Dual antenna array systems in tunnels: propagation channel properties

To increase the bit rate of mobile communications, it has been proposed to use an antenna array at both the transmitter site and the receiver. The main problem is that the propagation paths between the different transmitting and receiving elements must be uncorrelated to be able to recover the signal. Usually, in an indoor environment, multiple reflections on randomly distributed obstacles may give rise to independent and identically distributed Rayleigh channels. The transposition of this technique for applications in tunnels requires a detailed study of the propagation characteristics and of the properties of the channel matrix. Indeed, uncorrelated channels can only be obtained if, first, a large number of modes is supported by the tunnel and, secondly, if the position of each array element and their spacing, is such that the weighting of these modes is more or less uniformly distributed. To improve the performance of data transmission in subway tunnels, many experiments have been carried out in the Paris subway at a frequency of 900 MHz. Since numerous cables and pipes are hung on the walls and the configuration of the metro line is very complicated, all presented results are based on measurements of propagation characteristics. A parametric study was performed to point out the influence of the position and structure of the antenna array on the correlation between paths and, consequently, on the expected maximum bit rate.