Cellular repeater
Encyclopedia
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 boost
ing 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 for broadcasting by the network providers, but are much smaller, usually intended for use in one building. 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
. 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 over the coming years, particularly in the USA. This is due to the combination of the poor network coverage
in some areas, and the large scale departure from the land-line system. The CTIA – The Wireless Association (formerly the Cellular Telecommunications & Internet Association) had predicted that by 2007 30% of phone users in the US would be mobile only - more than 60 million lines. This combined with the low population density (compared with Europe and Japan) means that many people will have to use some method to improve their home signal.
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.
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.
(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.
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 (for picorepeaters typically from around 5 dBm
(3.2 mW)). Standard GSM channel selective repeater (operated by telecommunication operators for coverage of large areas and big buildings) has output power around 2W, high power repeaters (e.g., NodeG from Andrew) offering output power around 10W). The power gain is calculated by the following equation:
Boost
-Science, technology and mathematics:* Automotive:** Boost, positive manifold pressure in cars, see Turbocharger#Pressure increase.*** a loose term for turbo or supercharger** A slang term meaning to start a vehicle, see jump start...
ing the cell phone reception to the local area by the usage of a reception antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
, a signal amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
and an internal rebroadcast antenna. These are similar to the cellular broadcast towers used for broadcasting by the network providers, but are much smaller, usually intended for use in one building. Modern cellular repeater amplifiers rebroadcast cellular signals inside the building. The systems usually use an external, directional antenna
Directional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
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
Signal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
. 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 over the coming years, particularly in the USA. This is due to the combination of the poor network coverage
Coverage (telecommunication)
In telecommunications, the coverage of a radio station is the geographic area where the station can communicate. Broadcasters and telecommunications companies frequently produce coverage maps to indicate to users the station's intended service area. Coverage depends on several factors, such as...
in some areas, and the large scale departure from the land-line system. The CTIA – The Wireless Association (formerly the Cellular Telecommunications & Internet Association) had predicted that by 2007 30% of phone users in the US would be mobile only - more than 60 million lines. This combined with the low population density (compared with Europe and Japan) means that many people will have to use some method to improve their home signal.
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.
External directional antenna
Although some of the less expensive models do not include an external directional antennaDirectional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
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 antennaMonopole antenna
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...
(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.
Signal amplifier
All models will include a signal amplifier. Even the cheaper home-use models (typically band selective) now provide 20dB - 50dB gain and many of the more expensive models provide over 50dB. 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 decibelDecibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
scale is measured on a logarithmic scale
Logarithmic scale
A logarithmic scale is a scale of measurement using the logarithm of a physical quantity instead of the quantity itself.A simple example is a chart whose vertical axis increments are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4...
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 (for picorepeaters typically from around 5 dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
(3.2 mW)). Standard GSM channel selective repeater (operated by telecommunication operators for coverage of large areas and big buildings) has output power around 2W, high power repeaters (e.g., NodeG from Andrew) offering output power around 10W). The power gain is calculated by the following equation:
-
For repeater is needed to secure sufficient isolation between donor and service antenna. When the isolation is lower than actual gain + reserve (typically 5-15dB) then repeaters is in loop oscillation.
Also cheap models are equipped by automatic gain reduction in case of poor or weak isolation. In case of poor isolation the device works but with low gain, and coverage is poor.
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)).
Isolation can be also improved by integrated feature called ICE (interference cancellation equipment) offered in some products (e.g., NodeG, RFWindow). Activating of this feature has negative impact to internal delay (higher delay => prox. +5us up to standard rep. delay) and consequently to shorter radius from donor site, where could be repeater used.
By amplification and filtration there is some delay (typically between 5us to 15us). It depends on the type of repeater and used features. Additional delay form point of view of propagation means additional distance. 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).
It is reason why somewhere with sufficient levels repeater doesn't work. After repeating you have better (or excellent) coverage but you can't access to network. User is from network point of view too far.
There is also problem with noise amplification (especially in UL) and desensitization of the donor site.
Amateur installation of the pico/mini repeaters can be harmful for many reasons:- Poor choice of donor site may not improve signal
- Using the wrong antenna and improperly installing the repeater without paying attention to minimizing interference (e.g. installing the repeater in a higher place than necessary, being shaded by buildings (e.g., by wall installation or suppression of side/back lobes by chimney, etc.)) and without sense to donor site selection.
- Use of a poor device for signal generation, causing noiseNoise (radio)In radio reception, noise is the superposition of white noise and other disturbing influences on the signal, caused either by thermal noise and other electronic noise from receiver input circuits or by interference from radiated electromagnetic noise picked up by the receiver's antenna...
and inter-modulation products. - Repeating only part of the band, such as in cases where the operator is using wider band (e.g., EGSM) or more bands and the repeater does not support EGSM or is only for 900GSM. Operators can operate also EGSM or GSM900+GSM1800 layers with single BCCH (Siemens(SAG) commonly supports the BCCH feature. In the case of improper repeater support, many calls may drop).
Rural areas
In many rural areas the housing density is too low to make construction of a new base stationBase stationThe term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
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 interferenceMultipath interferenceMultipath interference is a phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and, under the right condition, the two components of the wave interfere...
, 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 mallShopping mallA shopping mall, shopping centre, shopping arcade, shopping precinct or simply mall is one or more buildings forming a complex of shops representing merchandisers, with interconnecting walkways enabling visitors to easily walk from unit to unit, along with a parking area — a modern, indoor version...
s. 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 antennaDirectional antennaA directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
s 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 interferenceMultipath interferenceMultipath interference is a phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and, under the right condition, the two components of the wave interfere...
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 ionosphereIonosphereThe ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
as shortwave radio waves do, cell phone waves cannot travel via the ionosphere.
See DiffractionDiffractionDiffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
and AttenuationAttenuationIn physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
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 phoneSatellite phoneA satellite telephone, satellite phone, or satphone is a type of mobile phone that connects to orbiting satellites instead of terrestrial cell sites...
systems, allowing the satphones to be used indoors without a clear line of sight to the satellite.
Approval in the USA by the FCC
Although there are some products on the market in the USA which are still pending Federal Communications CommissionFederal Communications CommissionThe Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(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.
Approval in the UK by Ofcom and the UK market
In the UK there is no third party licencing regime in place even for low powered devices for home use. 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. The UK regulator, Ofcom specifically states that; "Repeater devices transmit or re-transmit in the cellular frequency bands. Only the mobile network operators are licensed to use equipment that transmits in these bands. Installation or use of repeater devices by anyone without a licence is a criminal offence under Section 8 of the WT Act 2006."
This has not detered the UK market for signal boosters with an explosion of web retailers offering a wide range of solutions from DIY installs to installations. This has largely been driven by the tightening of energy efficicency regulations in building construction. The use of materials that mitigate heat loss such as metallic foil laminated insulation board and energy efficient glazing with a metallic anodised coating also have high signal attenuation properties.
The window for any such regulation of the UK market as in the US, is likely passed as the quantity of repeaters already shipped and in place in the UK is estimated at the tens of thousands. Repeaters operating in rural and less densely populated areas do not pose a quantifiable problem. 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. The lack of regulatory guidance has led to the situation where most installtions are simply 'plug and play' and have been installed by non technical persons who have no appreciation or understanding of even the most basic RF principles.
There are well established vendors who have worked for some time in this area who work for, and with the networks to provide in building coverage solutions. Ofcom suggest that "Anyone wishing to improve coverage in a particular area is advised to contact their network provider."
At present, the willingness and capability of network operators to initiate the process to providing an in building solution varies greatly. Some network operators have dedicated teams that will visit a customers site to assess the cause of the problems experienced and offer to put in place a solution provided by a third party vendor. However this is generally limited to large corporates and the public sector, with the expense being borne by the client. The disparity in approach and the lack of a cross network operator working group means that sites requiring a 'wideband' coverage solution for all networks is not currently possible.
See also
- Base Station SubsystemBase Station SubsystemThe base station subsystem is the section of a traditional cellular telephone network which is responsible for handling traffic and signaling between a mobile phone and the network switching subsystem...
- Cellular networkCellular networkA cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...
- Coverage noticer
- Dead zone (cell phone)
- WavesWAVESThe WAVES were a World War II-era division of the U.S. Navy that consisted entirely of women. The name of this group is an acronym for "Women Accepted for Volunteer Emergency Service" ; the word "emergency" implied that the acceptance of women was due to the unusual circumstances of the war and...
- Cel-FiCel-FiCel-Fi is a device, similar to a cellular repeater, designed to eliminate in-building dead zones and improve indoor mobile phone reception.Created by Nextivity, Inc., the Cel-Fi systems are designed with smart antenna technology to seek out the best available signal to maximize signal gain to phone...