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4G severn tunnel?
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RailUK Forums
I first noticed the 4G signal in the Severn Tunnel a couple of months back. There definitely never used to be a mobile signal (the catering trolley would park up while in the tunnel because their card machine didn't work without a connection). If a leaky feeder has been installed I'm surprised there hasn't been any publicity for it. Very welcome, all the same.
hexagon789
Veteran Member
ADComms assisted Network Rail in installing six mobile repeaters in the Severn Tunnel some months back.
Four major telecoms providers also had input in the project.
Four major telecoms providers also had input in the project.
Would those be the four national real (not virtual) mobile networks in the UK (O2, EE, Vodaphone and Three)? or something else?
hexagon789
Veteran Member
I can't find the article on AdComms website now, but I think those were indeed the four.
Searching google for adcom severn tunnel pulled up a link to https://adcomms.ltd/project/severn-tunnel/ the page seems to have disappeared off their website, but archive.org has it. Seems the article is 5 years old,
web.archive.org
Severn Tunnel - ADComms
ADComms was contracted by Network Rail Telecom to work with them and Mobile Network Operator, EE, to modify the existing Severn Tunnel radio infrastructure by designing and installing a Neutral Host Distributed Antenna System.
web.archive.org
tfw756rider
On Moderation
I've never thought of mobile signal that technically before. I'm guessing you're also a radio enthusiast!
What's an ESN?
I take 5 MHz to be the radio channel bandwidth - and 5 MHz is the minimum, the same as in 3G. This radio "bandwidth" really is the width of a radio band, not a data capacity (bit rate), but it is a factor limiting the data capacity.
On my work phone, which is on Vodafone, I get full signal through the Severn Tunnel, but no data. On my personal phone, which is on ID Mobile, so Three's network, I get nothing at all.
I assumed the signal was there for making emergency calls however data would be quite handy for tracking tight connections at Severn Tunnel Junction.
I assumed the signal was there for making emergency calls however data would be quite handy for tracking tight connections at Severn Tunnel Junction.
The Severn Tunnel has had mobile coverage for Emergency Services Network (ESN) since 2019, after ADComms was contracted by Network Rail Telecom to work with them and EE to modify the existing Severn Tunnel radio infrastructure by designing and installing a Neutral Host Distributed Antenna System.
This is not a repeater solution in the normal sense, but a neutral-host distributed antenna system (meaning it is not unique to any particular operator), with mobile network radios in a comms room on the English side of the tunnel, antennas found inside the tunnel, as well as an infill antenna on the English tunnel portal for better handover.
The application and ADComms website reference repeaters, but these are to rebroadcast the dedicated signal which is broadcast into the tunnel rather than to rebroadcast the signal from outside of the tunnel into the tunnel like you might see for an in-building "signal booster". This was done so that a replacement of the existing leaky feeder cable in the tunnel wasn't needed, and neither was installing a brand new cable just for this.
web.archive.org
It used to be the case for many years that this coverage was only for ESN users, but EE customers could "sneakily" (unintentionally) roam onto this network if they were on a phone call prior to entering the tunnel. If they were not, the network would refuse them access to the ESN network.
Around September 2025, the solution was also enabled for EE's consumer network, and started broadcasting under their network identifier (PLMN) as well, so all their customers can now use it while travelling through the tunnel too.
The deployment should be 5 MHz paired UL/DL (FDD) of Band 20 800 MHz 4G LTE, as well as 20 MHz paired of Band 3 1800 MHz 4G LTE. My understanding is the 1800 MHz deployment is effectively unusable, though, with high error rate.
You can see further details in the planning application for the project:
There is no coverage for other mobile networks in the tunnel today. If people report seeing this, it's likely an error, the phone lying about having signal (common if you are connected to WiFi with WiFi Calling enabled), or the phone displaying the signal of another network to reflect that something is available for emergency calls which can be made on any network rather than the network you pay to use.
This is not a repeater solution in the normal sense, but a neutral-host distributed antenna system (meaning it is not unique to any particular operator), with mobile network radios in a comms room on the English side of the tunnel, antennas found inside the tunnel, as well as an infill antenna on the English tunnel portal for better handover.
The application and ADComms website reference repeaters, but these are to rebroadcast the dedicated signal which is broadcast into the tunnel rather than to rebroadcast the signal from outside of the tunnel into the tunnel like you might see for an in-building "signal booster". This was done so that a replacement of the existing leaky feeder cable in the tunnel wasn't needed, and neither was installing a brand new cable just for this.
Severn Tunnel - ADComms
ADComms was contracted by Network Rail Telecom to work with them and Mobile Network Operator, EE, to modify the existing Severn Tunnel radio infrastructure by designing and installing a Neutral Host Distributed Antenna System.
web.archive.org
It used to be the case for many years that this coverage was only for ESN users, but EE customers could "sneakily" (unintentionally) roam onto this network if they were on a phone call prior to entering the tunnel. If they were not, the network would refuse them access to the ESN network.
Around September 2025, the solution was also enabled for EE's consumer network, and started broadcasting under their network identifier (PLMN) as well, so all their customers can now use it while travelling through the tunnel too.
The deployment should be 5 MHz paired UL/DL (FDD) of Band 20 800 MHz 4G LTE, as well as 20 MHz paired of Band 3 1800 MHz 4G LTE. My understanding is the 1800 MHz deployment is effectively unusable, though, with high error rate.
You can see further details in the planning application for the project:
There is no coverage for other mobile networks in the tunnel today. If people report seeing this, it's likely an error, the phone lying about having signal (common if you are connected to WiFi with WiFi Calling enabled), or the phone displaying the signal of another network to reflect that something is available for emergency calls which can be made on any network rather than the network you pay to use.
Not just "a" factor, but probably the single most important factor.
The shannon-hartley theorem defines the theoretical maximum capacity of a channel based on certain assumptions, it says.
C = B Log₂(S/N)
In practice this forumula tends to serve as an upper-limit for channel capacity. The logarithm in there means that improvements in SNR have relatively little impact on capacity, while improvements in channel bandwidth have a massive impact on capacity.
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