TY - JOUR TI - Multi-Hop RIS-Empowered Terahertz Communications: A DRL-Based Hybrid Beamforming Design AU - Huang, Chongwen AU - Yang, Zhaohui AU - Alexandropoulos, George C. and AU - Xiong, Kai AU - Wei, Li AU - Yuen, Chau AU - Zhang, Zhaoyang AU - Debbah, AU - Merouane JO - IEEE Journal on Selected Areas in Communications PY - 2021 VL - 39 TODO - 6 SP - 1663-1677 PB - IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC SN - 0733-8716, 1558-0008 TODO - 10.1109/JSAC.2021.3071836 TODO - Array signal processing; Attenuation; Wireless communication; Spread spectrum communication; Relays; Optimization; Nonlinear optics; Terahertz communication; reconfigurable intelligent surface; 6G; Massive-MIMO; multi-hop; multiuser; beamforming; deep reinforcement learning; alternating optimization TODO - Wireless communication in the TeraHertz band (0.1-10 THz) is envisioned as one of the key enabling technologies for the future sixth generation (6G) wireless communication systems scaled up beyond massive multiple input multiple output (Massive-MIMO) technology. However, very high propagation attenuations and molecular absorptions of THz frequencies often limit the signal transmission distance and coverage range. Benefited from the recent breakthrough on the reconfigurable intelligent surfaces (RIS) for realizing smart radio propagation environment, we propose a novel hybrid beamforming scheme for the multi-hop RIS-assisted communication networks to improve the coverage range at THz-band frequencies. Particularly, multiple passive and controllable RISs are deployed to assist the transmissions between the base station (BS) and multiple single-antenna users. We investigate the joint design of digital beamforming matrix at the BS and analog beamforming matrices at the RISs, by leveraging the recent advances in deep reinforcement learning (DRL) to combat the propagation loss. To improve the convergence of the proposed DRL-based algorithm, two algorithms are then designed to initialize the digital beamforming and the analog beamforming matrices utilizing the alternating optimization technique. Simulation results show that our proposed scheme is able to improve 50% more coverage range of THz communications compared with the benchmarks. Furthermore, it is also shown that our proposed DRL-based method is a state-of-the-art method to solve the NP-hard beamforming problem, especially when the signals at RIS-assisted THz communication networks experience multiple hops. ER -