Hybrid Beamforming for Terahertz Wireless Communications: Challenges, Architectures, and Open Problems

Terahertz (THz) communications are regarded as a pillar technology for 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 antenna arrays are proposed to enable an enticingly high array gain. In UM-MIMO systems, hybrid beamforming stands out for its great potential in promisingly high data rate and reduced power consumption. In this article, challenges and features of the THz hybrid beamforming design are investigated, in light of the distinctive THz peculiarities. Specifically, we investigate that the spatial degree-of-freedom is poor, which is caused by the extreme sparsity of the THz channel. The blockage problem caused by the huge reflection and scattering losses is studied. We analyze the challenges led by the array containing 1024 or more antennas, including the requirement for dynamic subarray architecture, strict energy efficiency, and propagation characterization based on spherical-wave propagation mechanisms. Owning to multi-ten-GHz bandwidth, beam squint effect could cause tens of dB array gain loss. Inspired by these facts, three novel THz-specific hybrid beamforming architectures are presented, including widely-spaced multi-subarray, dynamic array-of-subarrays, and dynamic-subarray with fixed-true-time-delay 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.