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 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 many technical rooms have to be covered by RF – a point made by Mariusz Gerlach, strategic marketing manager at SEE Telecom, manufacturer and designer of repeaters and systems for in-building and tunnel coverage for TETRA and other wireless systems. “Tunnels are, in most cases, multi-band systems with TETRA, Tetrapol, UHF, VHF and FM channels represented”, he said.

FM broadcast channels are an important way of communicating with the public in a tunnel or building in case of emergency. Handover while leaving/entering the tunnel should be designed within the coverage plan, which can be simulated for all systems in the tunnel in advance.

Distributed antennas are for the most part used for GSM, because of their improved propagation of higher frequencies inside the tunnel pipe, Gerlach explained. But if an accident occurs, such as a multi-vehicle pile-up or train crash, GSM signals become attenuated by the physical obstructions. TETRA and other emergency systems cannot, though, afford to be lost at times of emergency. In most cases, TETRA is distributed via radiating cable, which is often protected by a concrete bench beside the tunnel wall to shield the cable from mechanical and fire damage.

Redundancy is a very important element of any emergency communications system. The repeaters used in a tunnel – best protected in technical rooms inside the tunnels and not mounted directly in the tunnel pipe – are fed from two BTS via two different fibre cables. If one repeater fails, enough power remains in the other repeaters to send TETRA and other signals further into the tunnel. Battery back-up is a must.

A little RF goes a long way

Another engineering company specializing in this field is Intelligent RadIo Solutions (IRIS). Managing director Daniel Lewing, said: “Regardless of whether we’re dealing with a tunnel or a building, new build or existing structure, the real issue is one of making TETRA work behind walls, through any surface, or under the ground. We want to end up with the same coverage inside as outside and have effectively, therefore, to physically remove the effects of the structure from the equation.

“The first thing we look at is how to get coverage into the structure, and then whether the requirement is one of a capacity fix or a coverage fix. For an emergency services requirement, for instance, the need will be one of coverage.
For an airport campus scenario, it’s capacity that will be the major requirement for the customer – in which case we must ensure the coverage from the new site does not affect the overall network plan. The signal must not get out. So you then need to marry the new network with the existing network handover profiles.”

David Giles, IRIS general manager, added, “What we are trying to do is minimize any negative impact on the user experience. For the network operator, this means ensuring the upgrade does not affect the overall frequency re-use patterns. A little RF will go a long, long way.”

But whether it’s a coverage, or capacity, deployment, Lewing echoed the sentiments of Analysys Mason’s David Taylor: “No single solution fits all scenarios. This should come as no surprise when you consider, for instance, that handover characteristics of all the technologies are different, and each scenario may combine them differently.”
One final issue which was extremely important but often forgotten, Lewing said, was that once re-tuning the network infrastructure by the network operator takes place, channelized repeaters in the tunnel will also need re-tuning or new channels created.

‘Simple laws of physics’

Tunnels and buildings are challenging for several reasons, according to Robert Froger, sales engineer at iBwave. “Simple laws of physics mean that the many obstructions encountered in a tunnel or indoor environment will prevent radio waves from propagating as well as they do in free space.
(中国集群通信网 | 责任编辑：陈晓亮)