Pilot transmission and channel estimation for multiple transmitters

• US 7,418,046 B2
• Filed: 12/22/2004
• Issued: 08/26/2008
• Est. Priority Date: 07/22/2004
• Status: Expired due to Fees

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Gore, Dhananjay Ashok, Agrawal, Avneesh, Kadous, Tamer
• Primary Examiner(s): Phu; Phuong
• Application Number:
  US11/022,146
• Publication Number:
  US 20060018411A1
• Time in Patent Office: 1,343 Days
• Field of Search: 375/140, 375/144, 375/346, 375/349, 375/350, 370/328, 370/441
• US Class Current: 375/260
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/0212 of impulse response
• CPC Class Codes: H04L 25/0226 sounding signals per se
• CPC Class Codes: H04L 27/261 Details of reference signals

Abstract

Each transmitter is assigned a time-only pilot code, a frequency-only pilot code, or a time-frequency pilot code to use for pilot transmission. The pilot codes may be pseudo-random, orthogonal, and/or cyclic-shift codes. To obtain a channel estimate for a transmitter using a time-frequency pilot code composed of a time-only code and a frequency-only code, a receiver multiplies a set of received symbols for each symbol period with a set of code values for the frequency-only code to obtain a set of detected symbols and performs an IDFT on the set of detected symbols to obtain an initial impulse response estimate. The receiver performs code matching on multiple initial impulse response estimates derived for multiple symbol periods with the time-only code to obtain a final impulse response estimate for the desired transmitter. The receiver retains the first L channel taps and zeroes out remaining channel taps, where L is the expected channel length.

Claims

1. A method of performing channel estimation in a wireless communication system, comprising:

deriving a plurality of initial impulse response estimates for a plurality of symbol periods; and

performing code matching on the plurality of initial impulse response estimates with a first code for a transmitter to obtain a final impulse response estimate for the transmitter.

2. The method of claim 1, wherein the deriving the plurality of initial impulse response estimates comprises

obtaining a set of received symbols for a set of frequency subbands in each of the plurality of symbol periods, and

performing an inverse transform on the set of received symbols for each symbol period to derive an initial impulse response estimate for the symbol period.

3. The method of claim 1, wherein the deriving the plurality of initial impulse response estimates comprises

obtaining a set of received symbols for a set of frequency subbands in each of the plurality of symbol periods, wherein at least two different sets of frequency subbands are used for pilot transmission, and

performing an inverse transform on the set of received symbols for each symbol period to derive an initial impulse response estimate for the symbol period.

4. The method of claim 1, further comprising:

removing a phase ramp in each of the plurality of initial impulse response estimates.

5. The method of claim 1, further comprising:

determining a set of frequency subbands used for pilot transmission in each symbol period, wherein at least two different sets of frequency subbands are used for pilot transmission in the plurality of symbol periods; and

removing a phase ramp in each initial impulse response estimate for each symbol period based on the set of frequency subbands used for pilot transmission in the symbol period.

6. The method of claim 1, wherein the performing code matching comprises, for each tap position among a plurality of tap positions,

identifying a plurality of channel taps at the tap position in the plurality of initial impulse response estimates,

multiplying the plurality of channel taps with a plurality of code values for the first code to obtain a plurality of intermediate values, and

accumulating the plurality of intermediate values to obtain a channel tap at the tap position in the final impulse response estimate.

7. The method of claim 1, further comprising:

deriving a plurality of sets of detected symbols for the plurality of symbol periods based on a second code for the transmitter.

8. The method of claim 7, wherein the deriving the plurality of initial impulse response estimates comprises

performing an inverse transform on a set of detected symbols for each symbol period to derive an initial impulse response estimate for the symbol period.

9. The method of claim 7, wherein the deriving the plurality of sets of detected symbols comprises, for each of the plurality of symbol periods,

determining a set of code values based on the second code and a set of frequency subbands used for pilot transmission in the symbol period, and

multiplying a set of received symbols for the symbol period with the set of code values to obtain a set of detected symbols for the symbol period.

10. The method of claim 1, further comprising:

retaining first L channel taps in the final impulse response estimate, where L is an integer greater than one; and

setting remaining channel taps in the final impulse response estimate to zero.

11. The method of claim 1, further comprising:

setting channel taps in the final impulse response estimate with magnitude below a predetermined threshold to zero.

12. An apparatus in a wireless communication system, comprising:

a channel estimator operative to derive a plurality of initial impulse response estimates for a plurality of symbol periods and to perform code matching on the plurality of initial impulse response estimates with a first code for a transmitter to obtain a final impulse response estimate for the transmitter.

13. The apparatus of claim 12, further comprising:

a demodulator operative to provide a set of received symbols for a set of frequency subbands in each of the plurality of symbol periods, and

wherein the channel estimator is operative to perform an inverse transform on the set of received symbols for each symbol period to derive an initial impulse response estimate for the symbol period.

14. The apparatus of claim 12, wherein the channel estimator is operative to, for each tap position among a plurality of tap positions,

identify a plurality of channel taps at the tap position in the plurality of initial impulse response estimates,

multiply the plurality of channel taps with a plurality of code values for the first code to obtain a plurality of intermediate values, and

accumulate the plurality of intermediate values to obtain a channel tap at the tap position in the final impulse response estimate.

15. The apparatus of claim 12, wherein the channel estimator is operative to retain first L channel taps in the final impulse response estimate and to set remaining channel taps in the final impulse response estimate to zero, where L is an integer greater than one.

16. The apparatus of claim 12, wherein the channel estimator is operative to derive a plurality of sets of detected symbols for the plurality of symbol periods based on a second code for the transmitter and to perform an inverse transform on a set of detected symbols for each symbol period to derive an initial impulse response estimate for the symbol period.

17. The apparatus of claim 12, wherein the first code for the transmitter is orthogonal to at least one other first code for at least one other transmitter.

18. The apparatus of claim 12, wherein the first code for the transmitter is pseudo-random with respect to at least one other first code for at least one other transmitter.

19. The apparatus of claim 16, wherein the second code for the transmitter is pseudo-random with respect to at least one other second code for at least one other transmitter.

20. The apparatus of claim 16, wherein the second code for the transmitter is a cyclic-shift code.

21. An apparatus in a wireless communication system, comprising:

means for deriving a plurality of initial impulse response estimates for a plurality of symbol periods; and

means for performing code matching on the plurality of initial impulse response estimates with a first code for a transmitter to obtain a final impulse response estimate for the transmitter.

22. The apparatus of claim 21, wherein the means for deriving the plurality of initial impulse response estimates comprises

means for obtaining a set of received symbols for a set of frequency subbands in each of the plurality of symbol periods, and

means for performing an inverse transform on the set of received symbols for each symbol period to derive an initial impulse response estimate for the symbol period.

23. The apparatus of claim 21, wherein the means for performing code matching comprises, for each tap position among a plurality of tap positions,

means for identifying a plurality of channel taps at the tap position in the plurality of initial impulse response estimates,

means for multiplying the plurality of channel taps with a plurality of code values for the first code to obtain a plurality of intermediate values, and

means for accumulating the plurality of intermediate values to obtain a channel tap at the tap position in the final impulse response estimate.

24. The apparatus of claim 21, further comprising:

means for retaining first L channel taps in the final impulse response estimate, where L is an integer greater than one; and

means for setting remaining channel taps in the final impulse response estimate to zero.

25. The apparatus of claim 21, further comprising:

means for deriving a plurality of sets of detected symbols for the plurality of symbol periods based on a second code for the transmitter, and

wherein the means for deriving the plurality of initial impulse response estimates comprises means for performing an inverse transform on a set of detected symbols for each symbol period to derive an initial impulse response estimate for the symbol period.

26. An apparatus in a wireless communication system, comprising:

a demodulator operative to provide a plurality of sets of received symbols for a plurality of symbol periods; and

a channel estimator operative to derive a plurality of sets of detected symbols based on the plurality of sets of received symbols and a cyclic-shift code for a transmitter, to derive at least one initial impulse response estimate based on the plurality of sets of detected symbols, and to derive a final impulse response estimate for the transmitter based on the at least one initial impulse response estimate.

27. The apparatus of claim 26, wherein the channel estimator is operative to form a sequence of detected symbols with the plurality of sets of detected symbols and to perform an inverse transform on the sequence of detected symbols to derive a single initial impulse response estimate.

28. The apparatus of claim 26, wherein the channel estimator is operative to derive a plurality of initial impulse response estimates for a plurality of code values for a second code for the transmitter and to perform code matching on the plurality of initial impulse response estimates with the second code.

29. An apparatus in a wireless communication system, comprising:

means for deriving a plurality of sets of detected symbols based on a plurality of sets of received symbols for a plurality of symbol periods and a cyclic-shift code for a transmitter;

means for deriving at least one initial impulse response estimate based on the plurality of sets of detected symbols; and

means for deriving a final impulse response estimate for the transmitter based on the at least one initial impulse response estimate.

30. The apparatus of claim 29, wherein the means for deriving at least one initial impulse response estimate comprises

means for forming a sequence of detected symbols with the plurality of sets of detected symbols, and

means for performing an inverse transform on the sequence of detected symbols to derive a single initial impulse response estimate.

31. The apparatus of claim 29, wherein the means for deriving the at least one initial impulse response estimate comprises, for each code value among a plurality of code values for a second code for the transmitter,

means for forming a sequence of detected symbols with at least one set of detected symbols for which the code value applies, and means for performing an inverse transform on the sequence of detected symbols to derive an initial impulse response estimate for the code value.

32. The apparatus of claim 31, wherein the means for deriving the final impulse response estimate comprises

means for performing code matching on a plurality of initial impulse response estimates, derived for the plurality of code values, with the second code.