CBTC and WiFi: an agreement at last!
Discovering CBTC…
CBTC (Communication-Based Train Control) is a railway communication and signalling system for metros and suburban trains. Currently, communication between a metro train and the command centre in charge of traffic management is by radio waves at 5.9 GHz. The main feature of CBTC is its ability to make train operation automatic, as on the Paris metro line 1. This critical system has redundancy to guarantee safety, including double radio access points along the tracks and modems at both ends of the train. The architecture guarantees high line availability by absorbing the hazards inherent to all radio systems (interference, faults, etc.).
CBCT history
In France, the first Paris metro lines to be equipped with CBTC using the 5.9 GHz frequency band were lines 3 (in 2010) and 1 (in 2011). Since then, CBTC has spread throughout France, in the metro in Rennes, Lyon, Lille and Marseille, as well as across Europe, in Copenhagen and Brussels for example, and in Vienna and Frankfurt soon.
It was only in March 2020 that the use of CBTC in the 5,915-5,935 MHz frequency band was harmonised by the CEPT, and then in October 2020 by the European Commission, 10 years after the first European CBTC line went into service. In November 2020, the CEPT also harmonised the 5,945-6,425 MHz frequency band for RLANs (in practice, WiFi and Bluetooth), and that harmonisation was endorsed by the European Commission in June 2021.
Observations and adjustments
Before any harmonisation occurs, the CEPT checks the coexistence with radio applications on adjacent frequencies. Now, portable RLAN equipment can produce out-of-band emissions which technical studies have shown can interfere with CBTC receivers even though the frequency bands are 10 MHz apart, particularly those onboard automatic metros. In the event of prolonged disruption, the train could stop and traffic on the line would be suspended. To avert that risk, a temporary agreement was reached between administrations in 2020, imposing an out-of-band emission limit of -45 dBm/MHz on portable RLANs below 5,935 MHz until 1st January 2025. The limit was then to be raised to -37 dBm/MHz if further studies did not confirm the appropriateness and proportionality of -45 dBm/MHz. That's four years to see how things are going…
But as the years went by, no consensus was found! The plenary ECC in Kristiansand in June 2024 then decided to set up a specific working group, the VLP CG, chaired by Ms Andrea Mora (ANFR), to assess whether a tried-and-tested solution might not be suitable: indeed, in the United States, technical requirements had been defined by the FCC to protect roadside ITS below the RLAN band, and appeared to be satisfactory. There are two of them: use channels above 6,105 MHz in priority, and set up power control. Those constraints could therefore simply be carried over to protect CBTC in Europe. Furthermore, adopting the same rules would prevent market fragmentation between North America and Europe, which remains a concern for the WiFi industry.
Work and agreement
Thanks to the work of the VLP CG, the plenary ECC in Vienna in November 2024 was fortunately able to identify a solution that satisfied all the administrations, particularly France, which has been operating CBTC for over a decade. Portable RLAN devices will therefore see their out-of-band emissions constraints below 5,935 MHz relaxed to -37 dBm/MHz, provided they select channels above 6,105 MHz first. Otherwise, the previous limit of -45 dBm/MHz will continue to apply. A ”considering” indicates that the implementation of power control by these devices will also be essential. That approach has been validated by studies carried out by the RATP, which also confirm that giving priority to channels above 6,105 MHz has a much greater protective effect on CBTC protection than power control.
The agreement brilliantly concludes the work on coexistence between CBTC and 6 GHz RLAN, which had been mobilising European frequency managers since May 2017! It is the result of close collaboration between the ANFR, the RATP, the SNCF and the UITP, who were able to obtain the support of other administrations in the rail sector, who in turn raised awareness among national spectrum managers. Building consensus sector by sector, nation by nation, is becoming increasingly necessary to make it possible for effective decisions to be made in these areas of growing technical complexity: this case has demonstrated its exemplary effectiveness.
Cautiously, the European Commission nevertheless published a decision proposing to extend the -45 dBm/MHz limit for one year. However, as soon as our agreement is transposed into a European Commission decision, the new system will come into force: probably before mid-2025.