Much of our consulting work involves tracking down interference sources and resolving interference problems. We perform some of this work for public safety agencies, but most of our interference hunting is done for the four national wireless operators (AT&T Mobility, Verizon Wireless, Sprint and T-Mobile).
The interference problem is multifaceted. Recent work for the City of Oakland involved interference from intentional emitters (cell sites) that affected public safety radios operating on adjacent frequencies. In this case, the receiver has only a finite capacity to reject signals on nearby frequencies (i.e., no filter is a “brick wall”) and the growth of cellular sites and numerous radio carriers at each cell site contributes to this problem.
Interference at the cell site (as opposed to from it) is no less important to the wireless operator. Most of this interference originates from unintentional emitters. Our firm specializes in hunting down and eliminating interference. The problem primarily occurs in the 800 MHz cellular reverse link (cell phone to cell site) bands, but we also see interference in the 700 MHz LTE bands and occasionally in the PCS and AWS bands. This interference comes from a variety of sources, including the following:
- Passive intermodulation (PIM) at the cell site, caused by unintentional non-linear mixing of transmit frequencies in corroded joints, loose connectors, etc. This is often the easiest to identify provided one has the proper test equipment.
- Booster amplifiers (sometimes called BDAs) that sometimes oscillate when there is inadequate isolation between input and output. Some of these are found in residences and inside vehicles and are designed to boost cell phone coverage for the user.
- Utility power transformers (occasionally)
- Florescent light ballasts (occasionally)
- Cable TV set top boxes, especially those used in hotel rooms
- Cordless phones and any devices operating in the 902-928 MHz license-free band
Pericle uses sophisticated software and test equipment to identify and track down interferers, but experience is often the most valuable tool. The work is inherently labor intensive, but one can get to the root of the problem quickly by recognizing potential interference sources by sight. Direction finding is an important first step, but it does not solve the problem completely. One must still track the interference down and correct it. Our engineers can tell some harrowing tales of uncooperative property owners.
One recent interference source was a cellular booster amplifier at a U.S. Park Service building at Moose Crossing in the Grand Teton National Park. See below. This amplifier was causing interference to multiple cell sites in Jackson Hole.
We do this work across the western U.S., including Colorado, New Mexico, Wyoming, Utah, Idaho, Montana, Nevada, southern California, Washington State, Oregon, Alaska and Hawaii.