Non-linear equalizer system and method

• US 7,031,382 B2
• Filed: 10/23/2001
• Issued: 04/18/2006
• Est. Priority Date: 10/24/2000
• Status: Active Grant

• Current Assignee: Harris Global Communications, Inc. (L3Harris Technologies, Inc.)
• Original Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Inventors: Hessel, Clifford, Norris, James A.
• Primary Examiner(s): Tran, Khai
• Application Number:
  US09/983,110
• Publication Number:
  US 20020054633A1
• Time in Patent Office: 1,638 Days
• Field of Search: 375/229, 375/230, 375/348, 375/346, 375/316, 375/332, 375/329, 708/322, 708/323
• US Class Current: 375/229
• CPC Class Codes: H04L 2027/0067 Phase error detectors
• CPC Class Codes: H04L 27/2332 using a non-coherent carrier

Abstract

A system and method for compensating for angular distortion of a received waveform is disclosed. One embodiment measures the angular position of a current symbol, the preceding symbol and the succeeding symbol, calculates the phase shift between the current symbol and a preceding symbol and the phase shift between the current symbol and a succeeding symbol, uses the phase shifts to enter a 2-D lookup table to extract a phase correction factor, then uses the phase correction factor to modify the angular position of the current symbol.

Claims

1. A method of compensating for distortion of a received waveform containing digital symbols at one of a predetermined number of phase angles comprising the steps of:

(a) receiving at least three successive symbols;

(b) measuring the phase angle of a one of said received symbols;

(c) determining for said one received symbol a reverse phase shift between said one received symbol and an immediately preceding symbol and a forward phase shift between said one received symbol and an immediately succeeding symbol;

(d) modifying the phase angle of said one received symbol as a function of both the reverse and forward phase shifts to thereby compensate for phase distortion of the received waveform.

2. The method of claim 1 wherein the received waveform is a continuous phase modulation waveform.

3. The method of claim 2 wherein the received waveform is 4-ary modulated.

4. The method of claim 2 wherein the received waveform is 8-ary modulated.

5. The method of claim 2 wherein the received waveform is 16-ary modulated.

6. The method of claim 2 wherein the received waveform is in a 25 KHz channel.

7. The method of claim 6 wherein the received waveform is in a satellite communication channel.

8. The method of claim 1 wherein the step of modifying the phase angle of said one received symbol comprises the steps of:

(i) providing a first lookup table, indexed by reverse phase shift, of reverse phase correction factors for all possible phase angles of the immediately preceding symbol and thus all possible reverse phase shifts;

(ii) providing a second lookup table, indexed by forward phase shift, of forward phase correction factors for all possible phase angles of the immediately succeeding symbol and thus all possible forward phase shifts;

(iii) extracting from said first lookup table a reverse phase correction factor for said reverse phase shift determined in step (c);

(iv) extracting from said second lookup table a forward phase correction factor for said forward phase shift determined in step (c);

(v) multiplying the phase angle of said one received symbol by the reverse phase correction factor extracted from said first lookup table and by the forward phase correction factor extracted from said second lookup table.

9. The method of claim 8 wherein the step of providing said first lookup table comprises the steps of:

(1) determining the difference between the expected phase angle of a received symbol and the phase angle of the immediately preceding symbol to thereby determine an expected reverse phase shift;

(2) determining the difference between the actual phase angle of the received symbol and the phase angle of the immediately preceding symbol to thereby determine an actual reverse phase shift;

(3) dividing the expected reverse phase shift by the actual reverse phase shift to thereby determine a reverse error ratio;

(4) multiplying the reverse error ratio by the actual reverse phase shift to thereby determine a reverse phase correction factor;

(5) entering the reverse phase correction factor in said first lookup table indexed by said actual reverse phase shift;

(6) repeating steps (1) through (5) for each possible reverse phase shift.

10. The method of claim 9 wherein steps (5) and (6) are replaced with the following steps:

(5) iterating steps (1) through (4) at least one time and averaging the values for the reverse phase correction factor for each iteration to thereby determine an average reverse phase correction factor;

(6) entering the average reverse phase correction factor in said first lookup table indexed by said actual reverse phase shift;

(7) repeating steps (1) through (6) for each possible reverse phase shift.

11. The method of claim 8 wherein the step of providing said second lookup table comprises the steps of:

(1) determining the difference between the expected phase angle of a received symbol and the phase angle of the immediately succeeding symbol to thereby determine an expected forward phase shift;

(2) determining the difference between the actual phase angle of the received symbol and the phase angle of the immediately succeeding symbol to thereby determine an actual forward phase shift;

(3) dividing the expected forward phase shift by the actual forward phase shift to thereby determine a forward error ratio;

(4) multiplying the forward error ratio by the actual forward phase shift to thereby determine a forward phase correction factor;

(5) entering the forward phase correction factor in said second lookup table indexed by said actual forward phase shift;

(6) repeating steps (1) through (5) for each possible forward phase shift.

12. The method of claim 11 wherein steps (5) and (6) are replaced with the following steps:

(5) iterating steps (1) through (4) at least one time and averaging the values for the forward phase correction factor for each iteration to thereby determine an average forward phase correction factor;

(6) entering the average forward phase correction factor in said first lookup table indexed by said actual forward phase shift;

(7) repeating steps (1) through (6) for each possible forward phase shift.

13. The method of claim 1 wherein the step of modifying the phase angle of said one received symbol comprises the steps of:

(i) determining a phase correction factor as a function of the reverse and forward phase shifts;

(ii) modifying the phase angle of said one received symbol by the phase correction factor.

14. The method of claim 13 wherein the phase correction factor is a linear function of the combination of the reverse and forward phase shifts.

15. The method of claim 13 wherein the phase correction factor is added to the phase angle of said one received symbol.

16. The method of claim 13 wherein the phase correction factor is determined from a lookup table.

17. The method of claim 13 wherein the phase correction factor is proportional to the magnitude of the reverse and forward phase shifts.

18. The method of claim 1 wherein steps (c) and (d) are replaced with the following steps:

(c) determining a phase correction factor as a function of the reverse phase shift and the forward phase shift;

(d) multiplying the phase angle of said one received symbol by the phase correction factor to thereby compensate for distortion of the received waveform.

19. The method of claim 18 wherein the phase correction factor is chosen to minimize the mean square difference between the expected phase angle of said one received symbol and the actual phase angle of said one received symbol.

20. The method of claim 19 wherein said difference is a weighted difference.

21. The method of claim 20 wherein the weighted difference is a function of the amplitude of said one received symbol.

22. The method of claim 20 wherein the weighted difference is a function of the square of the amplitude of said one received symbol.

23. The method of claim 18 wherein the phase correction factor is chosen to minimize the absolute value of the difference between the expected phase angle of said one received symbol and the actual phase angle of said one received symbol.

24. The method of claim 23 wherein said difference is a weighted difference.

25. The method of claim 24 wherein the weighted difference is a function of the amplitude of said one received symbol.

26. The method of claim 24 wherein the weighted difference is a function of the square of the amplitude of said one received symbol.

32. The method of claim 20 wherein the weighted difference is a function of the square of the amplitude of said one received symbol.

27. A method of equalizing a received waveform containing digital symbols at one of a predetermined number of phase angles comprising the steps of:

(a) receiving at least three successive symbols;

(b) measuring the phase angle of the received symbols;

(c) determining for one of the received symbols a reverse phase shift between said one received symbol and an immediately preceding symbol;

(d) determining for said one received symbol a forward phase shift between said one received symbol and an immediately succeeding symbol;

(e) providing a two-dimensional lookup table of phase correction factors for all possible combinations of forward and reverse phase angles and thus all possible combinations of reverse and forward phase shifts, indexed by reverse phase shift and forward phase shift;

(f) extracting from said lookup table a phase correction factor for the reverse phase shift determined in step (c) and the forward phase shift determined in step (d);

(g) modifying the phase angle of said one received symbol by the phase correction factor from step (f) to thereby equalize the received waveform.

28. The method of claim 27 wherein the received waveform is a continuous phase modulation waveform.

29. The method of claim 27 wherein the phase correction factors populating the lookup table in step (e) are chosen to minimize the mean square difference between the expected phase angle of said one received symbol and the actual phase angle of said one received symbol.

30. The method of claim 29 wherein said difference is a weighted difference.

31. The method of claim 30 wherein the weighted difference is a function of the amplitude of said one received symbol.

33. The method of claim 27 wherein the phase correction factors populating the lookup table in step (e) are chosen to minimize the absolute value of the difference between the expected phase angle of said one received symbol and the actual phase angle of said one received symbol.

34. The method of claim 33 wherein said difference is a weighted difference.

35. The method of claim 34 wherein the weighted difference is a function of the amplitude of said one received symbol.

36. The method of claim 34 wherein the weighted difference is a function of the square of the amplitude of said one received symbol.

37. A method of reducing intersymbol interference in the form of phase error of a received waveform containing digital symbols at one of a predetermined number of phase angles comprising the steps of:

(a) receiving successive symbols;

(b) measuring the phase angle of a one of said received symbols;

(c) determining for each of a first predetermined number of received symbols preceding said one received symbol a reverse phase shift;

(d) determining for each of a second predetermined number of received symbols succeeding said one received symbol a forward phase shift;

(e) modifying the phase angle of said one received symbol as a function of the first predetermined number of reverse phase shifts and the second predetermined number of forward phase shifts to thereby reduce the intersymbol interference of the received waveform.

38. The method of claim 37 wherein the first predetermined number of received symbols equals the second predetermined number of received symbols.

39. The method of claim 37 wherein the received waveform is a continuous phase modulation waveform.

40. A non-linear equalizer for compensating for distortion of a received waveform containing digital symbols at one of a predetermined number of phase angles comprising:

a receiver for receiving at least three successive symbols;

measuring means for determining the phase angle of a one of said received symbols;

means for determining for said one received symbol a reverse phase shift between said one received symbol and an immediately preceding symbol and a forward phase shift between said one received symbol and an immediately succeeding symbol;

means for modifying the phase angle of said one received symbol as a function of both the reverse and forward phase shifts to thereby compensate for phase distortion of the received waveform.

41. The non-linear equalizer of claim 40 wherein the received waveform is a continuous phase modulation waveform.

42. The non-linear equalizer of claim 40 wherein the means for modifying the phase angle of said one received symbol includes extracting a phase correction factor from a lookup table.

43. The non-linear equalizer of claim 42 wherein the lookup table is a two-dimensional lookup table indexed by reverse phase shift and forward phase shift.

44. The non-linear equalizer of claim 43 wherein the lookup table is populated with phase correction factors determined from previously-received symbols.

45. The non-linear equalizer of claim 44 wherein the phase correction factors in said lookup table are dynamically updated with information derived from received symbols.

46. An angular distortion compensator for modifying the phase angle of a digital symbol in a received waveform containing digital symbols at one of a predetermined number of phase angles comprising:

a receiver for receiving at least three successive symbols;

means for measuring the phase angle of said received symbols;

means for determining a phase angle correction factor as a function of the difference in phase angles between one of said received symbols and an immediately preceding symbol and said one of said received symbols and an immediately succeeding symbol;

modifying means for modifying the phase angle of said one of said received symbols by the phase angle correction factor to thereby modify the phase angle.

47. The compensator of claim 46 wherein the received waveform is a continuous phase modulation waveform.

48. The compensator of claim 46 wherein the modifying means adds the phase angle correction factor to the phase angle of said one of said received symbols.

49. The compensator of claim 46 wherein the phase angle correction factor is chosen to minimize the mean square difference between the expected phase angle of said one of said received symbols and the actual phase angle of said one of said received symbols.

50. The compensator of claim 49 wherein said difference is a weighted difference.

51. The compensator of claim 50 wherein the weighted difference is a function of the amplitude of said one of said received symbols.

52. The compensator of claim 50 wherein the weighted difference is a function of the square of the amplitude of said one of said received symbols.

53. The compensator of claim 46 wherein the phase correction factor is chosen to minimize the absolute value of the difference between the expected phase angle of said one of said received symbols and the actual phase angle of said one of said received symbols.

54. The compensator of claim 53 wherein said difference is a weighted difference.

55. The compensator of claim 54 wherein the weighted difference is a function of the amplitude of said one of said received symbols.

56. The compensator of claim 54 wherein the weighted difference is a function of the square of the amplitude of said one of said received symbols.