Fading types by delay spread and Doppler spread

Fading is the rapid fluctuation of the amplitude, phase or multipath delay of a radio signal over a short time or short distance, caused by interference between two or more versions of the transmitted signal arriving at slightly different times (multipath).

Small-scale fading is classified independently by two mechanisms: multipath time delay spread and Doppler spread.

Based on multipath delay spread (time dispersion)

1. Flat fading

• Bs ≪ Bc and Ts ≫ στ (signal bandwidth smaller than coherence bandwidth).
• All frequency components fade together; spectral shape is kept, only gain changes.
• No ISI. Amplitude usually Rayleigh distributed. Example: narrowband voice channel.

2. Frequency selective fading

• Bs > Bc and Ts < στ.
• Different frequencies fade differently; the channel causes ISI and the received signal is distorted.
• Needs equalizers, RAKE or OFDM. A common rule: frequency selective if Ts < 10στ.

Based on Doppler spread (frequency dispersion)

3. Fast fading

• Tc < Ts and BD > Bs: the channel changes within one symbol.
• Causes frequency dispersion (time-selective fading) and signal distortion. Occurs at very low data rates or high speeds.

4. Slow fading

• Tc ≫ Ts and BD ≪ Bs: channel is nearly constant over many symbols.
• Channel changes slowly compared with the symbol rate.

The two classifications are independent, so a channel can be, for example, flat and slow fading at the same time.