Channel reciprocity of compact antenna array and the calibration

Channel reciprocity is regarded as one of the most important advantages of TDD systems. However, the uplink (UL) and downlink (DL) channels are not actually reciprocal because of different characteristics of transmitters (TXs) and receivers (RXs). When the antenna elements are placed with large distance, the Radio Frequency (RF) gain mismatch of TXs and RXs is the dominant un-reciprocal factor, but if the antenna elements are placed close, such as compact antenna array, mutual coupling (MC) should be considered. In general, the MC effects for a transmitting and receiving array are different, even if the physical geometry of the array remains unchanged. Conventional reciprocity calibration methods mainly target at compensating RF gain mismatch and regard reciprocity error as diagonal matrix. However, the reciprocity error is no longer diagonal for compact antenna array due to the un-reciprocal MC of TXs and RXs, and thus the conventional calibration methods do not work in this scenario. In this paper, a comprehensive reciprocity error model is proposed firstly, and a combined MMSE and dot-division calibration scheme is then presented for calibrating MC and RF mismatch at the same time. Link level simulation results demonstrate the efficiency of the proposed scheme. The calibration method can well calibrate the UL and DL channel reciprocity, and the performance can be boosted up to around 1dB~2dB less than the ideal reciprocal case.