In a radio link, a part of the energy radiated by the transmitting antenna reaches the receiving station through different paths. Along these paths, interaction may occur between the electromagnetic field and various objects. Possible interactions are specular reflection on long plane surfaces, diffuse scattering from surfaces exhibiting small irregularities or from objects of small size, transmission through dense material, shadowing by obstacles, etc. At the position of the receiving antenna, the resulting electric field is spread both in time delay and direction. In radio systems using digital modulation schemes the properties of the time-varying, frequency-dispersive radio channel are important to know. In such radio environments not only the narrow-band signal is relevant but also the time delays and angles-of-arrival of the individual multipath components are important. Multipath signals are not seen as interferers only; in some systems their energy is used to improve the signal level.