shows that proper RF planning and system design well ahead
of the deployment is a key factor to optimise installation.
“Co-ordination with civil works for cable routing,
leaky cable location and indoor antennas installation is
critical to make sure that the right information is delivered
and received on time by the civil works contractors
and radio system designers to reach an ecient radio
design co-ordination and to avoid a costly design and
implementationprocess.”
When a metro goes down it is enormously disruptive, so
it is important the radio equipment is robust and reliable.
“We put resilient cores in for metro systems,” says Skinner.
“ese solutions will maintain communications in degraded
modes with geographical redundant switches and bring the
system back in minutes. ere is a lot of resilience built
into TETRA anyway and also now on the cyber security
sidetoo.”
Signal distribution
One of the main challenges is providing a consistent signal
throughout the miles of tunnels. Distributed antenna
system (DAS) connected to radiating (leaky feeder) cable
is the normal way to distribute the signals evenly down the
tunnels, according to Ingo Flomer, business development
director at Cobham Wireless.
“In developed countries I’d say 90 per cent are covered
by DAS with multiple radios and operators. Metros have
long tunnels, so a multi-radio, multi-operator approach is
the only ecient way to do it. e other advantage of DAS
is that you can continue to distribute the signal coverage
outside thetunnel.”
Flomer explains that when metro trains exit a tunnel
there can be handover diculties with the transmission from
inside the tunnel to the outside world. “Suppose there is
a trac jam at the exit of the tunnel. at can occupy the
entire cell capacity and you can’t make a call.
“If the train is using this cell, all calls will drop, but if
you continue the DAS strategy outside the tunnel with
a dedicated cell for the metro, you will get continuity of
coverage and adequate capacity for the metro train and its
passengers,” he says.
In the Middle East, Cobham’s idDAS system is being used
to provide public safety communications on a metro system.
“ey are using LTE on two bands dedicated for public
safety use, along with a UHF one for TETRA and one for
P25. It is the rst mixed-use system of its kind. ere are
500 radio units, making it the biggest public safety system
for metros as far as we know,” says Flomer.
Looking ahead, Flomer says Cobham’s idDAS system is
5G-ready, but he points out that 4G still has to be deployed
in any signicant way. Where he does see a big change
happening is in the way DAS is fed, moving from power-
The sheer pace at
which new metro
lines are being
built represents
a signicant
opportunity for
TETRA vendors
and suppliers
In developed countries, I’d say 90
per cent are covered by DAS. Metros
have long tunnels, so a multi-radio,
multi-operator approach is the only
efcient way to do it
hungry full base stations to just using the base band unit
connected to a multi-sector digital hub, connecting in turn
to digital remote units.
“Our virtual RAN idDAS solution will oer massively
reduced power consumption,” says Flomer. “We have done
feasibility tests and the metro industry is asking for it, as it
makes the communications systems more ecient for both
the opex and the capex side.”
Signalling systems
Traditionally, metro signalling was controlled by xed-block,
track-circuit-based systems, but many metros have now
switched to Communication-Based Train Control (CBTC)
moving-block signalling systems. Wireless technology is used
to pinpoint the exact locations of all the trains and ensures
they maintain the correct safe operating distance between
each other.
CBTC enables real-time train control information to
allow the distance, or headway, between trains to be safely
reduced to as little as 60 seconds, meaning more trains can
be run, thereby boosting capacity on the line.
Wi-Fi is the dominant technology used for CBTC, which
is made up of three integrated networks: the train onboard
network; the train-to-trackside radio network, which
generally uses Wi-Fi; and the trackside backbone network
situated along the tracks.
e onboard control unit or computer sends train control
information to the trackside network at regular intervals
and works together with the Automatic Train Protection
(ATP) system, which controls the safety functions such as
emergency braking, and the Automatic Train Operating
(ATO) system, which controls the train driving functions
such as doors, starting, stopping, accelerating and braking.
PMR manufacturers have worked with signalling
companies to develop a CBTC system using TETRA, but
the solution is not being adopted. “It is technically possible
26 www.criticalcomms.com June 2019
Adobe Stock/VILevi
/www.criticalcomms.com