LiFi Applications

21) LiFi and Underground Trains (Tube, Metro, Subway, etc..)

Some underground transport networks around the world offer internet access inside their tunnels or their underground trains. Moscow’s Metro offers both WiFi and mobile phone network to its passengers since 2014. New York has had fast and reliable WiFi since 2017. 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 WiFi 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. However, some underground transport networks have yet to make that jump possibly due to high implementation costs and their transport network infrastructure.

As of 2021, the iconic London Tube network has yet to offer internet connectivity inside their underground trains. This is due to multiple causes. Firstly is cost. “Technically, it is straightforward, although expensive, to deliver WiFi 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.

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. WiFi 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 WiFi 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.

Well, Moscow for its part, uses a WiFi signal with high-frequency radio waves that provide high data rates. These signals work in the line of sight, and that’s doable because tunnels in Moscow are wider and straighter than those in London.

Also in Moscow, the WiFi 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 instruments in the back and at the front of each train that is linked by cables.

WiFi installation and implementation on the Moscow underground network were not easy or 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 would not work in London because TfL’s tube trains simply don’t have enough space to install any of the on-the-train infrastructures. An alternative approach would be to install cables called ‘leaky feeders’ along the length of the tunnel. 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 Wi-Fi,” says Griffin.

TfL tested a range of frequencies – 800MHz, 1800MHz, 2100MHz and 2600mHz. Tube tunnels differ in length between stations. For example, 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 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. This makes WiFi impractical, which requires a signal at 2400MHz.

Another option that can be considered for London Underground trains is the use of LiFi. LiFi can deliver high-speed on-board internet access as well as accommodate a large number of passengers on its LiFi network without compromising connectivity. The cost of implementing a LiFi solution inside the trains will be much cheaper than WiFi installation though installing LiFi on London Tube tunnels could prove a challenge due to their small size. However, LiFi could be useful for driverless trains, automatic train systems and signalling. Signify, Oledcomm, pureLiFi and many more LiFi companies are hoping that LiFi on trains will come at a faster pace in the next decade.