I. Introduction
Massive multiple-input multiple-output (MIMO) systems are one of the essential technologies for the beyond-fifth-generation (B5G) and six-generation (6G) wireless communications. However, a large number of radio frequency (RF) chains connected to active antennas in the conventional fully-digital transmitter will incur huge power consumption and high complexity, which has motivated the work on hybrid analog-digital beamforming structures [1], [2], [3], [4] to decrease the number of RF chains by moving some signal processing operations into the analogue domain. However, the undesired high sidelobes may be caused in these hybrid beamforming structures. Hence, to mitigate the adverse effect of the sidelobes in the transmit beam, different techniques were proposed in the hybrid beamforming architecture [5], [6], [7]. Nevertheless, the cost of the analog network in conventional hybrid beamforming structures is still high for the massive number of RF components, such as dividers, combiners, phase shifters and the connections between them, which calls for more effective and innovative solutions [8].