  Terahertz (THz) communications with multi-GHz bandwidth are envisioned as a
key technology for 6G systems. Ultra-massive (UM) MIMO with hybrid beamforming
architectures are widely investigated to provide a high array gain to overcome
the huge propagation loss. However, most of the existing hybrid beamforming
architectures can only utilize the multiplexing offered by the multipath
components, i.e., inter-path multiplexing, which is very limited due to the
spatially sparse THz channel. In this paper, a widely-spaced multi-subarray
(WSMS) hybrid beamforming architecture is proposed, which improves the
multiplexing gain by exploiting a new type of intra-path multiplexing provided
by the spherical-wave propagation among k widely-spaced subarrays, in addition
to the inter-path multiplexing. The resulting multiplexing gain of WSMS
architecture is k times of the existing architectures. To harness WSMS hybrid
beamforming, a novel design problem is formulated by optimizing the number of
subarrays, subarray spacing, and hybrid beamforming matrices to maximize the
spectral efficiency, which is decomposed into two subproblems. An optimal
closed-form solution is derived for the first hybrid beamforming subproblem,
while a dominant-line-of-sight-relaxation algorithm is proposed for the second
array configuration subproblem. Extensive simulation results demonstrate that
the WSMS architecture and proposed algorithms substantially enhance the
spectral efficiency and energy efficiency.
