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Several years ago, passive intermodulation (PIM) was a virtually unknown performance metric in distributed antenna systems (DAS). Today it is increasingly recognized as one of the most critical requirements for optimum system performance. Hypersensitive antennas and radios, multiple frequency overlays and more components in the RF path create an environment in which the margin for error regarding PIM continues to shrink. Given the high susceptibility of current DAS systems, even small levels of PIM distortion can significantly impact network performance, as measured by upload speed.
Outdoor macro sites were the first deployment scenarios where PIM issues had to be tackled. High power levels from the base transceiver station ports and a more complex RF path to the antennas—including jumpers, filters and tower mounted amplifiers—contribute to generating PIM, which can be very detrimental to the quality of wireless service. Due to the limited uplink transmit power of mobile terminals, the uplink receive sensitivity is a critical parameter to optimize for outdoor scenarios to allow a balanced downlink/uplink maximum path-loss. Best practices for macro site deployments have been defined over the past few years.
High and reliable data throughput values are even more important in DAS environments, such as stadiums, where there are many components in the RF path that can contribute to PIM generation. The minimum PIM specification for each and every component is improving continually. PIM specifications for RF components (splitters, couplers, etc.) and antennas have transitioned from –140 dBc to –150 dBc and now are moving to –153 dBc and –160 dBc. With the passive components—such as splitters, hybrid couplers, and directional couplers—being placed closer to the signal sources in these systems, it is critical that the PIM specification for these devices are at the highest levels.
Passive components used in RF signal distribution networks have wideband frequency support. Therefore, multiband and multicarrier signals from DAS remote unit output ports can mix together at every passive stage and generate a large variety of detrimental PIM products falling in multiple uplink bands. As such, PIM requirements for these passive components must be more stringent.
Want to learn more? Read my new white paper “PIM Requirements Must Increase to Support Evolving DAS Systems” (brief registration required). Leave me a comment if you have any questions, and I will be sure to respond.