Methods and apparatus for equalization and decoding of digital communications channels using antenna diversity

• US 5,862,192 A
• Filed: 12/28/1995
• Issued: 01/19/1999
• Est. Priority Date: 12/31/1991
• Status: Expired due to Term

• Current Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Huszar, Stephen Russell, Seshadri, Nambirajan
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): DEPPE, BETSY LEE
• Application Number:
  US08/580,218
• Time in Patent Office: 1,118 Days
• Field of Search: 375/341, 375/347, 375/348, 375/346, 375/340, 375/232, 375/262, 375/267, 371/43.8, 371/43.7
• US Class Current: 375/347
• CPC Class Codes: H03M 13/25 Error detection or forward error correction by signal space coding, i.e. adding redundancy in the signal constellation, e.g. Trellis Coded Modulation [TCM] modulation codes H03M13/31
• CPC Class Codes: H04L 1/0047 Decoding adapted to other signal detection operation in conjunction with sequence estimation or equalization H04L25/03286
• CPC Class Codes: H04L 1/0054 Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms
• CPC Class Codes: H04L 25/0212 of impulse response
• CPC Class Codes: H04L 25/025 using least-mean-square [LMS] method
• CPC Class Codes: H04L 25/03267 with decision feedback equalisers
• CPC Class Codes: H04L 25/03292 with channel estimation circuitry
• CPC Class Codes: H04L 25/03324 Provision of tentative decisions

Abstract

An equalizer/decoder for a communication channel presenting impairments to error-free reception of transmitted symbols includes a modified Viterbi decoder operating on samples of received signals in a plurality of equivalent subchannels. Estimates of each of the subchannels are updated using a locally best estimate in the Viterbi trellis processing, thereby avoiding delay in updating channel estimates in a rapidly changing channel such as a digital cellular communications channels. A leaky predictor channel updating technique also proves advantageous in an alternative embodiment.

Claims

1. A method for decoding a time varying signal representing a sequence of symbols having been supplied to a communications channel at a symbol rate, said method comprising the steps of:

receiving said time varying signal at each one of a plurality of antennas;

sampling said received time varying signal at a sampling rate of R/T, wherein R≧

2, and generating a set of R sampled values representative of said sampled time varying signal at each of said plurality of antennas;

generating locally a set of values for each one of a plurality of sets of candidate symbol sequences, each of said values representing at least a partial estimate of an output of said communications channel as received at least one of said plurality of antennas; and

comparing each of said sample sets with one or more of said value sets and, in response thereto, selecting the sequence of symbols supplied to said communications channel and received at at least one of said plurality of antennas.

2. The method as set forth in claim 1 wherein said comparing step further includes the step of utilizing one or more multi-state trellises to generate said set of values for at least one of said plurality of sets of candidate symbol sequences.

3. The method as set forth in claim 2 wherein said utilizing step further includes the step of using a Viterbi algorithm.

4. The method as set forth in claim 2 wherein said utilizing step further includes the step of traversing said one or more multi-state trellises.

5. The method as set forth in claim 1 wherein said selectively comparing step further includes the step of forming an error signal reflecting a difference between at least a subset of said set of samples and a subset of said set of values for each of said candidate symbol sequences.

6. The method as set forth in claim 5 wherein said selectively comparing step further includes the step of selecting as a symbol supplied to said communications channel a symbol in the candidate symbol sequence yielding a minimum value for said error signal for said plurality of antennas.

7. The method as set forth in claim 2 further including the step of updating at least a portion of one or more of said multi-state trellises using a minimum value for said error signal.

8. The method as set forth in claim 1 wherein at least one particular candidate symbol sequence, which is representative of at least one output of said communications channel as received at at least one of said plurality of antennas, includes a plurality of symbols and said generating step further includes the step of producing a current value for a current symbol of said particular candidate symbol sequence, said current value affected by intersymbol interference caused by at least one symbol supplied to said communications channel prior to said current symbol.

9. The method as set forth in claim 8 wherein said producing step further includes the step of using at least a first prior value at a time k-1 and at least a second prior value at a time k to generate said current value for said current symbol at a time k+1.

10. The method as set forth in claim 5 wherein said generating step further includes the step of determining each of said partial estimates of said output of said communications channel based on a respective partial estimate of characteristics of said communications channel.

11. The method as set forth in claim 10 further including the step of updating said partial estimates of the characteristics of said communications channel in response to said error signal.

12. The method as set forth in claim 11 wherein said error signal is determined by determining a difference between each of said set of samples for at least one of said plurality of antennas and a noiseless estimate of the channel output.

13. The method as set forth in claim 12 wherein said noiseless estimate of the channel output is generated in response to a tentative decision as to the symbol actually supplied to said communications channel.

14. The method as set forth in claim 13 further including the step of updating each of said partial estimates of the characteristics of said communications channel in response to a signal representing a prediction based on the current estimates of said characteristics of said communications channel.

15. A decoder for decoding a time varying signal representing a sequence of symbols having been supplied to a communications channel at a symbol rate, said decoder comprising:

a plurality of antennas operative to receive said time varying signal; and

a circuit coupled with said plurality of antennas and operative to;

sample said received time varying signal at a sampling rate of R/T wherein R≧

2, and generate a set of R sampled values representative of said sampled time varying signal at each of said plurality of antennas;

generate a set of values for each one of a plurality of sets of candidate symbol sequences, each said value representing at least a partial estimate of an output of said communications channel as received at at least one of said plurality of antennas; and

compare each of said sample sets with one or more of said value sets and, in response thereto, select the sequence of symbols supplied to said communications channel and received at at least one of said plurality of antennas.

16. The decoder as set forth in claim 15 wherein said circuit is further operative to utilize one or more multi-state trellises to generate said set of values for at least one of said plurality of sets of candidate symbol sequences.

17. The decoder as set forth in claim 15 wherein said circuit is further operative to use a Viterbi algorithm.

18. The decoder as set forth in claim 16 wherein said circuit is further operative to traverse said one more multi-state trellises.

19. The decoder as set forth in claim 16 wherein said circuit is further operative to form an error signal reflecting a difference between at least a subset of said set samples and a subset of said set of values for each of said candidate symbol sequences.

20. The decoder as set forth in claim 19 wherein said circuit is further operative to select as a symbol supplied to said communications channel a symbol in the candidate symbol sequence yielding a minimum value for said error signal for said plurality of antennas.

21. The decoder as set forth in claim 19 wherein said circuit is further operative to update at least a portion of one or more of said multi-state trellises using a minimum value for said error signal.

22. The decoder as set forth in claim 15 wherein at least one particular candidate symbol sequence, which is representative of at least one output of said communications channel as received at at least one of said plurality of antennas, includes a plurality of symbols and said circuit is further operative to produce a current value for a current symbol of said particular candidate symbol sequence, said current value affected by intersymbol interference caused by at least one symbol supplied to said communications channel prior to said current symbol.

23. The decoder as set forth in claim 22 wherein said circuit is further operative to use at least a first prior value at a time k-1 and at least a second prior value at a time k to generate said current value for said current symbol at a time k+1.

24. The decoder as set forth in claim 19 wherein said circuit is further operative to determine each of said partial estimates of said output of said communications channel based on a respective partial estimate of characteristics of said communications channel.

25. The decoder as set forth in claim 24 wherein said circuit is further operative to update said partial estimates of the characteristics of said communications channel in response to said error signal.

26. The decoder as set forth in claim 25 wherein said circuit is further operative to determine said error signal by determining a difference between each of said set of samples for at least one of said plurality of antennas and a noiseless estimate of the channel output.

27. The decoder as set forth in claim 26 wherein said noiseless estimate of the channel output is generated in response to a tentative decision as to the symbol actually supplied to said communications channel.

28. The decoder as set forth in claim 27 wherein said circuit is further operative to update each of said partial estimates of the characteristics of said communications channel in response to a signal representing a prediction based on the current estimates of said characteristics of said communications channel.