data on a fundamental channel to enhance a pilot channel estimate, which is subsequently utilized by a dot product module in demodulating a supplementary data channel.

Description:
CROSS REFERENCE 
     This application is a continuation of Ser. No. 09/310,232, filed May 12, 1999, now U.S. Pat. No. 6,414,988 B1, issued Jul. 2, 2002, entitled “Amplitude and Phase Estimation Method in a Wireless Communication System,” and currently assigned to the assignee of the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The current invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method of compensating for phase and amplitude distortion of multiple signals transmitted through a single channel. 
     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), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. Techniques for distinguishing different concurrently-transmitted signals in multiple access communication systems are also known as channelization. The spread spectrum modulation technique of CDMA has significant advantages over other multiple access techniques. 
     The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, and in U.S. Pat. No. 5,751,761, entitled “SYSTEM AND METHOD FOR ORTHOGONAL SPREAD SPECTRUM SEQUENCE GENERATION IN VARIABLE DATA RATE SYSTEMS”, both assigned to the assignee of the present invention and incorporated by reference herein. Code division multiple access communications systems have been standardized in the United States in Telecommunications Industry Association TIA/EIA/IS-95-A, entitled “MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM”, hereafter referred to as IS-95 and incorporated by reference herein. 
     The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled “The cdma2000 ITU-R RTT Candidate Submission”, hereafter referred to as cdma2000 and incorporated by reference herein. A second of these proposals was issued by the European Telecommunications Standards Institute (ETSI), entitled “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission”. And a third proposal was submitted by U.S. TG 8/1 entitled “The UWC-136 Candidate Submission” (referred to herein as EDGE). The contents of these submissions is public record and is well known in the art. 
     In the CDMA demodulator structure used in some IS-95 systems, the pseudonoise (PN) chip interval defines the minimum separation two paths must have in order to be combined. Before the distinct paths can be demodulated, the relative arrival times (or offsets) of the paths in the received signal must first be determined. The demodulator performs this function by “searching” through a sequence of offsets and measuring the energy received at each offset. If the energy associated with a potential offset exceeds a certain threshold, a demodulation element, or “finger” may be assigned to that offset. The signal present at that path offset can then be summed with the contributions of other fingers at their respective offsets. The use of CDMA searchers is disclosed in U.S. Pat. No. 5,764,687, entitled “MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein. 
     In the CDMA receiver structure used in some IS-95 systems, data passing from transmitter to receiver is divided into frames which are transmitted at fixed time intervals. Depending on the varying amount of data to be transmitted during each interval, the transmitter places the data into one of several sizes of frame. Since each of these frame sizes corresponds to a different data rate, the frames are often referred to variable-rate frames. The receiver in such a system must determine the rate of each received frame to properly interpret the data carried within the received frame. Such rate determination methods often include the generation of frame quality metrics, which may be used to assess the level of uncertainty associated with the determined frame rate. Methods of performing rate determination and generating frame quality metrics are disclosed in U.S. Pat. No. 5,751,725, entitled “METHOD AND APPARATUS FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein. 
     Signals in a CDMA system may be complex PN spread as described in U.S. patent application Ser. No. 08/856,428, entitled “REDUCED PEAK TO AVERAGE TRANSMIT POWER HIGH DATA RATE IN A CDMA WIRELESS COMMUNICATION SYSTEM,” filed Apr. 9, 1996, assigned to the assignee of the present invention and incorporated by reference herein, and in accordance with the following equations: 
     
       
           I=I′PN   I   +Q′PN   Q   (1)  
       
     
     
       
           Q=I′PN   Q   −Q′PN   I .  (2)  
       
     
     where PN I  and PN Q  are distinct PN spreading codes and I′ and Q′ are two channels being spread at the transmitter. 
     As described in cdma2000, transmission signals are constructed utilizing orthogonal Walsh coding, with one Walsh code used to transmit a pilot sub-channel signal. The orthogonal Walsh sub-channels used to construct such transmission signals are added together before being transmitted, and travel through the same transmission channels or pathways before being received at the receiver. Each transmission channel, by its inherent nature, alters the phase and amplitude of the signals passing through it, and also adds a component of thermal noise. These channel characteristics change with any movement by transmitter or receiver, but may vary over time even when both receiver and transmitter are stationary. Channel characteristics generally change very slowly compared with the data symbols transmitted through the channel. 
     Some CDMA receivers employ circuits which estimate the phase and amplitude distortion of the channel. These estimates are then used to compensate for channel distortion, enabling more accurate decoding and demodulation of the received signals. One such circuit for estimating phase and amplitude of a channel, and performing a dot product of that output with the demodulated data signal, is described in detail in U.S. Pat. No. 5,506,865, entitled “PILOT CARRIER DOT PRODUCT CIRCUIT”, assigned to the assignee of the present invention and incorporated by reference herein. In that described implementation, an all-zero pilot channel is received and used to estimate the channel characteristics. The resultant channel estimates are then used to convert demodulated signals to scalar digital values. 
     All CDMA signals transmitted on orthogonal sub-channels cause mutual interference to each other, as well as acting as jammers for adjacent cell areas. To enable coherent demodulation of orthogonal sub-channel signals, one sub-channel is often dedicated as a pilot carrier. As detailed in aforementioned U.S. Pat. No. 5,506,865, the pilot carrier is used in the receiver to produce estimates of the channel characteristics. The accuracy of these channel estimates is dependent on the strength of the pilot channel signal. Unfortunately, the pilot channel carries no data, so it is desirable to minimize the pilot transmit power. Conventionally the pilot power relative to the data signal power is selected by balancing between these two factors such that the best overall system performance can be achieved. For this reason, a method of producing accurate channel estimates which does not require increased pilot signal strength is highly desirable. 
     SUMMARY OF THE INVENTION 
     The present invention describes a method and apparatus for improving the performance of a receiver that receives multiple sub-channel signals transmitted together through a common propagation path, also called a transmission channel. In order to compensate for phase and amplitude distortion introduced into the signals by the transmission channel, the receiver uses a pilot sub-channel signal to estimate the phase and amplitude distortion of the transmission channel. The process of estimating of distortion inherent in the transmission channel is called channel estimation, which is used to produce channel estimates. The invention includes a novel method of utilizing data carrying sub-channels (not the pilot sub-channel) to improve the accuracy of channel estimates. The present invention is applicable to any communication system employing simultaneous transmission of multiple sub-channels and coherent demodulation. 
     The sub-channel signals within an information signal may be either time division multiplexed (TDMed) or code division multiplexed (CDMed). The exemplary embodiment describes the present invention in the context of the reverse link proposed in cdma2000. Because of overriding commonalties in channel structure, the present invention is equally applicable to reception of the reverse link transmissions according to the candidate submission proposed by the European Telecommunications Standards Institute (ETSI), entitled “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission” (hereafter WCDMA). Moreover, the present invention is equally applicable to reception of the forward link of these systems. 
     In cdma2000, the data-bearing sub-channels include a high data rate (e.g. 76.8 kbps) supplemental channel and a low data rate (e.g. 9.6 kbps) fundamental channel. The nominal power of the pilot channel is optimized for demodulation of the fundamental channel (e.g., ¼ of the fundamental channel power). In order to enable proper demodulation of the high data rate supplemental channel, the cdma2000 standard proposes to increase the pilot power beyond nominal levels when the supplemental channel is in use. In addition, the cdma2000 standard proposed to use different levels of pilot power depending on which of several available data rates the supplemental channel is using. 
     Varying the pilot power according to data rate causes other difficulties in system design. For example, it requires the receiver to know the data rate in advance in order for the power control loop to behave correctly. This also makes the selection of searching/finger locking more difficult. Moreover, it is desirable to reduce the pilot overhead to improve overall system performance if it can be done without sacrificing demodulation performance. 
     By enabling the formation of channel estimates based on the fundamental channel signal, the present invention enables a system to achieve superior supplementary channel demodulation performance. If enough channel estimate information can be extracted from the fundamental channel, acceptable supplementary channel demodulation performance may be achieved without varying the pilot power at all. Because the fundamental signal can be transmitted with as much as 4 times the power of the pilot signal, a channel estimate formed using both signals is much more accurate than an estimate based on the pilot signal alone. Subsequent demodulation using the more accurate channel estimate will have improved performance as well. 
     In cdma2000, the transmit power of the fundamental channel is four times that of the nominal pilot. The combined power of the pilot and fundamental channels would be five times the power of just the nominal pilot channel. A combined channel estimate derived from both the nominal pilot and fundamental channels would be accurate enough for demodulating a cdma2000 supplemental channel. Though increasing the pilot power whenever the supplemental channel is in use would still be an option, it may not be necessary given the enhanced accuracy of the combined channel estimate. 
     The added accuracy of a channel estimate extracted from the received fundamental channel depends on the use of a correct reference signal, which is optimally identical to the transmitted fundamental channel signal. Any inaccuracy in the decoded symbols used in forming fundamental channel estimates will degrade the quality of the combined channel estimate. Though the supplemental channel is likely to be a packet data channel, which has a high tolerance for frame errors, it may still be desirable to minimize the frame error rate when demodulating the supplemental channel. 
     In the preferred embodiment of the invention, the received fundamental channel signal is first deinterleaved and forward error correction (FEC) decoded to take advantage of the transmitter&#39;s complementary FEC encoding and interleaving functions. Then, the corrected symbol stream is re-encoded and re-interleaved to produce an ideal replica of the transmitted signal for use as a reference signal by the channel estimator. 
     In an alternative embodiment of the invention, fundamental channel power is increased as necessary to reduce the fundamental channel error rate. Because decreasing the fundamental channel error rate produces a more accurate channel estimate, increasing fundamental channel power also results in a reduced error rate when demodulating the supplemental channel. When the data rate ratio between the supplemental and the fundamental channels is large, a slight increase in fundamental channel power has little effect on the total transmitted power and hence causes little degradation. 
     In a more general sense, the present invention can be used where a single channel of information is transmitted. In an alternate embodiment using a single data channel, the channel is artificially split into two physical channels, which are transmitted synchronously at different data rates. Upon receipt, the low rate channel is first demodulated and decoded using pilot based channel estimates. The decoded bits are then re-encoded and used to improve the channel estimates used to coherently demodulate the high data rate supplemental channel. This scheme may enable data throughput which draws nearer to the theoretical capacity limit in a fading environment. 
    
    
     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 basic components of a wireless communication system incorporating an embodiment of the invention. 
     FIG. 2 is a block diagram of a preferred embodiment of the invention in a wireless transmitter. 
     FIG. 3 is a block diagram of a preferred embodiment of the invention in a wireless receiver. 
     FIG. 4 is a block diagram of an exemplary channel estimator circuit. 
     FIG. 5 is a block diagram of an exemplary channel estimate combiner. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows the present invention in the context of a wireless communication system. In the exemplary embodiment, subscriber station  2  transmits several code division multiplexed signals through a transmission channel  8  to a base station transceiver subsystem (BTS)  4  through receive antenna  6 . In the exemplary embodiment of a cdma2000 or WCDMA reverse link, the code division multiplexed channels are distinguished from one another using orthogonal coding. This method of providing orthogonal coding is described in detail in aforementioned copending U.S. patent application Ser. No. 08/856,428. 
     In the exemplary embodiment, the three types of CDMA signals transmitted from subscriber station  2  to base station transceiver subsystem  4  are pilot  10 , fundamental  12 , and supplemental  14 . In the exemplary embodiment, the signals transmitted from subscriber station  2  are code division multiple access communication signals including a pilot channel, a fundamental channel, and a supplemental channel, as defined in cdma2000. The generation and transmission of code division multiple access communication signals is well known in the art and is described in detail in the aforementioned U.S. Pat. No. 5,103,459 and in the IS-95 specification. 
     The subscriber station  2  is shown as a mobile station, but could also be a wireless modem, wireless local loop subscriber station, a BTS, or any other wireless communication equipment which transmits multiple synchronous sub-channels. The receiver station  4  is shown as a BTS, but could also be a wireless subscriber station or any other receiver which coherently demodulates multiple sub-channels. The method and apparatus for simultaneously receiving multiple transmissions is well known in the art. In the exemplary embodiment, the signals transmitted from subscriber station  2  are received at BTS  4  using a RAKE receiver, the implementation of which is well known in the art and is described in the aforementioned U.S. Pat. No. 5,109,390. 
     FIG. 2 shows subscriber station  2  capable of transmitting multiple synchronous sub-channels in accordance with one embodiment of the invention. In FIG. 2, pilot, supplemental, and fundamental channel signals are produced for transmission on orthogonal sub-channels. 
     The pilot channel is a known, constant transmitted waveform, and therefore carries no data. For this reason, forward error correction and interleaving are unnecessary on the pilot channel. The pilot channel is sent directly into a Walsh spreader  110  which spreads the data according to a pilot channel Walsh function W P , thus producing a Walsh covered pilot channel signal. The Walsh covered pilot channel signal is then sent to a relative gain module  116 , which adjusts the amplitude of the covered pilot channel signal relative to the signals carried by other orthogonal transmit sub-channels. In the preferred embodiment, the pilot channel Walsh function is the all-zero Walsh code, the pilot channel Walsh spreader  110  is omitted, and a DC signal is sent directly into relative gain module  116 . 
     The fundamental channel data is first sent to a forward error correction (FEC) encoder  102 , which produces an encoded fundamental channel signal. The resultant encoded fundamental channel signal is sent to an interleaver  106 , which produces an interleaved fundamental channel signal. The interleaved fundamental channel signal is then sent to the Walsh spreader  112 , which spreads the data according to a fundamental channel Walsh function W F , thus producing a covered fundamental channel signal. The covered fundamental channel signal is then sent to a relative gain module  118 , which adjusts the amplitude of the covered fundamental channel signal relative to the signals carried by other orthogonal transmit sub-channels. 
     The supplemental channel data is first sent to a forward error correction (FEC) encoder  104 , which produces an encoded supplemental channel signal. The resultant encoded supplemental channel signal is sent to an interleaver  108 , which produces an interleaved supplemental channel signal. The interleaved supplemental channel signal is then sent to the Walsh spreader  114 , which spreads the data according to a supplemental channel Walsh function W S , thus producing a covered supplemental channel signal. The covered supplemental channel signal is then sent to a relative gain module  120 , which adjusts the amplitude of the covered supplemental channel signal relative to the signals carried by other orthogonal transmit sub-channels. 
     Though the preferred embodiment shown uses orthogonal Walsh functions to accomplish sub-channel coding, one skilled in the art will appreciate that the sub-channel coding could also be accomplished using TDMA or PN coding without departing from the current invention. In an embodiment utilizing PN coding, the reference signals W S , W P , and W F  are replaced by PN codes corresponding to the supplemental, pilot, and fundamental channels respectively. 
     One skilled in the art will appreciate that the FEC modules  102  and  104  could employ any of a number of forward error correction techniques without departing from the current invention. Such techniques include turbo-code encoding, convolutional coding, or other form of coding such as block coding. In addition, the interleavers  106  and  108  could utilize any of a number of interleaving techniques, including convolutional interleaving, turbo-interleaving, block interleaving and bit reversal interleaving. Turbo code encoders and turbo interleavers are described in aforementioned cdma2000 specification. 
     The output of each relative gain module  116 ,  118 , and  120  is then sent to the PN spreader module  122 . The output of the PN spreader module  122  is then sent to transmitter  124 . Transmitter  124  provides additional control of transmit gain by varying the gain of the entire composite signal received from PN spreader module  122  before transmitting the signal through antenna  126 . 
     In an alternative embodiment, the optional relative gain module  116  is omitted, and the pilot signal is sent directly to the PN spreader module  122 . The gains of other channels are adjusted with respect to the gain of the pilot channel. One skilled in the art will appreciate that the two methods of controlling relative gains of the channels, using the system including relative gain module  116  or without relative gain module  116 , are functionally equivalent. 
     One skilled in the art will appreciate that any sub-channel signal may be “turned off” by causing its effective transmit gain to equal zero. This may be accomplished by so configuring its respective relative gain module  116 , 118 , or  120 . The same result may be obtained by discontinuing the progress of the sub-channel signal through the PN spreader, such as with a logic switch. One skilled in the art will appreciate that one may use either method of setting a sub-channel&#39;s effective transmit gain to zero without departing from the present invention. 
     PN spreader  122  spreads the orthogonal channel signals using a pseudorandom generated spreading sequence and sends the resultant composite signal to the transmitter  124  for transmission through the antenna  126 . In the preferred embodiment, the PN spreader  122  utilizes complex PN spreading, as described in aforementioned U.S. patent application Ser. No. 08/856,428. As shown in FIG. 33 of aforementioned cdma2000 specification, the PN spreader  122  may additionally rotate the signals of the fundamental and supplemental channel outputs of gain modules  118  and  120  by 90 degrees relative to the pilot channel signal output by gain module  116  prior to performing PN spreading. 
     One skilled in the art will appreciate that PN spreader  122  could produce one complex spread signal for each input signal, allowing relative gain modules  116 ,  118 , and  120 , to be placed after PN spreader  122  and before 
     In an alternative embodiment, the relative gains applied by relative gain modules  116 ,  118 , and  120  are controlled dynamically by gain control processor  128 . The gain of each module may be altered according to data rates of the channels. For example, the pilot channel gain may be increased when data is being transmitted on both the fundamental and the supplemental channel. Or, the fundamental channel gain may be increased when data is being transmitted on the supplemental channel. 
     FIG. 3 shows a preferred embodiment of the invention as used in a wireless receiver. The composite signal containing three orthogonal sub-channels is received through the antenna  200  and is downconverted in the receiver  202 . The resultant downconverted signal is then sent to the complex PN despreader  204  to produce I and Q component samples used in subsequent processing. Complex PN despreader operates in accordance with aforementioned in U.S. patent application Ser. No. 08/856,428. The operation of fundamental channel estimation apparatus  250 , pilot channel estimation apparatus  252 , and channel estimate combiner  230  are explained in detail below. 
     The I and Q component samples are sent to a Walsh despreader  206 , which uses the same Walsh function W F  used to spread the fundamental channels in the Walsh spreader  112 . A Walsh despreader contains The Walsh despreader  206  produces I and Q components for the decovered fundamental channel. 
     The I and Q component samples are sent to a Walsh despreader  206 , which uses the same Walsh function WE used to spread the fundamental channels in the Walsh spreader  112 . The Walsh despreader  206  produces I and Q components for the decovered fundamental channel. 
     FIG. 4 shows an exemplary embodiment of a channel estimator  218 . The complex input signal is provided to channel estimator  218  as I and Q sample streams. The I samples are mixed with a reference signal in mixer  302   a , to extract a real component of the complex input signal. The output of mixer  302   a  is provided to noise rejection filter  304   a  to remove noise from the extracted real component. In mixer  302   b , The Q samples are mixed with the same reference signal as used in mixer  302   a  in order to extract an imaginary component of the complex input signal. The output of mixer  302   b  is provided to noise rejection filter  304   b  to remove noise from the extracted imaginary component. One skilled in the art will appreciate that the noise rejection filters  304  may be implemented as low-pass filters, matched filters, or accumulators without departing from the current invention. 
     The reference signal used in a channel estimator  218  could be real, imaginary, or complex. In an alternative embodiment of a channel estimator  218  appropriate for use with a complex reference signal, mixers  302  are complex multipliers (which may also be called complex mixers), each having both real and imaginary outputs. The real outputs of mixers  302  are then summed before being filtered in real-component filter  304   a . The imaginary outputs of mixers  302  are summed before being filtered in imaginary-component filter  304   b . In the same fashion, complex multipliers could be used in a Walsh spreader or despreader to allow the use of complex Walsh codes as reference functions during spreading and despreading. Walsh spreading using complex Walsh codes is known as complex Walsh spreading, and Walsh despreading using complex Walsh codes is known as complex Walsh despreading. 
     In the proposed cdma2000 standard, the pilot channel is transmitted 90 degrees out of phase with the fundamental and supplemental channels. In the preferred embodiment, therefore, the pilot channel estimator  218   a  rotates its output by 90 degrees. This rotation may be accomplished in many ways, including multiplying the reference by an imaginary value, or by rotating the real and imaginary outputs of noise rejection filters  304 . The same end result may also be accomplished by rotating the signals of the fundamental and supplementary channels without departing from the current invention. Also, the relative rotation of the pilot channel in relation to the fundamental and supplementary channels may be positive or negative without departing from the current invention. 
     Together, the extracted real and imaginary components constitute a channel estimate vector containing amplitude and phase information for any signal component which correlates with the reference signal. The quality of the channel estimate depends on the degree of correlation between the received complex input signal and the reference signal. To achieve the highest degree of correlation between the received complex input signal and the reference signal, the reference signal used by the receiver must exactly match that transmitted by the transmitter, for example Walsh code W p  in the case of the pilot channel. Any difference between the reference signal and the transmitted signal can cause inaccuracy in the channel estimate. 
     In an IS-95 system, the pilot Walsh code W p  is an all-zero Walsh code, in which case a channel estimate can be made using just a pair of filters, as is described in aforementioned U.S. Pat. No. 5,506,865. In this case, pilot channel Walsh spreader  110  is omitted from the transmitter. The channel estimator in the receiver could then be implemented such that the mixers  302  could be omitted from pilot channel estimator  218   a . A channel estimator for an all-zero Walsh code pilot, consisting of filters without mixers, is also known as a pilot filter. The embodiment of the channel estimator depicted in FIG. 4, however, allows the use of a pilot Walsh code other than the all-zero Walsh code. 
     Together, the Pilot I and Pilot Q signals are used as an estimate of the amplitude and phase characteristics of the ODMA transmission channel  8 . The resultant Pilot I and Pilot Q, along with the decovered fundamental channel I and Q components are provided to dot product module  208 . Dot product module  208  which computes the scalar projection of the fundamental channel signal onto the pilot channel estimate vector, in accordance with the circuit described in aforementioned U.S. Pat. No. 5,506,865. Because the pilot channel signal  10 , the fundamental channel signal  12 , and the supplemental channel signal  14  have traversed the same propagation path  8 , the channel induced phase error is the same for all three signals. 
     This phase error is removed by performing the dot product operation described in aforementioned U.S. Pat. 5,506,865. In the exemplary embodiment, the fundamental channel is coherently demodulated in a dot product module  208  using a pilot channel estimate. The dot product module produces a scalar signal for each symbol period, which is indicative of the magnitude of the fundamental channel signal that is in phase with the pilot signal received through the transmission channel  8 . 
     The fundamental channel symbols output by the dot product module  208  is then sent into deinterleaver  210 , which performs the inverse of the function of transmit interleaver  106 . The resultant deinterleaved signal is then sent to forward error correction (FEC) decoder  212 . Decoder  212  performs the inverse function of the FEC encoder  102  and outputs a forward error corrected signal. 
     The corrected signal output by decoder  212  is also sent to an encoder  224 , which re-encodes the signal using the same FEC function as the transmitter FEC encoder  102 . In this way, encoder  224  produces an ideal representation of the transmitted fundamental signal. This ideal representation is then sent to an interleaver  226 , which performs the same function as the transmitter interleaver  106 , producing an ideal representation of the interleaved fundamental channel data transmitted by subscriber station  2 . 
     The I and Q component samples produced by Walsh despreader are also input into delays  220 , which produce I and Q components which are synchronized with the output of the interleaver  226 . Delays  220  are designed to compensate for the delays introduced by the dot product module  208 , the deinterleaver  210 , the decoder  212 , the encoder  224 , and the interleaver  226 . 
     The synchronized I and Q components output by delays  220  are then sent, along with the output of interleaver  226 , into channel estimator  218   b . Channel estimator  218   b  uses the output of interleaver  226  as a reference signal, and uses the outputs of delays  220  as the I and Q sample stream from which it forms a channel estimate output. 
     The corrected bits output by FEC decoder  212  are re-encoded and re-interleaved to produce a reference signal which has a higher probability of matching what was actually transmitted on the fundamental channel. By using this more reliable reference signal as input for channel estimator  218   b , the accuracy of fundamental channel estimates produced by channel estimator  218   b  is improved. 
     In a suboptimal embodiment, instead of using deinterleaver  210 , decoder  212 , encoder  224 , and interleaver  226  to create an ideal representation of the fundamental channel signal, the output of dot product module  208  could be provided directly to channel estimator  218   b . In this case, delay elements  220  would only compensate for the time required to perform the dot product operation in dot product module  208 . However, the fundamental channel estimator would not gain the error correction benefits of the bypassed components. 
     The complex output components of the pilot channel estimator  218   a  are subjected to delay elements  222  to compensate for the delay inherent in performing channel estimation using the fundamental channel signal. The channel estimation parameters produced by processing of the fundamental channel is sent, along with the delayed channel estimation parameters from the delay elements  220  and  222  into channel estimate combiner  230 . Channel estimate combiner  230  combines the channel estimation data for both pilot and fundamental channel processing and produces output containing a third, combined channel estimate. As the characteristics of the transmission channel change over time, pilot channel estimator  218   a  and channel estimator  218   b  provide updated channel estimates to channel estimate combiner  230 , which updates the combined channel estimation output accordingly. 
     In the preferred embodiment, the output of decoder  212  sent to encoder  224  is additionally sent to control processor  216 . Control processor  216  produces frame rate information for each received frame of data. Control processor  216  also performs validity checking of the received frames. Control processor  216  produces a fundamental channel quality metric based on the results of its rate determination and validity checking of received data. The fundamental channel quality metric is used to assign an appropriate weighting factor to the fundamental channel estimate in relation to the weighting factor assigned to the pilot channel estimate. The fundamental channel quality metric varies based on the validity of received frames based on the correctness of the CRC. Since different rate frames may also use different numbers of CRC bits, or have varying degrees of frame error checking protection, control processor  216  may additionally vary the fundamental channel quality metric according to received frame rate. 
     Control processor  216  is also connected to encoder  224 . Control processor  216  sends frame rate information to encoder  224  for use in re-encoding the data received from decoder  212 . 
     In the exemplary embodiment, channel estimate combiner  230  is a weighted-average combiner, which produces the combined channel estimation signal by performing a weighted average of the pilot and fundamental channel estimates in accordance with the following equations: 
     
       
           R   COMB   =XR   PILOT +(1 −X ) R   FUND   (3)  
       
     
     
       
           I   COMB   =XI   PILOT +(1 −X ) I   FND   (4)  
       
     
     where R COMB  and I COMB  are the real an imaginary components of the combined channel estimate, R PILOT  and I PILOT  are the real an imaginary components of the pilot channel estimate, R FUND  and I FUND  are the real an imaginary components of the fundamental channel estimate, and X is a scaling factor. The scaling factor X has a value from 0 to 1. A scaling factor value of 1 results in a combined channel estimate which is equal to the pilot channel estimate. A scaling factor value of 0 results in a combined channel estimate which is equal to the fundamental channel estimate. The value of X represents a first multiplier, which is multiplied by the pilot channel estimate to produce a scaled channel estimate for the pilot channel. The value of (1−X) represents a second multiplier, which is multiplied by the fundamental channel estimate to produce a scaled channel estimate for the fundamental channel. The two scaled channel estimates are added together to produce the combined channel estimate. FIG. 5 shows an exemplary implementation of channel estimate combiner  230  configured according to equations (3) and (4). A pilot channel estimate multiplier  402  multiplies the scaling factor X by the real and imaginary components of the pilot estimate R PILOT  and I PILOT . The output of pilot channel estimate multiplier  402  is a scaled pilot channel estimate. A fundamental channel estimate multiplier  404  multiplies the second multiplier (1-X) by the real and imaginary components of the fundamental channel estimate R FUND  and I FUND . The output of fundamental channel estimate multiplier  404  is a scaled fundamental channel estimate. The real components of the scaled pilot channel estimate and the scaled fundamental channel estimate are summed in summer  406  to provide the real portion of the combined channel estimate R COMB . The imaginary components of the scaled pilot channel estimate and the scaled fundamental channel estimate are summed in summer  408  to provide the imaginary portion of the combined channel estimate I COMB . 
     Channel estimate combiner  230  additionally uses the fundamental channel quality metric provided by control processor  216  as a dynamic weighting factor to the channel estimates produced from the fundamental channel. When the fundamental channel quality metric indicates a high rate of frame errors, channel estimate combiner  230  increases the value of the scaling factor X. When frame errors occur, therefore, the combined channel estimate used for demodulating the supplemental channel is derived more from the pilot channel estimate and less from the fundamental channel estimate. In an alternative embodiment, a frame error causes the value of scaling factor X to be equal to 1 until a valid frame is received. 
     In an alternative embodiment of the invention, control processor  216  includes a smoothing module, which performs smoothing, or low-pass filtering, of the fundamental channel quality metric before it is sent to channel estimate combiner  230 . This smoothing helps to make the weighted average performed by channel estimate combiner  230  less susceptible to high-frequency noise inherent in the channel. 
     In yet another embodiment of the current invention, the receiver knows the relative gains used by relative gain modules  116  and  118  when transmitting the pilot and fundamental channel signals. In this embodiment, the value of X is adjusted such that the ratio of the first multiplier over the second multiplier is equal to the ratio of the transmit gain of the pilot channel over the transmit gain of the fundamental channel. 
     In the preferred embodiment, the fundamental channel quality metric provided by control processor  216  to channel estimate combiner  230  is synchronized with the reference signal provided to channel estimator  218   b . This can be accomplished by incorporating a delay or buffer into control processor  216 . Control processor  216  may also perform a smoothing function to the fundamental channel quality metric before providing it to channel estimator  218   b . In the preferred embodiment, however, the fundamental channel quality metric produced by control processor  216  is not smoothed, and may change suddenly on frame boundaries. 
     The I and Q component samples used as input to Walsh despreader  236  are sent through delay elements  232 , which serve to synchronize the output of Walsh despreader  236  with the output of channel estimate combiner  230 . Delay elements  232  could instead be placed between Walsh despreader  238  and dot product module  238  without departing from the present invention. Walsh despreader  236  uses the Walsh function W S  used by the transmitter&#39;s Walsh spreader  114 , and produces decovered supplemental channel I and Q components. These decovered supplemental channel components, along with the combined channel estimation signal from channel estimate combiner  230 , are used as input for dot product module  238 . 
     Dot product module  238  computes the magnitude of the projection of the supplemental channel signal onto the combined channel estimate vector, resulting in a scalar projection output. The output of dot product module  238  is then deinterleaved in deinterleaver  240 , which performs the inverse function of interleaver  108 . The output of deinterleaver  238  is provided to decoder  242 , which performs the inverse function of interleaver  104 . 
     Throughout the wireless receiver portrayed in FIG. 3, one skilled in the art will appreciate that any of the delay elements  220 ,  222 , or  232  could be implemented as accumulators or buffers without departing from the current invention. In addition, one skilled in the art will appreciate that pairs of delay elements, for example delay elements  232   a  and  232   b,  may be implemented separately, or combined into a single delay module which performs the same function, without departing from the current invention. 
     Though the preferred embodiment shown uses orthogonal Walsh functions to accomplish sub-channel decoding, one skilled in the art will appreciate that the sub-channel decoding could also be accomplished using TDMA or PN coding without departing from the current invention. In an embodiment utilizing PN coding, reference signals W S , W P , and W F  are replaced by PN codes corresponding to the supplemental, pilot, and fundamental channels respectively.