In the previous 3 parts of this series, I’ve had a look at the superb LTE/5G network coverage that has been installed in the tube in central London in 2024. One main question that had to remain unanswered at the time was how much capacity has actually been put into place. On one end of possible options, all four network operators could share bandwidth and many stations and tunnels in between could use the same cells. On the other end of possible options, each network operator could use its own spectrum, and stations and tunnels would be covered individually. When I was recently back in London, I took a few hours to have a closer look.
How Many Places does a Cell Site Cover?
It turned out that from a capacity point of view, the solution chosen for the London tube is on the very positive side of potential implementation options. It turned out that every station has its own cell site per operator. In other words, each station has the full capacity of what is deployed in the different frequency bands. Each tunnel segment also has its individual cell, or rather two individual cells with a handover in between. The next tunnel segment then re-uses the cell from the opposite side of the platform until the middle. But in essence, each tunnel segment has a cell of its own.
Spectrum Sharing?
I ran my tests on two of the four networks, and both of them used their individual spectrum holdings. So in the worst case, the other two network operators might share that spectrum, but it is more likely they also use their own spectrum. That means that not only is a massive amount of spectrum used, but capacity is fabulous. So how many people could be in a tunnel segment simultaneously? According to Wikipedia, a single train can carry up to 1000 passengers. In case two trains are in the same tunnel segment between two stations, i.e. one train in each direction, that’s 2000 simultaneous customers spread over four networks. That means 500 customer per network, and not all of them will be active at the same time. Not a low number but nothing a network needs to be afraid of.
In the stations themselves, I guess there will be fewer people than what a train can carry, because not everybody gets in and out at a station. However, more people will be in the walkway tunnels to and from the platforms which are also covered. I only had a look at the platforms, so I can’t say if additional cells are used for other parts of a station.
So that’s the first important message of this post: Capacity is massive, every station and every tunnel segment has individual cells. Impressive!
Station / Platform Spectrum Use
So let’s come to the coverage itself, which is different in stations and tunnels. While none of the tunnel segments have 3.5 GHz n78 5G coverage, not all but many stations had the technology installed. One network operator had 100 MHz + 40 MHz of n78 5G installed and used it together with LTE band 1 and 3 (15 + 15 MHz). That’s 170 MHz of aggregated bandwidth.
The other network operator I had a look at could only use 40 MHz of n78 5G 3.5 GHz spectrum, because that’s all he has. However, this spectrum was combined with 5 LTE carriers (5CA…) for a total aggregated bandwidth of 130 MHz. Also quite cool. Add to this a similar amount of bandwidth deployed by the other two operators.
Having said all of this, it’s not a surprise that on platforms on which this amount of spectrum was deployed, I could easily get datarates of 800+ Mbps during an off-peak time. But off-peak in London during the day still means a lot of people, so I would expect that even during peak times, service is still excellent. Something to test next time.
Tunnel Spectrum Use
In the tunnels between the stations, capacity was significantly lower. One network operator used LTE band 3 and LTE band 1 in the tunnel with a total aggregated bandwidth of 40 MHz. No 5G. The other operator used LTE band 3 and 5G band n28 with a total aggregate bandwidth of 30 MHz. Speed tests, again during an off-peak time during the day, still resulted in 150 Mbps. I’m not sure why less spectrum was used inside the tunnels, perhaps it’s a matter of maximum downlink transmit power? Just a wild guess. Still, a very good result.
Summary
I’m really excited that the London tube got state of the art LTE/5G network coverage with massive capacity that should serve commuters well for many years to come. If anybody has details on why less spectrum is used in the tunnels, please consider leaving a comment. And finally, dear UK network operators, upgrade train stations and airports to a similar capacity because networks there are totally overloaded and slow!