Over the last decade, railway industry has seen a huge transition from conventional railway signalling systems to modern, communication-based signalling systems. Communications-based train control (CBTC) is a modern communication-based system that uses radio communication to transfer timely and accurate train control information. CBTC is the choice of mass-transit railway operators today, with over a hundred systems currently installed worldwide. The safety-related, time-critical applications such as train control impose stringent reliability and availability requirements on the radio communication technology used. IEEE 802.11 Wi-Fi, despite being originally developed for stationary users within a limited area, has prevailed as the de-facto radio technology for CBTC. Unfortunately, very limited literature is publicly available on this topic due to the highly competitive nature of the railway industry. We believe that this paper fills the much-needed gap. It aims to present a comprehensive tutorial, as well as a survey of the state-of-the-art, of CBTC and the role of radio communication in it. The operation and fundamental components of a CBTC system are discussed. A summary of the evolution of the communication technologies used for modern railway signalling is presented. The benefits and drawbacks of using a radio communication technology, particularly Wi-Fi, and the challenges it introduces, are discussed. Best practices in the design of a CBTC radio network and the measures to optimize its availability are discussed, while using the currently in-progress Copenhagen S-train CBTC project as a reference. An overview of the CBTC standardization efforts, as well as the IEEE CBTC standard-frequently overlooked due to its limited scope-is included. This paper is concluded by providing a number of potential directions for future work.
