le antenna is at a much lower height than the base station antenna, and there may be high buildings or hills in the area. Thus, the signal undergoes diffraction in reaching the mobile antenna. This phenomenon is also known as 'shadowing' because the mobile receiver is in the shadow of these structures. Building and Vehicle Penetration When the signal strikes the surface of a building, it may be diffracted or absorbed. If it is to some extent absorbed the signal strength is reduced. The amount of absorption is dependent on the type of building and its environment: the amount of solid structure and glass on the outside surface, the propagation characteristics near the building, orientation of the building with respect to the antenna orientation, etc. This is an important consideration in the coverage planning of a radio network. Vehicle penetration loss is similar, except that the object in this case is a vehicle rather than a building. Propagation of a Signal Over Water Propagation over water is a big concern for radio planners. The reason is that the radio signal might create interference with the frequencies of other cells. Moreover, as the water surface is a very good reflector of radio waves, there is a possibility of the signal causing interference to the antenna radiation patterns of other cells. Propagation of a Signal Over Vegetation (Foliage Loss) Foliage loss is caused by propagation of the radio signal over vegetation, principally forests. The variation in signal strength depends upon many factors, such as the type of trees, trunks, leaves, branches, their densities, and their heights relative to the antenna heights. Foliage loss depends on the signal frequency and varies according to the season. This loss can be as high at 20 dB in GSM 800 systems. RADIO NETWORK PLANNING PROCESS Fading of the Signal As the signal travels from the transmitting antenna to the receiving antenna, it loses strength. This may be due to the phenomenon of path loss as explained above, or it may be due to the Rayleigh effect. R