Cellular repeater
From Wikipedia, the free encyclopedia
A cellular repeater or wireless cellular signal booster, called a bi-directional amplifier (BDA) in the wireless telecommunications 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) predict 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 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.
Contents |
[edit] Typical components
[edit] 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.
[edit] 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 not depend on what angle the cell phone is at with regards to the aerial's alignment.
[edit] Signal 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 thousand fold 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:
[edit] Reasons for weak signal
[edit] 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.
[edit] 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 somewhat. Some solid foam insulation used for roofs or exterior wall has foil backing, which can reduce transmittance. Some materials have peaks in their absorption spectra which massively decrease signal strength.
[edit] Building size
Large buildings, such as warehouses, hospitals and factories, often have no cellular reception further than a few metres 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.
[edit] 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 due to the fact the different wavelengths of the different frequencies interfere destructively at different points. Directional antennae 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.
[edit] 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.
[edit] Different operating frequencies
There are two frequency bands used by mobile telephones in the U.S. They are around 850 MHz and 1900 MHz. Many cellular repeaters only work with one of the two radio bands, although dual band models are available. Network operators normally only use one of the two frequency bands available, apart from Cingular, who uses the old 850 MHz AT&T network as well as their original 1900 MHz one.
[edit] 850 MHz
Used by Verizon Wireless, Pace Wireless, Cellular One and Alltel. Frequencies range 824 MHz to 894 MHz and are classified as Ultra High Frequency or UHF electromagnetic radiation. Cingular (since they purchased AT&T) uses this frequency as well as the 1900 MHz - this is why cell phones by this provider often find two networks.
[edit] 1900 MHz
Used by T-Mobile, Sprint PCS, Cingular, Virgin Mobile, Dobson Communications and some Verizon markets. Frequencies range from 1850 MHz to 1990 MHz and are sometimes classified as belonging to the low energy microwave region of the electromagnetic spectrum, although they are also known as ultra-high frequency waves, due to the overlap of the ranges.
[edit] 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.
