Want a quick digital detox? In London, all you need to do is take the Tube. But, from next year, Transport for London (TfL) says it will roll out 4G coverage on the Underground. It’s a start, but London is still some way behind its big city rivals.

That 4G coverage will join the Tube’s Wi-Fi network, which, since 2012, has let people get online while travelling through London’s myriad of Underground stations. Unlike the metro networks in many other major cities around the world though, in London you can browse only while you’re at a station. So if you’re sitting on the Tube and have that all-important email to send, you will be off-the-grid while you’re in the tunnel; as soon as your train pulls in, it’s a mad dash to find the network and connect, in the hope your train doesn’t pull out just as you hit the ‘send’ button.


But why does London have much worse connectivity than most other public transport systems below ground? And why is there zero phone connectivity? Take Moscow’s Metro, where both Wi-Fi and mobile phone network have been available since 2014. New York has had fast and reliable Wi-Fi since 2017 – admittedly only at stations, but to make up for that the Metro system also has 4G, while the carriages at least have Bluetooth beacons that provide arrival times to customers using the MYmta app. Rome has excellent Wi-Fi service on most of its lines. Tokyo, Barcelona, Hong Kong and Melbourne all provide connectivity in tunnels. In South Korea, Seoul has even been trialling connectivity using the mmWave spectrum, which is expected to be a key part of next-generation 5G networks.

London’s failure to connect has multiple causes. First is cost. “Technically, it is straightforward, although expensive, to deliver Wi-Fi in stations,” says Matthew Griffin, head of commercial telecoms at TfL. To install it, individual access points have to be placed within the station ceiling or hidden in voids, with flat antennas providing the signal.

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While this sounds simple, it’s very expensive to lay cabling to reach all these access points. “This cabling needs to carry a significant amount of internet traffic to manage a reliable and consistent service, one terabyte per day on the Tube, and requires careful engineering to ensure it can be delivered without interfering with other station infrastructure,” says Griffin.

In tunnels, the process is much more difficult. Some sections of the Tube are more than 150 years old and its tunnels very narrow, which means there is little space to install any extra equipment. Wi-Fi uses radio waves, which work great when they can move in a straight line and have plenty of space (say, up to and down from a satellite, or through your living room). But they run into trouble when they hit solid matter.


London’s Tube tunnels twist and turn, so any Wi-Fi radio waves would not be able to penetrate walls or go around corners. To deliver mobile connectivity on, say, the Northern Line, TfL would have to install an enormous number of access points – which is both uneconomical because of the cost of equipment, and unreliable as it will be tricky to maintain all these access points in such a confined space, says Griffin.

So why don’t the metro networks of other cities face the same problems? Well, Moscow for its part, uses a Wi-Fi signal with high frequency radio waves that provide high data rates. These signals work in line of sight, and that’s doable because tunnels in Moscow are wider and straighter than those in London.

Also in Moscow, the Wi-Fi repeaters are not in tunnels but on trains. The signal arrives through antennas on the outside, which increase the height of trains by three to five centimetres and effectively decrease the distance between the train and the tunnel to the right in the direction of movement by 30 to 40cm. Access points are managed by autonomous 'controllers', two small instrument in the back and at the front of each train that are linked by cables.

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The train network data is transmitted to base stations via a train-tunnel radio channel, says a spokesperson of the Wi-Fi provider MaximaTelecom. “Base stations along the tracks are arranged so that the train moves in a solid radio field. The average distance between them is about 700 meters,” she says. About 35 per cent of Muscovite commuters connect to the Metro’s Wi-Fi network every day.


Installing Wi-Fi on the Moscow underground was neither easy nor cheap. The Moscow Department of Transport and the Moscow Metro worked with MaximaTelecom to equip the systems across the whole 330 kilometres of tunnels snaking below the Russian capital. To get some return on this huge investment, the operator forces users to watch a minute of unskippable adverts before connecting (unless they switch to the ads-free premium option). The alternative is to use the cellular network, which offers fair to good connectivity in most tunnels.

This solution, however, would not work in London, because TfL’s tube trains simply don’t have enough space to install any of the on-the-train infrastructure. An alternative approach would be to install cables called ‘leaky feeders’ along the length of the tunnel – basically, a very large antenna cable with slots for the signal to get in and out. Tests on the Waterloo & City line in 2017 found that this method “would allow us to deliver good mobile phone coverage, but would not work well for WiFi,” says Griffin.

Blame physics. The lower the frequency, the further a signal will travel through the cable. TfL tested a range of frequencies – 800MHz, 1800MHz, 2100MHz and 2600mHz. Tube tunnels vary in length between stations, but the vast majority are in the 1km range. The Waterloo & City line runs for 2.2km, so it was an ideal testbed for the effectiveness of signal propagation. “We fed the signal in at both ends, which avoided the need to have equipment in the middle of tunnels,” says Griffin. That’s because there is rarely space for any equipment to safely be installed, providing power can be difficult and, if the equipment goes wrong, we may have to wait a long time to fix it. As some Tube lines run all through the night during weekends, any fault would have to wait until Monday morning to be fixed.

TfL found that for more than 80 per cent of tunnels it would be possible to provide full coverage using frequencies from 800MHz through to 2100MHz. Most parts of the network could be be covered by 800MHz base stations, while a couple of sections would need some additional equipment to make it work. Beyond 2100MHz, however, the signal’s reach declines sharply; at 2600MHz it’s less than 100m. That effectively rules out Wi-Fi, which requires a signal at 2400MHz.



On the upside, the tests showed that it would be feasible to deliver a mobile phone signal even in the most challenging parts of the network, says Griffin. Last year, TfL shortlisted four telecoms firms bidding for the contract. By early 2020, some lines in central London may be offering 4G connectivity for voice signals.

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That’s only half the solution, though. A train carriage is just like a big Faraday cage, which blocks or at least weakens electromagnetic signals, including 4G radio waves. “Because of this, most operators will still build an on-train Wi-Fi network for passengers and use antennas on the roof to ‘backhaul’ on-train wireless data from passengers to the trackside, using 4G or Wi-Fi spectrum,” says Richard Osborne, strategic solutions specialist at Cisco.

If 4G is used from train to track, there will be an additional challenge: if passengers can access the 4G service directly, then the network operator will be unable to control the bandwidth they use. Just a few passengers trying to stream Netflix could overwhelm the service, says Osborne. That’s why any network operator will hope that they can build a dedicated data connection into each train, where they can control the access point and thus the bandwidth used by passengers. But TfL disagrees, saying that all passengers are expected to have good 4G experience on the Tube, based on trials.

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