line of sight (GSM, UMTS) (LOS)
Line of sight (LOS) between transmitter and receiver plays an essential role wherever wave propagation can only be ensured if there is an unimpaired link between transmitter and receiver.
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.