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Fading : Slow & Fast and Large & Small Scale Fading


LARGE SCALE FADING

The term 'Large scale fading' is used to describe variations in received signal power over a long distance, usually just considering shadowing. Assume that a transmitter (say, a cell tower) and a receiver  (say, your smartphone) are in constant communication. Take into account the fact that you are in a moving vehicle. An obstacle, such as a tall building, comes between your cell tower and your vehicle's line of sight (LOS) path. Then you'll notice a decline in the power of your received signal on the spectrogram. Large-scale fading is the term for this type of phenomenon.


SMALL SCALE FADING

 Small scale fading is a term that describes rapid fluctuations in the received signal power on a small time scale. This includes multipath propagation effects as well as movement-induced Doppler frequency shifts. The statistics of small scale fading in industrial contexts can be described as Rician fading, and the Rician K-factor values for various factory conditions were estimated. We're talking abut the industrial environment and the Rician k-factor because there's a lot more signal reflection and refraction than in other contexts or environments. 


Slow fading

Because of the Doppler shift. When the signal bandwidth is much lesser than the Doppler spread. Slow fading occurs when the channel changes faster than the modulated symbol rate. What causes the channel to change? Doppler shift is responsible for that. Go through the formula of Doppler shift. 


Fast fading

Because of Doppler shift, particularly when the Doppler spread is equal to or larger than the signal bandwidth. The additive or subtractive nature of waveforms with varying phases can also produce fast fading. In simpler words, fast fading occurs when the channel changes faster than the modulated symbol rate.


Also read about

[1] Flat fading versus Frequency selective fading

[2] Doppler Shift

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