Comparison of RIS-Assisted and DF Relay-Assisted NOMA Communication Systems

초록

Reconfigurable intelligent surface (RIS)-assisted non-orthogonal multiple access (NOMA) communication networks represent a current research hotspot. This study presents a comprehensive performance comparison between RIS-assisted and decode-and-forward (DF) relay-assisted NOMA systems in an urban environment. Complete system models are developed for both configurations, and their performance is evaluated across multiple metrics, including transmission power, decoding strategy, number of RIS elements, and energy efficiency under varying data rate requirements. The results reveal that in RIS-assisted systems, the effective user channel quality, and thus the optimal decoding order, can change with the number of RIS elements, requiring dynamic adaptation to maintain fairness and maximise throughput. Furthermore, while the DF relay-assisted system performs better at low to moderate data rates, the RIS-assisted system outperforms it when the data rate exceeds a certain threshold. Notably, as the target data rate increases, the number of RIS elements required to surpass DF relay performance decreases sharply, with only a single RIS element needed when the rate exceeds 8 bit/s/Hz. Although the DF relay-assisted system achieves higher energy efficiency at moderate data rates, the RIS-assisted system becomes the most energy-efficient option in high-rate scenarios, because of its passive design and low perelement power consumption. © 2025 IEEE.

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