I. Introduction
Massive multiple-input multiple-output (MIMO) is one of the key enabling technologies advanced to meet the exacting target performance requirements of the fifth generation (5G) of wireless systems [1]. Due to the use of millimetre-wave (mmWave) signals, the antenna arrays of massive MIMO systems will be smaller and include more antenna elements than previous technologies [2]. However, using more antennas requires having more radio-frequency (RF) chains, which in turn is problematic because the latter are expensive and power-hungry [2], [3]. One way to cope with this constraint is to employ a hybrid architecture, wherein an analog beamformer is used to reduce the effective number of RF signals prior to the application of baseband processing [4], [5].