Estimation of time-domain calibration parameters to restore MIMO-TDD channel reciprocity

Spatial interweave cognitive radio opportunistically exploits spatial holes to enable transmission of secondary systems. The latter form carefully aligned spatial beams so that there is no interference to the primary system. This requires acquisition of the (spatio-temporal) channel state (CS) between the secondary emitter and the primary receiver. CS can be acquired by using a multi-input multi-output time-division-duplex (MIMO-TDD) setup thanks to channel reciprocity inherent in such systems. Unfortunately, global reciprocity is often jeopardized by the nonreciprocal radio frequency front-ends (RF) at the two ends of the link. Restoration of reciprocity to compensate for the RF calls for on-line (and hence low complexity) calibration parameters estimation. In this paper, we propose an approach to estimate the space-time-domain parameters restoring the channel reciprocity in a frequency selective MIMO-TDD context. Accounting for antenna coupling effects, we find accurate space-time-domain calibration parameters and demonstrate real-time calibration process in an OFDM system.