STAR-RIS-Enabled Multi-Path Beam Routing with Passive Beam Splitting

Reconfigurable intelligent surfaces can be densely deployed in the environment to create multi-reflection line-ofsight (LoS) links between base stations (BSs) and users, thereby significantly enhancing the BS coverage. However, conventional RISs can only achieve half-space reflection, which limits the LoS path diversity. In contrast, simultaneously transmitting and reflecting RISs (STAR-RISs) can split incident signals into reflected and transmitted signals pointing to different half spaces simultaneously, thereby creating more LoS paths. Hence, in this paper, we study a new multi-STAR-RIS-aided wireless communication system, where a multi-antenna base station (BS) transmits to a single-antenna user by exploiting the signal beam routing over a set of cascaded LoS paths each formed by multiple STAR-RISs. We aim to jointly optimize the active beamforming at the BS, the BS’s power allocation over different paths, the number of beam-routing paths, the selected STAR-RISs for each path, as well as their amplitude and phase shifts for transmission/reflection, such that the received signal power at the user is maximized. However, this problem is particularly 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 solutions 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. Simulation results show that our proposed STAR-RIS-enabled beam routing can achieve much better performance than the conventional beam routing with reflection-only RISs.