In this paper, we consider the simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted THz communications with three-side beam split. Except for the beam split at the base station (BS), we analyze the double-side beam split at the STAR-RIS for the first time. To relieve the double-side beam split effect, we propose a time delayer (TD)-based fully-connected structure at the STAR-RIS. As a further advance, a low-hardware complexity and low-power consumption sub-connected structure is developed, where multiple STAR-RIS elements share one TD. Meanwhile, considering the practical scenario, we investigate a multi-STAR-RIS and multi-user communication system, and a sum rate maximization problem is formulated by jointly optimizing the hybrid analog/digital beamforming, time delays at the BS as well as the double-layer phase-shift coefficients, time delays and amplitude coefficients at the STAR-RISs. Based on this, we first allocate users for each STAR-RIS, and then derive the analog beamforming, time delays at the BS, and the double-layer phase-shift coefficients, time delays at each STAR-RIS. Next, we develop an alternative optimization algorithm to calculate the digital beamforming at the BS and amplitude coefficients at the STAR-RISs. Finally, the numerical results verify the effectiveness of the proposed schemes.

翻译：本文研究了同时透射与反射可重构智能表面（STAR-RIS）辅助的三侧波束分裂太赫兹通信系统。首次分析了除基站（BS）波束分裂外，STAR-RIS处的双侧波束分裂效应。为缓解该双侧波束分裂效应，提出了一种基于时间延迟器（TD）的STAR-RIS全连接结构。进一步，开发了低硬件复杂度与低功耗的子连接结构，其中多个STAR-RIS单元共享一个TD。同时，考虑实际场景，研究了多STAR-RIS与多用户通信系统，通过联合优化混合模拟/数字波束赋形、基站时延、STAR-RIS双层相移系数、时延及幅度系数，构建了和速率最大化问题。基于此，首先为每个STAR-RIS分配用户，随后推导了基站模拟波束赋形与时延、STAR-RIS双层相移系数及时延；接着开发了交替优化算法以计算基站数字波束赋形与STAR-RIS幅度系数。数值结果验证了所提方案的有效性。