  Terahertz (THz) communications with multi-GHz bandwidth are envisioned as a
key technology for 6G wireless systems. While suffering from huge propagation
loss, large arrays of sub-millimeter wavelength antennas can be realized in
ultra-massive (UM) MIMO systems to overcome the distance limitation. However,
channel sparsity and low spatial degree-of-freedom of the THz channel limit the
spatial multiplexing gain and hence the achievable rate. In this paper, a
widely-spaced multi-subarray (WSMS) hybrid beamforming architecture is proposed
for THz UM-MIMO systems to improve the multiplexing gain. In each subarray, the
antennas are critically-spaced to utilize the inter-path multiplexing and
beamforming gains, while the subarrays are widely-spaced to harvest the novel
intra-path multiplexing gain through exploiting the phase differences over the
subarrays. By exploring the THz channel peculiarity, a dominant-LoS-relaxation
(DLR) method is proposed to balance the multiplexing and beamforming, and a
block-diagonal vectorization-based (BD-VEC) algorithm is developed to solve the
hybrid beamforming problem. Extensive simulation results demonstrate that the
multiplexing gain is improved by a factor of the number of subarrays in the THz
WSMS system. The low-complexity DLR method maximizes the achievable rate
near-optimally. Compared to existing hybrid beamforming algorithms, the BD-VEC
algorithm achieves higher spectral efficiency with substantially lower
computational complexity.
