Fading

In wireless communications, fading is the variation of signal attenuation over variables like time, geographical position, and radio frequency.

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 of multiple copies of the transmitted signal, each traversing a different path.

Each signal copy will experience differences in attenuation, delay and phase shift while traveling from the source to 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.

Channels with a large Doppler spread have signal components that are each changing independently in phase over time.

Since fading depends on whether signal components add constructively or destructively, such channels have a very short coherence time.

Typically the slowly-varying channels based on jakes model of Rayleigh spectrum [5] is used for block fading in an OFDM system.

Selective fading manifests as a slow, cyclic disturbance; the cancellation effect, or "null", is deepest at one particular frequency, which changes constantly, sweeping through the received audio.

Certain modulation schemes such as orthogonal frequency-division multiplexing (OFDM) and code-division multiple access (CDMA) are well-suited to employing frequency diversity to provide robustness to fading.

Upfade is a special case of fading, used to describe constructive interference, in situations where a radio signal gains strength.

In such cases, the probability of experiencing a fade (and associated bit errors as the signal-to-noise ratio drops) on the channel becomes the limiting factor in the link's performance.