Software-Defined Radio for Positive Train Control
Authors: Adams Jon T. , Li Jia-Ru 
Published: July 2013

United States (US) Freight and Passenger railroads and transit entities are preparing for the deployment of Positive Train Control (PTC) systems as soon as the end of 2012, and by Federal mandate, by the end of 2015. PTC depends on a reliable and robust wireless data link between train and wayside equipment, train and network access points, and waysides and network access points. While, for the wireless component, some spectrum has been acquired and initial functionality defined, there will be lessons learned and new expectations as systems are rolled out over this period and beyond. Since the average service life of radio equipment in rail and transit applications is in excess of 10–15 years, and a system design may remain in place for 20–25 years, the challenge is to ensure that wireless systems are flexible enough to adapt to technology changes that may occur during the radio system’s long service life. Software-Defined Radio (SDR) is the appropriate approach to deal with this future uncertainty.
The worldwide cellular communications standards are a great example: 3GPP established its first 3G specification, Release-99, in the year 2000; there have been 7 full specification releases since then, demonstrating the challenges of predicting future use based on current expectation. Deployment and long-term maintenance account for the majority of system cost: when system deployment is expensive and time-consuming, the needs for flexible design are crucial. The maintenance cost should be added and weighted higher in the calculation for total cost of ownership. One can argue that fixed design is cheaper; however, the total cost of ownership may end up much higher when changes are needed before the end of the product’s expected service life. In Information Technology, the system performance doubles approximately every 18–24 months. Thoughtful radio design can leverage this generational improvement.
To address the issue of broad interoperability and a forward-looking specification, radio experts, including transit systems integrators, Federal agencies, end device and chipset manufacturers, and researchers, from around the world have teamed up under the aegis of the Institute of Electrical and Electronic Engineers (IEEE) to establish a Positive Train Control group for the RF/PHY/MAC based upon the well adopted IEEE 802.15.4 standard. The outcome of this effort will ensure the maturity of a PTC specification and long term maintenance of this mission-critical PTC wireless interface protocol. With any new specification development, continuous vetting, modification and upgrade will occur over coming years to improve the functionality and stability. SDR architectures allow the radio system to follow the standard.
SDR allows railroads to start system validation and hardware qualification now, in order to meet the required Federal deadline. Since SDR provides a continuous path for interoperability as radios can reprogram and reconfigure on the fly, the hardware can remain unchanged while interoperability and advances in standards can be met through software upgrade.