User Selection for Multiuser MIMO Systems Based on Block Diagonalization in Wide-Range SNR Environment

Multiuser (MU) - multiple input multiple output (MIMO) techniques are attractive to increase the downlink spectrum efficiency since they can accept a small number of antennas at each user station (STA). In MU-MIMO systems, the transmission performance is affected by the STA combination that is spatially multiplexed in the same frequency and same time slot. Thus, the AP needs to determine the optimum STA subset and the number of spatially multiplexed STAs from the channel state information. In this paper, we propose a user selection scheme that uses an iterative algorithm to determine the number and the optimal combination of multiplexed STAs. The proposed scheme chooses STAs one by one so as to improve the throughput of all STAs in the subset compared to that achieved in single-user MIMO transmission. Thus, all STAs have a fair chance of being selected even when their SNRs vary widely. Furthermore, the iterative algorithm has lower calculation complexity than the exhaustive search approach. The index employed for user selection is also simplified by using the signal-space vectors of the channel matrices. Computer simulations confirm that the proposed method improves the increase in STA throughput over single-user MIMO throughput in a wide-range of SNR values while equalizing STA selection probabilities.