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Top 3 things you need to know about underground telecommunications

Posted by Marcello Perricone on Jul 19, 2019

Brooklyn Bridge and Lower Manhattan NY gray sky day

Nowadays, passenger comfort goes beyond swift journeys and good seats. The invention of the internet and the smartphone connected virtually anyone to anywhere at any place or time, from the comfort of their own property to the middle of a national park, and that constant connection has become a requirement of live in both the professional and home lives of most people around the globe.

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That modern life necessity is expected both on public transport and long-range rail routes, creating immense pressure on operators and service providers to fulfil it. So in an age of constant technological advancement and evermore demanding customers, what do you need to know about communications?

crossrail tunnel


Radio transmissions, like waves on water, use air as a medium to move through the physical space and reach somewhere else. While radio transmissions are electromagnetic waves and can even move through vacuum, they still can’t penetrate certain materials or thickness without considerable signal loss; like an ocean wave hitting a sea wall, these radio signals can be stopped by constructs and buildings of any kind, and the ground itself is a fantastic signal blocker.

As a result, underground transmitters and receivers are needed to spread the network below the surface, regardless of transmission types – cell phones, wi-fi, and walkie-talkies all need amplifiers in order to reach the coverage needed in subway networks.

CBTC CITYFLO is also installed in Kuala Lumpur

Nowadays, the problem is intensified due to the speed of our connections. The jump from 3G and 4G and the upcoming change to 5G all require new and more sophisticated protocols, able to use wider bands and higher frequencies to transmit data. But as technology increases and communications are replaced with faster and more sophisticated protocols, signal penetration drops considerably – and so does area coverage. What used to need a single tower to cover a few square miles might need two transmitters in 4G, and as many as 6 in 5G.

Therefore, befit hardware is necessary to cover vast stretches of underground effectively, vastly increasing the engineering work required and the price of such projects. Wireless antennas, gaped cables, and routers must be installed on platforms, tunnels, and trains to ensure a contiguous coverage of the network.

tunnel rail cables by Sonny Sixteen from Pexels

What’s wrong with that?

The problem with that is two-fold: one, the price of covering underground installations balloons as network requirements increase exponentially, forcing service providers to either plan ahead for future expansion or force operators to perform costly retrofits to update an infrastructure that can no longer serve its purpose. Two, the required engineering effort depends on uninterrupted access to the system for a long time – an extremely complicated idea for public transport systems that almost run non-stop 24/7.

As a result, installation becomes an extremely tiresome proposition. Network providers need to compete with several other ongoing maintenance and installation projects going on during the short downtime of underground networks, and must fight for priority every time something big needs to be done. The bigger the project and the larger the number of equipment required, the longer the timeframe and the bigger problem – and the cost.

CTA_Night Chicago Transit Authority Chicago L tracks in the Chicago Loop at the Adams Wabash station at night


Once all those problems are dealt with, the network starts to operate and service thousands of customers a day, if not per hour. As a result of all that hardware and the constant weight of continuous transmissions, the threat of airwave saturations arises. As signalling and control systems evolve and change from analogous to digital, they rely more and more in constant communication – which creates a problem when their frequencies match that of other devices nearby.

In 2012, Shenzhen Metro in China was repeatedly forced to halt operations for hours due to an overlap on Wi-Fi and CBTC systems, creating a huge security breach that needed to be immediately fixed. While places like Boston and Los Angeles in the US have established standards that prevent those issues from happening, international collaborative locations with frequent interchange of technology and services, like Europe, are more susceptible to frequency conflict and saturation due to the sheer amount of different equipment involved.

At the moment, there are both technological and legislative solutions being sought to it, such as an European standard to be adopted by every single EU country. These range from simple frequency determinations to systems that block and allow traffic depending on transport intensity and locations, and should start coming to fruition in the upcoming years.

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Topics: Telecommunications, SRW Featured, 5G, Underground

Marcello Perricone

Written by Marcello Perricone

The Editor of SmartRail World and Transport Security World.

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