Coverage Analysis of Hybrid FSO/RF Communication Systems with Selection Combining in High-Speed Railway Scenarios

Modern transportation systems impose stringent requirements on high-speed railway (HSR) communications in terms of high data rate and high reliability. Conventional radio frequency (RF) links are limited by the scarcity of spectrum resources, whereas free-space optical (FSO) links are highly vulnerable to atmospheric turbulence and weather conditions. To overcome the limitations of a single transmission medium, this paper proposes and investigates a hybrid FSO/RF selection combining (SC) communication system for narrow-strip cells in HSR scenarios. First, a geometric coverage model is established for heterogeneous base-station deployment, together with a location-aware activation mechanism that enables adaptive activation of base stations according to train positions. Then, by taking practical railway operating conditions into account, the FSO link is modeled to capture the combined effects of atmospheric turbulence, pointing errors, and atmospheric attenuation, while the RF link is modeled as a composite channel incorporating path loss, shadow fading and small-scale fading. Based on this framework, exact closed-form expressions for the system edge coverage probability (ECP) and the percentage of cell coverage area (CCA) are derived. For the sake of comparisons, theoretical expressions of the standalone FSO and standalone RF links are also obtained. Finally, Monte-Carlo simulations are conducted to verify the validity of the derived analytical results. The numerical comparisons demonstrate excellent agreement between the analytical predictions and simulation results, with average relative deviations of only 0.047% for ECP and 0.095% for the percentage of CCA. Compared with either the standalone FSO system or standalone RF system, the proposed hybrid architecture achieves superior coverage performance across different transmit powers, weather conditions, and cell diameters, offering a highly reliable and robust networking solution for future seamless broadband HSR communication systems.