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This paper describes how railway signaling systems that transmit control information via rails have been deployed in many applications, for example, digital automatic train control (ATC) systems for controlling train speed. Since the performance of digital ATC systems depends on the signal transmission speed, recently there have been many studies aimed at realizing high-speed data transmission. However, it is difficult to increase the transmission speed because rails have strong attenuation at frequencies above about 10 kHz. Therefore, the present transmission speed limit of digital ATC using rails is about 300 bps. To overcome these limitations, the paper aimed to increase the transmission speed by improving the modulation method. In this paper, we propose a new modulation method combining code-division multiple access (CDMA) and quadrature amplitude modulation (QAM), called the CDMA-QAM method. This method transmits QAM symbols allocated based on a CDMA signal. It has the advantage that QAM errors can be recovered if they are in a range where they can be absorbed by CDMA demodulation. The paper developed a CDMA-QAM rail transmission system based on digital signal processors. The paper conducted a field experiment using an actual rail and verified that the system can achieve favorable constellation characteristics with about 20 dB attenuation of the received signal at a carrier frequency of 3 kHz.

Development of a high-speed rail transmission system using digital signal processors for railway signalling