Fast Fading: It varies quickly with the frequency. Fast fading originates due to effects of constructive and destructive interference patterns which is caused due to multipath.

Fast fading refers to changes in signal strength between a transmitter and receiver as the distance between the two changes by a small distance of about one-half a wavelength.

Doppler spread leads to frequency dispersion and time selective fading.

Fast Fading results due to following:

• High Doppler Spread.
• Coherence Time < Symbol Period.
• Channel impulse response changes rapidly within the symbol duration.
• Occurs if Ts > Tc, Bs < BD.
• It occurs for very low data rates.

Slow Fading: It does not vary quickly with the frequency. It originates due to effect of mobility. It is result of signal path change due to shadowing and obstructions such as tree or buildings etc.

Slow fading refers to changes in signal strength between a transmitter and receiver as the distance between the two changes by a larger distance, well in excess of a wavelength.

Slow Fading results due to following:

• Low Doppler Spread.
• Coherence Time >> Symbol Period.
• Impulse response changes much slower than the transmitted signal.
• It occurs if Ts << Tc, Bs >>BD.