The Invisible Spectrum
What you can’t see can cost you.
By Deborah Jackson
Years ago, while I was living on a quiet cul-de-sac, my burglar alarm began turning me into the neighborhood nuisance by randomly sounding off its siren. It began happening two or three times a month, often during the day but sometimes in the evening. To me, the occurrences were puzzling because there was no sign of intruders or other triggers. My next-door neighbors were flat-out annoyed, especially if I wasn’t home to immediately turn off the alarm. Finally, after six months, my neighbor finally noticed that the alarm went off every time her daughter-in-law visited. With that clue, we figured out that the keyless entry system on the visitor’s new car transmitted on the same frequency as the emergency call button on my burglar alarm.
That solved mystery led me to wonder why radio frequency interference (RFI) incidents are not more common. After all, the electromagnetic radio frequency spectrum is a limited resource that is becoming even scarcer with the development of more services and applications: RFID to track packages, cell phones, GPS locators, customized weather reports, Wi-Fi Internet, broadcast TV and radio, microwave ovens, and so on.
Radio regulations (RR) are used to manage the allocation of the radio frequency spectrum, allowing the myriad everyday services we enjoy to coexist with minimal RFI. In the United States, regulatory responsibility for the radio spectrum is shared between the Federal Communications Commission (FCC) and the National Telecommunications and Information Administration (NTIA). The FCC, which is an independent regulatory agency, administers the spectrum for nonfederal use (i.e., state and local government, and commercial, private business, and personal use). The NTIA, which is an operating unit of the Department of Commerce, administers the spectrum for federal use (e.g., the U.S. Army, the FAA, and the FBI). The two groups’ oversight involves amending the radio regulations to ameliorate interference cropping up among regulated uses as well as fining industries, radio stations, and individuals operating in violation of the RR.
Fines and penalties for violating stipulations in spectrum allocation can be quite hefty. In 2018, for example, a cellphone company had to pay more than $614 million to the FCC for operating approximately 1,000 cell phones at unlicensed frequencies. With penalties ranging from a few thousand dollars to millions of dollars, depending on the violation, it’s critical that engineers designing RF spectrum applications be aware of the RR, especially since the lack of awareness of RF spectrum regulation does not accord protection from penalties.
The design engineer must also be aware of amendments to the RR that affect allowable frequencies. For example, an RR amendment was made for microwave ovens, which are designed to operate at a frequency of 2.45 GHz. To reduce RFI, the perforated metal shield in the glass door of your microwave oven is designed to block radiation. Despite heavy shielding, the ovens’ high-power emissions still sometimes interfered with neighboring frequencies, so the frequency allocation was modified to include guard bands on either side of the emission band to limit interference.
Another example of an RR amendment benefited biomedical devices, which historically operated at frequencies assigned to the television channel UHF bands (470–668 MHz). Broadcast television was the primary licensee; medical device use was licensed in the absence of broadcasting. Because frequent interference was impairing device performance, the RR were updated in 2002 to designate new bands for the Wireless Medical Telemetry Service (WMTS), specifically 608–614 MHz, 1395–1400 MHz, and 1427–1432 MHz. Allocating these bands should decrease the chance of interference from television broadcasters.
Many academicians developing medical devices are still unaware of many RR, including the 2002 amendment that changed frequency allocations for medical devices. Consequently, they risk being penalized for transmitting without a license at frequencies outside the assigned bandwidth.
In the age of the Internet of Things and with the pervasiveness of high-tech medical devices, more of our engineered systems require an updated awareness of RF spectrum management. Unfortunately, most engineering programs lack a course on RF spectrum management introducing the RR and penalty avoidance. Access a short video introduction at https://tinyurl.com/RFspectrumIntro.
Deborah Jackson is a program manager in the National Science Foundation’s Engineering Directorate. The views expressed in the article do not necessarily represent those of NSF or of the U.S. government.