HK1060669B - Method and device for performing demodulation of information signal - Google Patents

Description

Method and apparatus for demodulating information signal

Background

I. Technical Field

The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved method of compensating for phase and amplitude distortion of a plurality of signals transmitted over a single channel.

II. Description of the Prior Art

The use of Code Division Multiple Access (CDMA) modulation techniques is one of several techniques that facilitate communication among a large number of system users. There are other multiple access communication system technologies in the art, such as: time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and AM modulation schemes such as Amplitude Compression Single Sideband (ACSSB). A technique for distinguishing between different simultaneously transmitted signals in a multiple access communication system is also known as channelization. CDMA spread spectrum modulation techniques have significant advantages over other multiple access techniques.

The use of CDMA techniques in MULTIPLE ACCESS COMMUNICATION SYSTEMs is disclosed in U.S. patent No. 4,901,307, entitled "forward speech COMMUNICATION SYSTEM using a mobile station OR TERRESTRIAL REATERS," which is assigned to the assignee of the present invention and is incorporated herein by reference. The use of CDMA technology in multiple access communication SYSTEMs is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS INA CDMA CELLULAR TELEPHONE System," and U.S. Pat. No. 5,751,761, entitled "SYSTEM AND METHOD FOR the same software speech SEQUENCE DATA RATE SYSTEM," both of which are assigned to the assignee of the present invention and are incorporated herein by reference. Code division multiple access communication SYSTEMs have been standardized in the United states by the telecommunication industry Association TIA/EIA/IS-95-A entitled "Mobile STATION-BASE STATION communication FOR Dual-MODE Wireless communication System," hereafter referred to as the IS-95 standard, and incorporated herein by reference.

The recent international union for telecommunications requests the submission of the proposed method for providing high-rate data and high-quality voice services over wireless communication channels. The first recommendation was made by The "Telecommunications industry Association," entitled "The CDMA2000 ITU-R RTT conference sub," hereafter referred to as CDMA2000, and incorporated herein by reference. A second recommendation is made by the European Telecommunications Standards Institute (ETSI) entitled "ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candida Transmission". A third proposal has been made by U.S. TG 8/1 entitled "The UWC-136Candidate Transmission" (referred to herein as EDGE). These recommendations are publicly recorded files, and are well known in the art.

In some CDMA demodulator structures used in IS-95 systems, the pseudo-noise (PN) chip time interval defines the minimum separation that the two paths must have in order to be combined. Before the different 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" for the sequence of offsets and measuring the received energy at each offset. If the energy associated with a potential offset exceeds a certain threshold value, a demodulation element, or "finger," may be assigned to the offset. The signal occurring at that path offset can then be summed with the contributions at the corresponding offsets of the other fingers. The use of a CDMA searcher is disclosed in U.S. patent 5,764,687 entitled "Mobile demodulator FOR A SPREAD Spectrum MULTIPLE ACCESS System," which is assigned to the assignee of the present invention and incorporated herein by reference.

In CDMA receiver architectures used in some IS-95 systems, data communicated from a transmitter to a receiver IS divided into frames that are transmitted at fixed time intervals. The transmitter places the data in one of the frames of different sizes, depending on the amount of data to be transmitted in each time interval. Since each frame size corresponds to a different data rate, the frames are often referred to as variable rate frames. The receiver in such a system must determine the rate of each received frame in order to correctly interpret the data carried in the received frame. Typically such rate determination methods include generating a frame quality metric that can be used to estimate a degree of uncertainty associated with the determined frame rate. A METHOD OF performing rate determination and generating frame quality metrics is disclosed in U.S. patent No. 5,751,725, entitled "METHOD and apparatus FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE rate communication SYSTEM," which is assigned to the assignee OF the present invention and is incorporated herein by reference.

The possibility of complex PN spreading of signals in CDMA SYSTEMs is described in U.S. patent application 08/856,428 entitled "REDUCED PEAK TO AVERAGE TRANSMIT POWER HIGH DATA RATE INA CDMA WIRELESS COMMUNICATION SYSTEM," which is assigned TO the assignee of the present invention and incorporated herein by reference, and which uses the following equation:

I＝I’PN_I+Q’PN_Q (1)

Q＝I’PN_Q-Q’PN_I (2)

wherein, PN_IAnd PN_QAre different PN spreading codes and I 'and Q' are the two channels spread at the transmitter.

As described in cdma2000, the transmitted signal is constructed using orthogonal walsh codes, one of which is used to transmit the pilot subchannel signal. The orthogonal walsh subchannels used to form such transmitted signals are summed prior to transmission and propagated through the same transmission channel or path prior to reception at the receiver. Each transmission channel changes the phase and amplitude of the signal passing through it due to its inherent characteristics and also adds a thermal noise component. These channel characteristics change with any movement of the transmitter or receiver, but even if both the receiver and transmitter are stationary, they change over time. The channel characteristics typically change very slowly compared to the data symbols transmitted over the channel.

Some CDMA receivers use circuitry that estimates the phase and amplitude distortions of the channel. These estimates are then used to compensate for channel distortion, enabling more accurate decoding and demodulation of the received signal. One such CIRCUIT for estimating the phase and amplitude of a channel and DOT-PRODUCT the output with a demodulated data signal is described in detail in U.S. patent No. 5,506,865, entitled "PILOT CARRIER DOT PRODUCT CIRCUIT", assigned to the assignee of the present invention and incorporated herein by reference. In the described embodiment, an all-zero pilot channel is received and used to estimate the channel characteristics. The resulting channel estimate is then used to convert the demodulated signal to a scalar digital value.

All CDMA signals transmitted on orthogonal subchannels cause mutual interference between each other and act as interferers to neighboring cells. To enable coherent demodulation of the orthogonal subchannel signals, one subchannel is typically dedicated to the pilot carrier. As detailed in the above-mentioned us patent 5,506,865, pilot carriers used in the receiver are used to generate estimates of the channel characteristics. The correctness of these channel estimates depends on the strength of the pilot channel signal. Unfortunately, the pilot channel does not carry data, and it is therefore desirable to minimize the pilot transmit power. Typically, one chooses the pilot power relative to the data signal power by balancing these two factors so that the best overall system performance can be achieved. Therefore, it is highly desirable to have a method for generating a correct channel estimation value without increasing the pilot signal strength.

Summary of The Invention

A method and apparatus for improving the performance of a receiver that receives a plurality of sub-channel signals transmitted together over a common propagation path (also referred to as a transmit channel) is described. To compensate for phase and amplitude distortions introduced into the signal by the transmit channel, the receiver uses the pilot subchannel to estimate the phase and amplitude distortions of the transmit channel. The distortion estimation process inherent in the transmission channel is referred to as channel estimation, and the channel estimation is used to generate a channel estimation value. The present invention includes a new method of using data-carrying sub-channels (not pilot sub-channels) to improve the accuracy of channel estimates. The present invention may be applied to any communication system using simultaneous transmission of multiple subchannels and coherent demodulation.

The subchannel signals in the information signal may be time division multiplexed (TDMed) or code division multiplexed (CDMed). The exemplary embodiment describes the present invention in the reverse link scenario proposed in cdma 2000. Because of The prevalence of overload in The channel structure, The present invention is equally applicable to reception of reverse link transmissions, according to The Candidate claims (submissions) proposed by The European Telecommunications Standards Institute (ETSI) entitled "The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-RTT cancer subscription" (hereinafter WCDMA). Furthermore, the present invention is equally applicable to forward link reception for these systems.

In cdma2000, the subchannels carrying data include supplemental channels (supplemental channels) at high data rates (e.g., 76.8kbps) and fundamental channels at low data rates (e.g., 9.6 kbps). For demodulation of the baseband channel, a pilot channel of limited power (e.g., baseband channel power) is preferably used. To enable proper demodulation of supplemental channels at high data rates, the cdma2000 standard suggests increasing the pilot power above the nominal level when supplemental channels are used. In addition, the cdma2000 standard recommends using different levels of pilot power depending on which of several available data rates the supplemental channel is using.

Varying the pilot power based on the data rate creates other difficulties in system design. For example, it requires that the receiver know the data rate a priori in order for the power control loop to function properly. This in turn makes the selection of search/finger lock more difficult. In addition, pilot overhead reduction is required to improve overall system performance, which, of course, does not sacrifice demodulation performance.

The present invention enables a system to achieve superior supplemental channel demodulation performance by enabling the formation of channel estimates based on a baseband channel signal. If enough channel estimation information can be obtained from the fundamental frequency channel, acceptable supplemental channel demodulation performance can be achieved without changing the pilot power at all. Because the baseband signal can be transmitted at a power that is 4 times as high as the pilot signal, the channel estimate formed using the two signals is much more accurate than an estimate based on the pilot signal alone. At the same time, demodulation using more accurate channel estimates will result in improved performance.

In cdma2000, the transmit power of the baseband channel is 4 times the nominal pilot. The combined power of the pilot channel and the fundamental frequency channel will be 5 times the power of the nominal pilot channel only. The combined channel estimates derived from both the nominal pilot channel and the baseband channel are sufficiently accurate for demodulation of the cdma2000 supplemental channel. While increasing the pilot power at any time when the supplemental channel is used is still an option, it does not necessarily improve the accuracy of the combined channel estimates.

Additional accuracy in obtaining channel estimates from the received baseband channel is related to using the correct reference signal, which ideally is the same as the transmitted baseband channel signal. Any inaccuracy in the decoded symbols used in forming the baseband channel estimate will degrade the quality of the combined channel estimate. Although the supplemental channel may be a packet data channel with high tolerance for frame errors, it is desirable to minimize the frame error rate when demodulating the supplemental channel.

In the preferred embodiment of the present invention, the received baseband channel signal is first deinterleaved and Forward Error Correction (FEC) decoded to take advantage of the auxiliary FEC encoding and interleaving functions of the transmitter. The corrected symbol stream is then 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 another embodiment of the invention, the baseband channel power is increased as needed to reduce the error rate of the baseband channel. Increasing the fundamental channel power also results in a decrease in error rate when demodulating the supplemental channel, since decreasing the fundamental channel error rate produces a more accurate channel estimate. When the data rate ratio between the supplemental channel and the fundamental channel is large, slightly increasing the fundamental channel power has little effect on the total transmit power and thus the likelihood of degradation is also small.

In a more general sense, the present invention may be used in the case of transmitting single channel information. In another embodiment using a single data channel, the channel is artificially split into two channels that are transmitted synchronously at different data rates. On reception, the low rate channel is first demodulated and decoded using a pilot-based channel estimate. The decoded data bits are then re-encoded and used to improve the channel estimate used for coherent demodulation of the high data rate supplemental channel. This scheme enables data throughput in fading environments to approach theoretical capacity limits.

Brief Description of Drawings

The features, objects, and advantages of the present invention are described in detail below with reference to the accompanying drawings. Like reference numerals refer to like parts throughout the several views of the drawings. Wherein:

fig. 1 is a diagram of the basic elements of a wireless communication system of one embodiment of the present invention;

fig. 2 is a block diagram of a preferred embodiment of the present invention in a wireless transmitter;

fig. 3 is a block diagram of a preferred embodiment of the present invention in a wireless receiver;

fig. 4 is a block diagram of an example channel estimator circuit.

Detailed description of the preferred embodiments

Fig. 1 shows the invention in the case of a wireless communication system. In the exemplary embodiment, subscriber station 2 transmits a plurality of code division multiplexed signals over transmit channel 8 to Base Transceiver Subsystem (BTS)4 via receive antenna 6. In the exemplary embodiment of the cdma2000 or WCDMA reverse link, orthogonal codes are used to distinguish the code division multiplexed channels from each other. The method of providing orthogonal coding is described in detail in the above-mentioned co-pending U.S. patent application 08/856,428.

In the exemplary embodiment, the three types of CDMA signals transmitted from subscriber station 2 to base transceiver subsystem 4 are pilot signal 10, baseband signal 12, and supplemental signal 14. In the exemplary embodiment, the signal transmitted from subscriber station 2 is a code division multiple access communication signal that includes a pilot channel, a fundamental frequency channel, and a supplemental channel, as defined in cdma 2000. 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. patent 5,102,459 and IS-95.

Subscriber station 2 is shown as a mobile station but may also be a wireless modem, a wireless local loop subscriber station, a BTS (base transceiver subsystem), or any other wireless communication device that transmits multiple simultaneous sub-channels. The receiver station 4 in the figure is a BTS but may also be a wireless subscriber station or any other receiver that coherently demodulates a plurality of sub-channels. Methods and apparatus for simultaneously receiving multiple transmitted signals are well known in the art. In the present exemplary embodiment, the signal transmitted from the subscriber station 2 is received at the BTS 4 using a RAKE receiver, which is well known in the art and can be found in the aforementioned U.S. patent No. 5,109,390.

Fig. 2 shows a subscriber station 2 capable of transmitting multiple synchronization sub-channels according to one embodiment of the present invention. In fig. 2, a pilot channel signal, a supplemental channel signal, and a fundamental channel signal are generated for transmission on orthogonal subchannels.

The pilot channel is a known, transmitted constant waveform and therefore does not carry any data. Therefore, forward error correction and interleaving on the pilot channel are not necessary. The pilot channel is passed directly to a walsh spreader 110, which is based on the pilot channel walsh function W_pTo spread the data and thereby generate walsh covered pilot channel signals. The walsh covered pilot channel signal is then passed to a relative gain block 116 which adjusts the amplitude of the covered pilot channel signal relative to the signal carried by the orthogonal transmit subchannel. In the preferred embodiment, the pilot channel walsh function is an all zero walsh code, the pilot channel walsh spreader 110 is omitted, and the DC signal is passed directly to the relative gain block 116.

The baseband channel data is first passed to a Forward Error Correction (FEC) encoder 102, which generates an encoded baseband channel signal. The resulting encoded baseband channel signal is passed to an interleaver 106 which generates an interleaved baseband channel signal. The interleaved baseband channel signals are then passed to a Walsh spreader 112, which performs a Walsh function on the baseband channel signals according to a Walsh function W_FTo spread the data to produce an overlaid baseband channel signal. The covered to baseband channel signal is then passed to a relative gain module 118, which adjusts the amplitude of the covered baseband channel signal relative to the signals carried by the other orthogonal transmit subchannels.

The supplemental channel data is first passed to a Forward Error Correction (FEC) encoder 104, which generates an encoded supplemental channel signal. The resulting encoded supplemental channel signal is passed to interleaver 108, which generates an interleaved supplemental channel signal. The interleaved supplemental channel signal is then passed to a Walsh spreader 114, which performs a Walsh function W on the supplemental channel signal_STo spread the data to produce an overlaid supplemental channel signal. The overlaid supplemental channel signal is then passed to a relative gain module 120, which adjusts the amplitude of the overlaid supplemental channel signal relative to the signal carried by the other orthogonal transmit sub-channels.

Although the preferred embodiment shown uses orthogonal walsh functions to implement subchannel coding, those skilled in the art will appreciate that subchannel coding may also be implemented using TDMA or PN coding without departing from the invention. In one embodiment employing PN coding, the reference signal W_P、W_PAnd W_FAre replaced by PN codes corresponding to the supplemental channel, the pilot channel, and the baseband channel, respectively.

Those skilled in the art will appreciate that any forward error correction technique may be used by FEC modules 102 and 104 without departing from the invention. These techniques include turbo code encoding, convolutional encoding, or other forms of encoding such as block encoding. Further, interleavers 106 and 108 may utilize any of a number of interleaving techniques, including convolutional interleaving, turbo interleaving, block interleaving, and bit reversal interleaving. The above-mentioned cdma2000 specification describes a turbo encoder and a turbo interleaver.

The output of each relative gain block 116, 118, and 120 is then passed to a PN spreader block 122. The output of the PN spreader module 122 is then passed to a transmitter 124. Transmitter 124 provides additional transmit gain control by varying the gain of the entire combined signal received from PN spreader module 122 before transmitting the signal through antenna 126.

In another embodiment, the optional relative gain module 116 is omitted and the pilot signal is passed directly to the PN spreader module 122. The gain of the other channels is adjusted relative to the gain of the pilot channel. Those skilled in the art will appreciate that the two methods of controlling the relative gain of the channels using either a system that includes the relative gain module 116 or a system that does not include the relative gain module 116 are functionally equivalent.

Those skilled in the art will appreciate that the signal may be turned "off" by making the effective transmit gain of any subchannel signal equal to zero. This may be accomplished by configuring their respective relative gain modules 116, 118, or 120 as such. The same result can be achieved by a process of disconnecting the sub-channel signals through the PN spreader, such as with a logic switch. Those skilled in the art will appreciate that various methods of setting the effective transmit gain of a subchannel to zero may be employed without departing from the invention.

PN spreader 122 spreads the orthogonal channel signal with a spreading sequence that forms a pseudo-random number and conveys the resulting combined signal to transmitter 124 for transmission via antenna 126. In the preferred embodiment, PN spreader 122 utilizes complex PN spreading, as described in the above-mentioned U.S. patent application 08/856,428. As in the cdma2000 specification shown in fig. 33 above, PN spreader 122 may rotate the signals of the supplemental channel outputs of gain modules 118 and 120 relative to the pilot channel signal output by gain module 116 by 90 degrees before performing PN spreading.

Those skilled in the art will appreciate that for each input signal, PN spreader 122 may generate a complex spread signal such that relative gain blocks 116,118, and 120 are placed after PN spreader 122 and before transmitter 124.

In another embodiment, the relative gains applied by the relative gain modules 116, 118, and 120 are dynamically controlled by the gain control processor 128. The gain of each module may be changed according to the data rate of the channel. For example, when data is transmitted on both the fundamental channel and the supplemental channel, the pilot channel gain can be increased. Alternatively, the baseband channel gain may be increased when data is transmitted on the supplemental channel.

Fig. 3 illustrates a preferred embodiment of the use of the present invention in a wireless receiver. A mixed signal containing three orthogonal sub-channels is received by antenna 200 and down-converted in receiver 202. The resulting downconverted signal is then passed to complex PN despreader 204 to produce I and Q component samples for use in subsequent processing. The complex PN despreader operates in accordance with the above-mentioned U.S. patent application 08/856,428. The operation of the fundamental channel estimation device 250, the pilot channel estimation device 252, and the channel estimation combiner 230 is described in detail below.

The I and Q component samples are passed to a walsh despreader 206 which uses the same walsh function W_FThe baseband channel in walsh spreader 112 is spread. Walsh despreader 206 produces the resulting I and Q components for decovering the baseband channel.

The I and Q component signals are also input to a pilot channel estimator 218a, which generates filtered pilot I and Q samples. The pilot channel estimator 218a in the figure has a walsh code W_PWhich corresponds to the walsh code used to spread the pilot channel in walsh spreader 110.

Fig. 4 illustrates an example embodiment of the channel estimator 218. The complex input signal is provided to the channel estimator 218 as a stream of I and Q samples. In mixer 302a, the I samples are mixed with a reference signal to obtain the real part of the complex input signal. The output of the mixer 302a is supplied to a noise suppression filter 304a, and noise is removed from the acquired real number component. In mixer 302b, the Q samples are mixed with the same reference signal used in mixer 302a to obtain the imaginary part of the complex input signal. The output of the mixer 302b is supplied to a noise suppression filter 304b, which removes noise from the acquired imaginary component. Those skilled in the art will appreciate that the noise suppression filter 304 may be constructed with a low pass filter, matched filter, or accumulator without departing from the invention.

The reference signal used in the channel estimator 218 may be real, imaginary, or complex. In another embodiment of channel estimator 218 adapted for use with complex reference signals, mixer 302 is a complex multiplier (also referred to as a complex mixer), each having two outputs, real and imaginary. The real output of mixer 302 is then summed before filtering in real part filter 304 a. The imaginary output of mixer 302 is summed prior to filtering in imaginary part filter 304 b. In the same manner, a complex multiplier can be used in either the walsh spreader or despreader so that a complex walsh code can be used as a reference function during both the spreader and despreader. 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 transmitted pilot channel is 90 degrees out of phase with the fundamental channel and the supplemental channel. Thus, in the preferred embodiment, pilot channel estimator 218a rotates its output by 90 degrees. This rotation may be accomplished in a number of ways, including multiplying the reference signal by an imaginary value, or by rotating the real and imaginary outputs of the noise suppression filter 304. The same end result is achieved by rotating the signals of the fundamental channel and the supplemental channel without departing from the invention. Also, the relative rotation of the pilot channel with respect to the fundamental channel and the supplemental channel may be positive and negative without departing from the invention.

The acquired real and imaginary parts together constitute a channel estimateA vector comprising magnitude and phase information for all signal components associated with a reference signal. The quality of the channel estimate is related to the degree of correlation between the received complex input signal and the reference signal. In order to obtain the highest degree of correlation between the received complex input signal and the reference signal, the reference signal used by the receiver must be correctly matched to the signal transmitted by the transmitter, e.g. the walsh code W in the case of a pilot channel_P. Any difference between the reference signal and the transmitted signal will result in an incorrect channel estimate.

In IS-95 system, pilot Walsh code W_PAre all zero walsh codes, in which case channel estimation can be performed using only one pair of filters, as described in the above-mentioned U.S. patent 5,506,865. In this case, pilot channel walsh spreader 110 is omitted from the transmitter. The channel estimator may then be formed in the receiver such that mixer 302 may be omitted from pilot channel estimator 218 a. A channel estimator piloted by the filter without the all zero walsh code of the mixer is known as a pilot filter. However, the embodiment of the channel estimator depicted in fig. 4 makes it possible to use pilot walsh codes instead of all-zero walsh codes.

Meanwhile, the amplitude and phase characteristics of the CDMA transmission channel 8 are estimated using the pilot I signal and the pilot Q signal. The resulting pilot I and pilot Q and the decovered baseband channel I and Q components are provided to a dot product module 208. The dot product module 208 calculates a scalar projection of the fundamental channel signal onto the pilot channel estimate vector according to the above-described circuitry as described in the above-mentioned U.S. patent 5,506,865. Since the pilot channel signal 10, the fundamental channel signal 12 and the supplemental channel signal 14 have all traversed the same propagation path 8, the phase errors of the 3 signals resulting from the channel are identical.

Such phase errors are removed by performing the dot product operation described in the above-mentioned U.S. Pat. No. 5,506,865. In the present exemplary embodiment, the pilot channel estimate is used to coherently demodulate the baseband channel in the dot product module 208. The dot product module generates a scalar signal for each symbol period that represents the amplitude of the fundamental channel signal in phase with the pilot signal received via the transmit channel 8.

The baseband channel symbols output by the dot product block 208 are then passed to a deinterleaver 210, which performs the inverse of the function of the transmit interleaver 106. The resulting de-interleaved signal is then passed to a Forward Error Correction (FEC) decoder 212. The decoder 212 performs the reverse function of the FEC encoder 102 and outputs a forward error correction signal.

The correction signal output by decoder 212 is also passed to encoder 224, which re-encodes the signal using the same FEC function (function) as transmitter FEC encoder 102. In this manner, the encoder 224 produces an ideal representation of the transmitted baseband signal. This ideal representation is then passed to interleaver 226, which performs the same function as transmitter interleaver 106 to produce an ideal representation of the interleaved baseband channel data transmitted by subscriber station 2.

The samples of the I and Q components produced by the walsh despreader are also input to a delay 220 which produces I and Q components synchronized with the output of an interleaver 226. The design of the delay device 220 enables compensation of the delay 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 the delay 220 are then passed to the channel estimator 218b along with the output of the interleaver 226. Channel estimator 218b uses the output of interleaver 226 as a reference signal and the output of delay 220 as a stream of I and Q samples from which a channel estimate output is formed.

The correction data bits output by the FEC decoder 212 are re-encoded and re-interleaved to produce a reference signal that has a higher probability of matching to the signal actually transmitted on the baseband channel. Using this more reliable reference signal as an input to channel estimator 218b improves the accuracy of the baseband channel estimate generated by channel estimator 218 b.

In a sub-optimal embodiment, the dot product module 208 may be provided directly to the channel estimator 218b instead of using the deinterleaver 210, decoder 212, encoder 224, and interleaver 226 to generate the ideal representation of the baseband channel signal. In this case, the delay element 220 only compensates for the time required to perform the dot-product operation in the dot-product module 208. However, the baseband channel estimator no longer has the error correction advantage of the bypass element.

The complex output portion of pilot channel estimator 218a passes through delay element 222 to compensate for the delay inherent in performing channel estimation using the baseband channel signal. The channel estimation parameters generated by processing the baseband channel are passed into channel estimation combiner 230 along with the delayed channel estimation parameters from delay elements 220 and 222. Channel estimate combiner 230 combines the two channel estimate data for the pilot channel and the baseband channel processes and produces an output that includes a third, combined channel estimate. As the characteristics of the transmit channel change over time, pilot channel estimator 218a and channel estimator 218b provide updated channel estimates to channel estimate combiner 230, which updates the combined channel estimate output accordingly.

In the preferred embodiment, the output of decoder 212 that is passed to encoder 224 is additionally passed to control processor 216. Control processor 214 generates frame rate information for each received data frame. The control processor 216 also performs a validity check on the received frame. The control processor 216 generates a baseband channel quality metric based on the results of its rate decisions and validity checks of the received data. The baseband channel quality metric is used to assign an appropriate weighting factor to the baseband channel estimate relative to the weighting factor assigned to the pilot channel estimate. The baseband channel quality metric varies according to the validity of the received frame, which is based on CRC correctness. Since different rate frames may also use different numbers of CRC bits, or have different degrees of frame error detection protection, the control processor 216 may additionally vary the baseband channel quality metric depending on the received frame rate.

The control processor 216 is also connected to an encoder 224. Control processor 216 passes the frame rate information to encoder 224 for re-encoding the data received from decoder 212.

In the present exemplary embodiment, channel estimate combiner 230 is a weighted average combiner that generates a combined channel estimate signal by performing a weighted average of the pilot channel estimate and the baseband channel estimate according to the following equation:

R_COMB＝XR_PILOT+(1-X)R_FUND (3)

I_COMB＝XI_PILOT+(1-X)I_FUND (4)

wherein R is_COMBAnd I_COMBIs to combine the real and imaginary parts, R, of the channel estimate_PILOTAnd I_PILOTIs the real and imaginary part, R, of the pilot channel estimate_FUNDAnd I_FUNDIs the real and imaginary parts of the fundamental channel estimate and X is a scaling factor. The scaling factor X has a value from 0 to 1. A scale factor value of 1 makes the combined channel estimate equal to the pilot channel estimate. A scale factor value of 0 makes the combined channel estimate equal to the baseband channel estimate. The value of X represents a first multiplier that is multiplied by the pilot channel estimate to produce a proportional channel estimate for the pilot channel. The value (1-X) represents a second multiplier that is multiplied by the baseband channel estimate to produce a scaled channel estimate for the baseband channel. The two proportional channel estimates are added together to produce a combined channel estimate.

Channel estimate combiner 230 additionally uses the baseband channel quality metric provided by control processor 216 as a dynamic weighting factor for the channel estimate generated from the baseband channel. The channel estimate combiner 230 increases the value of the scaling factor X when the baseband channel quality metric gives a high frame error rate. Therefore, when a frame error occurs, the combined channel estimate used to demodulate the baseband channel is derived more from the pilot channel estimate and less from the baseband channel estimate. In another embodiment, a frame error results in the value of the scale factor being equal to 1 until a valid frame is received.

In another embodiment of the invention, the control processor 216 includes a smoothing module that performs a smoothing, or low pass filtering, on the baseband channel quality metric before it is passed to the channel estimate combiner 230. This smoothing helps make the weighted average performed by the channel estimate combiner 230 less susceptible to high frequency noise inherent in the channel.

In yet another embodiment of the present invention, the receiver knows the relative gains used by the relative gain modules 116 and 118 when transmitting the pilot channel signal and the baseband channel signal. In this embodiment, the value of X is adjusted so that the ratio of the first multiplier to the second multiplier equals the ratio of the transmit gain of the pilot channel to the transmit gain of the baseband channel.

In the preferred embodiment, the baseband 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 may be accomplished by combining a delay or buffer in the control processor 216. The control processor 216 may also perform a smoothing process on the baseband channel quality metric before providing it to the channel estimator 218 b. However, in the preferred embodiment, the baseband channel quality metric generated by the control processor 216 is not smoothed and may change abruptly at frame boundaries.

The I and Q component samples, which are used as inputs to the walsh despreader 236, are passed through a delay element 232, the delay element 232 acting to synchronize the output of the walsh despreader 236 with the output of the channel estimate combiner 238. The delay element 232 may alternatively be placed between the walsh despreader 236 and the dot product module 238 without departing from the invention. Using a transmitter for the Walsh despreader 236Walsh function W used by walsh spreader 114_SAnd generates decovered supplemental channel I and Q components. These decovered supplemental channel components are used as inputs to the dot product module 238 along with the combined channel estimate signal from the channel estimate combiner 230.

The dot product module 238 computes the magnitude of the supplemental channel signal projection onto the combined channel estimate vector, producing a scalar projection output. The output of the dot product module 238 is then deinterleaved in a deinterleaver 240, the deinterleaver 240 performing the inverse function of the interleaver 108. The output of the deinterleaver 240 is provided to a decoder 242 which performs the inverse function of the interleaver 104.

With the wireless receiver shown in fig. 3, those skilled in the art will appreciate that any of the delay elements 220, 222, or 230 may act as an accumulator or buffer without departing from the invention. Furthermore, those skilled in the art will appreciate that pairs of delay elements, such as delay elements 232a and 232b, may be formed separately or combined into a single delay module performing the same function without departing from the invention.

Although the preferred embodiment shown uses orthogonal walsh functions to implement subchannel decoding, those skilled in the art will appreciate that subchannel decoding may also be implemented using TDMA or PN coding without departing from the invention. In embodiments using PN codes, the reference signal W is replaced by PN codes corresponding to the supplemental channel, the pilot channel, and the baseband channel, respectively_S、W_PAnd W_F。

Claims (55)

1. A method of demodulating an information signal, wherein the information signal is received over a channel having a channel characteristic, and wherein the information signal comprises a pilot signal, a first data carrying signal, and a second data carrying signal, the method comprising:

performing a first estimation on the channel characteristics according to the pilot signal to provide a pilot channel estimation value;

performing second estimation on the channel characteristics according to the first data carrying signal to provide a data channel estimation value;

the pilot channel estimate and the data channel estimate are combined to provide a combined channel estimate.

2. The method of claim 1 further comprising generating a scalar projection of said information signal in accordance with said combined channel estimate.

3. The method of claim 1, further comprising pseudonoise despreading the information signal.

4. The method of claim 3, wherein the pseudo-noise despreading is complex pseudo-noise despreading.

5. The method of claim 1 wherein said second estimating comprises generating a scalar projection of said information signal in accordance with said pilot channel estimate.

6. The method of claim 5, wherein said second estimating further comprises generating an ideal representation of said first data-carrying signal.

7. The method of claim 6, wherein said generating an ideal representation comprises:

deinterleaving the first data-carrying signal to provide a deinterleaved signal; and

interleaving the de-interleaved signal.

8. The method of claim 6, wherein said generating an ideal representation comprises:

decoding the first data carrying signal to provide a decoded signal; and

the decoded signal is encoded.

9. The method of claim 1, further comprising introducing a delay in the pilot channel estimate for synchronization between the pilot channel estimate and the data channel estimate.

10. The method of claim 1, wherein the combining comprises:

multiplying the pilot channel estimate by a pilot multiplier to produce a scaled pilot channel estimate;

multiplying the data channel estimate by a data multiplier to produce a de-scaled data channel estimate; and

adding the scaled pilot channel estimate to the scaled data channel estimate to provide the combined channel estimate.

11. The method of claim 10, wherein a ratio of the pilot multiplier to the data multiplier is based on a ratio of a gain used to transmit the pilot signal to a gain used to transmit the first data carrying signal.

12. The method of claim 10, further comprising generating the pilot multiplier and the data multiplier.

13. The method of claim 10 further comprising varying a ratio of the pilot multiplier to the data multiplier based on a data rate of the first data carrying signal.

14. The method of claim 10 further comprising varying a ratio of said pilot multiplier to said data multiplier based on a frame quality metric of said first data carrying signal.

15. The method of claim 1 wherein said first estimating comprises filtering said information signal to provide said pilot channel estimate.

16. The method of claim 15 wherein said first estimating further comprises multiplying said information signal by a reference pilot code.

17. The method of claim 1, wherein the second estimating comprises: :

generating a scalar projection of said information signal in accordance with said pilot channel estimate to provide a scalar information signal;

decoding said scalar information signal to provide a decoded signal;

encoding the decoded signal to provide an ideal representation of the first data-carrying signal; and

multiplying said information signal by said ideal representation to provide said data channel estimate.

18. The method of claim 17, wherein the second estimating further comprises:

de-interleaving said scalar information signal prior to said decoding; and

the ideal representation is interleaved prior to the multiplication.

19. An apparatus for demodulating an information signal, wherein the information signal is received over a channel having channel characteristics, and wherein the information signal comprises a pilot signal, a first data carrying signal, and a second data carrying signal, the apparatus comprising:

first means for estimating said channel characteristics from a received pilot signal to provide a pilot channel estimate;

a second means for estimating said channel characteristics in response to said first data carrying signal to provide a data channel estimate;

means for combining the pilot channel estimate with the data channel estimate to provide a combined channel estimate.

20. The apparatus of claim 19 further comprising generating a scalar projection of said information signal according to the combined channel estimates.

21. The apparatus of claim 19 further comprising means for pseudonoise despreading said information signal.

22. The apparatus of claim 19 further comprising performing complex pseudonoise despreading on said information signal.

23. The apparatus of claim 19 wherein said second means for estimating comprises means for generating a scalar projection of said information signal in accordance with said pilot channel estimate.

24. The apparatus of claim 23, wherein said second means for estimating further comprises means for generating an ideal representation of said first data-carrying signal.

25. The apparatus of claim 24, wherein said means for generating an ideal representation comprises:

means for deinterleaving the first data-carrying signal to provide a deinterleaved signal; and

means for interleaving the deinterleaved signals.

26. The apparatus of claim 24, wherein said means for generating an ideal representation comprises:

means for decoding said first data carrying signal to provide a decoded signal; and

and means for encoding the decoded signal.

27. The apparatus of claim 19 further comprising means for introducing a delay in the pilot channel estimate to provide synchronization between the pilot channel estimate and the data channel estimate.

28. The apparatus of claim 19, wherein said combining means comprises:

means for multiplying the pilot channel estimate by a pilot multiplier to produce a scaled pilot channel estimate;

means for multiplying the data channel estimate by a data multiplier to produce a scaled data channel estimate; and

means for adding the scaled pilot channel estimate to the scaled data channel estimate to provide the combined channel estimate.

29. The apparatus of claim 28 further comprising means for generating the pilot multiplier and the data multiplier.

30. The apparatus of claim 28 further comprising means for varying the ratio of said pilot multiplier to said data multiplier based on the data rate of said first data carrying signal.

31. The method of claim 28 further comprising means for varying a ratio of said pilot multiplier to said data multiplier based on a frame quality metric of said first data carrying signal.

32. The apparatus of claim 19 wherein said first estimating means comprises means for filtering said information signal to provide said pilot channel estimate.

33. The apparatus of claim 32 wherein said first estimating means further comprises means for multiplying said information signal by a reference pilot code.

34. The apparatus of claim 19, wherein said second estimating means comprises:

means for generating a scalar projection of said information signal in accordance with said pilot channel estimate to provide a scalar information signal;

means for decoding said scalar information signal to provide a decoded signal;

means for encoding said decoded signal to provide an ideal representation of said first data carrying signal; and

means for multiplying said information signal by said ideal representation to provide said data channel estimate.

35. The apparatus of claim 34, wherein said second estimating means further comprises:

means for de-interleaving said scalar information signal prior to said coding; and

means for interleaving said ideal representation prior to said multiplying.

36. An apparatus for demodulating an information signal, wherein the information signal is received over a channel having a channel characteristic, and wherein the information signal comprises a pilot signal, a first data carrying signal, and a second data carrying signal, the apparatus comprising:

pilot channel estimation means for estimating the channel characteristics based on the pilot channel signal to provide a pilot channel estimation value;

data channel estimation means for estimating said channel characteristics from said first data channel signal to provide a data channel estimate;

a channel estimate combiner that combines the pilot channel estimate with the data channel estimate to produce a combined channel estimate.

37. The apparatus of claim 36 further comprising a first dot product module for modifying a phase of said information signal based on said combined channel estimate to produce a stream of subchannel symbols.

38. The apparatus of claim 36 further comprising a dot product module for generating a scalar projection of said information signal in accordance with said combined channel estimate.

39. The apparatus of claim 36 further comprising a pseudo-noise despreader for multiplying said information signal by a pseudo-noise code.

40. The apparatus of claim 39 wherein said pseudo-noise despreader is a complex pseudo-noise despreader for multiplying said information signal by a complex pseudo-noise code.

41. The apparatus of claim 36 wherein said data channel estimation device comprises a dot product module for generating a scalar projection of said information signal in accordance with said pilot channel estimate to provide a scalar information signal.

42. The apparatus of claim 41, wherein said data channel estimation device further comprises means for generating an ideal representation of said first data-carrying signal based on said scalar information signal.

43. The apparatus of claim 42, wherein said data channel estimation device further comprises:

a de-interleaver for de-interleaving said scalar information signal to provide a de-interleaved signal; and

an interleaver that interleaves the de-interleaved signal.

44. The apparatus of claim 42, wherein said data channel estimation device further comprises:

a decoder for decoding said scalar information signal to provide a decoded signal; and

an encoder for encoding the decoded signal.

45. The apparatus of claim 36, further comprising delaying means for introducing a delay in the pilot channel estimate to provide synchronization between the pilot channel estimate and the data channel estimate.

46. The apparatus of claim 36 wherein said channel estimate combiner is a weighted average combiner.

47. The apparatus of claim 36 wherein said channel estimate combiner is a weighted average combiner for providing said combined channel estimates according to the following equation:

R_COMB＝X*R_PILOT+(1-X)*R_DATA

I_COMB＝X*I_PILOT+(1-X)*I_DATA

wherein R is_COMBAnd I_COMBIs the real part of the imaginary component of the combined channel estimate, and R_PILOTAnd I_PILOTIs the real part of the imaginary component of the pilot channel estimate, R_DATAAnd I_DATAIs the real part of the imaginary component of the data channel estimate and X is the scaling factor.

48. The apparatus of claim 47, wherein the weighted average combiner is operative to employ a value of X based on a ratio of a gain value used to transmit the pilot signal and a gain value used to transmit the first data-carrying signal.

49. The apparatus of claim 47 further comprising a control processor for providing a value of X to said weighted average combiner.

50. The apparatus of claim 49 wherein said control processor is operative to adjust the value of X based on a data rate of said first data carrying signal.

51. The apparatus as claimed in claim 47 wherein said control processor is operative to adjust said X value based on a frame quality metric of said first data carrying signal.

52. The apparatus of claim 36 wherein said pilot channel estimation device comprises a filter for filtering said information signal to provide said pilot channel estimate.

53. The apparatus of claim 52, wherein the pilot channel estimation device comprises a mixer that multiplies the information signal by a reference pilot code.

54. The apparatus of claim 36, wherein said data channel estimation device comprises:

a dot product module for multiplying said information signal by said pilot channel estimation device to provide a scalar information signal;

a decoder for decoding said scalar information signal to provide a decoded signal;

an encoder for encoding the decoded signal to provide an ideal representation of the first data carrying signal; and

a mixer for multiplying said information signal by said ideal representation to provide said data channel estimate.

55. The apparatus of claim 54, wherein said data channel estimation device further comprises:

a de-interleaver for de-interleaving said scalar information signal; and

an interleaver that interleaves the ideal representation.