Beamforming design and user grouping for FDD massive MIMO systems

This paper considers a frequency-division-duplexing (FDD) massive multiple-input multiple-output (MIMO) system that operates with two-stage beamforming based on user grouping. In this system, we propose a first stage beamformer and a user grouping method for the two-stage beamforming. The proposed first stage beamformer maximizes an approximate sum rate calculated from the second-order statistic of the channels. The proposed user grouping method improves both the spatial orthogonality among different groups and the effective channel gain of in-group users. Simulation results show the sum rate gain achieved by both the proposed first stage BF and user grouping methods, compared to conventional methods.