[citation needed] A "donor antenna" is typically installed by a window or on the roof a building and used to communicate back to a nearby cell tower.
An omnidirectional antenna (which broadcast in all directions) is typically used for a repeater system that amplify coverage for all cellular carriers.
Depending on attenuation from obstacles, the advantage of using an omnidirectional antenna is that the signal will be equally distributed in all directions.
[citation needed] Isolation can be also improved by integrated feature called ICE (interference cancellation equipment) offered in some products (e.g., NodeG, RFWindow).
In many rural areas the housing density is too low to make construction of a new base station commercially viable.
Any building that has a significant thickness of concrete, or a large amount of metal used in its construction, will attenuate the signal.
Even in urban areas (which usually have strong cellular signals throughout), there may be dead zones caused by destructive interference of waves.
Directional antennas can be helpful at overcoming this issue since they may be used to select a single path from several (see Multipath interference for more details).
It used to be legal to use the low power devices available for home and small scale use in commercial areas (offices, shops, bars, etc.).
In May 2011, Ofcom stated the following: Installation or use of repeater devices (as with any radio equipment) is a criminal offence unless two conditions are satisfied: Under WT Act 2006 section 1.15, the wireless act also allows an exemption if the device does not "involve undue interference with wireless telegraphy".