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TEST &TEST & MEASUREMEN T
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ACCURATE TESTING OF CELLULAR AND DAS AMPLIFIERS
ACCURATE TESTIN G O F CELL ULAR AND DAS AMPLIFIER S Wa C
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ellular amplifiers and antenna systems (DA
roduct ma ge provide signal in obstruct
ed areas such S) are used to distributed
as building interiors and behind
geologic features like hills, valleys, etc. In-building systems are especially common, because industry analysis of network usage shows that about 75% of cellular usage comes from within buildings. Modern buildings are often built to “green” standards for energy savings. These high-efficiency buildings employ metalised glass for retention or exclusion of heat, but metalised glass also impedes the flow of RF energy, resulting in attenuation of desired signals on both uplink and downlink paths. Cellular amplifiers and DAS systems are deployed to address these issues. It should be noted that a cellular amplifier or DAS does not increase system capacity; it simply makes the available resources of th e serving system more effective.
Adjusted properly, cellular amplifiers will improve performance and increase subscriber satisfaction. However, if misaligned, these devices can generate interference and cause network
performance issues. Proper alignment at time of install is needed to maximise performance and prevent causing more problems than are solved. A cellular amplifier that has gone i nto feedback can cause numerous problems for subscribers. It is important to
remember that an amplifier boosts both the intended signal and any noise in the receiver passband and adds some noise of its own. You want just enough gain, and no more.
For cellular systems, bi-directional amplifiers are used. Ensuring isolation between the Donor and Server antennas is critical so that RF feedback does not occur This isolation figu re should exceed the adjusted gain of the
.
amplifier by at least 10dB and ideally 15dB or more.
DAS is a specialised case of cellular amplifier architecture where, instead of a single server antenna, there are several Server antennas fed from a
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connected to a portable signal analyser capable of measuring control channel power.
central amplifier (Figure 1). Along the path between the amplifier and server antennas, there may be coaxial splitters/combiners and possibly
amplifiers and attenuators. Calculation of proper system gain must take into
account the additive dB gain of any path amplifiers, minus the losses from coax, connectors, splitters/combiners, etc. When installing cellular amplifiers, it is important to specify and use high quality equipment. Good amplifiers will be more immune to self-oscillation and will have adjustable gain for each intended band o f operation. Determination of correct gain settings is unique to each installation, and care must be taken to ensure that the gain is set based on the maximum expected downlink signal likely to be received at the donor antenna. This is best done by analysing the control channel’s signal power because this resource is transmitted at constant power and doesn’t vary with utilisation. Ideally, this me asurement is done wit h the amplifier’s donor antenna mounted in place and with the donor antenna coax
Figure 1: Figure 1: Repeator driving a DAS Repeator driving a D
Measurement of amplifier gain can be done with a cable and antenna analyser such as the Anritsu BTS Master MT8220T, using the “2-port Gain” measurement mode. 2-port Gain is an amplitude-only measurement and phase will not be analysed, so a formal calibration isn’t needed. In this mode , the output port can generate signals with levels of either -7dBm (High Power) or -40dBm (Low Power) into the donor antenna port. For the gain measurement, use the Low Power level and further reduce the RF Out signal level using an attenuator to drive the amplifier input to approximately the level of the expected downlink signal. The MT8220T’s RF In port can handle up to -5dBm, but for best accuracy, the input signal should be adjusted t o about -10dBm, so an adjustable
attenuator should also be used on the amplifier’s output. Be sure that this attenuator is capable of safely handling the maximum power level, which the amplifier could potentially generate at maximum input when set to maximum gain! It may be appropriate to use a high-power fixed attenuator followed by an adjustable attenuator.
Figure 2: Figure 2:
Measurement of attenuators
Measurement of attenuators
Before making gain or isolatio n measurements, a 2-port or “thru” calibration must be done using all components except the cellular amplifier.
A properly installed and adjusted cellular amplifier or DAS can provide network coverage in problematic
locations. It is important to analyse the signals which need amplification and adjust the amplifier using a test instrument to determine the right amount of gain Paying close attentio n to antenna installati surrounding physica
.
l environment can on and the
help mitigate isolation issues. Antritsu
https://www.anritsu.com/en-GB T: 01582 433280
https://www.anritsu.com/en-GB
/ ELECTRONICS
ELECTRONICS
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