Is fading only for multipaths?
Thank you in advance.
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Just adding to the first answer...
Regarding the scale, fading is often classified as:
Regarding the speed. Determining whether the fading is fast or slow is relative to the application that is to be transmitted over that channel. Better to check on D. Tse Book, I recommend chapter 2.
You don't get fading in free-space because, from a point-source antenna, EM waves travel in all directions equally unless limited by an antenna with directionality (high gain).
Objects cause fading when they disrupt the main path that a radio wave takes towards a receiver. One object can reduce the signal on the main path and another object can cause a reflection of energy that wouldn't normally have been received. Because the paths are different lengths they can become out of phase and cancel completely.
How different does the length have to be? One half of one wavelength is the critical difference in path length to cause a problem.