Channel calibration for a time division duplexed communication system

• US 20050128953A1
• Filed: 01/27/2005
• Published: 06/16/2005
• Est. Priority Date: 10/25/2002
• Status: Active Grant

• Current Assignee: John W. Ketchum, Mark S. Wallace, Steven J. Howard, Walton J. Rodney
• Original Assignee: John W. Ketchum, Mark S. Wallace, Steven J. Howard, Walton J. Rodney
• Inventors: Howard, Steven J., Ketchum, John W., Walton, J. Rodney, Wallace, Mark S.
• Granted Patent: US 8,570,988 B2
• Time in Patent Office: Days
• US Class Current: 370/241
• CPC Class Codes: H04B 17/21 for calibration; for correcting measurements
• CPC Class Codes: H04B 7/0421 utilizing implicit feedback, e.g. steered pilot signals
• CPC Class Codes: H04B 7/0615 of weighted versions of same signal
• CPC Class Codes: H04B 7/0619 using feedback from receiving side feedback signaling for adaptive modulation/coding H04L1/0001
• 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 7/0854 using error minimizing algorithms, e.g. minimum mean squared error [MMSE], "cross-correlation" or matrix inversion
• CPC Class Codes: H04L 1/0001 Systems modifying transmission characteristics according to link quality, e.g. power backoff adaptive data allocation for multicarrier modulation H04L5/0044; controlling transmission power for radio systems H04W52/04
• CPC Class Codes: H04L 1/0003 by switching between different modulation schemes
• CPC Class Codes: H04L 1/0009 by adapting the channel coding H04L1/1812 takes precedence
• CPC Class Codes: H04L 1/0017 where the mode-switching is based on Quality of Service requirement
• CPC Class Codes: H04L 1/0059 Convolutional codes
• CPC Class Codes: H04L 1/0061 Error detection codes
• CPC Class Codes: H04L 1/0068 by puncturing
• CPC Class Codes: H04L 1/0071 Use of interleaving interleaving per se H03M13/27
• CPC Class Codes: H04L 1/06 using space diversity
• CPC Class Codes: H04L 1/0618 Space-time coding
• CPC Class Codes: H04L 1/08 by repeating transmission, e.g. Verdan system H04L1/1858 and H04L1/189 take precedence
• CPC Class Codes: H04L 1/16 in which the return channel carries supervisory signals, e.g. repetition request signals
• CPC Class Codes: H04L 2001/0093 Point-to-multipoint
• CPC Class Codes: H04L 2025/03414 : Multicarrier
• CPC Class Codes: H04L 2025/03426 : transmission using multiple-input and multiple-output channels
• CPC Class Codes: H04L 25/0204 : of multiple channels
• CPC Class Codes: H04L 25/0224 : using sounding signals
• CPC Class Codes: H04L 25/0226 : sounding signals per se
• CPC Class Codes: H04L 25/0242 : using matrix methods
• CPC Class Codes: H04L 25/0244 : with inversion
• CPC Class Codes: H04L 25/03343 : Arrangements at the transmitter end
• CPC Class Codes: H04L 27/2601 : Multicarrier modulation systems
• CPC Class Codes: H04L 27/261 : Details of reference signals
• CPC Class Codes: H04L 27/2647 : Arrangements specific to the receiver only equalisation H04L27/01
• CPC Class Codes: H04L 5/1469 : using time-sharing
• CPC Class Codes: H04W 16/28 : using beam steering
• CPC Class Codes: H04W 52/42 : in systems with time, space, frequency or polarisation diversity
• CPC Class Codes: H04W 52/50 : at the moment of starting communication in a multiple access environment

Abstract

Techniques are described to calibrate the downlink and uplink channels to account for differences in the frequency responses of the transmit and receive chains at an access point and a user terminal. In one method, pilots are transmitted on the downlink and uplink channels and used to derive estimates of the downlink and uplink channel responses, respectively. Correction factors for the access point and correction factors for the user terminal are determined based on (e.g., by performing matrix-ratio computation or minimum mean square error (MMSE) computation on) the downlink and uplink channel response estimates. The correction factors for the access point and the correction factors for the user terminal are used to obtain a calibrated downlink channel and a calibrated uplink channel, which are transpose of one another. The calibration may be performed in real time based on over-the-air transmission.

Claims

1. A method of calibrating communication links in a wireless time division duplexed (TDD) communication system, comprising:

obtaining a channel response estimate for a downlink channel from an access point to a user terminal;

obtaining a channel response estimate for an uplink channel from the user terminal to the access point; and

determining correction factors for the access point and correction factors for the user terminal based on the channel response estimates for the downlink and uplink channels, the correction factors for the access point and the correction factors for the user terminal being used to obtain a calibrated downlink channel response and a calibrated uplink channel response.

2. The method of claim 1, further comprising:

applying the correction factors for the access point on a transmit side, or a receive side, or both the transmit and receive sides at the access point.

3. The method of claim 1, further comprising:

applying the correction factors for the user terminal on a transmit side, or a receive side, or both the transmit and receive sides at the user terminal.

4. The method of claim 1, wherein the determining the correction factors for the access point and the correction factors for the user terminal comprises determining the correction factors for the access point and the correction factors for the user terminal based on the following equation:

Ĥ

_up{circumflex over (K)}_ut=(Ĥ

_dn{circumflex over (K)}_ap)^T,

where Ĥ

_dn is a matrix for the channel response estimate for the downlink channel,

Ĥ

_up is a matrix for the channel response estimate for the uplink channel,

{circumflex over (K)}_ap is a matrix for the correction factors for the access point,

{circumflex over (K)}_ut is a matrix for the correction factors for the user terminal, and “

T”

denotes a transpose.

5. The method of claim 4, further comprising:

deriving correction factors for a transmit side of the access point and correction factors for a receive side of the access point based on the following equation;

{circumflex over (K)}_ap={circumflex over (K)}_tap{circumflex over (K)}_rap<sup>−

1</sup>,

where {circumflex over (K)}_tap is a matrix for the correction factors for the transmit side of the access point, and

{circumflex over (K)}_rap is a matrix for the correction factors for the receive side of the access point.

6. The method of claim 5, further comprising:

setting the matrix {circumflex over (K)}_tap or the matrix {circumflex over (K)}_rap to an identity matrix.

7. The method of claim 5, further comprising:

setting the matrix {circumflex over (K)}_tap or the matrix {circumflex over (K)}_rap to an arbitrary matrix.

8. The method of claim 4, further comprising:

deriving correction factors for a transmit side of the user terminal and correction factors for a receive side of the user terminal based on the following equation;

{circumflex over (K)}_ut={circumflex over (K)}_tut{circumflex over (K)}_rut<sup>−

1</sup>,

where {circumflex over (K)}_tut is a matrix for the correction factors for the transmit side of the user terminal, and

{circumflex over (K)}_rut is a matrix for the correction factors for the receive side of the user terminal.

9. The method of claim 4, wherein the determining the correction factors for the access point and the correction factors for the user terminal comprises

computing a matrix C as an element-wise ratio of the matrix Ĥ

_up over the matrix Ĥ

_dn, and

deriving the matrices {circumflex over (K)}_ap and {circumflex over (K)}_ut based on the matrix C.

10. The method of claim 9, wherein the deriving the matrix {circumflex over (K)}_ut comprises

normalizing each of a plurality of rows of the matrix C,

determining a mean of the plurality of normalized rows of the matrix C, and

forming the matrix {circumflex over (K)}_ut based on the mean of the plurality of normalized rows.

11. The method of claim 9, wherein the deriving the matrix {circumflex over (K)}_ap comprises

normalizing each of a plurality of columns of the matrix C,

determining a mean of inverses of the plurality of normalized columns of the matrix C, and

forming the matrix {circumflex over (K)}_ap based on the mean of the inverses of the plurality of normalized columns.

12. The method of claim 4, further comprising:

deriving the matrices {circumflex over (K)}_ap and +E,circ K_ut based on minimum mean square error (MMSE) computation.

13. The method of claim 4, further comprising:

deriving the matrices {circumflex over (K)}_ap and {circumflex over (K)}_ut based on minimum mean square error (MMSE) computation to minimize a mean square error (MSE) given by the following equation

|H_upK_up−

(H_dnK_ap)^T|².

14. The method of claim 1, wherein the determining the correction factors for the access point and the correction factors for the user terminal comprises

deriving a first set of matrices of correction factors for the access point for a first set of frequency subbands, and

interpolating the first set of matrices to obtain a second set of matrices of correction factors for the access point for a second set of frequency subbands.

15. The method of claim 1, wherein the determining the correction factors for the access point and the correction factors for the user terminal comprises

deriving a first set of matrices of correction factors for the user terminal for a first set of frequency subbands, and

interpolating the first set of matrices to obtain a second set of matrices of correction factors for the user terminal for a second set of frequency subbands.

16. The method of claim 1, further comprising:

transmitting a pilot on the uplink channel, wherein the uplink channel response estimate is derived based on the pilot transmitted on the uplink channel; and

receiving a pilot on the downlink channel, wherein the downlink channel response estimate is derived based on the pilot received on the downlink channel.

17. An apparatus in a wireless time division duplexed (TDD) communication system, comprising:

means for obtaining a channel response estimate for a downlink channel from an access point to a user terminal;

means for obtaining a channel response estimate for an uplink channel from the user terminal to the access point; and

means for determining correction factors for the access point and correction factors for the user terminal based on the channel response estimates for the downlink and uplink channels, the correction factors for the access point and the correction factors for the user terminal being used to obtain a calibrated downlink channel response and a calibrated uplink channel response.

18. The apparatus of claim 17, further comprising:

means for applying the correction factors for the access point on a transmit side, or a receive side, or both the transmit and receive sides at the access point.

19. The apparatus of claim 17, further comprising:

means for deriving correction factors for a transmit side of the access point and correction factors for a receive side of the access point based on the correction factors for the access point.

20. The apparatus of claim 17, further comprising:

means for applying the correction factors for the user terminal on a transmit side, or a receive side, or both the transmit and receive sides at the user terminal.

21. The apparatus of claim 17, further comprising:

means for deriving correction factors for a transmit side of the user terminal and correction factors for a receive side of the user terminal based on the correction factors for the user terminal.

22. The apparatus of claim 17, wherein the means for determining the correction factors for the access point and the correction factors for the user terminal comprises

means for performing minimum mean square error (MMSE) computation on the channel response estimates for the downlink and uplink channels to determine the correction factors for the access point and the correction factors for the user terminal.

23. The apparatus of claim 17, wherein the means for determining the correction factors for the access point and the correction factors for the user terminal comprises

means for performing matrix ratio computation on the channel response estimates for the downlink and uplink channels to determine the correction factors for the access point and the correction factors for the user terminal.

24. A method of calibrating communication links in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising:

transmitting a pilot on a first communication link from a first station to a second station;

obtaining a channel response estimate for the first communication link derived based on the pilot transmitted on the first communication link;

receiving a pilot on a second communication link from the second station;

deriving a channel response estimate for the second communication link based on the pilot received on the second communication link; and

determining correction factors for the first station and correction factors for the second station based on the channel response estimates for the first and second communication links, the correction factors for the first station and the correction factors for the second station being used to obtain a calibrated channel response for the first communication link and a calibrated channel response for the second communication link.

25. The method of claim 24, further comprising:

applying the correction factors for the first station on a transmit side, or a receive side, or both the transmit and receive sides at the first station.

26. The method of claim 24, further comprising:

sending the correction factors for the second station to the second station.

27. The method of claim 24, further comprising:

updating the correction factors for the first station based on calibration with a plurality of second stations.

28. An apparatus in a wireless time division duplexed (TDD) communication system, comprising:

a transmit spatial processor to transmit a first pilot on a first communication link from a first station to a second station;

a receive spatial processor to receive a second pilot on a second communication link from the second station, to derive a channel response estimate for the second communication link based on the received second pilot, and to receive a channel response estimate for the first communication link derived based on the transmitted first pilot; and

a controller to determine correction factors for the first station and correction factors for the second station based on the channel response estimates for the first and second communication links, the correction factors for the first station and the correction factors for the second station being used to obtain a calibrated channel response for the first communication link and a calibrated channel response for the second communication link.

29. The apparatus of claim 28, wherein the controller performs minimum mean square error (MMSE) computation on the channel response estimates for the first and second communication links to determine the correction factors for the first station and the correction factors for the second station.

30. The apparatus of claim 28, wherein the controller performs matrix-ratio computation on the channel response estimates for the first and second communication links to determine the correction factors for the first station and the correction factors for the second station.

31. The apparatus of claim 28, wherein the controller derives correction factors for the transmit spatial processor and correction factors for the receive spatial processor based on the correction factors for the first station.

32. The apparatus of claim 28, wherein the controller updates the correction factors for the first station based on calibration with a plurality of second stations.

33. A method of transmitting data in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising:

applying correction factors for a first station on a transmit side, or a receive side, or both the transmit and receive sides at the first station;

transmitting a pilot on a first communication link from the first station to a second station; and

receiving a data transmission sent on a second communication link from the second station to the first station, wherein the data transmission is spatially processed based on a channel response estimate for the first communication link derived from the pilot transmitted on the first communication link.

34. The method of claim 33, further comprising:

spatially processing the received data transmission with a matched filter.

35. The method of claim 33, wherein the receiving the data transmission sent on the second communication link comprises

receiving the data transmission sent on a plurality of eigenmodes of the second communication link.

36. The method of claim 33, wherein the second station applies correction factors on a transmit side, or a receive side, or both the transmit and receive sides at the second station.

37. An apparatus in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising:

means for applying correction factors for a first station on a transmit side, or a receive side, or both the transmit and receive sides at the first station;

means for transmitting a pilot on a first communication link from the first station to a second station; and

means for receiving a data transmission sent on a second communication link from the second station to the first station, wherein the data transmission is spatially processed based on a channel response estimate for the first communication link derived from the pilot transmitted on the first communication link.

38. An apparatus in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising:

a transmit processor to transmit a pilot on a first communication link from a first station to a second station; and

a receive processor to receive a data transmission sent on a second communication link from the second station to the first station, wherein the data transmission is spatially processed based on a channel response estimate for the first communication link derived from the pilot transmitted on the first communication link, and wherein the transmit processor applies correction factors to the transmitted pilot, or the receive processor applies correction factors to the received data transmission, or both the transmit processor applies correction factors to the transmitted pilot and the receive processor applies correction factors to the received data transmission.

39. A method of transmitting data in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising:

transmitting a pilot on a first communication link from a first station to a second station;

receiving a data transmission sent on a second communication link from the second station to the first station, wherein the second station applies correction factors on a transmit side, or a receive side, or both the transmit and receive sides at the second station, and wherein the data transmission is spatially processed based on a channel response estimate for the first communication link derived from the pilot transmitted on the first communication link; and

processing the received data transmission with a minimum mean square error (MMSE) receiver at the first station.

40. The method of claim 39, wherein the receiving the data transmission sent on the second communication link comprises

receiving the data transmission sent on a plurality of eigenmodes of the second communication link.