Method and apparatus for estimation of orthogonal frequency division multiplexing symbol timing and carrier frequency offset

• US 7,218,691 B1
• Filed: 02/04/2002
• Issued: 05/15/2007
• Est. Priority Date: 03/05/2001
• Status: Expired due to Term

• Current Assignee: Marvell Asia Pte Limited (Marvell Technology Group Limited)
• Original Assignee: Marvell International Limited (Marvell Technology Group Limited)
• Inventors: Narasimhan, Ravi
• Primary Examiner(s): Ghebretinsae; Temesghen
• Application Number:
  US10/067,556
• Time in Patent Office: 1,926 Days
• Field of Search: 375/240.28, 375/356, 375/260, 375/355, 375/232, 375/350, 375/362, 375/343, 375/363, 375/147, 375/354, 375/326, 375/344, 375/149, 370/503, 370/208, 370/480, 370/210, 370/281, 370/290, 370/292, 370/295, 370/310, 370/203
• US Class Current: 375/344
• CPC Class Codes: H04L 25/0228 with direct estimation from sounding signals
• CPC Class Codes: H04L 27/2659 Coarse or integer frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/266 Fine or fractional frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 27/2675 Pilot or known symbols
• CPC Class Codes: H04L 27/2695 with channel estimation, e.g. determination of delay spread, derivative or peak tracking channel estimation H04L25/0202

Abstract

A system and method estimates symbol timing of long training symbols and coarse carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network. The system and method estimates the symbol timing and coarse carrier frequency offset during short training symbols of a preamble of a data packet. Fine carrier frequency offset is estimated during long training symbols. Channel estimates during a data portion of the data packet are adapted.

Claims

1. A method for estimating carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

receiving a preamble including at least two adjacent short training symbols;

sampling said at least two adjacent short training symbols of said preamble at a first rate; and

correlating said at least two adjacent short training symbols to generate a correlation signal.

2. The method of claim 1 further comprising normalizing said correlation signal to generate a normalized correlation signal.

3. The method of claim 2 wherein said normalizing step further comprises dividing said correlation signal by an energy of at least one of said adjacent short training symbols.

4. The method of claim 2 further comprising:

repeating said sampling, correlating and normalizing steps for all of said short training symbols; and

identifying a maximum value of said normalized correlation signal during said short training symbols.

5. The method of claim 4 further comprising multiplying said maximum value of said normalized correlation signal by a threshold value to identify left and right edges of a plateau defined by said normalized correlation signal.

6. The method of claim 5 wherein said threshold value is greater than zero and less than one.

7. The method of claim 5 further comprising identifying left and right time index values corresponding to said left and right edges.

8. The method of claim 7 further comprising identifying a center time index value using said left and right time index values.

9. The method of claim 8 further comprising using said center time index value and a correlation value corresponding to said center time index value to calculate said carrier frequency offset.

10. The method of claim 9 further comprising:

calculating a first value by dividing an imaginary component of said correlation value at said center time index value by a real component of said correlation value at said center time index value; and

calculating an arctangent of said first value.

11. The method of claim 1 wherein said method is a software method.

12. The method of claim 10 further comprising calculating said carrier frequency offset by dividing said arctangent by 2π

T_short wherein T_short is the period of said short training symbols.

13. The method of claim 1 wherein said preamble forms part of an orthogonal frequency division multiplex (OFDM) packet.

14. A method for estimating carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

receiving a preamble including a plurality of short training symbols;

sampling said short training symbols of said preamble using a sampling window; and

correlating samples from the short training symbols in a first half of said sampling window with samples from the short training symbols in a second half of said sampling window to generate a correlation signal.

15. The method of claim 14 wherein said sampling window has a period that is equal to a duration of two short training symbols.

16. The method of claim 14 further comprising normalizing said correlation signal to generate a normalized correlation signal.

17. The method of claim 14 wherein said method is a software method.

18. The method of claim 14 wherein said preamble forms part of an orthogonal frequency division multiplex (OFDM) packet.

19. A method for estimating carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

receiving a preamble including a plurality of short training symbols;

sampling said short training symbols of said preamble using a sampling window;

correlating a first half of said sampling window with a second half of said sampling window to generate a correlation signal;

normalizing said correlation signal to generate a normalized correlation signal; and

wherein said normalizing step further comprises dividing said correlation signal by an energy of at least one of said first and second halves of said sampling window.

20. The method of claim 19 further comprising repeating said sampling, correlating and normalizing steps for all of said short training symbols.

21. The method of claim 20 further comprising identifying a maximum value of said normalized correlation signal during said short training symbols.

22. The method of claim 21 further comprising multiplying said maximum value of said normalized correlation signal by a threshold value to identify left and right edges of a plateau defined by said normalized correlation signal.

23. The method of claim 22 wherein said threshold value is greater than zero and less than one.

24. The method of claim 22 further comprising identifying left and right time index values corresponding to said left and right edges.

25. The method of claim 24 further comprising identifying a center time index value using said left and right time index values.

26. The method of claim 25 further comprising using said center time index value and a correlation value corresponding to said center time index value to calculate said carrier frequency offset.

27. The method of claim 26 further comprising calculating a first value by dividing an imaginary component of said correlation value at said center time index value by a real component of said correlation value at said center time index value and calculating an arctangent of said first value.

28. The method of claim 27 further comprising calculating said carrier frequency offset by dividing said arctangent by 2π

T_short wherein T_short is the period of said short training symbols.

29. A method for estimating carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

sampling at least two adjacent short training symbols of a preamble of a data packet to generate a received signal;

quantizing sign bits of real and imaginary components of said received signal; and

correlating said quantized sign bits to generate a correlation signal.

30. The method of claim 29 further comprising generating a filtered sum of an absolute value of a real component of said correlation signal and an absolute value of an imaginary component of said correlation signal.

31. The method of claim 30 wherein said filtered sum is generated by a single pole filter.

32. The method of claim 30 further comprising identifying a local maximum value of said filtered sum during said short training symbols.

33. The method of claim 32 wherein said local maximum value is identified by updating and storing said filtered sums and by comparing at least one filtered sum to a prior filtered sum and to a subsequent filtered sum.

34. The method of claim 29 wherein said method is a software method.

35. The method of claim 30 further comprising identifying a maximum value of said filtered sum during said short training symbols.

36. The method of claim 35 further comprising identifying said maximum value by updating and storing said filtered sums and by comparing at least one filtered sum to a prior filtered sum and to a subsequent filtered sum.

37. The method of claim 36 further comprising:

identifying a time index value corresponding to said maximum value; and

identifying a correlation signal value corresponding to said time index value.

38. The method of claim 37 further comprising:

calculating an imaginary component of said correlation signal value corresponding to said time index value;

calculating a real component of said correlation signal value corresponding to said time index value;

dividing said imaginary component by said real component to generate a quotient; and

calculating an arctangent of said quotient to generate a coarse carrier frequency offset estimate.

39. The method of claim 38 further comprising:

multiplying said received signal by e<sup>−

jnω

Δ</sup>

where ω

_Δ

is said coarse carrier frequency offset estimate and n is a time index.

40. A method for estimating carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

sampling short training symbols of a preamble of a data packet to generate a received signal;

quantizing sign bits of real and imaginary components of said received signal;

generating a filtered sum of an absolute value of a real component of said correlation signal and an absolute value of an imaginary component of said correlation signal;

identifying a local maximum value of said filtered sum during said short training symbols; and

multiplying said local maximum value of said filtered sum by a threshold value to identify a right edge of a plateau wherein said threshold value is greater than zero and less than one.

41. The method of claim 40 further comprising:

identifying a right time index value corresponding to said right edge; and

calculating symbol timing from said right time index value.

42. A method for estimating carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

sampling short and long training symbols of a preamble of a data packet to generate a received signal;

quantizing sign bits of real and imaginary components of said received signal;

correlating said quantized sign bits of at least two adjacent short training symbols to generate a first correlation signal;

generating a symbol timing estimate that is based on said first correlation signal and identifies a start time of first and second ones of said long training symbols correlating said first and second long training symbols to generate a second correlation signal; and

calculating a fine carrier frequency offset from said second correlation signal.

43. The method of claim 42 wherein said step of calculating further comprises:

calculating an imaginary component of said correlation signal;

calculating a real component of said correlation signal;

dividing said imaginary component by said real component to generate a quotient; and

calculating an arctangent of said quotient to generate said fine carrier frequency offset estimate.

44. The method of claim 43 further comprising updating a sampling clock with said fine carrier frequency offset estimate.

45. A method for adapting a carrier frequency offset estimate in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

generating channel estimates for an orthogonal frequency division multiplexing subcarrier as a function of subcarrier index values during a data portion of an orthogonal frequency division multiplexing packet;

generating a complex number by summing a product of frequency domain signals and said channel estimates for each of said subcarrier index values and dividing said sum by a sum of a squared absolute value of said channel estimate for each of said subcarrier index values;

calculating an imaginary component of said complex number; and

adapting the carrier frequency offset estimate based on said imaginary component.

46. A method for adapting a carrier frequency offset estimate in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

generating channel estimates for an orthogonal frequency division multiplexing subcarrier as a function of subcarrier index values;

generating a complex number by summing a product of frequency domain signals and said channel estimates for each of said subcarrier index values and dividing said sum by a sum of a squared absolute value of said channel estimate for each of said subcarrier index values;

calculating an imaginary component of said complex number;

multiplying said imaginary component by an adaptation parameter to generate a product; and

adding said product to a prior carrier frequency offset estimate to produce an adapted carrier frequency offset estimate.

47. A carrier frequency offset estimator for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

sampling means for sampling at least two adjacent short training symbols of a preamble of a data packet to generate a received signal;

quantizing means for quantizing sign bits of real and imaginary components of said received signal; and

correlating means for correlating said quantized sign bits of at least two adjacent short training symbols to generate a correlation signal.

48. The carrier frequency offset estimator of claim 47 further comprising filtered sum generating means for generating a filtered sum of an absolute value of a real component of said correlation signal and an absolute value of an imaginary component of said correlation signal.

49. The carrier frequency offset estimator of claim 48 wherein said filtered sum is generated by a single pole filter.

50. The carrier frequency offset estimator of claim 48 further comprising first identifying means for identifying a local maximum value of said filtered sum during said short training symbols.

51. The carrier frequency offset estimator of claim 50 wherein said local maximum value is identified by updating and storing said filtered sums and by comparing at least one filtered sum to a prior filtered sum and to a subsequent filtered sum.

52. The carrier frequency offset estimator of claim 50 further comprising first multiplying means for multiplying said local maximum value of said filtered sum by a threshold value to identify a right edge of a plateau.

53. The carrier frequency offset estimator of claim 52 wherein said threshold value is greater than zero and less than one.

54. The carrier frequency offset estimator of claim 52 further comprising:

second identifying means for identifying a right time index value corresponding to said right edge; and

first calculating means for calculating symbol timing from said right time index value.

55. The carrier frequency offset estimator of claim 48 further comprising third identifying means for identifying a maximum value of said filtered sum during said short training symbols.

56. The carrier frequency offset estimator of claim 55 further comprising fourth identifying means for identifying said maximum value by updating and storing said filtered sums and by comparing at least one filtered sum to a prior filtered sum and to a subsequent filtered sum.

57. The carrier frequency offset estimator of claim 56 further comprising:

fifth identifying means for identifying a time index value corresponding to said maximum value; and

sixth identifying means for identifying a correlation signal value corresponding to said time index value.

58. The carrier frequency offset estimator of claim 57 further comprising:

second calculating means for calculating an imaginary component of said correlation signal value corresponding to said time index value;

third calculating means for calculating a real component of said correlation signal value corresponding to said time index value;

dividing means for dividing said imaginary component by said real component to generate a quotient; and

fourth calculating means for calculating an arctangent of said quotient to generate a coarse carrier frequency offset estimate.

59. The carrier frequency offset estimator of claim 58 further comprising:

second multiplying means for multiplying said received signal by e<sup>−

jnω

Δ</sup>

where ω

Δ

is said coarse carrier frequency offset estimate and n is a time index.

60. A carrier frequency offset estimator for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

sampling means for sampling short and long training symbols of a preamble of a data packet to generate a received signal;

quantizing means for quantizing sign bits of real and imaginary components of said received signal;

first correlating means for correlating said quantized sign bits of at least two adjacent short training symbols to generate a first correlation signal;

symbol timing generating means for generating a symbol timing estimate that is based on said first correlation signal and identifies a start time of first and second ones of said long training correlating means for correlating said first and second long training symbols to generate a second correlation signal; and

first calculating means for calculating a fine carrier frequency offset from said second correlation signal.

61. The carrier frequency offset estimator of claim 60 wherein said first calculating means calculates an imaginary component of said correlation signal, calculates a real component of said correlation signal, divides said imaginary component by said real component to generate a quotient, and calculates an arctangent of said quotient to generate said fine carrier frequency offset estimate.

62. The carrier frequency offset estimator of claim 61 further comprising updating means for updating a sampling clock with said fine carrier frequency offset estimate.

63. A carrier frequency offset estimate adaptor for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

channel estimate generating means for generating channel estimates for an orthogonal frequency division multiplexing subcarrier as a function of subcarrier index values during a data portion of an orthogonal frequency division multiplexing packet;

complex number generating means for generating a complex number by summing a product of frequency domain signals and said channel estimates for each of said subcarrier index values and dividing said sum by a sum of a squared absolute value of said channel estimate for each of said subcarrier index values;

first calculating means for calculating an imaginary component of said complex number;

adapting means for adapting the carrier frequency offset estimate based on said imaginary component.

64. A carrier frequency offset estimate adaptor for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

channel estimate generating means for generating channel estimates for an orthogonal frequency division multiplexing subcarrier as a function of subcarrier index values;

complex number generating means for generating a complex number by summing a product of frequency domain signals and said channel estimates for each of said subcarrier index values and dividing said sum by a sum of a squared absolute value of said channel estimate for each of said subcarrier index values;

first calculating means for calculating an imaginary component of said complex number; and

first multiplying means for multiplying said imaginary component by an adaptation parameter to generate a product.

65. The carrier frequency offset estimate adaptor of claim 64 further comprising first adding means for adding said product to a prior carrier frequency offset estimate to produce an adapted carrier frequency offset estimate.

66. A carrier frequency offset estimator for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

a sampler that samples at least two adjacent short training symbols of a preamble of a data packet to generate a received signal;

a quantizer that quantizes sign bits of real and imaginary components of said received signal; and

a correlator that correlates said quantized sign bits to generate a correlation signal.

67. The carrier frequency offset estimator of claim 66 further a filtered sum generator that generates a filtered sum of an absolute value of a real component of said correlation signal and an absolute value of an imaginary component of said correlation signal.

68. The carrier frequency offset estimator of claim 67 wherein said filtered sum is generated by a single pole filter.

69. The carrier frequency offset estimator of claim 67 further comprising a first identifier circuit that identifies a local maximum value of said filtered sum during said short training symbols.

70. The carrier frequency offset estimator of claim 69 wherein said local maximum value is identified by updating and storing said filtered sums and by comparing at least one filtered sum to a prior filtered sum and to a subsequent filtered sum.

71. The carrier frequency offset estimator of claim 69 further comprising a first multiplier circuit that multiplies said local maximum value of said filtered sum by a threshold value to identify a right edge of a plateau.

72. The carrier frequency offset estimator of claim 71 wherein said threshold value is greater than zero and less than one.

73. The carrier frequency offset estimator of claim 71 further comprising:

second identifier circuit that identifies a right time index value corresponding to said right edge; and

a first calculator that calculates symbol timing from said right time index value.

74. The carrier frequency offset estimator of claim 67 further comprising a third identifier that identifies a maximum value of said filtered sum during said short training symbols.

75. The carrier frequency offset estimator of claim 74 further comprising fourth identifying means for identifying said maximum value by updating and storing said filtered sums and by comparing at least one filtered sum to a prior filtered sum and to a subsequent filtered sum.

76. The carrier frequency offset estimator of claim 75 further comprising:

a fifth identifier that identifies a time index value corresponding to said maximum value; and

a sixth identifier that identifies a correlation signal value corresponding to said time index value.

77. The carrier frequency offset estimator of claim 76 further comprising:

a second calculator that calculates an imaginary component of said correlation signal value corresponding to said time index value;

a third calculator that calculates a real component of said correlation signal value corresponding to said time index value;

a divider that divides said imaginary component by said real component to generate a quotient; and

a fourth calculator that calculates an arctangent of said quotient to generate a coarse carrier frequency offset estimate.

78. The carrier frequency offset estimator of claim 77 further comprising:

a second multiplier that multiplies said received signal by e<sup>−

jnω

Δ</sup>

where ω

_Δ

is said coarse carrier frequency offset estimate and n is a time index.

79. A carrier frequency offset estimator for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

a sampler that samples short and long training symbols of a preamble of a data packet and generates a received signal;

a quantizer that quantizes sign bits of real and imaginary components of said received signal;

a first correlator that correlates said quantized sign bits of at least two adjacent short training symbols to generate a first correlation signal;

a symbol timing generator that produces a symbol timing estimate that is based on said first correlation signal and identifies a start time of first and second ones of said long training symbols;

a second correlator that correlates said first and second long training symbols to generate a second correlation signal; and

a first calculator that calculates a fine carrier frequency offset from said second correlation signal.

80. The carrier frequency offset estimator of claim 79 wherein said first calculator calculates an imaginary component of said correlation signal, calculates a real component of said correlation signal, divides said imaginary component by said real component to generate a quotient, and calculates an arctangent of said quotient to generate said fine carrier frequency offset estimate.

81. The carrier frequency offset estimator of claim 80 further comprising a sampling clock updater that updates a sampling clock with said fine carrier frequency offset estimate.

82. A carrier frequency offset estimate adaptor for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:

a channel estimator that generates channel estimates for an orthogonal frequency division multiplexing subcarrier as a function of subcarrier index values during a data portion of an orthogonal frequency division multiplexing packet;

a complex number generator that generates a complex number by summing a product of frequency domain signals and said channel estimates for each of said subcarrier index values and dividing said sum by a sum of a squared absolute value of said channel estimate for each of said subcarrier index values;

a first calculator that calculates an imaginary component of said complex number; and

an adaptor that adapts the carrier frequency offset estimate based on said imaginary component.

83. The carrier frequency offset estimate adaptor of claim 82 further comprising a first multiplier that multiplies said imaginary component by an adaptation parameter to generate a product.

84. The carrier frequency offset estimate adaptor of claim 83 further comprising a first adder that adds said product to a prior carrier frequency offset estimate to produce an adapted carrier frequency offset estimate.