Fast/Slow Fading: How are doppler shift and coherence time related?
Here's an intuitive explanation.
Imagine a communication system (wireless) in a static environment (maybe picture the inside of a closed warehouse). There will be an amount of fading, but it is static: it never changes, because the channel doesn't change.
For the channel to change, the multipath scenario must change, and that means that objects must move around (it can be the reflectors/scatterers, the antennas, or both). If they move relatively slowly, you have &quot;slow&quot; fading; if they move relatively quickly, the fading becomes &quot;fast&quot;.
Now, whenever you mix waves and moving objects, you get Doppler shifts. Furthermore, the faster the movement, the higher the Doppler. In consequence, you can measure how fast the channel is changing by either measuring how fast the objects are moving, or by how large the Doppler is.
Since the Doppler shift is easy to quantify, it is the preferred way to measure how fast a fading channel is changing.