1 Introduction
Hybrid beamforming has been a widely recognised solution for the implementation of massive MIMO and mmWave communications in 5G [1]–[6]. It combines the analogue beam-forming technique and the digital beamforming technique together, where analogue beamforming is performed first to re-duce the number of analogue channels, which are then converted into digital via a reduced number of digital converters (DACs), and after that digital beamforming can then be performed.