Most mobile communication occurs when there is no direct path between the base station antennas and the mobile user. The signal reflects off many objects along the path between the two. This propagation follows a Rayleigh probability distribution about the mean signal level:

R is the signal level, α the value of the peak in the distribution, with mean =  and median  . The median signal level is found by fitting measured data for various localities (town, small town, open country, etc.) into a prediction model. The signal will have large signal fades where the level drops rapidly. The Rayleigh model can be solved for the average distance between fades given the level. As a designer it is important to realize the magnitude of the problem:

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SCALE 1  Average distance between fades and depth of fade for a Rayleigh multipath.

SCALE 2  Average fade length and depth of fade for a Rayleigh multipath.

R is the fade level (ratio) and RM is the median signal level found from a propagation model. Scale 1 shows the relationship between the average distance between fades and the depth of fade for Rayleigh multipath. A mobile channel operating at 1.85 GHz ( = 16.2 cm) has a 15-dB fade every 2.75  which equals 44.5 cm, while 10-dB fades occur every 1.62  = 26.25 cm. The communication system must overcome these fades.

Fortunately, the deep fades occur over a short distance:

The signal fades and then recovers quickly for a moving user. Scale 2 shows the average fade length along a path given the depth of fade. For the 1.85-GHz channel the 15-dB fade occurs only over 0.06(16.2) = 0.97 cm, and the 10-dB fade length is

0.109(16.2) = 1.76 cm.

The solution to mobile communication multipath fading is found either in increasing the link margin with higher gain base station antennas or the application of diversity techniques. We use multiple paths between the user and the base station so that while one path experiences a fade, the other one does not. Diversity has no effect on

the median signal level, but it reduces the effects of the nulls due to the Rayleigh distribution propagation.