Multipath
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In telecommunication, multipath is the propagation phenomenon that results in radio signals' reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from terrestrial objects, such as mountains and buildings.
The effects of multipath include constructive and destructive interference, and phase shifting of the signal. This causes Rayleigh fading, named after Lord Rayleigh. The standard statistical model of this gives a distribution known as the Rayleigh distribution.
Rayleigh fading with a strong line of sight content is said to have a Rician distribution, or to be Rician fading.
In facsimile and television transmission, multipath causes jitter and ghosting, seen as a faded duplicate image to the right of the main image.
Source: from Federal Standard 1037C and from MIL-STD-188
In digital radio communications (such as GSM) multipath can cause errors and affect the quality of communications. The errors are due to Intersymbol interference (ISI). Equalisers are often used to correct the ISI. Alternatively, techniques such as orthogonal frequency division modulation and Rake receivers may be used.