A simultaneously transmitting and reflecting surface (STARS) aided terahertz (THz) communication system is proposed. A novel power consumption model is proposed that depends on the type and resolution of the STARS elements. The spectral efficiency (SE) and energy efficiency (EE) are maximized in both narrowband and wideband THz systems by jointly optimizing the hybrid beamforming at the base station (BS) and the passive beamforming at the STARS. 1) For narrowband systems, independent phase-shift STARSs are investigated first. The resulting complex joint optimization problem is decoupled into a series of subproblems using penalty dual decomposition. Low-complexity element-wise algorithms are proposed to optimize the analog beamforming at the BS and the passive beamforming at the STARS. The proposed algorithm is then extended to the case of coupled phase-shift STARS. 2) For wideband systems, the spatial wideband effect at the BS and STARS leads to significant performance degradation due to the beam split issue. To address this, true time delayers (TTDs) are introduced into the conventional hybrid beamforming structure for facilitating wideband beamforming. An iterative algorithm based on the quasi-Newton method is proposed to design the coefficients of the TTDs. Finally, our numerical results confirm the superiority of the STARS over the conventional reconfigurable intelligent surface (RIS). It is also revealed that i) there is only a slight performance loss in terms of SE and EE caused by coupled phase shifts of the STARS in both narrowband and wideband systems, and ii) the conventional hybrid beamforming achieves comparable SE performance and much higher EE performance compared with the full-digital beamforming in narrowband systems but not in wideband systems, where the TTD-based hybrid beamforming is required for mitigating wideband beam split.

翻译：提出了一种同时透射与反射表面（STARS）辅助的太赫兹（THz）通信系统。首先建立了一种依赖于STARS单元类型与分辨率的新型功耗模型。通过联合优化基站（BS）的混合波束赋形与STARS的被动波束赋形，分别最大化窄带与宽带太赫兹系统的频谱效率（SE）与能量效率（EE）。1）针对窄带系统，首先研究了独立相移STARS。利用惩罚对偶分解将复杂的联合优化问题解耦为一系列子问题，提出低复杂度的逐元素算法以优化基站的模拟波束赋形与STARS的被动波束赋形，并将该算法扩展至耦合相移STARS场景。2）针对宽带系统，基站与STARS的空间宽带效应因波束分裂问题导致性能显著下降。为此，在传统混合波束赋形结构中引入真延时器（TTD）以支持宽带波束赋形，提出基于拟牛顿法的迭代算法设计TTD系数。数值结果验证了STARS相较传统可重构智能表面（RIS）的优越性，并揭示：i）在窄带与宽带系统中，STARS的耦合相移仅导致频谱效率与能量效率的轻微性能损失；ii）窄带系统中传统混合波束赋形可实现与全数字波束赋形相当的频谱效率并具有更高的能量效率，而宽带系统中需采用基于TTD的混合波束赋形以缓解宽带波束分裂效应。