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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

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.

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1. A method for establishing peer-to-peer communications by a first user terminal (UT) in a wireless communication system, comprising: calibrating with an access point, by: obtaining an estimate of a downlink channel response;obtaining an estimate of an uplink channel response;determining first and

1. A method for establishing peer-to-peer communications by a first user terminal (UT) in a wireless communication system, comprising: calibrating with an access point, by: 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; andcalibrating a downlink channel and uplink channel based on each of the first and second sets of correction factors, respectively, to form a calibrated downlink channel and a calibrated uplink channel; anddetermining a scaling value indicative of an average difference between the estimate of the downlink channel response and the estimate of the uplink channel response, wherein the estimates for the downlink and uplink channel responses are normalized to account for receiver noise floor; andestablishing direct peer-to-peer communication with a second UT that has also calibrated with the same or a different access point, wherein the establishing is performed without further calibration between the first and second UTs. 2. The method of claim 1, wherein the first set of correction factors is used to scale symbols prior to transmission on the calibrated downlink channel and the second set of correction factors is used to scale symbols prior to transmission on the calibrated uplink channel. 3. The method of claim 1, wherein the first set of correction factors is used to scale symbols received on the calibrated downlink channel and the second set of correction factors is used to scale symbols received on the calibrated uplink channel. 4. The method of claim 1, wherein the first and second sets of correction factors are determined based on the following equation: Ĥup{circumflex over (K)}ut=(Ĥdn{circumflex over (K)}ap)T,where Ĥdn is a matrix for the estimate of the downlink channel response,Ĥup is a matrix for the estimate of the uplink channel response,{circumflex over (K)}ap is a matrix for the first set of correction factors,{circumflex over (K)}ut is a matrix for the second set of correction factors, and“T” 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 Ĥup over the matrix Ĥdn, andderiving 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, anddetermining 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, anddetermining 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 |Ĥup{circumflex over (K)}ut−(Ĥdn{circumflex over (K)}ap)T|2. 10. The method of claim 1, wherein the determining is performed at a each user terminal. 11. 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. 12. 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. 13. 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. 14. The method of claim 1, wherein the TDD system is a multiple-input multiple-output (MIMO) system. 15. The method of claim 1, wherein the TDD system utilizes orthogonal frequency division multiplexing (OFDM). 16. A method for establishing peer-to-peer communications by a first user terminal (UT) in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising: calibrating with an access point, by: transmitting a pilot on an uplink channel;obtaining an estimate of an uplink channel response derived based on the pilot transmitted on the uplink channel;receiving a pilot on a downlink channel;obtaining an estimate of a downlink channel response derived based on the pilot received on the downlink channel; anddetermining 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; andscaling symbols with the first set of correction factors prior to transmission on the downlink;scaling symbols with the second set of correction factors prior to transmission on the uplink channel; andestablishing direct peer-to-peer communication with a second UT that has also calibrated with the access point, wherein the establishing is performed without further calibration between the first and second UTs. 17. The method of claim 16, wherein the first and second sets of correction factors are determined based on a minimum mean square error (MMSE) computation. 18. The method of claim 16, wherein the first and second sets of correction factors are determined based on a matrix-ratio computation. 19. The method of claim 16, wherein the first set of correction factors is updated based on calibration with a plurality of user terminals. 20. A first user terminal (UT) in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising: means for calibrating with an access point, by: obtaining an estimate of a downlink channel response;obtaining an estimate of an uplink channel response; anddetermining 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; anddetermining a scaling value indicative of an average difference between the estimate of the downlink channel response and the estimate of the uplink channel response, wherein the estimates for the downlink and uplink channel responses are normalized to account for receiver noise floor; andmeans for establishing direct peer-to-peer communication with a second UT that has also calibrated with an access point, wherein the means for establishing is performed without further calibration between the first and second UTs. 21. In a system for calibrating downlink and uplink channels in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system including an access point, a first user terminal (UT) and a second UT, an access point comprising: a transmit (TX) spatial processor configured to transmit a pilot on the uplink channel for each of the first UT and the second UT;a receive (RX) spatial processor configured to, for each of the first UT and the second UT, obtain an estimate of an uplink channel response derived based on the pilot transmitted on the uplink channel, receive a pilot on the downlink channel, and obtain an estimate of a downlink channel response derived based on the pilot received on the downlink channel; anda controller configured to determine, for each of the first UT and the second UT, first and second sets of correction factors based on the estimates of the downlink and uplink channel responses, wherein a calibrated downlink channel for peer-to-peer communication between the first and second UTs is formed by using the first set of correction factors for the downlink channel, and a calibrated uplink channel for peer-to-peer communication between the first and second UTs is formed by using the second set of correction factors for the uplink channel, and the controller further configured to establish direct peer-to-peer communication between the first UT and the second UT without further calibration between them. 22. 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 direct peer-to-peer communication between the first and second user stations using steering without performing calibration between the first and second user stations, 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 direct peer-to-peer 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; andtransmitting 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. 23. The method of claim 22, 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; andtransmitting information between the first and second user stations using steering based on the steered pilot. 24. The method of claim 23 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. 25. The method of claim 23 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. 26. The method of claim 22 wherein the phase rotation is determined based on the steered pilot received from the second user station. 27. An apparatus for communication in a wireless system, comprising: means for 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 means for establishing direct peer-to-peer communication between the first and second user stations using steering without performing calibration between the first and second user stations, 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 the means for establishing the direct peer-to-peer 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; andtransmitting 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. 28. The apparatus of claim 27, 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; andmeans for transmitting information between the first and second user stations using steering based on the steered pilot. 29. The apparatus of claim 28 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. 30. The apparatus of claim 28 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. 31. The apparatus of claim 27 wherein the phase rotation is determined based on the steered pilot received from the second user station.