Fast fading vs slow fading-difference between fast,slow fading

This page on fast fading vs slow fading mentions difference between fast fading and slow fading.

Fading refers to variation in signal strength with respect to time as it is received at the antenna from the transmitter at distant end. The variation can be result of communication channel between the transmitter and receiver.

The figure depicts both fast fading and slow fading amplitude versus distance curve. As shown signals vary with time and location. It combines effect of both direct and indirect paths. Refer complete article on fading basics and fading types >>.

fast fading vs slow fading

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.

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 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

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RF and Wireless Terminologies