Non-line-of-sight propagation

Non-line-of-sight (NLOS) signal propagation, also referred to as beyond-line-of-sight propagation, is an electromagnetic signaling that uses advanced modulation techniques to compensate for signal obstacles for indirect communications between transmitting stations. This communication technique has been considered of greater importance as wireless traffic has increased, and the number of connected devices has continued to grow. In order to satisfy the demand for connected devices and faster wireless communications, two technologies have been used: radio wireless communication, such as Wi-Fi, and optical wireless communication.

In the case of NLOS propagation, it is used when a radio transmitter and receiver are not in the direct visual line of sight, and can be dealt with by using multiple paths in signal propagation, and with the use of antennas and related communication devices. Distance between transmitter and receiver can also play a significant role in lowering the receiving power of a signal. And in most computer networking systems, NLOS is a large concern that is often reduced by using relays at various points to maintain signal transmission around obstructions and without the loss of data or transmission quality.

NLOS propagation is necessary because of the obstacles in line-of-sight communications. These impairments can include tall buildings, trees, the physical landscape, and high-voltage power conductors. And while some obstacles absorb the signal, and other reflect the signal, they all limit the ability or quality of the signal to reach the receiver.

Example of the Fresnel Zone's in LOS propagation.

The first possible impairment to line-of-sight communications is the Fresnel Zone. This is a football-shaped area between the two tapered link end points that is required to keep clear of obstructions to ensure a quality link. The largest area of concern is the first Fresnel Zone, which surrounds the transmitter and receiver; while obstructions within that zone will not necessarily cause a transmission to drop, they can cause degradation of signal strength and intermittent impairment. Behavior of signals will also differ depending on the antenna polarization, such that a vertically polarized signal encountering an object in the first Fresnel Zone will invert and arrive at the antenna out of phase, degrading the signal.

Another possible impairment to line-of-sight communications are reflections from the ground or water local to a transmitter or receiver. The reflections from what is a ground plane can cause multipath interference and degrade the signal. In short wave range microwave transmission, the multipath phenomenon is dealt with using diverse antennas and complex algorithms to combine or reject signals based on whether they are received in or out of phase. For longer range links, the common way to deal with reflections is to raise the height of the antenna.

Another possible impairment to line-of sight-communications is the curvature of the Earth and the effect of atmosphere on signal propagation. For the curvature of the Earth, a transmitter at sea level can have a signal that reaches seven miles if unobstructed. Atmospheric conditions will also have varying effects on the line-of-sight communications, especially as the signal does not travel at a uniform height above the earth.

The way radio plane waves are affected by obstructions and the effect of that obstruction is dependent upon the wavelength, with obstructions falling broadly into three categories. The first category is an obstruction that is smaller than the incident wavelength, which causes negligible, if any, interference. When an obstruction is around the same size as the incident wavelength, the plane wave will diffract around and through it with minor attenuation. And if the obstruction is larger than the incident wavelength, the signal can be obstructed to a varying degrees depending on the obstruction's materials and their electrical characteristics.

NLOS, or beyond-line-of-sight, is a special case of NLOS often used by the military to describe NLOS conditions and techniques used to overcome obstacles in long-distance communications. The most common method for medium to long-range links are passive or active repeaters, which receive a signal from an originating transmitter and repeat it to an increase range.

Passive repeaters do not amplify a signal, but rather they reflect it to a desired area. These are often used to beam signals into areas isolated by terrain or other obstructions and are most useful if the original signal strength is strong enough to sustain the loss of transmission.

Active repeaters, similar to passive repeaters, transmit a signal around obstacles. But they differ from passive repeaters in that they amplify the original signal in order to preserve the signal quality while increasing the range.

Another method of dealing with NLOS is troposphere scatter ionospheric propagation (troposcatter), which uses the Earth's atmosphere as a reflector to propagate radio frequencies over the horizon. Troposcatter can increase the range of transmissions up to 300 miles. Whereas ionospheric propagation, similar to troposcatter, can cover more than 2,000 miles. However, both of these methods are vulnerable to atmospheric conditions and magnetic storms.

Simplified example of ionospheric propagation.

Ionospheric propagation is a main mode of NLOS radio propagation used in medium frequency and high frequency portions of the radio spectrum. The concept behind ionospheric propagation is the use of ionosphere to bounce or reflect communications between a transmitter and receiver that cannot see each other. This is used in two-way radio communication, although often satellite communications is used in this case as well, and as a backup for point-to-point radio communications.

Passive random reflections, much as the name suggests, are when a reflection is achieved when plane waves are subject to one or more reflective paths. The reflective paths can be caused by various objects, and the better the conductor the better the signal propagation. This technique is used by common communication services such as Wi-Fi, WiMax, MIMO, and mobile communications.

Example of ground wave propagation.

Ground wave propagation is a form of signal propagation that uses the surface of the ground to pass along signal waves and provide regional coverage on the long and medium wave bands. This is generally used to provide relatively local radio communications coverage, and is especially used by radio broadcast stations to cover a particular locality. However, a ground wave signal is made up of a number of constraints, including receivers getting a direct and reflected signal. In ground wave propagation, the signal is often subject to passive random reflections, and the signal tends to follow the curvature of the Earth and can enable coverage beyond the horizon.

Non-standard or anomalous propagation (anaprop) is a phenomenon that occurs when the refractive index is modified by changes in temperature gradient, pressure, or water vapor content, which create non-standard propagation conditions. This is capable of creating radar overreaches, where the normal range of radar is greatly extended due to weather conditions. Or in ducting, where the ground temperature falls but the upper levels of the atmosphere remain relatively warm, which creates a reverse of the normal negative temperature gradient that results in a ducting of the transmitted signal along the ground, again extending the range of the radar. This effect is also capable of causing a sub-refraction, in which radar coverage may be lost at lower elevation angles due to atmospheric effect.

Used to adapt a signal, beam-forming uses an adaptive antenna array to improve communications and achieve non-line-of-sight communications. Often this method uses a ducting layer from anomalous propagation with the antenna array able to aim the signal into the ducting layer which acts as a leaky waveguide, but offers a chance to greatly increase wave ranges. As well, this has been used to create energy efficient, non-line-of-sight free-space optical communication that can steer both light or signal around obstacles and reach a receiver through shaped or reflected signals.