Baseband interference canceling spread spectrum communications methods and apparatus

• US 6,574,270 B1
• Filed: 07/30/1999
• Issued: 06/03/2003
• Est. Priority Date: 07/30/1999
• Status: Expired due to Term

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Wang, Yi Pin Eric, Madkour, Mohamed F., Gupta, Someshwar C.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Lugo, David B.
• Application Number:
  US09/364,169
• Time in Patent Office: 1,404 Days
• Field of Search: 375/148, 375/144, 375/140, 375/141, 375/142, 375/147, 375/150, 375/346
• US Class Current: 375/148
• CPC Class Codes: H04B 1/71072 Successive interference cancellation

Abstract

Desired information is recovered from a communications signal that includes interference by processing the communications signal to produce a baseband signal and detecting a spreading sequence from the baseband signal. An estimate of an interfering signal component of the baseband signal associated with the detected spreading sequence is generated, and new version of the baseband signal is generated from a previous version of the baseband signal based on the generated estimate of the interfering signal component. The desired information is estimated from the new version of the baseband signal. The spreading sequence preferably is detected by resolving, e.g., descrambling, a version of the baseband signal into a plurality of signal components, respective ones of which are associated with respective propagation paths. The resolved plurality of signal components are correlated with a set of spreading sequences to generate respective sets of correlations for respective ones of the resolved signal components. The correlations are combined, preferably using maximal ratio combining (MRC) to generate a plurality of combined correlations, respective ones of which correspond to respective ones of the spreading sequences. The spreading sequence associated with the interfering signal component is detected from the generated combined correlations.

Claims

1. A method of recovering information from a communications signal that includes desired information and interference, the method comprising the steps of:

processing the communications signal to produce a baseband signal;

detecting a spreading sequence from the baseband signal;

generating an estimate of an interfering signal component of the baseband signal associated with the detected spreading sequence;

generating a new version of the baseband signal from a previous version of the baseband signal based on the generated estimate of the interfering signal component; and

estimating the desired information from the new version of the baseband signal.

2. A method according to claim 1, wherein said step of detecting a spreading sequence comprises the steps of:

resolving a version of the baseband signal into a plurality of signal components, respective ones of which are associated with respective propagation paths;

correlating the resolved plurality of signal components with a set of spreading sequences including the spreading sequence associated with the interfering signal component to generate respective sets of correlations for respective ones of the resolved signal components;

combining correlations to generate a plurality of combined correlations, respective ones of which correspond to respective ones of the spreading sequences; and

detecting the spreading sequence associated with the interfering signal component from the generated combined correlations.

3. A method according to claim 2:

wherein said step of resolving comprises the step of descrambling a version of the baseband signal according to a scrambling sequence to generate the resolved plurality of signal components;

wherein said step of generating an estimate of the interfering signal component comprises the steps of;

generating an estimate of a symbol transmitted according to the detected spreading sequence from one of the combined correlations that is associated with the detected spreading sequence; and

processing the estimate of the symbol according to the detected spreading sequence, the scrambling sequence and a channel estimate to generate an estimate of the interfering signal component.

4. A method according to claim 2:

wherein said step of resolving comprises the step of descrambling a version of the baseband signal according to a scrambling sequence to generate the resolved plurality of signal components;

wherein said step of detecting a spreading sequence associated with an interfering signal component comprises the step of detecting a plurality of spreading sequences from the resolved plurality of signal components, respective ones of the detected plurality of spreading sequences being associated with respective ones of a plurality of interfering signal components in the communications signal; and

wherein said step of generating an estimate of the interfering signal component comprises the steps of;

generating respective estimates of symbols encoded according to respective ones of the detected spreading sequences from respective ones of the combined correlations associated with the detected spreading sequences; and

processing the estimates of the encoded symbols according to the detected spreading sequences, the scrambling sequence and a channel estimate to generate an estimate of the plurality of interfering signal components; and

wherein said step of generating new version of the baseband signal comprises the step of generating a new version of the baseband signal from a previous version of the baseband signal and the generated estimate of the plurality of interfering signal components.

5. A method according to claim 4;

wherein said step of processing the estimates of the encoded symbols comprises the steps of;

spreading respective ones of the estimates of the encoded symbols according to corresponding respective ones of the spreading sequences;

combining the spread estimates of the encoded symbols to generate a composite signal;

scrambling the composite signal according to the scrambling sequence; and

processing the scrambled composite signal according to the channel estimate to generate a composite estimate of the plurality of interfering signal components; and

wherein said step of generating a new version of the baseband signal comprises the step of generating a new version of the baseband signal from a previous version of the baseband signal and the generated composite estimate of the plurality of interfering signal components.

6. A method according to claim 5, wherein said step of generating a new version of the baseband signal comprises the step of subtracting the composite estimate of the plurality of interfering signal components from a previous version of the baseband signal to generate a new version of the baseband signal.

7. A method according to claim 5, wherein said step of generating a new version of the baseband signal comprises the step of determining a projection of a previous version of the baseband signal in a direction in signal space orthogonal to the composite estimate of the plurality of interfering signal components.

8. A method according to claim 7, wherein said step of determining a new version of the baseband signal comprises the step of performing a Gram-Schmidt orthogonalization.

9. A method according to claim 4:

wherein said step of processing the estimates of the encoded symbols comprises the steps of;

spreading respective ones of the estimates of the encoded symbols according to corresponding respective ones of the spreading sequences;

scrambling the spread estimates of the encoded symbols according to the scrambling sequence to generate a plurality of scrambled spread signals; and

processing respective ones of the scrambled spread signals according to the channel estimate to generate respective estimates of respective one of the plurality of interfering signal components; and

wherein said step of generating a new version of the baseband signal comprises the step of generating a new version of the baseband signal from a previous version of the baseband signal and the generated estimates of the plurality of interfering signal components.

10. A method according to claim 9, wherein said step of generating a new estimate of the baseband signal comprises the step of determining a projection of a previous version of the baseband signal in a direction in signal space orthogonal to the generated estimates of the plurality of interfering signal components.

11. A method according to claim 10, wherein said step of determining a new version of the baseband signal comprises the step of performing a Gram-Schmidt orthogonalization.

12. A method according to claim 1, wherein said step of generating a new version of the baseband signal comprises the step of subtracting the estimate of the interfering signal component from a previous version of the baseband signal to generate the new version of the baseband signal.

13. A method according to claim 1, wherein said step of generating a new version of the baseband signal comprises the step of determining a projection of a previous version of the baseband signal in a direction in signal space orthogonal to the estimate of the interfering signal component.

14. A method according to claim 1, wherein the desired information is transmitted according to a desired spreading sequence, and wherein said step of estimating the desired information comprises the steps of:

resolving the new version of the baseband signal into a plurality of interference-canceled signal components, respective ones of which are associated with respective propagation paths;

correlating the resolved plurality of interference-canceled signal components with the desired spreading sequence to generate a set of correlations for the resolved interference-canceled signal components;

combining correlations of the set of correlations for the resolved interference-canceled signal components; and

estimating the desired information from the combined correlations.

15. A method according to claim 14, wherein said step of combining comprises the step of maximal ratio combining.

16. A method according to claim 2, wherein said step of correlating comprises the step of Walsh-Hadamard transforming each of the plurality of resolved signal components.

17. A method according to claim 2, wherein respective ones of the combined correlations indicate respective strengths for respective interfering signals associated with respective spreading sequences of the set of spreading sequences, and wherein said step of detecting a spreading sequence comprises the step of identifying a spreading sequence having an associated interfering signal satisfying a predetermined signal strength criterion.

18. A method according to claim 1:

wherein said step of detecting comprises the step of detecting a first spreading sequence associated with a first interfering signal component;

wherein said step of generating an estimate of the interfering signal component comprises the step of generating an estimate of the first interfering signal component from the detected first spreading sequence;

wherein said step of generating a new version of the baseband signal comprises the step of generating a first version of the baseband signal from a previous version of the baseband signal and the generated estimate of the first interfering signal component;

wherein said step of detecting further comprises the step of detecting a second spreading sequence associated with a second interfering signal component from the first version of the baseband signal;

wherein said step of generating an estimate of the interfering signal component further comprises the step of generating an estimate of the second interfering signal component for the detected second spreading sequence; and

wherein said step of generating a new version of the baseband signal further comprises the step of generating a second version of the baseband signal from the first version of the baseband signal and the generated estimate of the second interfering signal component.

19. A method according to claim 1, wherein said steps of generating an estimate of the interfering signal component and generating a new version of the baseband signal are repeatedly performed until an iteration criterion is satisfied.

20. A method according to claim 19, wherein the iteration criterion comprises at least one of an error rate or an iteration count.

21. A method according to claim 1, wherein said step of detecting a spreading sequence comprises the step of detecting spreading sequences on a symbol-by-symbol basis.

22. A method according to claim 1, wherein said step of detecting a spreading sequence comprises the step of intermittently detecting spreading sequences for selected symbol periods.

23. A method according to claim 22:

wherein said step of detecting a spreading sequence comprises the step of detecting the spreading sequence from a first portion of the baseband signal corresponding to a first transmitted symbol;

wherein said step of generating an estimate of an interfering signal component comprises the step of generating an estimate of the interfering signal component from a second portion of the baseband signal associated with a second transmitted symbol based on the detected spreading sequence;

wherein said step of generating a new version of the baseband signal comprises the step of generating a new version of the second portion of the baseband signal from a previous version of the second portion of the baseband signal and the generated estimate of the interfering signal component; and

wherein said step of estimating the desired information comprises the step of estimating the second transmitted symbol from the new version of the second portion of the baseband signal.

24. A method of recovering information from communications signal including interfering components, the method comprising the steps of:

correlating a version of the communications signal with a set of spreading sequences;

detecting a spreading sequence associated with an interfering signal component of the communications signal from correlations of the version of the communications signal with the set of spreading sequences;

generating an estimate of an interfering signal component of the communications signal associated with the detected spreading sequence;

generating a new version of the communications signal from the previous version of the communications signal based on the estimate of the interfering signal component; and

estimating information in the communications signal from the new version of the communications signal.

25. A method according to claim 24:

wherein said step of correlating comprises the steps of;

resolving the version of the communications signal into a plurality of multipath signal components; and

correlating each of the resolved multipath signal components with the set of spreading sequences to produce a plurality of sets of correlations, respective ones of which are associated with respective ones of the spreading sequences; and

wherein said step of detecting comprises the step of;

combining correlations in each of the sets of correlations; and

detecting the spreading sequence from the combined correlations.

26. A method according to claim 25:

wherein said step of resolving comprises the step of descrambling the version of the communications signal according to a scrambling sequence to generate the resolved plurality of signal components;

wherein said step of generating an estimate of the interfering signal component comprises the steps of;

generating an estimate of a symbol transmitted according to the detected spreading sequence from the combined correlation associated with the detected spreading sequence; and

processing the estimate of the symbol according to the detected spreading sequence, the scrambling sequence and a channel estimate to generate an estimate of the interfering signal component.

27. A method according to claim 24, wherein said step of generating a new version of the communications signal comprises the step of subtracting the estimate of the interfering signal component from a previous version of the communications signal to generate the new version of the communications signal.

28. A method according to claim 24, wherein said step of generating a new version of the communications signal comprises the step of determining a projection of a previous version of the communications signal in a direction in signal space orthogonal to the estimate of the interfering signal component.

29. A method according to claim 24, wherein said step of estimating information in the communications signal comprise the steps of:

resolving the new version of the communications signal into a plurality of multipath signal components;

correlating the resolved plurality of interference-canceled signal components with a desired spreading sequence to generate a set of correlations;

combining correlations of the set of correlations; and

estimating information in the communications signal from the combined correlations.

30. A method according to claim 24, wherein said steps of correlating, detecting a spreading sequence, generating an estimate of an interfering signal component, and generating a new version of the communications signal are repeatedly performed until an iteration criterion is satisfied.

31. A method according to claim 30, wherein the iteration criterion comprises at least one of an error rate or an iteration count.

32. A method according to claim 24, wherein said steps of correlating, detecting, generating an estimate of an interfering signal component, and generating a new version of the communications signal are performed on at least one of a symbol-by-symbol basis or a frame-by-frame basis.

33. An apparatus for recovering information from a communications signal that includes desired information and interference, the apparatus comprising:

means for processing the communications signal to produce a baseband signal;

means for detecting a spreading sequence from the baseband signal;

means for generating an estimate of an interfering signal component of the baseband signal associated with the detected spreading sequence;

means for generating a new version of the baseband signal from a previous version of the baseband signal based on the generated estimate of the interfering signal component; and

means for estimating the desired information from the new version of the baseband signal.

34. An apparatus according to claim 33, wherein said means for detecting a spreading sequence comprises:

means for resolving a version of the baseband signal into a plurality of signal components, respective ones of which are associated with respective propagation paths;

means for correlating the resolved plurality of signal components with a set of spreading sequences including the spreading sequence associated with the interfering signal component to generate respective sets of correlations for respective ones of the resolved signal components;

means for combining correlations to generate a plurality of combined correlations, respective ones of which correspond to respective ones of the spreading sequences; and

means for detecting the spreading sequence associated with the interfering signal component from the generated combined correlations.

35. An apparatus according to claim 34:

wherein said means for resolving comprises means for descrambling a version of the baseband signal according to a scrambling sequence to generate the resolved plurality of signal components;

wherein said means for generating an estimate of the interfering signal component comprises;

means for generating an estimate of a symbol transmitted according to the detected spreading sequence from one of the combined correlations that is associated with the detected spreading sequence; and

means for processing the estimate of the symbol according to the detected spreading sequence, the scrambling sequence and a channel estimate to generate an estimate of the interfering signal component.

36. An apparatus according to claim 34:

wherein said means for resolving comprises means for descrambling a version of the baseband signal according to a scrambling sequence to generate the resolved plurality of signal components;

wherein said means for detecting a spreading sequence associated with an interfering signal component comprises means for detecting a plurality of spreading sequences from the resolved plurality of signal components, respective ones of the detected plurality of spreading sequences being associated with respective ones of a plurality of interfering signal components in the communications signal; and

wherein said means for generating an estimate of the interfering signal component comprises;

means for generating respective estimates of symbols encoded according to respective ones of the detected spreading sequences from respective ones of the combined correlations associated with the detected spreading sequences; and

means for processing the estimates of the encoded symbols according to the detected spreading sequences, the scrambling sequence and a channel estimate to generate an estimate of the plurality of interfering signal components; and

wherein said means for generating new version of the baseband signal comprises means for generating a new version of the baseband signal from a previous version of the baseband signal and the generated estimate of the plurality of interfering signal components.

37. An apparatus according to claim 36:

wherein said means for processing the estimates of the encoded symbols comprises;

means for spreading respective ones of the estimates of the encoded symbols according to corresponding respective ones of the spreading sequences;

means for combining the spread estimates of the encoded symbols to generate, a composite signal;

means for scrambling the composite signal according to the scrambling sequence; and

means for processing the scrambled composite signal according to the channel estimate to generate a composite estimate of the plurality of interfering signal components; and

wherein said means for generating a new version of the baseband signal comprises means for generating a new version of the baseband signal from a previous version of the baseband signal and the generated composite estimate of the plurality of interfering signal components.

38. An apparatus according to claim 37, wherein said means for generating a new version of the baseband signal comprises means for subtracting the composite estimate of the plurality of interfering signal components from a previous version of the baseband signal to generate a new version of the baseband signal.

39. An apparatus according to claim 37, wherein said means for generating a new version of the baseband signal comprises means for determining a projection of a previous version of the baseband signal in a direction in signal space orthogonal to the composite estimate of the plurality of interfering signal components.

40. An apparatus according to claim 39, wherein said means for determining a new version of the baseband signal comprises means for performing a Gram-Schmidt orthogonalization.

41. An apparatus according to claim 36:

wherein said means for processing the estimates of the encoded symbols comprises;

means for spreading respective ones of the estimates of the encoded symbols according to corresponding respective ones of the spreading sequences;

means for scrambling the spread estimates of the encoded symbols according to the scrambling sequence to generate a plurality of scrambled spread signals; and

means for processing respective ones of the scrambled spread signals according to the channel estimate to generate respective estimates of respective one of the plurality of interfering signal components; and

wherein said means for generating a new version of the baseband signal comprises means for generating a new version of the baseband signal from a previous version of the baseband signal and the generated estimates of the plurality of interfering signal components.

42. An apparatus according to claim 41, wherein said means for generating a new estimate of the baseband signal comprises means for determining a projection of a previous version of the baseband signal in a direction in signal space orthogonal to the generated estimates of the plurality of interfering signal components.

43. An apparatus according to claim 42, wherein said means for determining a new version of the baseband signal comprises means for performing a Gram-Schmidt orthogonalization.

44. An apparatus according to claim 33, wherein said means for generating a new version of the baseband signal comprises means for subtracting the estimate of the interfering signal component from a previous version of the baseband signal to generate the new version of the baseband signal.

45. An apparatus according to claim 33, wherein said means for generating a new version of the baseband signal comprises means for determining a projection of a previous version of the baseband signal in a direction in signal space orthogonal to the estimate of the interfering signal component.

46. An apparatus according to claim 33, wherein the desired information is transmitted according to a desired spreading sequence, and wherein said means for estimating the desired information comprises:

means for resolving the new version of the baseband signal into a plurality of interference-canceled signal components, respective ones of which are associated with respective propagation paths;

means for correlating the resolved plurality of interference-canceled signal components with the desired spreading sequence to generate a set of correlations for the resolved interference-canceled signal components;

means for combining correlations of the set of correlations for the resolved interference-canceled signal components; and

means for estimating the desired information from the combined correlations.

47. An apparatus according to claim 46, wherein said means for combining comprises means for maximal ratio combining.

48. An apparatus according to claim 34, wherein said means for correlating comprises means for Walsh-Hadamard transforming each of the plurality of resolved signal components.

49. An apparatus according to claim 34, wherein respective ones of the combined correlations indicate respective strengths for respective interfering signals associated with respective spreading sequences of the set of spreading sequences, and wherein said means for detecting a spreading sequence comprises means for identifying a spreading sequence having an associated interfering signal satisfying a predetermined signal strength criterion.

50. An apparatus according to claim 33:

wherein said means for detecting comprises means for detecting a first spreading sequence associated with a first interfering signal component;

wherein said means for generating an estimate of the interfering signal component comprises means for generating an estimate of the first interfering signal component from the detected first spreading sequence;

wherein said means for generating a new version of the baseband signal comprises means for generating a first version of the baseband signal from a previous version of the baseband signal and the generated estimate of the first interfering signal component;

wherein said means for detecting further comprises means for detecting a second spreading sequence associated with a second interfering signal component from the first version of the baseband signal;

wherein said means for generating an estimate of the interfering signal component further comprises means for generating an estimate of the second interfering signal component for the detected second spreading sequence; and

wherein said means for generating a new version of the baseband signal further comprises means for generating a second version of the baseband signal from the first version of the baseband signal and the generated estimate of the second interfering signal component.

51. An apparatus according to claim 33, wherein said means for generating an estimate of the interfering signal component and said means for generating a new version of the baseband signal repeatedly generate estimates of interfering signal components and generate new versions of the baseband signal, respectively, until an iteration criterion is satisfied.

52. An apparatus according to claim 51, wherein the iteration criterion comprises at least one of an error rate or an iteration count.

53. An apparatus according to claim 33, wherein said means for detecting a spreading sequence comprises means for detecting spreading sequences on a symbol-by-symbol basis.

54. An apparatus according to claim 33, wherein said means for detecting a spreading sequence comprises means for intermittently detecting spreading sequences for selected symbol periods.

55. An apparatus according to claim 54:

wherein said means for detecting a spreading sequence comprises means for detecting the spreading sequence from a first portion of the baseband signal corresponding to a first transmitted symbol;

wherein said means for generating an estimate of an interfering signal component comprises means for generating an estimate of the interfering signal component from a second portion of the baseband signal associated with a second transmitted symbol based on the detected spreading sequence;

wherein said means for generating a new version of the baseband signal comprises means for generating a new version of the second portion of the baseband signal from a previous version of the second portion of the baseband signal and the generated estimate of the interfering signal component; and

wherein said means for estimating the desired information comprises means for estimating the second transmitted symbol from the new version of the second portion of the baseband signal.

56. A receiver, comprising:

a recursive baseband processor that detects a spreading sequence from the baseband signal, generates an estimate of an interfering signal component of the baseband signal associated with the detected spreading sequence, generates an interference-canceled version of the baseband signal from the baseband signal based on correlations of the baseband signal with a set of spreading sequences, and generates an estimate the desired information from the interference-canceled version of the baseband signal.

57. A receiver according to claim 56, wherein said recursive baseband processor comprises:

a descrambler operative to resolve a version of the baseband signal into a plurality of signal components, respective ones of which are associated with respective propagation paths;

a correlator responsive to said descrambler and operative to correlate the resolved plurality of signal components with a set of spreading sequences to generate respective sets of correlations for respective ones of the resolved signal components;

a maximal ratio combiner responsive to said correlator and operative to combine correlations to generate a plurality of combined correlations, respective ones of which correspond to respective ones of the spreading sequences;

a spreading sequence detector responsive to said maximal ratio combiner and operative to detect the spreading sequence associated with the interfering signal component from the generated combined correlation;

a symbol estimator responsive to said maximal ratio combiner and operative to generate a symbol estimate for the interfering signal component;

a spreader responsive to said spreading sequence detector and to said symbol estimator and operative to spread the generated symbol estimate according to the detected spreading sequence;

a scrambler responsive to said spreader and operative to scramble the spread symbol estimate according to the scrambling sequence;

a channel emulator responsive to said scrambler and operative to process the scrambled spread symbol estimate according to a channel estimate to generate an estimate of the interfering signal component; and

a baseband interference canceler responsive to said channel emulator and operative to generate a new version of the baseband signal from a previous version of the baseband signal and the generated estimate of the interfering signal component.

58. A receiver according to claim 57:

wherein said descrambler is further operative to resolve the new version of the baseband signal into a plurality of signal components, respective ones of which are associated with respective propagation paths;

wherein said correlator is further operative to correlate the resolved plurality of signal components of the new version of the baseband signal with a desired spreading sequence to generate a set of correlations for the resolved signal components of the new version of the baseband signal;

wherein said maximal ratio combiner is further operative to combine the set of correlations for the resolved signal components of the new version of the baseband signal to generate a decision statistic for symbol transmitted according to the desired spreading sequence and

wherein said recursive baseband processor further comprises a symbol estimator responsive to said maximal ratio combiner and operative to generate an estimate of a symbol transmitted according to the desired spreading sequence from the generated decision statistic.