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PIM generators include ferromagnetic materials, galvanic contacts, solder joints, screw connections, conductive coatings, rust, and antenna systems. Such concerns have driven manufacturers of RF/microwave passive components to design low PIM-performing products. Low-PIM construction requirements involve carefully designing every interconnection between components for low PIM and choosing materials that possess very low PIM.

Low-PIM parts like adapters, couplers, splitters, attenuators, switches, cables, and antennas are common. Designing for PIM can be a costly design process and expensive to implement. Generally, using low-PIM components does not ensure that a telecommunications site will be resilient to PIM.

External PIM generators as well as improper installation can be a large factor in producing PIM. Highly cluttered environments in particular are troubling for DAS and small-cell installations. Certain antenna designs, such as omni-directional antennas, could suffer from PIM without the ability to remove offenders from the environment. These factors have created an attractive market for quasi-omnidirectional antennas. Such antennas are composed of several highly directional, offset-radiating elements combined to form an envelope response that is similar, but not omni-directional.

With a common three-lobe structure indicative of such antennas, the system can be rotated to position a PIM generator in a null zone between lobes. This option still offers reasonable coverage in null zones for mobile devices, as the compound reflections could provide adequate reception. Scenarios like these make quasi-omnidirectional antennas a potential PIM fighting solution for fixed small-cell locations like telephone poles, street lights, power lines, and building edges.

Generally, DAS antennas have limited freedom for positioning. But a few tens of centimeters of location change could enable significantly lower PIM response. Some installers will do a PIM sweep when they install the DAS antennas and use an antenna on a movable pole to sense for the lowest PIM position. A technique like repositioning is effective with lower-power antennas, as PIM generators further from the antenna reflect less power and the separation distance grows. Clever methods, like this PIM spotting, may be essential for DAS installations for services like LTE-A and WiGig.

As of September 2013, a report by Network Digest noted that 1064 LTE mobile devices had been announced by 111 different manufacturers. This represents approximately 150% annual growth. Also, the Global Mobile Suppliers Association predicted that 260 commercial LTE networks in 93 countries would be in operation by the end of 2013. The network density needed to support the predicted traffic volume will only add to the requirements for PIM-driven design.

To enable LTE-A’s and WiGig’s high data rates, for example, the resistance of the modulation methods to noise and distortions has been compromised. Another factor to consider is the enhanced sensitivity and broader-band operation necessary for receivers in high-throughput services. Other methods to increase data rates proposed for LTE-A, such as carrier aggregation (CA), further weaken the service to PIM-based distortions.

CA is a proposed addition to the LTE-A standard that allows for multiple carrier reception. This method would increase the apparent throughput of data to the device. Yet this approach becomes another PIM concern, as several powerful antennas would need to broadcast in a single area and on different frequency bands to enable operation.

If a location had any PIM generators in the vicinity, it could potentially induce PIM responses to several of the transmitted signals. This would significantly hamper uplink data rates and could potentially disable the ability of one or multiple carriers to receive. To avoid this, a concerted effort from the installers and maintainers of these telecommunications systems would need to be arranged.

The goal of this effort would be to limit intersystem PIM production. This may be an impossible effort to undergo, as many system installers would be adding new systems or revamping old systems throughout an area. Different service providers also may be operating within the same area. As there is no standard for PIM maximums for external devices (and only for the maximum power output of transmitters), inter-site interference may become a significant issue. Such scenarios call for the development of an inclusive and extensive PIM standard.

The International Electro-technical Commission (IEC) developed a PIM testing standard in 1999, choosing two 20-W carriers with 5-kHz bandwidth as the standard for measuring PIM response. Additionally, third-order intermodulation distortion is designated the qualifying product for PIM response for mobile applications. The IEC produced an update to its standard in 2012, describing in more detail the process in which antennas, cables, connectors, filters, and assemblies would be tested (including dynamic testing). Considering the rapidly changing technologies, however, this standard may not prove adequate for PIM responses for many applications.

Applications that may require more stringent PIM maximums and testing procedures include those that use higher carrier power or where multiple carriers are operating, the bandwidth of the carrier is greater than 5 kHz, and intermodulation products may be of concern. The standard’s testing protocol also omits reasonable descriptions for vibration, wear and tear, installation error, temperature, and materials. Sweeping signal frequencies is another test typology that may reveal PIM, which might not be as easily identified under standard two-tone tests. An additional consideration is three-or-more tone testing for dense signal environments.

As the frequency spectrum is filled with greater traffic and data demands drive up transmitter and carrier densities, PIM-induced system degradation is of increasing concern. A system composed of low-PIM-designed components may not ensure a low-PIM final installation. DAS and small-cell systems may also be more prone to PIM losses, as the necessary receiver sensitivity makes them less resilient to the poor SNR caused by PIM. More detailed best practices for system design and installation—along with less forgiving PIM standards—may be an industry requirement in the near future to enable the latest wireless standards.

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