Editing Rayleigh fading (section)

===Average fade duration===
The average fade duration quantifies how long the signal spends below the threshold <math>\,\!\rho</math>. For Rayleigh fading, the average fade duration is:<ref name="rappaport" />
:<math>\mathrm{AFD} = \frac{e^{\rho^2}-1}{\rho f_d \sqrt{2\pi}}.</math>

The level crossing rate and average fade duration taken together give a useful means of characterizing the severity of the fading over time.

For a particular normalized threshold value <math>\rho</math>, the product of the average fade duration and the level crossing rate is a constant and is given by

:<math>\mathrm{AFD} \times \mathrm{LCR} = 1 - e^{-\rho^2}. </math>