Method and apparatus for reducing the effect of multipath propagation in a received signal

• US 6,792,035 B2
• Filed: 12/20/2000
• Issued: 09/14/2004
• Est. Priority Date: 12/21/1999
• Status: Active Grant

• Current Assignee: III Holdings 3, LLC
• Original Assignee: Nokia Mobile Phones UK Limited (Nokia Corporation)
• Inventors: Kontola, Ilkka
• Primary Examiner(s): Corrielus, Jean B.
• Application Number:
  US09/745,081
• Publication Number:
  US 20010012315A1
• Time in Patent Office: 1,364 Days
• Field of Search: 375/285, 375/130, 375/140, 375/142, 375/150, 375/346, 375/348, 375/350, 375/343, 375/144, 375/147, 375/148, 370/210
• US Class Current: 375/148
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7102 with transform to frequency domain

Abstract

The invention relates to a method for reducing the effect of multipath propagation in a receiver (1) in which a code-modulated spread spectrum signal is received. In the method, at least one reference code (r(x)) corresponding to a code used in modulation is used to form at least two reference signals with different phases. Correlation between a received signal and each reference signal is performed to form correlation values. The method also comprises the steps of forming a divider function and performing at least a deconvolution step, in which a first time-to-frequency transformation is performed on the corelation results to form first transform results, and the first transform results are divided by the values of the divider function to form a division result. The invention also relates to a receiver (1) for receiving a code modulated spread spectrum signal, and an electronic device (24) comprising at least a positioning receiver (1) with count (2) for receiving a code modulated spread spectrum signal.

Claims

1. A method for reducing the effect of multipath propagation of a code-modulated spread-spectrum signal on the identification of a directly propagated signal component, said multipath propagation occurring during transmission of said code-modulated spread-spectrum signal over a wireless link from a transmitter to a receiver, the method comprising performing the following steps at the receiver:

receiving the code-modulated spread-spectrum signal,

using at least one reference code (r(x)) corresponding to a code used in modulation of the code-modulated spread-spectrum signal to form n reference signals having different code phases, and

performing a correlation between the received signal and each of the n reference signals thus formed to produce n correlation values representative of a cross-correlation function,

characterized in that the method further comprises the steps of;

performing a first time-to-frequency domain transformation on the n correlation values to form n first transform results,

forming a frequency-domain divider function representative of a cross-correlation function that would be obtained by said correlation step in the absence of multipath propagation and

performing a deconvolution step, by dividing said first transform results by said divider function to form a division result comprising n values.

2. The method according to claim 1, characterized in that it further comprises the following steps:

a padding step, in which the division result formed in the deconvolution step is supplemented with a number of filler values,

an interpolation step, in which a frequency-to-time domain transformation is performed on the values of the padded division result to form m second transform results, and

a determination step to determine the code phase of the directly propagated signal component of the received signal on the basis of said m second transform results.

3. The method according to claim 2, characterized in that in the determination step, the m second transform results are searched to identify one or more maxima exceeding a predefined threshold level, wherein the maximum corresponding to the smallest code phase is interpreted as being indicative of the code phase of the directly propagated signal component of the received signal.

4. The method according to claim 1, characterized in that it further comprises the following steps:

a padding step, in which the division result formed in the deconvolution step is supplemented with a number of filler values,

an interpolation step, in which a frequency-to-time domain transformation is performed on the values of the padded division result to form m second transform results,

the method further comprising;

repeating said deconvolution step, padding step and interpolation step form k sets of m second transform results,

storing the m second transform results formed at each repetition in the form of a deconvolution matrix having m rows and k columns,

performing a frequency analysis step, in which each of the m rows of the deconvolution matrix, comprising k values, is subjected to a second time-to-frequency domain transformation so as to form k third transform results for each row,

storing the k third transform results formed for each of the m rows of the deconvolution matrix in the form of an analysis matrix having m rows and k columns, and

searching for maximum values in said analysis matrix to determine the directly propagated signal component of the received signal.

5. The method according to claim 4, characterized in that the third transform results stored in the analysis matrix are searched to identify one or more maxima exceeding a predefined threshold level, wherein an identified maximum having the smallest code phase of all identified maxima is interpreted as being indicative of a code phase of the directly propagated signal component of the received signal.

6. The method according to claim 5, characterized in that determination of the smallest code phase is made in two dimensions, on the basis of the code phase and frequency of the third transform results stored in said analysis matrix.

7. The method according to claim 1, characterized in that said first time-to-frequency domain transformation is a Fourier transform.

8. The method according to claim 4, characterized in that said second time-to-frequency domain transformation is a Fourier transform.

9. The method according to claim 2, characterized in that said filler values are zeros.

10. The method according to claim 2, characterized in that said filler values are placed in the initial and final parts of the division result in such a way that they are symmetrically distributed.

11. The method according to claim 2, characterized in that n is an integer power of two m and is the next larger or a higher power of two than n and m−

n filler values are added in said padding step.

12. The method according to claim 1, characterized in that said divider function is formed by:

forming a cross-correlation function model;

performing a time-to-frequency domain transformation on said cross-correlation function model; and

replacing values of the time-to-frequency domain transformation result of the cross-correlation function model less than a predetermined threshold value (ε

) with said threshold value (ε

).

13. The method according to claim 12, characterized in that said cross-correlation function model is an equilateral triangle.

14. The method according to claim 12, in which the received signal is transferred via RF stages of the receiver, characterized in that said cross-correlation function model takes into account at least the effects of said RF stages on the received signal.

15. The method according to claim 12, in which said received signal is transferred via IF stages of the receiver, characterized in that said cross-correlation function model takes into account at least the effects of said IF stages on the received signal.

16. The method according to claim 12, characterized in that said divider function is pre-calculated and stored in the receiver.

17. The method according to claim 1, characterized in that two or more spread spectrum signals are received, wherein the steps of the method are repeated for each spread spectrum signal received.

18. The method according to claim 17, characterized in that a directly propagated signal component is searched for from each of the signals received.

19. The method according to claim 1, characterized in that said frequency-to-time domain transformation is an inverse Fourier transform.

20. The method according to claim 1, characterized in that said filler values are zeros.

21. The method according to claim 4, characterized in that said filler values are placed in the initial and final parts of the division result in such a way that they are symmetrically distributed.

22. The method according to claim 4, characterized in that n is an integer power of two and m is the next larger or a higher power of two than n and m−

n filler values are added in said padding step.

23. The method according to claim 2, characterized in that two or more spread spectrum signals are received, wherein the steps of the method are repeated for each spread spectrum signal received.

24. The method according to claim 4, characterized in that two or more spread spectrum signals are received, wherein the steps of the method are repeated for each spread spectrum signal received.

25. The method according to claim 23, characterized in that a directly propagated signal component is searched for from each of the signals received.

26. The method according to claim 24, characterized in that a directly propagated signal component is searched for from each of the signals received.

27. The method according to claim 4, characterized in that said frequency-to-time domain transformation is an inverse Fourier transform.

28. A wireless receiver for receiving a code-modulated spread-spectrum signal, the receiver being configured to reduce the effect of multipath propagation of the code-modulated spread-spectrum signal on the identification of a directly propagated signal component, said multipath propagation occurring during transmission of said code-modulated spread-spectrum signal from a transmitter to the receiver, the receiver comprising:

a delay generator for forming n reference signals having different code phases on the basis of at least one reference code (r(x)) corresponding to a code used in the modulation of the code-modulated spread-spectrum signal, and

a correlator for performing a correlation between the received signal and each of the n reference signals formed by the delay generator to produce n correlation values representative of a cross-correlation function,

characterized in that the receiver further comprises;

a time-to-frequency domain transformer for performing a first time-to-frequency domain transformation on the n correlation values to form n first transform results,

a divider function forming unit for forming a frequency-domain divider function representative of a cross-correlation function that would be obtained by the correlator in the absence of multipath propagation, and

a deconvolution unit for dividing said first transform results by said divider function to form a division result comprising n values.

29. The receiver according to claim 28, characterized in that it further comprises:

a padder for adding a number of filler values to the division result formed by the deconvolution unit,

an interpolator for performing a frequency-to-time domain transformation on the values of the padded division result to form m second transform results, and

a searcher for determining the code phase of said directly propagated component of the received signal on the basis of said m second transform results.

30. The receiver according to claim 29, characterized in that the searcher is configured to search the m second transform results to identify one or more maxima exceeding a predefined threshold level, and is adapted to interpret an identified maximum having the smallest code phase of all identified maxima as being indicative of the code phase of the directly propagated component of the received signal.

31. The receiver according to claim 28, characterized in that it further comprises:

a padder for adding a number of filler values to the division result formed by the deconvolution unit,

an interpolator for performing a frequency-to-time domain transformation on the values of the padded division result to form m second transform results,

said deconvolution unit, said padder and said interpolator being configured to operate repeatedly to form k sets of m second transform results, the receiver further comprising;

a memory for storing the m second transform results formed at each repetition in the form of a deconvolution matrix having m rows and k columns,

a frequency analyser for applying a second time-to-frequency domain transformation to each of the m rows of the deconvolution matrix comprising k values so as to form k third transform results for each row,

a memory for storing the k third transform results formed for each of the m rows of the deconvolution matrix in the form of an analysis matrix having m rows and k columns, and

a searcher for searching for maximum values in said analysis matrix to determine the directly propagated signal component of the received signal.

32. The receiver according to claim 31, characterized in that the searcher is configured to search the third transform results stored in the analysis matrix to identify one or more maxima exceeding a predefined threshold level, and is adapted to interpret an identified maximum having the smallest code phase of all identified maxima as being indicative of the code phase of the directly propagated component of the received signal.

33. The receiver according to claim 32, characterized in that the searcher is adapted to determine the smallest code phase by performing a search in two dimensions, on the basis of the code phase and frequency of the third transform results stored in said analysis matrix.

34. The receiver according to claim 28, characterized in that said first time-to-frequency domain transformation is a Fourier transform.

35. The receiver according to claim 31, characterized in that said second time-to-frequency domain transformation is a Fourier transform.

36. The receiver according to claim 29, characterized in that said filler values are zeros.

37. The receiver according to claim 29, characterized in that said padder is adapted to place said filler values in the initial and final parts of the division result in such a way that they are symmetrically distributed.

38. The receiver according to claim 29, characterized in that n is an integer power of two and m is the next larger or a higher power of two than n and said padder is adapted to add m−

n filler values to the division result.

39. The receiver according to claim 28, characterized in that said divider function forming unit is adapted to form said divider function by:

forming a cross-correlation function model;

performing a time-to-frequency domain transformation on said cross-correlation function model; and

replacing values of the time-to-frequency domain transform result of the cross-correlation function model less than a predetermined threshold value (ε

) with said threshold value (ε

).

40. The receiver according to claim 39, characterized in that said cross-correlation function model is an equilateral triangle.

41. The receiver according to claim 39, in which the received signal is transferred via RF stages of the receiver, characterized in that said cross-correlation function model takes into account at least the effects of said RF stages on the received signal.

42. The receiver according to claim 39, in which the received signal is transferred via IF stages of the receiver, characterized in that said cross-correlation function model takes into account at least the effects of said IF stages on the received signal.

43. The receiver according to claim 28, characterized in that said divider function is pre-calculated and stored in the receiver.

44. The receiver according to claim 28, characterized in that it is a GPS receiver.

45. The receiver according to claim 29, characterized in that said frequency-to-time domain transformation is an inverse Fourier transform.

46. The receiver according to claim 28, characterized in that it is provided in a wireless communication device.

47. The receiver according to claim 31, characterized in that said filler values are zeros.

48. The receiver according to claim 31, characterized in that said padder is adapted to place said filler values in the initial and final parts of the division result in such a way that they are symmetrically distributed.

49. The receiver according to claim 31, characterized in that n is an integer power of two and m is the next larger or a higher power of two than n and said padder is adapted to add m−

n filler values to the division result.

50. The receiver according to claim 31, characterized in that said frequency-to-time domain transformation is an inverse Fourier transform.

51. An electronic device comprising a wireless receiver for receiving a code-modulated spread-spectrum signal, the receiver being configured to reduce the effect of multipath propagation of the code-modulated spread-spectrum signal on the identification of a directly propagated signal component, said multipath propagation occurring during transmission of said code-modulated spread-spectrum signal from a transmitter to the receiver, the receiver comprising:

a delay generator for forming n reference signals having different code phases on the basis of at least one reference code (r(x)) corresponding to a code used in modulation of the code-modulated spread-spectrum signal, and

a correlator for performing a correlation between the received signal and each of the n reference signals formed by the delay generator to produce n correlation values representative of a cross-correlation function,

characterized in that the receiver of the electronic device also comprises;

a time-to-frequency domain transformer for performing a first time-to-frequency domain transformation on the n correlation values to form n first transform results,

a divider function forming unit for forming a divider function representative of a cross-correlation function that would be obtained by the correlator in the absence of multipath propagation, and

a deconvolution unit for dividing said first transform results by said divider function to form a division result comprising n values.

52. The electronic device according to claim 51, characterized in that the receiver further comprises:

a padder for adding a number of filler values to the division result formed by the deconvolution unit;

an interpolator for performing a frequency-to-time domain transformation on the values of the padded division result to form m second transform results; and

a searcher for determining the code phase of said directly propagated component of the received signal on the basis of said m second transform results.

53. The electronic device according to claim 51, characterized in that it also comprises means for performing functions of a mobile communication device.

54. The electronic device according to claim 52, characterized in that the searcher is configured to search the m second transform results to identify one or more maxima exceeding a predefined threshold level, and is adapted to interpret an identified maximum having the smallest code phase of all identified maxima as being indicative of the code phase of the directly propagated component of the received signal.

55. The electronic device according to claim 51, characterized in that the receiver further comprises:

a padder for adding a number of filler values to the division result formed by the deconvolution unit,

an interpolator for performing a frequency-to-time domain transformation on the values of the padded division result to form m second transform results,

said deconvolution unit, said padder and said interpolator being configured to operate repeatedly to form k sets of m second transform results, the receiver further comprising;

a memory for storing the m second transform results formed at each repetition in the form of a deconvolution matrix having m rows and k columns,

a frequency domain analyser for applying a second time-to-frequency transformation to each of the m rows of the deconvolution matrix comprising k values so as to form k third transform results for each row,

a memory for storing the k third transform results formed for each of the m rows of the deconvolution matrix in the form of an analysis matrix having m rows and k columns, and

a searcher for searching for maximum values in said analysis matrix to determine the directly propagated signal component of the received signal.

56. The electronic device according to claim 55, characterized in that the searcher is configured to search the third transform results stored in the analysis matrix to identify one or more maxima exceeding a predefined threshold level, and is adapted to interpret an identified maximum having the smallest code phase of all identified maxima as being indicative of the code phase of the directly propagated component of the received signal.