CDMA multiple access interference cancellation using signal estimation

• US 6,404,760 B1
• Filed: 07/19/1999
• Issued: 06/11/2002
• Est. Priority Date: 07/19/1999
• Status: Expired due to Term

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Warrier, Dilip G., Holtzman, Jack
• Primary Examiner(s): Chin, Wellington
• Assistant Examiner(s): Phan, M.
• Application Number:
  US09/356,610
• Time in Patent Office: 1,058 Days
• Field of Search: 370/342, 370/346, 370/335, 370/561, 370/209, 370/320, 370/441, 370/286, 370/289, 370/406, 370/408, 370/407, 375/200, 375/205, 375/261, 375/227, 375/298, 375/346
• US Class Current: 370/342
• CPC Class Codes: H04B 1/7107 Subtractive interference cancellation
• CPC Class Codes: H04B 1/71072 Successive interference cancellation
• CPC Class Codes: H04B 2001/71077 Partial interference cancellation
• CPC Class Codes: H04B 2201/70701 featuring pilot assisted reception

Abstract

Several methods and corresponding apparatus to reduce multiple access interference in code division multiple access communications systems through successive interference cancellation techniques. An estimate of reliability of cancellation of a strongest interfering signal is formed from analysis of a pilot signal associated with the strongest signal. The estimate is used to derive a weight that is multiplied by a replica of the strongest signal to provide a weighted replica. The weighted replica is subtracted from a delayed version of the received channel. As a result, interference cancellation is robustly implemented when the strongest signal replica is most likely to be accurate and is not robustly implemented when the strongest signal replica is not likely to be accurate. This avoids combining a replica of the strongest signal with the received channel when the replica is more likely to contribute additional interference rather than reduce it.

Claims

1. In a communication system having a base station and a plurality of user stations that exchange communication signals between the base station and each of the plurality of users, a method for reducing multiple access interference between transmitted communication signals, the method comprising:

receiving a plurality of code division multiple access signals;

finding a first signal of the plurality of code division multiple access signals that has a greatest amplitude;

receiving a first pilot signal associated with the first signal;

determining parameters from the first pilot signal;

decoding first data from the first signal;

constructing a first replica of the first signal from the first data;

combining the first replica with parameters derived from the first pilot signal to provide a first subtractive signal; and

subtracting the first subtractive signal from the plurality of code division multiple access signals to provide a first reduced plurality of code division multiple access signals.

2. The method of claim 1, further comprising:

finding a subsequent signal of the reduced plural of code division multiple access signals that has a greatest amplitude;

receiving a subsequent pilot signal associated with the subsequent signal;

determining parameters from the subsequent pilot signal;

decoding the subsequent signal to provide subsequent data;

constructing a subsequent replica of the subsequent signal from the second data;

combining the subsequent replica with parameters derived from the subsequent pilot signal to provide a subsequent subtraction signal; and

subtracting the subsequent subtraction signal from the first reduced plurality of code division multiple access signals to provide a subsequent reduced plurality of code division multiple access signals.

3. The method of claim 1, further comprising:

performing RAKE analysis of at least one of the first pilot signal and the first signal to determine multipath contributions to the first signal; and

reconstructing a first replica of the first signal including multipath contributions to the first signal.

4. The method of claim 1 wherein combining the first replica with parameters derived from the first pilot signal to provide a first subtractive signal comprises:

determining an amplitude of the first pilot signal;

normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

deriving a weight from the normalized first amplitude; and

multiplying the first replica by the weight.

5. The method of claim 1 wherein combining the first replica with parameters derived from the first pilot signal to provide a first subtractive signal comprises:

determining an amplitude of the first pilot signal;

normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

using the normalized first amplitude as an argument of a hyperbolic tangent function to provide a weight; and

multiplying the first replica by the weight.

6. The method of claim 1 wherein combining the first replica with parameters derived from the first pilot signal to provide a first subtractive signal comprises:

determining an amplitude of the first pilot signal;

normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

using the normalized first amplitude as an argument x of a function F(x)=(1-cos(x))/2, 0<

x<

π

, F(x)=1, x≧

π

to providing a weight F(x); and

multiplying the first replica by the weight F(x).

7. The method of claim 1 wherein combining the first replica with parameters derived from the first pilot signal to provide a first subtractive signal comprises:

determining an amplitude of the first pilot signal;

normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

assigning a weight of less than one-half when the normalized first amplitude has a value of less than a first threshold and assigning a weight of greater than one-half when the normalized first amplitude has a value greater than a second threshold, the second threshold being greater than the first threshold; and

multiplying the first replica by the weight.

8. The method of claim 1 wherein determining parameters from the first pilot signal also includes:

decoding the first pilot signal to provide first pilot signal data;

comparing the first pilot signal data to stored pilot data to determine a number of errors in the first pilot signal data; and

comparing the number of errors to a lookup table to derive a weight.

9. The method of claim 1 wherein determining parameters from the first pilot signal also includes:

decoding the first pilot signal to provide first pilot signal data;

comparing the first pilot signal data to stored pilot data to determine a number of errors in the first pilot signal data; and

assigning a weight of between zero and one when the number of errors exceeds a first threshold number of errors but does not exceed a second threshold number of errors.

10. In a communication system having a base station and a plurality of user stations that exchange communication signals between the base station and each of the plurality of users, an apparatus for reducing multiple access interference between received communication signals, the apparatus comprising:

first means for selecting a first signal from a plurality of received code division multiple access signals;

first means, coupled to the first selecting means, for estimating a confidence level of a correctness of the first signal;

first means, coupled to the first estimating means, for constructing a first replica of the first signal;

first means, coupled to the first constructing means, for determining a weight for the first signal based the estimated confidence level to provide a first subtractive signal; and

first means, coupled to the first determining means, for subtracting the first subtractive signal from the plurality of received code division multiple access signals.

11. The apparatus of claim 10 wherein the first subtracting means includes first means for subtracting the first subtractive signal from the plurality of received code division multiple access signals to provide a first reduced plurality of code division multiple access signals, the apparatus further comprising:

second means, coupled to the first subtracting means, for selecting a subsequent signal from the reduced plurality of code division multiple access signals;

second means, coupled to the second selecting means, for estimating a confidence level of a correctness of the subsequent signal;

second means, coupled to the second estimating means, for constructing a subsequent replica of the subsequent signal;

second means, coupled to the second constructing means, for determining a weight for the subsequent signal based on the estimated confidence level to provide a subsequent subtraction signal; and

second means, coupled to the second determining means, for subtracting the subsequent subtraction signal from the first reduced plurality of received code division multiple access signals.

12. The apparatus of claim 10, further comprising:

second means, coupled to the first selecting means, for selecting a received first pilot signal associated with the first signal;

means, coupled to the second selecting means, for performing RAKE analysis of at least one of the first pilot signal and the first signal to determine multipath contributions to the first signal;

second means, coupled to the RAKE analysis means, for constructing a first replica of the first signal including multipath contributions to the first signal; and

means, coupled to the second constructing means, for forming the first subtractive signal using the first replica.

13. The apparatus of claim 10 wherein the first weight determining means comprises:

second means, coupled to the first selecting means, for selecting a first pilot signal associated with the first signal;

second means, coupled to the second selecting means, for estimating an amplitude of the first pilot signal;

means, coupled to the second estimating means, for normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

means, coupled to the normalizing means, for deriving a weight from the normalized first amplitude;

means, coupled to the first selecting means, for decoding first data from the first signal;

means, coupled to the decoding means, for constructing a first replica of the first signal from the first data; and

means, coupled to the constructing means, for multiplying the first replica by the weight to provide the first subtractive signal.

14. The apparatus of claim 10 wherein the means for deriving a weight comprises:

second means, coupled to the first selecting means, for selecting a first pilot signal associated with the first signal;

second means, coupled to the second selecting means, for estimating an amplitude of the first pilot signal;

means, coupled to the second estimating means, for normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

means, coupled to the normalizing means, for using the normalized first amplitude as an argument of a hyperbolic tangent function to provide a weight;

means, coupled to the first selecting means, for decoding first data from the first signal;

means, coupled to the decoding means, for constructing a first replica of the first signal from the first data; and

means, coupled to the constructing means, for multiplying the first replica by the weight to provide the first subtractive signal.

15. The apparatus of claim 10 wherein the means for deriving a weight comprises:

second means, coupled to the first selecting means, for selecting a first pilot signal associated with the first signal;

second means, coupled to the second selecting means, for estimating an amplitude of the first pilot signal;

means, coupled to the second estimating means, for normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

means, coupled to the normalizing means, for using the normalized first amplitude as an argument x of a function F(x)=(1-cos(x))2, 0<

x<

π

, F(x)=1, x≧

π

to providing a weight F(x);

means, coupled to the first selecting means, for decoding first data from the first signal;

means, coupled to the decoding means, for constructing a first replica of the first signal from the first data; and

means, coupled to the constructing means, for multiplying the first replica by the weight F(x) to provide the first subtractive signal.

16. The apparatus of claim 10 wherein the means for deriving a weight comprise:

second means, coupled to the first selecting means, for selecting a first pilot signal associated with the first signal;

second means, coupled to the second selecting means, for estimating an amplitude of the first pilot signal;

means, coupled to the second estimating means, for normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

means, coupled to the normalizing means, for assigning a weight of less than one-half when the normalized first amplitude has a value of less than a first threshold and assigning a weight of greater than one-half when the normalized first amplitude has a value greater than a second threshold, the second threshold being greater than the first threshold;

means, coupled to the first selecting means, for decoding first data from the first signal;

means, coupled to the decoding means, for constructing a first replica of the first signal from the first data; and

means, coupled to the constructing means, for multiplying the first replica by the weight to provide the first subtractive signal.

17. The apparatus of claim 10 wherein the first selecting means comprises first means for selecting a first signal from a plurality of received code division multiple access signals by a base station.

18. The apparatus of claim 10 wherein the means for estimating a confidence level of a correctness of the first signal also includes:

second means, coupled to the first selecting means, for selecting a first pilot signal associated with the first signal;

means, coupled to the second selecting means, for decoding the first pilot signal to provide first pilot signal data;

first means, coupled to the decoding means, for comparing the first pilot signal data to stored pilot data to determine a number of errors in the first pilot signal data; and

second means, coupled to the first comparing means, for comparing the number of errors to a lookup table to derive a weight.

19. A computer-readable medium having instructions stored thereon to cause computers in a communication system to perform a method, wherein the system includes at least a base station exchanging signals with a plurality of user stations, the method comprising:

selecting a first signal from a plurality of received code division multiple access signals;

determining a confidence level of a correctness of the first signal;

determining a weight for the first signal based the determined confidence level to provide a first subtractive signal; and

subtracting the first subtractive signal from the plurality of received code division multiple access signals.

20. The article of manufacture of claim 19 wherein subtracting the first subtractive signal from the plurality of received code division multiple access signals includes subtracting the first subtractive signal from the plurality of received code division multiple access signals to provide a first reduced plurality of code division multiple access signals, the method further comprising:

selecting a subsequent signal from the reduced plurality of code division multiple access signals;

determining a confidence level of a correctness of the subsequent signal;

determining a weight for the subsequent signal based on the determined confidence level to provide a subsequent subtraction signal; and

subtracting the subsequent subtraction signal from the first reduced plurality of received code division multiple access signals.

21. The article of manufacture of claim 19, further comprising:

selecting a received first pilot signal associated with the first signal;

performing RAKE analysis of at least one of the first pilot signal and the first signal to determine multipath contributions to the first signal;

reconstructing a first replica of the first signal including multipath contributions to the first signal; and

forming the first subtractive signal using the first replica.

22. The article of manufacture of claim 19 wherein determining a weight comprises:

selecting a first pilot signal associated with the first signal;

determining an amplitude of the first pilot signal;

normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

deriving a weight from the normalized first amplitude; and

multiplying the first replica by the weight to provide the first subtractive signal.

23. The article of manufacture of claim 19 wherein determining a weight comprises:

selecting a first pilot signal associated with the first signal;

determining an amplitude of the first pilot signal;

normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

using the normalized first amplitude as an argument of a hyperbolic tangent function to provide a weight;

reconstructing a first replica of the first signal from the first data; and

multiplying the first replica by the weight to provide the first subtractive signal.

24. The article of manufacture of claim 19 wherein determining a weight comprises:

selecting a first pilot signal associated with the first signal;

determining an amplitude of the first pilot signal;

normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

using the normalized first amplitude as an argument x of a function F(x)=(1-cos(x))/2,0<

x<

π

, F(x)=1, x≧

π

to providing a weight F(x);

reconstructing a first replica of the first signal; and

multiplying the first replica by the weight F(x) to provide the first subtractive signal.

25. The article of manufacture of claim 19 wherein determining a weight comprises:

selecting a first pilot signal associated with the first signal;

determining an amplitude of the first pilot signal;

normalizing the amplitude of the first pilot signal to provide a normalized first amplitude;

assigning a weight of less than one-half when the normalized first amplitude has a value of less than a first threshold and assigning a weight of greater than one-half when the normalized first amplitude has a value greater than a second threshold, the second threshold being greater than the first threshold;

reconstructing a first replica of the first signal; and

multiplying the first replica by the weight to provide the first subtractive signal.

26. The article of manufacture of claim 19 wherein selecting a first signal from a plurality of received code division multiple access signals comprises selecting a first signal from a plurality of received code division multiple access signals by a base station.

27. The article of manufacture of claim 19 wherein determining a confidence level includes:

comparing a first pilot signal to stored pilot data to determine a number of errors in the first pilot signal data; and

determining a weight based on the number of errors.

28. A communication system having a base station and a plurality of user stations that exchange communication signals between the base station and each of the plurality of users, including an apparatus comprising:

a receiver capable of receiving a plurality of code division multiple access signals and an associated plurality of pilot signals, ranking the signals in order of decreasing amplitude and identifying a first signal having a greatest amplitude;

an estimator that determines parameters from a first pilot signal associated with the first signal coupled to the receiver;

a first decoder that decodes first data from the first signal coupled to the receiver;

a first spreader that provides a first replica of the first signal from the first data, the first spreader coupled to the first decoder;

a first combiner that combines the first replica with parameters derived from the first pilot signal to provide a first subtractive signal, the first combiner coupled to the first spreader and the estimator; and

a first subtracter that subtracts the first subtractive signal from the plurality of code division multiple access signals to provide a first reduced plurality of code division multiple access signals, the first subtracter coupled to the first combiner and to the receiver.

29. The communication system of claim 28, wherein the receiver identifies a subsequent signal having a greatest amplitude from the first reduced plurality of code division multiple access signals, the apparatus further comprising:

a second estimator that determines parameters of a subsequent pilot signal associated with the second signal;

a second decoder, coupled to the receiver, that decodes the subsequent signal to provide subsequent data;

a second spreader, coupled to the second decoder, that provides a subsequent replica of the subsequent signal from the subsequent data;

a second combiner, coupled to the second spreader and to the second estimator, that combines the subsequent replica with parameters derived from the subsequent pilot signal to provide a subsequent subtraction signal; and

a second subtracter that subtracts the subsequent subtraction signal from the first reduced plurality of code division multiple access signals to provide a subsequent reduced plurality of code division multiple access signals.

30. The communication system of claim 28 further comprising:

a RAKE analyzer coupled to the receiver that provides a multipath analysis of received signals; and

a multipath signal reconstructor, coupled to the RAKE analyzer, that reconstructs a first replica of the first signal including multipath contributions to the first signal.

31. The communication system of claim 28 wherein the first combiner comprises:

an amplitude estimator, coupled to the receiver, that determines an amplitude of the first pilot signal;

a normalizer, coupled to the amplitude estimator, that normalizes the amplitude of the first pilot signal to provide a normalized first amplitude;

a weighting circuit, coupled to the normalizer, that uses the normalized first amplitude as an argument x of a function F(x)=(1-cos(x))/2,0<

x<

π

, F(x)=1, x≧

π

to providing a weight F(x); and

a multiplier, coupled to the weighting circuit, that multiplies the first replica by the weight F(x).

32. The communication system of claim 28 wherein the first combiner comprises:

an amplitude estimator, coupled to the receiver, that determines an amplitude of the first pilot signal;

a normalizer, coupled to the amplitude estimator, that normalizes the amplitude of the first pilot signal to provide a normalized first amplitude;

a weighting circuit, coupled to the normalizer, that assigns a weight of less than one-half when the normalized first amplitude has a value of less than a first threshold and that assigns a weight of greater than one-half when the normalized first amplitude has a value greater than a second threshold, the second threshold being greater than the first threshold; and

a multiplier, coupled to the weighting circuit, that multiplies the first replica by the weight.

33. The communication system of claim 28 wherein the receiver comprises a base station.

34. The communication system of claim 28 wherein the first estimator includes:

a decoder coupled to the receiver that decodes the first pilot signal to provide first pilot signal data; and

logic circuitry coupled to the decoder that compares the first pilot signal data to stored pilot data to determine a number of errors in the first pilot signal data and to derive a weight.

35. The communication system of claim 28 the first estimator includes:

a decoder that decodes the first pilot signal to provide first pilot signal data, the decoder coupled to the receiver;

logic circuitry, coupled to the decoder, that compares the first pilot signal data to stored pilot data to determine a number of errors in the first pilot signal data; and

a weighting circuit, coupled to the logic circuitry, that assigns a weight of zero when the number of errors in the first pilot signal data exceeds a threshold number of errors.

36. An apparatus comprising:

a receiver capable of receiving a plurality of code division multiple access signals and selecting a first signal;

a first estimator, coupled to the receiver, that determines a confidence level of correctness associated with the first signal;

a first weighting circuit, coupled to the first estimator and to the receiver, that determines a weight for the first signal based on the determined confidence level to provide a first subtractive signal; and

a first subtracter, coupled to the first weighting circuit and to the receiver, that subtracts the first subtractive signal from the plurality of received code division multiple access signals.

37. The apparatus of claim 36, wherein the first subtracter provides a first reduced plurality of received code division multiple access signals and the receiver identifies a subsequent signal from the first reduced plurality of code division multiple access signals, the radio further comprising:

a second estimator, coupled to the receiver, that determines a confidence level of correctness associated with the subsequent signal;

a second weighting circuit that determines a weight for the subsequent signal based on the determined confidence level of the subsequent signal to provide a subsequent subtractive signal; and

a second subtracter, coupled to the second weighting circuit and to the receiver, that subtracts the subsequent subtractive signal from the first reduced plurality of received code division multiple access signals.

38. The apparatus of claim 36 further comprising:

a RAKE analyzer coupled to the receiver that provides a multipath analysis of received signals; and

a multipath signal reconstructor, coupled to the RAKE analyzer, that reconstructs a first replica of the first signal including multipath contributions to the first signal to provide the first subtractive signal.

39. The apparatus of claim 36 wherein the first weighting circuit comprises:

an amplitude estimator, coupled to the receiver, that determines an amplitude of a first pilot signal associated with the first signal;

a normalizer, coupled to the amplitude estimator, that normalizes the amplitude of the first pilot signal to provide a normalized first amplitude;

a weighting selection circuit, coupled to the normalizer, that uses the normalized first amplitude as an argument x of a function F(x)=(1-cos(x))/2,0<

x<

π

, F(x)=1, x≧

π

to providing a weight F(x); and

a multiplier, coupled to the weighting selection circuit, that multiplies the first replica by the weight F(x) to provide the first subtractive signal.

40. The apparatus of claim 36 wherein the first weighting circuit comprises:

an amplitude estimator, coupled to the receiver, that determines an amplitude of the first pilot signal;

a normalizer, coupled to the amplitude estimator, that normalizes the amplitude of the first pilot signal to provide a normalized first amplitude;

a weighting selection circuit, coupled to the normalizer, that assigns a weight of less than one-half when the normalized first amplitude has a value of less than a first threshold and that assigns a weight of greater than one-half when the normalized first amplitude has a value greater than a second threshold, the second threshold being greater than the first threshold; and

a multiplier, coupled to the weighting circuit, that multiplies the first replica by the weight to provide the first subtractive signal.

41. The apparatus of claim 36 wherein the first estimator comprises:

a first decoder that decodes a first pilot signal associated with the first signal;

a second decoder that decodes first data from the first signal; and

a spreader that re-spreads the first data to provide a replica of the first signal;

wherein the first estimator determines parameters from the first pilot signal to provide the confidence level of correctness.