Multipath propagation

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

Refraction

bending of radio waves as they propagate through the atmosphere (propagates faster at higher altitude)

Diffraction

bending of waves at the edge of an impenetrable body that is large compared to wavelength of radio wave

Rician fading

best characterizes a situation where there is a direct LOS path in addition to a number of indirect multipath signals.

If phases add destructively

the signal level relative to noise declines, making detection more difficult

Atmospheric absorption

water vapor and oxygen contribute to attenuation

Selective fading

affects unequally the different spectral components of a radio signal

Reflection

occurs when signal encounters a surface that is large relative to the wavelength of the signal

The Effects of Multipath Propagation

Multiple copies of a signal may arrive at different phases

Multipath

obstacles reflect signals so that multiple copies with varying delays are received

Scattering

occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

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.

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.

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.

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.