Phase Shift Configuration Strategies for Unbalanced T&R Users in STAR-RIS-Aided NOMA

Abstract

This letter considers a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided non-orthogonal multiple access (NOMA) system in the presence of unbalanced transmission and reflection (T&R) users. It takes into account the correlated T&R phase shifts and proposes various phase shift configuration strategies, namely blind phase shifting (BPS), STAR-RIS partitioning (SPS), secondary user STAR-RIS partitioning (SSPS), and near-field broadcasting (NBS). The ergodic sum rate is then analyzed under the mentioned strategies. Numerical and simulation results demonstrate the NBS strategy's superiority over other strategies. However, its effectiveness is constrained by the distance and size of the STAR-RIS, making it more suitable for short-range applications. The results also reveal that the SPS strategy outperforms the BPS and SSPS strategies in low-power scenarios and with limited STAR-RIS elements allocated for the secondary user. The SSPS strategy shows substantial improvement in high-power scenarios, making it well-suited for systems such as millimeter-wave and terahertz wireless communications.