RIS-aided NOMA networks: Mitigating multipath fading and Doppler effects for beyond 5G wireless systems

Abstract

In beyond-5G wireless systems, user mobility causes Doppler-driven phase rotation, which changes the way the received signal behaves over time by superimposing multiple paths. Doppler doesn’t just cause a frequency shift; it mainly controls the time-varying complex-envelope magnitude through constructive and destructive interference between propagation routes. This research creates and tests a paradigm for modeling channels that takes into account mobility in reconfigurable intelligent surface (RIS)-assisted cooperative non-orthogonal multiple access (NOMA) systems, both when there is a line of sight (LoS) and when there is not (NLoS). Closed-form formulations for the instantaneous received complex envelope are obtained by explicitly integrating distance evolution, Doppler frequency, and RIS phase control. The model shows that envelope fluctuations are mostly caused by phase changes across small observation windows, whereas large-scale route loss stays mostly the same. Simulation results validate that RIS does not eradicate Doppler but rather modifies its expression by requiring coherent phase alignment among reflected components. This phase control greatly reduces deep fades, stabilizes the envelope amplitude, and cuts down on uncontrolled amplitude modulation in both the temporal and frequency domains. Doppler-spectrum and FFT analysis further show that RIS keeps the location of the Doppler peak while making the spectral profile more organized and less scattered. This means that destructive multipath interference is less likely to happen. In circumstances devoid of direct linkages, RIS-enabled reflection enhances array gain and raises the minimum envelope level, hence augmenting resilience against fading caused by mobility. The suggested paradigm makes a clear physical connection between phase evolution caused by the Doppler effect, envelope dynamics, and performance measures at higher levels. This gives a physics-consistent basis for studying and constructing RIS-assisted wireless systems in places where people move about a lot.