Multipath propagation

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Atmospheric absorption
water vapor and oxygen contribute to attenuation
Multipath
obstacles reflect signals so that multiple copies with varying delays are received
Refraction
bending of radio waves as they propagate through the atmosphere (propagates faster at higher altitude)
Reflection
occurs when signal encounters a surface that is large relative to the wavelength of the signal
Diffraction
bending of waves at the edge of an impenetrable body that is large compared to wavelength of radio wave
Scattering
occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less
The Effects of Multipath Propagation
Multiple copies of a signal may arrive at different phases
If phases add destructively
the signal level relative to noise declines, making detection more difficult
Intersymbol interference (ISI)
One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit, causing interference
Fast fading
refers to changes in signal strength between a transmitter and receiver as the distance between the two changes by a small distance of about one-half a wavelength.
Slow fading
refers to changes in signal strength between a transmitter and receiver as the distance between the two changes by a larger distance, well in excess of a wavelength.
Flat fading
or nonselective fading, is that type of fading in which all frequency components of the received signal fluctuate in the same proportions simultaneously.
Selective fading
affects unequally the different spectral components of a radio signal
Rayleigh fading
occurs when there are multiple indirect paths between transmitter and receiver and no distinct dominant path, such as an LOS path. This represents a worst case scenario.
Rician fading
best characterizes a situation where there is a direct LOS path in addition to a number of indirect multipath signals.
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