Non-line-of-sight propagation

Non-line-of-sight (NLOS) or near-line-of-sight is radio transmission across a path that is partially obstructed, usually by a physical object in the innermost Fresnel zone.

Many types of radio transmissions depend, to varying degrees, on line of sight (LOS) between the transmitter and receiver. Obstacles that commonly cause NLOS conditions include buildings, trees, hills, mountains, and, in some cases, high voltage electric power lines. Some of these obstructions reflect certain radio frequencies, while some simply absorb or garble the signals; but, in either case, they limit the use of many types of radio transmissions, especially when low on power budget.