Cellular repeater

A cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-directional amplifier (BDA) as commonly named in the wireless telecommunications industry, is a device used for boosting the cell phone reception to the local area by the usage of a reception antenna, a signal amplifier and an internal rebroadcast antenna.

Modern cellular repeater amplifiers rebroadcast cellular signals inside the building.

The systems usually use an external, directional antenna to collect the best cellular signal, which is then transmitted to an amplifier unit which amplifies the signal, and retransmits it locally, providing significantly improved signal strength.
One advantage of cellular repeaters is an increase in the cell phone's battery life due to the lower power required to broadcast the signal to the local bi-directional amplifier, due to its proximity to the phone.

This is because the antenna can be oriented and located outside to provide the best possible signal, usually aligned with the nearest cell tower.

Generally speaking the larger the external antenna the better the signal - although even a small, correctly oriented external antenna should provide better signal than the internal antenna on any cell phone.
The better systems will generally include an internal monopole antenna (although the type of antenna is far from standardised) for rebroadcasting the signal internally - the advantage of using a monopole antenna is that the signal will be equally distributed in all directions (subject, of course, to attenuation from obstacles).

Because all radio antennas are intrinsically polarized, cell phones perform best when their antennas are oriented parallel to the booster's antenna - although within reasonable proximity the booster's signal will be strong enough that the orientation of the cell phone's antenna will not make a significant difference in usability.
Excellent high-power models (not home usage - smart and expensive technology of the operators) offering gain around 100dBm (ICE function is welcomed as an improvement of the radio isolation between donor and service antenna).

However, since the decibel scale is measured on a logarithmic scale a 30dB gain represents a one thousandfold signal power increase - meaning the total amplification of a repeater with greater than around 50dB is likely to be useless without a good, well aligned antenna.

The power gain is calculated by the following equation: For repeater is needed to secure sufficient isolation between donor and service antenna.
The isolation is possible to improve by antenna type selection, in macro environment by angle between donor and service antenna (ideally 180°), space separation (typically vertical distance in case of the tower installation between donor and service antenna is several meters), inserting of the attenuating environment (smart placement of the donor and service antenna, e.g., between donor and service antenna is wall, placement of the metal mesh), reduction of reflections - in front of the donor antenna no near obstacle (like tree, metal-sheet building, glasshouse, or house)).

Because of the cellular network has form principle reduced cell size (depends on the technology and activated features typically X*10 km (for standard GSM 35 km), urban FDD/TDD network 20 km) usage of repeater virtually moving user to bigger distance: radio distance = real distance + (repeater delay in us) *0.3 km (delay of RF signal in air is 3.3us/km).
As a result, the only way to obtain strong cell phone signal in these areas is usually to install a home cellular repeater.

In these cases the installation of a cellular repeater will generally massively increase signal strength just due to the amplifier, even a great distance from the broadcast towers.

Some construction materials very rapidly attenuate cell phone signal strength.

Even in urban areas which usually have strong cellular signals throughout, there are often dead zones caused by destructive interference of waves which have taken different paths (caused by the signal bouncing off buildings etc.)

Because the frequencies which cell phones use are too high to reflect off the ionosphere as shortwave radio waves do, cell phone waves cannot travel via the ionosphere.
Repeaters are available for all the different GSM frequency bands, some repeaters will handle different types of network such as multi-mode GSM and UMTS repeaters however dual- and tri-band systems cost significantly more.

The use of a mobile signal booster in the UK is only permitted by the mobile networks who hold the licences in the cellular bands.

However, in cities and areas where many cells exist from each operator, the use of devices ranging from small mobile signal boosters to high power repeaters can cause loss of coverage through interference and desensitisation of the cells.