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

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