FM Interference to 700 MHz LTE Service in Question

Wireless companies with 700MHz licenses have complained of FM interference to their LTE service.

Fletcher Heald & Hildreth’s Peter Tannenwald blogs about whether FMs should be forced to pay to correct that.

The spectrum in question used to be TV Channels 52 and up. That’s been reallocated to 700 MHz wireless services.

Wireless operators using high-gain LTE antenna systems and high-gain LTE receivers say they’ve experienced interference from nearby FMs, but Tannenwald says you might wonder how that could be, since FM stations operate in the 88–108 MHz band. However, “every radio transmitter emits not only its primary signal but also multiples — two times, three times, four times the frequency and on up. Do the math: stations operating anywhere from 88.1 MHz to 100.5 MHz will generate 8th harmonics somewhere in the 700 MHz wireless band.”

The wireless carriers want the FMs to suppress their harmonic in the 700 MHz band. The Enforcement Bureau has told at least one FM it has the responsibility to correct the problem, but Tannenwald says it’s not clear the particular station is at fault.

He asked the advice of Gary Cavell of Cavell Mertz & Associates, who points to the “extreme sensitivity” of the LTE high-gain antennas. Cavell notes the gear provides reliable service from handsets operating at a distance from LTE towers, “so it’s attractive to wireless providers because it reduces the number of cells required to cover an area.” But that also means “the LTE systems can be disturbed by FM emissions well below the floor that FM stations are required by the rules to maintain. We have heard of at least one wireless carrier demanding that the FM station suppress harmonic radiation to -105 dB, or less than 2 one-millionths of a watt (0.000002 watt) for a 50 kW station.”

Cavell and his team believe the emissions may be leaking from the transmitter cabinets. How to fix this? Shielding may block the transmitter’s air intake but shielding the exhaust areas with screening may help. A Faraday Cage seems to do the trick, Cavell believes. 

Getting back to who is responsible for mitigation, Tannenwald says past FCC policy has been to rely on making the last group that shares the particular spectrum responsible for mitigation. In the FM/700 MHz case, the wireless carriers would be “last in,” he says, however the Enforcement Bureau seems to have reached the opposite conclusion in the specific situation he’s referring to.

Stay tuned to see how this shakes out, he advises, as more 700 MHz operations are deployed.


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Comment List:

What, -105dB ? Tell me of one cable system that contains their signal to that level? Part 15 type rules should apply here where 100mW is a more reasonable leakage signal. And, when in doubt, I feel that "we were here first" rules should usually apply. An FM station with no changes since 1972 shouldn't have to remediate a 700MHz issue; but that's just my opinion.
By Bob Veeus on 6/28/2013

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