Interference Hunter


Interference hunting

Hunting down and eliminating interference

When we launched our US telecommunications practice in 2009, 3G was still the dominant technology. But with the deployment of 4G, the carriers needed additional spectrum to cope with the explosion of data traffic. Many additional spectrum blocks were re-purposed for 4G, namely in the 700MHz, AWS and PCS bands. Carriers invested billions of dollars. However, not all previous users vacated their bands and many cellular sites were subject to severe interference of unknown origin.

We initially pitched the idea of hunting down and eliminating interference as a professional service to AT&T in the New York area on a Friday in 2010. We got invited to a meeting the next Monday and from then on, we expanded this service across the continental US and across all major US carriers (except for Sprint initially).

Typically, when a site becomes affected by interference, it raises alarms. Symptoms are bad throughput and dropped calls. Once the carriers’ engineers investigate, they can see that the source of the problems lies in the spectrum: someone or something is interfering with the frequencies and making so much noise, that cell phone and tower cannot talk to each other anymore.

When our engineers start investigating the site, we “climb" to the roof or the top of a tower (we don’t actually climb) and measure the incoming spectrum using portable spectrum analyzers. These units have highly directional antennas, so we can point them in different directions to see where the noise comes from. This is where the hunt begins: climbing different elevated locations, we try to get a cross-bearing of the source. Once we have identified the building, the hunt continues indoors until we find the culprit device. Sometimes, the interference is only intermittent, which makes targeting the source really hard.

It’s a real end-to-end service: we localize the emitting device and try to convince its owner to shut it down. Typically, conditions at the cell site improve once the correct source is eliminated.

In rare cases, when owners of an interfering device refuse to shut them down, we hand the case over to the legal department of the rightful spectrum owner. In that case, it is the FCC who has the final say.

Over the last 10 years, we have found and shut-down all sorts of interference sources: wireless microphones in churches, non-FCC-approved video cameras (and DECT phones), cable-TV cables without proper shielding, soda vending machines, FM stations and many more. Probably the most spectacular hunt was when we found the LEDs of a new large video wall on Times Square in Manhattan to be emitting in the 700MHz band, effectively blocking the service. We had to climb into the video wall, many floors above ground, to prove the interference to the owner. In the end, the wall was shut down and the LEDs were replaced.

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