Uplink Sum-Rate Optimization in RIS-Assisted NOMA Systems with Spatial Correlation

This paper addresses the uplink sum-rate maximization in RIS-assisted power-domain NOMA systems. The aim is to optimize RIS phase shifts under unit-modulus constraints, resulting in a non-convex problem with coupled variables. To tackle this, an alternating optimization (AO) method is proposed, in which the RIS configuration is updated for one user at a time, keeping others fixed. Each step uses a phase-only conjugate gradient method (CGM), inspired by adaptive array processing. Various models for spatial correlation among the RIS elements are explicitly considered. Simulations show that the proposed AO-CGM approach improves both the sum-rate and the user fairness compared to conventional methods.