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 330 MHz high-frequency (HF) band used for ionospheric propagation, as well as the 300 MHz3 GHz ultra-high-frequency (UHF) and the 330 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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