Information about Line Of Sight

Line-of-sight propagation refers to electro-magnetic radiation travelling in a straight line. The rays are therefore deviated or reflected by obstructions and cannot travel over the horizon. Beyond that, material disperses the rays.

Radio signals, like all electromagnetic radiation, travel in straight lines. However, at low frequencies (below approximately 2 MHz or so) ground effect transmission causes significant diffraction, allowing photons to partially follow the Earth's curvature, thus enabling AM radio signals in low-noise environments to be received well after the transmitting antenna has dropped below the horizon. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the F1/F2 Layer, thus giving radio transmissions in this range a potentially global reach (see shortwave radio).

However, at higher frequencies, neither of these effects apply, and so any obstruction between the transmitting and receiving antenna will block the signal, just like the light that the eye senses. Therefore, as the ability to visually sight a transmitting antenna (with regards to the limitations of the eye's resolution) roughly corresponds with the ability to receive a signal from it, the propagation characteristic of high-frequency radio is called "line-of-sight" as per radio wave propagation is called as "radio horizon".

In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM radio, at comparatively low freqencies of around 100 MHz using immensely-powerful transmitters, easily propagates through buildings and forests.

Line of sight propagation as a prerequisite for radio distance measurements

Travel time of radio waves between transmitters and receivers can be measured disregarding the type of propagation. But, generally, travel time only then represents the distance between transmitter and receiver, when line of sight propagation is the basis for the measurement. This applies as well to RADAR, to Real Time Locating and to LIDAR.

This rules: Travel time measurements for determining the distance between pairs of transmitters and receivers generally require line of sight propagation for proper results. Whereas the desire to have just any type of propagation to enable communication may suffice, this does never coincide with the requirement to have strictly line of sight at least temporarily as the means to obtain properly measured distances. However, the travel time measurement may be always biased by multi-path propagation including line of sight propagation as well as non line of sight propagation in any random share. A qualified system for measuring the distance between transmitters and receivers must take this phenomenon into account. Thus filtering signals traveling along various paths makes the approach either operationally sound or just tediously irritating.

Impairments to line-of-sight propagation

Low-powered microwave transmitters can be foiled by a few tree branches, or even heavy rain or snow.

If a direct visual fix cannot be taken, it is important to take into account the curvature of the Earth when calculating line-of-sight from maps.

The presence of objects not in the direct visual line of sight can interfere with radio transmission. This is caused by diffraction effects: for the best propagation, a volume known as the first Fresnel zone should be kept free of obstructions.

Reflected radiation from the ground plane also acts to cancel out the direct signal. This effect, combined with the free-space r^-2 propagation loss to a r^-4 propagation loss. This effect can be reduced by raising either or both antennas further from the ground: the reduction in loss achieved is known as height gain.

How do cellphones work, then?

The frequencies used by cellphones are in the line-of-sight range. So how do they work in cities? The answer is a combination of the following effects:

• r⁻⁴ propagation over the rooftop landscape
• diffraction into the "street canyon" below
• multi-path reflection along the street
• diffraction through windows, and attenuated passage through walls, into the building
• reflection, diffraction, and attenuated passage through internal walls, floors and ceilings within the building

The combination of all these effects makes the cellphone propagation environment highly complex, with multipath effects and extensive Rayleigh fading. These problems are tackled using:

• many base stations (a phone can typically see six at any given time)
• rapid handoff between base stations
• extensive error correction and detection in the radio link

See also

• Non-line-of-sight propagation

• Real Time Locating