CA2552443C

CA2552443C - Multichannel cdma subtractive interference canceler

Info

Publication number: CA2552443C
Application number: CA2552443A
Authority: CA
Inventors: Donald L. Schilling; John Kowalski; Shimon Moshavi
Original assignee: InterDigital Technology Corp
Current assignee: InterDigital Technology Corp
Priority date: 1994-07-26
Filing date: 1995-07-05
Publication date: 2010-10-05
Anticipated expiration: 2015-07-05
Also published as: US7167464B2; SE532856C2; ES2137140T3; HK1030697A1; ES2342903T3; CN1373583A; ES2151461T1; FI117612B; EP0772928A4; SE521137C2; US20020172172A1; CN1165125C; DK176193B1; US7242675B2; CN1252957C; CN1490966A; NZ290723A; CN1154770A; DK200700410A; ATE460785T1

Abstract

A plurality of channels are received as a received signal. Each channel is associated with a code. For each of the plurality of channels, others of the plurality of channels are subtracted from the received signal and a result of that subtracting is despread as data for that channel.

Description

MULTICHANNEL CDMA SUBTRACTIVE INTERFERENCE CANCELER

Division of 2,369,499 filed 1995/07/05, which is a Division of 2,342,454 filed 1995/07/05, which is a Division of 2,195,534, filed 1995/07/05.

BACKGROUND OF THE INVENTION
This invention relates to spread-spectrum communications, and more particularly to an interference canceler and method for reducing interference in a direct sequence, code division multiple access receiver.

DESCRIPTION OF THE RELEVANT ART
Direct sequence, code division multiple access., spread-spectrum communications systems are capacity limited by interference caused by other simultaneous users. This is compounded if adaptive power control is not used, or is used but is not perfect.
Code division multiple access is interference limited.
The more users transmitting simultaneously, the higher the bit error rate (BER)-., Increased capacity requires forward error correction (FEC)- coding, which in turn, increases the data rate and limits capacity.

SUMMARY OF THE . INVENTION
A general object of the invention is to reduce noise resulting from N-1 interfering signals in a direct sequence, spread-spectrum code division multiple access receiver.
The present invention, as embodied and broadly described herein, provides a spread-spectrum code division multiple access. (CDMA) interference canceler for reducing interference in.a spread-spectrum CDMA receiver. having. N
channels.. Each of the N channels is spread-spectrum processed by a distinct chip-code signal. The chip-code signal, preferably, is derived from 'a distinct pseudo-noise (PN) sequence, which may be generated 'from a distinct chip codeword. The interference canceler partially cancels N-1 interfering COMA channels, and provides a signal-to-noise ratio.(SNR) improvement of approximately.N/PG, where PG' is the processing gain. Processing gain is the ratio of the chip rate divided by the bit rate. By canceling or reducing interference, the SNR primarily may be due to thermal noise, and residual, interference-produced noise. Thus, the SNR may increase, lowering the BER, which reduces the demand for a FEC encoder/decoder.
The interference canceler, for a particular channel, includes a plurality of despreading means, a plurality of spread-spectrum-processing means, subtracting means, and channel-despreading means. Using a plurality of chip-code signals, the plurality of despreading means despreads the spread-spectrum CDMA signals as a plurality of despread signals, respectively. The plurality of spread-spectrum-processing means uses a timed version of the plurality of chip-code signals, for spread-spectrum processing the plurality of despread signals, respectively, with a chip-code signal corresponding to a respective despread signal. The timed version of a chip-code signal may be generated by delaying the chip-code signal from a chip-code-signal generator.
Alternatively, a matched filter may detect a particular PN sequence in the spread-spectrum CDMA signal. A chip-code-signal generator may use the detected signal from the matched filter to trigger a timed version of the chip-code signal.
For recovering a particular CDMA channel using an ith chip-code signal, the subtracting means subtracts from the spread-spectrum CDMA signal, each of the N-1 spread-spectrum-processed-despread signals, thereby generating a subtracted signal. The N-1 spread-spectrum-processed-despread signals do not include the spread-spectrum-processed-despread signal of the irh channel corresponding to the ith chip-code signal. The channel-despreading means despreads the subtracted signal with the ith chip-code signal.
According to a first broad aspect of the present disclosure there is disclosed a remote unit having a receiver for recovering data transmitted over a plurality of channels employing a wireless code division multiple access communication technique, the receiver comprising: a unit for receiving the plurality of channels as a received signal, each channel associated with a code;
a unit for despreading the plurality of channels as a plurality of despread received signals; a unit for respreading each channel despread signal with a respective channel code wherein the respreading channel code is aligned to a -2a-timing of each of the despread received signals; and a unit for subtracting for each of the plurality of channels, others of the plurality of channels from the received signal and despreading a result of that subtraction by a delayed version for a first chip-code signal by a first channel mixer and filtered by a first channel filter as data for that channel; and wherein the despreading further comprises passing the result of the unit for subtracting through the first channel mixer together with a code of a channel excluded from the subtraction operation, and wherein the receiving, despreading, respreading, and subtracting units repeat their operation employing the despread result obtained during an initial performance of the operation and combining the result from the repeated operation and passing the combined results through a decision device.
According to a second broad aspect of the present disclosure there is disclosed a remote unit receiver for recovering data transmitted over a plurality of channels employing a wireless code division multiple access communication technique, comprising: a unit for receiving the plurality of channels as a received signal, each channel associated with a code; a unit for despreading the plurality of channels as a plurality of despread received signals; a unit for respreading each channel despread signal with a respective channel code wherein the respreading channel code is aligned to a timing of each of the despread received signals; and for each of the plurality of channels, a subtractor for subtracting others of the plurality of channels from the received signal and despreading a result of that subtraction by a delayed version for a first chip-code signal by a first channel mixer and filtered by a first channel filter as data for that channel; and wherein the despreading further comprises passing the result of the unit for subtracting through the first channel mixer together with a code of a channel excluded from the subtraction operation, and wherein the receiving, despreading, respreading, and subtracting units repeat their operation employing the despread result obtained during an initial performance of the operation and combining the result from the repeated operation and passing the combined results through a decision device.

Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention, and together with the description serve to explain the principles of the invention.
FIG. 1 is a block diagram of the spread-spectrum CDMA interference canceler using correlators;
FIG. 2 is a block diagram of the spread-spectrum CDMA interference canceler for processing multiple channels using correlators;
FIG. 3 is a block diagram of the spread-spectrum CDMA interference canceler using matched filters;
FIG. 4 is a block diagram of the spread-spectrum CDMA

interference canceler for processing multiple channels using matched filters;
FIG. 5 is a block diagram of the spread-spectrum CDMA
interference canceler having multiple iterations for processing multiple channels;
FIG. 6 illustrates theoretical performance characteristic for Eb/tl = 6 dB;
FIG. 7 illustrates theoretical performance characteristic for Eb/r7 = 10 dB;
FIG. 8 illustrates theoretical performance -characteristic for Eb/t) = 15 dB;
FIG. 9 illustrates theoretical performance characteristic for Eb/rl = 20 dB;
FIG. 10 illustrates theoretical performance characteristic for Eb/r7 = 25 dB;
FIG. 11 illustrates theoretical performance characteristic for Eb/ri = 30 dB;
FIG. 12 is a block diagram of interference cancelers connected together;
FIG. 13 is a block diagram combining the outputs of the interference cancelers of FIG. 12;
FIG. 14 illustrates simulation performance characteristics for asynchronous, PG = 100, Equal Powers, EbN = 30 dB;
FIG. 15 illustrates simulation performance characteristics for asynchronous,. PG = 100, Equal Powers, EbN = 30 dB;
FIG. 16 illustrates simulation performance characteristics for asynchronous, PG = 100, Equal Powers, EbN = 30 dB; and FIG. 17 illustrates simulation performance characteristics for asynchronous, PG = 100, Equal Powers, EbN = 30 dB.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference now is made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals indicate like elements throughout the several views.
In the exemplary arrangement shown in FIG. 1, a spread-spectrum code division multiple access (CDMA) interference canceler is provided for reducing interference in a spread-spectrum CDMA receiver having N channels. The present invention also works on a spread-spectrum code division multiplexed (CDM) system. Accordingly, without loss of generality, the term spread-spectrum CDMA signal, as used herein, includes spread-spectrum .CDMA signals and spread-spectrum-CDM signals. In a personal communications service, the interference canceler may be used at a base station or in a remote unit such as a handset.
FIG. .1 illustrates the interference canceler for the first channel, defined by the first chip-code signal. The interference canceler includes a plurality of despreading means, a plurality of timing means, a plurality of spread-spectrum-processing means, subtracting means, and first channel-despreading means.
Using a plurality of chip-code signals, the plurality of despreading means despreads the received spread-spectrum .CDMA signals as a plurality of despread signals, respectively. In FIG. 1 the plurality of despreading means is shown as first despreading means, second despreading means, through Nth despreading means. The first despreading means includes a first correlator, which is embodied, by way of example, as a first mixer 51, first chip-code-signal generator 52, and a first integrator 54. The first integrator_54-alternativery may be a first lowpass filter or a first bandpass filter. The first mixer 51 is coupled between the input 41 and the first chip-code-signal generator 52' and the first integrator 54.
The second despreading means includes a second correlator, which is embodied, by way of example, as second mixer 61, second chip-code-signal generator 62 and second integrator 64. The second integrator 64 alternatively may be a second lowpass filter or a second bandpass filter. The second mixer 61, is coupled between the input 41, the second chip-code-signal generator 62, and the second integrator 64.
The Nth despreading means is depicted as an Nth correlator shown, by way of example, as Nth mixer 71, and Nth chip-code-signal generator 72, and Nth integrator 7-4. The Nth integrator 74 alternatively may be an Nth lowpass filter or an Nth bandpass filter. The Nth mixer 71 is coupled between t- input 41, the Nth chip-code-signal generator 72 and the Nth it.egrator 74. --F-z is well known in the art, the first through Nth despr::_..._ing means maybe embodied as any device which can despread a channel in a spread-spectrum signal.
The plurality of timing means may be embodied as a 2.0_ plurality of delay devices 53, 63, 73. A first delay device 53 has a delay time T, which is approximately the same as the integration time Tb of first integrator 54, or time constant of the first lowpass filter or first bandpass filter. A second delay device 63 has a time delay T, which is approximately the same as the integration time Tb of .second integrator 64, or time constant of the second lowpass filter or second bandpass filter. Similarly, the Nth delay device 73 has a time delay T, which is approximately the same as the integration time Tb of Nth integrator 74, or time constant of the Nth lowpass filter or Nth bandpass filter.
Typically, the integration times of the first integrator 54, second integrator 64 through Nth integrator 74 are the same.
If Iowpass filters are used, then typically the-time constants of the first lowpass filter, second lowpass filter through Nth lowpass filter are the same. If bandpass filters are used, then the time constants of the first bandpass filter, second bandpass filter through Nth bandpass filter are the same.
The plurality of spread-spectrum-processing means regenerates each of the plurality of despread signals as a plurality of spread-spectrum signals. The plurality of spread-spectrum-processing means uses a timed version, i.e.
delayed version, of the plurality of chip-code signals, for spread-spectrum processing the plurality of despread signals, respectively, with a chip-code signal corresponding to a respective despread signal. The plurality of spread-spectrum-processing means is shown, by way of example, as a first processing mixer 55, a second processing mixer 65, through an Nth processing, mixer 75. The first processing mixer 55 is coupled to the first integrator 54, and through a first delay device 53 to the first chip-code-signal generator 52. The second processing mixer 65 is coupled to the second integrator 64, and through the second delay device 63 to the second chip-code-signal generator 62. The Nth processing mixer 75 is coupled to the Nth integrator 74 through the delay device 73 to the Nth chip-code-signal generator 72.
For reducing interference to a channel using an ith chip-code signal of the spread-spectrum CDMA signal, the subtracting means subtracts, from the spread-spectrum CDMA
.signal, each of the N-1 spread-spectrum-processed-despread signals not corresponding to the ith channel. The subtracting means thereby generates a subtracted signal.
The subtracting means is shown as a first subtractor 150.
The first subtractor 150 is shown coupled to the output of the second processing mixer 65, through the Nth processing mixer 75. Additionally, the first subtractor 150 is coupled through a main delay device 48' to the input 41.
The ith channel-despreading means despreads the subtracted signal with the ith chip-code signal as the ith channel. The first channel-despreading means is shown as a first channel mixer 147. The first channel mixer 147 is coupled to the first delay device 53, and to the first subtractor 150. The first channel integrator 146 is coupled to the first channel mixer 147.
The first chip-code-signal generator 52, the second chip-code-signal generator 62, through the Nth chip-code-signal generator 72 generate a first chip-code signal, a second chip-code signal, through a Nth chip-code signal, respectively. The term "chip-code signal" is used herein to mean the spreading signal of a spread-spectrum signal, as is well known in the art. Typically the chip-code signal is generated from a pseudorandom (PN) sequence. The first chip-code signal, the second chip code signal, through the Nth chip-coae signal might be generated from a first PN
sequence, a second PN sequence, through a Nth PN sequence, respectively. The first PN sequence is defined by or generated from a first chip codeword, the second PN sequence is defined by or generated from a second chip codeword, through the Nth PN sequence is defined by or generated from a Nth chip-codeword. Each of the first chip codeword, second chip codeword through Nth chip codeword is distinct, i.e. different from one another. In general, a chip codeword can be the actual sequence of_a PN sequence, or used to define settings for generating the PN sequence. The .settings might be the delay taps of shift registers, for example.
A first channel of a received spread-spectrum CDMA
signal at input 41 is despread by first mixer 51 as a first despread signal, using the first chip-code signal generated by first chip-code-s-ignal generator 52. The first despread signal from the first mixer 51 is filtered through first integrator 54. First integrator 54 integrates fora time Tb, the time duration of a symbol such as a bit. At the same time, the first chip-code signal is delayed by time T

by delay device 53. The delay time T is approximately equal to the integration time Tb plus system or component delays.
Systems or component delays are usually small, compared to integration time Tb.
The delayed version of the first chip-code signal is processed with the first despread signal from the output of the first integrator 54 using the first spreading mixer 55.
The output of the first spreading mixer 55 is fed to subtractors other than first subtractor 150 for processing the second through Nth channels of the spread-spectrum CDMA
signal.
For reducing interference to the first channel of the spread-spectrum CDMA signal, the received spread-spectrum CDMA signal is processed by the second through Nth despreaders as follows. The second channel of the spread-spectrum CDMA signal is despread by the second despreading means. At the second mixer 61, a second chip-code signal, generated by the second chip-code-signal generator 62, despreads the second channel of the spread-spectrum CDMA
signal. The despread second channel is filtered through second integrator 64. The output of the second integrator 64 is the second despread signal. The second despread signal is spread-spectrum processed by second processing mixer 65 by a delayed version of the second chip-code signal. The second chip-code signal is delayed through .delay device 63. The delay device 63 delays the second chip-code signal by time T. The second channel mixer 65 spread-spectrum processes a timed version, i.e. delayed version, of the second chip-code signal with the filtered version of the second spread-spectrum channel from second integrator 64. The term "spread-spectrum process" as used herein includes any method for generating a spread-spectrum signal by mixing or modulating a signal with a chip-code signal. Spread-spectrum processing may be done by product devices, EXCLUSIVE-OR gates, matched filters, or any other device or circuit as is well known in the art.
Similarly, the Nth channel of the spread-spectrum CDMA
signal is despread by the Nth despreading means.
Accordingly, the received spread-spectrum CDMA signal has the Nth channel despread by Nth mixer 71, by mixing the spread-spectrum CDMA signal with the Nth chip-code signal from Nth chip-code-signal generator 72. The output of the Nth mixer 71 is filtered by Nth integrator 74. The output of the Nth integrator 74, which is the Nth despread signal, is a despread and filtered version of the Nth channel of the spread-spectrum CDMA signal. The Nth despread signal is spread-spectrum processed by a delayed version of the Nth chip-code signal. The Nth chip-code signal is delayed through Nth delay device 73. The Nth processing mixer 75 spread-spectrum processes the timed version, i.e. a delayed version, of the Nth chip-code signal with the Nth despread signal.
At.the first subtractor 150, each. of_ the outputs- of the second processing mixer 65 through the Nth processing mixer 75 is subtracted from a timed version, i.e. a delayed version, of the spread-spectrum CDMA signal from input 41.
The delay of the spread-spectrum CDMA signal is timed through the first main delay device 48. Typically, the delay of the first main delay device 48 is time T, which is approximately equal to the integration time of the first .integrator 5.4.. through Nth integrattor 74.
At the output of the. first..subtractor.150, is. generated a first subtracted signal. The first subtracted signal, for the first channel of the spread-spectrum CDMA signal, is defined herein to be the outputs from the second processing mixer 65 through Nth processing mixer 75, subtracted from the delayed version of the spread-spectrum CDMA signal. The second subtracted signal through Nth subtracted signal are similarly defined.

The delayed version of the first chip-code signal from the output of first delay device 53 is used to despread the output of the first subtractor 150. Accordingly, the first subtracted signal is despread by the first chip-code signal by first channel.mixer 147. The output of the first channel mixer 147 is filtered by first channel integrator 147. This produces an output estimate d1 of the first channel of the spread-spectrum CDMA signal.
As illustratively shown in FIG. 2, a plurality of subtractors 150, 250, 350, 450 can be coupled appropriately to the input 41 and to a first spreading mixer 55, second spreading mixer 65, third spreading mixer, through an Nth spreading mixer 75 of FIG. 1. The plurality of subtractors 150, 250, 350, 450 also are coupled to the main delay device 48 from the input 41_ This-arrangement can generate a. first subtracted signal from the first subtractor 150,. a second subtracted signal from the second subtractor 250, a third subtracted signal from the third subtractor 350,__, through an Nth subtracted signal from an Nth subtractor 450.
The outputs of the first subtractor 150, second subtractor 250, third subtractor 350, through the Nth subtractor 450 are each coupled to a respective first channel mixer 147, second channel mixer 247, third channel mixer 347, through Nth channel mixer 447. Each of the channel mixers is coupled to a delayed version of the first chip-code signal, g,(t-T), second chip-code signal, g2(t-T), third chip-code signal, g3(t-T), through Nth chip-code signal, g,(t-T).- The outputs of each of the respective first channel mixer 147, second channel mixer 247, third channel mixer 347, through Nth channel mixer 447 are coupled to a first channel integrator 146, second channel integrator 246, third channel integrator 346 through Nth channel integrator 446,- respectively.- ' At the -output of each of the channel integrators is produced an estimate of the respective first channel d1 , second channel d2, third channel d3, through Nth channel dN.
Referring to FIG. 1, use of the present invention is illustrated for the first channel of the spread-spectrum CDMA signal, with the understanding that the second through Nth CDMA channels work similarly. A received spread-spectrum CDMA signal at input 41 is delayed by delay device 48 and fed to the first subtractor 150. The spread-spectrum CDMA signal has the second channel through Nth channel despread by second mixer 61 using the second chip-code signal, through the Nth mixer 71 using the Nth chip-code signal. The respective second chip-code signal through the Nth chip-code signal are generated by the second chip-code-signal generator 62 through the Nth chip-code-signal generator 72. The second channel through Nth channel are despread and filtered through the second integrator-.64 through the Nth integrator 74, respectively. The despreading removes, partially or totally, the non-despread channels at the outputs of each of the second integrator 64 through Nth integrator 74.
In a preferred embodiment, each of the chip-code signal used for the first chip-code-signal generator 52, second chip-code-signal generator 62 through the Nth chip-code-signal generator 72, are orthogonal to each other. Use of chip-code signals having orthogonality however, is not required for operation of the present invention. When using orthogonal chip-code signals, the despread signals have the respective channel -plus noise at the output of each of the integrators. With orthogonal chip-code signals, theoretically the mixers remove channels orthogonal to the despread channel. The respective channel is spread-spectrum processed by the respective processing mixer.
At the output of the second processing mixer 65 through the Nth processing mixer 75 is a respread-version of the second channel through the Nth channel, plus noise components contained therein. Each of the second channel through Nth channel is then subtracted from the received spread-spectrum CDMA signal by the first subtractor 150.
The first subtractor 150 produces the first subtracted signal. The first subtracted signal is despread by a delayed version of the first chip-code signal by first channel mixer 147, and filtered by first channel filter 146.
Accordingly, prior to despreading the first channel of the spread-spectrum CDMA signal, the second through Nth channels plus noise components aligned with these channels are subtracted from the received spread-spectrum CDMA signal.
As illustratively shown in FIG. 3, an alternative embodiment of the spread-spectrum CDMA interference canceler includes a plurality of first despreading means, a plurality of spread-spectrum-processing means, subtracting means, and second despreading means. In FIG. 3, the plurality of despreading means is shown as first despreading means, second despreading means through Nth despreading means. The first despreading means is embodied as a first matched filter 154.
The first matched filter 154 has an impulse response matched to the first chip-code signal, which is used to spread-spectrum process and define the first channel of the spread-spectrum CDMA signal. The first matched filter 154 is coupled to the input 41.
The second despreading means is shown as second matched filter 164. The second matched filter 164 has an impulse .response matched to the second chip-code signal, which. is used to spread-spectrum process and define the second channel of the spread-spectrum CDMA signal. The second .matched filter 164 is coupled to the input 41.
The Nth despreading means is shown an Nth matched filter 174. The Nth matched filter has an impulse response matched to the Nth chip-code signal, which is used to spread-spectrum process and define the Nth` channel of the spread-spectrum CDMA signal. The Nth matched filter is coupled to the input 41.

The term matched filter, as used herein, includes any type of matched filter that can be matched to a chip-code signal. The matched filter may be a digital matched filter or analog matched filter. A surface acoustic wave (SAW) device may be used at a radio frequency (RF) or intermediate frequency (IF). Digital signal processors and application specific integrated circuits (ASIC) having matched filters may be used at RF, IF or baseband frequency.
In FIG. 3, the plurality of spread-spectrum-processing means is shown as the first processing mixer 55, the second processing mixer 65, through the Nth processing mixer 75.
The first processing mixer 55 may be coupled through a first adjustment device 97 to the first chip-code-signal generator 52. The second processing mixer 65 may be coupled through the second adjustment device 98 to the second chip-code-signal generator 62. The Nth processing mixer 75 may be coupled through the Nth adjustment device 73 to the Nth chip-code-signal generator 72.. The first adj_ustment..device 97,, second adjustment device 98 through Nth adjustment device 99 are optional, and are used as an adjustment for aligning the first chip-code signal, second chip-code signal through Nth chip-code signal with the first despread signal, second despread signal through Nth despread signal, outputted from the first matched filter 154, second matched filter 164 through Nth matched filter *174, respectively.
The subtracting means is shown as the first subtractor 150. The first subtractor 150 is coupled to the output of the second processing mixer 65, through the Nth processing mixer 75. Additionally, the first subtractor 150 is coupled through the main delay device 48 to the input 41.
The first channel-despreading means is shown as a first channel-matched filter 126. The first channel-matched filter 126 is coupled to the first subtractor 150. The first channel-matched filter 126 has an impulse response matched to the first chip-code signal.

A first channel of a received spread-spectrum CDMA
signal, at input 41, is despread by first matched filter 154. The first matched filter 154 has an impulse response matched to the first chip-code signal. The first chip-code signal defines the first channel of the spread-spectrum CDMA
signal, and is used by the first chip-code-signal generator 52. The first chip-code signal may be delayed by adjustment time r by adjustment device 97. The output of the first matched filter 154 is spread-spectrum processed by the first processing mixer 55 with the first chip-code signal. The output of the first processing mixer 55 is fed-to subtractors other than the first subtractor 150 for processing the second channel through Nth channel of the spread-spectrum CDMA signals.
For reducing- interference to the first spread-spectrum channel, the received spread-spectrum CDMA signal is processed by the second despreading means through Nth despreading means as follows. The second matched filter 164 has an impulse response matched to the second chip-code -signal. The second chip-code signal defines the second channel of the.spread-spectrum CDMA signal, and is used by the second chip-code-signal generator 62. The second matched filter 164 despreads the second channel of.the spread-spectrum CDMA signal. The output of the second matched filter 164 is the second despread signal. The .second despread signal triggers second chip-code-signal generator 62. The second despread signal also is spread-spectrum processed by second processing mixer 65 by a timed version of the second chip-code signal. The timing of the second chip-code signal triggers the second despread signal from the second matched filter 164.
Similarly, the Nth channel of the spread-spectrum CDMA
signal is despread-by the Nth despreading means.
Accordingly, the received spread-spectrum CDMA signal has the Nth channel despread by Nth matched filter 174. The output of the Nth matched,filter 174 is the Nth despread signal, i.e. a despread and filtered version of the Nth channel of the spread-spectrum CDMA signal. The Nth despread signal is spread-spectrum processed by a timed version of the Nth chip-code signal. The timing of the Nth chip-code signal is triggered by the Nth despread signal from the Nth matched filter 174. The " Nth processing mixer 75 spread-spectrum processes the timed version of the Nth chip-code signal with the Nth despread signal.
At the first subtractor 150, each of the outputs of the second processing mixer 65 through the Nth processing mixer 75 are subtracted from a delayed version of the spread-spectrum CDMA signal from input 41. The delay of the spread-spectrum CDMA signal is timed through delay device 48. The time of delay device 48 is set to align the second through Nth spread-spectrum-processed-despread signals for subtraction from the spread-spectrum CDMA signal. This generates at the output of the first subtractor 150, a first subtracted signal. The subtracted signal is despread by the first channel-matched filter 126. This produces an output estimate d, of the first channel of the spread-spectrum CDMA
signal.
As illustrated in Fig. 4, a plurality of subtractors 150, 250, 350, 450 can be coupled appropriately to the output from a first processing mixer, second processing ,mixer, third processing mixer, through a Nth processing mixer, and to a main delay device from the input. A first subtracted signal is outputted from the first subtractor 150, a second subtracted signal is outputted from the second subtractor 250, a third subtracted signal is outputted from the third subtractor 350, through an-Nth subtractor signal is outputted from an Nth subtractor 450.
The output of the first subtractor 150, second subtractor 250, third subtractor 350, through the Nth subtractor 450 are each coupled to a respective first channel-matched filter 126, second channel-matched filter 226, third channel-matched filter 326, through Nth channel-matched filter 426. The first channel-matched filter 126, second channel-matched filter 226, third channel-matched filter 326 through Nth channel-matched filter 426 have an impulse response matched the first chip-code signal, second chip-code signal, third chip-code signal, through Nth chip-code signal, defining the first channel, second channel, third channel through Nth channel, respectively, of the spread-spectrum CDMA signal. At each of the outputs of the respective first channel-matched filter 126, second channel-matched filter 226, third channel-matched filter 326, through Nth channel-matched filter 426, is produced an estimate of the respective first channel d,, second channel d2, third channel d3, through Nth channel dn.
In use, the present invention is illustrated for the first channel of the spread-spectrum CDMA signal, with the understanding that the second channel through Nth channel work similarly. A received spread-spectrum CDMA signal at input 41 is delayed by delay device 48 and fed to subtractor 150. The same spread-spectrum CDMA signal has the second through Nth channel despread by the second matched filter 164 through the Nth matched filter 174. This despreading removes the other CDMA channels from the respective despread channel. In a preferred embodiment, each of the chip-code signals used for the first channel, second channel, through the Nth channel, is orthogonal to the other chip-code signals. At the output of the first matched filter 154, second matched filter 164 through N`h matched filter 174, are the first despread signal, second despread signal through Nth despread signal, plus noise.
The respective channel is spread-spectrum processed by the-processing mixers. Accordingly, at the output of the second processing mixer 65 through the Nth processing mixer 75 is a spread version of the second despread signal through the Nth despread signal, plus noise components contained therein. Each of the spread-spectrum-processed-despread signals, is then subtracted from the received spread-spectrum CDMA.signal by the first subtractor 150. This produces the first subtracted signal. The first subtracted signal is despread by first channel-matched filter 126.
Accordingly, prior to despreading the first channel of the spread-spectrum CDMA signal, the second channel through Nth channel plus noise components aligned with these channels, are subtracted from the received spread-spectrum CDMA
signal.
As is well known in the art, correlators and matched filters may be interchanged to accomplish the same function.
FIGS. 1 and 3 show alternate embodiments using correlators or matched filters. The arrangements may be varied. For example, the plurality of despreading means may be embodied as a plurality of matched filters, while the channel despreading means may be embodied as a correlator.
Alternatively, the plurality of despreading means may be a combination of matched filters and correlators. Also, the spread-spectrum-processing means my be embodied as a matched filter or SAW, or as EXCLUSIVE-OR gates or other devices for mixing a despread signal with a chip-code signal. As is well known in the art, any spread-spectrum despreader or demodulator may despread the spread-spectrum CDMA signal.
The particular circuits shown in FIGS. 1-4 illustrate the invention by way of example.
The concepts taught in FIGS. 1-4 may be repeated, as shown in FIG. 5. FIG. 5 illustrates a first plurality of interference cancelers 511, 512, 513, a second plurality of interference cancelers 521, 522, 523, through an Nth plurality of interference cancelers 531, 532, 533. Each plurality of interference cancelers includes appropriate elements as already disclosed, and referring to FIGS. 1-4.
The input is delayed through a delay device in each interference canceler.
The received spread-spectrum CDMA signal has interference canceled initially by the first plurality of interference cancelers 511, 512, 513, thereby producing a first set of estimates, i.e. a first estimate d11, a second estimate d12, through an Nth estimate d1N, of the first channel, second channel through the Nth channel, of the spread-spectrum CDMA signal. The first set of estimates can have interference canceled by the second plurality of interference cancelers 521, 522, 523. The first set of estimates -d11, d12, ... , d1N, of the f irst channel, second channel through Nth channel, are input to the second plurality of interference cancelers, interference canceler 521, interference canceler 522 through Nth interference canceler 523 of the second plurality of interference cancelers. The second plurality of interference cancelers thereby produce a second set of estimates, i.e. d21, d22, .... d2N, of the first channel, second channel, through Nth channel. Similarly, the second set estimates can pass through a third plurality of interference cancelers, and ultimately through an Mth set of interference cancelers.531, 532, 533, respectively.
The present invention also includes a method for reducing interference in a spread-spectrum CDMA receiver having N chip-code channels. Each of the N channels is .identified by a distinct chip-code signal. The method comprises the steps of despreading, using a plurality of chip-code signals, the spread-spectrum CDMA signal as a plurality of despread signals, respectively. Using a timed version of the plurality of chip-code signals, the plurality of despread signals are spread-spectrum processed with a chip-code signal corresponding to a respective despread signal. Each of the N-1 spread-spectrum-processed-despread signals, is subtracted from the spread-spectrum CDMA signal, with the N-1 spread-spectrum-processed-despread signals not including a spread-spectrum-processed signal of the ith despread signal, thereby generating a subtracted signal.
The subtracted signal is despread to generate the itn channel.
The probability of error Pe for direct sequence, spread-spectrum CDMA system is:

Pe = 2erfc(aSNR)h where erfc is complementary error function, SNR is signal--to-noise ratio, and 1 < a < 2. The value of a depends on how a particular interference canceler system is designed.
The SNR after interference cancellation and method is given by:

SNR = (PG/N)R+1 1+ (PG/N) R+1 1 1-(N/PG ) R+t Eb/r7 1-N/PG

where N is the number of channels, PG is the processing gain, R is the-number of repetitions of the interference canceler, Eb is energy per information bit and r7 is noise power spectral density.
FIG. 6 illustrates theoretical performance characteristic, of the interference canceler and method for when Eb/r1 = 6 dB. The performance characteristic is illustrated for SNR out of the interference canceler, versus PG/N. The lowest curve, for R = 0, is the performance without the interference canceler The curves, for R = 1 and R = 2, illustrates improved performance for using one and two iterations of the interference canceler as shown in FIG. 5. As PG/N -+ 1, there is insufficient SNR to operate.
If PG > N, then the output SNR from the interference canceler approaches Eb/r7. Further, if (N/PG)R`' << 1, then SNR -+ (Eb/r7) (1 - N/PG) .

FIG. 7 illustrates the performance characteristic for when Eb/n = 10 dB. FIG. 7 illustrates that three iterations of the interference canceler can yield a 4 dB improvement with PG/N = 2.
FIG. 8 illustrates the performance characteristic for when Eb/n = 15 dB. With this bit energy to noise ratio, two iterations of the interference canceler can yield 6 dB
improvement for PG/N = 2.
FIG. 9 illustrates the performance characteristic for when Eb/r) = 20 dB. With this bit energy to noise ratio, two iterations of the interference canceler can yield 6 dB
improvement for PG/N = 2. Similarly, FIGS. 10 and 11 shows that one iteration of the interference canceler can yield more than 10 dB improvement for PG/N = 2.
The present invention may be extended to a plurality of interference cancelers. As shown in FIG. 12, a received spread-spectrum signal, R(t), is despread and detected by CDMA/DS detector 611. Each of the channels is represented as outputs 001, 002, 003, . . . . Dom. Thus, each output is a despread, spread-spectrum channel from a received spread-spectrum signal,.R(t).
Each of the outputs of the CDMA/DS detector 611 is passed through a plurality of interference cancelers 612, 613, . . . , 614, which are serially connected. Each of the spread-spectrum channels passes through the interference canceling processes as discussed previously. The input to each interference canceler is attained by sampling and holding the output of the previous stage once per bit time.
For channel i, the first interference canceler samples the output of the CDMA/DS detector at time t=T+r,. This value is held constant as the input until t=2T+r; at which point the next bit value is sampled. Thus, the input waveforms to the interference canceler are estimates, d^ , (t-,r of the original data waveform, d,(t-r1), and the outputs are second estimates, d^^;(t-r1). The M spread-spectrum channel outputs Ooi, i=1, 2, ..., M, are passed through interference canceler 612 to produce a new corresponding set of channel outputs 0 i= 1, 2, ..., M.
As shown in FIG. 13, the outputs of a particular spread-spectrum channel, which are at the output of each of the interference cancelers, may be combined. Accordingly, combiner 615 can combine the output of the first channel which is from CDMA/DS detector 611, and the output O>> from the first interference canceler 612, and the output 021 from the second interference canceler 613, through the output 0 from the Nth interference canceler 614. Each output to_ be combined is of the corresponding bit. Therefore "s" bit time delays are inserted for each 0S1. The combined outputs are then passed through the decision device 616. This can be done for each spread spectrum channel, and therefore designate the outputs of each of the combiners. 615, 617, 619 as averaged outputs 0, for channel one, averaged output O2 for channel two, and averaged output OM for channel M. Each of the averaged outputs are sequentially passed through decision device 616, decision device 618, and decision device 620. Preferably, the averaged outputs have multiplying factor cj which may vary according to a particular design. In a preferred embodiment, c _ 1/2j.
This allows the outputs of the various interference cancelers to be combined in a particular manner.
FIGS. 14-17 illustrate simulation performance characteristics for the arrangement of FIGS. 12 and 13.
FIGS. 14-17 are for asynchronous channel (relative time delays are uniformly distributed between 0 and bit time, T), processing gain of 100, all users have equal powers, and thermal signal to noise ratio (EbN-of 30 dB). Length 8191 Gold codes are used for the PN sequences.
In FIG. 14, performance characteristic of each of the output stages of FIG. 12 is shown. Thus, SO represents the BER performance at the output of CDMA/DS detector 611, Si -2'3- -represents the BER performance at the output of interference canceler 612, S2 represents the BER performance at the output of interference canceler 613, etc. No combining of the outputs of the interference cancelers are used in determining the performance characteristic shown in FIG. 14.
Instead, the performance characteristic is for repetitively using interference cancelers. As a guideline, in each of the subsequent figures the output for each characteristic of CDMA/DS detector 611 is shown in each figure.
FIG. 15 shows the performance characteristic when the output of subsequent interference cancelers are combined.
This is shown for a particular channel. Thus, curve SO is the output of the CDMA/DS detector 611. Curve Si represents the BER performance of the average of the outputs of CDMA/DS
detector 611 and interference canceler 612. Here Co = C1,=
1/2 Cj = 0, j not equal to zero, one. Curve S2 represents the BER performance of the average output of interference canceler 613 and interference canceler 612. Curve S2 is determined using the combiner shown in FIG. 13. Here, C1 and CZ are set equal to 1/2 and all other C, set to zero.
Similarly, curve S3 is the performance of the output of a second and third interference canceler averaged together.
Thus, curve S3 is the performance characteristic of the average between. output of a second and third interference canceler. Curve S4 is the performance characteristic of the .average output. of a third and fourth interference canceler.
Only two interference cancelers are taken at a time for determining a performance characteristic of an average output of those particular interference cancelers. FIG. 16 shows the regular outputs for the CDMA/DS detector 611, and a first and second interference canceler 612, 613.
Additionally, the average output of the CDMA/DS detector 611 and the.first interference canceler 612 is shown as S1 AVG.
The BER performance of the average of the outputs of the first interference canceler 612 and the second interference canceler 613 is shown as the average output S2 AVG.
FIG. 17 shows performance characteristic correspondence for those of Fig. 16, but in terms of signal to-noise ratio in decibels(dB).
It will be apparent to those skilled in the art that various modifications can be made to the spread-spectrum CDMA interference canceler and method of the instant invention without departing from the scope or spirit of the invention, and it is intended that the present invention cover modifications and variations of the spread-spectrum CDMA interference canceler and method provided they come within the scope of the appended claims and their equivalents.

Claims

1. A remote unit having a receiver for recovering data transmitted over a plurality of channels employing a wireless code division multiple access communication technique, the receiver comprising:
a unit for receiving the plurality of channels as a received signal, each channel associated with a code;
a unit for despreading the plurality of channels as a plurality of despread received signals;
a unit for respreading each channel despread signal with a respective channel code wherein the respreading channel code is aligned to a timing of each of the despread received signals; and a unit for subtracting for each of the plurality of channels, others of the plurality of channels from the received signal and despreading a result of that subtraction by a delayed version for a first chip-code signal by a first channel mixer and filtered by a first channel filter as data for that channel;
and wherein the despreading further comprises passing the result of the unit for subtracting through the first channel mixer together with a code of a channel excluded from the subtraction operation, and wherein the receiving, despreading, respreading, and subtracting units repeat their operation employing the despread result obtained during an initial performance of the operation and combining the result from the repeated operation and passing the combined results through a decision device.

2. The remote unit receiver of claim 1 wherein the subtracting unit includes, for each channel:

a unit for despreading the received signal with the others channel codes;

a unit for respreading the despread others channel codes; and a unit for subtracting from the received signal the respread other channels.

3. The remote unit receiver of claim 2 wherein the unit for despreading is a mixer.

4. The remote unit receiver of claim 2 wherein the unit for despreading is a matched filter.

5. A remote unit receiver for recovering data transmitted over a plurality of channels employing a wireless code division multiple access communication technique, comprising:
a unit for receiving the plurality of channels as a received signal, each channel associated with a code;
a unit for despreading the plurality of channels as a plurality of despread received signals;
a unit for respreading each channel despread signal with a respective channel code wherein the respreading channel code is aligned to a timing of each of the despread received signals; and for each of the plurality of channels, a subtractor for subtracting others of the plurality of channels from the received signal and despreading a result of that subtraction by a delayed version for a first chip-code signal by a first channel mixer and filtered by a first channel filter as data for that channel; and wherein the despreading further comprises passing the result of the unit for subtracting through the first channel mixer together with a code of a channel excluded from the subtraction operation, and wherein the receiving, despreading, respreading, and subtracting units repeat their operation employing the despread result obtained during an initial performance of the operation and combining the result from the repeated operation and passing the combined results through a decision device.

6. The remote unit receiver of claim 5 further comprising:
for each channel, a first mixer operatively coupled to the input for despreading the received signal with that channel code;
for each channel, a second mixer for respreading that channel despread signal; and wherein for each channel, the subtractor subtracts from the received signal the respread channels other than that channel.

7. The remote unit of claim 5 further comprising:

for each channel, a matched filter operatively coupled to the receiving unit for despreading the received signal with that channel code;
for each channel, a mixer for respreading that channel despread signal; and wherein for each channel, the subtractor subtracts from the received signal the respread channels other than that channel.