May 17, 2016
After a slow start, small cell deployments are finally beginning to take off in many regions across the United States. Due to the high volume of sites required and the fact that these sites are much closer to the ground, site appearance is playing a significant role in the approval process. Cities like San Francisco have published guidelines for wireless operators and FAQs for their citizens to communicate aesthetic requirements for Small Cells on Street Light and Transit Poles.
In many cases, operators are deploying small cells on existing infrastructure such as light poles (figure 1, at right) or utility poles within the community. To improve the aesthetics of these sites, small diameter, cylindrical antennas are often deployed with shrouds to hide RF cabling and combiners. Radios are either discretely mounted to the side of the pole or installed in radio cabinets on the ground. To prevent the site from looking like a lollipop, operators are often encouraged to mount antennas at an elevation approximately equal to the existing streel light luminaires. While consumers will like the appearance of the site, they might not be thrilled with the performance of their mobile device. That’s because passive intermodulation (PIM) can be a significant issue in this scenario.
Let’s shed some light on why PIM is a potential problem in these installations. Most modern luminaires are constructed using arrays of Light Emitting Diodes (LEDs) to reduce energy consumption. Placing LED lights in the main beam of a small cell antenna is just asking for trouble. PIM is known to be generated by “environmental diodes” such as loose metal-to-metal contacts and rusty metal surfaces. When actual diodes are placed in the antenna main beam, PIM levels can be high enough to cripple small cell performance, especially at sites with high transmit power.
A PIM Experiment
An obvious question is, “how close can I place an LED luminaire to a small cell antenna and still achieve acceptable results?” To test this, an experiment was conducted using a typical cobra-head style luminaire and a 2’ tall, 65° panel antenna. The luminaire was placed at 3 elevations relative to the antenna – straight in front of the antenna, 2 feet below the antenna and 4 feet below the antenna. For each luminaire elevation, the antenna was moved in even steps away from the luminaire. PIM was measured using a 700 MHz and 1900 MHz PIM analyzer. A test power level of 5 W was used to represent typical small cell radio transmitters.
Not surprisingly, the results (figure 2) clearly validate that PIM can be a serious issue with LED style luminaires, especially at lower frequencies. Assuming a pass/fail level of -115 dBm, the luminaire should be located at least 2 feet below the antenna and a distance of approximately 6 feet beyond the antenna to prevent harmful interference. Farther distances will be required for higher transmit power and when higher gain antennas are deployed.
An analyzer such as the Anritsu PIM Master™ MW82119B battery-operated, high power, portable passive intermodulation analyzer is well suited for validating PIM performance of small cell sites deployed on light poles. Battery operation allows the test to be easily conducted at the small cell radio output and features such as Distance-to-PIM (DTP) enable operators to quickly locate PIM sources beyond the antenna. For more information on how to test passive intermodulation, visit our PIM technology page.