RIS-Assisted Multihop FSO/RF Hybrid System for Vehicular Communications over Generalized Fading

Reconfigurable intelligent surface (RIS) is a promising technology to avoid signal blockage by creating line-of-sight (LOS) connectivity for free-space optical (FSO) and radio frequency (RF) wireless systems. There is limited research on the use of multiple RIS between a source and destination for wireless communications. This paper analyzes the performance of a RIS-assisted multi-hop transmission for vehicular communications by employing multiple RIS to enable LOS communication and reliable connectivity for a hybrid FSO and RF system. We develop an analytical framework to derive statistical results of the signal-to-noise ratio (SNR) of a multi-RIS communication system over general fading models. We use decode-and-forward (DF) and fixed-gain (FG) relaying protocols to mix multi-RIS transmissions over RF and FSO technologies, and derive probability density and distribution functions for both the relaying schemes by considering independent and non-identical double generalized gamma (dGG) distribution models for vehicular RF transmissions and atmospheric turbulence for FSO system combined with zero-boresight pointing errors. We analyze the performance of a moving vehicle connected to one of the RIS modules by deriving exact analytical expressions of the outage probability, average bit-error rate (BER), and ergodic capacity in terms of Fox's H-function. We present asymptotic analysis and diversity order of the outage probability in the high SNR regime to provide a better insight into the system performance. We use computer simulations to demonstrate the effect of multiple RIS modules, fading parameters, and pointing errors on the RIS-aided multi-hop transmissions for the considered vehicular communication system.