Cellular repeater

A cellular repeater (also known as cell phone signal booster or amplifier), in the cell phone industry, is a type of bi-directional amplifier 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. 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 can rebroadcast cellular signals inside a building. The systems usually use an external, directional antenna to collect the best cellular signal, which is then transmitted using coaxial cable to a signal amplifier unit, and then transmitted using more coaxial cable to an interior antenna which retransmits it to the users, thus 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. Some political jurisdictions require these systems in all new large buildings to support emergency use (i.e. police, fire and paramedic).

There are two types of signal boosters, or repeaters

1. Analog Signal Boosters
Most common type of amplifiers. These signal boosters are usually analog repeaters that use traditional technology to amplify (usually) all frequencies from cell phone carriers. They are almost always wide band (or broadband) repeaters. Such boosters are usually sold with a kit of outdoor antenna and cable and require an installation. These are sometime called BDA units, short for Bi-Directional Amplifier.

2. Smart Signal Booster
This is a new category of Signal Boosters. This new category of modern boosters utilizes powerful all-digital baseband processors to clean the signal before it is rebroadcast (hence the "Smart" in the name Smart Signal Booster). Most of the Smart Signal Boosters have gains of 100 dB (compared to analog booster's gain of 63 to 70 dB) and are carrier-specific (hence they can have higher gain in the US according to new FCC regulations). Some of these new boosters, while more expensive than traditional analog booster, are plug and play: they usually don't need the installation of outdoor antennas (usually the donor antenna is inside the booster box). Most of the time, Smart Signal Boosters are carrier-specific and are under consideration by the FCC's new rules starting in 3/2014.

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. Consequently, the market value of world's cellular repeater suppliers such as Huaptec, Phonetone, Stella Doradus, etc. will increase.

One advantage of cellular repeaters is an increase in the cell phone's battery life since it requires lower power 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 to the booster's signal, it 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 20 dB - 50 dB gain and many of the more expensive models provide over 50 dB. Excellent high-power models (used for by commercial operators) offer a gain of around 100 dBm (Interference cancellation equipment function is an improvement of the radio isolation between donor and service antenna). However, since decibels are logarithmic a 30 dB gain represents a one thousandfold signal power increase - meaning the total amplification of a repeater with greater than around 50 dB 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 repeaters (operated by telecommunication operators for coverage of large areas and big buildings) have output power around 2 W, high power repeaters (e.g., NodeG from Andrew) have output power around 10 W. The power gain is calculated by the following equation:

$\quad P_{\mathrm {dB} }=10\log _{10}\left({\frac {P}{P_{0}}}\right)\$

A repeater is needed to secure sufficient isolation between donor and service antenna. When the isolation is lower than actual gain + reserve (typically 5-15 dB) then the repeater is in loop oscillation and this oscillation can cause serious interference to the cellular network outside the building.

Also, cheap models are equipped with 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 may be improved by antenna type selection in a macro environment: by adjusting the angle between the donor and service antennas (ideally 180°), space separation (typically the vertical distance in the case of the tower installation between donor and service antenna is several meters), insertion into an attenuating environment (e.g. installing a metal mesh between donor and service antennas), and/or reduction of reflections (no near obstacles in front of the donor antenna such as trees or buildings).

Isolation can be also improved by integrated feature called ICE (Interference Cancellation Equipment) offered in some products (e.g., NodeG, RFWindow). Activation of this feature has a negative impact on internal delay (higher delay => prox. +5 us up to standard rep. delay) and consequently a shorter radius from donor site, where a repeater could be used.

Amplification and filtration introduce a delay (typically between 5 and 15 us), depending on the type of repeater and features used. Additional distance adds propagation delay. 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 moves the user to a bigger distance: radio distance = real distance + (repeater delay in us) *0.3 km (delay of RF signal in air is 3.3us/km). This is reason why, with sufficient levels, a repeater doesn't work. After repeating there will be better (or excellent) coverage, but the network can not be accessed. The user, from network point of view, is too far.

There is also problem with noise amplification (especially in UL) and desensitization of the donor site.

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 it 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 with a great distance from the broadcast towers.

Building construction material

Some construction materials rapidly attenuate cell phone signal strength. Older buildings, such as churches, which may use lead in their roofing material, will very effectively block any signal. Any building which has a significant thickness of concrete, or a large amount of metal used in its construction, 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 have 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 details.

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 ionosphere.

See Diffraction and Attenuation for more details.

Different operating frequencies

Repeaters are available for all the different GSM frequency bands. Some repeaters will handle different types of networks, 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.

Regulation in Australia

General
A cellular repeater is referred to as a mobile phone repeater or mobile repeater, and communicated in that way by Regulators.

A mobile phone repeater operates within apparatus or spectrum licensed radio-frequency bands. Mobile repeaters are also used and managed by mobile carriers as part of their general network, and authorised under its PMTS licence.

Subscribers (mobile phone end users) wanting to use a mobile repeater must (with the exception of a passive mobile repeater) obtain permission from the mobile carrier under a third-party arrangement. Subscribers that operate a mobile repeater without the mobile carrier’s permission, runs the risk of causing harmful interference to the mobile network, and a number of contraventions to the Radiocommunications Act 1992 such as Unlicensed Operation and Possession. Section 474.6 of the Criminal Code Act 1995 may apply as the person unlawfully operates the mobile repeater in the carriers network which can be taken to be an action of tamper or interference. Absolute liability applies to the physical element of circumstance of the offence, that the facility is owned or operated by a carrier, a carriage service provider or a nominated carrier.

Suppliers
Australian Suppliers of Mobile Repeaters are now required to maintain records of repeater sales and location information to assist the ACMA in its investigation of interference complaints by being able to access detailed information about those devices supplied.

The amendment to the Regulations, Regulation 38B specifies the ‘particulars’ which the supplier is required to record under paragraph 301(1)(b) of the Act. In summary, the ‘particulars’ include: licence holder or authorisation agreement details (for example the licence number, date of issue and expiry, full name of the licensee or a duplicate of the licence) repeater device details (for example the device’s serial or model number) and information concerning the actual transaction (for example, the date of supply, particulars regarding the identity of the recipient, such as photo identification). 　

Illegal supply of repeaters
The changes to legislation has not stopped the supply and proliferation of unauthorised Mobile Phone repeaters into Australia. There is ongoing misleading information from overseas sellers continuing to target Australian Consumers, and they're using search engine advertised marketing such as Google Adwords, to entice consumers to purchase and use their mobile repeaters that are illegal to possess or operate under Australian Law. The changes has only affected Australian Suppliers that can be directly governed under Australian Jurisdiction. The ACMA and Australian carriers reported a number of instances of illegal mobile repeaters they found which caused interference, through Twitter feeds and online news services.

Legitimate suppliers
At the time of writing this literature, there was only one known importer that has agreements with all three Australian Carriers, other than the mobile carriers themselves (Optus Telstra and Vodafone), that supply Mobile Phone repeaters that can be authorised under a third party agreement. The repeaters are manufactured by Nextivity Inc. The mobile repeaters are imported by Powertec Telecommunications Australia Ltd and distributed by a number of retailers and installers across the country. The repeaters are sold under the Cel-Fi brand to subscribers.

Applicable Standards
At the time of writing this literature, there were no Regulatory Australian Telecommunications Standards applicable to mobile repeaters. However the 3rd Generation Project Partnership (3GPP) developed technical specification TS25.143 to allow manufacturers to assess performance of the repeaters against the specification as a benchmark.

Other applicable standards that may apply are the Radiocommunications EMC Standard 2008 for external power supplies, and ARPANSA standard relates to the electromagnetic radiation exposure to humans.

The Regulators links
Links currently do not work as the ACMA has not updated changed urls! There are strict restrictions for operation in Australia, further information can be found at:

• Mobile phone repeaters fact sheet; and
• Mobile phone boosters fact sheet.

Approval in the USA by the FCC

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.).

On February 20, 2013, the FCC released a Report & Order, thus establishing two Safe Harbors and defining the use of “network safe” consumer boosters on licensed spectrum. The Safe Harbors represent a compromise solution between Technology Manufacturers and Wireless Operators. It is widely considered a landmark decision which was many years in the making. Only a few companies have a product compatible with the new FCC report and order.

The FCC has defined two types of repeaters:
1. Wide-band (or broadband) signal boosters are usually repeaters that amplify all frequencies from cell phone carriers. Because interferences can be generated from such boosters, the manufacturers who apply to the FCC must limit their gain (among other things), to 70 dB (for the low LTE 700Mhz bands) to 72 dB (for higher frequencies such as AWS). By limiting the system gain, such boosters are only useful when the outdoor signal is relatively high, and need a complex outdoor installation of specific antennas.
2. Carrier specific (or provider specific) signal boosters. These boosters are only designed to boost those frequencies (and signal) that belong to a particular carrier. Usually, such carrier specific boosters do not produce interferences on other carrier's frequencies, and are allowed to have much larger system gains (sometimes 100 dB). In these conditions, such devices boost signal in a larger coverage area, and can still be efficient when outdoor carrier signals are weak, but are only boosting the signal for the carrier it is designed to operate.

The new rules from the FCC should start on March 1, 2014. Here are the rules.

Approval in the UK by Ofcom and the UK market

Many websites and forums state Ofcom have banned the use of mobile boosters. This does not appear to be the case and in May 2011 Ofcom provided clarification. This was stated as follows:

Installation or use of repeater devices (as with any radio equipment) is a criminal offence unless two conditions are satisfied:

1. That the equipment is CE marked, indicating that the manufacturer has declared it complies with all relevant EU regulatory requirements, including the Radio equipment and Telecommunications Terminal Equipment (R&TTE) Directive;

2. That the use of the equipment is specifically authorised in the UK, either via a licence or by regulations made by Ofcom to exempt the use from licensing.

Possible Clarification due late 2015.

The wireless act also allows under WT Act 2006 section 1.15 an exemption if the device does not "involve undue interference with wireless telegraphy" They have entered into a discussion with www.mobilerepeatergermany.com to give guidelines after an EU case was brought against Ofcom claiming exemption under WT Act 2006 section 1.15 and Ofcom lost and has to undergo a review of the guidelines. See EU case summary here. http://www.monckton.com/publication/recall-support-services-limited-et-al-v-secretary-state-culture-media-sport/

It is expected to follow the US style regulations where a mobile repeater must have protection built in against interference and will allow people install quality units but put cheaper ones out of business.

Ofcom 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 deterred 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 efficiency 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 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. Most installations 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 Subsystem
• Femtocell - type of cellular repeater
• Home Node B - a femtocell
• Coverage noticer
• Dead zone (cell phone)
• Waves
• Cell Phone Signal Booster
• Cel-Fi Signal Booster