 For a particular normalized threshold value formula_10, the product of the average fade duration and the level crossing rate is a constant and is given by

The Doppler power spectral density of a fading channel describes how much spectral broadening it causes.This shows how a pure frequency, e.g., a pure sinusoid, which is an impulse in the frequency domain, is spread out across frequency when it passes through the channel.It is the Fourier transform of the time-autocorrelation function.