  Free-space optic (FSO) is a promising solution to provide broadband Internet
access for high-speed trains (HSTs). Besides, reconfigurable intelligent
surfaces (RIS) are considered as hardware technology to improve performance of
optical wireless communication systems. In this paper, we propose a
RIS-assisted FSO system to provide access connectivity for HTSs, as an upgrade
for the existing direct and relay-assisted FSO access setups. Our motivation is
mainly based on well-proven results indicating that a RIS-assisted optical
wireless system, with a large enough number of RIS elements, outperforms a
relay-assisted one thanks to its programmable structure. We firstly compute the
statistical expressions of the considered RIS-assisted FSO channels under weak
and moderate-to-strong fading conditions. Then, the network's average
signal-to-noise ratio and outage probability are formulated based on the
assumed fading conditions, and for two fixed- and dynamic-oriented RIS coverage
scenarios. Our results reveal that the proposed access network offers up to
around 44% higher data rates and 240% wider coverage area for each FSO base
station (FSO-BS) compared to those of the relay-assisted one. The increase of
coverage area, on average, reduces 67% the number of required FSO-BSs for a
given distance, which results in fewer handover processes compared to the
alternative setups. Finally, the results are verified through Monte-Carlo
simulations.
