Attenuation (redirected from fading)

Loss of signal power in a transmission.

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Attenuation

The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities. Various mechanisms can give rise to attenuation. Among the most important are geometrical attenuation, absorption, and scattering.

For unconfined radiation from a point source in free space, the power density (watts per square meter) decreases in proportion to the square of the distance. The power densities, I₁ and I₂, at distances r₁ and r₂ from the source, are related by Eq. (1).

(1) 

If the signal, in a parallel beam so that there is no geometrical attenuation, passes through a lossy medium, absorption reduces the power level, I, exponentially with distance, x, according to Eq. (2), where a

(2) 

is the attenuation coefficient. See Absorption of electromagnetic radiation, Sound absorption

Scattering is said to occur if the power is not absorbed in the medium but scattered from inhomogeneities. See Scattering of electromagnetic radiation

More complicated situations occur with guided waves, such as acoustic waves in pipes or electromagnetic waves in transmission lines or waveguides, where absorption may take place and irregularities may cause reflection of some power.

In electric circuits, constituent elements are often described as attenuators when they reduce the level of signals passing through them.

Attenuation is usually measured in terms of the logarithm of the power ratio, the units being the neper or the decibel. See Decibel

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(communications)

attenuation - The progressive reduction in amplitude of a signal as it travels farther from the point of origin. For example, an electric signal's amplitude reduces with distance due to electrical impedance. Attenuation is usually measured in decibels Attenuation does not imply appreciable modification of the shape of the waveform (distortion), though as the signal amplitude falls the signal-to-noise ratio will also fall unless the channel itself is noise free or the signal is amplified at some intermediate point(s) along the channel. ["Networking Essentials, second edition", Microsoft Corporation, pub. Microsoft Press 1997].