On Hybrid Cascaded RIS-Assisted Downlink NOMA-ISAC Systems

Abstract

This letter proposes a hybrid cascaded reconfigurable intelligent surface (RIS) architecture for downlink non-orthogonal multiple access (NOMA)-enabled integrated sensing and communication (ISAC) systems with a dual-functional radar-communication (DFRC) base station, where a passive RIS assists both sensing and phase control while an active RIS provides signal amplification. By jointly optimizing the cascaded passive-active RIS coefficients and NOMA beamforming under radar signal-to-noise ratio (SNR) constraints, we develop a low-complexity alternating optimization framework based on weighted minimum mean-square error (WMMSE), penalty method, and rotation phase algorithm. Closed-form solutions are derived for the active RIS amplification coefficients. Simulation results demonstrate that the proposed scheme achieves superior weighted sum-rate and sensing performance compared to dual-passive RIS and orthogonal multiple access (OMA) baselines, revealing a favorable sensing-communication tradeoff.