Ookla Study Reveals Quality of 4G and 5G Mobile Signals on London Underground

Network benchmarking giant Ookla, which collects data from consumers via their popular broadband and mobile Speedtest.net service, has today published one of the first independent studies of real-world 4G and 5G mobile (mobile broadband) coverage quality on the London Underground. The good news is that it works and exists exactly where TfL says it should.

Boldyn Networks, using kit from Nokia and others, currently holds a 20-year concession deal with Transport for London (TfL), which was signed in June 2021 (here). The agreement allows them to deploy new mobile network infrastructure across the whole Tube (London Underground), DLR and Elizabeth line network.

The project has made good progress over the past few years and is currently expected to complete by around the end of 2026, which makes now a good time to examine whether or not its delivering on its promises. As of August 2026, over 60% of the 121 below-ground stations were live and the rest of the sites are expected to be live by year-end.

To test how real the impact is, Ookla says they analysed more than 50 million background signal scans collected across Greater London between 25th March and 30th April 2026, using a new method that identifies high-confidence below-ground measurements at massive scale (this is needed because otherwise it’s difficult to distinguish below-ground from above-ground signals between users). Overall, the study found that the availability of mobile signals aligns closely with the line sections TfL’s official coverage map show as live.

Key Findings of Ookla’s London Underground Study

➤ Almost one in five below-ground connections is already on 5G.
Some 19.8% of below-ground scans connected over 5G, against 16.5% above ground citywide, because the system was built 5G-ready from day one. Every operator shows a healthy below-ground 5G share, from 12.8% on Three UK to 26.5% on O2.

➤ Signal strength below ground is comparable to the street above.
Median 4G signal strength is -96 dBm on below-ground platforms, against -98 dBm at street level citywide (readings closer to zero are stronger). A like-for-like comparison within the same small areas shows the street still wins by 2 to 3 dB, but holding within a couple of decibels of the open street inside 19th-century tunnels reflects a network built into the stations and tunnels themselves.

➤ The signal holds to 30 meters down, and 5G is live at the deepest measured platforms.
Median 4G power readings stay between -94 and -96 dBm across the first 30 meters of estimated depth, falling to -101 dBm beyond that, while the share of connections on 5G declines from 21% in the shallowest band to 14% in the deepest. Where 5G is present below ground it is strong, with a median 5G reading of -93 dBm, stronger than the above-ground citywide 5G median of -98 dBm.

➤ All four UK operators are live on one shared network, and customers use it in proportion.
Below-ground connections split EE 33%, Three 24%, Vodafone 21%, and O2 20%, with the remaining 2% on MVNOs such as Sky and on international visitors roaming onto UK networks. No other UK mass transit deployment has matched that tenancy mix. London locked it in before the first major post-pilot expansion, with all four networks signed by July 2022 and the Emergency Services Network (ESN) providing a separately funded public service workstream.

Sadly, Ookla’s study was solely focused on signal strength and coverage, but didn’t attempt to examine mobile broadband speeds, latency or call stability etc. In that sense we don’t yet have a good idea of how well the new mobile infrastructure actually performs across the underground and via different mobile operators, but hopefully such data will emerge in the future.

Ookla’s Full London Underground Study
https://www.ookla.com/articles/london-underground-5g