MIMO and diversity techniques in tunnels

To improve the performances of a communication link, both in terms of capacity and bit error rate, MIMO technique is a solution which has been widely studied. However, in road or railway tunnels, the waves are guided by the structure and the angular spread of the rays, both at the transmitting side and at the receiving side, is rather small. Nevertheless, improvement of the capacity based on the Shannon theorem was already outlined and explained by introducing the concept of modal diversity instead of space diversity. However, to point out the interest of using MIMO, its performances must be compared to those of other well known diversity techniques, in terms not only of capacity but also of bit error rate. The objective of this contribution is, in a first part, to present the main channel characteristics of the propagation in tunnel which will be used, in a second part, to explain the relative performances of diversity techniques. The channel matrices introduced in the software simulating the link are those measured in a straight rectangular tunnel at 2.8 GHz.