Code-division-multiple-access (DS-CDMA) channels with aperiodic codes

• US 7,376,106 B2
• Filed: 11/27/2002
• Issued: 05/20/2008
• Est. Priority Date: 11/27/2002
• Status: Active Grant

• Current Assignee: Google LLC (Alphabet Inc.)
• Original Assignee: International Business Machines Corporation
• Inventors: Sheinvald, Jacob, Karnin, Ehud, Ben-David, Shay
• Primary Examiner(s): Nguyen; Duc M.
• Assistant Examiner(s): GENACK, MATTHEW W
• Application Number:
  US10/306,916
• Publication Number:
  US 20040100928A1
• Time in Patent Office: 2,001 Days
• Field of Search: None
• US Class Current: 370/335
• CPC Class Codes: H04B 1/7105 Joint detection techniques, e.g. linear detectors
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/0236 using estimation of the other symbols

Abstract

A method for channel estimation, includes receiving an input signal due to a superposition of waveforms encoding symbols transmitted respectively by a plurality of transmitters in a common frequency band. Each transmitter among the plurality of transmitters modulates the symbols with a respective spreading code. The input signal is sampled at sampling intervals over the duration of an observation period, thus providing a sequence of samples. The sequence of complex samples is processed to estimate a channel response for each of the transmitters by maximizing, individually for each of the transmitters and based on the respective spreading code of the transmitter, a likelihood of correctly decoding the transmitted symbols using the estimated channel response.

Claims

1. A method for channel estimation, comprising:

receiving an input signal due to a superposition of waveforms encoding symbols transmitted respectively by a plurality of transmitters in a common frequency band, wherein each transmitter among the plurality of transmitters modulates the symbols with a respective spreading code;

sampling the input signal at sampling intervals over the duration of an observation period to provide a sequence of samples;

processing the sequence of samples to generate a respective initial estimate of a channel response for each of the transmitters by maximizing, individually for each of the transmitters and based on the respective spreading code of the transmitter, a likelihood of correctly decoding the transmitted symbols using the estimated channel response;

based on the initial estimate, determining decision values corresponding to the symbols transmitted by each of the transmitters;

re-estimating the channel response for each of the transmitters based on the decision values of the symbols; and

decoding the symbols transmitted by the plurality of transmitters using the re-estimated channel response.

2. A method according to claim 1, wherein processing the sequence of complex samples comprises:

computing respective fingerprints of the transmitters, based on the respective spreading code and the sequence of samples; and

determining the channel response for each of the transmitters so as to maximize a product of the respective fingerprints with decision values of the symbols transmitted by the transmitter.

3. A method according to claim 2, wherein determining the channel response comprises assembling a respective matrix of the fingerprints for each of the transmitters, and finding a singular value decomposition (SVD) of the respective matrix.

4. A method according to claim 3, wherein finding the SVD comprises deriving a unitary matrix from the matrix of the fingerprints, and estimating the channel response based on the unitary matrix.

5. A method according to claim 4, wherein estimating the channel response comprises determining the decision values of the symbols based on the unitary matrix, and computing the estimated channel response from the decision values.

6. A method according to claim 4, wherein estimating the channel response comprises computing the estimated channel response based on the unitary matrix and on a singular value determined by the SVD.

7. A method according to claim 3, wherein the transmitters transmit known pilot signals in addition to the encoded symbols, and wherein assembling the respective matrix comprises augmenting the matrix to account for the pilot signals, and wherein finding the SVD comprises computing the SVD of the augmented matrix so as to estimate the channel response using the pilot signals.

8. A method according to claim 2, wherein the symbols belong to a real-valued constellation, and wherein determining the channel response comprises maximizing the product of the respective fingerprints with the decision values while restricting the decision values to real values.

9. A method according to claim 1, wherein processing the sequence of complex samples comprises estimating a respective delay time for each of the transmitters by maximizing the likelihood of correctly decoding the transmitted symbols over a range of possible delay times.

10. A method according to claim 1, wherein sampling the input signal comprises generating the samples at a sample rate that is substantially equal to a chip rate of the spreading code.

11. A method according to claim 1, wherein sampling the input signal comprises generating the samples at a sample rate that is substantially greater than a chip rate of the spreading code.

12. A method according to claim 1, wherein receiving the input signal comprises receiving the signal through multiple antennas, and wherein processing the sequence of complex samples comprises estimating the channel response from each of the transmitters to each of the multiple antennas.

13. A method according to claim 1, wherein the waveforms comprise code-division multiple access (CDMA) waveforms.

14. A method according to claim 1, wherein the spreading codes comprise complex-valued codes.

15. A method according to claim 1, wherein re-estimating the channel response comprises iteratively repeating the steps of processing the sequence of complex samples to determine the decision values and re-estimating the channel response until a convergence criterion is satisfied.

16. A method according to claim 1, wherein the transmitted symbols belong to a predetermined constellation, and wherein processing the sequence of complex samples to determine the decision values comprises finding soft decision values corresponding to the symbols while constraining the soft decision values to a range of values to which the constellation belongs, and projecting the soft decision values onto the constellation to estimate the transmitted symbols.

17. A method according to claim 16, wherein the constellation is real-valued, and wherein constraining the soft decision values comprises requiring the soft decision values to be real.

18. A method according to claim 16, wherein the symbols in the constellation have a fixed magnitude and a variable phase, and wherein constraining the soft decision values comprises constraining the soft decision values to a circle in a complex plane.

19. A method according to claim 1, wherein the transmitters transmit known pilot signals in addition to the encoded symbols, and wherein processing the sequence of complex samples to estimate the channel response comprises estimating the channel response using the pilot signals, and wherein processing the sequence of complex samples to determine the decision values comprises estimating the decision values independent of the pilot signals.

20. A multi-user receiver, comprising:

input circuitry, coupled to receive an input signal due to a superposition of waveforms encoding symbols transmitted respectively by a plurality of transmitters in a common frequency band, wherein each transmitter among the plurality of transmitters modulates the symbols with a respective spreading code, and to sample the input signal at sampling intervals over the duration of an observation period to provide a sequence of samples;

channel estimation circuitry, coupled to receive and process the sequence of samples so as to estimate a channel response for each of the transmitters by maximizing, individually for each of the transmitters and based on the respective spreading code of the transmitter, a likelihood of correctly decoding the transmitted symbols using the estimated channel response; and

multi-user detection circuitry, which is arranged to process the sequence of samples using the estimated channel response for all the transmitters to determine decision values corresponding to the symbols transmitted by each of the transmitters,

wherein the channel estimation circuitry is arranged to re-estimate the channel response for each of the transmitters based on the decision values of the symbols, for use by the multi-user detection circuitry in decoding the symbols.

21. A receiver according to claim 20, wherein the channel estimation circuitry is arranged to compute respective fingerprints of the transmitters, based on the respective spreading code and the sequence of samples, and to determine the channel response for each of the transmitters so as to maximize a product of the respective fingerprints with decision values of the symbols transmitted by the transmitter.

22. A receiver according to claim 21, wherein the channel estimation circuitry is arranged to determine the channel response by assembling a respective matrix of the fingerprints for each of the transmitters, and finding a singular value decomposition (SVD) of the respective matrix.

23. A receiver according to claim 22, wherein the channel estimation circuitry is arranged to derive a unitary matrix from the matrix of the fingerprints, and to estimate the channel response based on the unitary matrix.

24. A receiver according to claim 23, wherein the channel estimation circuitry is arranged to determine the decision values of the symbols based on the unitary matrix, and to compute the estimated channel response from the decision values.

25. A receiver according to claim 23, wherein the channel estimation circuitry is arranged to compute the estimated channel response based on the unitary matrix and on a singular value determined by the SVD.

26. A receiver according to claim 22, wherein the transmitters transmit known pilot signals in addition to the encoded symbols, and wherein the channel estimation circuitry is arranged to augment the matrix to account for the pilot signals, and to compute the SVD of the augmented matrix so as to estimate the channel response using the pilot signals.

27. A receiver according to claim 21, wherein the symbols belong to a real-valued constellation, and wherein the channel estimation circuitry is arranged to determine the channel response by maximizing the product of the respective fingerprints with the decision values while restricting the decision values to real values.

28. A receiver according to claim 20, wherein the channel estimation circuitry is arranged to estimate a respective delay time for each of the transmitters by maximizing the likelihood of correctly decoding the transmitted symbols over a range of possible delay times.

29. A receiver according to claim 20, wherein the input circuitry is arranged to sample the input signal at a sample rate that is substantially equal to a chip rate of the spreading code.

30. A receiver according to claim 20, wherein the input circuitry is arranged to sample the input signal at a sample rate that is substantially greater than a chip rate of the spreading code.

31. A receiver according to claim 20, wherein the input circuitry is coupled to receive the input signal through multiple antennas, and wherein the channel estimation circuitry is arranged to estimate the channel response from each of the transmitters to each of the multiple antennas.

32. A receiver according to claim 20, wherein the waveforms comprise code-division multiple access (CDMA) waveforms.

33. A receiver according to claim 20, wherein the spreading codes comprise complex-valued codes.

34. A receiver according to claim 20, wherein the multi-user detection circuitry and the channel estimation circuitry are arranged to iteratively repeat determining the decision values and re-estimating the channel response until a convergence criterion is satisfied.

35. A receiver according to claim 20, wherein the transmitted symbols belong to a predetermined constellation, and wherein the multi-user detection circuitry is arranged to find soft decision values corresponding to the symbols while constraining the soft decision values to a range of values to which the constellation belongs, and to project the soft decision values onto the constellation to estimate the transmitted symbols.

36. A receiver according to claim 35, wherein the constellation is real-valued, and wherein the multi-user detection circuitry is arranged to constrain the soft decision values to be real.

37. A receiver according to claim 35, wherein the symbols in the constellation have a fixed magnitude and a variable phase, and wherein the multi-user detection circuitry is arranged to constrain the soft decision values to a circle in a complex plane.

38. A receiver according to claim 20, wherein the transmitters transmit known pilot signals in addition to the encoded symbols, and wherein the channel estimation circuitry is arranged to estimate the channel response using the pilot signals, and wherein the multi-user detection circuitry is arranged to estimate the decision values independent of the pilot signals.