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an inescapable fact is that trans- mission rates of 100 Mbit/s and higher will expose network infra- structure and services, like never before, to the eff ects of Passive Intermodulation (PIM). Now accepted as an industry


Passive intermodulation D


Interfering but not insurmountable: Peter Jackson show how to prevent passive intermodulation on multi-channel radio sites from damaging the user’s experience


espite all the fanfare about fourth genera- tion (4G) networks,


Such ‘passive intermodulation’


benchmark in determining the health of a cell site, this form of radio frequency (RF) interference must be addressed by every net- work operator, particularly when something as simple as a rusty bolt can be the cause. T ere is no escaping the fact that operators must minimize the eff ects of PIM with a formal strategy.


Causes and impact PIM is a phenomenon which can impact any RF-based communi- cations system. But it is only in recent years, with the boom in personal mobile communications, that the cellular industry, with its complex frequency plans, shared base station sites using high trans- mitter power levels and sensitive receivers, has had to address the problem of PIM. Non-linear systems, which are


composed of active components, i.e. those with an external power source, are typically subject to in- termodulation [mixing of frequen- cies]. T e very nature of being active, however, means such inter- modulation can be actively fi ltered. However, passive or linear compo- nents, such as connectors, cable as- semblies and antennas, which have no power source, can sometimes create intermodulation too.


Peter Jackson is Director – Europe for Communication Components Inc. (CCI)


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can stem from a loose connector, minor damage to an antenna, or poorly maintained equipment, such as a rusty/oxidized bolt or component. It results in two or more frequencies ‘clashing’ with one another in the transmission path, thereby impacting service quality. Where two or more carriers


share the same downlink path in a wireless network by cell-site shar- ing, PIM’s undesired, non-linear signal energy can cause signifi cant interference in the uplink receive band, which can lead to reduced receiver sensitivity. To the mobile phone user, this


multi-carrier interaction, typically caused by incompatible, poorly installed or damaged network hardware confi gurations at the base station, can manifest itself as a loss in voice audio quality, de- creased data speeds, or, in extreme circumstances, in dropped calls, an inability to make or receive calls, or even to use data services at all.


Multiple carriers PIM signals exist in this scenario as a result of the combined transmis- sion of multiple carrier frequencies within a transmission line path, and the objective for any mobile operator’s team of engineers must be to ensure that these levels occur at amplitudes below the base sta- tion’s receiver sensitivity. T e amplitude of these unde- sired signals is directly infl uenced by the fi delity of


the


transmission line path, including nents along


all and metal-to-metal compo- junctions


that path. Poor contact


junctions can, for exam- ple, create additional non-


LAND mobile October 2011


linearities resulting in PIM, and these can come from something as simple as under-torqued male-to- female DIN 7-16 connector mates, as well as irregular contact surfaces such as poorly manufactured con- nectors and surface oxidation.


PIM or interference? When taking PIM measurements at a site, it is sometimes diffi cult to distinguish PIM energy generated as a result of internally transmit- ted carrier signals from external interference signals coming from outside the antenna. PIM testing is intended to be


performed within a site’s trans- mission line path from the radio to the antenna. When antenna manufacturers test antennas for PIM performance, measurements are taken in an anechoic chamber, where the presence of external in- terference signals is not possible. In the fi eld, however, external


interference signals can often be construed as PIM signals, because they occasionally fall within the uplink receive band, and can come from sources such as adjacent cell sites, old TV transmitters, or the presence of metallic structures near the site. Adjacent cell site or TV transmitter interference can be identifi ed using a spectrum analyser and comparing spectrum responses between sectors.


Testing for PIM To


better represent real-traffi c


network conditions, PIM meas- urements should be per- formed at the BTS radio power


level, or slightly higher. Testing for PIM


at 40W has been demonstrated as a more stringent network test and a device tested at 40W actually per- forms 13 dB better than a device tested, for example, at 2W, even though both devices meet the de- sired –106 dBm PIM performance level. However, a device that meets the desired PIM performance at 2W may well fail if subjected to higher power levels of 20W or 40W. PIM tests that are performed


at low power can also mask the presence of PIM, and although performing PIM testing at 40W is considered to be a more stringent test then is currently required, it exposes a cell site’s PIM vulner- abilities in a signifi cantly more quantitative manner, leaving little room for conjecture as to the integ- rity of the devices or components under test. Meeting the 20W PIM specifi cation today at 40W gives operators and contractors more measurement confi dence and al- lows room for growth.


A 4G world In a future where MNOs will increasingly be sharing RF in- frastructure at the cell site, an eff ective PIM-management strat- egy will ensure this can take place without losses or compromise on performance – the result is a win- win of capex and opex savings for both the sharing networks. MNOs will be able to prevent this interference from causing QoS issues on their networks, thereby delivering good customer experi- ence and ensuring the continued loyalty of their subscribers in an unforgiving competitive arena.


This portable PIM analyser by CCI provides precise measurements to verify the integrity of any system or component at a cell site under high-power conditions


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