Reconfigurable Intelligent Surface Assisted Unified NOMA Framework

This article investigates the application of reconfigurable intelligent surface (RIS) to assist a unified non-orthogonal multiple access (RIS-U-NOMA) framework. More specifically, the proposed RIS-U-NOMA can be applied to RIS assisted code-domain NOMA (RIS-CD-NOMA) and power-domain NOMA (RIS-PD-NOMA). To characterize the performance of RIS-U-NOMA framework, we derive new approximated expressions of outage probability and ergodic rate for a pair of users. Based on the asymptotic expressions, the diversity orders and high signal-to-noise ratio (SNR) slopes for the nearby and distant users are obtained in details. In addition, the system throughput of RIS-U-NOMA is discussed in the delay-limited and delay-tolerant transmission modes. Monte Carlo simulations are provided to verify the analytical results and indicate that: 1) The outage behaviors of RIS-U-NOMA are better than that of RIS assisted orthogonal multiple access (RIS-OMA) and conventional full/half-duplex decode-and-forward relays; 2) The RIS-U-NOMA networks are competent to obtain superior performance in ergodic rate and achieve steeper slope than RIS-OMA; and 3) The delay-limited and delay-tolerant throughput of RIS-CD-NOMA outperform that of RIS-PD-NOMA.