Non-orthogonal Multiple Access with RISs

This chapter focuses on exploiting reconfigurable intelligent surfaces (RISs) in multi-user networks employing non-orthogonal multiple access (NOMA). Two application scenarios for RIS-assisted multi-user networks have been presented. In particular, for RIS-assisted single-antenna multi-user networks, a resource allocation problem was formulated to maximize the system throughput by jointly optimizing the channel assignment, decoding order, power allocation and reflection coefficients. By invoking alternating optimization, successive convex approximation and many-to-one matching, a suboptimal resource allocation algorithm was proposed. For RIS-assisted multiple-antenna multi-user networks, a joint optimization problem over active beamforming, passive beamforming and power allocation coefficients was considered. To tackle the resultant non-convex optimization, an alternative iterative algorithm was proposed. Numerical results illustrate that the proposed RIS-assisted NOMA networks outperforms the conventional NOMA system without the RIS.