Radio Based Train Control Systems for the APM Market

Radio is the new communication technology that can appropriately service the APM market. Radio is wireless which means it's inconspicuous and unobtrusive, yet ever-present and accessible. It is also a proven and secure communication medium. How is radio utilized in a train control system? Radio is used to transmit data between the wayside applications controlling the trains and the trains themselves. This data exchange must be persistent in order to ensure smooth and regulated train operation. There is one problem though — several radios are required to provide signal coverage over the entire guideway. How is persistent data communication achieved? As a train moves through the system, handovers must occur between the onboard radio and those deployed adjacent to the track. This is accomplished through proper radio spacing and careful adjustment of the roaming and joining threshold parameters of the onboard radio. Seamless handovers, without data loss, has been proven at speeds of up to 130km/h. The Alcatel SelTrac train control system, as deployed on the Las Vegas Monorail system, makes use of open standard frequency hopping radios in the unlicensed 2.4GHz band. Las Vegas is an RF rich environment with a lot of interference in this public band. Dealing with this interference has been a challenge but convincingly overcome. Several techniques were applied including disregarding, avoiding, and contending with interference. Alcatel's arsenal to mitigate against interference includes, optimization of radio settings to disregard certain interference, correct antenna selection and positioning to avoid interference, and use of radios with frequency hopping and collision avoidance mechanisms to contend with interference. These techniques will be elaborated upon in the final paper. The combination of high speed trains, high speed radio handovers and high levels of interference has nevertheless resulted in insignificant latencies and has had an inconsiderable effect on data throughput. This has only been possible through the correct selection of equipment and a well architected system design. A radio environment opens the door to the breaching of security through intrusion and emulation. Security violations are subverted through the use of open standard authentication techniques involving dynamic key management. These techniques will be explained in the final paper. The skepticism over the use of radio for APM CBTC is no longer legitimate as evidence of it's viability becomes apparent. It is proven and secure.