Down the tube with Airwave

The emergency services now have full radio communications in the London Underground. Catherine Haslam reports on how it was done

Down the tube with Airwave
The London Underground has 270 stations and 249 miles of track, which makes it the longest metro system in the world. It is also one of the busiest, carrying over one billion passengers every year, according to Transport for London.

Around 45% of the network is underground, creating a unique and highly challenging communications environment encompassing train, station and control staff, as well as emergency services personnel. This is now being covered by two separate but related public radio systems using TETRA (Terrestrial Trunked Radio) technology.

LUL's (London Underground Limited) commercial system is called Connect. It began operation in 2006 and now serves around 12,000 users and 700 trains. The needs of the emergency services have been met with an extension of the Airwave service, which is used nationally by all police forces as well as ambulance and some fire services.

The task Airwave faced in creating a network for the Underground was far from straightforward. It needed to be accessible by the British Transport Police (BTP), Metropolitan Police, City of London Police, London Ambulance Service and the London Fire Brigade, and be able to support seamless handover from above to below ground and vice versa.

Connectivity
The network comprises of Airwave-specific base stations in 125 deep underground stations, and connectivity in the underground tunnels is achieved by 'piggybacking' on the Connect network.

Both systems operate in the 380-400MHz band, making this sharing arrangement possible, with the leaky feeder cables covering the entire spectrum band and the systems operating on different frequencies within that band. Both Connect and Airwave also use Motorola infrastructure, although this was a purely commercial decision as TETRA is standard and therefore vendor equipment is interoperable.

Deployment in the tunnels was incredibly complicated as the system installation had to be conducted in the hours when the Underground rail network was closed. With just a four hour engineering window each day when work can be carried out, the Connect network took years to deploy and a separate network for Airwave was not feasible.

However, Airwave does have a separate transmission network and network management centre, and it also installed its own base stations in all 125 of the deep stations, which in itself was a practical and technical challenge.

Simply finding appropriate locations for the base stations has been difficult, as space is at a premium in the stations. Once that had been done, Airwave also had to deploy the system during engineering hours to avoid disturbing the running of the Underground train network. Despite this, deployment started in earnest after the completion of a pilot in July 2007 and was finished by October 2008, five months ahead of schedule.

In terms of coverage and availability, the automated network testing was supplemented by physical tests - Airwave staff used handsets in every part of every station. It was a time consuming process but one that ensures the emergency services are always connected to the network. The handover optimisation was also tested on every entry and exit to every station, and continues to be tested to balance transmission power and receive sensitivity to ensure effective handover. As Ray Mason, Airwave's programme manager, explains: 'This is an ongoing process as any changes to the above ground Airwave network affect the handover to underground.' Airwave therefore has a full time
engineer dedicated to handover optimisation and the National Policing Improvement Authority (NPIA) has its own engineers as well.

Fixing faults
A final element of the network is provided by seven emergency vehicles. These are sited at locations outside of London and can be brought in to provide coverage in times of emergency, when elements of the network are damaged or out of service. In total, the LUL Airwave extension costs around £115m for the full life of the system. This includes maintenance and operation of the network until 2018.

However, as Airwave's Josh Berle explains, the standard service level agreements associated with the Airwave network above do not apply, as the piggybacking arrangement and access limitations mean that Airwave cannot guarantee fault fixing in the way it can above ground where it has sole responsibility for the network.

The complicated contract requires 'reasonable endeavour' in Airwave co-operating with LUL Connect to fix faults and although the terms may appear vague, the relationship is strong and working well, according to Superintendent Phil Short from the NPIA. He explains that a blame culture has been avoided: 'Availability on the network reflects the above ground targets set by Airwave.'

User benefits
The BTP did have its own system underground prior to the introduction of Airwave but it meant they had to carry two radios. Once underground they could not communicate with the system above ground, other emergency services, or LUL staff. The Airwave LU system provides all these services and also supplies coverage in the tunnels and not just in the stations, a benefit that is particularly useful for major events such as escorting fans to football matches. This all makes for a safer policing operation and has a direct effect on the experience of passengers using the Underground and staff working there.

Including covert security forces, 3,500 talk groups have secure access to services. Because of the security services' involvement, even Airwave - as the operator - does not know all the user groups. The BTP is the primary user with up to 1,500 users.

Airwave is in the process of covering a further 15 tunnels on the London Underground and providing additional capacity for 46 stations, which will help in the build up to 2012. Eight stations already have uplifted capacity and Stratford is being worked on. In addition, work is ongoing to mesh the BTP systems for all the rail services involved including Network Rail, the Docklands Light Railway, Eurostar and Javelin, with the Underground network.

In terms of users, Berle says most additional emergency service personnel will use existing user groups as these do not put additional strain on the thin layer of capacity, although some new user groups will need to be added.

The Airwave deployment is working very well according to Superintendent Short, who concludes that the only complaints received from the BTP primary users are for above ground stations that are covered by the national Airwave network. 'The coverage is so good underground, the BTP in London have got used to that level of service.'

What is TETRA?
TETRA (TErrestrial Trunked RAdio) is an ETSI developed standard for Professional Mobile Radio systems. It is a TDMA-based technology designed for mission critical communications and it is used by emergency services, transport providers and government agencies all over the world. Only in the US has it failed to gain a significant foothold.

Its most significant characteristic is group calling mode, which allows literally thousands of user groups to have access on the same system, end-to-end encryption and instant call set up with push-to-talk functionality. The set-up time for a single node call is typically less than 250msec, while for a one-to-many group call it is generally within 0.5 seconds. This compares with the seven to 10 seconds that are typically required for a GSM
Written by Wireless magazine
Wireless magazine

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