Contents I. Introduction
Despite the great potential of providing abundant spectral resources for Six-Generation (6G) and beyond systems [1], millimeter-wave (mmWave) and terahertz (THz) signals suffer from extremely worse propagation conditions raised by high path loss, atmospheric absorption, and weather attenuation, leading to short transmission distances [2]. Therefore, a large-scale antenna array with high beamforming gain is generally employed to compensate for this propagation loss. Digital beamforming over a large-scale array raises high hardware and energy costs because it requires massive radio frequency (RF) components. Analog beamforming provides a low-cost, low-complexity alternative by using a single RF chain, but it suffers from poor performance and hardware constraints. Accordingly, hybrid beamforming [3] is widely recognized as the most suitable structure for implementing mmWave and THz transceivers. Using a few RF chains and a phase-shifter network, it achieves high performance comparable to digital beamforming with much lower hardware complexity.
