Editing Line-of-sight propagation (section)

== Mobile telephones ==
Although the frequencies used by [[mobile phone]]s (cell phones) are in the line-of-sight range, they still function in cities. This is made possible by a combination of the following effects:
* {{frac|1|''r''<sup>&nbsp;4</sup>}} propagation over the rooftop landscape{{clarify|date=June 2016}}
* diffraction into the "street canyon" below
* [[multipath fading|multipath]] 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 mobile phone propagation environment highly complex, with [[multipath fading|multipath]] effects and extensive [[Rayleigh fading]]. For mobile phone services, these problems are tackled using:
* rooftop or hilltop positioning of base stations
* many [[base station]]s (usually called "cell sites"). A phone can typically see at least three, and usually as many as six at any given time.
* "sectorized" antennas at the base stations. Instead of one antenna with [[omnidirectional antenna|omnidirectional]] coverage, the station may use as few as 3 (rural areas with few customers) or as many as 32&nbsp;separate antennas, each covering a portion of the circular coverage. This allows the base station to use a directional antenna that is pointing at the user, which improves the [[signal-to-noise ratio]]. If the user moves (perhaps by walking or driving) from one antenna sector to another, the base station automatically selects the proper antenna.
* rapid [[handoff]] between base stations (roaming)
* the radio link used by the phones is a digital link with extensive [[Error detection and correction|error correction and detection]] in the digital protocol
* sufficient operation of mobile phone in tunnels when supported by split cable antennas
* local repeaters inside complex vehicles or buildings

A [[Faraday cage]] is composed of a conductor that completely surrounds an area on all sides, top, and bottom. Electromagnetic radiation is blocked where the wavelength is longer than any gaps. For example, mobile telephone signals are blocked in windowless metal enclosures that approximate a Faraday cage, such as elevator cabins, and parts of trains, cars, and ships. The same problem can affect signals in buildings with extensive steel reinforcement.

[[File:Repeater-schema.svg|thumb|right|Two stations not in line-of-sight may be able to communicate through an intermediate [[radio repeater]] station.]]