Online Underground: the technology behind the Tube’s forthcoming 4G network

While passengers have been able to access Wi-Fi on the Underground’s platforms since 2012, the current system provides no connectivity in the tunnels and access to phone calls and SMS messages is restricted.

The 4G networks up for tender will solve this, as well as providing important communication avenues for emergency services, which are currently lacking in the tunnels, and will be compatible with the long-delayed Emergency Services Network when it eventually launches.

Ingo Flomer from UK-based Cobham Wireless, which is bidding to build the distribution system, said the new network will be upgradable to 5G once it’s ready and will be compatible with older cellular all the way down to 2G.

“TfL (Transport for London) have long needed a project to bring coverage and data into the London Underground, which is standard in most metros of the world, but has been delayed for many reasons - part political and part technical,” he said.

The delay in setting up a working cellular network has been partly blamed on the age of the Underground system, which is the oldest in the world. An underground section of the Metropolitan line first opened in 1863 between Paddington and Farringdon.

The age of the system means that many of the tunnels on the older lines are much narrower than modern networks and there is little space to fit the additional equipment and cabling that is necessary to provide mobile coverage.

Some of the stations themselves are also quite pokey and each one will need at least one box of equipment to drive the signal down the cabling.

Even when the infrastructure rollout is in full swing, there will only be short windows of “three or four hours” in the middle of the night to lay the cables, Flomer said.

The cables themselves are known as ‘leaky feeders’ and essentially comprise a standard coaxial cable that can emit and receive radio waves.

The cable is ‘leaky’ in that it has gaps or slots in its outer conductor to allow the radio signal to leak into or out of the cable along its entire length.

Flomer describes it as “basically a very long antenna that radiates RF on every centimetre of the cable” with a maximum distance of about 800m. Low-frequency bands will also be used, despite their relatively limited data throughput, because they have the best reliability in tunnels.

This tech can achieve dependable data connections in high-speed trains travelling at 200mph he said - a far cry from the average 45-65mph speeds at which London Underground trains travel.

Data usage is typically quite low inside train carriages, Flomer said, so this is generally not a problem, although upgraded 4.5G or 5G networks should still be able to transmit in the hundreds of megabits.

The whole system will also be shared amongst operators unlike above-ground infrastructure, where most large networks have their own set of mobile towers.

The winner of the tender is expected to be chosen by TfL in the next few months, although Flomer hopes that Cobham will have the upper hand, being a British-based company. While the full list of companies up for tender has yet to be revealed, he also believes that a “big Chinese telecom manufacturer” will submit a bid.

When pressed for comment, TfL said that Huawei - a company which fits the above description - “are not one of the four consortiums shortlisted to install 4G onto the London Underground,‎” although it admitted that Huawei equipment may be used in the final rollout.

Huawei has come under scrutiny in recent months over its links to the Chinese Government and concerns that it could abuse its position to undermine data privacy. 

Flomer estimates that the distribution system could cost TfL up to £40m, although the cabling will cost “multiples” of this figure and will make up the bulk of the cost of the network installation.

He said it will be “an investment in the future” and a “huge step forward for London” that will bring its metro system in line with other major Western cities.