INTRODUCTIONRadiocommunications between mobiles are expected to experience very large expansion over the next few years, and it is predicted that urban traffic will shift more and more to underground routes due to the space shortage in big cities. This explains the interest in radiotransmission in shadow areas such as tunnels and underground parkings.Radiopropagation inside electromagnetic shadow areas is burdened with strong attenuation, multipath, and the waveguide and masking effects. These particular conditions require specific solutions generally based on leaky cables. Appropriate system design can solve most problems encountered from 100 kHz up to more than 1 GHz.This paper begins with a brief description of the already existing applications of tunnel retransmission equipments and analyses the specific requirements of GSM retransmission. This last point is important for the RTI applications based on cellular radio developed in several DRIVE Projects.This work has been carried out in the framework of the European DRIVE 1 CERACS and DRIVE 2 ICAR Projects. [7] Up to now, tunnel retransmission equipments have been used in many countries for the following applications : 2. WORKING PRINCIPLES [4], [5], [6] andOne-wav . .