This can result in either constructive or destructive interference, amplifying or attenuating the signal power seen at the receiver. Each signal copy will experience differences in attenuation, delay and phase shift while traveling from the source to the receiver. As a result, the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path. The presence of reflectors in the environment surrounding a transmitter and receiver create multiple paths that a transmitted signal can traverse. In wireless systems, fading may either be due to multipath propagation, referred to as multipath-induced fading, weather (particularly rain), or shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading. A fading channel is a communication channel that experiences fading. Fading is often modeled as a random process. These variables include time, geographical position, and radio frequency. In wireless communications, fading is variation of the attenuation of a signal with the various variables. Time is shown on the horizontal axis, frequency on the vertical axis and signal strength as grey-scale intensity. Term in wireless communications Frequency-selective time-varying fading causes a cloudy pattern to appear on a spectrogram.
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