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TGn Sync Calibration for Beamforming
Month 2002 doc.: IEEE /xxxr0 November 2004 TGn Sync Calibration for Beamforming John S. Sadowsky Intel Corporation W. Chandler Blvd. Chandler, AZ USA John S. Sadowsky, Intel John Doe, His Company
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November 2004 Overview The TGn Sync proposal includes optional BF (beamforming) transmission modes. This requires a calibration to equalize response differences between the Tx and Rx RF and analog chains of a given STA. This contribution does not provide an exhaustive treatment of calibration. Rather, we provide mathematical details for one important case; the case of an AP transmitting BF on the downlink to a low cost client. Our design criterion, for this example case, is to minimize complexity in the client. A future revision of the TGn Sync proposal will include the details of this calibration procedure, as well as other protocols to support bi-directional BF. John S. Sadowsky, Intel
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Terminology Correction Calibration
Month 2002 doc.: IEEE /xxxr0 November 2004 Terminology Correction The correction is a complex coefficient applied to the transmit streams on a per antenna – per subcarrier basis. The purpose of the correction is to equalize response difference between transmit and receive processing paths (up to an unknown constant across antennas). Calibration The process of calculating correction coefficients. John S. Sadowsky, Intel John Doe, His Company
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November 2004 Tx Corrections The correction matrix is a diagonal matrix of per antenna correction coefficients that is applied after beam-forming trans-formations and before IFFT processing. There is a different correction matrix for each subcarrier. We have suppressed subcarrier dependence in the notation. John S. Sadowsky, Intel
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Frequency Domain Model
November 2004 Frequency Domain Model Tx chain response matrix for device i: Rx chain response matrix for device i: Physical antenna-to-antenna channel for transmission from device i to device j: John S. Sadowsky, Intel
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November 2004 Tx Calibration The purpose of calibration is to select the correction coefficients so that for each device we have Since these are diagonal matrices, we can also express this element-wise as The constant is constant across antennas (but may vary across subcarriers). It is not necessary to know as beam-forming is phase invariant. Definition: The composite channel is Under calibration Mismatch ratio for antenna k of STA i John S. Sadowsky, Intel
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Reciprocity of Composite Channel
November 2004 Reciprocity of Composite Channel Reciprocity of physical channel: Reciprocity of composite channel, up to an unknown constant: by reciprocity of physical channel John S. Sadowsky, Intel
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Calibration Uncorrected composite channel:
November 2004 Calibration Uncorrected composite channel: Since A and B are diagonal matrices, we can work element-wise: Thus STA 1 mismatch ratio STA 2 mismatch ratio reciprocity X John S. Sadowsky, Intel
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Calibration (2) Calibration of STA 2 uses both and .
November 2004 Calibration (2) Calibration of STA 2 uses both and Select fixed k1. For k2 = 0, …, N-1 because k1 is fixed John S. Sadowsky, Intel
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Two Types of STA Correction Capabilities
November 2004 Two Types of STA Correction Capabilities Basic BF-Rx only STA Does not transmit BF Does not apply any corrections BF-Tx&Rx STA STA can transmit and receive BF Must have Tx correction capability John S. Sadowsky, Intel
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Basic DL Only BF Solution
November 2004 Basic DL Only BF Solution BF transmission BF only in DL (AP → client) Client is a BF-Rx only STA Client Calibration Exchange AP sends sounding PPDU to client on DL May or may not transmitted with correction Client responds with sounding packet (no correction) containing DL channel estimates from previous AP PPDU in data payload AP has both UL and (uncorrected) DL channel estimates AP calculates both AP and client correction coefficients If not already calibrated, AP my also calculate its own correction coefficients as well John S. Sadowsky, Intel
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Basic Rx Only Solution Packet Exchange
November 2004 Basic Rx Only Solution Packet Exchange Sounding packets from client (carrying RAC or RTS, and/or ACK) provide uncorrected composite UL channel AP corrects UL channel estimate by applying client Tx correction coefficients AP calculates SVD for corrected UL channel transpose AP transmits SVD using AP correction coefficients Key features of basic Rx solution Minimal complexity at the client Client requirements are only that the client transmit sounding packets and participate in the calibration protocol exchange including transmitting channel estimates in protocol packet data payload. AP complexity AP must apply Tx corrections to DL BF transmissions, and AP must store client correction coefficients for UL channel estimate correction John S. Sadowsky, Intel
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Summary and Conclusion
November 2004 Summary and Conclusion Basic Rx-only BF Tx correction capability is not mandatory This eliminates complex multiplies form low cost clients Mandatory requirements for basic STA Ability to transmit sounding packets Calibration exchange Respond to inquiry by sending channel estimates Additional Calibration Protocols Support for bi-directional BF Modification of Rx-only protocol is straight-forward John S. Sadowsky, Intel
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