RIS-Empowered Relays for Cooperative NOMA

To harness the benefits of non-orthogonal multiple access (NOMA) and reconfigurable intelligent surfaces (RISs), we propose a novel RIS-empowered decode-and-forward (RIS-DF) relaying scheme tailored to cooperative NOMA transmissions. This integration is a promising direction for improving communication reliability, extending coverage, and enhancing the overall performance in sixth-generation (6G) wireless networks. This paper focuses on enhancing signal reception of the cell-edge users with weak channel conditions by deploying multiple RISs within cooperative NOMA systems. In this setting, we investigate system outage performance and derive closed-form analytical expressions for both cooperative NOMA and its orthogonal multiple access (OMA) counterpart, which is used as a benchmark for comparison. To validate the proposed solutions, numerical results are provided to demonstrate the performance gains of the proposed scheme compared to the existing conventional DF relays developed for cooperative NOMA. Further, the impact of RIS placement within the system on performance is also examined, offering useful practical design insights.