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.

翻译：TeraHertz乐队(0.1-10 THz)的无线通信(0.1-10 THz)被认为是未来第六代(6G)无线通信系统的关键赋能技术之一,其规模超出了大规模多重输入多重输出(Massive-MIMO)技术。然而,Thz频率的高度传播衰减和分子吸收往往限制了信号传输的距离和覆盖范围。从最近为实现智能无线电传播环境而实现的可重新配置智能表面(RIS)的突破中受益,我们提议为多式RAIS协助的多式通信网络建立一个新的混合式的解决方案,以改善THZ频带频率的覆盖范围。特别是,部署多个被动和可控的RIS系统来协助基站和多个单安纳用户之间的传输。我们调查了BS的数字化组合矩阵的联合设计,以及RIS的拟议模拟矩阵组合,利用了基于深度通信的升级学习(DRL)的最新进展来消除传播损失。为了改进基于DRIS-波段的多式信号覆盖范围范围,我们拟议的数字-LRIS-RIS(L)信号定位模型的升级方法也用来显示我们模拟的模型的模型的升级方法。