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The Characterization of Fading Channels

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Channel degradations are sometimes assumed to be relatively benign. That is, the channel might suffer only from additive white Gaussian noise (AWGN), and experience no degradation due to intersymbol interference (ISI). For many systems, such as mobile radios experiencing fading, such a benign starting point fails to illustrate the most challenging features of fading channels. This article attempts to characterize and organize the fundamental fading phenomena and the types of degradation that they bring about.
This article is excerpted from Digital Communications: Fundamentals and Applications, Second Edition (Prentice-Hall, 2001, ISBN 0-13-084788-7).
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When the mechanisms that cause fading in communication channels were first modeled in the 1950s and 1960s, the principles developed were primarily applied to over-the-horizon communications covering a wide range of frequency bands. The 3—30 MHz high-frequency (HF) band used for ionospheric propagation, as well as the 300 MHz—3 GHz ultra-high-frequency (UHF) and the 3—30 GHz super-high-frequency (SHF) bands used for tropospheric scatter, are examples of channels that are affected by fading phenomena. Although the fading effects in mobile radio channels are somewhat different from those encountered in ionospheric and tropospheric channels, the early models are still quite useful in helping to characterize the fading effects in mobile digital communication systems. This article emphasizes so-called Rayleigh fading, primarily in the UHF band, which affects mobile systems such as cellular and personal communication systems (PCS). The primary goal is to characterize the fading channel and in so doing to describe the fundamental fading manifestations and types of degradation.

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Editor's Note: This article is offered in PDF in order to provide consistency and clarity to the in-text equations.

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