line of sight
Introduction
Line of sight propagation (or visual range propagation, its name in English, Line of Sight, or its acronym LOS is sometimes used) refers to a characteristic of the radiation of electromagnetic radiation (radio, laser or infrared) or the propagation of acoustic waves, which means that the waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions that travel in straight lines. Rays or waves can be diffracted, refracted, reflected or absorbed by the atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles.
Unlike line-of-sight propagation, at low frequencies (below about 3 MHz due to diffraction), radio waves can travel as surface waves, which follow the contour of the Earth. This allows AM radio stations to transmit beyond the horizon. Additionally, frequencies in the shortwave bands between about 1 and 30 MHz can be refracted back to Earth by the ionosphere, called ionospheric wave propagation or "ionospheric bounce", giving radio transmissions in this range a potentially global reach.
Any obstruction between the transmitting antenna (transmitter) and the receiving antenna (receiver "Radio (receiver)") will block the signal, as well as the light that the eye can sense. However, at frequencies above 30 MHz (VHF (up to 300 MHz) and UHF (up to 900 MHz) radio links) and at lower levels of the atmosphere have some tolerance to obstacles so they can be NLOS links. In microwave radio links, which operate at frequencies higher than 900 MHz, it is not possible to have a stable and well-behaved link if there are obstacles between the receiving antenna and the transmitting antenna in their Fresnel zone. Therefore, since the visual ability of a transmitting antenna (without taking into account the limitations of the resolution of the eye) corresponds approximately to the ability to receive a radio signal from it, the propagation characteristic at these frequencies is called "line of sight". The farthest possible propagation point is called the "radio horizon."
In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and strength of the transmitted signal (a function of both the transmitter and the characteristics of the antenna). FM radio transmissions, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.
In a digital communication such as Wifi, the links must transport a large amount of data (kbps/s), so they need a high bandwidth, which means a higher frequency of the radio carrier (2.4 GHz or 5.8Ghz). For this reason, in these technologies it is very important to consider the existence of LOS.