Sum Rate Maximization via STAR-RIS Element Allocation in OFDM Systems

In this paper, we propose a sum rate maximization algorithm for a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted multiple-input single-output orthogonal frequency division multiplexing (MISO-OFDM) system, where the proposed algorithm updates the sub-carrier allocation, transmission and reflection coefficients, and sub-carrier power allocation. In specific, the transmission and reflection coefficients of the STAR-RIS are updated through a novel STAR-RIS element allocation technique. The STAR-RIS element allocation technique allocates each element to two sub-carriers, where one sub-carrier serves a single user-equipment (UE) in the transmission region of the STAR-RIS, and the other sub-carrier serves a single UE in the reflection region of the STAR-RIS. Through the element allocation process, the phase coefficients of the STAR-RIS are obtained in closed-form, and the amplitude coefficients of the STAR-RIS, which are to satisfy the law of conservation of energy, are obtained by iteratively solving a properly defined problem that has a closed-form solution, decreasing the complexity of the overall algorithm. Simulation results show that the proposed algorithm achieves high sum rate performance close to an optimization-based benchmark even with low complexity.