Taking responsibility for harmful interference
My conversations with operators around the country reveal that incidents of harmful electrical interference are on the rise. Some operators report that they are spending two days a week tracking down sources of harmful interference and trying to clean up the spectrum. Although the causes vary somewhat, the question remains, what is the FCC doing about it? The answer is, sadly, bupkis.
Remember the old days, when the FCC at least gave the appearance of wanting to help? Old FCC field office records gave the impression that a fleet of vans would encircle the offender like a pack of sharks, DF dorsal fins breaking the sky, and pounce upon the interfering facility with demands for immediate inspection and correction.
This enforcement was sometimes viewed as intrusive by otherwise well-meaning operators whose systems had slipped their tuning, or whose systems were sending spurious signals that the operators hadn’t detected. The presence of the FCC served as a necessary impetus for operators to do something, and do it quickly.
The problem is so basic that it gets few headlines, and that’s the reason why it has been allowed to get worse. Today, the FCC is “Broadway,” with shows, critics, plots, characters and stars. Nobody wants to talk about gum under the seats while the actors are performing. Rather than keeping order so the industry can do its thing on stage, the ushers_the regulators_have become part of the act.
Digital masking rules This publication has the word “Technology” in the title and, judging by the rest of the editorial content, the other writers are much more qualified than some cheesy lawyer to speak on technical issues. But speaking on behalf of cheesy lawyers (Our meetings require the entire state of Kansas, where there’s plenty of standing room.), we sometimes notice things first.
During the past few years, analog operators have been complaining about adjacent-channel digital systems, that broadcast signals with square wave configurations that are often incompatible with adjacent analog operations. The square wave overlaps the analog sine wave, and the energy pulsed into the adjacent analog wave desensitizes the analog receiver, which is attempting to capture the entire analog wave.
When this phenomenon is discovered by analog operators and is pointed out to digital operators, some of them respond with “Our equipment is operating according to specs.” This translates roughly into, “It’s your problem”_an interesting approach to spectrum management. (An approach that sometimes is mirrored by the FCC.)
The truth is that it is the digital operator’s problem, particularly in view of the FCC rules that require operators to take reasonable precautions to prevent harmful interference. Although operating a digital system with type-accepted equipment might be a good start in meeting the rules, the method of operation is as important as the equipment used. Therefore, if an operator knows that his system is causing harmful interference because of a system design that includes selecting the location and power used for each transmit frequency within a multichannel digital system, the creator of the problem has the burden of finding a solution.
Most of these situations that were resolved (and many are still ongoing) were fixed by the digital operator implementing a different frequency use plan or adding filtering. In other words, “Good fences make good neighbors,” but where this problem continues unabated, it can be severely disrupting to operators and cause numerous headaches for customers of the analog systems. So what is the better answer?
In Washington, DC, a place where few answers are found, one group is gathering information for the purpose of proposing rules to be adopted by the FCC regarding the operation of digital systems adjacent to analog systems. The object is to update the “masking rules” for digital operations. Masking refers to the distribution of radio energy within a frequency channel. The less energy there is that extends to the channel edges, the less likely it is to interfere with adjacent channels. The current masking rules for some services are the same for both analog and digital operations. Therefore, when an operator makes the switch from analog to digital, he is allowed to use the same masking rules.
If more practical masking rules were adopted, digital and analog operators could live in better harmony, without the increasing spectrum chafe. Of course, this would require some enforcement of the new rules by the FCC_which might be hoping for too much. At least a new masking requirement might cause equipment to be built that would be more compatible within the actual operating environment.
In praise of filters Another source of the interference problem is the failure by technicians to install filters. This problem is seen quite often in the installation of additional channels on 900MHz paging tranmsitters. The carrier wants to use a single transmitter to meet its construction requirement for multiple paging frequencies. The technician is directed to install the additional channels, but neglects to install additional filters to upgrade the transmitter into “good neighbor” status for the use of the additional frequencies.
The failure to install the filters creates a sloppy signal that splashes over the band and onto adjacent channel systems. Bingo! Harmful interference is caused that is difficult for the operators of adjacent-channel systems to pinpoint_and even more difficult for them to explain to the offending carrier. Although most readers of this publication like to think of themselves as RF professionals, the truth is that you often might be harassed by your company’s executives and marketing managers to “Get it done today,” even if today you lack the filters necessary to do the job right. You know when the channels are being installed that they might cause a problem without proper filters. Yet, sometimes you do the job you are paid to do, rather than the one you know is correct.
To solve this problem takes professionalism and ethics. RF engineers and technicians must expand their vision to consider not only how their systems are working, but how the operation of their systems might affect others.
Tell me more If you want to help solve the increasing incidents of harmful electrical interference, your input is sorely needed. You know the “whys” and the “hows” better than anyone inside the FCC because you have to live with the problems every day. Please send your stories, solutions, technical observations, and suggestions to me, Robert Schwaninger at 1835 K Street, Suite 650, Washington DC 20006, or fax it to 202-659-0071. I’ll pass it on to the group working on the problem.
Schwaninger, MRT’s regulatory consultant, is a partner in the law firm of Brown and Schwaninger, Washington, DC. He is a member of the Radio Club of America.
