Gain of spatial diversity techniques in tunnel environment
Résumé
It is well known that diversity techniques improve the signal to noise ratio and are thus widely used both for SIMO or MIMO configurations. Their performances, described in the literature, are based on the assumption of a Rayleigh or Rice fading distribution of the electric field. However, in tunnels, the channel characteristics are quite different than in indoor or urban environment due to the guiding effect of the geometrical structure. Therefore, one can wonder if diversity or adaptive beamforming techniques remain an attractive way to improve the SNR, keeping in mind their possible complexity. Measurements have been carried out in a long road tunnel at 2.8 GHz to deduce the MIMO channel matrices. The channel characteristics, expressed in terms of correlation between array antenna elements, distribution of the singular values of the MIMO transfer matrices and eigen directions, are studied as a function of the distance between the transmitter and the receiver. Then diversity gains for different diversity techniques and eigen-beamforming are compared. A simple approach based on broadside antenna arrays has also been studied and it is shown that, in terms of SNR, it gives the best compromise between efficiency and complexity.