# Determining the Optimal Positions of Infill Balise Groups for ERTMS/ETCS Level 1 Applications
###### Abstract
For radio transmission, Global System for Mobile Communications -- Railway (GSM-R) and Future Railway Mobile Communication System (FRMCS) can be used, with the first approaching its technical end of life and the latter set to be its enhanced replacement [12]. For ETCS level 1, it is specified in [13] that infill information can be transmitted to the train by means of balise groups (Eurobalises), Euroloop, or radio infill. Their respective specifications can be found in [14; 15; 16; 17]. Eurobalises only transmit information at distinct locations (spot transmission), whereas Euroloop and radio infill provide a continuous transmission of infill over a limited distance, e.g., in the approach zone of a main signal. This potentially reduces the system latency in responding to a main signal changing from a \"stop\" aspect (closed main signal) to a \"clear\" aspect (open main signal) and therefore increases capacity. Although both Euroloop and radio infill have been implemented and are in use today on select infrastructures, recent deployments of ETCS level 1 all exclusively use balise groups to transmit infill information, indicating a better cost-to-performance or complexity-to-performance ratio than those of semi-continuous solutions. As a downside, providing infill information to a train is only possible at distinct locations where the infill balise groups are located. The current TSI for control-command and signaling systems [18] do in fact prohibit the new deployment of Euroloops and Radio Infill Units (RIUs) with specific, minimal exceptions. This highlights the need to optimize the locations of the infill balise groups to increase capacity. The literature review and work below were therefore focused solely on using standard Eurobalises to transmit infill information via the so-called air gap [19] from the trackside infrastructure to the train. Most of the European countries implementing ETCS have focused on deploying ETCS level 2, as it benefits the infrastructure manager in further reducing the necessary lineside equipment such as full lineside signaling to just static marker boards. However, such lines need to have redundant GSM-R or FRMCS equipment for the necessary continuous radio communication in place--expenditures many non-federally owned infrastructure managers are not willing or able to afford. Unfortunately, high costs due to the complexity of the system and especially due to the need to upgrade or completely replace existing interlockings, the rollout has to date been much slower than anticipated. For example, the National Implementation Plan of Germany [20] published in 2017 does not reflect the current state of deployment anymore. The current migration strategy [21] for the network of Germany's federally owned infrastructure manager DB InfraGO AG covers the planned commissioning of ETCS until the end of 2029. Until then, lines with a combined length of 5090 km shall be equipped with ETCS, which equals around 15% of the total line length of 34,065 km and covering most of the core parts of the network with the highest number of trains. The non-federally owned infrastructure managers, of which Germany has about 190 [22], are therefore to date cautious with the implementation of ETCS, as long as no concrete planning is underway for DB's connecting lines. Their preferred option is to deploy a low-complexity, cost-effective solution to enable interoperable operations and therefore plan to implement ETCS level 1 Full Supervision. In the neighboring country of Austria, federal railway Osterreichische Bundesbahnen (OBB) has commissioned around 300 km of ETCS level 2 and will increase that number to about 3700 km until 2038 to cover their core network [23]. The remaining network might in part be fitted with ETCS level 1. Banedanmark, the state-owned infrastructure manager in Denmark, aims to have ETCS level 2 deployed on all lines by the year 2033 [24]. Countries with (almost) complete coverage of active ETCS on their standard gauge networks include Switzerland and Luxembourg.