Mixed RF/FSO Relay Networks: RIS-Equipped RF Source vs RIS-Aided RF Source

This letter proposes and evaluates the performance of reconfigurable intelligent surface (RIS)-equipped source mixed radio frequency (RF)/free space optical (FSO) relay network with opportunistic RF source scheduling. Closed-form analytical approximations are derived for the outage probability and average symbol error probability (ASEP) assuming Rayleigh and Gamma-Gamma fading models for the RF and FSO channels, respectively. Moreover, the system is studied at the high signal-to-noise ratio (SNR) regime, where the diversity order and coding gain are provided. Results illustrate that at high SNR values, the system performance is dominated by the worst hop and the diversity order is equal to $G_{d}=\mathrm {min}(KN,\alpha,\beta,\zeta ^{2})$ , where ${K}$ is the number of RF sources, ${N}$ is the number of reflecting elements at each source, $\alpha $ and $\beta $ are the atmospheric turbulence parameters of the FSO link, and $\zeta ^{2}$ is a measure for the alignment quality of the FSO source and receiver. In addition, findings show that for the same $G_{d}$ , ${N}$ is more impactful on the performance than ${K}$ through the coding gain $G_{c}$. Furthermore, the RIS-equipped source scenario where RIS is internally implemented in RF sources is shown to outperform the RIS-aided scenario where RIS is used as relay in terms of both $G_{d}$ and $G_{c}$.