Performance and Phase Optimization of Energy-Harvesting Enabled Multi-User RIS-Assisted Cooperative NOMA Systems Over κ–μ Shadowed Fading

Abstract

Reconfigurable intelligent surfaces (RISs) and non-orthogonal multiple access (NOMA) are often analyzed under idealized assumptions such as fully active RIS panels, perfect phase control, and cooperative relaying models that neglect RIS control energy. These assumptions can overestimate reliability in hardware-constrained deployments. This paper develops an energy-neutral RIS-assisted cooperative downlink NOMA framework in which the RIS activates only a subset of reflecting elements according to a frame-level harvested-energy budget, while the near user $U_{1}$ employs power-splitting simultaneous wireless information and power transfer (PS–SWIPT) to support decode-and-forward (DF) relaying. The model captures an SNR-dependent active-element count, finite-resolution RIS phase control, imperfect successive interference cancellation (SIC), and the coupling between the Phase-1 relay decoding gate and the Phase-2 forwarding power. A low-complexity weighted RIS phase-alignment rule is used to balance cascaded gains across users while supporting the SIC chain at $U_{1}$ . Using finite mixture-Gamma surrogates, a tractable near-user outage expression and a gate-conditioned far-user outage approximation are developed under DF relaying with maximal-ratio combining (MRC). Monte-Carlo simulations for a four-user system and a two-user specialization validate the analysis and quantify the effects of energy-neutral RIS activation, phase quantization, SWIPT power splitting, imperfect SIC, nonlinear energy harvesting, and imperfect CSI. The results provide design guidelines for RIS–NOMA reliability under practical RIS power-consumption and energy-harvesting constraints.