Propagation channel measurements and analysis at 2.4 GHz in subway tunnels

The reliability of communication systems in a subway tunnel requires an absolute understanding of the propagation channel in this environment. From an insightful look at the subway channel and especially when compared with the other underground propagation environments such as mines, roads and railway tunnels, an important feature of the subway tunnel is the existence of stations in the tunnel. These stations cause extra reflection in the channel and also increase the delay spread observed. Although this effect has been largely neglected in available propagation channel literature, this is an important concept that ought to be explored. In this work, the authors present channel measurements conducted close to a station in a subway tunnel at 2.4 GHz, using a self-developed frequency domain channel sounder. To aid the authors analysis, three typical propagation regions are defined: line-of-sight (LOS), non-LOS and far-LOS, and also use a larger observation delay window to investigate the long delay clusters caused by the existence of stations in subway tunnel. From the analysis of the power delay profile (PDP), root-mean-square delay spread and scattering function in the three propagation regions, the authors have been able to infer a long delay cluster of about 2 μs, which the authors attributed to multipath propagation of signal along the tunnel and reflections in the station. The Saleh-Valenzuela model is used to characterise the long clustering phenomenon observed in the PDP. The results show that the time spreading in subway tunnel is more significant because of the station compared with other underground tunnels, which leads to substantial signal distortion.