 The dis- tribution is a Rayleigh distribution in amplitude and an exponential distribution in power. Relative to mean power level, signal will be below 10 dB one-tenth of the time, below -20 dB 1 percent of the time, below -30 dB 0.1 percent of the time, and SO on. The autocorrelation distance is about half a wavelength, so it is a pretty safe bet that moving a wavelength away from a low point will give you a better signal. 3 Finally, the 3 Multipath fading has some cyclic behavior with high and low points about a wavelength apart, but we will not be too far astray if we think of this fading as an autoregressive process in three dimensions. And no, you do not need to know much about three-dimensional autoregressive processes to be a good radio engineer. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Signal Quality Key Radio Concepts fading depends on frequency as well as on location, with an autocorrelation frequency change related to the range of the multiple paths. Where there is a significant line-of-sight path from the transmitter to the receiver, the distribution shifts from a Rayleigh distribution to a Ricean distribution. The Ricean distribution is just a Rayleigh distribution with a constant term added to it. A Ricean- distributed signal can get close to zero, just as a Rayleigh-distributed signal can, but it is less likely. Ricean multipath fading is less likely to be severe, and the larger the line- of-sight component, the more favorable the Ricean statistics get. There are several techniques for dealing with fading built into cellular systems. There are two techniques that rely on diversity, where two receivers are placed far enough apart that their fading is uncorrelated. When the two receivers are only a few wavelengths apart on the same tower, we call it microscopic diversity. There are two basic forms of microscopic di