Tunnel vision

Confined spaces present a unique challenge for RF engineers. Andy Gamble, of Simoco Group, considers what needs to be done to ensure reliable communication

Confined spaces come in a variety of shapes and sizes, each one presenting its own unique challenge. Road and rail tunnels, service tunnels, underground stations and even ships need careful planning and consideration when implementing a reliable radio communications system. Here, we look at the challenges that arise when working in tunnel networks, and the preparation needed to overcome them.

Plan to succeed

Is it a tunnel or is it an underpass? The UK Department of Transport Design Manual for Roads and Bridges Vol 2 section 9 BD78/99 defines a road tunnel as a subsurface highway structure of 150m or more in length but a similar definition could apply for any tunnel be it rail, pedestrian, cable or even a long passageway on a ship. A tunnel’s shape and length has a major influence on the design and performance of a radio network – a simple antenna in the middle of a 150m road tunnel may suffice but for 1.5km it just won’t work.

In the first stages of designing an RF communication system, there are a number of considerations, such as how to ensure continuous coverage of the entire length of the tunnel. 

So what is meant by ‘coverage’? Coverage is something that just works, or a system that provides predictable and measurable signal levels reliably throughout with little interference and with a tunnel full of a train or buses for example. Reliable system operation means that wherever a user goes then the radio will work, so it’s important to ensure there is a safety margin built in to the design. However this is really the same as designing any system. 

A tunnel can present some unique problems, as often a single radiating infrastructure has to provide coverage for a large number of different services, which may include PMR channels across several bands, cellular telephone services and services intended for domestic use such as AM/FM broadcast and DAB. Add to this some requirements for ease of system operation and maintenance under harsh environmental conditions, and you have a complex set of requirements from a variety of services. Try and put this into a nice new shiny structure and then you have the architects to appease as well plus the radio system providers.

Field strength requirements

During the design and planning process, it is essential to know what is required in the tunnel network. As well as the type and size of the area to be covered there are the field strength requirements for various services to be considered – all essential considerations for designing a successful confined space scheme.

In order to provide a signal along the entire length of a tunnel, you need to understand how radio signals behave in confined spaces. While the obvious solution would be to install antennas at each end of the tunnel, the process of transmitting radio frequencies to the entire tunnel is not that simple. Obstructions from a moving train for example, or the sheer length of the tunnel, could mean unacceptable attenuation of the signal and an unreliable system. In many cases, there may also need to be a degree of surface coverage, particularly in areas such as station entrances or road tunnel portals, 

but equally too much external coverage may also be a problem particularly in central city locations. 

In general terms, the distance from confined space to surface coverage should be minimal,  which is important for systems that require a cellular element. Mobile cellular network operators providing services in a tunnel may prefer cell handover to take place underground to avoid overloading the surface system when a potentially large number of users try and change cells as the train exits the tunnel.

The important consideration is the power required to radiate signals across entire networks. Careful calculation of the system power budgets need to be carried out to determine power levels needed, which will determine cable types to be used and give predictions of signal levels, against the customers’ requirements for system coverage. 

Cabling matters

The solution of choice for providing radio communication in tunnels is the radiating cable, effectively a coaxial cable with a number of holes in it where signal leaks out (hence the term ‘leaky feeder’), with small antennas along its length. This can be equated to holes in a water pipe. Radio signals leak out of the ‘hole’ in the same way that water leaks from holes in a pipe. However, there are two major losses to consider when designing a system. 

Firstly there is coupling loss, the loss of signal from the cable to the user. High coupling loss means lower RF level to the user but generally lower longitudinal loss along the length of the cable, while low coupling loss means more signal to the user, but less signal to reach the end of the tunnel. Longitudinal loss relates to how much signal is lost along the way. 

It is important to get the balance right to ensure all portables within the tunnel receive optimal signal, while transmitting the signal along the entire length of the tunnel for complete coverage. Lower longitudinal losses generally mean larger diameter cables with a consequent increase in material and installation costs.

Radiating cables should be installed with careful planning, particularly when specifying the spacing of the cable from walls, metal cable trays and other objects. Failure to adhere to manufacturer’s recommendations could lead to severe reductions in performance with higher longitudinal and unpredictable coupling loss variations. One area that can catch out the unwary is the installation of radiating cables above false ceilings. Many of these ceilings are comprised of fibreboard with a metallic backing or metal foil interior and this type of tile will have a disastrous effect on the radiation from the cable.

Source of signals

Implementing a communication system in tunnels requires a number of decisions to be made, one of which is whether to use a dedicated base station or cell enhancer as a means of determining the source of signals that are fed into the confined space network. Dedicated base station equipment can serve the underground network alone, or using a cell enhancer (also called an off air repeater) can take a signal from the surface and rebroadcast it underground. 

Implementing a cell enhancer, which is essentially a high gain amplifier designed to receive and retransmit signals from a local site, can lead to its own set of problems – high gain amplifiers connected effectively to antennas can exhibit a nice feedback loop. This technique is widely used to provide coverage for emergency services in small road tunnels. The off air repeater is the common way of providing coverage of FM Band 2 broadcast and DAB services – even AM on long and medium wave are used in some locations.

Special consideration should be taken when handling cabling in railway tunnels, which have high voltage overhead line equipment (OHLE) or powered third rail systems. The radiating cables will be running parallel to the OHLE over considerable distances. The electromagnetic field produced is capable of inducing high voltages into the radiating cables, which may only be less than three metres away. This can be a considerable hazard for maintenance technicians, which is why DC blocks at intervals and appropriate earthing systems must be installed. All fault conditions need to be evaluated – it’s not a good idea for the radiating cable to provide the return path for powering an electric train.

Finding the right partner

Providing a reliable and robust RF communication in a tunnel network is not a simple task, and requires a significant amount of planning and testing. While the technology is vitally important, it is also necessary to find a provider that understands the entire process, from planning and design to installation and maintenance. Mobile radio specialists can help put together a design to suit individual needs, carry out rigorous testing on-site up until the point it is operational, then provide ongoing support through the life of the system.