Degraded signals are the bane of a wireless system, yet the RF environment increasingly is cluttered with potential hazards to clear signals. One of the most fundamental signal threats is passive intermodulation interference, or PIM.
Passive intermodulation distortion exists at some level in all passive systems. But when inferior parts are used or incorrect installation of equipment occurs, the distortion can result in degraded system performance. Therefore, RF engineers should pay particular attention to avoiding errors in installation procedures and inferior materials.
The first step in assessing the vulnerability of a system is the intermodulation study. As performed by LBA, this study encompasses all radiators in the area, on all frequencies likely to interact. Highly computation-intensive, a massive number of frequency combinations are evaluated to determine those combinations capable of interfering within the pass bands of the system receivers. These intermodulation interferences are determined to the 5th or 7th order, as desired by the client.
Third-order PIM gives the highest level of self-interference, followed by 5th and 7th order. The third-order products can spread by three times the channel bandwidth; thus, a 10 MHz LTE signal would interfere over at least 30 MHz. Not too long ago, careful frequency planning could eliminate concerns about PIM. Unfortunately, in LTE and 4G networks, the probability of interference is enhanced—making PIM a larger concern.
PIM is caused by a myriad of things, most of them sitting in plain view: site guy wires, rusty materials, poor quality plating on connectors, and contaminated conducting and dielectric materials. In today’s systems, there are more components than ever in the RF transmission path, including cables, antennas, and amplifiers. Each additional component is another potential source of signal degradation. This multiplicity of components contributes to increasing loss and liabilities in system performance.
Components of antenna systems should be a PIM focus during installation. Installers should look for poor quality and damaged jumpers, adapters, and connectors. Furthermore, while analyzing the overall system performance of a cluster of cell sites, engineers can help reduce the threat of PIM by carefully and methodically testing base station equipment.
Find an FAQ on Interference at http://www.lbagroup.com/associates/intfaq.php.
Monitoring and field testing of PIM has changed through the years. Historically, PIM of components was measured and evaluated within a properly isolated anechoic chamber. Various ad hoc field tests were used to attempt to isolate egregious local problems. However, new test equipment and procedures allow detailed on-site antenna measurements with portable PIM generators / test analyzers. However, where PIM is emanating from non-system components, the old field tests and engineering experience may still be needed to find the offending generator!
Some other suggestions for avoiding PIM include properly installing coaxial cable connectors. A clean, square, properly deburred, and debris-free cable is required for low PIM. Corroded joints that radiate intermodulation distortion products obviously should not pass muster. Even rust on a tower itself can be problematic in respect to PIM. For antennas to operate as designed for maximum RF propagation, no internal or external corrosion or unwanted mechanical stresses should be acceptable. Rooftop and stealth environments have their own problems like loose air handler panels, signage, supports, and stealth materials themselves.
For mobile wireless networks to perform properly and reach their full operating potential, each included antenna must also comply with its design standard. When an overall antenna system does not operate to its standard in conjunction with the base station equipment, several detrimental outcomes can occur: lost revenue, customer complaints resulting in churn, and increased infrastructure cost, as well as a reduction in cell site coverage area. For these reasons, PIM is a growing concern among wireless operators.
LBA Group Inc. offers RF engineering design and performance consulting that can assist with the process of producing and maintaining clear signals. These solutions include distributed antenna systems (DAS), RF safety compliance and training modules, and PIM evaluations. For more information contact Mike Britner at 252-757-0279 or firstname.lastname@example.org.
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