Apparatus and methods for implementing a split equalizer filter for sparse channels

• US 7,801,210 B2
• Filed: 01/24/2006
• Issued: 09/21/2010
• Est. Priority Date: 01/24/2006
• Status: Expired due to Fees

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Corporation
• Inventors: Zhang, Jianzhong, Heikkila, Markku J.
• Primary Examiner(s): Wang; Ted M
• Application Number:
  US11/339,093
• Publication Number:
  US 20070171960A1
• Time in Patent Office: 1,701 Days
• Field of Search: 375/229, 375/231, 375/232, 375/233, 375/147
• US Class Current: 375/231
• CPC Class Codes: H04B 1/7101 with estimation filters
• CPC Class Codes: H04B 1/7113 Determination of path profile
• CPC Class Codes: H04L 25/0216 with estimation of channel length
• CPC Class Codes: H04L 25/03038 with a non-recursive structure H04L25/03031 takes precedence
• CPC Class Codes: H04L 25/03292 with channel estimation circuitry

Abstract

The invention concerns methods and apparatus for performing channel equalization in a direct-sequence spread spectrum communications system. The methods and apparatus of the present invention are particularly applicable in situations where communication occurs over a channel experiencing multipath interference associated with paths having significantly different lengths. The impulse response-delay profile of such a channel is typically sparse, that is, dominated by a relatively few and widely spaced taps or clusters of closely spaced taps. In one aspect, equalization filter coefficients are calculated based on a channel estimate derived from a single cluster of closely-spaced channel taps collectively having the greatest percentage of energy as indicated by the impulse response-delay profile. In another aspect, equalization filter coefficients are calculated for each of two clusters of closely-spaced channel taps, where the two clusters of closely-spaced channel taps are separated by a delay spread indicative of a sparse channel. In a further aspect where channel equalization coefficients are calculated for each of two clusters of closely-spaced channel taps, the equalization coefficients are jointly optimized during the calculation process.

Claims

1. An apparatus, comprising at least one processor;

and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following;

estimating an impulse response-delay profile of a channel using a known signal, the channel being a direct sequence spread spectrum communications channel;

determining if the impulse response-delay profile is indicative of a sparse channel;

if the channel impulse response-delay profile is indicative of a sparse channel, selecting a first channel impulse response cluster using a pre-determined selection criterion, wherein the first channel impulse response cluster is separated from other channel impulse response clusters by a delay difference indicative of the sparse nature of the channel;

aligning a first filter window with the first channel impulse response cluster;

calculating channel equalization coefficients for the first filter window; and

using the channel equalization coefficients to perform channel equalization.

2. The apparatus of claim 1, wherein the pre-determined criterion comprises selecting a channel impulse response cluster as the first channel impulse response cluster in dependence on which channel impulse response cluster has a greatest percentage of energy associated with the desired signal as indicated by the channel impulse response-delay profile.

3. The apparatus of claim 1, where the channel equalization coefficients are calculated using a linear minimum mean square error criterion.

4. The apparatus of claim 1, wherein the first filter window is wider than the first channel impulse response cluster in terms of the delay difference reflected in the first channel impulse response cluster.

5. The apparatus of claim 1, the computer program code configured to, with the at least one processor and the memory, further cause the apparatus to perform at least the following:

prior to aligning the first filter window with the first channel impulse response cluster, calculating a width of the first filter window based on relative delays associated with impulse response components comprising the first channel impulse response cluster.

6. The apparatus of claim 1, wherein when the first filter window is aligned with the first channel impulse response cluster, the first filter window is positioned so that the first channel impulse response cluster is displaced towards the rightmost portion of the first filter window in order to reduce interpath interference effects between channel impulse response components comprising the first channel impulse response cluster when the channel equalization coefficients associated with the first filter window are calculated.

7. The apparatus of claim 1, the computer program code configured to, with the at least one processor and the memory, further cause the apparatus to perform at least the following:

selecting a second channel impulse response cluster using the pre-determined selection criterion, wherein the second channel impulse response cluster is separated from the first channel impulse response cluster by a delay difference indicative of the sparse nature of the channel;

aligning a second filter window with the second channel impulse response cluster;

calculating channel equalization coefficients for the second filter window; and

using the channel equalization coefficients associated the second filter window to perform channel equalization.

8. The apparatus of claim 7, wherein the pre-determined criterion comprises selecting a channel impulse response cluster as the second channel impulse response cluster in dependence on which channel impulse response cluster has a second-greatest percentage of energy associated with the known signal as indicated by the channel impulse response-delay profile.

9. The apparatus of claim 7, the computer program code configured to, with the at least one processor and the memory, further cause the apparatus to perform at least the following:

while calculating channel equalization coefficients for the first and second filter windows, jointly optimizing the channel equalization coefficients associated with the first and second filter windows.

10. The apparatus of claim 7, the computer program code configured to, with the at least one processor and the memory, further cause the apparatus to perform at least the following:

positioning a third filter window with respect to the first and second filter windows, wherein only one of the first or second filter windows is leftmost and whereby the third filter window is positioned to the left of the leftmost one of the first or second filter windows;

calculating channel equalization coefficients for the third filter window; and

using the channel equalization coefficients associated with the third filter window to perform channel equalization operations on signals received by a mobile terminal.

11. The apparatus of claim 7, where the channel equalization coefficients are used for performing chip level equalization, and wherein using the channel equalization coefficients associated with the first filter window result in a first chip estimate and using the channel equalization coefficients associated with the second filter window result in a second chip estimate, the operations further comprising:

combining the chip estimates using a maximum ratio combining method.

12. The apparatus of claim 1, where the apparatus is embodied in a mobile terminal.

13. The apparatus of claim 1, where the apparatus is embodied in a signal processing component.

14. A method comprising:

estimating an impulse response-delay profile of a channel using a known signal, the channel being a direct sequence spread spectrum communications channel;

determining if the impulse response-delay profile is indicative of a sparse channel;

if the channel impulse response-delay profile is indicative of a sparse channel, selecting a first channel impulse response cluster using a pre-determined selection criterion, wherein the first channel impulse response cluster is separated from other channel impulse response clusters by a delay difference indicative of the sparse nature of the channel;

aligning a first filter window with the first channel impulse response cluster;

calculating channel equalization coefficients for the first filter window; and

using the channel equalization coefficients to perform channel equalization.

15. The method of claim 14, wherein the pre-determined criterion comprises selecting a channel impulse response cluster as the first channel impulse response cluster in dependence on which channel impulse response cluster has a greatest percentage of energy associated with the desired signal as indicated by the channel impulse response-delay profile.

16. The method of claim 14, where the channel equalization coefficients are calculated using a linear minimum mean square error criterion.

17. The method of claim 14, wherein the first filter window is wider than the first channel impulse response cluster in terms of the delay difference reflected in the first channel impulse response cluster.

18. The method of claim 14, further comprising:

prior to aligning the first filter window with the first channel impulse response cluster, calculating a width of the first filter window based on relative delays associated with impulse response components comprising the first channel impulse response cluster.

19. The method of claim 14, wherein when the first filter window is aligned with the first channel impulse response cluster, the first filter window is positioned so that the first channel impulse response cluster is displaced towards the rightmost portion of the first filter window in order to reduce interpath interference effects between channel impulse response components comprising the first channel impulse response cluster when the channel equalization coefficients associated with the first filter window are calculated.

20. The method of claim 14, further comprising:

selecting a second channel impulse response cluster using the pre-determined selection criterion, wherein the second channel impulse response cluster is separated from the first channel impulse response cluster by a delay difference indicative of the sparse nature of the channel;

aligning a second filter window with the second channel impulse response cluster;

calculating channel equalization coefficients for the second filter window; and

using the channel equalization coefficients associated the second filter window to perform channel equalization.

21. A computer readable memory tangibly encoded with a computer program executable by a processor to perform actions comprising:

estimating an impulse response-delay profile of a channel using a known signal, the channel being a direct sequence spread spectrum communications channel;

determining if the impulse response-delay profile is indicative of a sparse channel;

if the channel impulse response-delay profile is indicative of a sparse channel, selecting a first channel impulse response cluster using a pre-determined selection criterion, wherein the first channel impulse response cluster is separated from other channel impulse response clusters by a delay difference indicative of the sparse nature of the channel;

aligning a first filter window with the first channel impulse response cluster;

calculating channel equalization coefficients for the first filter window; and

using the channel equalization coefficients to perform channel equalization.

22. The computer readable memory of claim 21, wherein the pre-determined criterion comprises selecting a channel impulse response cluster as the first channel impulse response cluster in dependence on which channel impulse response cluster has a greatest percentage of energy associated with the desired signal as indicated by the channel impulse response-delay profile.

23. The computer readable memory of claim 21, wherein when the first filter window is aligned with the first channel impulse response cluster, the first filter window is positioned so that the first channel impulse response cluster is displaced towards the rightmost portion of the first filter window in order to reduce interpath interference effects between channel impulse response components comprising the first channel impulse response cluster when the channel equalization coefficients associated with the first filter window are calculated.

24. The computer readable memory of claim 21, where the actions further comprise:

selecting a second channel impulse response cluster using the pre-determined selection criterion, wherein the second channel impulse response cluster is separated from the first channel impulse response cluster by a delay difference indicative of the sparse nature of the channel;

aligning a second filter window with the second channel impulse response cluster;

calculating channel equalization coefficients for the second filter window; and

using the channel equalization coefficients associated the second filter window to perform channel equalization.