Electromagnetic propagation in tunnels

Abstract

Introduction of wireless communications systems in railway communications are at the origin of this thesis. Ifercat, the company in charge of the development of Línia 9 of Barcelona Metro, decided that wireless systems were employed in order to increase efficiency and safety. For this reason, characterisation of ISM 5.8GHz band in tunnel environments for broad band wireless train communications becomes necessary. Tunnel environments constitute harsh environments due to humidity, obstacles, power systems, moving trains, curves¿ The Automatic Train Control system requires a 20MHz bandwidth for train-to-ground video transmission in order to get on-board information and surveillance.

Given that Línia 9 was at the early stages of its development at the beginning of the study, a dual-polarised spectral simulator was developed first. Spectral techniques work in both spatial and spatial-frequency domain and are extremely adaptable to changes in the tunnel cross section as the wave front passes down the tunnel. Efficiency of this technique comes from the well-known properties of FFT algorithms. Spectral techniques provide good near-field predictions and can model different antenna configurations easily. On the other hand, boundary conditions present some issues that must be overcome. Long tunnels also represent a problem in terms of required memory space.

The parabolic equation has been used to enhance the performance of spectral techniques far from the source. They complement each other well because parabolic conditions require smooth variations in one direction, thus far from the source, where only field components propagating parallel to the tunnel axis remain, in order to provide accurate results. Application of Leontovich boundary conditions ensures proper solution at the change of media and its low computational cost permits acceleration of predictions.

These two techniques are then combined to verify the measurement campaigns developed at metro tunnels during the thesis. MIMO schemes are used to enhance the system throughput and simulation predictions are compared to measurements with good results. The work presented in this thesis consisted first on implementing both simulators and verifying their correct behaviour with theoretical analytical solutions. Secondly, predictions are compared with measurement campaigns carried out in Barcelona Metro environments. The study focuses on EM attenuation, field distribution, fading characterisation, antenna location and MIMO processing at the frequency band of interest.