Channel estimation for an OFDM communication system with inactive subbands

• US 8,391,413 B2
• Filed: 12/19/2003
• Issued: 03/05/2013
• Est. Priority Date: 12/19/2003
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Mantravadi, Ashok, Khandekar, Aamod, Teague, Edward H., Kadous, Tamer
• Primary Examiner(s): Puente, Eva
• Application Number:
  US10/741,524
• Publication Number:
  US 20050135509A1
• Time in Patent Office: 3,364 Days
• Field of Search: 375/340, 375/354, 375/227, 375/316, 375/260, 375/144, 370/208, 370/206, 370/203, 370/210
• US Class Current: 375/316
• CPC Class Codes: H04L 25/0218 with detection of nulls
• CPC Class Codes: H04L 25/022 of frequency response
• CPC Class Codes: H04L 25/0232 by interpolation between sounding signals

Abstract

For channel estimation in a spectrally shaped wireless communication system, an initial frequency response estimate is obtained for a first set of P uniformly spaced subbands (1) based on pilot symbols received on a second set of subbands used for pilot transmission and (2) using extrapolation and/or interpolation, where P is a power of two. A channel impulse response estimate is obtained by performing a P-point IFFT on the initial frequency response estimate. A final frequency response estimate for N total subbands is derived by (1) setting low quality taps for the channel impulse response estimate to zero, (2) zero-padding the channel impulse response estimate to length N, and (3) performing an N-point FFT on the zero-padded channel impulse response estimate. The channel frequency/impulse response estimate may be filtered to obtain a higher quality channel estimate.

Claims

1. A method of efficiently estimating a frequency response of a wireless channel in a wireless OFDM communication system in which only M subbands centered among N total subbands are used for data/pilot transmission with N−

M subbands at either end comprising inactive subbands, the method comprising;

(a) obtaining an initial frequency response estimate for subbands in a first set of uniformly spaced apart by N/P subbands based on channel gain estimates for a second set of subbands within the centered M subbands, where P is an integer greater than one and N is total subbands, and wherein the first set includes at least one subband not included in the second set;

(b) deriving a time-domain channel impulse response estimate for the wireless channel based on the initial frequency response estimate; and

(c) deriving a final frequency response estimate for the wireless channel based on the channel impulse response estimate.

2. The method of claim 1, further comprising:

deriving the channel gain estimates for the second set of subbands based on pilot symbols received on the subbands in the second set.

3. The method of claim 1, wherein the deriving the time-domain channel impulse response estimate includes performing a P-point inverse fast Fourier transform (IFFT) on the initial frequency response estimate to obtain the channel impulse response estimate.

4. The method of claim 1, wherein the deriving a final frequency response estimate includes zero padding the channel impulse response estimate to length S, where S is an integer greater than or equal to P and is a power of two, and

performing an S-point fast Fourier transform (FFT) on the zero-padded channel impulse response estimate to obtain the final frequency response estimate.

5. The method of claim 4, wherein S is equal to total number of subbands in the system.

6. The method of claim 1, further comprising:

performing extrapolation based on the received pilot symbols to obtain at least one channel gain estimate for the at least one subband not included in the second set, and wherein the initial frequency response estimate includes the at least one channel gain estimate.

7. The method of claim 1, further comprising:

performing interpolation based on the received pilot symbols to obtain at least one channel gain estimate for the at least one subband not included in the second set, and wherein the initial frequency response estimate includes the at least one channel gain estimate.

8. The method of claim 1, further comprising:

obtaining a channel gain estimate for each of the at least one subband based on a channel gain estimate for a nearest subband.

9. The method of claim 1, further comprising:

obtaining a channel gain estimate for each of the at least one subband based on a weighted sum of the channel gain estimates for the second set of subbands.

10. The method of claim 1, wherein the channel impulse response estimate includes P taps, and wherein selected ones of the P taps are set to zero.

11. The method of claim 1, further comprising:

filtering the channel impulse response estimate, and wherein the final frequency response estimate is derived based on the filtered channel impulse response estimate.

12. The method of claim 1, further comprising:

filtering the final frequency response estimate to obtain a higher quality frequency response estimate for the wireless channel.

13. The method of claim 1, wherein the wireless communication system is an orthogonal frequency division multiplexing (OFDM) communication system.

31. The method of claim 1, further comprising:

obtaining a channel gain estimate for each of the at least one subband based on a weighted sum of the channel gain estimates for the second set of subbands, wherein the weighted sum is determined based on a least-squares or a minimum mean square error (MMSE) criterion.

14. An apparatus in a wireless communication system for efficiently estimating a frequency response of a wireless channel in a wireless OFDM communication system in which only M subbands centered among N total subbands are used for data/pilot transmission with N−

M subbands at either end comprising inactive subbands, comprising;

a demodulator operative to provide received symbols; and

a processor operative to obtain an initial frequency response estimate for subbands in a first set of uniformly spaced apart by N/P subbands based on channel gain estimates for a second set of subbands within the centered M subbands derived from the received symbols, where P is an integer greater than one and N is total subbands, wherein the first set includes at least one subband not included in the second set,

derive a time-domain channel impulse response estimate for the wireless channel based on the initial frequency response estimate, and

derive a final frequency response estimate for the wireless channel based on the channel impulse response estimate.

15. The apparatus of claim 14, wherein the processor is further operative to perform extrapolation or interpolation based on the received pilot symbols to obtain at least one channel gain estimate for the at least one subband not included in the second set, and wherein the initial frequency response estimate includes the at least one channel gain estimate.

16. The apparatus of claim 14, wherein the processor is further operative to set selected ones of P taps for the channel impulse response estimate to zero.

17. The apparatus of claim 14, wherein the processor is further operative to filter the channel impulse response estimate, and wherein the final frequency response estimate is derived based on the filtered channel impulse response estimate.

18. An apparatus in a wireless communication system for efficiently estimating a frequency response of a wireless channel in a wireless OFDM communication system in which only M subbands centered among N total subbands are used for data/pilot transmission with N−

M subbands at either end comprising inactive subbands, comprising;

means for obtaining an initial frequency response estimate for subbands in a first set of uniformly spaced apart by N/P subbands based on channel gain estimates for a second set of subbands within the centered M subbands, where P is an integer greater than one and N is total subbands, and wherein the first set includes at least one subband not included in the second set;

means for deriving a time-domain channel impulse response estimate for the wireless channel based on the initial frequency response estimate; and

means for deriving a final frequency response estimate for the wireless channel based on the channel impulse response estimate.

19. The apparatus of claim 18, further comprising:

means for performing extrapolation based on the received pilot symbols to obtain at least one channel gain estimate for the at least one subband not included in the second set, and wherein the initial frequency response estimate includes the at least one channel gain estimate.

20. The apparatus of claim 18, further comprising:

means for setting selected ones of P taps for the channel impulse response estimate to zero.

21. The apparatus of claim 18, further comprising:

means for filtering the channel impulse response estimate, and wherein the final frequency response estimate is derived based on the filtered channel impulse response estimate.

22. A method of efficiently estimating a frequency response of a wireless channel in a wireless OFDM communication system in which only M subbands centered among N total subbands are used for data/pilot transmission with N−

M subbands at either end comprising inactive subbands, comprising;

a) obtaining an initial frequency response estimate for subbands in a first set of uniformly spaced apart by N/P subbands based on channel gain estimates for a second set of subbands within the centered M subbands, where P is an integer greater than one and N is total subbands, and wherein the first set includes at least one subband not included in the second set;

b) deriving a time-domain channel impulse response estimate for the wireless channel based on the initial frequency response estimate;

c) filtering the channel impulse response estimate over a plurality of symbol periods; and

d) deriving a final frequency response estimate for the wireless channel based on the filtered channel impulse response estimate.

23. The method of claim 22, wherein the filtering of the channel impulse response estimate is performed separately for each of L taps, where L is an integer greater than one and less than P.

24. The method of claim 22, wherein the filtering is based on a finite impulse response (FIR) filter or an infinite impulse response (IIR) filter.

25. The method of claim 22, wherein the filtering is based on a causal filter.

26. The method of claim 22, wherein the filtering is based on a non-causal filter.

27. A method of efficiently estimating a frequency response of a wireless channel in an orthogonal frequency division multiplexing (OFDM) communication system in which only M subbands centered among N total subbands are used for data/pilot transmission with N−

M subbands at either end comprising inactive subbands, the method comprising;

obtaining an initial frequency response estimate for subbands in a first set of uniformly spaced apart by N/P subbands based on channel gain estimates derived from pilot symbols received on a second set of subbands within the centered M subbands, where P is an integer greater than one and N is total subbands, and wherein the first set includes at least one subband not included in the second set;

performing a P-point inverse fast Fourier transform (IFFT) on the initial frequency response estimate to obtain a time-domain channel impulse response estimate;

zero padding the channel impulse response estimate to length N, where N is an integer greater than P and is a power of two; and

performing an N-point fast Fourier transform (FFT) on the zero-padded channel impulse response estimate to obtain a final frequency response estimate for the wireless channel.

28. The method of claim 27, further comprising:

performing extrapolation based on the received pilot symbols to obtain at least one channel gain estimate for the at least one subband not included in the second set, and wherein the initial frequency response estimate includes the at least one channel gain estimate.

29. The method of claim 27, further comprising:

selecting and setting P−

L of the P taps to zero, wherein P and L are integers and L less than P.

30. A processor readable memory for storing instructions operable to:

efficiently derive an initial frequency response estimate of a wireless channel in a wireless OFDM communication system, in which only M subbands centered among N total subbands are used for data/pilot transmission with N−

M subbands at either end comprising inactive subbands for a first set of uniformly spaced apart by N/P subbands in an orthogonal frequency division multiplexing (OFDM) communication system based on channel gain estimates for a second set of subbands within the centered M subbands, where P is an integer greater than one and N is total subbands, and wherein the first set includes at least one subband not included in the second set;

perform a P-point inverse fast Fourier transform (IFFT) on the initial frequency response estimate to obtain a time-domain channel impulse response estimate;

zero pad the channel impulse response estimate to length N, where N is an integer greater than P and is a power of two; and

perform an N-point fast Fourier transform (FFT) on the zero-padded channel impulse response estimate to obtain a final frequency response estimate for a wireless channel in the system.

32. A non-transitory computer readable medium having computer executable instructions stored thereon, which when executed by a machine cause the machine to perform operations comprising:

(a) obtaining an initial frequency response estimate for subbands in a first set of uniformly spaced apart by N/P subbands based on channel gain estimates for a second set of subbands within the centered M subbands, where P is an integer greater than one and N is total subbands, and wherein the first set includes at least one subband not included in the second set;

(b) deriving a time-domain channel impulse response estimate for the wireless channel based on the initial frequency response estimate; and

(c) deriving a final frequency response estimate for the wireless channel based on the channel impulse response estimate.

33. The non-transitory computer readable medium of claim 32, having further computer executable instructions stored thereon, which when executed by the machine cause the machine to perform further operations comprising:

deriving the channel gain estimates for the second set of subbands based on pilot symbols received on the subbands in the second set.

34. The non-transitory computer readable medium of claim 32, wherein the deriving the time-domain channel impulse response estimate includes performing a P-point inverse fast Fourier transform (IFFT) on the initial frequency response estimate to obtain the channel impulse response estimate.

35. The non-transitory computer readable medium of claim 32, wherein the deriving a final frequency response estimate includes zero padding the channel impulse response estimate to length S, where S is an integer greater than or equal to P and is a power of two, and

performing an S-point fast Fourier transform (FFT) on the zero-padded channel impulse response estimate to obtain the final frequency response estimate.

36. The non-transitory computer readable medium of claim 35, wherein S is equal to total number of subbands in the system.

37. The non-transitory computer readable medium of claim 32, having further computer executable instructions stored thereon, which when executed by the machine cause the machine to perform further operations comprising:

performing extrapolation based on the received pilot symbols to obtain at least one channel gain estimate for the at least one subband not included in the second set, and wherein the initial frequency response estimate includes the at least one channel gain estimate.

38. The non-transitory computer readable medium of claim 32, having further computer executable instructions stored thereon, which when executed by the machine cause the machine to perform further operations comprising:

performing interpolation based on the received pilot symbols to obtain at least one channel gain estimate for the at least one subband not included in the second set, and wherein the initial frequency response estimate includes the at least one channel gain estimate.

39. The non-transitory computer readable medium of claim 32, having further computer executable instructions stored thereon, which when executed by the machine cause the machine to perform further operations comprising:

obtaining a channel gain estimate for each of the at least one subband based on a channel gain estimate for a nearest subband.

40. The non-transitory computer readable medium of claim 32, having further computer executable instructions stored thereon, which when executed by the machine cause the machine to perform further operations comprising:

obtaining a channel gain estimate for each of the at least one subband based on a weighted sum of the channel gain estimates for the second set of subbands.

41. The non-transitory computer readable medium of claim 32, wherein the channel impulse response estimate includes P taps, and wherein selected ones of the P taps are set to zero.

42. The non-transitory computer readable medium of claim 32, having further computer executable instructions stored thereon, which when executed by the machine cause the machine to perform further operations comprising:

filtering the channel impulse response estimate, and wherein the final frequency response estimate is derived based on the filtered channel impulse response estimate.

43. The non-transitory computer readable medium of claim 32, having further computer executable instructions stored thereon, which when executed by the machine cause the machine to perform further operations comprising:

filtering the final frequency response estimate to obtain a higher quality frequency response estimate for the wireless channel.

44. The non-transitory computer readable medium of claim 32, wherein the wireless communication system is an orthogonal frequency division multiplexing (OFDM) communication system.

45. The non-transitory computer readable medium of claim 32, having further computer executable instructions stored thereon, which when executed by the machine cause the machine to perform further operations comprising:

obtaining a channel gain estimate for each of the at least one subband based on a weighted sum of the channel gain estimates for the second set of subbands, wherein the weighted sum is determined based on a least-squares or a minimum mean square error (MMSE) criterion.