Average Sum-Rate Maximization of Coupled Phase-Shift STAR-RIS-Assisted SWIPT-NOMA System

Abstract

Coupled phase-shift simultaneously transmitting and reflecting reconfigurable intelligent surface (CPS-STAR-RIS) is regarded as an economically efficient alternative to STAR-RIS with an independent phase-shift model. In this letter, we investigate a simultaneous wireless information and power transfer (SWIPT) system with non-orthogonal multiple access (NOMA) assisted by CPS-STAR-RIS. To maximize the average sum-rate (ASR) of the system, we propose an alternating optimization (AO) scheme to jointly optimize the active beamforming vectors at the base station (BS) and the transmission and reflection coefficients at the CPS-STAR-RIS. Specifically, we first equivalently represent the ASR maximization problem by applying the weighted minimum mean square error (WMMSE) algorithm to address the coupling of variables. Subsequently, the successive convex approximation (SCA) algorithm and the penalty dual decomposition (PDD) algorithm are adopted to tackle the optimization challenges of the active beamforming vectors at the BS and the coefficients at the CPS-STAR-RIS, respectively. Simulations show that compared with the conventional RIS (CRIS) and the OMA schemes, the proposed scheme significantly enhances the ASR.