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What is line of sight (LOS) and why is it important?


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What is line of sight?

Line of sight (LOS) is the imaginary line between an observer and a target. In communication, the line of sight is the direct path from a transmitter to a receiver and the obstacles in that path. A clear line of sight is important for high speed communication.

Line of sight is the straight path between two points. This is called “line of sight” because if a person stands at one point, the LOS will be the path they see. If an object is blocking the view, it will be considered out of sight or obstructed line of sight.

The concept of lines of sight has been important for much of history. Imagine a guard on top of a watchtower. Their high-positioning will give them an unobstructed view, or clear lines of sight, over a large area. If he lights a fire at the signal, everyone in the area will have a clear vision to see the fire and sound the alarm.

The curvature of the Earth has been the limiting factor in the lines of sight. As the two objects move further away, Earth obstructs the LOS between them. Eventually it will disappear over the horizon and break the LOS.

line of sight in wireless communication

Line of sight is an important factor in wireless communication. Some forms of wireless transmission are completely blocked when anything comes between the transmitter and the receiver. Other forms of transmission can penetrate less dense objects such as walls and buildings, but are blocked by larger objects such as mountains.

Most wireless transmission uses radio waves, which travel in a straight line from the transmitter. Holding the transmitter high will give it a clear line of sight. Like the visible lines of sight, the curvature of the Earth will eventually block a radio wave. It sets the limit on how far any radio tower can transmit on its own.

Signal reflection or refraction can be used to extend the usable range of a radio signal beyond line of sight. Reflecting radio waves from Earth’s ionosphere at night was an early way to extend radio transmissions beyond the LOS.

Communication satellites revolutionized wireless transmission. Their position above the Earth gives them a clear view of terrestrial communication towers. One tower transmits to a satellite, and the satellite sends a signal to the second tower beyond the LOS of the first tower. The higher a satellite’s orbit, the more Earth it can cover. A single satellite in geostationary orbit has a LOS of more than half the Earth’s surface. Low Earth orbit (LEO) satellites can only cover a relatively small area.

Satellites often work together in constellations to cover the required area. There are tradeoffs between the higher cost of high orbits in satellites, greater latency for higher orbits, and the number of satellites needed to cover an area. For example, Starlink is known for its high speed and low latency but requires thousands of satellites, while Huguenet uses only one satellite but with greater latency.

How satellite orbits compare
Diagram showing how satellite orbits compare

The ability of a radio signal to tolerate obstructed line of sight is determined by the signal’s wavelength. The longer the wavelength, the greater its ability to penetrate through obstacles. Conversely, the shorter the wavelength, the less barrier it can pass through, but the more data it can transmit.

Line of sight must be considered when placing a Wi-Fi access point. Placing them high will limit obstructions from the furniture. The thickness and structure of the walls is a major factor. The slower 2.4 GHz band will pass through walls better than Wi-Fi AC and Wi-Fi 6’s faster 5 GHz band.

Sight lines are also very important for cellular base stations. They are often placed on the highest point in a city, such as over water towers. 5G-NR data will have different efficiencies depending on the wavelength used. Wideband signals cover longer distances and penetrate buildings well but will not provide the fastest speeds. Narrowband offers more speed but doesn’t work as well inside buildings. The fastest forms of 5G rely heavily on clear lines of sight. Millimeter waves can also be blocked by holding a phone in hand.

Cellular vs Millimeter wave.
The diagram shows the difference between cellular and millimeter wave.

Clear vision is required for most point-to-point wireless backhaul. Microwave data transmission can be blocked or degraded by obstructing objects. Fog or rain can also impair performance.

See all: transceiver, troposphere diffusion, mean radius of earth, propagation delay, RFID



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