Non-line-of-sight propagation

Non-line-of-sight (NLOS) and near-line-of-sight are radio transmissions across a path that is partially obstructed, usually by a physical object in the innermost Fresnel zone. Non-line-of-sight (NLOS) and near-line-of-sight are radio transmissions 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. Lower power levels at a receiver reduce the chance of successfully receiving a transmission. Low levels can be caused by at least three basic reasons: low transmit level, for example Wi-Fi power levels; far-away transmitter, such as 3G more than 5 miles (8.0 km) away or TV more than 31 miles (50 km) away; and obstruction between the transmitter and the receiver, leaving no clear path.