201310 – Alvarez & Roman – ETCS L2 and CBTC over LTE – Convergence of the Radio Layer in Advanced Train Control Systems

A general trend in modern Train Control Systems is the use of increasingly similar hardware platforms to implement different applications. More and more, the on-board equipment needed to deploy a mass transit CBTC system is, if not effectively the same, at least equivalent to the equipment used for ETCS Level 2 rollouts. A similar process is taking place trackside, with Eurobalises being adopted for CBTC and Zone Controllers or Interlockings being revamped into RBCs. It is mostly at the application level where these systems really begin to differ, as if CBTC systems were about to become a series of customised ATO applications on top of what basically is a generic ETCS-like ATP system.

This integration tendency begs a question: what will happen with the radio layer? Today, nearly all ETCS Level 2 systems use GSM-R as their radio carrier technology, with a few anecdotal instances of TETRA usage. At the same time, nearly all CBTC systems use radio networks based on IEEE 802.11 (Wi-Fi). The main reason for this difference is historical – with GSM-R being developed by European authorities as part of the ERTMS specification, and Wi-Fi being chosen as a “cheap and dirty” unlicensed band solution for railways that are mostly underground.

This paper explores the forces that underpin the trend to move away from those radio layers. It also identifies LTE as a technology that seems to be, according to current market trends and to technical reasons, the obvious successor to GSM-R and the best alternative to replace Wi-Fi in safety critical applications. The paper finally presents some of the integration challenges that train control system engineers will face in the coming years in trying to make the transition from their current radio interfaces to the latest radio carrier technology around, and how enhanced capabilities of the radio layer may open the box for oncoming innovations in Train Control Systems.