Combining multi-RIS and relay for performance improvement of multi-user NOMA systems

Abstract

In this paper, we propose a theoretical framework to examine the performance of a non-orthogonal multiple access (NOMA) system with multiple users under the support of multiple advanced reconfigurable intelligent surfaces (RISs) and a traditional relay. We derive the analytical expressions of main performance metrics of the proposed multi-RIS-and-relay aided multiple-user NOMA system (so called the RIS-R-NOMA system), including the outage probability (OP) and the achievable data rate (ADR) over Nakagami-m fading channels. We conduct the Monte-Carlo simulations to confirm the exactness of our mathematical expressions. Numerical results reveal that the OP of the proposed RIS-R-NOMA system is significantly lower than the OPs of the traditional NOMA systems without either RIS or relay (the R-NOMA and RIS-NOMA systems), and the ADR of the proposed RIS-R-NOMA system is much higher than the ADR of the R-NOMA system. Importantly, at high transmit power, the ADRs of both RIS-R-NOMA and R-NOMA systems are similar due to the property of NOMA. Additionally, the effects of various parameters such as the number of reflecting elements (REs), the locations of the RISs, the power allocation coefficients, and the data transmission rates on the performance of the RIS-R-NOMA system are demonstrated.