User Grouping and Hybrid RF/Baseband Precoding for Multi-User Massive MIMO Systems

In this paper, we investigate hybrid precoders for massive MIMO systems with the objective to reduce the number of radio-frequency (RF) chains. The users are first partitioned into groups that share similar angles of departure (AoDs) to design an angular-based RF-beamforming stage. Then a multi-user-Multiple Input Multiple Output (MIMO) baseband precoding stage is developed with reduced channel state information (CSI) dimension in massive MIMO systems. We develop a systematic approach to simultaneously determine the number of groups and to cluster the users accordingly. The partition is done by minimizing the inter-group interference using a similarity graph. We also incorporate a simple transfer block between the digital precoding and RF-beamforming stages to reduce the number of RF chains in massive MIMO systems. For a sufficiently large number of antennas and over a wide range of signal-to-noise ratio (SNR), the proposed hybrid RF/baseband precoding schemes can offer a sum-rate performance approaching that of the single-stage full digital optimal precoder, while keeping the number of RF chains equal to the number of users. Next, we propose a simple way to design the RF and baseband digital precoding stages in the case of multiple scatterings to fully exploit the multiple reflections that occur in dense urban environments. The proposed hybrid 2-stage RF/baseband schemes significantly outperform other hybrid RF/baseband precoding schemes using orthogonal multiplexing.