Subarray Hybrid Precoding for Massive MIMO Capacity Maximization.

An effective solution for reducing complexity and power consumption of massive MIMO systems is hybrid beam-forming (HB) using analog precoding at RF and digital precoding at the baseband. Researchers have studied HB, assuming optimal antenna implementation, ideal environment, and full rank effective channel. However, the rank of the effective channel matrix could be deficient due to implementation limitations and other factors, which would degrade the performance of the system. The objective of this study is to maintain a high rank effective channel with independent beams sending data streams to the receiver. When the weights of the RF beamformer are properly selected, the structure of the multipath propagation channel will be exploited by the transmitted beams, which maximize the system capacity. This paper presents a new set of solutions for HB that performs nearly as a fully digital beamformer in terms of MIMO multiplexing gains.