The mobility of the future is digital and connected - this applies especially to rail transport. The German Aerospace Center (DLR) is developing new solutions and technologies for this purpose. With the aim of increasing safety, comfort and efficiency in rail transport, a DLR project team has investigated communication systems that enable secure wireless communication between trains, trainsets and carriages under the conditions of interfering communication systems of the road traffic. The approximately one-year research study was successfully completed in May 2021.
The project "V2X-DuRail" (Vehicle-to-Everything-Funk für digitale, urbane Zugkommunikation) was funded with 97,923 € by the German Federal Ministry of Transport and Digital Infrastructure as part of the mFUND innovation initiative.
"Our research helps to better understand and avoid interference between the various wireless radio systems in road and rail traffic. This increases safety for all road and rail vehicles," explains project leader Dr. Dina Bousdar from the DLR Institute of Communications and Navigation. "Based on our results, a cost-effective and reliable train control system can be implemented that uses V2X communications, i.e., a radio link from one train to other trains and, if necessary, also to road vehicles at level crossings."
Among others, cooperative intelligent transport systems (C-ITS) are designed to make level crossings safer by alerting cars of approaching trains and vice versa. In rail transport, the main focus is on expanding train control and train management systems with wireless communication. In parallel, cars are increasingly being equipped with C-ITS. Wireless communications can be used to better utilize the infrastructure and avoid collisions with other trains or cars.
However, the C-ITS in road and rail traffic can influence each other. This can lead to interference, especially in urban areas with high vehicle density. In addition, the environment influences the signal propagation, such as urban canyons or bridges. Reliable wireless communication is the key for future safety-critical applications.
Various CBTC (Communications Based Train Control) systems are used for regional and metro lines. There are no binding standards for CBTC so far and many commercial implementations use WLAN (Wireless Local Area Network). However, the frequency bands used for this purpose are also heavily used for Internet access and consumer electronics. Other systems use frequency bands which are exclusively allocated for the application; these frequency bands are limited and their use is expensive.
That is why spectrum regulatory bodies are initiating spectrum sharing and multiple uses: Subways run not only under, but also above ground and usually parallel to roads. Whereas cars use the traffic space of roads. Hence, a common standard based on C-ITS for road and rail traffic becomes possible.
This is where the V2X-DuRail project comes in: V2X communications, familiar from road traffic, is being investigated as a harmonized technical solution. The focus is on the reliable coexistence of wireless communication of C-ITS in road and rail traffic based on V2X communication in the 5.9 GHz band. The term "V2X communication" refers to vehicle-to-everything communication systems that enable direct radio communication between vehicles and the road infrastructure without the need of a centralized control, as is the case with "classic mobile radio". As a result, V2X radio already achieves extremely low end-to-end delays and is therefore suitable for safety-critical traffic applications. Currently, there are two competing solutions for V2X communication on the market: One is the WLAN-based ITS-G5, the other one the 4G-based LTE C-V2X. Each of these technologies is currently being further developed in the corresponding standardization bodies of IEEE and 3GPP and will be specified accordingly in the coming months. It is therefore particularly important that the interference measurements also take these future V2X communication systems into account.
As the highlight of the research project, the V2X-DuRail team carried out an extensive measurement campaign in Berlin from March 4 to 7, 2021. For this purpose, 16 scientists conducted measurements with special V2X communication technology, a technology-independent radio measurement system (Channel Sounder), as well as various tracking receivers and sensors. In a total of 50 test scenarios in the city of Berlin, they investigated which factors affect radio transmission and to what extent.
For these measurements, the experts from the DLR Institute of Communication and Navigation used the "advanced TrainLab," an experimental train operated by Deutsche Bahn. In addition, two special equipped cars were on the road. All vehicles were equipped with various communication and localization systems. A base station was also set up near a suburban train station to emulate CBTC systems. The team performed various maneuvers with the vehicles in different environments. The measurement equipment recorded which signals were received from the cars on the train and in what quality. In this way, the project team specifically tested situations in which interference can occur and the wireless communication systems of road and rail can interfere with each other.
Since the measurement campaign, DLR's Vehicular Application Group has been evaluating the measurement data. The following figure shows an example of the position of the test vehicles and the base station on a map during a test scenario between Berlin-Tempelhof in the west and Berlin-Neukölln in the east. The thin solid lines show the "line of sight" signal between the vehicles, even when it is shadowed by other objects. This allows the researchers to assess which objects have weak or strong effects on the communication signals and interference. Further information can be found here: https://elib.dlr.de/143201/
"We managed to perform one hundred percent of the planned test scenarios, during the V2X-DuRail measurement campaign. The test scenarios include different settings of the interfered CBTC train to base station link (IEEE 802.11a, ITS-G5, LTE C-V2X) and the interference source in the road vehicles (ITS-G5)," summarizes Dr. Dina Bousdar.
The results were contributed to standardization bodies of the European Telecommunications Standards Institute (ETSI). Within the framework of the "ETSI Special Task Force (STF) 603", further V2X field tests with the Paris metro were organized in fall 2021. Similar to the completed "V2X-DuRail" project, the aim of the measurements is to measure and model the interference of road communication on CBTC based rail communication in order to define protection zones in the immediate vicinity of the tracks and mitigation procedures for road communication in a second step. DLR was contributing decisively to the field tests in Paris and was participating with research vehicles and staff. An identical measurement setup as for V2X-DuRail was used. The preparation of the V2X-DuRail measurement campaign, the measurement setup, the lessons learned on the course of the field tests as well as the first evaluation of the measurement data were directly incorporated into ETSI STF 603. DLR scientists Dr. Stephan Sand and Dr. Fabian de Ponte Müller, who were actively involved in both projects, ensured a continuous flow of information between the two project teams. During the first half of 2021, a close exchange between V2X-DuRail and ETSI STF 603 took place.
As part of the mFUND funding program, the BMVI has been supporting research and development projects related to data-based digital innovations for Mobility 4.0 since 2016. Project funding is supplemented by active professional networking between stakeholders from politics, business, administration and research and the provision of open data on the mCLOUD portal. For more information, visit www.mfund.de.
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