  Terahertz (THz) communications are regarded as a pillar technology for the 6G
systems, by offering multi-ten-GHz bandwidth. To overcome the huge propagation
loss while reducing the hardware complexity, THz ultra-massive (UM) MIMO
systems with hybrid beamforming are proposed to offer high array gain. Notably,
the adjustable-phase-shifters considered in most existing hybrid beamforming
studies are power-hungry and difficult to realize in the THz band. Moreover,
due to the ultra-massive antennas, full channel-state-information (CSI) is
challenging to obtain. To address these practical concerns, in this paper, an
energy-efficient dynamic-subarray with fixed-phase-shifters (DS-FPS)
architecture is proposed for THz hybrid beamforming. To compensate for the
spectral efficiency loss caused by the fixed-phase of FPS, a switch network is
inserted to enable dynamic connections. In addition, by considering the partial
CSI, we propose a row-successive-decomposition (RSD) algorithm to design the
hybrid beamforming matrices for DS-FPS. A row-by-row (RBR) algorithm is further
proposed to reduce computational complexity. Extensive simulation results show
that, the proposed DS-FPS architecture with the RSD and RBR algorithms achieves
much higher energy efficiency than the existing architectures. Moreover, the
DS-FPS architecture with partial CSI achieves 97% spectral efficiency of that
with full CSI.
