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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 Many 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 “There 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 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. The simplest passive installation is a passive repeater – essentially two antennas, one outdoor and one indoor, connected by a length of feeder cable. The 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. “The very nature of the moving target makes the art of call handling that much more important, for railway applications”, Taylor said. “So, not only do RF signals need to remain strong at the deepest/farthest point, but being able to hand over a call from one cell or base station to another inside a tunnel without being dropped is crucial.” In the case of newly-built tunnels, TETRA technology and capabilities are likely to be included in tunnel construction as part of a communications bundle from the outset. Taylor cited new tunnels in Singapore, where a ‘leaky feeder’ provides commercial and mass communications and a second leaky feeder carries signals for public sector communications, including TETRA. Indeed, the Singapore Civil Defence Force operates via the same feeder – though using different frequency bands – as that used by the police. “If you are doing work as an upgrade to provide any form of tunnel communications”, Taylor warned, “adding infrastructure after initial construction will be much more expensive because access to a ‘live’ operational tunnel can only be for a few hours at night. That means much longer project timeframe costs, coupled with the increased costs associated with late-night working.” Base station ‘hotel’ He explained that in the past, VHF, UHF and other communications signals have often been propagated between sets of base stations, starting at the tunnel portal or entrance, with the signals then funnelled down one leaky feeder. Signal decay towards the centre of the tunnel has been mitigated using the same feeder fed, from both ends, by a base station at each entrance. As the signal from one base station decays, it allows switching from one signal to the other, much like a cellular handover.
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