16 Sep Installation of a Cell Extender
There are many names for an indoor cell extender. Commonly it is referred to as a Cell Booster but in actual fact it is a fully functional low powered cellphone repeater. The industry refers to it as a mini, a pico or a SOHO repeater or a cell extender. There are indoor and outdoor types. There are different formats like CDMA, GSM, DSC, UMTS There are also combinations of the mentioned frequency formats and they are referred to as dual & even tri-band repeaters. All repeaters should be regarded as the network operator’s responsibility. The network and the frequency spectrum is their licensed property over which they supply specific services and for which the end user pays revenue. The quality of equipment used on these networks is their responsibility as well as to provide coverage throughout. Installations of any kind should be carried out by a fully qualified network technician or an appointed contractor.
Should a residence or business not have indoor cellphone coverage, their preferred network operator whose services have been assigned to should be consulted prior to making such a decision? Only if this service is not forthcoming due to whatever reason, should a resident consider installation of an indoor cell extender.
• It must be remembered that this devise is an indoor repeater and has been designed to provide signal coverage indoors.
• Coverage within homes, lodges, storerooms, workshops, factories, shopping malls, basements, offices, tunnels, all fall under the category of indoor coverage.
• These are systems are not for repeating signal over any distances outdoors, but with use of multiple antennas and line amplifiers, large indoor or secluded arias can enjoy converge.
• Even though it is an indoor device it still needs to be installed by a qualified installer.
• Even though cell boosters are obtainable over the counter as a DIY kit it is austerely advised that you not purchase one and try to install it yourself.
Before a cell extender is installed, have the RSSI (Received Signal Strength Indication) measured. This is done by a trained installer measuring the RSSI using a portable mast, high quality cable and antenna. This is erected and the measurement is done using a portable spectrum analyzer, a portable signal strength meter or software used on a lap top or cell phone. Full signal may be indicated on your mobile, but no coverage is available from the network you need to connect with, so also determining which operators’ signal is being measured is also relevant.
The RSSI is vendor dependant – in other words, the network being connected to can fluctuate, it can improve of deteriorate. BTS output power levels can be changed or an antenna can be turned a little into another direction, the network could erect another BTS (Base Transmitting Station) or cellphone tower in the vicinity. A building or a bridge could have been built in between the BTS and the home or business shielding it totally. Because high frequencies are LOS (Line of Sight) dependant, these factors can change and individuals can be affected drastically. All these factors should be measured and recorded.
The RSSI measurement should be done at a height of not less than 7 meters from the ground. This is the minimum installation height of an outdoor antenna on a cell extender. Ground level RSSI is irrelevant for this purpose. It is imperative that you do have some signal on the site of installation – if there is no signal there is nothing to amplify. But, no signal at ground level does not mean there is no signal, there could be good signal just above the tree line.
What must be achieved? These measurements are curtail in determining what the outdoor (donor or up/link) gain should be. How many BTS towers in the coverage aria and their situation is information needed in selecting the correct antenna type to communicate between the BTS and the repeater. This criteria is not catered for in DIY kits. Antenna types, their radiation patterns and their strength (Gain) are not common knowledge to the layman. Once this is established, a link budget calculation can be calculated and the correct cell extender recommended. This will ensure a cell extender that does what it is designed to do and that is to give you indoor coverage. Incorrect calculation and unqualified installation will very probably lead to a system that oscillates, creates noise on the network and gives very limited coverage.
Typical signal strength evaluation One bar signal on you cell phone can be amplified to full strength with a cell extender. Signal strength is measure in dBm. Here is a rough guideline that could assist. Remember, this is what the user is experiencing at ground level. He may have full signal on a hill 100 meters from home.
5 Bar = -40-80 dBm very good
4 Bar = -85 dBm good
3 Bar = -90 dBm usable
2 Bar = -95 dBm may break up and drop calls
1 Bar = -100dBm poor
Theoretically using a Coiler TG900 27dBm cell extender with 60dB gain and selected antennas a site with -90dBm can be amplified to -27dBm which will provide coverage to a normal sized home. Between 7~10met radius on the GSM900 frequency is common. Much larger coverage arias have been achieved regularly but one tends to be very careful in sketching coverage when RF is being applied as it is not very predictable and so many variations apply.
Is it usable signal? There are weak signals, poor quality signals, noisy signals, reflected signals etc. The parable applies “garbage in, amplified garbage out”. The better the RSSI before amplification, the better the coverage and the quality of the amplified signal. By reflected signal we mean the signal is bounced off a stationary object such as a mountain cliff or a building such as a silo etc. These signals are not always stable. Often this signal is varying in strength. Reflected signal can be amplified, but the repeater will not necessarily stabilize the varying signal.
Which Network Operator’s signal is being amplified? Network Operators in SA often use the same infrastructure but many rural high sites host only one operators BTS. It is necessary therefore to ensure that the relevant base station provides the operator’s service subscribed to. The broadband GSM repeater amplifies the whole GSM900 frequency spectrum and the other operator’s signal is also amplified. This can mean that the user could have excellent operator 2 signal but weak operator 1 signal. The Service Providers/Network Operators use band selective transmitters and receivers (BTS) to control their own customer’s billing and added services. This means only that operator’s dedicated channels in the frequency spectrum will be enhanced and not their oppositions. This also assists them in identifying who is responsible when a problem arises.
Which services are amplified? By services we refer to:
• GSM900 (Global System for Mobile Communications) – voice, GPRS and 2G (2nd Generation) in SA
• GSM1800 or DCS (Digital Cellular Service) used for EDGE (Enhanced Data rates for Global Evolution) – Data and high density arias for voice in SA
• UMTS2200 (Universal Mobile Telecommunications System) commonly uses W -CDMA (Wideband Code Division Multiple Access), for 3G (3rd Generation) or HSDPA (High-Speed Downlink Packet Access)
Because each of these technologies is being run on a specific frequency bandwidth, a repeater for that bandwidth or combination repeater engineered for two or more bandwidths is needed to enhance each format. GSM900 on the 900MHz frequency, DCS on the 1800MHz and UMTS on the 2.1GHz frequency. South Africa operates a Tri-Band network in its major suburban areas where all three services are available. In rural and minor suburb arias a dualband (GSM900 & DCS) or a singleband GSM900 & GPRS network should be available. The UMTS cell extender cannot amplify 3G if the local BTS is not a UMTS carrier even if the end user’s mobile is 3G enabled. He will have to be satisfied with GPRS or Edge.
Why is all signal not available in all arias? No network available. A network is rolled out according to a network plan. Services for arias are planned for and provided according to the potential market. The network operator is a business with shareholders all demanding the best dividends on their investments. A BTS to provide these services to the end user will not be erected for one or two farmers with the potential to generate R6000pm calls/sms’s made by themselves and their workers. Where there is substantial inhabitancy of an aria a market for network services is created and accordingly coverage will be provided.
No coverage even though network is available due to distance. Approximate distance a frequency travels when generated at prescribed power level. (dBm power ratio in decibels)What is not commonly understood is that different frequencies have different wave lengths and are transmitted at different power levels as determined by the governing broadcasting body. The power level is determined due to the danger of Radio Frequency Radiation (RF) hazards. 1800MHz is already a low microwave frequency and 2.1 GHz is close to the 2.4GHz frequency most microwaves ovens operate on. Should this frequency be amplified indiscriminately, a real health hazard will be inevitable. Should you fall outside this radius you will experience weak signal.
No coverage even though network is available due to obstacles.
• A second point not commonly known is that the higher the frequency (the narrower the wave length) the more difficult it’s penetration of obstacles, like trees, buildings, steel structures like roofs and hills etc.
A suck of the thumb indication over an obstacle-free field (LOS or line of site)
• GSM900 (900MHz) transmitted from a BTS should give a coverage distance/radius of 16km
• DCS1800(1800MHz) transmitted from a BTS should give a coverage distance/radius of 7km
• UMTS (2200MHz) transmitted from a BTS should give a coverage distance/radius of 3km
3G and HSDPA have a lot of challenges in the cities centres due to penetration. E.g. Accompany could have great 3G connectivity – then a bridge, a building or the Gautrain overhead rail is built across the up-town suburb between the BTS and your point of reception, and that is the end of the 3G signal. One only needs to move offices down the hall to lose a 3G signal and this is due to the penetration phenomena.
What should become clear is that it is no easy task giving coverage to every person in a diverse country such as South Africa. It is even more of a challenge to provide all the service available with cellphone technology coming up with new formats and medias such as “push technology” and video conferencing. It is not only about the phone but about the signal.
If need assistance, do not talk to just anyone. Talk to the right people.
By Shaun C. J. Vermaak