Reconfigurable intelligent surfaces (RISs) can be densely deployed in the environment to create multi-reflection line-of-sight (LoS) links for signal coverage enhancement. However, conventional reflection-only RISs can only achieve half-space reflection, which limits the LoS path diversity. In contrast, simultaneously transmitting and reflecting RISs (STAR-RISs) can achieve full-space reflection and transmission, thereby creating more LoS paths. Hence, in this paper, we study a new multi-STAR-RIS-aided communication system, where a multi-antenna base station (BS) transmits to multiple single-antenna users by exploiting the signal beam routing over a set of cascaded LoS paths each formed by multiple STAR-RISs. To reveal essential insights, we first consider a simplified single-user case, aiming to maximize its received signal power by jointly optimizing the active beamforming at the BS, the BS's power allocation over different paths, the number of selected beam-routing paths, the selected STAR-RISs for each path, as well as their amplitude and phase shifts for transmission/reflection. However, this problem is difficult to be optimally solved as different paths may be intricately coupled at their shared STAR-RISs. To tackle this difficulty, we first derive the optimal solution to this problem in closed-form for a given set of paths. The clique-based approach in graph theory is then applied to solve the remaining multi-path selection problem efficiently. Next, we extend the proposed clique-based method to the multi-user case to maximize the minimum received signal power among all users, subject to additional constraints on the disjointness of the selected paths for different users. Simulation results show that our proposed STAR-RIS-enabled beam routing outperforms the conventional beam routing with reflection-only RISs in both single- and multi-user cases.

翻译：可重构智能表面（RIS）可在环境中密集部署，以创建多反射视距（LoS）链路来增强信号覆盖。然而，传统的仅反射型RIS只能实现半空间反射，这限制了视距路径的多样性。相比之下，同时透射和反射型智能表面（STAR-RIS）能够实现全空间的反射与透射，从而创建更多的视距路径。因此，本文研究一种新型的多STAR-RIS辅助通信系统，其中多天线基站（BS）通过利用一组级联视距路径上的信号波束路由，向多个单天线用户传输信号，每条路径由多个STAR-RIS构成。为揭示核心机理，我们首先考虑一个简化的单用户场景，旨在通过联合优化BS处的主动波束成形、BS在不同路径上的功率分配、所选波束路由路径的数量、每条路径所选的STAR-RIS及其透射/反射的幅度与相位偏移，来最大化用户的接收信号功率。然而，由于不同路径可能在其共享的STAR-RIS处复杂耦合，该问题难以获得最优解。为克服此困难，我们首先针对给定路径集推导了该问题的闭式最优解。随后应用图论中基于团的方法来高效求解剩余的多路径选择问题。接着，我们将所提出的基于团的方法扩展到多用户场景，以最大化所有用户中最小的接收信号功率，并受限于不同用户所选路径需互不相交的附加约束。仿真结果表明，在单用户和多用户场景下，我们所提出的STAR-RIS赋能波束路由方案均优于传统的仅反射型RIS波束路由方案。