SER-Effective Constellation Scaling and Rotation in STAR-RIS-Assisted Uplink NOMA

Error-rate performance is one of the important metrics in any communication system. In this letter, we leverage on the ability of the simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) to alter distinctively the channel characteristics of its reflection and transmission zone, enhancing the performance of non-orthogonal multiple access (NOMA). Specifically, to improve the symbol-error rate (SER) performance of an uplink NOMA system when users are employing QAM modulation, we propose to adjust the phase-shift and amplitude parameters of the STAR-RIS based on the distance-maximizing constellation scaling and rotation. The proposed schemes are explained using a simplified model, and simulations show that they significantly improve the SER performance of the system compared to other baseline schemes.