At high frequencies in the microwave range, wave propagation is impaired by a lack of line-of-sight; in extreme cases, no reception is possible. Examples of this are satellite communication, directional radio and mobileradio. Typical phenomena for a lack of line-of-sight in mobile communications are deviations (deflection) of the radio beam from the LOS link, radio shadows or radio holes.
Radio transmission over line-of-sight links is impaired by free- space attenuation. This increases quadratically with frequency and distance. In some applications, such as optical directional radio or satellite transmission, a line-of-sight link is mandatory. There are various standards in which line-of-sight as well as non-line-of-sight( NLOS) are included in the scope of definition.