  Terahertz (THz) communications are regarded as a pillar technology for the
sixth generation (6G) wireless systems, by offering multi-ten-GHz bandwidth. To
overcome the short transmission distance and huge propagation loss,
ultra-massive (UM) MIMO systems that employ sub-millimeter wavelength antennas
array are proposed to enable an enticingly high array gain. In the UM-MIMO
systems, hybrid beamforming stands out for its great potential in promisingly
high data rate and reduced power consumption. In this paper, challenges and
features of the THz hybrid beamforming design are investigated, in light of the
distinctive THz peculiarities. Specifically, we demonstrate that the spatial
degree-of-freedom (SDoF) is less than 5, which is caused by the extreme
sparsity of the THz channel. The blockage problem caused by the huge reflection
and scattering losses, as high as 15 dB or over, is studied. Moreover, we
analyze the challenges led by the array containing 1024 or more antennas,
including the requirement for intelligent subarray architecture, strict energy
efficiency, and propagation characterization based on spherical-wave
propagation mechanisms. Owning up to hundreds of GHz bandwidth, beam squint
effect could cause over 5~dB array gain loss, when the fractional bandwidth
exceeds 10%. Inspired by these facts, three novel THz-specific hybrid
beamforming architectures are presented, including widely-spaced
multi-subarray, dynamic array-of-subarrays, and true-time-delay-based
architectures. We also demonstrate the potential data rate, power consumption,
and array gain capabilities for THz communications. As a roadmap of THz hybrid
beamforming design, multiple open problems and potential research directions
are elaborated.
