Energy Efficiency Optimization for Distributed STAR-RIS-Aided Communication Systems with Coupled Phase-Shift

Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) can achieve full-space coverage of the signal compared to conventional reconfigurable intelligent surface (RIS). However, in existing works, the problem of energy efficiency optimization for multiple STAR-RISs-aided multi-user communication systems with the coupled phase-shift (CPS) constraint remains unsolved. To this end, this letter proposes a novel joint optimization framework for solving this problem. To overcome this complex nonlinear, nonconvex issue, we decompose the main problem into two individual subproblems: phase-shift optimization, and base station (BS) beamforming design. Penalty dual decomposition (PDD)-based and successive convex approximation (SCA) methods are employed to solve the two subproblems in the alternating optimization process. Simulation outcomes demonstrate that the proposed approach surpasses both centralized deployment strategies and traditional RIS-assisted systems.