Power Allocation for STAR-RIS-Assisted C-V2X System with Discrete Phase Shifts

In this paper, a power optimization problem with discrete phase shifts is presented for energy splitting (ES), mode switching (MS) and time switching (TS) in a STAR-RIS-assisted downlink MISO 3D non-stationary cellular vehicle-to-everything(C-V2X) scenario, where base station(BS) transmits signals to client vehicles via a beacon vehicle equipped with simultaneously transmitting and reflecting reconfigurable intelligent surface(STAR-RIS). Since STAR-RIS introduces discrete phase constraints, for solving this non-convex problem, minimum Euclidean distance criterion, majorization-minimization (MM) and penalty function algorithm are utilised to jointly optimize active and passive beamforming matrices to satisfy the quality of service (QoS) of client vehicles. Numerical results show that TS protocol has better performance in reducing power in the proposed C-V2X scenario, whether in continuous or discrete phases. Moreover, we examine the impact of quantization bit number on the power performance loss compared with continuous phases and give advice on engineering applications.