London Underground’s mobile coverage plan

Of remote radio heads and leaky feeders: some technical details of the London Underground cellular trials.

The most recent London Underground cellular trial was carried out using radiating cable in a 2.1km length of tunnel, with multiple low power radio heads located in Waterloo and Bank stations.

Four different radiating cables (also known as leaky feeders) from two different manufacturers were tested in the tunnel environment. TFL said the trial showed that there was no interference with any of the existing railway signalling and radio systems at these frequencies.

O2 and Vodafone permitted testing in the 800MHz, 1800MHz & 2100MHz bands in tunnels, and 1800MHz, 2100MHz & 2600MHz bands at Waterloo and Bank stations.

One operational aspect the trial uncovered is that MNO installation teams are not considered qualified to work in railway tunnel environments. Therefore, installation will be carried out by TFL-approved teams and then the infrastructure provided to MNOs, with the interface to the MNO base station being positioned in locations outside of the “core operational estate”. In essence, mobile operators’ base station hotels that will provide radio capacity to the remote radio units will be offsite.

Options for in-tunnel coverage include high power antennas that blast coverage down a tunnel, a series of boosters or remote antennas (DAS) distributing capacity within the tunnel, as well as radiating cable. Radiating cable also requires signal amplifiers along its length, usually every few hundred metres, but these are smaller than the hardware required for a DAS.

London’s tube tunnels, narrower  than modern builds, make it hard to attach remote antennas and repeaters to tunnel walls. And as the tunnels are also often bendier than modern rail systems, they are hard to cover with a high power directional approach. This high power base station approach was, TMN understands, the approach offered as a subsidised solution by Huawei in 2011.

It seems that has left radiating cable as the choice of technology in this instance, with a distributed radio head approach for high density areas in the stations.

TFL plans to award the contract for building and operating the neutral host network in summer 2018, with commercial negotiations with the operators to begin after. That could mean “incremental” coverage within the tube from early 2019.