Iterative Grouping Calibration for Channel Reciprocity in TDD MU-MIMO Systems

In this paper, we propose a new iterative grouping calibration algorithm for channel reciprocity in MU-MIMO (multiuser multiple-input multiple-output) systems. We assume the system operate in Time-division duplex (TDD). Because the TDD channel is reciprocal, we can use the uplink channel to estimate the downlink channel. The free-space electromagnetic channel can be regard as reciprocal channel in TDD channel. However, the generalized channel do not only consist of free-space electromagnetic channel, but also include transceiver circuitry. And in general case, the transceiver circuitry isn't perfect. For that reason, the reciprocity of generalized channel is destroyed. In this paper, we set a base station with one antenna to be the reference user, then we divide the MIMO BS antennas into N groups. We assume the users and every antenna group reform a new user-antenna group, the scope of transceiver circuitry coherence time in different user-antenna group are different. We only calibrate one user-antenna group in one free-space electromagnetic channel coherence time. The iterative grouping calibration algorithm in this paper can improve the channel capacity by transmitting pilot and signal at the same time slot. And the iterative grouping calibration algorithm goes especially well with massive mimo systems and high-speed mobile environment for its flexible antenna grouping.