Fading Effects due to Doppler spread:

Fast Fading:

In a fast fading channel, the channel impulse response changes rapidly within the symbol duration of the signal. Due to Doppler spreading, signal undergoes frequency dispersion leading to distortion. Therefore a signal undergoes fast fading if

TS>> TC

Where TC is the coherence time and

BS >> BD

Where BD is the Doppler spread. Transmission involving very low data rates suffers from fast fading.

Slow Fading:

In such a channel, the rate of the change of the channel impulse response is much less than the transmitted signal. We can consider a slow faded channel a channel in which channel is almost constant over at least one symbol duration. Hence

TS< And

BS>>BD

We observe that the velocity of the user plays an important role in deciding whether the signal experiences fast or slow fading.