Apparatus and methods for intereference cancellation in spread spectrum communications systems

• US 6,501,788 B1
• Filed: 01/22/1999
• Issued: 12/31/2002
• Est. Priority Date: 01/22/1999
• Status: Expired due to Term

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Wang, Yi-Pin Eric, Khayrallah, Ali
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Kim, Kevin
• Application Number:
  US09/235,470
• Time in Patent Office: 1,439 Days
• Field of Search: 375/144, 375/347, 375/349, 375/148, 375/150, 375/142, 375/346
• US Class Current: 375/148
• CPC Class Codes: H04B 1/7103 the interference being multiple access interference
• CPC Class Codes: H04J 11/0023 Interference mitigation or co-ordination direct sequence spread spectrum [DSSS] systems H04B1/7097; frequency hopping H04B1/713; allocation criteria for ingress interference avoidance H04L5/0062; frequency allocation criteria for requirements on out-of-channel emissions H04L5/0066; arrangements for removing intersymbol interference or baseband equalisers H04L25/03006; peak power aspects in multicarrier modulation H04L27/2614; power management H04W52/00; traffic scheduling H04W72/54, H04W72/541
• CPC Class Codes: H04J 13/0048 Walsh

Abstract

A symbol is transmitted in a communications medium according to a first spreading sequence of a set of spreading sequences, for example, CDMA channelization codes, which may be used to transmit symbols in the system. A communications signal is received from the communications medium, for example, at a mobile terminal. The received communications signal is resolved into a plurality of multipath signal components by, for example, descrambling the received signal according to a cell-specific scrambling code. The resolved plurality of signal components is correlated with the set of spreading sequences to generate a respective set of correlations for a respective one of the resolved signal components, a respective one of the sets of correlations including a respective correlation of a resolved signal component with the first spreading sequence. An interference component associated with a second spreading sequence of the set of spreading sequences, identified based on the correlations of the resolved signal components with the set of possible spreading sequences, is cancelled from the correlations of the plurality of resolved signal components with the first spreading sequence, thus generating a set of interference-cancelled correlations of the resolved signal components with the first spreading sequence. The transmitted symbol is estimated from the set of interference-cancelled correlations. Preferably, the set of spreading sequences is orthogonal. More preferably, the set of spreading sequences comprises a Hadamard code, and the correlations of the set of possible spreading sequences with the resolved signal components is generated using a fast Hadamard transform.

Claims

1. In a spread spectrum communications system, a method of recovering a symbol transmitted in a communications medium according to a first spreading sequence of a set of spreading sequences that may be used to transmit symbols in the system, the method comprising the steps of:

receiving a communications signal from the communications medium;

resolving the received communications signal into a plurality of signal components, a respective one of which is associated with a respective propagation path;

correlating the resolved plurality of signal components with the set of spreading sequences to generate a respective set of correlations for a respective one of the resolved signal components, a respective one of the sets of correlations including a respective correlation of a resolved signal component with the first spreading sequence;

canceling an interference component associated with a second spreading sequence of the set of spreading sequences from the correlations of the plurality of resolved signal components with the first spreading sequence based on the correlations of the plurality of resolved signal components with the second spreading sequence to generate a set of interference-cancelled correlations of the resolved signal components with the first spreading sequence; and

estimating the transmitted symbol from the set of interference-cancelled correlations.

2. A method according to claim 1, wherein said step of canceling comprises the step of identifying a second spreading sequence based on the correlations of the set of spreading sequences with the resolved signal components.

3. A method according to claim 1, wherein the set of spreading sequences is orthogonal.

4. A method according to claim 3, wherein the set of spreading sequences comprises a Hadamard code, and wherein said step of correlating comprises the step of fast Hadamard transforming each of the plurality of resolved signal components.

5. A method according to claim 1, wherein the spread spectrum communications system comprises a plurality of geographic cells, and wherein said step of resolving comprises the step of correlating the received communications signal with a set of time-shifted versions of a scrambling code to generate the resolved plurality of signal components.

6. A method according to claim 1, wherein said step of canceling comprises the steps of:

combining correlations for resolved signal components to determine respective energies for respective spreading sequences; and

identifying a second spreading sequence having an associated energy meeting a predetermined criterion.

7. A method according to claim 6, wherein said step of combining comprises the step of maximal ratio combining.

8. A method according to claim 6, wherein said step of identifying comprises the step of identifying a second spreading sequence having a highest associated energy.

9. A method according to claim 6, wherein said step of identifying comprises the step of identifying a second spreading sequence having an associated energy exceeding a predetermined threshold.

10. A method according to claim 6, wherein said step of canceling further comprises the steps of:

generating a symbol estimate for an interferer signal transmitted according to the identified second spreading sequence;

generating a power estimate for the interferer signal transmitted according to the identified second spreading sequence; and

determining a respective interference component for a respective one of the correlations of the resolved signal components with the first spreading sequence from the second spreading sequence, the symbol estimate for the second spreading sequence and the power estimate for the second spreading sequence.

11. A method according to claim 10:

wherein said step of resolving comprises the steps of;

resolving a received communications signal into a first plurality of signal components for a first symbol period; and

resolving a received communications signal into a second plurality of signal components for a second symbol period;

wherein said step of identifying comprises the step of identifying a second spreading sequence for the first symbol period from the first resolved plurality of signal components; and

wherein said step of canceling comprises the steps of;

correlating the second resolved plurality of signal components with the first and second spreading sequences to generate respective correlations of the second resolved plurality of signal components with the first and second spreading sequences; and

canceling an interference component from the correlations of the second resolved plurality of signal components with the first spreading sequence based on the correlations of the second resolved plurality of signal components with the second spreading sequence to generate a plurality of interference canceled correlations of the second resolved plurality of signal components with the first spreading sequence; and

wherein said step of estimating comprises the step generating an estimate of a symbol for the second symbol period from the interference-cancelled correlations of the plurality of resolved signal components for the second symbol period with the first spreading sequence.

12. A method according to claim 6:

wherein said step of correlating comprises the step of correlating resolved signal components for each of a succession of symbol periods for the first spreading sequence with each of the spreading sequences; and

wherein said step of identifying comprises the step of identifying a second spreading sequence for each of the symbol periods.

13. A method according to claim 1, wherein said step of estimating comprises the step of combining the interference-cancelled correlations of the resolved signal components and the first spreading sequence to generate an estimate of the transmitted symbol.

14. A method according to claim 13, wherein said step of combining comprises the step of maximal ratio combining.

15. In a code division multiple access (CDMA) communications system in which a base station and terminals communicate within a cell according to a cell-specific scrambling code and a terminal-specific spreading sequence selected from a set of spreading sequences of a Hadamard code, a method of recovering a symbol transmitted in a communications medium according to a first spreading sequence, the method comprising the steps of:

receiving a communications signal from the communications medium;

descrambling the received communications signal according to the scrambling code to resolve the received communications signal into a plurality of multipath signal components;

fast Hadamard transforming the plurality of multipath signal components to generate respective sets of correlations of the plurality of multipath signal components with the set of spreading sequences, a respective one of the sets of correlations including a respective correlation of a multipath signal component with the first spreading sequence;

canceling an interference component associated with a second spreading sequence of the set of spreading sequences from the correlations of the plurality of multipath signal components with the first spreading sequence based on the correlations of the plurality of multipath signal components with the second spreading sequence to generate a set of interference-cancelled correlations of the resolved multipath signal components with the first spreading sequence; and

estimating the transmitted symbol from the set of interference-cancelled correlations of the multipath signal components with the first spreading sequence.

16. A method according to claim 15, wherein the symbol is transmitted from a base station, and wherein said steps of receiving, fast Hadamard transforming, canceling and estimating are performed at a terminal.

17. A method according to claim 15, wherein said step of canceling comprises the step of identifying a second spreading sequence based on the correlations of the set of spreading sequences with the multipath signal components.

18. A method according to claim 15, wherein said step of canceling comprises the steps of:

combining correlations for multipath signal components to determine respective energies for respective spreading sequences; and

identifying a second spreading sequence having an associated energy meeting a predetermined criterion.

19. A method according to claim 18, wherein said step of combining comprises the step of maximal ratio combining.

20. A method according to claim 18, wherein said step of identifying comprises the step of identifying a second spreading sequence having a highest associated energy.

21. A method according to claim 18, wherein said step of identifying comprises the step of identifying a second spreading sequence having an associated energy exceeding a predetermined threshold.

22. A method according to claim 18, wherein said step of canceling further comprises the steps of:

generating a symbol estimate for an interferer signal transmitted according to the identified second spreading sequence;

generating a power estimate for interferer signal transmitted according to the identified second spreading sequence; and

determining a respective interference component for a respective one of the correlations of the multipath signal components with the first spreading sequence from the second spreading sequence, the symbol estimate for the second spreading sequence and the power estimate for the second spreading sequence.

23. An apparatus for recovering a symbol transmitted in a communications medium in a spread spectrum communications according to a first spreading sequence of a set of a spreading sequences, the apparatus comprising:

means for receiving a communications signal from the communications medium;

means for resolving the received communications signal into a plurality of signal components, a respective one of which is associated with a respective propagation path;

means for correlating the resolved plurality of signal components with the set of spreading sequences to generate a respective set of correlations for a respective one of the resolved signal components, a respective one of the sets of correlations including a respective correlation of a resolved signal component with the first spreading sequence;

means for canceling an interference component associated with a second spreading sequence of the set of spreading sequences from the correlations of the plurality of resolved signal components with the first spreading sequence based on the correlations of the plurality of resolved signal components with the second spreading sequence to generate a set of interference-cancelled correlations of the resolved signal components with the first spreading sequence; and

means for estimating the transmitted symbol from the set of interference-cancelled correlations of the resolved signal components with the first spreading sequence.

24. An apparatus according to claim 23, wherein said means for canceling comprises means, responsive to said means for correlating, for identifying a second spreading sequence associated with an interferer signal based on the correlations of the set of spreading sequences with the resolved signal components.

25. An apparatus according to claim 23, wherein the set of spreading sequences is orthogonal.

26. An apparatus according to claim 25, wherein the set of spreading sequences comprises a Hadamard code, and wherein said means for correlating comprises means for fast Hadamard transforming each of the plurality of resolved signal components.

27. An apparatus according to claim 23, wherein the spread spectrum communications system comprises a plurality of geographic cells, and wherein said means for resolving comprises means for descrambling according to a cell-specific scrambling code to generate the resolved plurality of signal components.

28. An apparatus according to claim 23, wherein said means for canceling comprises:

means, responsive to said means for correlating, for combining correlations of resolved signal components to determine respective energies for respective spreading sequences; and

means, responsive to said means for combining, for identifying a second spreading sequence having an associated energy meeting a predetermined criterion.

29. An apparatus according to claim 28, wherein said means for combining comprises means for maximal ratio combining.

30. An apparatus according to claim 28, wherein said means for identifying comprises means for identifying a second spreading sequence having a highest associated energy.

31. An apparatus according to claim 28, wherein said means for identifying comprises means for identifying a second spreading sequence having an associated energy exceeding a predetermined threshold.

32. An apparatus according to claim 28, wherein said means for canceling further comprises:

means, responsive to said means for identifying and to said means for combining, for generating a symbol estimate for the identified second spreading sequence;

means, responsive to said means for identifying and to said means for combining, for generating a power estimate for the identified second spreading sequence; and

means, responsive to said means for generating a symbol estimate and to said means for generating a power estimate, for determining a respective interference component for a respective one of the correlations of the resolved signal components with the first spreading sequence from the second spreading sequence, the symbol estimate for the second spreading sequence and the power estimate for the second spreading sequence.

33. An apparatus according to claim 32:

wherein said means for resolving comprises;

means for resolving a received communications signal into a first plurality of signal components for a first symbol period; and

means for resolving a received communications signal into a second plurality of signal components for a second symbol period;

wherein said means for identifying comprises means for identifying a second spreading sequence for the first symbol period from the first resolved plurality of signal components; and

wherein said means for canceling comprises;

means for correlating the second resolved plurality of signal components with the first and second spreading sequences to generate respective correlations of the second resolved plurality of signal components with the first and second spreading sequences; and

means for canceling the determined interference component from the correlations of the second resolved plurality of signal components with the first spreading sequence based on the correlations of the resolved second plurality of signal components with the second spreading sequence to generate a plurality of interference canceled correlations of the second resolved plurality of signal components with the first spreading sequence; and

wherein said means for estimating comprises means for generating an estimate of a symbol for the second symbol period from the interference-cancelled correlations of the plurality of resolved signal components for the second symbol period with the first spreading sequence.

34. An apparatus according to claim 28:

wherein said means for correlating comprises means for correlating resolved signal components for each of a succession of symbol periods for the first spreading sequence with each of the spreading sequences; and

wherein said means for identifying comprises means for identifying a second spreading sequence for each of the symbol periods.

35. An apparatus according to claim 23, wherein said means for estimating comprises means for combining the interference-cancelled correlations of the resolved signal components and the first spreading sequence to generate an estimate of the transmitted symbol.

36. An apparatus according to claim 35, wherein said means for combining comprises means for maximal ratio combining.

37. An apparatus for recovering a symbol from a communications signal, the apparatus comprising:

a multipath resolver operative to resolve the communications signal into a plurality of signal components, a respective one of which is associated with a respective propagation path;

a spreading sequence correlator responsive to said multipath resolver and operative to correlate resolved signal components with a set of spreading sequences to generate a respective set of correlations for a respective one of the resolved signal components, a respective one of the sets of correlations including a respective correlation of a resolved signal component with a first spreading sequence;

an interference canceler responsive to said spreading sequence correlator and operative to cancel an interference component associated with an interferer using a second spreading sequence of the set of spreading sequences from the correlations of the plurality of resolved signal components with the first spreading sequence based on the correlations of the plurality of resolved signal components with the second spreading sequence to generate a set of interference-cancelled correlations of the resolved signal components with the first spreading sequence; and

a symbol estimator responsive to said interference canceler and operative to estimate the transmitted symbol from the set of interference-cancelled correlations of the resolved signal components with the first spreading sequence.

38. An apparatus according to claim 37, wherein the set of spreading sequences is orthogonal.

39. An apparatus according to claim 38, wherein the set of spreading sequences comprises a Hadamard code, and wherein said spreading sequence correlator comprises a fast Hadamard transformer.

40. An apparatus according to claim 37, wherein said multipath resolver comprises a descrambler.

41. An apparatus according to claim 37, wherein said interference canceler comprises means for identifying a second spreading sequence based on the correlations of the set of spreading sequences with the resolved signal components.

42. An apparatus according to claim 37, wherein said interference canceler comprises:

a maximal ratio combiner operative to combine correlations for resolved signal components to determine respective energies for respective spreading sequences; and

means, responsive to said maximal ratio combiner, for identifying a second spreading sequence having an associated energy meeting a predetermined criterion.

43. An apparatus according to claim 42, wherein said means for identifying comprises means for identifying a second spreading sequence having a highest associated energy.

44. An apparatus according to claim 42, wherein said means for identifying comprises means for identifying a second spreading sequence having an associated energy exceeding a predetermined threshold.

45. An apparatus according to claim 42, wherein said interference canceler comprises:

a symbol estimator, responsive to said maximal ratio combiner and to said means for identifying and operative to generate a symbol estimate for an interferer signal transmitted according to the identified second spreading sequence;

a power estimator, responsive to said maximal ratio combiner and to said means for identifying and operative to generate a power estimate for the interferer signal transmitted according to the identified second spreading sequence; and

means, responsive to said symbol estimator and to said power estimator, for determining a respective interference component for a respective one of the correlations of the resolved signal components with the first spreading sequence from the second spreading sequence, the symbol estimate for the second spreading sequence and the power estimate for the second spreading sequence.

46. In a code division multiple access (CDMA) communications system in which a base station and terminals communicate within a cell according to a cell-specific scrambling code and a terminal-specific spreading sequence of set of spreading sequences of a Hadamard code, an apparatus for recovering a symbol from a received communications signal, the apparatus comprising:

a descrambler operative to resolve the received communications signal into a plurality of multipath signal components;

a plurality of fast Hadamard transformers responsive to said descrambler and operative to generate respective sets of correlations of the plurality of multipath signal components with the spreading sequences, a respective one of the sets of correlations including a respective correlation of a multipath signal component with the first spreading sequence;

an interference canceler responsive to said plurality of fast Hadamard transformers and operative to cancel an interference component associated with a second spreading sequence of the set of spreading sequences from the correlations of the plurality of resolved signal components with the first spreading sequence based on the correlations of the plurality of resolved signal components with the second spreading sequence to generate a set of interference-cancelled correlations of the resolved multipath signal components with the first spreading sequence; and

a symbol estimator responsive to said interference canceler and operative to estimate the transmitted symbol from the set of interference-cancelled correlations of the resolved signal components with the first spreading sequence.

47. An apparatus according to claim 46, wherein said interference canceler comprises means for identifying a second spreading sequence based on the correlations of the set of spreading sequences with the multipath signal components.

48. An apparatus according to claim 46, wherein interference canceler comprises:

a maximal ratio combiner responsive to said plurality of fast Hadamard transformers and operative to combine the sets of correlations for the multipath signal components to determine respective energies for the spreading sequences; and

means, responsive to said maximal ratio combiner, for identifying a second spreading sequence having an associated energy meeting a predetermined criterion.

49. An apparatus according to claim 47, wherein said means for identifying comprises means for identifying a second spreading sequence having a highest associated energy.

50. An apparatus according to claim 47, wherein said means for identifying comprises means for identifying a second spreading sequence having an associated energy exceeding a predetermined threshold.

51. An apparatus according to claim 47, wherein said interference canceler further comprises:

means, responsive to said maximal ratio combiner and to said means for identifying, for generating a symbol estimate for an interferer signal transmitted according to the identified second spreading sequence;

means, responsive to said maximal ratio combiner and to said means for identifying, for generating a power estimate for the interferer signal transmitted according to the identified second spreading sequence; and

means, responsive to said means for generating a symbol estimate and to said means for generating a power estimate, for determining a respective interference component for a respective one of the correlations of the multipath signal components with the first spreading sequence from the second spreading sequence, the symbol estimate for the second spreading sequence and the power estimate for the second spreading sequence.