RIS-Assisted Multi-Aperture FSO Communication Network for High-Speed Train: Second-Order Statistical Analysis

This paper considers an optical reconfigurable intelligent surface (ORIS)-assisted free space optical (FSO) communication network to provide internet connectivity to the high speed train (HST). To maintain reliable FSO communication and compensate for severe pointing losses caused due to misalignment, we utilize a multi-aperture HST receiver that performs selection combining while achieving spatial diversity. In particular, we analyze the second-order statistics (SOS) at the HST receiver in the presence of atmospheric turbulence, foggy conditions, and misalignment error. We specifically develop the closed-form analytical expressions for the level crossing rate (LCR) and the average fade duration (AFD) of the system using the signal-to-noise ratio stochastic process under non-isotropic scattering environment. The effect of various system and channel parameters, including number of ORIS elements, number of receiving apertures, speed of the HST, misalignment error and fog parameters, mean angle-of-arrival, and degree of non-isotropic scattering, has been demonstrated on the LCR and AFD performances of the considered network. Further, by using finite-state Markov channel (FSMC) model, we derive packet error rate (PER) using the derived LCR expression. In order to analytically evaluate the optimum packet length for the FSMC model, we employ stop-and-wait automatic repeat request (SW-ARQ) protocol and maximize the system throughput. Additionally, it is shown numerically how different system settings affect the PER and throughput performances of the FSO-based HST communication network.