Multipath and tracking error reduction method for spread-spectrum receivers

• US 20020101912A1
• Filed: 12/01/2000
• Published: 08/01/2002
• Est. Priority Date: 12/01/1999
• Status: Active Grant

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Inventors: Enge, Per, Phelts, Robert Eric
• Granted Patent: US 6,868,110 B2
• Time in Patent Office: Days
• US Class Current: 375/148
• CPC Class Codes: G01S 19/22 Multipath-related issues
• CPC Class Codes: G01S 19/30 code related G01S19/246 takes precedence
• CPC Class Codes: H04B 1/7085 using a code tracking loop, e.g. a delay-locked loop
• CPC Class Codes: H04B 1/711 the interference being multi-path interference
• CPC Class Codes: H04B 1/7115 Constructive combining of multi-path signals, i.e. RAKE receivers

Abstract

In wireless communications and navigation systems such as GPS, pseudorange measurements are made between a signal transmitter and a signal receiver based on pseudorandom noise codes modulating the transmitted signals. In the receiver, a code is tracked by forming a correlation function between the received code and a local code replica and locating the peak of the function using a delay-lock loop (DLL). Multipath signals, which reflect from objects before arriving at the receiver, degrade the code tracking performance. Multipath-invariant points are locations of the correlation function whose properties are independent of multipath parameters, and they can therefore be located reproducibly in the presence of multipath. A multipath mitigation method consists of locating a multipath-invariant (MPI) point of an ideal autocorrelation function and measuring the distance between the MPI point and the DLL. The same MPI point is located in a received correlation function, and the distance between the point and the DLL, now affected by multipath, is measured. The difference between the ideal distance and the actual distance is the code tracking error resulting from multipath. The error is subtracted from the computed pseudorange or used to control the DLL. The method can be used to reduce the effects of all types of tracking error sources, such as signal transmission failure or code noise.

Claims

1. A method for mitigating the effect of multipath signals on envelope tracking of a primary signal by a spread-spectrum receiver, wherein said primary signal comprises a carrier signal modulated by an envelope, and wherein a received signal comprises said multipath signals and said primary signal, said method comprising:

a) locating an ideal multipath-invariant point in an ideal comparison function, thereby determining an ideal distance between said ideal multipath-invariant point and an ideal multipath-dependent point, wherein said ideal comparison function is computed from an ideal signal that does not comprise multipath signals;

b) locating a corresponding multipath-invariant point in an actual comparison function, thereby determining an actual distance between said corresponding multipath-invariant point and a corresponding multipath-dependent point, wherein said actual comparison function is computed from said received signal;

c) comparing said ideal distance and said actual distance; and

d) based on said comparison, mitigating the effect of said multipath signals on said envelope tracking of said primary signal.

2. The method of claim 1 wherein step (c) comprises subtracting said ideal distance from said actual distance to obtain a tracking error.

3. The method of claim 2 wherein step (d) comprises adjusting a computed pseudorange by said tracking error, wherein said pseudorange is computed in dependence on said envelope tracking and represents a distance between said receiver and a signal transmitter.

4. The method of claim 2, further comprising removing a bias in said tracking error.

5. The method of claim 2 wherein said receiver further tracks a plurality of additional received signals, and wherein said method further comprises:

performing steps (a), (b), and (c) for each additional received signal to obtain a plurality of additional tracking errors; and

averaging said tracking error and said plurality of additional tracking errors to obtain an average tracking error.

6. The method of claim 1 wherein step (d) comprises controlling a signal tracking loop that tracks said received signal.

7. The method of claim 1 wherein said ideal multipath-invariant point is in a plateau of said ideal comparison function.

8. The method of claim 7 wherein said ideal multipath-invariant point is at an edge of said plateau.

9. The method of claim 1 wherein said ideal multipath-invariant point is adjacent to a plateau of said ideal comparison function.

10. The method of claim 1 wherein said corresponding multipath-dependent point is a point sampled by a signal tracking loop that tracks said received signal.

11. The method of claim 1 wherein step (b) comprises:

bounding an interval containing said corresponding multipath-invariant point, wherein bounds of said interval are selected in dependence on theoretical tracking errors and on said ideal distance; and

narrowing said interval until said corresponding multipath-invariant point is located.

12. The method of claim 11 wherein narrowing said interval comprises performing a modified Fibonacci search.

13. The method of claim 1 wherein step (b) comprises sampling said actual comparison function at at least one point to obtain at least a first sample.

14. The method of claim 13 wherein sampling said actual comparison function comprises eliminating the effect of dynamics of a signal tracking loop on said at least one point using a low-distortion component of said received signal.

15. The method of claim 14 wherein said low-distortion component of said received signal is said carrier signal.

16. The method of claim 14, further comprising estimating a rate of divergence of said signal tracking loop from said low-distortion component.

17. The method of claim 16 wherein estimating said rate of divergence comprises obtaining dual-frequency measurements of said received signal.

18. The method of claim 13 wherein step (b) further comprises sampling said actual comparison function to obtain at least a second sample.

19. The method of claim 18 wherein step (b) further comprises:

subtracting said first sample from said second sample to obtain a discriminator value; and

comparing said discriminator value with a predetermined threshold value.

20. The method of claim 19 wherein said first sample is not affected by a main lobe of a primary signal comparison function.

21. The method of claim 19, further comprising repeating said sampling, subtracting, and comparing steps at different locations until said discriminator value is sufficiently close to said threshold value.

22. The method of claim 18 wherein step (b) further comprises:

forming a ratio between said first sample and said second sample; and

comparing said ratio with a predetermined threshold value.

23. The method of claim 18 wherein step (b) further comprises estimating a shape of said actual comparison function from at least said first sample and said second sample, and locating said corresponding multipath-invariant point in dependence on said shape.

24. The method of claim 1, further comprising:

locating one or more additional ideal multipath-invariant points in said ideal comparison function, thereby determining one or more additional ideal distances between said one or more additional ideal multipath-invariant points and said ideal multipath-dependent point; and

locating one or more additional corresponding multipath-invariant points in said actual comparison function, thereby determining one or more additional actual distances between said one or more additional corresponding multipath-invariant points and said corresponding multipath-dependent point.

25. The method of claim 24, further comprising comparing said one or more additional ideal distances and said one or more additional actual distances.

26. The method of claim 24, further comprising comparing said actual distance and said one or more additional actual distances.

27. The method of claim 1, further comprising repeating step (b) at a later time to obtain a plurality of additional actual distances, and averaging said actual distance and said plurality of additional actual distances to obtain an average actual distance.

28. The method of claim 1 wherein said actual comparison function is further computed from a function that is not a replica of said received signal.

29. The method of claim 1, further comprising calibrating said actual comparison function in dependence on an elevation angle of a signal transmitter.

30. The method of claim 1, further comprising normalizing said actual comparison function.

31. The method of claim 1 wherein said receiver is a Global Positioning System (GPS) receiver.

32. A method for measuring a tracking error generated by a spread-spectrum receiver tracking a received signal comprising an envelope modulating a carrier signal, said method comprising:

a) locating an ideal distortion-invariant point in an ideal comparison function, thereby determining an ideal distance between said ideal distortion-invariant point and an ideal distortion-dependent point, wherein said ideal comparison function is computed from an ideal signal that is not distorted;

b) locating a corresponding distortion-invariant point in an actual comparison function, thereby determining an actual distance between said corresponding distortion-invariant point and a corresponding distortion-dependent point, wherein said actual comparison function is computed from said received signal, wherein said received signal is at least partially distorted; and

c) subtracting said ideal distance from said actual distance to obtain said tracking error.

33. The method of claim 32, further comprising using said tracking error to improve tracking performance of said receiver.

34. The method of claim 32, wherein said tracking error is caused by a failure in transmission of said received signal, and wherein said distortion-invariant point has a property that is unaffected by said failure.

35. The method of claim 32, wherein said tracking error is caused by noise in said received signal, and wherein said distortion-invariant point has a property that is unaffected by said noise.

36. The method of claim 32, further comprising using said tracking error to improve tracking of a different received signal by said receiver.

37. The method of claim 32, further comprising removing a bias in said tracking error.

38. The method of claim 32, further comprising calibrating said actual comparison function in dependence on an elevation angle of a signal transmitter.

39. The method of claim 32, further comprising normalizing said actual comparison function.

40. The method of claim 32 wherein step (b) further comprises eliminating the effect of dynamics of a signal tracking loop on locating said distortion-invariant point.

41. The method of claim 40 wherein said effect is eliminated using a low-distortion component of said received signal.

42. The method of claim 41 wherein said low-distortion component of said received signal is said carrier signal.

43. The method of claim 41, further comprising estimating a rate of divergence of said signal tracking loop from said low-distortion component.

44. The method of claim 43 wherein estimating said rate of divergence comprises obtaining dual-frequency measurements of said received signal.

45. The method of claim 32 wherein said receiver further tracks a plurality of additional received signals, and wherein said method further comprises:

performing steps (a), (b), and (c) for each additional received signal to obtain a plurality of additional tracking errors; and

averaging said tracking error and said plurality of additional tracking errors to obtain an average tracking error.