Channels estimation for multiple input - multiple output, orthogonal frequency division multiplexing (OFDM) system

• US 20020122381A1
• Filed: 12/29/2000
• Published: 09/05/2002
• Est. Priority Date: 09/01/2000
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Tong, Wen, Ma, Jianglei, Wu, Shiquan
• Granted Patent: US 6,850,481 B2
• Time in Patent Office: Days
• US Class Current: 370/208
• CPC Class Codes: H04B 7/0669 using different channel coding between antennas space-time coding H04L1/0618
• CPC Class Codes: H04B 7/0697 using spatial multiplexing
• CPC Class Codes: H04B 7/0845 per branch equalization, e.g. by an FIR-filter or RAKE receiver per antenna branch rake receivers as such H04B1/7115
• CPC Class Codes: H04B 7/0874 using subgroups of receive antennas
• CPC Class Codes: H04B 7/0894 using different delays between antennas
• CPC Class Codes: H04L 1/0618 Space-time coding
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/022 of frequency response
• CPC Class Codes: H04L 25/0226 sounding signals per se
• CPC Class Codes: H04L 25/0232 by interpolation between sounding signals
• CPC Class Codes: H04L 27/2605 Symbol extensions, e.g. Zero Tail, Unique Word [UW]
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 5/0048 Allocation of pilot signals, i.e. of signals known to the receiver

Abstract

The distortion in the sub-carrier signals is determined by transmitting known values that are incorporated into the preamble portion of the frame and/or are incorporated into pilot symbols that are inserted into the data portion of the frame. The receiver typically receives these known values in a distorted form and then processes the distorted values together with the original known values to obtain a channel response. The channel response is then used to estimate the frequencies at which the channels are received.

Claims

1. A method of estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said method comprising:

extracting said portion of said signal to obtain a frequency domain data vector;

multiplying each element of said frequency domain data vector by a respective element of a known vector to obtain a channel response vector, each element in said known vector being a function of a respective one of said plurality of known values, each element in said channel response vector being indicative of a distortion value;

selecting respective ones of said channel response vector to form a plurality of further channel response vectors, each of said further channel response vectors corresponding to a respective one of said plurality of transmitters, and inserting, within said each of said further channel response vectors, at least one zero value before or after each non-zero valued element therein such that said elements of said each of said further channel response vectors correspond to a transmission pattern of said corresponding one of said plurality of transmitters;

interpolating said elements of said further channel response vectors to obtain a plurality of distortion values each corresponding to a respective zero value element of said further channel response vectors, and substituting said plurality of distortion values in place of said corresponding zero value element;

performing an inverse fast Fourier transform (IFFT) on each of said further channel response vectors to obtain a plurality of time domain further channel response vectors;

substituting a zero value in place of any element of said time domain further channel response vectors that has a value indicative of signal noise; and

performing a fast Fourier transform (FFT) on said time domain further channel response vectors to obtain a plurality of frequency response vectors.

2. The method of claim 1 wherein said signal comprises an Orthogonal Frequency Division Multiplexing (OFDM) signal.

3. The method of claim 1 wherein each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to said respective receiver.

4. The method of claim 1 wherein said respective receiver comprises one of a plurality of receivers and each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to a respective one of said plurality of receivers.

5. The method of claim 1 wherein said portion of said signals comprises a preamble portion, and said transmission pattern of said corresponding one of said plurality of transmitters comprises a transmission pattern of said preamble portion.

6. The method of claim 1 wherein said known values comprise sequences pseudo-noise (PN) values, each of said sequences of PN values corresponding to a respective one of said plurality of transmitters.

7. The method of claim 6 wherein each element of each of said sequences of PN values is defined by the relation:

C_m(i)=(−

1)^miexp(jπ

m²i/1024),

where i is the index of a respective element in said sequence and has a value i=1, . . . , 1024, m is an integer index value that is uniquely assigned to a respective one of said plurality of transmitters, and j=sqrt(−

1).

8. The method of claim 7 wherein said value m is a prime number that is selected from the values 3, 5, 7, 11, 13, 17, . . . , 1023.

9. The method of claim 1 wherein said extracting step comprises:

sampling said portion of said signal to obtain a data vector, and performing a fast Fourier transform (FFT) on said data vector to obtain said frequency domain data vector.

10. The method of claim 1 wherein each element in said known vector comprises a complex conjugate of a respective one of said plurality of known values.

11. A method of estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said method comprising:

extracting said portion of said signal to obtain a frequency domain data vector;

multiplying said frequency domain data vector by a known array to obtain a channel response vector, each element in said known array being a function of a respective one of said plurality of known values, each element in said channel response vector being indicative of a distortion value;

interpolating said elements of said channel response vector to obtain at least one distortion value corresponding to a respective zero value element of said further channel response vectors, and substituting said plurality of distortion values in place of said corresponding zero value element;

performing an inverse fast Fourier transform (IFFT) on said channel response vector to obtain a frequency domain channel response vector;

substituting a zero value in place of any element of said time domain further channel response vectors that has a value indicative of signal noise; and

performing a fast Fourier transform (FFT) on said time domain further channel response vectors to obtain a plurality of frequency response vectors.

12. The method of claim 11 wherein said signal comprises an Orthogonal Frequency Division Multiplexing (OFDM) signal.

13. The method of claim 11 wherein each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to said respective receiver.

14. The method of claim 11 wherein said respective receiver comprises one of a plurality of receivers and each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to a respective one of said plurality of receivers.

15. The method of claim 11 wherein said portion of said signals comprises at least two pilot values that are inserted within said signals at known locations therein.

16. The method of claim 15 wherein said known locations comprise predetermined sub-carrier channels.

17. The method of claim 15 wherein said at least two pilot values are transmitted during an interval, and at least two further values are transmitted during a further interval, each said at least two further values being a function of a complex conjugate of a respective one of said at least two pilot values.

18. The method of claim 17 wherein a row of said array comprises said at least two pilot values, and a further row of said array comprises said at least two further values.

19. The method of claim 15 wherein said multiplying step comprises the following relation:

20. The method of claim 19 wherein said plurality of receivers comprises two receivers having indices i=1, 2, respectively, and said multiplying step comprises the following relations:

21. The method of claim 11 wherein said extracting step comprises:

sampling said portion of said signal to obtain a data vector, and performing a fast Fourier transform (FFT) on said data vector to obtain said frequency domain data vector.

22. A method of estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said method comprising:

extracting said portion of said signal during at least portion of a plurality of times nT, to obtain a plurality of sample values r_l(nT), wherein i is an index of said respective receiver;

defining each of said plurality of sample values r_i(nT) as a function of a plurality of time domain channel response values ch_mi(kT) according to the relation;

$r_1<br><br>\left(n<br><br>\text{\hspace{1em}<br><br>}<br><br>T\right)=\underset{k=22}{\overset{22}{\sum <br><br>}}<br><br>{\mathrm{ch}}_{\mathrm{mi}}<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>T\right)<br><br>T_{n-k}+N_i<br><br>\left(n<br><br>\text{\hspace{1em}<br><br>}<br><br>T\right),$

wherein m is an index of a respective one of said plurality of transmitters that is transmitting a non-zero value at time nT, T_n-k is a corresponding one of said plurality of known values, and N_i(nT) is noise value for receiver i, thereby obtaining a plurality of equations;

solving said plurality of equations to obtain a plurality of solutions; and

averaging said plurality of solutions to obtain said plurality of time domain channel response values ch_mi(kT).

23. The method of claim 22 wherein said signal comprises an Orthogonal Frequency Division Multiplexing (OFDM) signal.

24. The method of claim 22 wherein each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to said respective receiver.

25. The method of claim 22 wherein said respective receiver i comprises one of a plurality of receivers and each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to a respective one of said plurality of receivers.

26. The method of claim 22 wherein said portion of said signals comprises a preamble portion.

27. The method of claim 22 wherein said plurality of transmitters comprises two transmitters, one of said two transmitters having m=1 and transmitting values {T₀, T₁, . . . , T₅₁₁} during a first part half of an OFDM interval and transmitting zero values during a remainder of said OFDM interval, another of said two transmitters having m=2 and transmitting values {T₅₁₂, T₅₁₃, . . . , T₁₀₂₃} during a second part of said OFDM interval and transmitting zero values during a further remainder of said OFDM interval, and for receiver i=1, said plurality of sample values r_i(nT) is defined according to the relations:

28. The method of claim 22 further comprising the step of performing a fast Fourier transform (FFT) on said plurality of time domain channel response values ch_mi(kT) to obtain a plurality of frequency domain channel response values.

30. The apparatus of claim 29 wherein said signal comprises an Orthogonal Frequency Division Multiplexing (OFDM) signal.

31. The apparatus of claim 29 wherein each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to said respective receiver.

32. The apparatus of claim 29 wherein said respective receiver comprises one of a plurality of receivers and each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to a respective one of said plurality of receivers.

33. The apparatus of claim 29 wherein said portion of said signals comprises a preamble portion, and said transmission pattern of said corresponding one of said plurality of transmitters comprises a transmission pattern of said preamble portion.

34. The apparatus of claim 29 wherein said known values comprise sequences of psuedo-noise (PN) values, each of said sequences of PN values corresponding to a respective one of said plurality of transmitters.

35. The apparatus of claim 34 wherein each element of each of said sequences of PN values is defined by the relation:

C_m(i)=(−

1)^mi exp(jπ

m²i/1024),

where i is the index of a respective element in said sequence and has a value i=1, . . . , 1024, m is an integer index value that is uniquely assigned to a respective one of said plurality of base stations, and j=sqrt(−

1).

36. The apparatus of claim 35 wherein said value m is a prime number that is selected from the values 3, 5, 7, 11, 13, 17, . . . , 1023.

37. The apparatus of claim 29 further configured to sample said portion of said signal to obtain a data vector, and means for perform a fast Fourier transform (FFT) on said data vector to obtain said frequency domain data vector.

38. The apparatus of claim 29 wherein each element in said known vector comprises a complex conjugate of a respective one of said plurality of known values.

40. The apparatus of claim 39 wherein said signal comprises an Orthogonal Frequency Division Multiplexing (OFDM) signal.

41. The apparatus of claim 39 wherein each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to said respective receiver.

42. The apparatus of claim 39 wherein said respective receiver comprises one of a plurality of receivers and each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to a respective one of said plurality of receivers.

43. The apparatus of claim 39 wherein said portion of said signals comprises at least two pilot values that are inserted within said signals at known locations therein.

44. The apparatus of claim 43 wherein said known locations comprise predetermined sub-carrier channels.

45. The apparatus of claim 43 wherein said at least two pilot values are transmitted during an interval, and at least two further values are transmitted during a further interval, each said at least two further values being a function of a complex conjugate of a respective one of said at least two pilot values.

46. The apparatus of claim 45 wherein a row of said array comprises said at least two pilot values, and a further row of said array comprises said at least two further values.

47. The apparatus of claim 43 wherein said multiplying means multiplies according to the following relation:

48. The apparatus of claim 47 wherein said plurality of receivers comprises two receivers having indices i=1, 2, respectively, and said multiplying means multiplies according to the following relations:

49. The apparatus of claim 39 further configured to:

sample said portion of said signal to obtain a data vector, and perform a fast Fourier transform (FFT) on said data vector to obtain said frequency domain data vector.

29. An apparatus for estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said apparatus being configured to:

extract said portion of said signal to obtain a frequency domain data vector;

multiply each element of said frequency domain data vector by a respective element of a known vector to obtain a channel response vector, each element in said known vector being a function of a respective one of said plurality of known values, each element in said channel response vector being indicative of a distortion value;

select respective ones of said channel response vector to form a plurality of further channel response vectors, each of said further channel response vectors corresponding to a respective one of said plurality of transmitters, and inserting, within said each of said further channel response vectors, at least one zero value before or after each non-zero valued element therein such that said elements of said each of said further channel response vectors correspond to a transmission pattern of said corresponding one of said plurality of transmitters;

interpolate said elements of said further channel response vectors to obtain a plurality of distortion values each corresponding to a respective zero value element of said further channel response vectors, and substitute said plurality of distortion values in place of said corresponding zero value element;

perform an inverse fast Fourier transform (IFFT) on each of said further channel response vectors to obtain a plurality of time domain further channel response vectors

substitute a zero value in place of any element of said time domain further channel response vectors that has a value indicative of signal noise; and

perform a fast Fourier transform (FFT) on said time domain further channel response vectors to obtain a plurality of frequency response vectors.

39. An apparatus for estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said apparatus being configured to:

extract said portion of said signal to obtain a frequency domain data vector;

multiply said frequency domain data vector by a known array to obtain a channel response vector, each element in said known array being a function of a respective one of said plurality of known values, each element in said channel response vector being indicative of a distortion value;

interpolate said elements of said channel response vector to obtain at least one distortion value corresponding to a respective zero value element of said further channel response vectors, and substitute said plurality of distortion values in place of said corresponding zero value element;

perform an inverse fast Fourier transform (IFFT) on said channel response vector to obtain a frequency domain channel response vector;

substitute a zero value in place of any element of said time domain further channel response vectors that has a value indicative of signal noise; and

perform an FFT on said time domain further channel response vectors to obtain a plurality of frequency response vectors.

50. An apparatus for estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said apparatus being configured to:

extract said portion of said signal during at least portion of a plurality of times nT, wherein n=0, 1, . . . , 1023, to obtain a plurality of sample values r_i(nT), wherein i is an index of said respective receiver;

define each of said plurality of sample values r_i(nT) as a function of a plurality of time domain channel response values ch_mi(kT) according to the relation;

$r_1<br><br>\left(\mathrm{nT}\right)=\underset{k=22}{\overset{22}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{ch}}_{\mathrm{mi}}<br><br>\left(\mathrm{kT}\right)<br><br>T_{n-k}+N_i<br><br>\left(\mathrm{nT}\right)$

wherein m is an index of a respective one of said plurality of transmitters that is transmitting a non-zero value at time nT, T_n-k is a corresponding one of said plurality of known values, and N_i(nT) is noise value for receiver i, thereby obtaining a plurality of equations; and

solve said plurality of equations to obtain a plurality of solutions; and

average said plurality of solutions to obtain said plurality of time domain channel response values ch_mi(kT).

51. The apparatus of claim 50 wherein said signal comprises an Orthogonal Frequency Division Multiplexing (OFDM) signal.

52. The apparatus of claim 50 wherein each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to said respective receiver.

53. The apparatus of claim 50 wherein said respective receiver i comprises one of a plurality of receivers and each of said plurality of transmitters transmits via at least one sub-carrier channel that is unique to said each transmitter and to a respective one of said plurality of receivers.

54. The apparatus of claim 50 wherein said portion of said signals comprises a preamble portion.

55. The apparatus of claim 50 wherein said plurality of transmitters comprises two transmitters, one of said two transmitters having m=1 and transmitting values {T₀, T₁, . . . , T₅₁₁} during a first part of an OFDM interval and transmitting zero values during a remainder of said OFDM interval, another of said two transmitters having m=2 and transmitting values {T₅₁₂, T₅₁₃, . . . , T₁₀₂₃} during a second part of said OFDM interval and transmitting zero values during a further remainder of said OFDM interval, and for receiver i=1, said plurality of sample values r_i(nT) is defined according to the relations:

56. The apparatus of claim 50 further comprising means for performing a fast Fourier transform (FFT) on said plurality of time domain channel response values ch_mi(kT) to obtain a plurality of frequency domain channel response values.

57. An apparatus for estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said apparatus comprising:

means for extracting said portion of said signal to obtain a frequency domain data vector;

means for multiply each element of said frequency domain data vector by a respective element of a known vector to obtain a channel response vector, each element in said known vector being a function of a respective one of said plurality of known values, each element in said channel response vector being indicative of a distortion value;

means for selecting respective ones of said channel response vector to form a plurality of further channel response vectors, each of said further channel response vectors corresponding to a respective one of said plurality of transmitters, and inserting, within said each of said further channel response vectors, at least one zero value before or after each non-zero valued element therein such that said elements of said each of said further channel response vectors correspond to a transmission pattern of said corresponding one of said plurality of transmitters;

means for interpolating said elements of said further channel response vectors to obtain a plurality of distortion values each corresponding to a respective zero value element of said further channel response vectors, and for substituting said plurality of distortion values in place of said corresponding zero value element;

means for performing an inverse fast Fourier transform (IFFT) on each of said further channel response vectors to obtain a plurality of time domain further channel response vectors;

means for substituting a zero value in place of any element of said time domain further channel response vectors that has a value indicative of signal noise; and

means for performing an fast Fourier transform (FFT) on said time domain further channel response vectors to obtain a plurality of frequency response vectors.

58. An apparatus for estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said apparatus comprising:

means for sampling said portion of said signal to obtain a frequency domain data vector;

means for multiplying said frequency domain data vector by a known array to obtain a channel response vector, each element in said known array being a function of a respective one of said plurality of known values, each element in said channel response vector being indicative of a distortion value;

means for interpolating said elements of said channel response vector to obtain a least one distortion value corresponding to a respective zero value element of said further channel response vectors, and for substituting said plurality of distortion values in place of said corresponding zero value element;

means for performing an inverse fast Fourier transform (IFFT) on said channel response vector to obtain a frequency domain channel response vector;

means for substituting a zero value in place of any element of said time domain further channel response vectors that has a value indicative of signal noise; and

means for performing an fast Fourier transform (FFT) on said time domain further channel response vectors to obtain a plurality of frequency response vectors.

59. An apparatus for estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said apparatus comprising:

means for sampling said portion of said signal during at least portion of plurality of times nT, wherein n=0, 1, . . . , 1023, to obtain a plurality of sample values r_i(nT), wherein i is an index of said respective receiver;

means for defining each of said plurality of sample values r_i(nT) as a function of a plurality of time domain channel response values ch_mi(kT)according to the relation;

$r_i<br><br>\left(\mathrm{nT}\right)=\underset{k=22}{\overset{22}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{ch}}_{\mathrm{mi}}<br><br>\left(\mathrm{kT}\right)<br><br>T_{n-k}+N_i<br><br>\left(\mathrm{nT}\right),$

wherein m is an index of a respective one of said plurality of transmitters that is transmitting a non-zero value at time nT, T_n-k is a corresponding one of said plurality of known values, and N_i(nT) is noise value for receiver i, thereby obtaining a plurality of equations;

means for solving said plurality of equations to obtain a plurality of solutions; and

means for averaging said plurality of solutions to obtain said plurality of time domain channel response values ch_ml(kT).