In this paper, we investigate the reconfigurable intelligent surface (RIS)-aided terahertz (THz) communication system with the sparse radio frequency chains antenna structure at the base station (BS). To overcome the beam split of the BS, different from the conventional single-layer true-time-delay (TTD) scheme, we propose a double-layer TTD scheme that can effectively reduce the number of large-range delay devices, which involve additional insertion loss and amplification circuitry. Next, we analyze the system performance under the proposed double-layer TTD scheme. To relieve the beam split of the RIS, we consider multiple distributed RISs to replace an ultra-large size RIS. Based on this, we formulate an achievable rate maximization problem for the distributed RISs-aided THz communications via jointly optimizing the hybrid analog/digital beamforming, time delays of the double-layer TTD network and reflection coefficients of RISs. Considering the practical hardware limitation, the finite-resolution phase shift, time delay and reflection phase are constrained. To solve the formulated problem, we first design an analog beamforming scheme including optimizing phase shift and time delay based on the RISs' locations. Then, an alternatively optimization algorithm is proposed to obtain the digital beamforming and reflection coefficients based on the minimum mean square error and coordinate update techniques. Finally, simulation results show the effectiveness of the proposed scheme.

翻译：本文研究了在基站采用稀疏射频链天线结构的可重构智能表面辅助太赫兹通信系统。为克服基站的波束分裂问题，不同于传统的单层真时延方案，我们提出了一种双层真时延方案，该方案可有效减少需要额外插入损耗和放大电路的大范围延迟器件数量。随后，我们分析了所提双层真时延方案下的系统性能。为缓解RIS的波束分裂问题，我们考虑采用多个分布式RIS替代单个超大规模RIS。在此基础上，通过联合优化混合模拟/数字波束赋形、双层TTD网络时延以及RIS反射系数，建立了分布式RIS辅助太赫兹通信的可达速率最大化问题。考虑实际硬件限制，对有限分辨率移相器、时延和反射相位进行了约束。为解决该问题，我们首先基于RIS位置设计了包含移相器优化与时延优化的模拟波束赋形方案。随后提出一种交替优化算法，基于最小均方误差和坐标更新技术获取数字波束赋形与反射系数。最后，仿真结果验证了所提方案的有效性。