Passive intermodulation (PIM) has always been around in wireless networks, and it has become a real problem as networks adopt higher orders of modulation and additional frequency bands. Yet many industry professionals are unaware of certain tricks of the trade that must be considered when field testing for PIM, according to Ray Butler, vice president of research and development for CommScope's active wireless products team.
PIM can result in degraded voice quality, dropped calls and reduced data throughput. PIM has become a significant problem with LTE not only because wideband channels are used but also because an entire band is used for transmit and receive functions.
"So any combination of signals in the transmit band could end up in the receive band, and they'll spread across the entire receive band," Butler told FierceWirelessTech. "And since you're only using one channel, you can't frequency-plan around it with LTE or 3G."
In addition, higher orders of modulation, such as 64 QAM, used with LTE are particularly sensitive to interference. Further, as operators deploy more frequency bands over which they deliver LTE services, there are increased opportunities for PIM interference. And the impact of PIM affects small cells as well as macrocells.
According to Butler, PIM can be caused by all kinds of "contaminated surfaces," including loose or dirty RF connectors. To avoid creating PIM in its antenna products, CommScope tries to avoid metal-to-metal connections, he said. For example, the company's SiteRise standard interface connects a remote radio unit (RRU) to a base station antenna via connector technology that provides an essentially PIM-free, blind-mate connection.
When it comes to field testing antennas, accurate results require testing in a pristine environment free of PIM sources that might invalidate the findings. Butler described testing conducted by his team in which false PIM failures were caused by things as simple as pointing the antenna at a fence or forklift, or having a man stand next to the antenna with a cell phone, keys and other items in his pockets. In fact, test equipment itself can even influence the results.
Butler said portable PIM chambers have started being used in the field so proper testing can be conducted without the impact of nearby metal objects in the environment.
In a white paper he wrote on PIM testing, Butler recommends installers validate their test equipment, use a low intermodulation (IM) load from the supplier of the IM test equipment and avoid resistive load elements such as braided cables, ensure cable ends are properly prepped, minimize the number of cable adapters, as well as ensure all connectors are properly torqued and not just hand-tightened.
- see this CommScope blog post
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