**1. Introduction**

A great deal of work is being done to optimize the performances of digital wireless communication systems, and most of the effort is focused on the urban environment where technologies evolve surprisingly fast. Deployment of new emerging technologies, all based on digital communications, first requires the knowledge of the physical layer in order to develop efficient antenna design and communication algorithms. Tunnels, including rail, road and pedestrian tunnels, even if they do not represent a wide coverage zone, must be taken into account in the network architecture, the tunnel either being considered part of a neighboring cell or as a cell itself. In order to cover the tunnel, two solutions have traditionally been proposed: the so-called "natural propagation" using antennas of small size, and leaky coaxial cables. However, the implementation of radiating cables is expensive, at least in long tunnels, because the diameter of the cable must be large enough to avoid a prohibitive attenuation in the 1-5 GHz band, which will be considered in this chapter. We will thus focus our attention on a link based on natural propagation.

Whatever the application, preliminary knowledge of the propagation phenomena is required. The first section of this chapter is thus devoted to a presentation of theoretical models, while in the second section the main narrow band and wideband double-directional channel characteristics, determined from numerous measurement campaigns, will be presented and interpreted. Since Multiple-Input Multiple-Output (MIMO) techniques may strongly improve the spectral efficiency and/ or decrease the error rate, keeping the transmitting power and the bandwidth constant; the last two sections will describe their performances in tunnels. Indeed, one can expect the degree of diversity of the channel to be, by far, quite different from its average value in an indoor environment due to the guiding structure of the tunnel. Applying the propagation model to MIMO allows the outlining of the main parameters playing an important role on the ergodic channel capacity, and introducing the so-called modal diversity. From measured channel matrices, predicted capacity is given for various tunnel and array configurations. The last paragraph of this chapter treats the robustness, in terms of error rate, of different MIMO schemes.
