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Passive intermodulation I


ntermodulation distortion (IMD) is not a new phenomenon. It was present


even in the earliest mobile networks, but the implications of distortion interference on early voice calls or small data transfers via text message were minimal. In modern data-driven networks, the problems IMD creates are far more pronounced. IMD can be caused by either


active or passive components in a base station. Distortion caused by active components, such as amplifiers, can be reduced or eliminated by filtering. Unfortunately, operators are now seeing increased passive intermodulation (PIM), which occurs in non-powered devices (such as cables or antennas). Tis cannot be filtered out. Te mechanics of PIM involves


the production of new, unwanted signal frequency components in the passive, non-linear devices commonly found in and around base stations. Tese new signals can land on existing transmission or reception frequencies on the same or an adjacent site, causing interference. PIM is usually the result of two (or more) high power tones mixing at device non-linearities such as junctions of dissimilar metals, corrosion or rust and even loose connectors. Te higher the signal amplitudes, the more pronounced the effect and the more prominent the interference that occurs. While standards are still under development, many vendors offer RF cables with PIM specifications of -140 decibels relative to the carrier.


Why is PIM a problem? With operators under growing financial pressure due to the strain of financing 4G deployments, they need to ensure faultless network performance. With air space rapidly becoming crowded and network data traffic


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increasing, the tolerance for PIM in networks is dropping fast. Regulators are squeezing spectrum capacity to its limit, shared base stations are becoming more common and the number of antennas used for transmit and receive functions are being reduced. Additionally, there are now more components than ever in the RF transmission path and each additional component is another potential source of PIM signal degradation. In older wireless networks, radio interference might lower the quality of a voice call, but subscribers would still be able to connect to the person they were calling and make out what was being said. However, in modern mobile networks, a site experiencing severe interference will hand-off traffic to another base station, meaning that subscribers can be suddenly cut off or even never receive a call at all. Te roots of PIM problems


often stretch back to the original construction of wireless networks, so careful network design and planning is essential to ensuring long-term, optimal performance. As operators invest in upgrading to 4G, new infrastructure is often overlaid on existing passive components, such as coaxial feeder cable, filters and base station antennas. Steps need to be taken to avoid potential signal degradation that can result from such overlays, especially when adding new frequency bands. Te current trend for operators to add more frequency bands to the RF path can create further opportunities for PIM to be generated. Interference caused by this can be minimised with specially designed and manufactured components. PIM can also be caused by


equipment that is not optimized for compatibility (for example, equipment purchased from different infrastructure vendors and brought together in the same base


station). Tus interference problems can be greatly reduced by choosing one equipment vendor. Choosing one vendor reduces deployment risks, lowers costs, improves network reliability, simplifies the network and provides one point of contact for any issues that may arise. Given that PIM can be difficult


to both detect and resolve, it is essential that operators employ certified, comprehensively trained installers. Tis can be a serious problem for operators in emerging markets, where highly skilled labour is often scarce or entirely unavailable. Equipment manufacturers can help this situation by simplifying products, so that they are not complex or difficult to install. Alternatively, choosing pre-integrated solutions can mean that they can be installed by less specialised personnel – a benefit for all, since qualified RF engineers can be costly. In developed countries


however, operators should partner with equipment vendors that have robust business partner programmes and provide expert installation for their own product offerings. Operators should expect high quality installation work, expert on-site advice and rock-solid accountability for any problems. Network service quality can also result from some operators choosing


Phil Sorsky, from CommScope, discusses solutions for intermodulation distortion


With air space becoming crowded and network data traffic increasing, the tolerance for passive intermodulation in networks is dropping fast


to pursue the false economy of reducing capital expenditure (CapEx) on new infrastructure by purchasing lower quality products. For example, the manufacturing quality of antennas is currently under-regulated which has led to 20-30 per cent of antenna links not meeting specifications in many networks, according to a recent Comsearch study. For wireless networks to achieve


their full operating potential, each sector within the network must perform up to its design standard. To achieve this, each RF component and interconnection of the RF path components must be properly installed, verified and maintained to ensure optimum performance over time. Te old adage of “buy cheap, buy twice” holds true.


PIM is here to stay Interference in general and PIM in particular, are issues that wireless operators will be dealing with for some time to come, while increasingly needing the networks to be as interference free as possible. In this context, it will be the


operators which have squeezed the highest possible level of performance from their networks that will not only survive the immediate crisis of falling revenues and increasing expenditures, but also thrive well into the future.


LAND mobile January 2013


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