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Publication numberCA2195534 C
Publication typeGrant
Application numberCA 2195534
PCT numberPCT/US1995/009652
Publication date9 Oct 2001
Filing date5 Jul 1995
Priority date26 Jul 1994
Also published asCA2195534A1, CA2342454A1, CA2342454C, CA2369499A1, CA2369499C, CA2552443A1, CA2552443C, CN1097368C, CN1154770A, CN1165125C, CN1223127C, CN1228937C, CN1229935C, CN1252957C, CN1337797A, CN1373581A, CN1373582A, CN1373583A, CN1490966A, CN1501611A, CN1501611B, CN1503493A, CN1503493B, DE772928T1, DE1033840T1, DE19581709T0, DE19581709T1, DE69528275D1, DE69528275T2, DE69535594D1, DE69535594T2, DE69536056D1, EP0772928A1, EP0772928A4, EP0772928B1, EP1033840A1, EP1033840B1, EP1860811A1, EP1860811B1, US5553062, US5719852, US6014373, US6259688, US6868076, US6868078, US6876665, US7027423, US7161919, US7164668, US7167462, US7167464, US7230938, US7242675, US20010019548, US20020167925, US20020172171, US20020172172, US20020172173, US20020186673, US20020191571, US20020196757, US20030002463, US20050157743, US20070258412, WO1996003819A1
Publication numberCA 2195534, CA 2195534 C, CA 2195534C, CA-C-2195534, CA2195534 C, CA2195534C, PCT/1995/9652, PCT/US/1995/009652, PCT/US/1995/09652, PCT/US/95/009652, PCT/US/95/09652, PCT/US1995/009652, PCT/US1995/09652, PCT/US1995009652, PCT/US199509652, PCT/US95/009652, PCT/US95/09652, PCT/US95009652, PCT/US9509652
InventorsDonald L. Schilling, John Kowalski, Shimon Moshavi
ApplicantDonald L. Schilling, John Kowalski, Shimon Moshavi, Interdigital Technology Corporation
Export CitationBiBTeX, EndNote, RefMan
External Links: CIPO, Espacenet
Spread spectrum interference canceler system and method
CA 2195534 C
Abstract
A spread-spectrum CDMA interference canceler for reducing interference in a DS/CDMA receiver having N chip-code channels. The interference canceler includes a plurality of correlators (54, 64, 74), a plurality of spread-spectrum-processing circuits (55, 65, 75), subtracting circuits (150), and channel correlators (146). Using a plurality of chip-code signals generated from chip codeword signal generators (52, 62, 72), the correlators (54, 64, 74) despreads the spread-spectrum CDMA signal as a plurality of despread signals. The plurality of spread-spectrum-processing circuits (55, 65, 75) uses a timed version of the plurality of chip-code signals generated from the delay devices (53, 63, 73), for spread-spectrum processing the plurality of despread signals. For recovering a code channel using an ith chip-code-signal, the subtracting circuits (150) subtracts from the spread-spectrum CDMA signal, each of the N-1 spread-spectrum-processed-despread signals thereby generating a subtracted signal. The channel-correlator (146) despreads the subtracted signal.
Claims(68)
1. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having N
channels, with each of the N channels identified by a distinct chip-code signal, comprising:
a plurality of interference cancelers, each of said plurality of interference cancelers including, means for generating a plurality of chip-code signals;
means for despreading, using the plurality of chip-code signals, a spread-spectrum CDMA signal as a plurality of despread signals, respectively;
means for timing the plurality of chip-code signals to generate a plurality of timed chip-code signals;
means, responsive to the plurality of timed chip-code-signals, for spread-spectrum processing the plurality of despread signals with the plurality of timed chip-code-signals, respectively;
means, for one of the channels, for subtracting from the spread-spectrum CDMA signal each of an N-1 plurality of spread-spectrum processed despread signals, with the N-1 plurality of spread-spectrum processed despread signals not including that one channel's spread-spectrum processed despread signal, thereby generating a subtracted signal;
and channel means for the one channel, for despreading the subtracted signal with the one channel's timed chip-code signal as a one channel signal.
2. The spread-spectrum CDMA interference canceler system as set forth in claim 1 with said despreading means including:
a filter;
a chip-code generator for generating a chip-code signal from a respective chip codeword; and a mixer coupled between said filter and said chip-code generator.
3. The spread-spectrum CDMA interference canceler system as set forth in claim 2 with said channel means including for the one channel:
a filter;
a chip-code generator for generating a chip-code signal from a chip codeword corresponding to the one channel signal; and a mixer coupled between said filter and said chip-code generator.
4. The spread-spectrum CDMA interference canceler system as set forth in claim 1 with said despreading means including a matched filter having an impulse response matched to a respective chip codeword.
5. The spread-spectrum CDMA interference canceler system as set forth in claim 4 with said channel means including for the one channel:
a filter;
a chip-code generator for generating a chip-code signal from a chip codeword corresponding to the one channel signal; and a mixer coupled between said filter and said chip-code generator.
6. The spread-spectrum CDMA interference canceler system as set forth in claim 4 with said channel means for the one channel including a matched filter having an impulse response matched to a chip codeword corresponding to the one channel.
7. The spread-spectrum CDMA interference canceler system as set forth in claim 1 with said channel means including for the one channel:
a filter;
a chip-code generator for generating a chip-code signal from a chip codeword corresponding to the one channel signal; and a mixer coupled between said filter and said chip-code generator.
8. The spread-spectrum CDMA interference canceler system as set forth in claim 1 with said despreading means including a digital signal processor with digital matched filter having an impulse response matched to a respective chip codeword.
9. The spread-spectrum CDMA interference canceler system as set forth in claim 8 with said channel means including for the one channel, a matched filter having an impulse response matched to a chip codeword corresponding to the one channel.
10. The spread-spectrum CDMA interference canceler system as set forth in claim 8 with said channel means including for the one channel, a surface acoustic wave (SAW) device having an impulse response matched to a chip codeword corresponding to the one channel.
11. The spread-spectrum CDMA interference canceler system as set forth in claim 1 with said channel means including for the one channel, a digital signal processor with digital matched filter having an impulse response matched to a chip codeword corresponding to the one channel.
12. The spread-spectrum CDMA interference canceler system as set forth in claim 1 with said despreading means including for each channel, a surface acoustic wave (SAW) device having an impulse response matched to a chip codeword corresponding to that channel.
13. The spread-spectrum CDMA interference canceler system as set forth in claim 12 with said channel means including for the one channel, a matched filter having an impulse response matched to a chip codeword corresponding to the one channel.
14. The spread-spectrum CDMA interference canceler system as forth in claim 12 with said channel means including for the one channel a chip-code generator for generating a chip-code signal from a digital matched filter having an impulse response matched to a chip codeword corresponding to the one channel.
15. The spread-spectrum CDMA interference canceler system as set forth in claim 12 with said channel means including for the one channel, a surface acoustic wave (SAW) device having an impulse response matched to a chip codeword corresponding to the one channel.
16. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the plurality of channels identified by a distinct chip-code signal, comprising:
a CDMA/DS detector for detecting and despreading a received spread-spectrum signal having a plurality of channels as a plurality of despread, spread-spectrum channels;
a plurality of serially connected interference cancelers each for processing the plurality of despread, spread-spectrum channels and for outputting a respective plurality of estimates of the plurality of channels; and a combiner for combining, using the outputs of the plurality of interference cancelers, a plurality of estimates of a particular channel to generate an averaged estimate.
17. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the plurality of channels identified by a distinct chip-code signal, comprising:
a CDMA/DS detector for detecting and despreading a received spread-spectrum signal having a plurality of channels as a plurality of despread, spread-spectrum channels;
a plurality of serially connected interference cancelers each for processing the plurality of despread, spread-spectrum channels and for outputting a respective plurality of estimates of the plurality of channels;

means for combining, using the respective plurality of estimates from said plurality of interference cancelers, for each channel, that channel's plurality of estimates to generate an averaged output for that channel; and decision means for processing each channel's averaged output.
18. A method for reducing interference in a spread-spectrum code division multiple access (CDMA) receiver having N channels, with each of the N channels identified by a distinct chip-code signal, using a plurality of interference cancelers, each of said plurality of interference cancelers including a plurality of chip-code generators for generating a plurality of chip-code signals and a plurality of timed devices for generating a plurality of timed-chip-code signals, comprising the steps, within each of said plurality of interference cancelers, of:
a. despreading, simultaneously, a plurality of spread-spectrum channels of a spread-spectrum CDMA signal as a plurality of despread signals using the plurality of chip-code signals, respectively;
b. spread-spectrum processing, simultaneously, using the plurality of timed-chip-code-signals, the plurality of despread signals, respectively, with each of the plurality of timed-chip-code signals corresponding to a respective one of the plurality of despread signals;
c. subtracting from the spread-spectrum CDMA signal, for each of the N

channels, each of a plurality of N-1 spread-spectrum-processed-despread signals not including that channel's spread-spectrum processed despread signal, thereby generating a subtracted signal;
d. despreading each subtracted signal with a timed chip-code signal as a channel signal, producing a first set of estimates of the N channels;
e. repeating steps a through d, a plurality of times using an additional plurality of interference cancelers per repetition, producing a plurality of sets of estimates of the N
channels.
19. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the plurality of channels identified by a distinct chip-code signal, comprising:
a plurality of interference cancelers, each of said interference cancelers including, a plurality of chip-code-signal generators for generating, simultaneously, a plurality of chip-code signals;
a plurality of correlators coupled to said plurality of chip-code generators through a plurality of mixers, each of said plurality of correlators responsive to a distinct chip-code signal of the plurality of chip-code-signals, for simultaneously despreading a plurality of spread-spectrum channels of a spread-spectrum CDMA signal as a plurality of despread signals, respectively;
a plurality of delay devices coupled to said plurality of chip-code-signal generators for delaying the plurality of chip-code signals as a timed plurality of chip-code signals, respectively;
a plurality of processing mixers coupled to said plurality of delay devices and to said plurality of correlators, responsive to the timed plurality of chip-code signals, for spread-spectrum processing, simultaneously, the plurality of despread signals, respectively, with a timed chip-code-signal corresponding to a respective despread signal, producing spread-spectrum-processed-despread signals;
a plurality of subtractors, each of said plurality of subtractors for subtracting from the spread-spectrum CDMA signal all but a particular one of the spread-spectrum-processed-despread signals, with the particular one of the spread-spectrum-processed-despread signals being different for each of said plurality of subtractors, thereby generating a plurality of subtracted signals; and a plurality of channel correlators for despreading the plurality of subtracted signals with a particular one of the plurality of timed chip-code signals, respectively, as a plurality of channel signals.
20. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the channels identified by a distinct chip-code signal, comprising:
a CDMA/DS detector for detecting and despreading a received spread-spectrum signal having a plurality of channels as a plurality of despread, spread-spectrum channels;
a plurality of serially connected interference cancelers each for processing the plurality of despread, spread-spectrum channels and for outputting a respective plurality of estimates of the plurality of channels; and means for combining, for each channel, that channel's plurality of estimates to generate an averaged output for that channel.
21. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the plurality of channels identified by a distinct chip-code signal, comprising:
a plurality of serially connected interference cancelers each for processing a plurality of despread, spread-spectrum channels and for outputting a plurality of estimates of a plurality of spread-spectrum channels corresponding to the plurality of despread, spread-spectrum channels, respectively;
means for combining the plurality of estimates to produce a plurality of averaged estimates; and decision means for processing the plurality of averaged estimates.
22. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the channels identified by a distinct chip-code signal, comprising:
a plurality of serially connected interference cancelers each for processing a plurality of despread, spread-spectrum channels and for outputting a plurality of estimates of a plurality of spread-spectrum channels corresponding to the plurality of despread, spread-spectrum channels, respectively;
a plurality of combiners for combining the plurality of estimates to produce a plurality of averaged estimates; and a plurality of decision devices for processing the plurality of averaged estimates.
23. A method for reducing interference in a spread-spectrum code division multiple access (CDMA) receiver having N channels, with each of the N channels identified by a distinct chip-code signal, using a first plurality of interference cancelers, comprising the steps of:
a. despreading, simultaneously, a plurality of spread-spectrum channels of a spread-spectrum CDMA signal as a plurality of despread signals, respectively;
b. spread-spectrum processing, simultaneously, using a timed version of a plurality of chip-code-signals, the plurality of despread signals, respectively, with a timed chip-code signal corresponding to a respective despread signal;
c. subtracting from the spread-spectrum CDMA signal, for each channel, each of a plurality of N-1 spread-spectrum-processed-despread signals, with the plurality of N-1 spread-spectrum-processed-despread signals not including a spread-spectrum processed despread signal of that channel's despread signal, thereby generating a subtracted signal;
d. despreading the subtracted signals with timed chip-code signals, producing a first set of estimates of the N channels;
e. inputting the set of estimates produced by the preceding step to a next plurality of interference cancelers and repeating steps a through d, using the next plurality of interference cancelers, producing a next set of estimates of the N
channels;
f. repeating step a at least one more time; and g. combining for one of the plurality of channels, that one channel's estimates to produce an average.
24. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the plurality of channels identified by a distinct chip-code signal, comprising:
a plurality of interference cancelers, each of said interference cancelers including, a plurality of chip-code-signal generators for generating, simultaneously, a plurality of chip-code signals;
a plurality of correlators, responsive to a plurality of distinct chip-code-signals, for simultaneously despreading a plurality of spread-spectrum channels of a spread-spectrum CDMA signal as a plurality of despread signals, respectively, each despread signal is an estimate for one of the plurality of channels;
a plurality of delay devices coupled to said plurality of chip-code-signal generators for delaying the plurality of chip-code signals as a timed plurality of chip-code signals, respectively;
a plurality of mixers, responsive to the timed plurality of chip-code signals, for spread-spectrum processing, simultaneously, the plurality of despread signals, respectively, with a timed chip-code-signal corresponding to a respective despread signal, producing spread-spectrum-processed-despread signals;
for each channel a first subtractor, for subtracting from the spread-spectrum CDMA signal, all but that channel's one of the spread-spectrum-processed-despread signals, thereby generating first subtracted signals;
for each channel a first channel correlator, for despreading that channel's first subtracted signal with a first timed chip-code signal as an estimate of that channel;
for each channel a first combiner, for combining that channel's estimates.
25. A spread-spectrum code division multiple access (CDMA) interference-canceler system for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the channels identified by a distinct chip-code signal, comprising:
a plurality of matched filters, responsive to a plurality of distinct chip-code-signals, for simultaneously despreading a plurality of spread-spectrum channels of a spread-spectrum CDMA signal as a plurality of despread signals, respectively, each despread signal corresponding to one of the channels and being on estimate for that one channel;
a plurality of interference cancelers, each of said interference cancelers including, a plurality of chip-code-signal generators, responsive to a plurality of despread signals from a plurality of matched filters, for generating, simultaneously, a timed plurality of chip-code signals, respectively;
a plurality of mixers, responsive to the plurality of despread signals from the plurality of matched filters and the timed plurality of chip-code signals from the plurality of chip-code-signal generators, respectively, for spread-spectrum processing, simultaneously, the plurality of despread signals with a respective timed chip-code signal, producing spread-spectrum-processed-despread signals, each spread-spectrum-processed-despread signal corresponding to one of the channels;
for each channel a subtractor, for subtracting from the spread-spectrum CDMA signal, all but that channel's one of the spread-spectrum-processed-despread signals, thereby generating a subtracted signal;
for each channel, a channel-matched filter for despreading the subtracted signal with a timed chip-code signal as an estimate of that channel;
for each channel a combiner for combining that channel's estimates.
26. A method for reducing interference in a spread-spectrum code division multiple access (CDMA) receiver having a plurality of channels, with each of the channels identified by a distinct chip-code signal, using a plurality of interference cancelers, comprising the steps of:
a. despreading, simultaneously, a plurality of spread-spectrum channels of a spread-spectrum CDMA signal as a plurality of despread signals, respectively;
b. spread-spectrum processing, simultaneously, the plurality of despread signals with a respective plurality of timed-chip-code signals, each of said plurality of timed-chip-code signals corresponding to a respective one of the plurality of despread signals, producing a spread-spectrum-processed-despread signal as an estimate for each channel;
c. for each channel, subtracting from the spread-spectrum CDMA signal, all but that one channel's spread-spectrum-processed-despread signals, thereby generating a subtracted signal;
d. for each channel, despreading that channel's subtracted signal with a timed-chip-code signal as an estimate of that channel;
e. for each channel, combining the estimates of that channel.
27. The method as set forth in claim 26, further comprising the steps of:
f. processing the estimates produced by step d, as a plurality of processed estimates;
g. inputting the plurality of processed estimates to a second plurality of interference cancelers to produce an additional estimate for each channel.
28. A spread-spectrum code division multiple access (CDMA) interference-canceler system having a plurality of interference cancelers for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the channels identified by a distinct chip-code signal, comprising:
a plurality of chip-code-signal generators for generating, simultaneously, a plurality of chip-code signals;
despreading means, responsive to a plurality of distinct chip-code-signals, for simultaneously despreading a plurality of spread-spectrum channels of a spread-spectrum CDMA signal as a plurality of despread signals, respectively;
a plurality of delay devices coupled to said plurality of chip-code-signal generators for delaying the plurality of chip-code signals as a plurality of timed-chip-code signals, respectively;
for each channel a spreading mixer for spread-spectrum processing a despread signal of that channel, of the plurality of despread signals, with a timed-chip-code signal, of the plurality of timed-chip-code signals, to produce a spread-spectrum-processed-despread signal for that channel;
for each channel, a subtractor, for subtracting from the spread-spectrum CDMA signal, all but that channel's spread-spectrum-processed-despread signal, thereby generating a subtracted signal;
for each channel, a channel correlator for despreading that channel's subtracted signal with a timed-chip-code signal as an estimate of that channel;
for each channel, a combiner for combining estimates of that channel as an averaged estimate; and decision means for processing each channel's averaged estimate.
29. The spread-spectrum CDMA interference-canceler system as set forth in claim 28, said plurality of delay devices comprising;
for each channel, a delay device for delaying a chip-code signal associated with that channel by a time delay to generate a timed-chip-code signal.
30. The spread-spectrum CDMA interference-canceler system as set forth in claim 28, said despreading means comprising:
a plurality of correlators.
31. The spread-spectrum CDMA interference-canceler system as set forth in claim 28, said despreading means comprising:
a plurality of matched filters.
32. The spread-spectrum CDMA interference-canceler system as set forth in claim 31, said plurality of delay devices comprising;
a plurality of adjustment device for aligning the chip-code signals with the despread signals.
33. A spread-spectrum code division multiple access (CDMA) interference-canceler system having a plurality of interference cancelers for reducing interference in a spread-spectrum CDMA receiver having a plurality of channels, with each of the channels identified by a distinct chip-code signal, comprising:
a plurality of matched filters, responsive to a plurality of distinct chip-code-signals, for simultaneously despreading a plurality of spread-spectrum channels of a spread-spectrum CDMA signal as a plurality of despread signals, respectively, each despread signal being an estimate for one of the channels;
a plurality of chip-code-signal generators, responsive to the plurality of despread signals from the plurality of matched filters, for generating, simultaneously, a plurality of timed-chip-code signals, respectively;
for each channel, a spreading mixer for spread-spectrum processing that channel's despread signal, of the plurality of despread signals, with the timed-chip-code signal associated with that channel, of the plurality of timed-chip-code signals, to produce a spread-spectrum-processed-despread signal;
for each channel, a subtractor, for subtracting from the spread-spectrum CDMA signal, all but that channel's spread-spectrum-processed-despread signal, thereby generating a subtracted signal;
for each channel, a channel-matched filter for despreading that channel's subtracted signal with a timed-chip-code signal as an estimate of that channel;
for each channel, a combiner for combining the estimates of that channel; and decision means for processing the combined estimates.
34. The spread-spectrum CDMA interference-canceler system as set forth in claim 33, said decision means comprising:
a plurality of decision devices for processing each channel's estimates.
35. A multi-channel, interference canceler for code division multiple access (CDMA) telecommunication systems for use in a communication receiver wherein each channel of the multi-channel, interference canceler comprises:
a plurality of despreading means, each for despreading a specific CDMA
channel from a received signal to produce a channel signal;
a subtracting means having inputs associated with all but a selected despreading means for outputting an interference reference signal with respect to all despread channels except a selected channel which corresponds to said specific CDMA
channel of said selected despreading means, by subtracting the all but the specific CDMA
channel signal from the received signal; and means for combining the channel signal produced by said selected despreading means with said interference reference signal and integrating a result of the combining to produce an interference canceled signal.
36. The multi-channel, interference canceler according to claim 35 further comprising:

for each channel despreading means, a respective channel subtracting means for producing an interference reference signal for said respective channel having inputs associated with all of the other said plurality of despreading means, by subtracting the all of the other said plurality of despreading means channel signals from the received signal; and for each respective channel subtracting means, a respective means for combining the channel signal produced by the despreading means with the interference reference signal and integrating a result of the combining to produce an interference canceled signal of said respective channel.
37. The multi-channel, interference canceler according to claim 35 wherein each of said plurality of despreading means further comprises:
a chip-code generator for generating a chip-code sequence for a channel from a respective chip codeword; and a mixer for despreading a specific CDMA channel with said chip-code sequence.
38. A multi-channel, interference canceler for code division multiple access (CDMA) telecommunication systems for use in a communication receiver wherein each channel of the multi-channel, interference canceler comprises:
a plurality of processing means, each for processing a specific CDMA channel;

a subtracting means having inputs associated with all but a selected processing means for outputting a subtracted signal, by subtracting the all but the selected processing means processed specific CDMA channel from a received signal; and match filtering means matched to a chip codeword of the specific CDMA
channel associated with said selected processing means for filtering said subtracted signal to produce an interference canceled signal of the specific CDMA channel associated with said selected processing means.
39. The multi-channel, interference canceler according to claim 38 further comprising:
for each processing means, a respective channel subtracting means having inputs associated with all of the other said plurality of processing means for producing a respective subtracted signal, by subtracting the all of the other processing means processed CDMA signals from the received signal; and for each respective channel subtracting means, a match filtering means matched to a respective chip codeword of the respective CDMA channel associated with the processing means which is not associated with the subtracting means inputs for filtering the respective subtracted signal to produce an interference canceled signal of the respective CDMA channel.
40. The multi-channel, interference canceler according to claim 38 wherein said plurality of processing means further comprises:
a matched filter matched to a channel corresponding to a respective chip codeword;
a chip-code generator for generating a chip-code sequence for the specific channel from a respective chip codeword; and a mixer coupled between said chip-code generator and an output of said matched filter.
41. The multi-channel, interference canceler according to claim 38 wherein each of said plurality of processing means further comprises:
a digital signal processor (DSP) configured as a digital matched filter having an impulse response matched to a channel corresponding to a respective chip codeword.
42. The multi-channel, interference canceler according to claim 38 wherein each of said plurality of processing means further comprises:
a surface acoustic wave (SAW) device having an impulse response matched to a channel corresponding to a respective chip codeword.
43. The multi-channel, interference canceler according to claim 40, wherein said plurality of processing means further comprises:
a delaying means coupled between said chip-code generator and said mixer.
44. A method for reducing interference in a multi-channel, code division multiple access (CDMA) telecommunication system communication receiver comprising the steps of:
despreading simultaneously a plurality of specific CDMA channels;
subtracting all but a selected despread channel of the plurality of channels from a received signal to produce an interference reference signal; and combining said selected despread channel with the interference reference signal and integrating a result of the combining to produce an interference canceled signal for said selected channel.
45. The method for reducing interference according to claim 44 wherein said step of combining further comprises the step of:
mixing said selected channel with said interference reference signal producing the result of the combined signal.
46. The method for reducing interference according to claim 44 further comprising performing said subtracting and said combining and integrating steps separately with respect to each of said specific CDMA channels such that each channel serves in turn as said selected channel.
47. A method for reducing interference in a multi-channel, code division multiple access (CDMA) telecommunications system communication receiver comprising the steps of:
processing a multichannel CDMA signal to produce a plurality of processed signals each processed with a different CDMA channel;
subtracting all but a selected one of said plurality of processed signals from a received signal to produce a subtracted signal; and filtering said subtracted signal with respect to a chip-code associated with the specific CDMA channel corresponding to said selected one of said plurality of processed signals to produce an interference canceled signal for the specific CDMA channel.
48. A method for reducing interference according to claim 47 further comprising performing said subtracting and said filtering steps for each processed signal such that each processed signal serves in turn as said selected one of said plurality of processed signals thereby producing interference canceled signals for each CDMA channel.
49. The multi-channel, interference canceler according to claim 35 wherein said subtracting means having an additional input in common with inputs of said plurality of processing means.
50. The multi-channel, interference canceler according to claim 38 wherein said subtracting means having an additional input in common with inputs of said plurality of despreading means.
51. The method for reducing interference according to claim 44 wherein the received signal is a multi-channel CDMA signal.
52. The method for reducing interference according to claim 47 wherein all but a selected one of said plurality of processed signals are subtracted from said multi-channel CDMA signal to produce a subtracted signal.
53. A method of receiving spread spectrum channel signals within a spread spectrum signal comprising:
a) despreading a received spread spectrum signal for each of a plurality of specific CDMA channels to produce a first estimate for each channel;
b) for each channel, despreading the estimate obtained from the prior step for all other channels to produce despread other channel signals and subtracting the despread other channel signals from the received signal and despreading the result to produce a next estimate for each channel;
c) repeating step b a selected number of times; and d) using the estimates for each channel produced by steps a, b and c to output a channel signal for each channel.
54. The method of claim 53 wherein step d further comprises for each channel summing the estimates produced by steps a, b and c for that channel to output the specific channel signals.
55. The method of claim 53 wherein step d further comprises for each channel multiplying each estimate produced by steps a, b and c for that channel by a factor prior to summing.
56. The method of claim 55 wherein for each channel each next estimate is multiplied by a factor with a value half of a value of a previous estimate's factor for that channel.
57. The method of claim 53 wherein each next estimate for each channel is delayed with respect to a previous estimate for that channel.
58. The method of claim 53 wherein each next estimate for each channel is delayed by a one bit time delay with respect to a previous estimate for that channel.
59. The method of claim 53 wherein step d further comprises for each channel averaging the estimates produced by steps a, b and c for that channel to output the specific channel signals.
60. The method of claim 53 wherein a number of estimates produced in steps a, b and c is at least two estimates.
61. A receiver for receiving spread spectrum channel signals within a spread spectrum signal comprising:
means for despreading a received spread spectrum signal for each of a plurality of specific CDMA channels to produce a first estimate of a series of estimates for each channel;
means for producing remaining estimates of the series for each channel by repeating a selected number of times, despreading a previous estimate for all other channels to produce despread other channel signals and subtracting the despread other channel signals from the received signal and despreading the result to produce a next estimate for each channel; and means using the series of estimates for each channel for outputting a channel signal for each channel.
62. The receiver of claim 61 wherein said outputting means further comprises means for summing for each channel the series of estimates for that channel to output the specific channel signals.
63. The receiver of claim 62 wherein said outputting means further comprises means for multiplying for each channel the series of estimates for that channel by a factor prior to summing.
64. The receiver of claim 62 wherein said outputting means further comprises means for multiplying for each channel each next estimate for that channel by a factor with a value half of a value of a previous estimate's factor.
65. The receiver of claim 62 further comprising means for delaying each next estimate for each channel with respect to a previous estimate for that channel.
66. The receiver of claim 62 further comprising means for delaying each next estimate for a channel by a one bit time delay with respect to a previous estimate for that channel.
67. The receiver of claim 58 wherein said outputting means further comprises means for averaging for each channel the series of estimates for that channel to output the specific channel signals.
68. The receiver of claim 58 wherein the series of estimates is at least two estimates.
Description  (OCR text may contain errors)

WO 96103819 ~ ; 34 r~

Spread Spectrum l"Le, re,ence Canceler System and Method BACKGRO~ND OF THE INVENTIQN
This invention relates to spread-spectrum ;cations, 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 AR~
Direct sequence, code division multiple access, spread-spectrum communications systems are capacity limited by interference caused by other simultaneous users. This is r _ 1~d 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 tBER). 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
rh~nn~l c. Each of the N rh~nn~l ~ 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 CDMA 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 r~nrGl; n~ or reducing __ ______ ~SLJEsTlTuTE SHEET (RULE 26) W096/03819 2 1 9 55 3 ~ s2 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 delay ng tne chip-code signal from a chip-code-signal generator. Alternatively, a matched filter may detect a particular PN sequence in the spread-spectrum CDNA 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.
~or 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-l spread-spectrum-processed-despread signals, thereby generating a subtracted signal. The N-l spread-spectrum-processed-despread signals do not include the spread-spectrum-processed-despread signal of the ith channel corresponding to the ith chip-code signal. The channel-despreading means despreads the subtracted signal with the ith chip-code signal.
The present invention also includes a method for reducing interference in a spread-spectrum CDMA receiver having N channels. The method comprises the steps o~

WO96/03819 3 .~1l~ SJ~ S~?

despreading, using a plurality of chip-code signals, the spread-spectrum CDMA signal as a plurality of despread signals, respectively; spread-spectrum processing, using a timed version of the plurality of chip-code signals, the plurality of despread signals, respectively, with a chip-code signal ~U~L~ onling to a respective despread signal;
subtracting from the spread-spectrum CDMA signal, each of the N-l spread-spectrum-processed-despread signals, with the N-l spread-spectrum-processed-despread signals not including a spread-spectrum-processed despread signal of the ith ~h~nn~l~, thereby generating a subtracted signal; and, despreading the subtracted signal having the i~h chip-code signal.
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. l 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 rh~nn~l ~ 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

W096/03819 2 ~ 9 5 5 3 4 P ~

interference canceler for processing multiple rh~nnPl~ using matched filters;
FIG. 5 is a block diagram of the spread-~e~ ." CDMA
interference rlnr~l~r having multiple iterations for processing multiple rh~nn~
FIG. 6 illustrates theoretical performance characteristic for Eb/~ = 6 dB;
FIG. 7 illustrates theoretical performance characteristic for Eb/~ = 10 dB;
FIG. s illustrates theoretical performance characteristic~for Eb/~ = 15 dB;
FIG. 9 illustrates theoretical performance characteristic for Eb/~ = 20 dB;
FIG. 10 illustrates theoretical performance characteristic for Eb/~ = 25 dB;
FIG. 11 illustrates theoretical performance characteristic for Eb/~ = 30 dB;
FIG. 12 is a block diagram of interference cancelers connected together;
FIG. 13 i5 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 = lOo, Equal Powers, EbN = 30 dB;
FIG. 16 illustrates simulation performance characteristics for asynchronous, PG = 100, Equal Powers, EbN = 30 dB; ana FIG. 17 illustrates simulation performance characteristics for asynchronous, PG = 100, Equal Powers, EbN = 30 dB.

WO96/03819 21~ 4 r~ ?
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DETAILED DESCRTPTION OF THE PREFERRED EMBODIMENTS
Reference now is made in detail to the present preferred ~ho~; ntS of the invention, examples of which are illustrated in the A~ -nying drawings, wherein like reference numerals indicate like elements throughout the several views.
In the exemplary arrangement shown in FIG. 1, a spread-~e~DLulll code division multiple access (CDMA) interference canceler i5 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-~e~Lulll-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 N~h 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 alternatively may be a first lowpass filter or a first b~n~rA~c filter. The first mixer 51 is coupled between the input 41 and the first chip-code-signal 2 1 9 5 5 3 4 I~ ?

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 h~n~r~cc 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 N~h mixer 71, and Nth chip-code-signal generator 72, and Nth integrator 74. The Nth integrator 74 alternatively may be an Nth lowpass filter or an Nth bandpa6s filter. The Nth mixer 71 is coupled between t l inpu_ 41, the Nth chip-code-signal generator 72 and the Nth ir,~egrator 74.
~s is well known in the art, the first through Nth despr -~ng means may be embodied as any device which can despread a channel in a spread-spectrum signal.
The plurality of timing means may be embodied as a 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 fiIter or first h~n~r~cc filter. A second delay device 63 has a time delay T, which is approximately the same as the integration time Tb ~f 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 cf Nth integrator 74, or time constant of the Nth lowpass filter or N'h hAn~r~cc filter.
Typically, the integration times of the first integrator 54, second integrator 64 through N~h integrator 74 are the same.
If lowpass filters are used, then typically the time constants of the first lowpass filter, second lowpass filter through Nth lowpass filter are the same. If h~n~p~cc WO96103819 2 1 9 5 5 ~ 4 P~ 5~-7 filters are used, then the time constants of the first bandpass filter, second bAn~rAcc filter through N~h 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-pr~c~qc;ng 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 proc~cc; ng 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 N~h integrator 74 through the delay device 73 to the N~h 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 3~ subtracted signal with the ith chip-code signal as the ith WO96103819 2 1 9 5 5 3 4 A ~ 111 5 ~, channel. The first channel-despreading means i8 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 N~h chip-code 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-signal generator 52. The first despread signal from the first mixer 51 is filtered through first integrator 54. First integrator 54 integrates for a 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

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_9_ 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 ~h~nn~l c of the spread-spectrum CDMA
signal.
For reducing interference to the first channel of the spread-spectrum CDMA signal, the received spread-~e~LLu-u CDMA signal is processed by the second through N~h 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-~e~LLu-" 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 WO96iO38l9 2 ~ 9 5 ~ r7 .

device or circuit as is well known in the art.
Similarly, the N~h channel of the spread-spectrum CDMA
signal i5 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 N~h integrator 74, which is the N~h despread signal, is a 10 despread and fl~tered version of the Nth channel of the spread-spectrum CDMA signal. The N~h despread signal is spread-spectrum processed by a delayed version of the N~h chip-code signal. The N~h chip-code signal is delayed through N~h delay device 73. The N~h processing mixer 75 15 spread-spectrum processes the timed version, i.e. a delayed version, of the N~h chip-code signal with the N~h despread signal.
At the first subtractor 150, each of the outputs of the second processing mixer 65 through the N~h processing mixer 20 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-~e~Lu-u 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 25 approximately equal to the integration time of the first integrator 54 through N'h integrator 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 30 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 N~h subtracted signal are similarly defined.

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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, 3.50, 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, gl(t-T), second chip-code signal, g2(t-T), third chip-code signal, g3(t-T), through Nth chip-code signal, gN(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 dz, third WO96103819 2 1 ~ F ~
.

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 ~h~nn~l ~ 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 ~e~-lu~, CDMA signal has the second channel through Nth channel despread by second mixer 61 uslng 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 ~mho~;r-nt~ 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 ~h~nnel ~ 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. ~ach of the second channel W096/03819 21 9S534 p ~

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 rh~nnelc plus noise ~nentS aligned with these rh~nn~lc are subtracted from the received spread-spectrum CDNA signal.
As illustratively shown in FIG. 3, an alternative rmho~; r -nt 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 N~h despreading means is shown an Nth matched filter 174. The Nth matched filter has an impulse response matched ~ to the N~h chip-code signal, which is used to spread-spectrum process and define the Nth channel of the spread-spectrum CDMA signal. The N'h matched filter is coupled to the input 41.

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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 int~ ~~;ate 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 adjustDent device 98 to the second chip-code-signal generator 62. The Nth proc~scing mixer 75 may be coupled through the Nth adjustment device 73 to the Nth chip-code-signal generator 72. The first adjustment 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.

WO 96/03819 ' 2 1 q 5 ~ ~ 4 ~ ~lIU~ .. '7 A first channel of a received spread-spectrum CDNA
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 ~ 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-~e~lul" CDNA 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 CDNA
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 Wo96/03819 ~i 9 5 5 3 4 . ~i~ r - 6s2 output of the N~h matched filter 174 i5 the Nth despread signal, i.e. a despread and filtered version of the N'h channel of the spread-spectrum CDMA signal. The N~h despread signal is spread-spectrum processed by a timed version of the Nth chip-code signal. The timing of the N~h chip-code signal is triggered by the N~h despread signal from the Nth matched filter 174. The Nth processing mixer 75 spread-spectru~ 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 d1 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 pror~c~;nrJ mixer, second processing mixer, third processing mixer, through a N'h 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 N'h 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 ~irst WO96103819 2 1 9 ~ 5 3 ~ P~

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 d1, 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 N'h channel despread by the second matched filter 164 through the Nth matched filter 174. This despreading removes the other CDMA rhAnnPl~ from the respective despread channel. In a preferred embodiment, each of the chip-code signals used for the first channel, second channel, through the N'h 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 N~h despread signal, plus noise.
The respective channel is spread-spectrum processed by the processing mixers. Accordingly, at the output of the second prncpccing mixer 65 through the N'h processing mixer 75 is a spread version of the second despread signal through WO96/03819 2 ~ ~ ~ 5 ~ 4 r~

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 CDNA 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 rh~nnol~, are subtracted from the received spread-spectrum CD~A
signal.
As is well known in the art, correlators and matched filters may be interchanged to accomplish the same function.
FIGS. l and 3 show alternate omho~ nts using correlators or matched filters. The arrangements may be varied. For example, the plurality of despreading means may be omho~;ed as a plurality of matched filters, while the channel despreading means may be embodied as a correlator.
Alternatively, the plurality of despreading means m7y 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 d~ tor may despread the spread-spectrum CDMA signal.
~he 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 2 1 9~534 WO96/03819 1~ 52 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 d1U, 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, ..., d,N, of the first 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, d2z, ..., 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 ~h~nnPl~. Each of the N ~h~nn~l c 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 WO96/03819 2 1 ~ ~ ~ 3 4 ~ 3~
*

including a spread-spectrum-processed signal of the ith despread signal, thereby generating a subtracted signal.
The subtracted signal is despread to generate the ith channel.
The prnhAhility of error Pe for direct sequence, spread-spectrum CDMA system is:

Pe = 2erfc(~SNR)~

where erfc is complementary error function, SNR is signal-to-noise ratio, and 1 < ~ ~ 2. The value of ~ depends on how a particular interference nAnr~l~r system is ~cign~.
The SNR after interference cancellation and method is given by:

SNR = (PG/N)R~1 +(PG/N) Rt1 1 l- (N/PG) R~1 Eb/~ 1-N~PG

where N is the number of ~hAnn~lc, PG is the processing gain, R is the number of repetitions of the interference canceler, Eb is energy per information bit and ~ is noise power spectral density.
FIG. 6 illustrates theoretical performance characteristic, of the interference canceler and method for when Eb/~ = 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 i uved 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.
I~ PG ~ N, then the output SNR from the interference canceler approaches Eb/~. Further, if (N/PG)R1 ~ 1, then SNR ~ (Eb/~ N/PG)-WO96/03819 2 1 9 5 5 3 4 r~l,L~
.

FIG. 7 illustrates the performance characteristic for when Eb/~ = 10 dB. FIG. 7 illustrates that three iterations of the interference canceler can yield a 4 dB ; u~ -nt with PG/N = 2.
FIG. 8 illustrates the performance characteristic for when Eb/~ = 15 dB. With this bit energy to noise ratio, two iterations of the interference canceler can yield 6 dB
ov~ t for PG/N = 2.
FIG. 9 illustrates the performance characteristic for when Eb/~ = 20 dB. With this bit energy to noise ratio, two iterations of the interference canceler can yield 6 dB
illl~uve -nt for PG/N = 2. Similarly, FIGS. 10 and 11 shows that one iteration of the interference canceler can yield more than 10 dB i uv L 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 ~hAnn~l~ is represented as outputs ~ol~ ~02 ' ~03 ' . . . ' ~Om . 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 ~h~nn~l s passes through the interference c~n~l; ng 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 CDMAtDS detector at time t=T+Tj. 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~j(t-rj), of the original data waveform, dj(t-rj), and the outputs are second estimates, d~~j(t-r;). The M spread-spectrum channel W096/03819 2 1 9 5 5 3 4 P~ , ,''7 .

outputs OOj, i=1, 2, ..., M, are passed through interference canceler 612 to produce a new corr~p~n~;ng set of channel outputs ~ 1i' 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 011 from the first interference canceler 612, and the output ~21 from the second interference canceler 613, through the output ~
from the Nth interference c~ r 614. Each output to be combined is of the corresponding bit. Therefore "s" bit time delays are inserted for each ~b1. 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 ~1 for channel one, averaged output ~2 for channel two, and averaged output OH for channel M. Each of the averaged outputs are se~uentially passed through decision device 616, decision device 618, and decision device 620. Preferably, the ~v~Lvg~d outputs have multiplying factor c; which may vary according to a particular design. In a preferred embodiment, cj = 1/2i.
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 e~ual powers, and thermal signal to noise ratio (EbN of 30 dB). Length 8191 Gold codes are used for the PN se~uences.
In FIG. 14, performance characteristic of each of the output stages of FIG. 12 is shown. Thus, S0 represents the BER performance at the output of CDMA/DS detector 611, S1 Wo96l038l9 2 ~ ~ 5 53 4 r~ 'J~
.

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 ,- 'ining of the outputs of the interference cancelers are used in det~rmining 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 S0 is the output of the CDMA/DS detector 611. Curve S1 represents the BER performance of the average of the outputs of CDMA/DS
detector 611 and interference canceler 612. ~ere C0 = C1 =
1/2 Cj = o, 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 detormin~d using the combiner shown in FIG. 13. Here, C
and C2 are set equal to 1/2 and all other Cj 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 det~rm;ning 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 WO96103819 ~ 5 3 4 . ~1/ ~ . . , .

canceler 613 is shown as the average output S2 AVG.
FIG. 17 shows performance characteristic corroGp~n~Pnce 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 prcvided they come within the scope of the appended claims and their equivalents.

Classifications
International ClassificationH04B1/707, H04B1/7107, H04B1/7093, H04B1/709, H04L7/00, H04B1/10, H04B7/216, H04B1/12
Cooperative ClassificationH04B1/7093, H04B1/709, H04B1/71075
European ClassificationH04B1/7107B, H04B1/709, H04B1/7093
Legal Events
DateCodeEventDescription
20 Jan 1997EEERExamination request
7 Sep 2015MKEXExpiry
Effective date: 20150706