If the path lengths for all the signals are very similar compared to
the wavelengths of the signal components, then the phase variations between components
will be small and they will all undergo very similar amounts of reinforcement or
cancellation. This is often termed flat fading. On the other
hand, if the difference in path length, is large the fading characteristics will vary
greatly even with small frequency separations.
The degree of correlation between fading over a range of frequencies is expressed in terms
of the coherence bandwidth which is defined as the bandwidth
over which the fading statistics are correlated to better than 90%.
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