On Performance of FRIS-Enabled Uplink NOMA

The fluid antenna system (FAS) introduces reconfigurability in both position and geometry, offering new degrees of freedom (DoFs) for future wireless networks. Extending this concept, fluid reconfigurable intelligent surfaces (FRIS) enable each reflecting element to dynamically reposition within a local subregion while applying a tunable phase shift, providing significantly greater flexibility than conventional reconfigurable intelligent surface (RIS) architectures. This letter proposes an FRIS-assisted uplink non-orthogonal multiple access (NOMA) multiuser communication framework that jointly maximizes the sum rate and the minimum user rate, thereby balancing throughput and fairness under quality of service (QoS) constraints. The resulting mixed-integer, non-convex optimization problems are efficiently addressed through a lightweight alternating-optimization (AO) algorithm. For fixed FRIS coefficients, a triangular feasibility test with backward recursion yields closed-form NOMA power updates, while for fixed transmit powers, a majorization-minimization (MM) step selects the top-Sentries along the ascent direction and projects the optimized phases onto ab-bit unit circle. Our results confirm that the proposed FRIS-enabled uplink NOMA design significantly outperforms conventional RIS-based baselines, and reveal fundamental trade-offs among sum rate, fairness, and QoS feasibility in FRIS-assisted systems.