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Running cabling in a few inches of space between trains and tunnels. Smashing apart rooms to rebuild fireproof walls. Drilling holes through foot-thick floors for cooling. Setting networking equipment on fire to see how it drips. This is how engineers at Canada Water station have spent the past few months, in a race to install 4G on the London Underground for the first time — and meet a tight political deadline.

It's the lament of the spoiled Londoner that there's no mobile connection on the Underground, as though it's perfectly reasonable to expect 4G service under the Thames in a tunnel that's part of a 157-year-old train network.


But Moscow, Seoul and Toyko have connected commutes, so why can't we? The difference between those networks and our own is tunnel size. It's not easy or cheap to install 4G networks in the space of a few inches, and that's only the first of plenty of hurdles facing Transport for London (TfL) engineers and contractor Installation Technology who are trying to bring mobile connections to the Underground for the first time.

"We've been working on the idea for the better part of ten years now with no solution," says Shashi Verma, chief technology officer for TfL. So what changed? The Home Office needed a new Emergency Services Network (ESN) and money suddenly appeared. Now, the systems for ESN and commercial 4G are being trialled in parallel in a project covering the tunnels and platforms from Westminster to just beyond North Greenwich, as well as most of the station areas.

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If all goes to plan, the system will go live in March, giving commuters connectivity in tunnels underground along that stretch of the Jubilee line. And if you look closely, you'll be able to see where it comes from.

Between stations of the Jubilee line, in the inches-wide gap between the train window and the wall, try to spot the black cables running along the tunnels. These are called "leaky feeders" and they are seeping radio connectivity into the tunnels, in the same way as a water pipe that's been methodically slashed would spray at intervals.


At the core of the cables is a copper wire, but the casing that normally protects the radio waves has regular apertures to passively leak out the signal.

"It's a fairly basic technology that's very low maintenance," says Martin Osborne, Head of Mobile Infrastructure Delivery at TfL. "We've been running systems using leaky feeder for 20 years."

Two leaky feeder cables have been installed: one for the commercial service and a second for the Home Office's emergency network. They join a leaky feeder cable already in place, used for drivers' radios — and all three need to squeeze into the few spare inches between the tunnel wall and the trains hurtling by, without interfering with each other's signal.

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To manage that, engineers used lasers to scan tunnels and 3D imaging to virtually install the cables, to see if it fit alongside trains - and discovered that they could risk placing them as close as 100mm away from each other without interference.


The team running thick cable from 500-metre drums in the dead of night when the tube isn't running have an even bigger problem to contend with: they might have connectivity, but it's not strong enough for anyone to really be able to use it.

Higher frequencies are faster and have more bandwidth, but need to be helped along by repeaters. While the emergency network happily operates at 800MHz via the leaky feeder alone, the signal strength is nowhere near enough for anyone on the train to use 4G.

TfL has built a hidden room with massive boxes of high-powered radio equipment, like a home router at a massive scale. That sends the data via a hub to convert the radio signal to optical, letting it hop onto a fibre line that also runs through the tunnels; look for the thinner, bright yellow cable along the walls. On the platforms, there are high-powered directional radios pointing into the tunnels, as well as low-power units above commuters’ heads, both of which connect to fibre and on to operators networks.

TfL has installed 4G server rooms to guarantee coverage in tunnels TfL

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The aim is that if you're on the train, your phone won't switch between systems, but if you emerge and head up escalators to the ticket hall, then you'll move onto a local network. "All we're doing is taking the signal from mobile operators and putting it through various different antennas and broadcasting it out across the tunnels and into the stations," says Osborne.

The eight stations in the trial are served by a "base station hotel" — a mini data centre for each of the four main 4G operators to keep their own separate equipment — at Canada Water station.

For the wider network, one will be needed every 15 kilometers or so, meaning as many as ten will be required. Here at Canada Water, EE has its own hotel in a nondescript room along the bus station, while the other three are setting up shop in a long corridor a few levels underground; as of our visit, only Vodafone and EE had installed equipment with weeks to go until launch.

All that may sound complex, but the system is straightforward to network specialists, who are using equipment bought off the shelf. "The challenge for us is doing it in a very difficult environment, underground, which is noisy, dirty, old and has no space," says Osborne. And that's spurred the design, with passive equipment such as leaky feeders used in locations where maintenance is a challenge, because no-one wants to have to do this work again.

To get fibre in, the pavement was ripped up. To install cooling in the other areas below ground, fist-sized holes were drilled through the concrete flooring.

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On a tour of the station, one of the engineers took visitors down a bright hallway in the hidden side of Canada Water station, slipping a key into a lock at the end. "We won't all fit in," he joked, pulling the door open to reveal not another hallway or room, but the equivalent of a broadband cabinet. For fire-safety, TfL had to build a foot-wide structure out of breeze-block to house a dozen cables plugged into ports.

EE's data hotel looks like any server room, with rows of racks filled with networking equipment. But it needed a new wall for fire containment and its networking kit needed to be fire tested. That means it was taken to a lab and lit on fire, to see how material would drip from it — important information for equipment installed over people's heads.

Fire isn't the only challenge. The Jubilee line is the newest part of the tube, but it still dates to 1998. Requirements were different then, so the network doesn't have enough power supply for all this additional equipment. Plus, the extra equipment gives off heat, meaning even more power is needed for cooling. To make it work, new power feeds had to be run from a local substation — not easy when external permission from local residents is required — and energy savings had to be found across other systems.

The installation team is already keeping an eye on temperatures outside the remit of the trial. Further down the line, trains often wait in tunnels outside Green Park station; because of the stop, trains pull hot air with them and leave some of it behind. One engineer said that spot is being monitored to see the temperature impact from a few radio boxes several stations away.

The heating and power would be less of an issue if the mobile operators were willing to upgrade their equipment to low-power alternatives. Had this project waited three years, that shift would likely already have happened, Osborne says.

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But as this is only a trial, it's difficult to demand operators invest in new equipment. For the full rollout, there are hopes that operators choose — or are forced — to upgrade to low-power kit. "We hopefully be using [this trial] as a lever to try to encourage the industry to move to low-power solutions," Osborne says. However, the arrival of 5G will only exacerbate the heating and power woes, as higher frequency equipment demands more energy and more antennas.

For now, commuters will have to make do with 4G. Progress may feel slow, but the Mayor of London Sadiq Khan has lit a fire under this project. Last year, the mayor's office got in touch with Verma. "We were told in no uncertain terms you have to build something," he said, referencing the 4G rollout.

While it may have pushed the project forward, the attention from on high means added pressure. "It has been very difficult and very challenging, especially in terms of time," he told the assembled installation team, gathered together to receive a safety award. One engineer told us the time pressure was the one part of the project he would have liked to have done differently.

The mad rush to finish this trial in March may have something to do with the mayoral election in May. But of course, Khan's pressure isn't the only reason this rollout is happening. Alongside the emergency network, there are plans to hand the system to a commercial supplier to implement, selling access to the mobile operators and bringing in much needed revenue to TfL. And, for Verma, there's a further motivation.

"Delivering a train service is kind of taken for granted," he said. "Our ability to delight people is very limited." Putting Wi-Fi into the stations ahead of the 2012 Olympics did just that, Verma says. He hopes that this new feat of engineering will be enough to spark a similar effect.

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