Performance Analysis of RIS-Assisted Relay Systems

Reconfigurable intelligent surface (RIS) is an emerging technology that can enhance service coverage and spectral efficiency in sixth-generation wireless networks. In contrast, relays are a traditional technique for achieving coverage extension. To explore potential benefits, we investigate three probable RIS-relay cascaded scenarios over Nakagami-$m$ fading channels, where the full-duplex relay is utilized for ensuring spectral efficiency. To evaluate the performance across different scenarios, we first derive their closed-form expressions for the outage probability. Next, we determine the respective diversity order using asymptotic approximations in the high signal-to-noise-ratio regime. Finally, we present the system throughput in delay-limited transmission. Simulation results validate our analysis and demonstrate significant performance differences among various cascaded configurations, with the multi-RIS-assisted relay scenario achieving the best performance. Furthermore, the considered RIS-decode-and-forward relay schemes outperform the corresponding RIS-amplify-and-forward relay schemes in the low-SNR regime.