CBTC and airwaves: How to solve the frequency saturation

One of the biggest downsides of the inexorable march of progress is chaos. As more and more devices start to communicate with each other and the modern world tries to adapt to a constantly interconnected reality, the airwaves are saturated to the point of exhaustion -- overlapping frequencies and overused transmission bands generate interference and sometimes straight up block signals from getting through, creating a messy tangle of conflicting waves.

To deal with that and the increasing problems it causes with train tracking technology, the International Association of Public Transport (UITP, from the French "Union Internationale des Transports Publics") are working on establishing on the European level an attribution of dedicated frequency band for CBTC systems. In association with bodies across the continent, the organisation is exploring multiple possible solutions to reduce the amount of interference and guarantee operational reliability.

"A few years ago, a metro line in China was stopped for two hours because users in the trains were using the Wi-Fi system in the same frequency as CBTC communications," says Frédéric Jans-Cooremans, Project Manage & Radio Spectrum Manager at STIB-MIVB. "The security of the communication couldn't be guaranteed, so all trains were stopped until the problem could be dealt with. We want to avoid that happening again, and certainly not on our existing and new automated metro lines."

To that end, Frédéric and the UITP are aiming at standardising the European bands across a working group called the "Spectrum User Group" -- a collection of operators and manufacturers. They're aiming at creating European rules which could then be adapt to other areas.

"Each company tends to use their own frequencies, which creates a lot of variations and sometimes problems when they use Wi-Fi channels," Frédéric says. "Passengers need Wi-Fi, so the conflicting frequencies can cause security and operational problems and require changes to be made."

"The problem is doing it afterwards takes a large amount of money -- it is incredibly expensive to change the communication through antennas and radios in each of the metros, alongside all the internal structure. We want to avoid that, so the plan is to standardise the frequency we use, and the main point here is the security of our passengers and the service we, as public transport operator, have to give to the public"

As it can be expected, standardising anything this important requires a lot of effort, so the team is working with the European Community and the European Telecommunication Standards Institute (ETSI), alongside several national regulators and the Rail Frequency Management group. However, the UITP's project also goes beyond regulation, exploring methods and tech that could help deal with the excess of concurrent waves that may limit or impair the performance of CBTC systems.

"We're working together to find ways to share frequencies between our Urban Rail ITS (Intelligent Transport System) and the Road ITS, and right now this could happen in two big, different ways," explains Frédéric. "One option is to put Warning Beacons along the tracks that keep track of when trains go past, and have them notify the Road ITS systems. That will allow the Road ITS to, for instance, reduce the power of their transmitter or limit the amount of data sent for a certain time -- like increasing transmission rates from every second to every 5 seconds, for example -- until the train clears the area."

"The other option would be a geographical zone, making use of a database in the actual ITS system that keeps track of the kind of traffic within a certain location," he continues. "When building a metro or rail line, we can often know where a tunnel exit will be more than five years in advance, which allows us to geographically tag locations to reduce transmission rates and stop Road ITS from broadcasting when a train is coming."

According to Frédéric, both solutions could be used separately or combined to suit best to each of the frequency users. a test run in Brussels that should be running within two years is already under way, and the UITP is working with multiple public transport companies, such as the RATP, SNCF, TfL, and STIB – as well as manufacturers like Alstom, Siemens, and Bombardier – with the goal of implementing a dedicated frequency band for the CBTC in the future and securing the existing lines running already today.

"This chance is very important, because there are many systems running not only in Europe, but also in Asia and in America," he says. "Train delays due to CBTC problems are a long process to recover from, so there's a need in the market for standardisation -- both operators and manufacturers want the assurance they can use those systems without creating any troubles, and we're doing the best we can to prevent that from happening in the future."

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