Radiating or Leaky Cable

The radiating cable, also called the leaky feeder, is a metallic wire that acts as a long antenna. The electromagnetic energy can be received or transmitted all along the cable, which is why it is well adapted to long narrow environments like corridors, elevators or tunnels. For example, in London, a radiating cable system is used in the underground for their internal communication network. In general, as represented in Figure 2.6, the radiating cable is directly connected to the base station.

2.5.1 Principle of Operation

A radiating cable is similar to a standard coaxial cable, however some tuned slots are posi-tioned on the surface of the outer conductor. A schematic representation of the transversal cut of such a cable is represented in Figure 2.7. The slots are tuned to the specific RF wavelength of operation or tuned to a specific radio frequency band. They will leak a part of the electromagnetic energy propagating in the cable in the form of electromagnetic waves. The antenna pattern is quasi-omni-directional in the transversal plan of the cable.

To reach a better efficiency, it is advised to leave a space between the cable and the walls. Moreover, metallic fixings and parts are not recommended because they affect the antenna pattern.

BS

Figure 2.6 Radiating cable radio coverage of a three-floor building

Polyethylene sheath Outer conductor Polyethylene skin Inner conductor Polyethylene dielectric Machined slots

Figure 2.7 Transversal cut of a leaky cable

If the cable is uniformly homogeneous, the ratio between the radiated energy and the energy propagating in the cable is constant all along the cable. The higher the losses in the cable, the more the ability to radiate energy is important. Some recent solutions have been proposed involving an adjustable coupling loss, allowing the user to adapt the radiated energy in complex environments [17].

This is why such a system has a limited range, especially in the high frequency range, where the losses are more important.

2.5.2 Deployment

As explained before, this technology is ideal for covering long narrow spaces. The main advantage of the radiating cable is that the energy is well distributed. For example, in a corridor, it provides homogeneous coverage all along the cable compared with the use of numerous base stations along the corridor, where the energy is distributed around the base stations. With a radiating cable, a single base station may be able to provide coverage over a large area, reducing the cost of system implementation.

A disadvantage is the difficult and expensive installation. The installation is time con-suming and it is not always easy to find the available space to install it. The cable must be aligned perfectly so that the slots can leak with minimum loss. Moreover the cable must not be installed directly against a wall but some space must be left, thanks to some special adaptors. Finally, especially inside tunnels (for example with trains making dust), the dirt degrades the performance of the cable. Therefore the cable must be regularly cleaned.

Due to the attenuation inside the cable, special configurations like cascaded BDAs and T-feed have been proposed in order to cover longer distances.

Cascaded BDAs

This solution to overcoming the problem of signal attenuation uses Bi-Directional Ampli-fiers (BDAs) when the maximal distance of cable to obtain a minimum quality is reached.

For long distances, BDAs are installed at certain intervals (see Figure 2.8), and their gains are configured so that the signal level is maintained at a certain level. In practice no more than three or four BDAs can be used because of the noise levels that are also amplified in the BDAs.

Radiating or Leaky Cable 29

BS BDA

Radiating Cable

BDA BDA

Figure 2.8 Radiating cable fed by cascaded BDAs

T-Feed

The T-feed uses an optical converter that converts the BS signal and distributes it via optical fibre, as represented in Figure 2.9. Each BDA has an optical interface to convert the optical signal and send it in both directions of the cable. This system can cover longer distances and has a better control of noise, because the different BDAs do not feed each other as in the cascaded BDAs configuration.

Comparison

As said before, the T-feed system is preferred because it reduces noise so can be used to cover longer distances. Moreover, because with T-feed the signal is sent in both directions of the BDAs, the resulting signal reaches higher levels and is more homogeneous. To illustrate this idea, in Figure 2.10 the attenuation along the cable with the two previous

BS

BDA BDA BDA

Radiating Cable Optical Fibre

Figure 2.9 T-fed radiating cable system

BDA1

Cascaded BDAs T-feed

BDA2 BDA3 Distance

Signal

Figure 2.10 Attenuation along a cable for the cascaded BDAs and the T-feed radiating cable systems

approaches is represented: for a similar distance between BDAs, the signal level is higher so the signal quality will be higher with the T-feed system.

2.5.3 Alternative to Radiating Cables

Radiating cable is an efficient solution for covering long distances, which is why it is widely used. However, some alternative solutions are possible, such as using a DAS along the environment. In [18], it is explained that radiating cable solutions outperform DAS because they contain coverage much better, but in practice, since the installation of this is not always straightforward for certain buildings, as well as installation costs and interference issues arising from the interaction with surrounding objects, DAS is still the preferred option in most installations.