RCR Wireless hosted a Local Speaker Series on November 9 in Dallas under the title “Mobile Broadband: Innovation and Opportunity.” I was one of three Andrew Solutions folks on a panel that fielded a variety of questions about the status of 3.5G and 4G network roll-outs, the role and requirements of in-building systems, and the need for more green, energy-efficient network solutions. But one of my main takeaways was the continued concern about PIM interference in next generation networks.

If you’re not familiar, PIM is passive intermodulation distortion, which is a type of wireless interference that can degrade signal quality at a cell site. PIM usually results from wireless signals mixing together; if they are in the same frequency as the desired wireless signal, PIM interference causes disruption. PIM is especially of concern in next generation networks because it is easier to disrupt data signals to a point where subscribers experience poor quality. That’s why PIM continues to be a main issue when it comes to quality of service and network optimization.

So how do you prevent PIM?

Well, there’s no one trick, since all sources of PIM in the entire cell site combine to yield the total PIM distortion for the system. Some keys to limiting PIM’s impact, though, include:

• Start with quality products ideally envisioned as a complete system, where connections will be clean, properly torqued, properly attached, and resistant to material degradation.

• Make sure products are installed correctly. Probably the most significant causes of PIM are due to improperly installed equipment. Putting together a clear RF Path requires definite know-how and experience. Make sure you are using qualified, well-trained installers.

• Remember that testing PIM levels is challenging because readings are sensitive to test equipment and other surroundings that can increase PIM, resulting in false failures. If you aren’t familiar with PIM testing requirements, you might want to check out the Andrew Institute’s new PIM class.

Obviously this is a pretty cursory overview of a complicated and reasonably concerning topic. PIM has been on Andrew Solutions’ radar for awhile, but there’s always more to be learned. As my experience at the recent RCR event attests, PIM is still on a lot of people’s minds.

What’s on your mind regarding PIM and the networks of today and tomorrow?

About the Author

Ray Butler

Ray Butler is vice president of Mobility Network Engineering at CommScope, responsible for wireless technical sales leadership in outdoor RF products. Before, Ray led the R&D team responsible for base station antennas, filters, combiners, remote radio heads and RF power amplifiers. He previously worked for Andrew Corporation as vice president of Base Station Antennas Engineering as well as Systems Engineering and Solutions Marketing. He has served as director of National RF Engineering with AT&T Wireless and vice president of Engineering, Research and Development, and International Operations at Metawave Communications, a smart antenna company. Ray was technical manager of Systems Engineering for Lucent Technologies Bell Laboratories, having also held other management positions responsible for the design of RF circuits, filters and amplifiers. Ray holds a Bachelor of Science degree in electrical engineering from Brigham Young University and a Master of Science in electrical engineering from Polytechnic University, and is a member of national engineering honor societies Eta Kappa Nu and Tau Beta Pi.

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2 comments for "Interference Concerns Still Top of Mind"
Sibasis Das

Hello Ray, Excellent efforts by Andrew in spreading awareness about PIM and provide real-time solutions and field testing. I had always wanted to see such an effort by any of passive OEMs, but hats-off again to Andrew for stealing the show! I do have a thought to share. I work for a BTS Antenna company in Tech Sales, in India ( am one of your competitors as well!), and am sorry to see very less and in fact no knowledge of PIM among Operators and OEMs like NSN, Ericsson in India. People are fantasized by VSWR and harp upon it endlessly, but no one pays attention to PIM. In fact, my question is when operators in India are steadfastly refusing to do trial of 6-port antennas for fear of outage, will they have enough motivation to allow Andrew to do PIM testing on site, which will involve removing antennas ( for doing fault analysis by POE), and of course the cost involved? My suggestion will be to try for minimizing timeline required for PIM testing on sites and also reduce the cost factor for PIM testing on sites. This will be a very high-end field test and people are bound to have apprehensions initially. Ignoring the competition between our companies, I really liked the PIM initiative started by Andrew and will like to see such high-end activities starting in India as well. Best Regards, Sibasis Das +91-9911418821

Ray Butler

Thanks for the comments. We are seeing the level of understanding and awareness of PIM in all parts of the world increasing. While the Return Loss sweeps will always be an important measure, there are potential issues in the RF path than can only be caught with a PIM measurement. One example is loose connectors in the RF - that were not properly torqued - and will cause network degradation if not immediately then over time. The PIM measurement is much more sensitive to component, assembly and installation errors. One solution to the PIM testing on site is to test the entire RF path with the antenna installed. The test frequencies need to be chosen carefully so as not to violate any regulatory agency licensing requirements. Also, it is typical to reduce the specification pass/fail threshold from the level for an individual component because of the summing of multiple PIM components in the path. By doing this, the overall timeframe can be reduced for on-site testing. I think as we work together to increase awareness of PIM, the net result will be improved wireless network performance and happier customers. Ray

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