Channel estimation for OFDM communication systems

• US 20050170783A1
• Filed: 04/07/2005
• Published: 08/04/2005
• Est. Priority Date: 10/29/2002
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Krishnan, Ranganathan, Kadous, Tamer
• Granted Patent: US 7,463,576 B2
• Time in Patent Office: Days
• US Class Current: 455/67.110
• CPC Class Codes: H04L 25/0212 of impulse response
• CPC Class Codes: H04L 25/022 of frequency response
• CPC Class Codes: H04L 25/023 with extension to other symbols
• CPC Class Codes: H04W 52/24 using SIR [Signal to Interference Ratio] or other wireless path parameters
• CPC Class Codes: H04W 52/325 Power control of control or pilot channels
• CPC Class Codes: H04W 52/34 TPC management, i.e. sharing limited amount of power among users or channels or data types, e.g. cell loading
• CPC Class Codes: H04W 52/42 in systems with time, space, frequency or polarisation diversity

Abstract

Techniques to estimate the frequency response of a wireless channel in an OFDM system. In one method, an initial estimate of the frequency response of the wireless channel is obtained for a first group of subbands based on a pilot transmission received via the subbands in the first group. An estimate of the impulse response of the wireless channel is then derived based on the initial frequency response estimate. An enhanced estimate of the frequency response of the wireless channel is then derived for a second group of subbands based on the impulse response estimate. The first and second groups may each include all or only a subset of the usable subbands. Subband multiplexing may be used to allow simultaneous pilot transmissions by multiple terminals on their associated groups of subbands.

Claims

1. A method for estimating a frequency response of a wireless channel, comprising

obtaining an initial estimate of the frequency response of the wireless channel for a first group of subbands;

and

deriving an enhanced estimate of the frequency response of the wireless channel for a second group of subbands based on the initial frequency response estimate, wherein the enhanced frequency response estimate is derived based on an estimate of an impulse response of the wireless channel that is implicitly or explicitly derived based on the initial frequency response estimate.

2. The method of claim 1, wherein the second group includes the subbands in the first group.

3. The method of claim 2, wherein the second group further includes at least one additional subband not in the first group.

4. The method of claim 1, wherein the initial channel frequency response estimate is obtained based on a pilot transmission received via the subbands in the first group.

5. The method of claim 4, wherein the initial frequency response estimate is obtained based on ratios of received pilot symbols over transmitted pilot symbols.

6. The method of claim 1, wherein the impulse response estimate is derived based on a least square estimate.

7. The method of claim 1, further comprising:

forming a first discrete Fourier transform (DFT) matrix for the subbands in the first group, and wherein the impulse response estimate is implicitly or explicitly derived based also on the first DFT matrix.

8. The method of claim 7, further comprising:

forming a second DFT matrix for the subbands in the second group, and wherein the enhanced frequency response estimate is derived based also on the second DFT matrix.

9. The method of claim 1, wherein the first group of subbands is a subset of all subbands usable for data transmission.

10. The method of claim 9, wherein the subbands in the first group are uniformly distributed across the usable subbands.

11. The method of claim 1, wherein transmit power for the subbands in the first group is scaled to maintain a total transmit power at a maximum allowed transmit power level.

12. The method of claim 11, wherein the transmit power for each subband in the first group is increased by a ratio of M/S relative to an average power level obtained by uniformly allocating the maximum allowed transmit power to all M subbands, where M is the number of subbands usable for data transmission and S is the number of subbands in the first group.

13. The method of claim 1, wherein the first group includes all subbands usable for data transmission.

14. The method of claim 1, wherein the second group includes all subbands usable for data transmission.

15. The method of claim 1, wherein the first group includes S subbands and the impulse response estimate for the wireless channel includes L taps, and wherein S is greater than or equal to L.

16. The method of claim 15, wherein S is approximately equal to 1.1*L.

17. The method of claim 1, wherein the subbands in the first and second groups are orthogonal subbands provided by orthogonal frequency division multiplexing (OFDM).

18. A method for estimating a frequency response of a wireless channel in an orthogonal frequency division multiplexing (OFDM) communication system, comprising

obtaining an initial estimate of the frequency response of the wireless channel for a first group of subbands based on a pilot transmission received via the subbands in the first group, wherein the first group of subbands is a subset of all subbands usable for data transmission;

and

deriving an enhanced estimate of the frequency response of the wireless channel for the usable subbands based on the initial frequency response estimate, wherein the enhanced frequency response estimate is derived based on an estimate of an impulse response of the wireless channel that is implicitly or explicitly derived based on the initial frequency response estimate, and wherein the enhanced frequency response estimate is further derived based on a first discrete Fourier transform (DFT) matrix for the subbands in the first group and a second DFT matrix for the usable subbands.

19. A method for estimating a frequency response of a wireless channel for each of a plurality of terminals in a wireless communication system, comprising

obtaining an initial estimate of the frequency response of the wireless channel for each of the plurality of terminals, wherein each of the plurality of terminal is associated with a respective one of a plurality of disjoint groups of subbands, and wherein the initial frequency response estimate for each terminal is obtained for the associated group of subbands based on a pilot transmission received via the subbands in the associated group;

and

deriving an enhanced estimate of the frequency response of the wireless channel for each of the plurality of terminals based on the initial frequency response estimate for the terminal, wherein the enhanced frequency response estimate for each terminal covers a particular set of subbands and is derived based on an estimate of an impulse response of the wireless channel for the terminal that is implicitly or explicitly derived based on the initial frequency response estimate for the terminal.

20. The method of claim 19, wherein the plurality of disjoint groups of subbands are formed from a plurality of usable subbands, and wherein the subbands in each of the plurality of disjoint groups are uniformly distributed across the plurality of usable subbands.

21. An apparatus operable to estimate a frequency response of a wireless channel, comprising

means for obtaining an initial estimate of the frequency response of the wireless channel for a first group of subbands;

and

means for deriving an enhanced estimate of the frequency response of the wireless channel for a second group of subbands based on the impulse response estimate, wherein the enhanced frequency response estimate is derived based on an estimate of an impulse response of the wireless channel that is implicitly or explicitly derived based on the initial frequency response estimate.

22. The apparatus of claim 21, wherein the second group includes the subbands in the first group and at least one additional subband not in the first group.

23. The apparatus of claim 21, wherein the impulse response estimate is derived based also on a first discrete Fourier transform (DFT) matrix for the subbands in the first group, and wherein the enhanced frequency response estimate is derived based also on a second DFT matrix for the subbands in the second group.

24. An access point in a wireless communication system, comprising:

a demodulator operative to receive pilot transmission from one or more terminals, wherein a plurality of disjoint groups of subbands are formed from a plurality of usable subbands, and wherein each of the one or more terminals transmits a pilot on a specific group of subbands that is selected from the plurality of disjoint groups of subbands and assigned to the terminal; and

a controller operative to obtain an initial estimate of a frequency response of an uplink channel for each of the one or more terminals, wherein the initial frequency response estimate for each terminal covers the group of subbands assigned to the terminal and is obtained based on the pilot transmission received from the terminal, and

derive an enhanced estimate of the frequency response of the uplink channel for each of the one or more terminals based on the initial frequency response estimate for the terminal, wherein the enhanced frequency response estimate for each terminal is derived based on an estimate of an impulse response of the wireless channel for the terminal that is implicitly or explicitly derived based on the initial frequency response estimate for the terminal.

25. The access point of claim 24, wherein the subbands in each of the plurality of disjoint groups are uniformly distributed across the plurality of usable subbands.

26. A terminal in a wireless communication system, comprising:

a demodulator operative to receive a pilot transmission on a first group of subbands; and

a controller operative to

obtain an initial estimate of a frequency response of a downlink channel for the first group of subbands based on the pilot transmission received via the subbands in the first group, and

derive an enhanced estimate of the frequency response of the downlink channel for a second group of subbands based on the initial frequency response estimate, wherein the enhanced frequency response estimate is derived based on an estimate of an impulse response of the wireless channel that is implicitly or explicitly derived based on the initial frequency response estimate.

27. The terminal of claim 26, wherein the second group includes the subbands in the first group and at least one additional subband not in the first group.