Addressing the PIM overlay challenge

With wireless communications fast becoming part and parcel of everyday life, wireless network providers need to maintain an efficient network that operates at optimal capacity.
Many operators try to do this by overlaying on existing networks. That means using existing tower equipment such as feeder coaxial cable lines and base station antennas.
In overlaying, operators encounter the phenomenon of passive intermodulation (PIM), which is the result of two or more wireless signals mixing together to create additional, undesired frequencies that cause interference or degrade transmission of desired signals in wireless networks. PIM may occur in passive non-linear radio frequency (RF) circuits with two or more common components. The two fundamental causes are current rectification at the conductor joints, and/or varying magnetic permeability due to the ferromagnetic material in or near the RF path.
PIM levels that approach those of thermal noise or other interference can desensitize the receiver, causing performance to deteriorate and usually results in signal blockage or loss of reception. The result is reduced network efficiency, channel capacity and bottom line profit, making PIM a critical threat and growing problem for network operators.
Also, antennas and radios are now hyper-sensitive and susceptible to smaller and smaller levels of distortion. A 1dB drop in uplink sensitivity due to PIM can reduce coverage by as much as 11%.
Operators are increasingly taking steps to avoid potential signal degradation that can result from overlays, especially when adding new frequency bands. Many carriers are addressing the problem by deploying PIM testing equipment to the field. However, using PIM equipment correctly requires greater care than standard voltage standing wave ratio site sweeps or distance-to-fault transmission line testing. But operators need not fret as long as they adopt a holistic and proactive approach towards dealing with PIM.
Firstly, network operators need to realize that PIM is not just confined to those involved with network testing. It affects everyone, from installers to those working on system design. One of the best ways to minimize the cost and problems created by PIM is to try and prevent it during field installation. Operators should invest in getting trained technicians to deal with connectorization and installation – two of the most prevalent causes of PIM.
PIM cannot be avoided entirely. Hence, operators should ensure that regular testing is conducted to see if unacceptable levels of intermodulation are detected in the RF path. Ideally, PIM testing should be done yearly as the RF path must be off line during PIM testing. However, the problem with taking PIM measurements is that test equipment and surroundings can skew readings, which could result in false failures.
Sources of irregularities or offending components should be identified and located as quickly as possible. If a component is isolated and identified as lacking PIM compliance or exceeding specified PIM performance levels, it should be replaced as soon as possible.
Operators can undertake some preventive maintenance by checking PIM levels when upgrades or changes are made to the RF path. Additionally, technicians should be sent for regular training on installation and connectorization.
Dipesh Rattan is manager of the microwave systems product line at CommScope’s wireless

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