Rail Engineer has reported on the London Underground Four Lines Modernisation (4LM) project on several occasions. This programme is the re-signalling and re-control of the Metropolitan, Hammersmith & City, Circle, and District lines to provide a Communications Based Train Control (CBTC) system incorporating Automatic Train Operation (ATO), Automatic Train Supervision (ATS) and Automatic Train Protection (ATP) with moving block to maximise train movement capacity.
The project was described in Issue 201 (Mar-Apr 2023), in an article which featured the SelTrac CBTC system previously deployed on the Northern and Jubilee lines and on the Docklands Light Railway. Following Hitachi’s acquisition of the Thales signalling business, Hitachi Rail is now responsible for delivering and supporting the Seltrac CBTC solution on 4LM. This project differs from the previous deployments by using radio-based train positioning and not track loops. The radio access network that underpins the signalling data exchange is extensive and is central to successful CBTC operation.
This article builds on Julien Puyon’s recent talk to the IRSE London & SE section and describes the design and implementation approach for the radio network.
Overview
The four lines represent 40% of the London Underground network, covering 150km of route, over 200 trains, and more than and 100 stations. Although classed as ‘Underground’ lines, the majority of the routes are above ground with the tunnel sections being built on the ‘cut and cover’ methodology, so they are near to the surface and carry both tracks. A control centre at Hammersmith was opened in 2018 and a progressive introduction of CBTC began with the Paddington Hammersmith section, rolling out to the central London areas and then finally to the extremities, the longest route being to Amersham in Buckinghamshire. A minimum of disruption to passengers was a demand made by the operating authorities, so the changeovers had to be carefully planned and tested with minimal line blockades during the commissioning periods.
The radio network
For the selection and design of any radio network, the frequency band of operation is the first consideration, taking account of spectrum availability and allocation, the application and its communications need including data rates, bandwidth, radio propagation requirements, and resilience considerations. For 4LM, this became clear early on with the decision to adopt a dedicated and purpose-designed radio access network engineered for CBTC communication in the operational Underground environment. The system requires sufficient bandwidth to support all the CBTC functionality to ensure reliable operation.
The various components of the CBTC where the Radio Access Network will be critical to successful operation are:
- The Vehicle Control Centre (VCC) which manages the area where the train is positioned, be it moving or stationary. Continuous and guaranteed communication is required between the VCC and the Vehicle On Board Controller (VOBC).
- Data Communication System (DCS) consisting of two sub systems: the Radio Access Network (RAN) and the core transmission network to connect the RAN back to the Hammersmith control centre.
- Wayside Radio Unit (WRU).
- Radio Access Point (AP) connected to the WRU and including aerials and RF cabling.
- Mobile Radio Unit (MRU) connected to the VOBC.
The CBTC system depends on these sub systems working reliably and correctly. All of these caused challenges during the design and testing phases which needed to be understood and resolved before implementation of CBTC could be achieved. In addition to the radio elements, the connections back to the control centre required a resilient backbone fibre network to be in place.
Designing the network
A first decision was whether to use tunnel-mounted aerials or radiating cable in the underground sections. An interference exercise indicated that radiating cable would be more vulnerable to picking up unwanted radio signals than aerials, so the latter was adopted. The aerials are directional and are relatively small at these higher frequencies. Coverage tests showed that aerials need to be spaced at roughly 200-metre intervals meaning around 1450 APs for the entire routes regardless of underground or surface locations. The link between the aerials and the train mobile relies on a direct line of sight to guarantee optimal signal quality.
Positioning the aerials underground had to overcome the problem of heritage platforms where historical factors made it forbidden to install APs in the middle of platforms. It was permitted to install APs at the platform edges, but this made positioning of the WRU difficult. Lack of clearance in the tunnels was another problem, and the existing structures were often cracked or had corroded beams or with water ingress. Obstruction of railway signal sighting was another factor as the trains would continue to be manually driven until the changeover. Getting the WRU as close as possible to the aerials was important to minimise the RF loss when receiving signals from a train.
Having worked through all of these, the resulting radio network had to comply with OFCOM regulations on power outputs and not cause interference to near neighbours operating in the same band which, as mentioned, is already congested.
Implementing the system
A design has to inevitably make some assumptions on the resulting system performance. For this project the V cycle process was used, enabling a robust validation process and subsequent system fine tuning. Validation required both static and moving train assessments. Static testing had to ensure that each AP is interacting with neighbouring APs. The moving-train testing had to ensure that the MRU configuration was optimised and that the CBTC would function when taking into account any signal degradation. The design will sometimes need changing once installation and testing takes place. Radio networks always need special attention because of the challenge in predicting radio coverage.
Testing needed to ensure that the emitted radio frequency (RF) output powers were commensurate with OFCOM regulations, that RF levels were strong enough to ensure reliable communication to trains and that the RF thresholds did not interfere with near neighbours through the static radio validation exercise.
Continuous monitoring of the radio signals is part of the static validation process, but additional testing took place every night to ensure compliance to the standards and to assess the impact of any radio degradation. This was part of the dynamic validation to ensure the following functionality:
- Packet loss – the data sent to the trains is in packets for spectrum efficiency but occasionally packets will be ‘lost’ and the loss rate has to be such that control of the CBTC operation is not impaired.
- Latency – the time it takes for a packet to do a ‘round trip’ between the control centre and the train.
- Consecutive communication loss – the duration of consecutive packet losses.
- General radio availability – the guarantee that an MRU always has a WRU to communicate with.
Another factor was to plan for the necessary staged introduction where some sections were fully commissioned while others were still in the testing stage. This meant having to make projections as to coverage likelihood in the overlap areas.
Continuous analysis and monitoring
Even when commissioned it is essential that the radio performance remains within the required specification. To do this, Big Data analysis is used to process billions of data inputs from across the network which are then assessed to identify the radio configurations from both the WRUs and the train MRUs.
From the data, further testing is often necessary to achieve optimised performance. Despite the careful RF planning, some changes were found to be necessary and consequently some site work to reposition aerials has taken place. All of this relates primarily to the robustness of the radio infrastructure. External influences will always occur, interference from unwanted sources being typical but can also emerge from changes to platform usage and situations of overcrowding.
Big data analysis is used for: verification of all AP installations; configuration tuning for both WRUs and MRUs; confirmation that the RF links are operational; confirmation that each AP meets the RF design threshold with its neighbours; and continuous monitoring of RF signals for degradation.
The same Big Data platform is used for other analytics that measure operational acceptability on the 4LM routes, such as: sub-system validation; monitoring of performance; system troubleshooting; corrective maintenance; system tuning.
The system is designed for both front and rear cab MRUs to receive the radio signal. The MRUs receive the data packets and apply deterministic selection logic based on reception quality metrics to prioritise the best available signal path. Some custom tools have been developed to monitor parameter change on the MRUs both between different train fitments and with communication to trains in differing circumstances.
System operation
The system must be robust enough for CBTC operation to be absolutely reliable, meaning that near-continuous communication is necessary since any interruption for only a few seconds will cause the emergency brake to be applied. The radio network needs constant monitoring to ensure acceptable performance. 70GBits of data are sent out every day and this data is constantly being checked for consistency which can lead to actionable modifications being determined.
The radio coverage is designed such that the loss of any one AP will not impact on system performance as adjacent APs have sufficient overlap coverage to fill the gap.
More tools are being developed and industrialised for engineer and technician usage into the future. The Big Data platform leads to data monitoring that supports decision making and new analytical capability. It is important for any new entrant that he/she learns and understands how to interpret the raw data. The platform provides the support to present complex raw data that can give actionable insight in a format that makes this possible.
In summary
So often with modern signalling systems like CBTC and ETCS, it is just assumed that the connectivity between train and control centre is the easy part and will always be there. GSM-R is a classic example but if this article has opened eyes as to the essential nature of the communication links and the considerable effort that goes into making sure they are robust, then it will have done its job.
The 4LM radio network has built on the success of communication links needed for the DLR, and the Jubilee and Northern lines which employ the Seltrac system. It has become a well-established system for metro modernisation with data driven monitoring becoming part of its ongoing features.
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