Thursday 28 March 2024

Helsinki Metro’s Cellular Network Pilot

Helsiki's radio network currently in use in the metro is being renewed in order to support the future train traffic control system. A cellular network pilot was carried out in 2022/23 with results published in April last year. Based on that it was decided that the new radio network will be implemented with mobile network technology, as it was seen as best suited to the needs of the new train traffic control system and the metro.

Quoting from the article:

The metro is still using many original (dating back over 40 years) systems that are nearing the end of their life-cycle. The current traffic control system, in particular, needs to be updated to ensure the reliable and safe operation of the metro in the future as well. Parts of the system that are now being updated include the train control system and track circuits.

The updating of the train control system will make it possible to increase the number of passengers of the metro by enabling shorter headways between trains than are currently possible. Shortening the headway between trains and other capacity-increasing measures are important, as transport forecasts indicate that the metro’s number of passengers will continue to increase. The current capacity of the metro is simply not enough to meet the increasing demand.

Metro systems have long service lives and their updates have far-reaching impacts. The updates to be implemented now will make it possible to operate the metro safely for another 40 years.

The results and observations from the 'Cellular Network Pilot' is available here. Quoting from that:

This innovative pilot demonstrated that a cellular based communication subsystem is suitable for train control as well as other metro systems applications. The pilot outcomes provided insights into the deployment of such systems and also confirmed the expectation that in order to meet the strict radio communication availability requirements necessary to support safety critical applications, at least two radio network layers should be present. These layers can be presented via implementation combinations of private and public networks including 5G SA slicing, depending on the current and future user requirements.

Ability to support signalling: The pilot test results showed that both the private network (4G or 5G) and the public network are suitable to support ATC performance requirements. In high public network load scenarios, it is advised that QoS is implemented to ensure the reliability of any safety critical streams.

Ability to support current systems: The pilot tests showed that the public network is suitable to support metro’s onboard existing systems. It was observed that when the public network was capacity stressed, with all applications present, the Wi-Fi stream could not reach its maximum intended capacity of 250Mbps. This was due to bandwidth limitations experienced during the Pilot tests and is re-lated to end-to-end connectivity restrictions and by the number of hops between end devices and the Mobile Network Operator’s core. Troubleshooting during the tests revealed that a considerable increase in capacity could be realistically achieved by addressing these limitations. 

Ability to support future systems: The pilot tests showed that the private network could not reliably service the critical CCTV stream due to the bandwidth limit of that network and the fact that the CCTV stream was duplicated over the two private routers. At the same time the VoIP stream could be reliably serviced indicating that if there was more capacity the issue with CCTV could be resolved. 

Private network deployment observations: In normal operation mode, the band used (2300 MHz) and the density of the radio units was demonstrated to fulfil the requirements for ATC and critical voice communication. For the private network, there was degradation of latency in the coverage area of three out of the four radio positions when these were offline. Most of the service degradation was affecting the Uplink and it was observed in areas were changes in radiating cable topology (changing positions/heights etc.) were occurring. Due to the private nature of the network, lack of external interference caused the system to perform better than expected in low signal situations. The two rooftop macro sites were able to provide good coverage and good handovers to the open track area when the radiating cable radio units in the same area were off. In the 5G SA mode all failures noted for the individual routers occur in areas where the radiating cable is on the opposite side of the respective router’s antennas.

Public network deployment observations: Signal quality and signal levels were good to excellent throughout the tunnel during all degraded mode scenarios. At the same time there were a few occurrences of longer than average delays in a certain handover area within the tunnel. This could be attributed to the geometry of the track, the size of the tunnel and the relevant positions of the directional anten-nas providing the coverage in this area which are lower than antennas on the roof of the train. These observations reveal that the radio design within the tunnel could be rationalised (less density but better located cells). Other results showed that the radio design needs to also consider that sufficient coverage is provided to allow handovers between tunnel and macro layers. An overarching observation was that for maximum redundancy the radio design should avoid designing private network cell edge areas at the same location as public network cell edge areas. By overlapping the network design, the reliability of the dual layer network can be maximised. A final observation is that routers/mobile gateways working in high availability mode and/or application devices that can manage packet duplication via multiple routers are recommended in order to increase data communication reliability.

You can read the whitepaper here.

WSP UK Transport & Infrastructure worked with Metropolitan Area Transport Ltd and its suppliers, providing technical leadership and assurance in the deployment of a pioneering 4G and 5G pilot in a brownfield metro environment. Digital connectivity and rail systems experts at WSP developed testing procedures and carried out an assessment of the most suitable technology and network layer combination using a range of key decision indicators. 

You can read more about their contribution here.

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