Harmonic Convergence? FM Interference to 700 MHz LTE Service

700 MHz licensees (with some apparent FCC support) complain of FM interference to hypersensitive LTE gear – but must FM licensees foot the bill to correct the problem?

The introduction of different species into an established ecosystem tends to be a dicey proposition.  Almost invariably, co-habitation requires the sharing of scarce resources.  And more often than not, the different species approach the whole sharing thing in different, not entirely compatible, ways.  The result: occasional dissatisfactions and frustrations – leading to occasional inter-species frictions and fisticuffs.

Take the RF spectrum ecosystem, for example.

Most inhabitants of the spectrum have historically figured out ways to coexist in relative peace (at least for the most part) – thanks largely to the fact that the potential impact of one service on another has been taken into account in the frequency allocation process. But as the demand for spectrum increases, and every little niche is filled up, it is becoming more difficult to avoid inter-service conflicts. And sure enough, the introduction of a recent new species – 700 MHz wireless systems using LTE equipment – seems to be causing some unexpected problems. 

Since January, 2012, spectrum that used to constitute TV channels 52 and up has been reallocated to 700 MHz wireless services. Television still occupies channels 51 and down (at least for the time being), and there has been much hand-wringing over how the relatively low power wireless services will be able to coexist in such close proximity to high-powered TV stations.

Now it turns out that another problem – less anticipated – has reared its ugly head. Wireless operators using high gain LTE antenna systems and high gain LTE receivers have experienced interference which, they claim, is caused not by TV but by nearby FM stations. 

FM stations? How can that be, since FM stations operate in the 88-108 MHz band, far away from 700 MHz?

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.

Wireless carriers have recently complained to a number of FM stations, demanding that the FM stations suppress their harmonic in the 700 MHz band. In at least one instance, that has led to the FCC’s issuance of an official Notice of Violation (NOV) directed to the FM station.  According to the NOV, the FM licensee is somehow violating the rules and is supposed to be taking corrective actions.

The problem is that it’s not at all clear that the FM licensee has done anything wrong.

According to the NOV, the FM station has been violating Section 73.317(a).  Allow us to quote that section in its entirety, so we’re all on the same page here:

(a) FM broadcast stations employing transmitters authorized after January 1, 1960, must maintain the bandwidth occupied by their emissions in accordance with the specification detailed below. FM broadcast stations employing transmitters installed or type accepted before January 1, 1960, must achieve the highest degree of compliance with these specifications practicable with their existing equipment. In either case, should harmful interference to other authorized stations occur, the licensee shall correct the problem promptly or cease operation.

You’ll note right off the bat that this section does not itself impose any particular operating limitation on FM stations; rather, it requires that they maintain their occupied bandwidth “in accordance with the specification detailed below”.  From what we hear from our friends in the consulting engineering universe, FM transmitters these days easily meet the various “specification[s] detailed below” in the rest of Section 73.317.

Section 73.317(d) tells FM licensees how strong harmonic emissions can be, and there’s no indication in the NOV that the targeted station was violating that particular standard.  Stations powered at 5 kW or more are required to suppress harmonics by 80 dB. In other words, if the 8th harmonic of a 50 kW station is 80 dB below the carrier on the main frequency – around five ten-thousandths of a one watt, or 0.0005 watt – the rule is satisfied.

One of our long-time friends, Gary Cavell (of the eponymous Cavell Mertz & Associates – a swell bunch of folks and excellent engineers, to boot) suggests that the interference observed by the wireless operators may arise from the extreme sensitivity of the LTE high gain antennas (juiced up by high gain amplifiers) they’re using.  Such gear provides reliable service from handsets operating at a distance from the LTE towers, so it’s attractive to wireless providers because it reduces the number of cells required to cover an area (yup, it reduces costs).  But that sensitivity can result in the LTE systems being 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.

If there is an interference problem here – and there may well be – is it the fault of FM broadcasters who may not be in any violation of the FCC’s Rules; or is it from the hyper-sensitivity of the wireless equipment in an existing RF environment that the FCC has blessed for decades?  

As it turns out, the FM emissions that appear to be causing the problem here may not be coming out of the FM antenna at all.  Rather, according to Professor Cavell (whose team has been looking into the issue), the emissions may be leaking from the FM transmitter cabinets, even when those transmitters are fully compliant with all technical specs.  Keep in mind that FM transmitter manufacturers have designed their equipment to comply with the FCC’s rule, not the demands of wireless carriers.

Why not just put up a shield to block the undesired emissions? Gary reports that shielding a (supposedly) interfering transmitter’s air intake and exhaust areas with screening seems to help some, but installation of a full-fledged Faraday Cage seems to do better.  Bad news: as Professor Cavell put it so that we could wrap our non-engineering minds around it, going that route “is not a cheap date”.  And whatever fix may eventually be used, it’ll cost time and effort on the part of the station’s engineer to figure out how best to mitigate the problem.

So the real question is who should be responsible for fixing whatever problem exists?  For the last several decades, at least, the Commission has imposed a “last-in” policy to handle interference problems that arise when one spectrum user’s newly-commenced operation causes or receives interference from other nearby spectrum users.  If all the various players are using gear that complies with all applicable rules, the “last-in” policy calls for the new kid on the block to fix things.  In the FM/700 MHz LTE situation, that would be the 700 MHz folks. (Of course, to avoid the problem in the first place, 700 MHz operators might want to opt for antenna sites that don’t happen to be close to any FM station whose 8th harmonic falls in the 700 MHz’s mobile-to-base band – if, that is, such sites happen to be available.)

But if the “last-in” policy applies here, the NOV doesn’t make much sense.  It seems, in knee-jerk fashion, to pin the blame on the FM broadcaster.  Exactly how the Enforcement Bureau’s Northeast Office reached that decision is not clear.  If the Bureau really thinks that the FM station’s equipment doesn’t satisfy the rules, it should say why it thinks that. 

But simply citing Section 73.317(a) without reference to any of the other, substantive, portions of that rule doesn’t seem to do the trick unless the Bureau has, without telling anybody, decided that the “last-in” policy is no longer in effect – or that the policy doesn’t apply when the “last-in” party happens to be a wireless operator and the other party is a mere FM broadcaster.  That would be unfortunate – and possibly unjustifiable, if it ever got to court without further due process, say, an intervening rulemaking to afford everyone adequate time to implement any new standards. Keep in mind, though, that the NOV came from an FCC Field Office; we don’t know whether they consulted with the FCC folks at home base in Washington. We also have not yet heard from the Media Bureau, which ideally isn’t likely to be in a hurry to put the squeeze on stations whose equipment is operating as designed and in compliance with the rules.

We hear that, in other situations, the FCC has not yet been called in.  Instead, some 700 MHz operators have sent their own nasty-grams to the FM stations calling on the FMers to correct the interference as if it’s a given that the FM licensee is responsible.  The good news there is that, in at least one such case that we’re aware of, the 700 MHz folks have seemed to be open to reason when the rules (and the longstanding “last-in” policy) are explained to them.  Of course, in such instances it’s useful – and probably the Right Thing to Do – for the FM operator to be cooperative in efforts to identify the precise source(s) of the interference and devise ways of fixing things.  But that cooperation does not necessarily require the FM licensee to bear any financial expense in that process, particularly if the FM licensee’s equipment complies with all applicable rules.

With the increasing deployment of 700 MHz operations nation-wide, it’s likely, if not certain, that this type of set-to will recur repeatedly.  FM licensees would be well-advised to consult knowledgeable engineering and legal counsel if a complainant (or the FCC) comes knocking on their door.

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Comments (6) Read through and enter the discussion with the form at the end
Tom Taggart - June 26, 2013 4:09 PM

Follow the big money. FCC has been trying to grab and resell UHF TV channels to the wireless folks for the last 5 years. FCC --and this administration, would shed no tears if all private broadcasters went away.

Steve Crowley - June 27, 2013 5:28 AM

It seems to me that regardless of last-in policies or specific emission limits, FM stations are still faced with this from Section 73.317(a):

"In either case, should harmful interference to other authorized stations occur, the licensee shall correct the problem promptly or cease operation."

Peter Tannenwald - June 27, 2013 7:30 AM

The question is whether the word "pre-existing" should be read as appearing before "other stations." FCC general interference principles do not obligate stations otherwise in compliance with the FCC's Rules to protect newcomers. They must cooperate, but that does not mean yielding to an incompatible new operator.

Jeremy Lansman - June 28, 2013 7:19 AM

So, I see Verizon is only 500 feet away. Harmonics can be generated by third party equipment, such as a nearby piece of metal touching another piece of metal, or a poor coax connector in one or the other system, or in a third party system. This sort of thing should be resolved engineer to engineer. Interference in the near field is not infrequent. What is infrequent is dragging the FCC into it. What went wrong was more than technical.

Peter Tannenwald - June 28, 2013 7:57 AM

Agreed, but we have had independent professional engineers on site from both the wireless carrier and the FM station in one case I am working on. So far, we have not found a solution, but we are not finished with our efforts.

Brad Lincicome - November 21, 2013 10:04 AM

So, I am developing a technology that has the potential to be interfered with by a medium that has been around for decades. Therefore, I have the right to make that medium change it's longstanding practices, simply because I did not foresee what impact that existing practice would have on my new technology.

What's wrong with this picture?

Sad.

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