ayleigh (or Rician) fading is due to the fast variation of the signal level both in terms of amplitude and phase between the transmitting and receiving antennas when there is no line-of-sight. Rayleigh fading can be divided into two kinds: multipath fading and frequency-selective fading. Arrival of the same signal from different paths at different times and its combination at the receiver causes the signal to fade. This phenomenon is multipath fading and is a direct result of multipath propagation. Multipath fading can cause fast fluctuations in the signal level. This kind of fading is independent of the downlink or uplink if the bandwidths used are different from each other in both directions. Frequency-selective fading takes place owing to variation in atmospheric conditions. Atmospheric conditions may cause the signal of a particular frequency to fade. When the mobile station moves from one location to another, the phase relationship between the various components arriving at the mobile antenna changes, thus changing the resultant signal level. Doppler shift in frequency takes place owing to the movement of the mobile with respect to the receiving frequencies. Interference The signal at the receiving antenna can be weak by virtue of interference from other signals. These signals may be from the same network or may be due to man-made objects. However, the major cause of interference in a cellular network is the radio resources in the network. There are many radio channels in use in a network that use common shared bandwidth. The solution to the problem is accurate frequency planning, which is dealt with later in the chapter. The mobile station may experience a slow or rapid fluctuation in the signal level in a radio network. This may be due to one or more of the factors discussed above, and as shown in Figure 2.8. These factors form the basis of cell coverage criteria. Figure 2.8 Factors affecting wave propagation: (1) direct signal; (2) diffraction; (3) vehicle penetration; (4) interference; (5) building penet