A Study on Digital/Analog Signals that Facilitate Analog ATC to Digital ATC Transition

Abstract

Owing to the need for high-speed, high-density, and comfortable rail transport, an automatic train control (ATC) system in which digital signal technologies are used to achieve wayside-to-onboard data communication via rails has been developed. However, it is not easy to apply digital signals to existing revenue lines, which already use analog signals, because the current frequency allocation cannot be changed due to the track circuit arrangement and interoperability situations. Meanwhile, existing analog signals that are transmitted contain information in the form of amplitude-modulation frequencies, which are envelope components of signal wave. Unless the modulation frequencies are varied, digital modulation signals such as phase-modulated signals or frequency-modulated signals can be used as carrier wave. Combined digital/analog signals consisting of digital modulation signals that are amplitude-modulated using existing signals frequencies, enable digital signals to be used in the revenue lines with analog ATC.We studied combined digital/analog signals and found that they satisfy the requirements of both analog and digital signals. Consequently, we conclude that signal transmission to existing ATC systems becomes possible by using an MSK (minimum shift keying) wave as a carrier wave of digital/analog signals and the modulation amplitude of carrier wave identical to that of an existing wave.