“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 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.

This 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. The 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.

“The 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. These 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.

“There should never be a ‘one-antenna-fits-all’ approach – and antenna designers and manufacturers need product offerings that cover many, if not all, eventualities.”

DELIVERING PREDICTABLE SIGNALS IN HARD-TO-GET-AT PLACES

The 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.
(中国集群通信网 | 责任编辑：陈晓亮)