Communicating underground

Stockholm’s metro has upgraded its 17 year-old communications system with a robust multi-purpose network installed by Axell Wireless, as Kate O’Flaherty discovers

Communicating underground

Stockholm’s metro has installed a highly robust multi-band system consisting of over 70 repeaters to provide radio coverage alongside the 105km long track. 

Swedish operator SL, which is responsible for overseeing Stockholm’s metro, commissioned Axell Wireless to install the network, replacing a 17 year-old system. 

The network had to support rail communications, through a completely redundant public safety system, and provide a public network for national mobile operators. 

Stockholm’s metro network consists of 100 underground stations, with 50 stations above ground. The metro’s public safety network had to support over one million journeys on a daily basis.  

‘The metro was a mixture of legacy VHF and UHF that weren’t fit for 21st century communication,’ says Ian Brown, CEO of Axell Wireless. ‘They wanted to upgrade them and they didn’t have coverage. They wanted something that would allow the transition to take place.’

Interference issues

The previous sub-com system had been impacted by the strain of supporting the metro network. The cables linking the system were over 15 years old, and there were a limited number of optical fibres available. The system was also struggling to supply air links to base stations above ground, because new buildings were causing interference issues for 160MHz band signals. 

Axell Wireless’ multi-band system supports the existing radio systems, while providing coverage for a TETRA public safety network for the emergency services, and a TETRA train communication and information network, as well as various commercial mobile radio services. 

DAS installed

Cables in tunnels are subjected to a huge amount of vibration, with chocks and connectors representing weak points that are often susceptible to degradation.

In order to meet the high reliability and availability requirements of the new Stockholm metro system, Axell employed a distributed antenna system based on fibre-fed repeaters. 

Using this method, a single base station can be positioned at each end of a section of tunnel, or group of tunnels, to feed repeaters placed at each underground station, connected through optical fibres. Antennas in ticket halls were replaced by leaky feeders to further streamline the network. 

‘Metros involve a lot of tunnels,’ Brown says. ‘There are some golden rules around putting coverage into them. First, make sure that as little equipment as possible is located within the metro; space is an issue and sometimes tunnels themselves are constrained. And power is an issue – you can’t always get power.’

Smooth transition

In order to facilitate optimum communications, the idea is to keep as much above ground as possible, says Brown. ‘In the Stockholm metro, they have very nice communications rooms, but that’s quite unusual. You normally have a base station hotel with as much equipment as you can and you connect them out.’

As metros are there to serve transport 24/7, maintenance is restricted, Brown says. This meant the old system had to run in parallel with the new one in order to keep supplying comprehensive coverage to the metro operator. 

Therefore, the project took two years to complete. ‘Stockholm wanted to do it in a phased manner – it was a very busy metro,’ Brown says. ‘We had to work hard as legacy systems were being switched off and they wanted both systems to operate, which was challenging.’

A smooth transition was facilitated with repeaters using software defined radio (SDR) and Axell Wireless’ channel selective CSR438 repeaters. 

To enhance SL’s visibility across the entire network, Axell Wireless installed its software manager Axell Element Manager (AEM), creating a balanced system with one technological interface. The supervision system is able to remotely shut down, or upgrade the network when needed. 

‘You want as much as possible away from tunnels,’ says Brown. ‘We’ve got some equipment that’s been in tunnels for 20 years; you need stuff that can withstand the conditions.’


Fibre optic repeater systems

Fibre optic repeater systems in TETRA networks overcome the challenge of underground locations by boosting and extending base station coverage over great distances to remote locations.

A reliable TETRA radio system must continue performing despite any failures to the system itself. This problem is solved by installing two independent base stations providing overlapping coverage underground. 

Each base station is configured to feed several repeaters placed inside a tunnel, with overlapping coverage between two adjacent repeaters. This means if a repeater fails, the repeater next to it will carry on providing coverage. 

 This provides the necessary critical resilience, ensuring that there are no coverage black spots for the  emergency services. Axell Wireless’ strong, yet flexible system allows the secure handover of RF  communications from one unit to the next, mitigating the risk of a base station or repeater suddenly  becoming damaged or destroyed during a crisis. 

Enter the void – GSM-R on the Botniabanan railway line

Alcatel-Lucent and Axell Wireless have equipped 13 tunnels, consisting of 25km of track, with radio communication systems on Sweden’s Botniabanan railway line.

The operator, Botniabanan AB, required a state-of-the-art communications solution for Sweden’s new high-speed line, a 190km track designed for trains travelling at up to 250km per hour.

Axell Wireless was chosen by Alcatel-Lucent to provide the active repeater equipment for the railway authorities, using GSM-R technology, as well as for the rescue organisations such as the fire brigade, which required analogue technologies. 

‘It was a new tunnel complex and these days you can’t open tunnels unless you have radio communications,’ Brown explains. ‘That came about as a result of a tunnel 10 years ago where there was a fire and the emergency services couldn’t communicate properly – since then it’s come into statute.’ 

A number of antennae with optimal mast units and remotes were used, covering different frequency bands and higher and lower power levels. Most of the 97 repeaters deployed were fibre-fed repeaters. Optical Master Units (OMUs) power the fibre-fed repeaters located inside the tunnel. 

Axell Wireless also provided the actual installation and commissioning of the equipment, as well as the delivery and set up of a management system. 

Written by Wireless magazine
Wireless magazine

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