coverage
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. “Te 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. Te 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. Tis 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. “While buildings present the challenge of inter-floor
interference, tunnels are more about severe multi-path propagation issues, as well as the disruptive effect of the presence of a train or other moving vehicle can have inside a tunnel. “Tunnels also cannot rely on signal penetration and,
therefore, need a dedicated coverage system. Effective penetration inside short tunnels can be achieved using a directional antenna pointing inside the tunnel from an entrance. However, leaky feeders tend to be the norm when it comes to longer ‘tubes’.” He added, “From a survey and installation perspective,
buildings and tunnels are typically either difficult to access, or are accessible only at certain times and are often security- sensitive venues, with restrictions as to who is allowed to enter and what they are allowed to do. As antennas can only be installed and cables routed in certain areas, there is a need to have a thorough knowledge of the venue’s architecture. “What’s more, as TETRA is all about safety, there is no
room for guesswork, and coverage targets must, therefore, be accurately mapped with the highest level of statistical accuracy (99 per cent coverage means nothing if you do not specify
Issue 4 2011 TE TRA TODAY
where that coverage is). While a cellular network can withstand a certain percentage of down- time, TETRA requires a permanent signal and, hence, the need to use redundancy techniques.” In an indoor coverage
scenario, Froger went on to explain, infrastructure often needs to support multiple technologies, if only to split the cost of equipment. As a result, the engineers dealing with indoor TETRA coverage often need to be proficient in other technologies and make use of versatile tools. And all of these challenges call for the use of tools that document a venue, tunnel or building, before, during and after the roll-out. Tis creates another challenge, according to Froger –
ensuring that the same information can be presented in different ways, depending on who looks at it. A designer, for example, might want to see a link budget report, down to each component, while the building owner or tunnel operator might want to see a colourful ‘heat’ map: different skills, different ways of representing the same system.
Overcoming by understanding Echoing much of this, Alan Dowen, at Artevea Digital, said: “Understanding the area of coverage and its definition is the key to success, and planning in line with these expectations is where we start. Te challenges of tunnels and buildings can all be overcome by understanding where the users need to access their system and what antenna system or repeater, for example, will provide this.” One grey area, he continued, is that when a wide area
system is involved, typically a city-wide or mainstream campus, it is often ‘expected’ to cover areas that are not clearly visible to the main system from the initial design outset. To overcome this, he explained, these areas can be filled in using repeaters with the correct antenna. “Te main point remains that seeing and setting the areas leads to a smoother path. Additionally, well lit, sufficient power, cable tray/lines and safe working environments need to be adhered to when installation and commissioning take place, so that no minor detail is missed and the antenna, for example, is well catered for before, during and after installation.”
Covering all eventualities Alan Spencer, global sales manager for Amphenol, offered a few words on the challenges of delivering TETRA in the structures under discussion, saying that the technical challenges are “more to do with a question of physics than a shortcoming in the TETRA network. Tese same issues apply for GSM networks. Depending on the structure, the materials used to build the structure and the distance from the base station, these factors will all greatly affect the signal level available inside.” He added that the aesthetic aspects of antenna design can
be as important to consider as the RF coverage plan. “For instance, in a rail tunnel with plenty of room, directional wideband yagi antennas could be used. Alternatively, a tube station might want a low-profile panel or a ceiling-mounted product. “Tere should never be a ‘one-antenna-fits-all’ approach
– and antenna designers and manufacturers need product offerings that cover many, if not all, eventualities.”
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Simulated in-building repeater coverage (illustration: iBwave)
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