Abstract. This paper describes the main electromagnetic characteristics of a radiating cable installed in a gallery. The emphasis is on the effect of the cable environment on the coherence bandwidth and on the delay spread of the radio channel. The direction of arrival of the waves inside the tunnel has also been deduced from the complex impulse response in order to optimize a method of localization of a mobile moving in the gallery.
IntroductionRadiating cables are widely used to ensure radio communications inside confined areas and, more precisely, in road or railway tunnels and sometimes inside buildings. A typical frequency band extends from 150 MHz to about 1 GHz since, beyond this frequency, the attenuation per unit length becomes prohibitive except for large cable diameters but which lead to large increases in weight and cost. An important feature of the radiating cable is to impose the mean value of the field attenuation per unit length inside the tunnel, whatever its shape and cross section. Furthermore, these cables usually only support voice or low-rate data transmission, and they are thus characterized from a narrowband analysis. However, there is now increasing demand not only to communicate with a mobile transmitter by increasing the data rate between onboard sensors and a control station but also to determine its position with rather good accuracy, typically of the order of few meters. To give only two examples, let us first mention the case of a worker walking in a sewerage system, the knowledge of his position being an important safety factor. Another application, which has been the starting point of this study, is to improve both safety conditions and productivity in underground mine operations by continuously monitoring the location of personnel and mobile equipment. One of the most critical points is to cover the main access galleries in which both pedestrians and heavy trucks move and also the other service galleries of smaller cross section. Indeed, the performance of a radiating cable is determined from standardized test procedures by putting the cable above the ground surface, assumed to be plane, and by measuring the field amplitude at various distances and discrete frequencies. However, the knowledge of the impulse response of the cable in its environment is a crucial point for the aimed application; this is the reason why extensive measurements have been performed in mine galleries. The most important results are presented and interpreted in this paper. First, we recall the main features of a radiating cable, and in a narrow frequency band, its performance is compared with that of discrete antennas radiating in the gallery. Then a wideband analysis of the radio channel, based on the use of a radiating cable, is presented in terms of coherence bandwidth and impulse responses. It is, of course, the usual approach for studying the natural propagation either in urban areas or in tunnels [Zhang and Hwang, 1998]. After determining the direction of arrival of the various rays incident on the ...
