User Selection for MU-MIMO Based on Channel Estimation and Spatial Orthogonality

This paper addresses interference management in downlink (DL) transmission of a multi-user multiple-input multiple-output (MU-MIMO) system in time division duplex (TDD) mode. A novel method of user selection is proposed based on a relaxed definition of spatial orthogonality, where the base station (BS) obtains channel state information (CSI) using pilot symbols in uplink (UL) transmission. Due to channel reciprocity in TDD mode, the BS selects user groups based on obtained CSI and maximum pairwise channel orthogonality for the DL transmission. Then, the BS employs linear beamformers including maximum ratio transmission (MRT) and zero forcing (ZF) to transmit the payload symbols for the users. Analytical and simulated results indicate the accuracy of the proposed method in terms of the achievable rate. Also for the proposed user selection method, the optimal training sequence length in UL channel is found to maximize sum rate among the selected users.