Free space optical communications (FSOC), pointing errors, reconfigurable intelligent surface (RIS), index modulation (IM), average bit error rate (ABER), Applied optics. Photonics, TA1501-1820, Optics. Light, QC350-467

This work investigates the performance of a hybrid communication system that integrates radio frequency (RF) and free-space optical (FSO) systems for backhaul networks. In this framework, both the RF and FSO links are enhanced by reconfigurable intelligent surfaces (RISs) and employ index modulation (IM) for link switching. The fading in the RF link is modeled using the Nakagami-m distribution, while the FSO link is affected by attenuation, turbulence, and pointing errors, with turbulence characterized by the Gamma-Gamma distribution. Link switching is performed through index modulation, where the selection of the transmission link conveys information bits. We derive exact closed-form expressions for the probability density function (PDF), cumulative distribution function (CDF), and moment-generating function (MGF) of the signal-to-noise ratio (SNR) for RIS-assisted FSO links. Utilizing these SNR statistics, we compute the outage probability (OP), average bit error rate (ABER), and ergodic capacity (EC) of the proposed hybrid RF/FSO system. In the numerical results, the performance of the proposed hybrid RF/FSO system is compared against other systems, including hybrid systems using index modulation without RISs, RIS-assisted FSO systems, and hybrid systems employing hard switching techniques. Finally, Monte Carlo simulations are carried out to validate the accuracy of the theoretical results.