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path loss
Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system.
This term is commonly used in wireless communications and signal propagation. Path loss may be due to many effects, such as free-space loss, refraction, diffraction, reflection, aperture-medium coupling loss, and absorption. Path loss is also influenced by terrain contours, environment (urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas.
==Causes==
Path loss normally includes ''propagation losses'' caused by the natural expansion of the radio wave front in free space (which usually takes the shape of an ever-increasing sphere), ''absorption losses'' (sometimes called penetration losses), when the signal passes through media not transparent to electromagnetic waves, ''diffraction losses'' when part of the radiowave front is obstructed by an opaque obstacle, and losses caused by other phenomena.
The signal radiated by a transmitter may also travel along many and different paths to a receiver simultaneously; this effect is called multipath. Multipath waves combine at the receiver antenna, resulting in a received signal that may vary widely, depending on the distribution of the intensity and relative propagation time of the waves and bandwidth of the transmitted signal. The total power of interfering waves in a Rayleigh fading scenario vary quickly as a function of space (which is known as ''small scale fading''). Small-scale fading refers to the rapid changes in radio signal amplitude in a short period of time or travel distance.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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