Using Fiber Optic-to-Radio Frequency (RF) Conversion for Communication-Based Train Control

The design of the Automatic Train Control (ATC) Radio System for the Bombardier CITYFLO* 650 (formerly called Flexiblok) has evolved into a system employing a fiber-optic based RF signal distribution system. In a communication-based train control system, the RF signal distribution system must achieve near-continuous signal coverage for the transit system’s route and at the same time afford high availability through redundant design. This paper begins with the background for the first-generation system using distributed line amplifiers, as were employed at San Francisco International Airport, Seattle-Tacoma International Airport, Dallas/Fort Worth International Airport, and SEPTA’s Subway-Surface Line in Philadelphia. The second-generation design uses a fiber optic-based approach that converts the base radio signal directly to an optical signal at the train control equipment room and recreates the RF signal up to 20 kilometers away. It is used for both radiating cable and line-of-sight antenna solutions. The advantages of this latest generation design are discussed, as well as how the approach is tailored for train control applications. The direction of RF technology in communication-based train control for the future is also addressed.