Multiple-access noise rejection filter for a DS-CDMA system

What is claimed is:

1. A direct-sequence multiple-access code division (DS-CDMA) communication system comprising:

(1) a multiplicity of transmitters, each including:

(a) a first signal generator that generates an analog baseband waveform signal having a plurality of bits, each bit having a time period of T.sub.B, said bits being digitally encoded with data that is to be transmitted,

(b) a second signal generator that generates a unique signature waveform signal having a spectrally inefficient power spectrum, said unique signature waveform signal being made up of a sequence of chip waveform signals, each chip waveform signal in said sequence of chip waveform signals having a duration of T.sub.C seconds, a polarity controlled by a unique spreading code, and a bandwidth substantially corresponding to an allowed channel bandwidth, said spectrally inefficient power spectrum being spectrally inefficient and substantially non-flat within a band .+-.1/T.sub.C,

(c) a first modulator that modulates each bit of the analog baseband waveform signal with the unique signature waveform signal to yield a direct sequence spread waveform signal,

(d) a first filter for filtering the direct sequence spread waveform signal,

(e) an RF generator that generates an RF carrier signal, said RF carrier signal having a carrier frequency that is the same for all of said multiplicity of transmitters,

(f) a second modulator that modulates said RF carrier signal with the filtered direct sequence spread waveform signal, and

(f) a transmitter that transmits the modulated RF carrier signal; and

(2) at least one base-station receiver comprising:

(a) an RF receiver that receives the transmitted modulated RF carrier signals from each of the multiplicity of transmitters,

(b) a second filter that filters the modulated RF carrier signal to improve the signal-to-noise ratio (SNR) and to compensate for the spectrally inefficient substantially non-flat power spectrum of the signature waveform signal within the band .+-.1/T.sub.C, and

(c) a spread spectrum receiver that processes the filtered modulated RF carrier signal to despread such signal in order to identify a particular signature waveform signal contained therein, downconvert such signal to remove the RF carrier therefrom, and integrate such signal over a bit time to determine its informational content, said informational content over several bit times comprising the digital data transmitted by a particular one of said multiplicity of transmitters.

2. The DS-CDMA communication system as set forth in claim 1 wherein the spectrally-inefficient substantially non-flat power spectrum of the chip waveform signal is shaped as a main lobe of a sinc-squared function within the band .+-.1/T.sub.C.

3. The DS-CDMA communication system as set forth in claim 2 wherein the first filter of each transmitter comprises a low pass filter having a frequency response band confined to the allowed channel bandwidth.

4. The DS-CDMA communication system as set forth in claim 1 wherein the second filter included within said base-station receiver comprises:

a matched filter having a frequency response that matches the non-flat power spectrum of the signature waveform signal, said modulated RF carrier signal being applied to an input of said matched filter; and

an adaptive filter coupled to an output of said matched filter, said adaptive filter having a transfer function that compensates for the spectrally-inefficient substantially non-flat power spectrum of the signature waveform signal within the band .+-.1/T.sub.C so as to provide a net frequency response of the filter means that is substantially flat over the allowed channel bandwidth, said substantially flat frequency response serving to improve the SNR.

5. The DS-CDMA communication system as set forth in claim 2 wherein said adaptive filter comprises an analog transversal filter.

6. The DS-CDMA communication system as set forth in claim 2 wherein said adaptive filter comprises:

a series of M-1 delay elements, where M is an integer greater than three, each delay element having an input signal line and an output signal line, with the signal on the output signal line being delayed by T.sub.H seconds from the signal appearing at the input signal line, the input signal line of a first delay element being connected to the output of said matched filter;

a series of M tap points, with one tap point being located on each side of each delay element:

means for generating a set of tap weight signals (.alpha..sub.i);

a series of M multiplier elements, each having first and second input lines and an output line, the first input line being connected to a respective one of said tap points, and the second input line being connected to receive a respective one of the "tap weight" signals, the output line having a product signal thereon that represents the product of the signals applied to the first and second input lines; and

a summing circuit coupled to the output lines of each multiplier element for accumulating all of the product signals generated by the multiplying means and producing an output signal representative of the sum of the product signals.

7. The DS-CDMA communication system as set forth in claim 6 wherein said means for generating a set of tap weight signals (.alpha..sub.i) includes a plurality of signal processing paths, each path including a multiplier element that multiplies the output signal by a tap point signal appearing on a respective one of said series of M tap points, a comparitor circuit that compares a product signal from the multiplier (110) with a specified reference signal h.sub.2 and determines the difference therebetween, and an integrator circuit that integrates an output signal from the comparitor circuit over a specified time period, with the resulting integrated signal comprising one of said tap weight signals .alpha..sub.i.

8. The DS-CDMA communication system as set forth in claim 6 wherein said integrator of each signal processing path comprises a low pass filter (LPF).

9. The DS-CDMA communication system as set forth in claim 2 wherein said adaptive filter comprises a discrete time adaptive filter.

10. A direct sequence code-division multiple-access (DS-CDMA) receiver for use with a plurality of transmitters, each transmitter being configured to asynchronously transmit a spectrally-inefficient CDMA signal (s.sup.(k) (t)) at the same carrier frequency, said spectrally-inefficient CDMA signal having a substantially non-flat power spectrum within a band .+-.1/T.sub.C, the CDMA signal transmitted by a particular transmitter having information data bits therein, of duration T.sub.B, encoded with a unique signature waveform that identifies the particular transmitter as the source of its transmitted CDMA signals, the signature waveform comprising a train of chip pulses separated by a time period T.sub.C, each having a polarity defined by a unique spreading code, with N.sub.C chip pulses being included within each data bit, whereby N.sub.C .times.T.sub.C =T.sub.B, the CDMA signals transmitted by any of said plurality of transmitters other than a transmitter of interest representing a form of noise, said DS-CDMA receiver including:

a front-end receiver that receives the spectrally-inefficient substantially non-flat CDMA signals transmitted by each of said transmitters within the band .+-.1/T.sub.C ;

a chip matched filter having a frequency response H*(.omega.) matched to a frequency response of each of said transmitters, said matched filter being coupled to the front-end receiver so as to filter the CDMA signals received by said front-end receiver;

a sampling circuit that samples an output of said chip matched filter at a prescribed sampling rate and produces a series signal x.sub.m ; and

an adaptive filter coupled to receive the series signal x.sub.m that generates an output series signal y.sub.m therefrom, said adaptive filter including means for compensating for the spectrally-inefficient substantially non-flat power spectrum of the CDMA signals within the band .+-.1/T.sub.C transmitted by a transmitter of interest so as to improve the signal-to-noise ratio (SNR) of the output series signal y.sub.m ;

a decimator that decimates the series signal y.sub.m and produces a series signal z.sub.n ;

a despreader that despreads the signal z.sub.n and identifies individual data bits that originated from a transmitter of interest; and

means for determining the informational content of the individual data bits obtained from the despreader.

11. The DS-CDMA receiver of claim 10 wherein said adaptive filter comprises a two-sided finite impulse response (FIR) filter having L taps per side.

12. The DS-CDMA receiver of claim 11 wherein said adaptive filter comprises:

a series of J-1 delay elements, where J is an integer greater than two, each delay element having an input signal line and an output signal line, with the signal on the output signal line being delayed by an amount T.sub.H from the signal appearing at the input signal line, the input signal line of a first delay element being connected to the output of said chip matched filter;

a series of J tap points with one tap point being located on each side of each delay element;

means for generating a set of tap weight signals (.alpha..sub.j);

a series of J multiplier elements; each having first and second input lines and an output line, the first input line being connected to a respective one of said tap points, and the second input line being connected to receive a respective one of the "tap weight" signals .alpha..sub.j, the output line having a product signal thereon that represents the product of the signals applied to the first and second input lines; and

a summer coupled to the output lines of each multiplier element for summing all of the product signals generated by the multiplying means and producing the series signal y.sub.m representative of the sum of the product signals.

13. The DS-CDMA receiver of claim 12 wherein said means for generating a set of tap weight signals (.alpha..sub.j) includes a plurality of signal processing paths, each path including a first multiplier element that multiplies the series signal y.sub.m by a tap point signal appearing on a respective one of said series of J tap points, a summing circuit that sums a product signal from the first multiplier element, multiplied by a -1 to change its polarity, with a specified reference signal h.sub.2, a second multiplier element that multiplies a sum signal generated by the summing circuit by a fixed constant .DELTA., and an integrator circuit that integrates an output signal from the summing circuit over the time T.sub.C, with the resulting integrated signal comprising one of said tap weight signals .alpha..sub.j.

14. The DS-CDMA receiver of claim 13 wherein said discrete time adaptive filter implements the adaptive function

.alpha..sup.n+1 =.alpha..sup.n +.DELTA.{h.sub.2 [0]-x[n](x.sup.T [n].alpha..sup.n)} (7)

where the constant .DELTA. controls the rate of convergence of the adaptive function, .alpha..sup.n represents the tap coefficients {.alpha..sub.-L, . . . .alpha..sub.L } of the FIR filter at time n, x[n] is a vector that represents the set of signals present on the tap points at time n, and h.sub.2 [0] is a vector representing the set of specified reference signals h.sub.2 applied to each summing circuit (130) of each signal processing path.

15. A code-division multiple access (CDMA) communication system that detects with an approximately maximized signal-to-noise ratio (SNR) whether the bits of a transmitted data signal used within such system represent a logical "1" or a logical "0", said CDMA communication system including:

a plurality of transmitters, each of which includes means for transmitting data signals having information data bits therein of bit time T.sub.B, encoded with a unique signature waveform, the signature waveform comprising a sequence of chip pulses separated by a time period T.sub.C, each of said plurality of transmitters transmitting at the same data rate and chip rate as are transmitted by others of the transmitters at the same time;

means within each transmitter for generating the sequence of chip pulses to define a unique chip sequence within each bit time of the data signal to be transmitted, said unique chip sequence serving to identify a particular transmitter from which the transmitted signal originates;

means for shaping the chip pulses of each sequence of chip pulses so that each has a spectrally-inefficient, substantially non-flat power spectrum within a frequency band of .+-.1/T.sub.C ;

means for transmitting the shaped sequence of chip pulses as part of each data bit that is transmitted by the particular transmitter, whereby each data bit transmitted is encoded with said unique chip sequence;

a base-station receiver;

means within said base-station receiver for receiving the sequence of chip pulses;

matched filter means within the receiver for filtering the sequence of chip pulses in accordance with a matched filter transfer function configured to match the power spectrum of the transmitted chip pulses;

means for sampling the sequence of chip pulses passed through the matched filter at a specified rate to produce a sampled series of pulses, x.sub.m (i);

linear filter means for filtering the sampled series of pulses, x.sub.m (i), in accordance with a prescribed transfer function so as to produce a sequence of pulses y.sub.m (i), said prescribed transfer function being adapted to: (a) compensate for the spectrally-inefficient substantially non-flat shape of the power spectrum within the band .+-.1/T.sub.C of the transmitted pulses, (b) compensate for the transfer function of the matched filter, and (c) produce a net transfer function for the series of pulses y.sub.m (i) that is substantially flat over all frequencies within the allowed frequency band, thereby maximizing the SNR of the series y.sub.m (i); and

means for determining whether the sequence of pulses y.sub.m (i) represents a data bit that is a logical "1" or a logical "0".

16. A method of detecting with a maximized signal-to-noise ratio (SNR) whether the bits of a transmitted data signal used within a code-division multiple access (CDMA) communications system represent a logical "1" or a logical "0", said CDMA communications system including a plurality of transmitters, each of which includes means for transmitting data signals at the same data rate and chip rate as may be transmitted by others of the transmitters at the same time, and a base-station receiver adapted to receive said transmitted data signals, said method comprising the steps of:

(a) generating a sequence of chip pulses separated by a time period T.sub.C that define a unique chip sequence within each bit time of the data signal to be transmitted, said unique chip sequence serving to identify a particular transmitter from which the transmitted signal originates;

(b) shaping the chip pulses of each sequence of chip pulses generated in step (a) so that each has a spectrally-inefficient substantially non-flat power spectrum within a frequency band .+-.1/T.sub.C ;

(c) transmitting the sequence of chip pulses shaped in step (b) as part of each data bit that is transmitted by the particular transmitter, whereby each data bit transmitted is encoded with said unique chip sequence;

(d) receiving the sequence of chip pulses at the receiver;

(e) passing the sequence of chip pulses received in step (d) through a matched filter, said matched filter having a transfer function adapted to match the power spectrum of the transmitted chip pulses;

(f) sampling the sequence of chip pulses passed through the matched filter in step (e) at a specified rate to produce a sampled series of pulses, x.sub.m (i);

(g) passing the sampled series of pulses, x.sub.m (i), through a linear filter to produce a sequence of pulses y.sub.m (i), said liner filter being configured to exhibit a transfer function that compensates for the spectrally-inefficient substantially non-flat shape of the power spectrum of the transmitted pulses within the frequency band .+-.1/T.sub.C, as well as the transfer function of the matched filter, to produce a net transfer function for the series of pulses y.sub.m (i) that is substantially flat over all frequencies within the frequency band .+-.1/T.sub.C, thereby maximizing the SNR of the series y.sub.m (i); and

(h) determining whether the sequence of pulses y.sub.m (i) represents a data bit that is a logical "1" or a logical "0".

17. The method of claim 16 wherein step (g) comprises

successively delaying the sampled series of pulses, x.sub.m (i), by an amount T.sub.H using a series of J-1 delay elements (122.sub.j), where J is an integer greater than three;

picking off the delayed sampled series of pulses as they pass through said delay elements (122.sub.j) at a series of J tap points (123.sub.j), with one tap point being located on each side of each delay element 122.sub.j ;

generating a set of tap weight signals (.alpha..sub.j);

multiplying a respective one of said picked off series of pulses available at said tap points (123.sub.j) by a respective one of the "tap weight" signals .alpha..sub.j to produce a respective product signal; and

summing all of the respective product signals to produce the series signal y.sub.m.

18. The method of claim 17 wherein the step of generating a set of tap weight signals (.alpha..sub.j) comprises:

multiplying the series signal y.sub.m by a tap point signal appearing at each of said series of J tap points (123.sub.j) to produce a series of first product signals;

multiplying the series of first product signals by -1 to reverse its polarity;

summing the first product signals, after multiplying by -1 to change their polarity, with a specified reference signal h.sub.2 to produce a series of difference signals;

multiplying the difference signals by a fixed constant .DELTA. to produce a series of second product signals; and

integrating each of the series of second product signals over the time T.sub.C to produce said set of tap weight signals .alpha..sub.j.

19. The method of claim 18 wherein step (g) comprises implementing the adaptive algorithm

.alpha..sup.n+1 =.alpha..sup.n +.DELTA.{h.sub.2 [0]-x[n](x.sup.T [n])}(7)

where the constant .DELTA. controls the rate of convergence of the adaptive algorithm, .alpha..sup.n represents the set of tap weight coefficients {.alpha..sub.-L, . . . .alpha..sub.L } at time n, x[n] is a vector that represents the set of signals present on the tap points (123.sub.j) at time n, and h.sub.2 [0] is a vector representing the set of specified reference signals h.sub.2 summed with the first product signals 1o produce the series of difference signals.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

The present invention relates to multiple-access communication systems, and more particularly to a multiple-access communication system that utilizes a direct-sequence code-division multiple-access (DS-CDMA) approach, thereby allowing a multiplicity of separate transmitters to efficiently access a stationary base-station receiver.

Multiple-access communication systems are typically designed for use with a relatively large number of separate transmitters (e.g., portable or mobile transmitters) that interface with at least one stationary receiver at one or more designated receiving locations. Such multi-access systems are commonly used with digital cellular telephones, personal communication services, wireless local area networks (LAN's), and the like. Because the receiver(s) of such systems must allow access to a large number of users, each having a transmitter, some means must be used to efficiently interconnect the multiple transmitters to the receiver(s), i.e., to efficiently utilize the available channel spectrum. Common techniques used to allow such multiple access include frequency division multiple access (FDMA), time division multiple access (TDMA), and code-division multiple access (CDMA). The present invention provides multiple access through a CDMA-based communication system.

A simplified model of a CDMA system is as follows: a common carrier frequency, modulated with data having a known bit time, is transmitted to a common receiver from each of several transmitters. All of the transmitters share the same carrier frequency. Each transmitter has its own low bandwidth information bearing signal. This signal is multiplied by a unique high bandwidth signature waveform, which makes it possible for the receiver to distinguish the desired signal from the other signals transmitted from the other transmitters. For purposes of the present application, it is assumed that the signature waveform consists of a sum of time-offset copies of a waveform called the "chip waveform." The signature waveform may be visualized as the result of convolving the chip waveform with a train of impulses, each of unit area, and of positive or negative polarity. The sequence of polarities included within the train of impulses is known in the art as a "spreading sequence" (or "spreading code"). The spreading sequence is unique to each transmitter, but the chip waveform is the same. If, as is common in the art, the spreading sequences appear statistically like random binary sequences, then the power spectrum of the resulting signature sequence is substantially the same as that of the original chip waveform. Thus, each transmitter sends a waveform of similar power spectrum over the channel, and a receiver which has knowledge of a spreading sequence used by a transmitter can distinguish the signal sent by that transmitter on that basis. The unit of time between impulses in the impulse train described above is known as a "chip time", T.sub.C, and the reciprocal of this time is known as the "chip rate". The present invention is not limited to the above model of DS-CDMA, but the description is simplified by such a model.

A common problem facing all multiple-access communication systems is accurately detecting the transmitted signal at the receiver after the signal has passed through a noisy channel, i.e., after the transmitted signal has been corrupted with noise. Such noise may include signals from other transmitters, thermal noise, and noise from other sources. A measure of the ability to accurately detect a signal in a noisy environment is the signal-to-noise ratio (SNR), defined as the ratio of the power of the desired signal divided by the power of all other undesired signals, measured at the final signal which is used to make a decision about the information bearing signal. A high SNR indicates that the integrity of the signal, when received at the receiver, has been more or less preserved, thereby enabling the individual bits of the signal to be detected above the noise with a low probability of error. It is thus a common objective of any communication system, including multiple-access communication systems, to achieve a high SNR, despite the noisy channels and mediums through which the transmitted signal may traverse as it propagates from the transmitter to the receiver.

In a DS-CDMA system, the transmitted signal is not only subject to additive white Gaussian noise (AWGN), a common form of noise in most communication channels, but is also subject to "multiple-access noise", i.e., noise resulting from the presence of other users who are transmitting at the prescribed carrier frequency and bit rates, but with different signature waveforms. To minimize the effect of the AWGN, it is known in the art to implement the receiver of a DS-CDMA system as a filter matched to the signature waveform of the user (transmitter) of interest. Unfortunately, such matched filter, while optimum for minimizing probability of bit error in AWGN, performs poorly when significant multiple-access noise is present. Thus, what is needed is a type of filter for use within a DS-CDMA receiver that performs acceptably in the presence of significant multiple-access interference.

An optimum multi-user receiver that minimizes multi-user noise is known. However, such optimum multi-user receiver is extremely complex. Numerous sub-optimal simplifications of such optimal structure have been proposed, however, such "simplifications" still require locking and despreading of some or all of the interfering signals, and hence also represent substantially complex circuitry. Thus, the matched filter receiver, despite its limitations, represents the most common method in practice.

Given a matched filter receiver structure, it is known that under certain broad conditions, the SNR in a CDMA system is maximized when all users transmit a signal with power evenly distributed over the entire allowed band. Thus, the power spectrum is a rectangular shape in the frequency domain, with extremely sharp dropoff at the edges of the allowed band (i.e., low "excess bandwidth"). Because this makes efficient use of the allowed band, such will hereafter be referred to as "spectrally efficient" signalling, or transmitting. Unfortunately, the time domain waveform corresponding to a rectangular block in the frequency domain is difficult to generate, especially at high chip rates. This results in a transmitter which is not only expensive to design and manufacture, but may be bulky to house and may consume more power than is desired. Because each user of such system employs a separate transmitter, the overall complexity and cost of the system thus increases dramatically.

It is evident, therefore, that what is needed is a practical, economical receiver structure that represents a simplification of prior art receiver systems. The present invention addresses the above and other needs.

SUMMARY OF THE INVENTION

A DS-CDMA communication system made in accordance with the invention includes a multiplicity of transmitters and at least one receiver. Each transmitter transmits its outgoing DS-CDMA signal using a spectrally inefficient power spectrum, i.e., a non-flat power spectrum, thereby simplifying the circuitry used within the transmitter, and allowing the transmitter to be less expensive and smaller than spectrally-efficient CDMA transmitters. The receiver receives the transmitted CDMA signal and operates thereon on a bit-by-bit basis, i.e., bit decisions are based on observation of the received waveform over approximately one bit time in the absence of knowledge of the other users' (transmitters') spreading codes, chip timing, and carrier phase.

The receiver includes a matched filter and an adaptive linear falter. The matched filter is designed to have a frequency response that matches the power spectrum of the transmitted CDMA signal, as is known in the art. The adaptive linear filter is configured to make the SNR for the spectrally-inefficient signature waveforms received from the transmitter approach asymptotically the SNR that would be received from a spectrally-efficient transmitted signal, at high signal to thermal noise ratios. Advantageously, because CDMA systems typically operate in a situation of high multiple-access noise and low thermal noise, this asymptotic result may be nearly realized in practice. Hence, the DS-CDMA system of the present invention advantageously simplifies the task of the transmitter (allowing transmission of spectrally-inefficient signature waveforms) while attaining a SNR performance nearly as good as spectrally-efficient spreading of the same bandwidth. Further, the use of the adaptive linear filter makes the system highly resistant to narrowband noise.

In accordance with one aspect of the invention, the adaptive linear filter utilized by the DS-CDMA receiver operates as an analog transversal filter on the incoming RF signal, before downconversion occurs to remove the RF carrier. Such filter advantageously allows the SNR to be nearly maximized when the number of CDMA users (i.e., the number of transmitters) is large (more than about 5 to 10). The adaptive filter includes a sequence of delay elements that delay the incoming signal by a prescribed amount. The delay may be realized, for example, using an analog delay line, in which case the delay provided is for a fixed time increment, selected to be an integer multiple of one cycle of the RF carrier signal. Tap points are provided after each delay, and the delayed signal from each tap, as well as the incoming signal, are multiplied by appropriate "tap weight" signals, or tap coefficients, and then summed. The tap coefficients are adaptively adjusted, using feedforward and feedback components of the incoming and delayed signals, so as to produce the set of tap coefficients that approximately maximize the average SNR.

In accordance with another aspect of the invention, after the signal has passed through the adaptive linear filter, the filtered signal is sampled to yield a signal series (e.g., at a rate defined by the chip rate) that enables the signature waveform included within the bit interval to be discerned. The signal series is then despread and summed to provide an output signal which is an appropriate decision statistic for the bit interval.

Thus, one embodiment of the invention may be characterized as a direct-sequence multiple-access code division (DS-CDMA) communication system. Such system includes a multiplicity of separate transmitters and at least one base-station receiver. Each of the mobile transmitters includes: (a) means for producing a unique binary spreading sequence; (b) means for generating from such binary spreading sequence a unique signature waveform, said unique signature waveform having a bandwidth substantially corresponding to an allowed channel bandwidth, and a non-flat power spectrum which rolls gradually at the band edges; (c) means for generating a low bandwidth analog baseband waveform signal encoded with digital data that is to be transmitted; (d) means for multiplying the unique signature waveform with the low bandwidth analog baseband waveform to yield a direct sequence spread waveform signal; (e) means for modulating a common RF carrier signal with the direct sequence spread waveform signal; and (f) means for transmitting the modulated RF carrier signal.

The base-station receiver of such communication system comprises: (a) an RF receiver that receives the transmitted modulated RF carrier signals from each of the multiplicity of transmitters, (b) a filter that filters the modulated RF carrier signal to maximize the SNR and to compensate for the non-flat power spectrum of the signature waveform, and (c) a spread spectrum receiver that processes the filtered modulated RF carrier signal to despread such signal in order to identify a particular signature waveform contained therein, downconvert such signal to remove the RF carrier therefrom, and integrate such signal over a bit time to determine the informational content thereof, i.e., whether such integrated signal represents a logical "1" or a "0". The filter includes a matched filter followed by an adaptive filter.

It is thus a feature of the present invention to provide a simplified DS-CDMA system that achieves a high SNR, despite the noisy channels and mediums through which the transmitted signal traverses as it propagates from the transmitter to the receiver.

It is another feature of the invention to provide such a DS-CDMA system that allows simplified transmitter circuits to be used by eliminating the need for flat-spectrum chip waveform pulses, and instead allows a transmitted chip waveform shape with a rounded spectrum.

It is a further feature of the invention to provide such a simplified DS-CDMA system that includes a receiver which compensates for a spectrally-inefficient shape of the transmitted pulse, which compensation provides a SNR that approximates that which could be obtained using a spectrally-efficient shape.

It is an additional feature of the invention to provide an adaptive filter for use within a DS-CDMA receiver that performs acceptably in the presence of significant multiple-access interference.

It is still a further feature of the invention to provide a practical and economical receiver structure that nearly maximizes the SNR, improves the probability of error, is highly resistant to narrowband noise, and makes bit decisions based on observation of the received waveform over one bit time in the absence of knowledge of the other users' (transmitters') spreading codes, chip timing, and carrier phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:

FIG. 1 illustrates the concept of mobile-to-base communications utilized by a DS-CDMA communication system;

FIG. 2 functionally depicts the concept of code-division multiple access (CDMA); showing how a CDMA signal made up of a plurality of chips at a chip time T.sub.C is used within each bit time, T.sub.B, of a data signal;

FIGS. 3A and 3B conceptually illustrate a spectrally efficient power spectrum and a spectrally inefficient power spectrum, respectively;

FIG. 4 is a simplified functional block diagram of a CDMA transmitter made in accordance with the invention;

FIG. 5 is a simplified functional block diagram of a CDMA receiver made in accordance with the invention;

FIG. 6 conceptually illustrates the result achieved by the invention relative to the frequency domain performance of a communication system made in accordance with the present invention, and in particular illustrates how an adaptive filter, having a transfer function identified as A(e.sup.j.omega.), used within the receiver compensates for the spectrally inefficient performance of the transmitter;

FIG. 7 depicts a model of the CDMA transmitter of the present invention;

FIG. 8 illustrates a model of the CDMA receiver of the present invention;

FIG. 9 shows a more detailed functional block diagram of the continuous-input adaptive filter used within the receiver of FIG. 5; and

FIG. 10 shows a more detailed block diagram of the adaptive linear filter included within the model of FIG. 8.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.

Referring first to FIG. 1, there is shown a communications system 10 that includes a plurality of separate transmitter units 12-1, 12-2, . . . 12-k (any one of which may hereafter be referred to as the transmitter 12 or transmitter unit 12), and a single base-station receiver 14. For many applications, the transmitter units 12 are mobile, as in a digital cellular telephone system. Hence, hereafter the system 10 may be referred to as a mobile-to-base station communication system. However, it is to be understood such description is only exemplary and that the transmitter units 12 need not be mobile.

Each of the transmitter units 12 is configured to transmit a signal that is modulated in accordance with a code-division multiple access (CDMA) scheme, as explained more fully below. The transmitted CDMA signals propagate, along respective signal paths 16-1, 16-2, . . . 16-k (referred to generically hereafter as the signal path 16), through a transmission medium 18, and are received at the base-station receiver 14. The transmission medium 18 may also be referred to as the communication channel. Although a single signal path is shown from each transmitter 12 through the medium 18 to the base-station receiver 14, it is understood that multiple signal paths may exist, e.g., due to reflections of the transmitted signal. Thus, it is common for a given transmitted signal to arrive at the base-station 14 through different signal paths. Such multiple paths create channel distortion in the received signals. Applicant's copending application, A FRACTIONALLY-SPACED EQUALIZER FOR A DS-CDMA SYSTEM, filed concurrently herewith, and incorporated herein by reference, addresses a preferred manner for handling such channel distortion.

The medium (or channel) 18 introduces noise into the transmitted signals received at the base-station 14. Additionally, there may be narrowband interference from a nearby narrowband communications system, or from hostile jamming. A major source of interference in a CDMA system is noise from other users of the system, known in the art as multiple-access interference. An important feature of the receiver 14 of the present invention is to alleviate the effects of such multiple-access noise, and to be highly resistant to narrowband noise, as well as to increase the signal-to-noise ratio (SNR) so that the transmitted signals can be detected at the receiver 14 with a low probability of error.

All efficient communication systems, such as the mobile-to-base station system shown in FIG. 1, utilize some technique for maximizing the channel efficiency, i.e., for maximizing the number of users (i.e., transmitter units) that may communicate through the system 10 without interfering with each other. As indicated in the above background discussion, many different "multiple use" schemes are known in the art. Such multi-access schemes include, for example, frequency division multiple access (FDMA) wherein each user is assigned a different transmission frequency; time division multiple access (TDMA), wherein each user transmits a signal that occupies a different time slot or space; and code-division multiple access (CDMA), wherein each user transmits a signal at the same frequency and time, but wherein each information-bearing signal is further encoded with a unique signature waveform. The communication system 10 of the present invention utilizes a CDMA scheme, and is particularly suited for a CDMA scheme that operates at a high data rate, although the invention may be used with any CDMA scheme, e.g., a CDMA-based cellular telephone system.

The basic concept of CDMA modulation is taught with respect to FIG. 2. Shown in the upper portion of FIG. 2 is a data waveform 20 that includes a plurality of bit times, T.sub.B. For purposes of FIG. 2, the waveform 20 is shown as a function of time and amplitude, with time being the horizontal axis, and amplitude being the vertical axis. The waveform 20 thus includes a sequence of data bits, each of duration T.sub.B, with each bit being either a digital "1" or a "0". A sequence of bits of a prescribed length (number of bits) is referred to as a digital "word". Typically, for most digital communication systems, a sequence of digital data words must be transmitted in a prescribed format in order to identify a particular user, and in order to provide other control and management information. For example, a cellular telephone system usually requires that each mobile transmitter transmit several informational words, each of a known or prescribed length, e.g., 48 bits. Even voice signals that are transmitted from the mobile transmitters to the base-station receiver once acquisition has been achieved and all the necessary control and management signals have been sent, may be converted to digital words prior to transmission. The waveform 20 thus comprises the low bandwidth information bearing signal. Only two complete bits of such information bearing signal are shown.

For clarity, the digital waveform 20 shown in FIG. 2 is depicted in an NRZ (non-return-to-zero) format (where a logical "1" maps to a -1 normalized amplitude, and a logical "0" maps to a +1 normalized amplitude). It is to be understood, however, that such representation is only exemplary. In practice, the information bearing signal 20 may be encoded in any appropriate manner, e.g., with Manchester (biphase) encoding, in order to shift the power spectral density of the transmitted waveform away from zero, or for other purposes.

In a CDMA scheme, each bit of the digital waveform 20 is further subdivided into a plurality of "chip times", T.sub.C, as shown in the lower portion of FIG. 2. Typically, there is a large number of chip times within each bit. About 127 to 255 chips per bit is typical. The signature waveform consists of a square-wave binary waveform signal 21 which assumes the value of +1 or -1 in each chip-time interval T.sub.C. The value assumed by the signature waveform during each interval T.sub.C is determined by the successive values of the binary spreading sequence (code) for a particular user. The signature waveform 21 is multiplied by the information bearing signal 20 to yield a direct sequence spread waveform 22.

For the example shown in FIG. 2, there are ten chips per bit. A chip waveform 23 comprises a rectangular (in time) pulse of duration T.sub.C and amplitude 1. The signature waveform signal 21 is obtained by repeating the chip waveform 23 at T.sub.C intervals, each occurrence being multiplied by successive elements of the spreading code {+1, -1, +1, +1, -1, -1, +1, -1, +1, -1}, with the spreading code being repeated each bit time T.sub.B. The information bearing signal 20 is multiplied by the signature waveform 22 to yield the direct sequence spread waveform. Thus, when the data bit is a logical "1" (or a -1 amplitude as shown in FIG. 2), the direct sequence spread waveform 22 comprises the inverse of the signature waveform 21. When the data bit is a logical "0" (or a "+1" amplitude as shown in FIG. 2), the direct sequence spread waveform 22 is the same as the signature waveform 21.

Each mobile transmitter 12 (FIG. 1) is configured to uniquely encode each bit time with a particular signature waveform. Such signature waveform thereafter serves to uniquely identify the bit as having originated from a particular transmitter. Thus, when multiple transmitters are used, and multiple bits are thus in the transmission medium at any given time, each bit carries its own unique signature waveform, which unique signature waveform may be considered as an "identification tag" that identifies the particular transmitter from which the bit originated. The presence of such "identification tag" thereafter conceptually provides a means for sorting all the bits received at the base-station receiver 14 so that the receiver processing circuits can determine which received bit signals came from which transmitters, thereby enabling multiple users to use the system at the same time.

A significant advantage of a CDMA-based system is that the transmitter units may operate asynchronously. Asynchronous operation significantly reduces the c