www.WirelessCommunication.NL Slide Show
  We know the spectrum. But how fast does the channel
change?
Wiener-Kinchine Theorem
  Power density spectrum of a random signal is the
Fourier transform of its autocorrelation 
Autocorrelation of components
  g(t
= 0) = local-mean power:  
In-phase component and its derivative are independent