Editing Fading (section)

==Key concepts==

The presence of reflectors in the environment surrounding a transmitter and receiver create multiple paths that a transmitted signal can traverse.  As a result, the receiver sees the [[Superposition principle|superposition]] of multiple copies of the transmitted signal, each traversing a different path. Each signal copy will experience differences in [[attenuation]], [[Propagation delay|delay]] and [[Phase (waves)|phase shift]] while traveling from the source to the receiver.  This can result in either [[Wave interference|constructive or destructive interference]], which amplifies or attenuates the signal power seen at the receiver.  Strong destructive interference is frequently referred to as a '''deep fade''' and may result in temporary failure of communication due to a severe drop in the channel [[signal-to-noise ratio]].

A common example of deep fade is the experience of stopping at a traffic light and hearing an FM broadcast degenerate into static, while the signal is re-acquired if the vehicle moves only a fraction of a meter.  The loss of the broadcast is caused by the vehicle stopping at a point where the signal experienced severe destructive interference.  Cellular phones can also exhibit similar momentary fades.

Fading channel models are often used to model the effects of electromagnetic transmission of information over the air in cellular networks and broadcast communication.  Fading channel models are also used in underwater acoustic communications to model the distortion caused by the water.