Channel calibration for a time division duplexed communication system

• US 20040085939A1
• Filed: 10/23/2003
• Published: 05/06/2004
• Est. Priority Date: 10/25/2002
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Howard, Steven J., Ketchum, John W., Walton, J. Rodney, Wallace, Mark S.
• Granted Patent: US 8,134,976 B2
• Time in Patent Office: Days
• US Class Current: 370/335
• CPC Class Codes: H04B 17/336 Signal-to-interference ratio [SIR] or carrier-to-interference ratio [CIR]
• 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 1/00 Arrangements for detecting or preventing errors in the information received correcting synchronisation H04L7/00
• 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 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 25/03006 : Arrangements for removing intersymbol interference
• 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 52/50 : at the moment of starting communication in a multiple access environment
• CPC Class Codes: H04W 74/04 : Scheduled access hybrid access H04W74/02

Abstract

Techniques 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 embodiment, pilots are transmitted on the downlink and uplink channels and used to derive estimates of the downlink and uplink channel responses, respectively. Two sets of correction factors are then determined based on the estimates of the downlink and uplink channel responses. A calibrated downlink channel is formed by using a first set of correction factors for the downlink channel, and a calibrated uplink channel is formed by using a second set of correction factors for the uplink channel. The first and second sets of correction factors may be determined using a matrix-ratio computation or a minimum mean square error (MMSE) computation. The calibration may be performed in real-time based on over-the-air transmission.

Claims

1. A method for calibrating downlink and uplink channels in a wireless communication system, comprising:

obtaining an estimate of a downlink channel response;

obtaining an estimate of an uplink channel response;

determining first and second sets of correction factors based on the estimates of the downlink and uplink channel responses; and

calibrating the downlink channel and uplink channel based on the first and second sets of correction factors, respectively, to form a calibrated downlink channel and a calibrated uplink channel.

2. The method of claim 1, wherein the first set of correction factors is used to scale symbols prior to transmission on the downlink channel and the second set of correction factors is used to scale symbols prior to transmission on the uplink channel.

3. The method of claim 1, wherein the first set of correction factors is used to scale symbols received on the downlink channel and the second set of correction factors is used to scale symbols received on the uplink channel.

4. The method of claim 1, wherein the first and second sets of correction factors are determined 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 estimate of the downlink channel response,

Ĥ

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

Ĥ

_ap is a matrix for the first set of correction factors,

Ĥ

_ut is a matrix for the second set of correction factors, and

“

^T”

the denotes a transpose.

5. The method of claim 4, wherein determining the first and second sets of correction factors includes:

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.

6. The method of claim 5, wherein the deriving the matrix {circumflex over (K)}_ut includes

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

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

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

7. The method of claim 5, wherein the deriving the matrix {circumflex over (K)}_ap includes

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

determining a mean of inverses of the plurality of normalized columns of the matrix C, and wherein the matrix {circumflex over (K)}_ap is formed based on the mean of the inverses of the plurality of normalized columns.

8. The method of claim 4, wherein the matrices {circumflex over (K)}_ut and {circumflex over (K)}_ap are derived based on a minimum mean square error (MMSE) computation.

9. The method of claim 8, wherein the MMSE computation minimizes a mean square error (MSE) given as

|Ĥ

_up{circumflex over (K)}_ut−

(Ĥ

_dn{circumflex over (K)}_ap)^T|².

10. The method of claim 1, further comprising:

determining a scaling value indicative of an average difference between the estimate of the downlink channel response and the estimate of the uplink channel response.

11. The method of claim 1, wherein the estimates for the downlink and uplink channel responses are normalized to account for receiver noise floor.

12. The method of claim 1, wherein the determining is performed at a user terminal.

13. The method of claim 4, wherein a first set of matrices of correction factors for the downlink channel is determined for a first set of subbands, the method further comprising:

interpolating the first set of matrices to obtain a second set of matrices of correction factors for the downlink channel for a second set of subbands.

14. The method of claim 1, wherein the estimates of the downlink and uplink channel responses are each obtained based on a pilot transmitted from a plurality of antennas and orthogonalized with a plurality of orthogonal sequences.

15. The method of claim 1 wherein the estimate of the uplink channel response is obtained based on a pilot transmitted on the uplink channel and wherein the estimate of the downlink channel response is obtained based on a pilot transmitted on the downlink channel.

16. The method of claim 1, wherein the TDD system is a multiple-input multiple-output (MIMO) system.

17. The method of claim 1, wherein the TDD system utilizes orthogonal frequency division multiplexing (OFDM).

18. A method for calibrating downlink and uplink channels in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising:

transmitting a pilot on the uplink channel;

obtaining an estimate of an uplink channel response derived based on the pilot transmitted on the uplink channel;

receiving a pilot on the downlink channel;

obtaining an estimate of a downlink channel response derived based on the pilot received on the downlink channel; and

determining first and second sets of correction factors based on the estimates of the downlink and uplink channel responses, wherein a calibrated downlink channel is formed by using the first set of correction factors for the downlink channel and a calibrated uplink channel is formed by using the second set of correction factors for the uplink channel.

19. The method of claim 18, wherein the first and second sets of correction factors are determined based on a minimum mean square error (MMSE) computation.

20. The method of claim 18, wherein the first and second sets of correction factors are determined based on a matrix-ratio computation.

21. The method of claim 18, wherein the first set of correction factors is updated based on calibration with a plurality of user terminals.

22. The method of claim 18, further comprising:

scaling symbols with the first set of correction factors prior to transmission on the downlink.

23. The method of claim 18, further comprising:

scaling symbols with the second set of correction factors prior to transmission on the uplink channel.

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

means for obtaining an estimate of a response of a downlink channel;

means for obtaining an estimate of a response of an uplink channel; and

means for determining first and second sets of correction factors based on the estimates of the downlink and uplink channel responses, wherein a calibrated downlink channel is formed by using the first set of correction factors for the downlink channel and a calibrated uplink channel is formed by using the second set of correction factors for the uplink channel.

25. A user terminal in a wireless time division duplexed (TDD) communication system, comprising:

an TX spatial processor operative to transmit a first pilot on an uplink channel;

an RX spatial processor operative to receive a second pilot on a downlink channel and derive an estimate of a downlink channel response based on the received second pilot, and to receive an estimate of an uplink channel response derived based on the transmitted first pilot; and

a controller operative to determine first and second sets of correction factors based on the estimates of the downlink and uplink channel responses, wherein a calibrated downlink channel is formed by using the first set of correction factors for the downlink channel and a calibrated uplink channel is formed by using the second set of correction factors for the uplink channel.

26. The user terminal of claim 25, wherein the controller is further operative to determine the first and second sets of correction factor based on a minimum mean square error (MMSE) computation.

27. The user terminal of claim 25, wherein the controller is further operative to determine the first and second sets of correction factor based on a matrix-ratio computation.

28. A method for communication in a wireless system, comprising:

calibrating one or more communication links between a plurality of user stations and one or more access points, based on one or more sets of correction factors derived from estimates of channel responses associated with the one or more communication links, the plurality of user stations including a first user station and a second user station; and

establishing communication between the first and second user stations using steering without performing calibration between the first and second user stations.

29. The method of claim 28 wherein establishing the communication between the first and second user stations comprises:

sending, from the first user station, a pilot and a request to establish a communication link with the second user station;

sending, from the second user station, a steered pilot and an acknowledgment in response to receiving the pilot and the request from first user station;

transmitting information between the first and second user stations using steering based on the steered pilot.

30. The method of claim 29 wherein the request to establish the communication comprises an identifier of a basic service set to which the first user station belongs and an identifier of the first user station.

31. The method of claim 29 wherein the acknowledgment comprises an identifier of the second user station, an identifier of a basic service set to which the second user station belongs, and a data rate indicator.

32. The method of claim 28 wherein the one or more access points includes a first access point associated with a first basic service set (BSS) and a second access point associated with a second BSS, wherein the first user station is calibrated with respect to the first access point and the second user station is calibrated with respect to the second access point, and wherein establishing the communication between the first and second user stations comprises:

sending, from the first user station, a pilot and a request to establish a communication link with the second user station;

sending, from the second user station, a steered pilot and an acknowledgment in response to receiving the pilot and the request from first user station; and

transmitting information between the first and second user stations using steering that is adjusted to compensate for a phase rotation caused by calibration of the first and second user stations with respect to different access points.

33. The method of claim 32 wherein the phase rotation is determined based on the steered pilot received from the second user station.

34. An apparatus for communication in a wireless system, comprising:

means for alibrating one or more communication links between a plurality of user stations and one or more access points, based on one or more sets of correction factors derived from estimates of channel responses associated with the one or more communication links, the plurality of user stations including a first user station and a second user station; and

means for establishing communication between the first and second user stations using steering without performing calibration between the first and second user stations.

35. The apparatus of claim 34 wherein establishing the communication between the first and second user stations comprises:

means for sending, from the first user station, a pilot and a request to establish a communication link with the second user station;

means for sending, from the second user station, a steered pilot and an acknowledgment in response to receiving the pilot and the request from first user station;

means for transmitting information between the first and second user stations using steering based on the steered pilot.

36. The apparatus of claim 35 wherein the request to establish the communication comprises an identifier of a basic service set to which the first user station belongs and an identifier of the first user station.

37. The apparatus of claim 35 wherein the acknowledgment comprises an identifier of the second user station, an identifier of a basic service set to which the second user station belongs, and a data rate indicator.

38. The apparatus of claim 34 wherein the one or more access points includes a first access point associated with a first basic service set (BSS) and a second access point associated with a second BSS, wherein the first user station is calibrated with respect to the first access point and the second user station is calibrated with respect to the second access point, and wherein establishing the communication between the first and second user stations comprises:

sending, from the first user station, a pilot and a request to establish a communication link with the second user station;

sending, from the second user station, a steered pilot and an acknowledgment in response to receiving the pilot and the request from first user station; and

transmitting information between the first and second user stations using steering that is adjusted to compensate for a phase rotation caused by calibration of the first and second user stations with respect to different access points.

39. The apparatus of claim 38 wherein the phase rotation is determined based on the steered pilot received from the second user station.