coverage
Reaching the parts other signals can’t reach
Photo: Airwave
Delivering strong RF signals at even the furthest, deepest points of a tunnel, mine or building complex is a challenge faced by service providers and equipment vendors alike. Tim Guest consulted some experts within the industry for their advice and opinions
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any of us have tried to use our mobile phones or to continue our radio listening to as we’ve driven into road tunnels, or as the train we’ve been riding has
disappeared into the darkness of a railway tunnel. Typically, our mobile calls have been dropped, or we end up rejoining the one o’clock news on the car radio just in time for the weather report. But while cellular or FM/AM radio signals may not be
of critical importance, ensuring that the signals delivering TETRA communications to first responders remain strong and uninterrupted – in the event, for example, of an emergency deep inside a tunnel – is imperative and potentially life-saving.
One size does not fit all “Tere is a need for public safety and emergency personnel to have ubiquitous coverage, it’s an operational necessity, not a luxury”, said, David Taylor, lead consultant at the telecommunications consultancy Analysys Mason. “However, every tunnel is unique and a bespoke communications solution/plan must be designed for each. One size does not fit all.” Another lead consultant at the same company, Adrian Dain, said that poor signal propagation from the outdoor
Above: for the recently-opened Hindhead road tunnel in Surrey, England, a 1·8km
twin bore which bypasses a notorious traffic bottleneck, Team Simoco designed and implemented an extensive multi-band communication system. Besides supplying coverage for Airwave, the nationwide shared TETRA network for the emergency services, this system supports fireground radio, mobile phone services, digital
audio broadcasting (DAB), voice break-in and a dedicated PMR radio system. The installation includes an additional Airwave base station in the tunnel
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environment into a tunnel or building is the first key issue that needs to be addressed. “Very short tunnels may receive adequate signal via propagation into the tunnel portals, and buildings may receive adequate signal by propagation through windows or door apertures and through the walls”, he explained. But it is longer tunnels and buildings with thick walls and/
or basements which will, according to Dain, undoubtedly contain areas without suitable coverage. Solutions to meet these challenges, he said, fall into two
broad categories: passive and active. Te simplest passive installation is a passive repeater – essentially two antennas, one outdoor and one indoor, connected by a length of feeder cable. Te outdoor antenna is typically a high-gain, directional antenna, which creates all the gain in the system. Active systems, Dain continued, range from narrow-band,
on-frequency, bidirectional amplifiers, which are used to increase the system gain in both the uplink and the downlink, to antenna systems using conventional feeder, RF-over-fibre, radiating feeder, and distributed antenna systems, with the signal source for each being a cell enhancer or even a complete indoor base station.
Two different propositions Taylor stressed that he viewed the provision of communications in tunnels as a “very different proposition” to delivering coverage for enclosed spaces. Older tunnels, he said, posed different challenges to ‘new build’ tunnels, although all posed a linear challenge typically made more complicated by moving targets, such as traffic and trains, which only become stationary when an incident of some kind occurs.
TE TRA TODAY Issue 4 2011
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