The phenomena which influence radio wave propagation can generally be described by four basic mechanisms: Reflection, diffraction, penetration and scattering. For the practical usage of propagation models in real scenarios these mechanisms must be described by approximations.

Predicting the propagation characteristics between two antennas inside a building is important especially for the design of wireless local area networks (WLAN). Also the installation of cellular systems with indoor base stations involves the usage of indoor propagation models.

The indoor propagation channel differs considerably from the outdoor one. The distance between transmitter and receiver is shorter due to high attenuation caused by internal walls and furniture and often also because of the lower transmitter power. The short distance implies shorter delay of echoes and consequently lower delay spread. The temporal variations of the channel are slower compared to mobile antennas moving with a car. As it is the case in outdoor systems, there are several important propagation parameters to be predicted. The path loss and the statistical characteristics of the received signal envelope are most important for coverage planning applications. The wide-band and time variation characteristics are essential for evaluation of the system performance.