Can You Hear Me Now? LTE Interference.
This article deviates significantly from my usual Tips & Tricks. The subject is much more detailed, because the topic is rapidly becoming an issue that many broadcasters are, or will be, facing at some point. It requires a lot of information from sources well beyond my usual range of expertise, so I’ve relied on several experts who are much more versed than I am on this matter.
There has been much discussion recently about LTE interference; problems being experienced and reported by mobile carriers as interference to the receiving equipment in their facilities. This can be a challenge for FM broadcast stations, because some of these receivers are trying to pick up a signal that’s well down into the noise floor; -100dB or even lower in some cases. This means that, even if your station is totally legal according to government requirements, you could still have a harmonic in the LTE band (700-850 MHz, depending on your geographic location and the carrier involved) that is high enough to create an issue.
Obviously, the mobile carriers have an interest in putting transceiver sites as far away from each other as possible; the fewer that need to be installed, the lower the costs. In addition, time commitments may play a factor – so again, fewer sites equates to less time. Finally, in defense of the cell carriers, it is safe to assume that they have to deal with the same challenges erecting towers as broadcast stations; the “not in my backyard” syndrome, so fewer sites translate to fewer challenges in that regard. The end result of sites being spaced as far apart as theoretically possible is that signals arriving to them from handheld devices are at extremely low levels – hence the necessity to be able to receive a signal that is near the noise floor.
A lot of the talk I see, points to Section 73.317 of the FCC’s rules. Section 73.317(d) requires that an FM transmitter harmonics be more than 80dB below carrier. However, there is also a sentence in Section 73.317(a) which states that, “should harmful interference to other authorized stations occur, the licensee shall correct the problem promptly or cease operation.”
To try to sort out this apparent discrepancy between Sections 73.317(a) and 73.317(d), I turned to attorney John Garziglia of the Washington, DC firm, Womble Carlyle, which represents numerous broadcast stations. John says that with respect to Section 73.317 compliance, the FCC’s Media Bureau takes the position that, if an FM station is in compliance with the spurious emissions limits stated in Section 73.317, there is no FCC issue. In support of this proposition, the FCC’s Media Bureau, if asked, will point to a 2007 spurious emissions case regarding WBTG-FM, Sheffield, Alabama (available on the FCC website). While this case involved allegations of first-adjacent, rather than harmonic, interference, this case supports the general proposition that if spurious emissions are below Section 73.317 limits, then there is no FCC issue.
One other argument that has been posted is the FCC’s “first in” policy, where if your station was on the air first and the cell carrier installed the LTE site later, then it is the cell carrier’s responsibility to remedy any interference they might see. John notes that this policy has its genesis in the FCC’s 1947 Midnight Sun Broadcasting Co. case (available on the FCC website). In this AM licensing case from Anchorage, Alaska, a newly-granted construction permit was conditioned upon the new facility not causing any cross-modulation or intermodulation interaction between two AM transmitters with the new facility permittee responsible for correcting any issues.
While the FCC’s “first in” policy is a good argument if you are the existing FM station with a new LTE facility locating nearby, what happens if you decide to upgrade your transmitter at some point in the future, or replace the antenna? At that point, from an equipment perspective you have just become the “new kid on the block”. From the legal perspective, John observes that under the FCC’s “first in” policy, even assuming the new transmitter or antenna is Section 73.317 rule-compliant, there arguably could be an FCC issue. But this is also where local law may come into play. There are a variety of legal theories regarding nuisance which might entitle an aggrieved party to damages or relief from interference circumstances created by a newcomer.
In the case of an existing LTE facility and a new FM station, or a change in an existing FM’s emissions due to a facility change, the existing LTE licensee may very well have a civil claim against the FM licensee if interference is caused to LTE reception. John observes that if he was representing the FM licensee in such a circumstance, his first retort would be that this issue is controlled by FCC rules and policy. But, a local court may not be willing to defer to the FCC. If there was significant damage caused by harmonic interference to existing LTE operations from newly-generated FM station emissions, a court might find an FM station liable to an existing LTE facility licensee under a nuisance theory no matter how low down below the noise floor the subject emissions might be.
Therefore, it is a good idea to get as educated as possible, whether it should be a problem or not. If you want some additional information, Scott Baxter, a wireless industry consultant in Nashville, presented a paper to the AFCCE (Association of Federal Communications Consulting Engineers) in 2013 that includes a wealth of information on LTE configuration and interference issues, including two case studies. You can find his paper here – it is definitely worth taking some time to read. Scott can be reached at [email protected] with any questions regarding his paper. He’s also indicated that any of our readers are more than welcome to reach out for any additional information, or to peruse the various courses and presentations on his site.
From Scott’s presentation, we learn that, “OFDM-based LTE technology pushes spectral efficiency and data throughput close to the Shannon limit, but in so doing becomes particularly vulnerable to low-level uplink interference.” The Shannon limit is the digital equivalent of the Nyquist theorem – effectively providing a means of calculating the maximum possible data throughput over a communication channel with a specific noise floor. Raise the noise floor and lose data – or add noise in the form of on-channel interference (such as the harmonic of an FM broadcast station) and achieve the same data loss.
Ultimately, there are two potential causes of emissions at the 7th-9th harmonics: radiation through the FM antenna bay(s), or directly from the transmitter cabinet, referred to as ‘cabinet-radiation’. Regarding antenna radiation, this is primarily a function of transmitter harmonic attenuation (filter) and tuning of any additional external components (channel combiners, the antenna itself). Sometimes, adding additional tuned circuits (such as the external low pass filter from a previous transmitter, if available) can help achieve some additional attenuation. In other cases, external reject filters might be necessary. Finally, nothing will stop the fact that we’re radiating several kW of RF and if that is what overloads the receiver at the LTE site, the solution will need to be on their end, although it is worth noting that this is very unlikely to be the case, as LTE equipment has very good rejection of out of band signals.
With respect to cabinet radiation, this is something that is governed by various specifications worldwide. The CE (European) specification is significantly more stringent than the FCC and Industry Canada requirements, so it might be beneficial to confirm that any new transmitter you install is CE compliant. This is NOT a guarantee that there will be no LTE interference, but this is a battle that is fought a few dB at a time and CE compliance is a good start. It’s worth noting here that all of Nautel’s current models meet CE specifications – I believe that’s true of most major manufacturers, but it is a very good idea to verify this when buying.
For older transmitters, Walker Sisson, a contract engineer in southwest Michigan, has provided us with a description of one of his successes in reducing cabinet radiation and overall LTE conflict in a site where the LTE antennas and FM antenna are co-located, about 10 feet apart on the same tower! The transmitter in this case was one of our older FM5 systems, but some of Walker’s efforts could be applied to other systems quite easily. Overall, the efforts outlined below resulted in a decrease of objectionable signal at the LTE receive antenna from a highly objectionable level of -78 dBm to less than -106 dBm, a change of 28dB and getting the FM problem into the noise floor.
Walker stresses it is necessary to take a planned, scientific approach for resolution. Since transmitter manufacturers carefully filter the normal RF output, suspect the cabinet-radiation possibility FIRST. Operate the transmitter into a dummy-load with all of the transmitter’s normal panels in place. If cellular / LTE interference is found while feeding just a dummy load, the problem is ‘cabinet-radiation’ and that must be cured first since it is probably more significant than the RF to antenna. In this case the cabinet radiation from the transmitter on the ground was some 25 dB higher at the LTE receive antenna, 250 feet in the air, than the harmonics out the coax port to the FM transmit antenna, located only ten feet away! This transmitter does a very good job of suppressing radiated harmonics through its amplifier stages, thus any improvements made in this area would be invisible until the first part was done.
One of the first things that Walker discovered was that several of the RF bypass capacitors on the DC power lines to the power amplifiers had failed by vaporizing, such that just the leads of the capacitors remained. That allowed a high level of RF, containing harmonics into the LTE range to be present on the wiring harness. That RF had no filtering and radiated from cabinet openings. Simply going through each amplifier and ensuring that all bypass caps were functional, replacing the bad ones, made a significant difference.
The next step was to provide shielding for openings – the biggest offenders being air intakes and exhaust. It is important to note that this needs to be done with an eye toward NOT restricting airflow, which could end up creating more problems than it solves.
The resolution on the top exhaust port was simple hardware cloth – available at Home Depot or most any hardware store. Walker used a 3-layer sandwich of hardware-cloth on the outside and finer window screen in the center, allowing for physical strength over the large opening (photo 1). Do not overlook the openings on the top of other brands of transmitters.
Always exercise extreme care when working on operating transmitters as lethal high-voltage or current may be present just below the surface where a screwdriver or fastener may venture.
Note that an additional improvement was made by adding hardware cloth around the flange, holding it with a hose clamp and tapering it outward to overlap the horizontal screen on the top of the transmitter, securing with bolts (not shown in photo 1). As mentioned above, every dB counts and there was some leakage around the ground lead and the poor fit to the riser.
Next, Walker treated the air intakes by placing just a single layer of hardware cloth over the air-intake filter frames. Note that this may have an effect on airflow, which may need to be countered by increasing positive air pressure at the air intake, or there would be a risk of increased fan/blower failures. In Walker’s case of the FM5, no change in the exhaust air temperature was measured, suggesting no flow restriction. However, dust build-up will be more critical on the finer mesh.
The next big source of RF leakage was identified as the meters on the front of the transmitter (visible in photo 1). Think of the front-panel cutouts for the meters as RF openings since the plastic meters do no shielding. The solution for this case was to build a shield inside the cabinet around the meters (photo 2) – note the phenolic squares used to prevent accidental contact between the cage and the meter terminals.
Finally, some leakage was detected from the underside of the power modules – Walker’s solution here was some brass stock from a hobby-store on each power module, formed to press against the chassis and held in place by the module handle (photo 3).
At this point, undesired signal at the cell carrier’s receive antenna was down by at least 30 dB from the initial reading when the FM transmitter was operating into the dummy load, as it was into the noise-floor of the test equipment at less than -110 dBm. However, operating into the broadcast antenna revealed interference to the LTE at around -102 dBm, which was a few dB higher than desired. Thus, it was apparent the interference was actually going up the broadcast coax. The LTE carrier mentioned they had supplied a pass-cavity to an FM station. That did not seem to be a practical solution for a harmonic problem – to install a hi-Q device to suppress a ninth harmonic. Walker installed a line-section style low-pass filter left over from an old CCA transmitter and the noise disappeared into the LTE noise-floor. At the end of the night, the cellular techs and their supervisors walked away very happy, making notes about the model number of the old Phelps-Dodge filter from the CCA rig. In Walker’s case he had the advantage of a spectrum analyzer to sniff out cabinet radiation. The involved LTE carrier also was cooperative by having their site tech monitor their equipment’s ‘intake’ of the FM interference on a couple occasions as the steps were taken. Prepare before asking for site-tech’s presence so that progress can be tested without the carrier waiting idly.
One note on this, the cell company involved in this particular case seemed to be quite aware that they were asking for a huge accomplishment, beyond the design parameters of the equipment, and appeared to be amicable to paying for devices to accomplish the improvements they desired. This may not be the case in every situation, but other cases I have encountered have gone along similar lines – so it might pay to be nice and see if there is a way any conflict of this nature can be resolved cooperatively. During a telephone call with Gray Frierson Haertig, a consultant from Portland, he indicated that this was in agreement with what he has seen – that there may be some bluster at the beginning, but in the end, there is a willingness to work with the broadcaster to resolve any situation.
Other options that may be used in some situations would include copper tape on any panel seams where it’s unlikely that the panels will need to be removed, copper finger stock on panels which are removed for maintenance, or even shielding the entire transmitter room, with copper sheets or even conductive paint, such as can be found here or here.
Regarding the conductive paint, to quote Curtis Flick (an engineer from Ohio), “As good as foil, but far easier. Paint the wall, run a strap up the wall 6 inches or so. Secure with drywall screws. Touch-up paint overlapping the strap. Done. In the event the wall cracks, breaking the copper conductivity, slap some more paint over the crack. Fixed. You can do a color finish coat of any latex over it.” Thanks for the tip, Cowboy – remember, you never know who’s paying attention!
Ultimately we are all subject to the laws of physics, so there may well be cases that cannot be solved by any measure of hardware cloth, copper tape, conductive paint for screening rooms or any other solution. However, it is nice to have at least a few ideas for when the situation arises at your station!
Special thanks to John Garziglia of Womble Carlyle for his extensive input on the legal aspects and to Walker Sisson for the practical solutions described above. Additional thanks to Scott Baxter and Gray Frierson Haertig for technical background, as well as to Harold Hallikainen for providing a fantastic resource. Honourable mention to Curtis “Cowboy” Flick, for the paint reference lifted from the broadcast list at www.radiolists.net. One of the best things I’ve learned about this industry in almost 25 years, is that when I don’t know the answer to a question (which does happen quite often), I’m fortunate enough to have a wide variety of friends and acquaintances – almost always I can find somebody who can shed some light on the situation!
That’s it for this episode, hard to believe we’re coming in to snow season here in the Great White North. Until next time, stay well, work safe and happy engineering!
Jeff Welton, has worked with Nautel for 25+ years. He is currently the Nautel Sales Manager for U.S. Central Region but previously he spent 16.5 years as a Nautel Customer Service Technician.
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