Retrieving channel reciprocity for coordinated multi-point transmission

Due to the difficulty of self antenna calibration among different base stations (BSs), the uplink-downlink channels are no longer reciprocal in time division duplex coordinated multi-point transmission with joint processing (CoMP-JP) systems. To retrieve the channel reciprocity, we propose an inter-BS antenna calibration method. Specifically, we estimate the inter-BS ambiguity factors with weighted least square criterion by using the uplink and downlink channel estimates and the received signal of a calibration sounding signal of multiple supporting users or multiple frames. We then analyze the mean square error of the proposed estimator, and compare the performance of the estimators with multiple supporters and with multiple frames. Simulation results show that the channel reciprocity is largely retrieved by the proposed antenna calibration method, which provides substantial throughput gain over the CoMP-JP systems without calibration.