Cellular repeater

A cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of "bi-directional amplifier (BDA)" as commonly munications industry, is a device used to boost the cell phone reception to the local area by the usage of a reception antenna, a signal amplifier and an internal rebroadcast antenna. These are similar to the cellular broadcast towers used to broadcast by the network providers, but are much smaller, usually intended for use by one building. Modern cellular repeater amplifiers work by rebroadcasting 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. The more advanced models often also allow multiple cell phones to use the same repeater at the same time, so are suitable for commercial as well as home use.

The market for cellular repeaters is expected to grow rapidly in the US over the coming years. This is due to the combination of the poor network coverage in some areas, and the large scale departure from the land-line system. By 2007 the CTIA (Cellular Telecommunications & Internet Association) had predicted that 30% of phone users in the US will be mobile only - more than 60 million lines. This combined with the low population density (compared with Europe and Japan); the fact that only 41% of the US population is urban; the low cell site density; and the fact that the cell network is split into over ten major providers Fact|date=November 2007 means that many people will have to use some method to improve their home signal.

Other advantages of cellular repeaters include an increase in the cell phone's battery life and a lower level of radiation emitted by the handset - both caused by the lower power required to broadcast the signal to the local bi-directional amplifier, due to its proximity to the phone.

Typical components

External directional antenna

Although some of the less expensive models do not include an external directional antenna they are crucial to providing significant signal strength gain. 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. These can either be fitted by professionals or will include a signal strength monitor for easy alignment.

Internal rebroadcast antenna

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.

ignal amplifier

All modern models will include a signal amplifier. Even the cheaper home-use models now provide around 30dB gain and many of the more expensive models provide over 50dB. However, it should be noted that 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. This is due to the difficulty of filtering the correct signal out from the background noise, which will be amplified equally, and the limiting maximum signal power of the amplifier (typically from around 5 dBm or 3.2 mW). The power gain is calculated by the following equation:

::$quad\; P\_mathrm\{dB\}\; =\; 10\; log\_\{10\}\; left(frac\{P\}\{P\_0\}\; ight)$

Reasons for weak signal

Rural areas

In many rural areas the housing density is too low to make construction of a new base station commercially viable. In these cases it is unlikely that the service provider will do anything to improve reception, due to the high cost of erecting a new tower. As a result, the only way to obtain strong cell phone signal in these areas is usually to install a home cellular repeater. In flat rural areas the signal is unlikely to suffer from multipath interference, so will just be heavily attenuated by the distance. 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.

Building construction material

Some construction materials very rapidly attenuate cell phone signal strength. Older buildings, such as churches, which use lead in their roofing material will very effectively block any signal. Any building which has a significant thickness of concrete or amount of metal used in its production will attenuate the signal. Concrete floors are often poured onto a metal pan which completely blocks most radio signals. Some solid foam insulation and some fiberglass insulation used in roofs or exterior walls has foil backing, which can reduce transmittance. Energy efficient windows and metal window screens are also very effective at blocking radio signals. Some materials have peaks in their absorption spectra which massively decrease signal strength.

Building size

Large buildings, such as warehouses, hospitals and factories, often have no cellular reception further than a few meters from the outside wall. Low signal strength is also often the case in underground areas such as basements and in shops and restaurants located towards the centre of shopping malls. This is caused by both the fact that the signal is attenuated heavily as it enters the building and the interference as the signal is reflected by the objects inside the building. For this reason in these cases an external antenna is usually desirable.

Multipath interference

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.). These usually have an area of a few blocks and will usually only affect one of the two frequency ranges used by cell phones. This is because the different wavelengths of the different frequencies interfere destructively at different points. Directional antennas are very helpful at overcoming this since they can be placed at points of constructive interference and aligned so as not to receive the destructive signal. See Multipath interference for more.

Diffraction and general attenuation

The longer wavelengths have the advantage of being able to diffract to a greater degree so are less reliant on line of sight to obtain a good signal, but still attenuate significantly. 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 ionospohere.

See Diffraction and Attenuation for more.

Different operating frequencies

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. Repeater systems are available for certain Satellite phone systems, allowing the satphones to be used indoors without a clear line of sight to the satellite.

FCC approval

Although there are some products on the market which are still pending FCC approval, they should have no problem gaining it. Although a license was originally required to broadcast at cell phone frequencies, it is legal to use the low power devices available today for home and small scale use in commercial areas (offices, shops, bars etc.). Many models already have FCC approval.

ee also

* Base Station Subsystem
* Cellular network
* Cell phone
* Coverage noticer
* Dead zone (cell phone)
* Coiler
* Waves

External links

* [http://www.deadcellzones.com Dead Cell Zones] Web site listing poor coverage areas in USA including information on the causes of poor indoor coverage