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In-tunnel coverage: Inside knowledge

Radio coverage in transport tunnels is essential for safety as well as tunnel management. Richard Lambley explores recent innovations in technology and practice

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For consumers and professional users alike radio coverage in enclosed spaces is increasingly seen as essential – and nowhere more so than in road and railway tunnels, where highly reliable systems are needed for smooth operations and to protect travellers. 

Sporadic but alarming incidents in tunnels have reminded the public of the potential dangers and alerted tunnel operators to the need for reliable communications, both to prevent mishaps and to manage clear-up operations. Major incidents recently have included fires inside the railway tunnel between France and England in 2008, a 24-tonne tunnel ceiling collapse that killed a car passenger in Boston in 2006, a blaze that killed 11 people in Switzerland’s Gotthard road tunnel in 2001, and the 1999 fire in the Mont Blanc road tunnel linking France and Italy that led to the deaths of 38 people.

In addition to these news-making events there have been thousands of minor collisions and blockages. But even when lives are not in danger tunnel incidents can be costly because of the disruption to transport routes. A good communications system is an essential tool for limiting the inconvenience.

Specialist solutions
In new tunnel projects dedicated installations for underground communications are being included in design requirements from the outset. “While in the past it might have just been fireground and emergency communications, now there is also pressure to include 4G and Wi-Fi, which complicates the specialist coverage solution,” says Adrian Dain, principal consultant at Mason Advisory, a London-based engineering consultancy that works on tunnel projects worldwide. “And a lot of road tunnels have voice breakthrough – [the ability] to make announcements over commercial radio frequencies in the event of an emergency.”

To meet coverage needs in this challenging environment system designers have a variety of technologies at their disposal. “We have many possibilities,” explains Ludovic Rousseaux, a radio network engineer with Airbus Defence and Space. “We have to choose between extra base stations or off-air repeaters. We have to choose how we will ensure the coverage – for example, antennas or radiating cables. We also choose whether the distribution network is active or passive, with RF cables or fibre optic, [and] the level of redundancy that is required. Sometimes we need to select a mix of solutions.”

A key requirement is the level of radio coverage expected. Rousseaux says customers typically ask for 95 to 99 per cent tunnel coverage. “After that we generally see some coverage redundancy requirements, because if equipment is down – because of an accident or non-availability – we have to propose a redundant solution,” he adds. “Very often we also have requirements about expandability: if the tunnel is extended later or a second bore is to be created we have to ensure that the system will be able to support additional antennas or we will be able to connect additional cables.” 

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Airbus Defence and Space designed a radio coverage system serving more than 200 kilometres of tunnels for the Paris Metro. It has also provided coverage in the city’s ring-road tunnels for police and fire services

Active and passive
“A starting point on the projects I’ve worked with is how much kit we are allowed to put in the tunnel,” says Dain. “Particularly with rail, clearance is a big issue. There’s very little space inside the bore, so there’s a push to put only passive equipment in the tunnel itself and all active equipment in places that are little bit more accessible.”

At its simplest, a tunnel system can consist of an outside antenna relaying an off-the-air signal into the interior via another antenna or a radiating cable (‘leaky feeder’), to provide distributed radio coverage along the length of the tunnel. “There is often a drive to make the radiating cable sections as long as they can be,” comments Dain. “With lower frequency services that’s usually not too much of a problem – at around 400 MHz you can go for a kilometre or a kilometre-and-a-half without any active equipment to re-boost the signal.

“But as you go up the frequency bands you find that the distance you can cover without active equipment gets shorter. We are now looking at Wi-Fi and 2.6 GHz, which tend to be the top end of what we will put on radiating cable systems. If you have those services on board 500 metres might be the maximum length you can manage before you need more active equipment.”

More complex installations may support multiple communications and broadcast services (such as TETRA or Tetrapol, analogue PMR, public mobile phone services, AM, FM and digital radio) in their various frequency bands, with dedicated base stations serving a system of remote antennas (a distributed antenna system or DAS). These may be backed up by reserve units and standby power supplies in case of failure. Many modern schemes employ optical fibre distribution, with an optical master unit (OMU) at the hub of the network. The OMU converts radio frequency signals into modulated light and back again, enabling signals to be exchanged efficiently via fibre cables with remote antenna units throughout the tunnel. 

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Cabling inside the 57-kilometre Gotthard Base tunnel in Switzerland, part of a new high-speed transport axis linking northern and southern Europe; image: © AlpTransit Gotthard 

Distributing the signal
“When we have a small tunnel it’s preferable to use off-air repeaters,” says Rousseaux. “It will be cheaper and will generally cover the need. But for longer tunnels – more than 10 kilometres – we prefer to use optical master units. There is also a need to ensure a certain degree of redundancy and resilience. For example, if there is a big accident in one of the tunnels it might destroy the base station of the active equipment. So we feed the distributed antenna system from both ends. In that case, if we lose one connection the other will still be available.”

“Distributed antennas can be a lot cheaper than radiating feeder cables, particularly at higher frequencies where the cable has to become bigger to reduce the attenuation per metre,” adds Dain. “There’s a cut-over point, and that cut-over point depends on how much space you’ve got in the tunnel as well. DAS systems now come with a single integrated unit that will take a fibre cable and it provides the whole RF stage, so that you can run a very easy to install fibre cable through the tunnel to each DAS point. There you have a small box that drives your RF stage, and then you have the antennas.

“The cost of DAS has come down, which has moved the break between DAS and radiating feeder cable systems. The cost of RF-over-fibre [looks like it’s] about to come down, so you can put in place a kind of neutral hosting system using fibre and distributed antennas that’s all quite broadband, and add discrete radio systems to that at a late stage.”

In railway tunnels the propagation of radio signals can be blocked by trains themselves. To deal with this some DAS installations are designed to work with on-board equipment. For example, a signal could be picked up by an antenna on the train and then re-radiated. “That’s particularly popular for things like Wi-Fi backhaul but it’s also sometimes used for other in-train systems such as condition monitoring and passenger information,” adds Dain.

Disturbance, interference
Designers have also responded to changes in specifications for tunnel construction, such as those resulting from the big Mont Blanc tunnel fire. “Not only do we have to provide redundancy but also to consider fire-retardant feeders,” says Rousseaux. “You also need to provide micro and macro redundancy. Micro is inside the equipment – for example, dual redundant power supplies. With macro, for example, we feed the different antenna systems and/or leaky feeders with two radio sources so that we have two cells repeated throughout the tunnel.”

In multi-band systems one potential problem requiring careful attention is interference. “We have to take care with intermodulation products especially,” Rousseaux continues. “They can happen between different carriers – they may intermodulate and generate interference on the uplink. This is something we have to calculate beforehand. The most difficult thing is to mix FM radio with TETRA signals. The FM radios will mix with our TETRA carriers and create third-order intermodulation products on the uplink at very high levels, and this can desensitise the whole system.

“We have to reduce the feed-in power to mitigate the issue – but what we generally recommend is to avoid using FM [signals], especially if they are separated by 10 MHz because it will create intermodulation products in our frequency
band on the uplink. We recommend not carrying FM on the same cable.”

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Control room of the Gotthard Base Tunnel. Radio coverage in the tunnel is part of Switzerland’s national Polycom emergency services network; credit: ATOS

Intelligent digital DAS

Meanwhile, a new approach to DAS is being pioneered by Cobham Wireless, whose versatile idDAS architecture for LTE is now being extended to public safety applications. “We will launch a product at the end of this year that combines TETRA 400 and LTE in any band that is available,” explains product manager Ingo Flomer. “You can choose up to four bands in total: one could be TETRA and then you could have three LTE bands, or two TETRA and two LTE bands.

“The cabinet that we use is designed for four bands, but that is a physical limit of the cabinet. The digital technology that we are using allows us to cascade each unit, so the remote  units can be cascaded at the same location, for example, which is important to create MIMO [multiple-input, multiple-output] or to generate more than four bands.”

Heart of the installation is the base station hotel or base station room, where capacity is supplied by conventional base station equipment. “The RF signals then go into a digitiser,” continues Flomer. “We call it MTDI [multiple technology digital interface]. We use the same protocol that is used by base station manufacturers to connect their remote radio units.”

In contrast to conventional DAS systems, where these streams are hardwired to individual antenna units, the idDAS system connects them via a router. “The important thing is that we can steer capacity,” Flomer emphasises. “It is a totally flexible approach; a cloud-run architecture where you can address the sectors to destination points. You can do that on an hourly or a daily basis, and this is how you react to events. In a stadium, for example, you don’t need capacity when they’re in idle mode most of the year.”

Capacity, he adds, is a factor that will increase in significance with the introduction of LTE into public safety communications. “TETRA was pretty straightforward with time slots and user groups. But LTE is data-centric and it is walking away from the group approach. It means capacity becomes an issue even in public safety.”

Connections from the router to the remote antenna units can be in Cat6 copper cable or in multi-mode or single-mode fibre, according to need. With TETRA links up to 40 kilometres long are possible, and with LTE up to 60 kilometres.

Though idDAS may sound like a complex system that could be too costly for simpler tunnel installations, Flomer claims the opposite is true. “We have customers doing significant rollouts in Norway for tunnels. Those tunnels need typically one or two remote units. The problem they had was they needed an off-air repeater to pick up the signal first, bring it into the master unit, and then feed it to the remote units. With idDAS we can take a remote unit and change the software in it to make it a master unit. And then you have, in one box, the RF front end and the converter to fibre. It’s IP65, and it’s lower in cost than an analogue DAS in a comparable situation.”

Moreover, he adds: “It can be cheaper for a small tunnel to use idDAS because you don’t need base stations for it – you usually pick up the signal from outside.”

Nonetheless, an idDAS remote unit is somewhat more expensive than an analogue remote unit, he acknowledges. “I’m talking about a 10 to 20 per cent gap here,” he says. “But it has much more functionality. Another good thing about idDAS is that you can daisy-chain or cascade the fibre. So you connect your first link to the first remote unit and then you can connect from that remote unit to the next one... and if you are worried about redundancy you can create a ring – you connect the last remote back to the router.”

The picocell option
For underground railway systems especially, another technology option is picocell coverage. In the past a picocell system would have been hugely expensive, but today the cost of picocells has dropped to the extent that they are becoming a good solution for sites such as station platforms, or even tunnels. Picocells can be connected using ordinary ethernet cable or gigabyte fibre, without the expense of radio frequency cables or concern over constraints such as bend radius, especially in retrofitting schemes for older tunnels.

Since today’s picocell devices embody software-defined radios it becomes possible to radiate several services, including 3G and 4G for the connected passenger as well as emergency communications. Here, much will depend on the commercial model adopted by the system operator. But in this way the public’s appetite for mobile communications might drive the provision of emergency communications coverage by creating opportunities for infrastructure sharing. 

“Critical radio communications is one of the ways we can keep these old tunnels working,” declares Mason Advisory’s Adrian Dain. “Modern tunnels are designed with things like SOS telephones and safe spaces for people to run to if there is a problem, and maybe even places where you can pull over. Those safety features just weren’t thought of when they built [older] tunnels. So often they are more reliant on special coverage solutions than modern tunnels, and they use comms to make up for some deficiencies.”

Reliable, resilient communications are critical in tunnel environments, and operators’ requirements are changing in response to greater demand for bandwidth-hungry applications. On the consumer side, while some of us may enjoy the brief freedom from mobile devices that comes as part and parcel of many underground journeys, the vast majority would rather be able to stream their favourite TV series during their morning commute.

This is creating new issues for installers, who are rising to the challenge with novel techniques and new technology. While it is still relatively early days the hunger for data will only continue to grow, making in-tunnel connectivity a must-have.


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