Method and apparatus for performing signal correlation at multiple resolutions to mitigate multipath interference

• US 20030072356A1
• Filed: 08/13/2002
• Published: 04/17/2003
• Est. Priority Date: 05/18/2001
• Status: Active Grant

• Current Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Original Assignee: Global Locate Incorporated (Broadcom, Inc.)
• Inventors: Abraham, Charles, Fuchs, Donald L.
• Granted Patent: US 7,006,556 B2
• Time in Patent Office: Days
• US Class Current: 375/148
• CPC Class Codes: G01S 1/045 Receivers
• CPC Class Codes: G01S 19/22 Multipath-related issues
• CPC Class Codes: G01S 19/235 Calibration of receiver components
• CPC Class Codes: G01S 19/254 relating to Doppler shift of satellite signals
• CPC Class Codes: G01S 19/256 relating to timing, e.g. time of week, code phase, timing offset
• CPC Class Codes: G01S 19/30 code related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/37 Hardware or software details of the signal processing chain
• CPC Class Codes: G06F 17/15 Correlation function computation including computation of convolution operations arithmetic circuits for sum of products per se, e.g. multiply-accumulators G06F7/5443; digital filters, e.g. FIR, IIR, adaptive filters H03H17/00
• CPC Class Codes: H04B 1/70752 Partial correlation
• CPC Class Codes: H04B 1/70757 with increased resolution, i.e. higher than half a chip H04B1/70758 takes precedence
• CPC Class Codes: H04B 1/7077 Multi-step acquisition, e.g. multi-dwell, coarse-fine or validation
• CPC Class Codes: H04B 1/709 Correlator structure
• CPC Class Codes: H04B 1/7095 Sliding correlator type
• CPC Class Codes: H04B 2201/70715 with application-specific features

Abstract

A method and apparatus for mitigating multipath interference in a digital signal correlator forms values representative of magnitude or power of a correlation response between a digital signal and a pseudorandom reference code. The earliest peak in the correlation response is then selected as the correct peak to compute a time delay.

Claims

1. A method of mitigating multipath interference in a digital signal correlator comprising:

forming values representative of magnitude or power of a correlation response between a digital signal and a pseudorandom reference code;

selecting an earliest peak in the correlation response to compute a time delay.

2. The method of claim 1, wherein the selecting step comprises:

determining a plurality of maxima for the correlation response; and

selecting the earliest maximum in the plurality of maxima as the earliest peak.

3. The method of claim 2, wherein the step of determining a plurality of maxima comprises:

performing differential analysis on the correlation response.

4. The method of claim 1, wherein the selecting step comprises:

computing a slope of the correlation response on an ascending edge thereof;

providing a mean noise level of the correlation response;

computing the intersection of the slope and the mean noise level; and

using the intersection to determine the earliest peak.

5. The method of claim 1, further comprising:

determining whether the correlation response is affected by one or more multipath signals.

6. The method of claim 5, wherein the step of determining whether the correlation response is affected by one or more multipath signals comprises:

computing a first slope of the correlation response on an ascending edge thereof;

computing a second slope of the correlation response on a descending edge thereof;

comparing the first slope with the second slope.

7. The method of claim 5, wherein the step of determining whether the correlation response is affected by one or more multipath signals comprises:

computing a first intersection of an ascending edge of the correlation response with a mean noise level of the correlation response;

computing a second intersection of a descending edge of the correlation response with the mean noise level of the correlation response;

computing the width between the first intersection and the second intersection.

8. The method of claim 1, wherein the forming step comprises:

sampling the digital signal to produce a set of digital signal samples having a selected sample spacing;

correlating the set of digital signal samples with the pseudorandom reference code to produce the correlation response.

9. The method of claim 8, wherein the correlating step comprises:

(a) dividing the pseudorandom reference code into a plurality of code segments;

(b) selecting a code segment;

(c) forming an inner product between the selected code segment and the set of digital signal samples to produce a partial correlation;

(d) repeating steps (b) and (c) to produce a plurality of partial correlations;

(e) summing the plurality of partial correlations to produce the correlation response.

10. The method of claim 9, wherein the pseudorandom reference code is a C/A code of the global positioning system (GPS).

11. The method of claim 10, wherein the pseudorandom reference code is generated by extending a code segment by replicating a C/A code bit to an integer number of samples.

12. The method of claim 11, further comprising:

generating the code segments from a lookup table by selecting one of a fixed number of non-overlapping segments of the C/A code.

13. The method of claim 9, wherein the inner product is formed by multiplying each of the digital signal samples with a respective chip of the selected code segment and summing each multiplication result with other multiplication results to produce the partial correlation.

14. The method of claim 9, wherein the sample spacing is at lease 1/2 of a C/A code chip.

15. A receiver of global positioning system (GPS) signals comprising:

an RF/IF converter for filtering and frequency translating a received GPS signal to form an IF signal;

an analog-to-digital converter for digitizing the IF signal;

a tuner for removing Doppler shift form the digitized signal and producing an in-phase (I) and a quadrature (Q) signal;

a decimation circuit for subsampling the I and Q signals;

a correlation processor for correlating the subsampled I and Q signals with a C/A reference code to produce values representative of magnitude or power of a correlation response; and

a processor for selecting an earliest peak in the correlation response to compute a time delay.

16. The receiver of claim 15, wherein the processor is further configured to:

determine a plurality of maxima for the correlation response; and

select the earliest maximum in the plurality of maxima as the earliest peak.

17. The receiver of claim 16, wherein the processor is further configured to:

perform differential analysis on the correlation response.

18. The receiver of claim 15, wherein the processor is further configured to:

compute a slope of the correlation response on an ascending edge thereof;

provide a mean noise level of the correlation response;

compute the intersection of the slope and the mean noise level; and

use the intersection to determine the earliest peak.

19. The receiver of claim 15, wherein the processor is further configured to:

determine whether the correlation response is affected by one or more multipath signals.

20. The receiver of claim 19, wherein the processor is further configured to:

compute a first slope of the correlation response on an ascending edge thereof;

compute a second slope of the correlation response on a descending edge thereof;

compare the first slope with the second slope.

21. The receiver of claim 19, wherein the processor is further configured to:

compute a first intersection of an ascending edge of the correlation response with a mean noise level of the correlation response;

compute a second intersection of a descending edge of the correlation response with the mean noise level of the correlation response;

compute the width between the first intersection and the second intersection.

22. An apparatus for mitigating multipath interference in a digital signal correlator comprising:

means for forming values representative of magnitude or power of a correlation response between a digital signal and a pseudorandom reference code;

means for selecting an earliest peak in the correlation response to compute a time delay.

23. The apparatus of claim 22, wherein the means for selecting comprises:

means for determining a plurality of maxima for the correlation response; and

means for selecting the earliest maximum in the plurality of maxima as the earliest peak.

24. The apparatus of claim 23, wherein the means for determining a plurality of maxima comprises:

means for performing differential analysis on the correlation response.

25. The apparatus of claim 22, wherein the means for selecting comprises:

means for computing a slope of the correlation response on an ascending edge thereof;

means for providing a mean noise level of the correlation response;

means for computing the intersection of the slope and the mean noise level; and

means for using the intersection to determine the earliest peak.

26. The apparatus of claim 22, further comprising:

means for determining whether the correlation response is affected by one or more multipath signals.

27. The apparatus of claim 26, wherein the means for determining whether the correlation response is affected by one or more multipath signals comprises:

means for computing a first slope of the correlation response on an ascending edge thereof;

means for computing a second slope of the correlation response on a descending edge thereof;

means for comparing the first slope with the second slope.

28. The apparatus of claim 26, wherein the means for determining whether the correlation response is affected by one or more multipath signals comprises:

means for computing a first intersection of an ascending edge of the correlation response with a mean noise level of the correlation response;

means for computing a second intersection of a descending edge of the correlation response with the mean noise level of the correlation response;

means for computing the width between the first intersection and the second intersection.