Subarray hybrid precoding with finite‐resolution PSs for massive MIMO capacity maximisation

Abstract

The addition of analogue processing to the digital precoding, known as hybrid beamforming (HB), is an efficient solution for massive multiple-input multiple-output (MIMO) systems. The design of a phase shifter (PS) network plays an important role in the complete precoder operation because it necessitates accurate components for realising precise phases, and that can be costly. Finite resolution PSs are good alternatives because they need simpler hardware implementation than those with infinite resolution. However, the degradation of performance of a MIMO system with very low-resolution PSs is significant. Although recent studies suggest adding extra radio frequency (RF) chains to substitute for the accuracy of the PSs, it is complex, expensive and not energy efficient. This study demonstrates that HB with low-resolution PSs can realise a high performance without increasing the number of RF chains. The authors proposed solution relies on a proper selection of the weights of the RF beamformer, hence exploiting the structure of the multipath propagation channel to maximise the system capacity. They also show that separating antennas from each other by sufficient distance, results in a less correlated channel, and thus, a minimal loss in the capacity and the antenna gains are assured.