coverage “Te 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. Tat 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,


happen if that site fails. Does the enhancer have to connect to another site? Systems must be versatile enough to go from handling a very low level of maybe two or three services calls a day, to a situation where, in the event of a major incident, they are carrying many simultaneous TETRA calls.


Coverage when it’s needed In tunnels, not only the main tubes but escape routes and


it allows switching from one signal to the other, much like a cellular handover. In metro or underground scenarios, where short distances


are involved, this configuration typically also provides coverage in platform and escalator areas, and such a set-up would, for example, enable TETRA handsets to communicate with each other from deep in the tunnel to the station areas, and vice versa. In the case of long tunnels, or when infrastructure cannot


be located at the station, Taylor suggested the use of a ‘base station hotel’ as one solution, where the base station is remote from the tunnel but connected by optical fibre cable to the tunnel infrastructure – perhaps with repeaters in the tunnel space or ventilation shafts. Where a cell enhancer is being used to provide the signals


in the tunnel, rather than a base station, decisions have to be taken as to the ‘donor’ site for the enhancer, and what will


Above: this design by Artevea shows a generic installation for an underground mass transit rail system, on a line 24 km long. The network supports voice and data communication linking the trains with a central control and monitoring station. Radiating (‘leaky’) feeder cables running through the tunnel form the antennas for Artevea’s T-Matrix TETRA radio site equipment


Delivering predictable signals in hard-to-get-at places T


he main challenges in delivering TETRA coverage in any confined location relate to the need to reach the highest


level of resilience, redundancy and quality of service: so stated Håkan Samuelsson, chief technical officer of Axell Wireless. “In a tunnel environment”, he continued, “these requirements


can be met by planning the system with overlapping coverage from two independent base stations. If one base station fails, coverage is still provided by the other. “Each base station is configured to feed several repeaters


placed inside a tunnel. The distance between the repeaters is calculated so that overlapping coverage exists between two adjacent repeaters and if one should fail, the other repeater situated next to the failed unit will continue providing coverage. “Either antennas or radiating cable can be used in tunnels,


although signal level fluctuations are usually higher when antennas are used and signal propagation from a tunnel antenna can be affected by large vehicles blocking the signals and causing interference. Tunnel coverage systems using radiating cables have a more predictable signal distribution. “The active equipment in a tunnel system must have


emergency power, usually consisting of large batteries. Tunnel coverage systems using base stations connected to a large number of repeaters via optical fibres have been installed in tunnels up to 20 km in length. Theoretically, a tunnel 40 km in length could be covered by fibre-fed repeaters connected to one or two TETRA base stations. “A similar concept and system design can also be deployed


Issue 4 2011 TE TRA TODAY


to support TETRA coverage in-building”, Samuelsson continued, “but in this scenario the use of antennas is more common. “Distributed antenna systems (DAS), which use one or several


repeaters, are most common for indoor solutions. The repeaters are connected via an outdoor antenna to the nearest base station (a so called off-air repeater) and one or several antennas inside the building provide coverage for any TETRA terminals used inside. The use of DAS provides better coverage with less signal variation throughout the building – although if the structure is large, and a large number of TETRA users are expected inside, then a dedicated base station may be used for the indoor system. If this is the case, then the system will resemble the type that would be deployed to support TETRA coverage in a tunnel.”


Reliable coverage: Håkan Samuelsson with a TETRA in-building repeater by Axell Wireless. Recent installations by the company include a large multi-band system for the Stockholm Metro, supporting train communications as well as the emergency services


15


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48