## Frequency flat vs frequency selective fading

#### A challenging phenomenon of multipath propagation is that the degree of cancellation or reinforcement of signals changes for the different frequencies within a data signal. This is because the relative phase shift between two frequency components undergoing identical path delays will be different as the wavelength of the two components is altered. The effect is known as frequency selective fading and gives rise to notches in the frequency response of the channel. |

#### 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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