I. Introduction
High-speed Internet connectivity is becoming an integral and indispensable part in our daily lives. Broadband data services are expected to be provisioned in high-speed trains (HSTs). Thus, providing Internet access in HSTs is one of the main incentives for the railway operators to attract more passengers. Currently, radio frequency wireless technologies are used to provide Internet access to passengers [1]– [3]. However, owing to the limitation of radio wave bandwidth and interference [4], [5], the existing infrastructure based on radio frequency technology such as Wi-Fi/WiMAX can provide peak data rates of up to 54/75 Mbps theoretically, which drop to lower than 10 Mbps in real scenarios [6], and thus cannot satisfy users’ growing data traffic demand [7]. In this situation, Free Space Optics (FSO) becomes a viable alternative wireless access technology to meet the increasing demand for high quality multi-media services in HSTs. Since the available frequency of the FSO technology is over 300 GHz which is license-free all over the world [8], it is promising to provision high data communications in HSTs. For example, an FSO system with a faster handover mechanism has been proposed to achieve a data rate in excess of 500 Mbps in HSTs [9]. Moreover, owing to Line-of-Sight (LOS), FSO is immune to the impact of multi-path propagation and interference from other transmitters, which remarkably degrade the system performance of radio frequency technologies [10] .