FCC to ask for input on possible regulations
Rosenworcel said she will propose launching “a new inquiry to explore receiver performance and standards.” The notice of inquiry would ask for public input on possible solutions, including new “regulatory requirements,” Rosenworcel said.
She went on:
This inquiry would ask how receiver improvements could provide greater opportunities for access to spectrum. It would explore how these specifications could come in the form of incentives, guidelines, or regulatory requirements—in specific frequency bands or across all bands. And it would seek comment on legal authority and market-based mechanisms that could help create a more transparent and predictable radio-frequency environment for all spectrum users—new and old.
Rosenworcel said she will make the proposal in April. While the FCC still has a 2-2 partisan deadlock, the Democratic chair has bipartisan support. Rosenworcel thanked Republican Commissioner Nathan Simington “for his leadership on these issues and his willingness to work with me on a path forward.”
Simington issued a statement applauding “Chairwoman Rosenworcel’s decision to consider exploring a new regulatory framework for commercial spectrum allocations.” Simington’s statement went on to cite the problem with altimeters:
An approach that looks at both the receiver and transmitter ends of the equation is the only framework truly capable of timely accommodating the interests of federal users of spectrum, and other incumbents. We see a lot of value in getting to a place where conflicts such as the C-band/altimeter fight are headed off at the pass.
This model will provide all interested parties sufficient advanced warning about problematic band edges adjacent to any new commercial spectrum. Clear rights regarding interference protection can provide incentives for innovation and collaboration among spectrum users in a way that avoids regulatory dictate.
FCC urged aviation industry to act two years ago
When the FCC voted to allow cellular transmissions in the C-band in February 2020, the commission adopted power limits and the 220 MHz guard band between 5G and altimeters. “The technical rules on power and emission limits we set for the 3.7 GHz service and the spectral separation of 220 megahertz should offer all due protection to services in the 4.2-4.4 GHz band,” the FCC said.
While the FCC found no evidence that “harmful interference would likely result under reasonable scenarios,” it urged the aviation industry to conduct more testing on altimeters. “[F]urther analysis is warranted on why there may even be a potential for some interference given that well-designed equipment should not ordinarily receive any significant interference (let alone harmful interference) given these circumstances,” the FCC said. Despite that warning, the FAA and aviation industry seemingly weren’t prepared for the 5G transmissions when the rollout occurred almost two years later.
Receiver design is fundamental problem
“Fundamentally, the problem is a design issue with the aviation industry’s radar altimeters,” Dennis Roberson, who runs a technology consulting firm and is a research professor at Illinois Institute of Technology, told lawmakers during a House subcommittee hearing last month. When altimeters were first designed decades ago, “they had very low-power neighbors, i.e., satellites beaming their information to the earth from very distant orbits,” he said.
Roberson continued:
Since the altimeters operate on a radar principle looking for a signal reflected from the ground, their receivers couldn’t detect the very low-power neighboring satellite signals. This led the early designers of the altimeters to decide they really could ignore their assigned spectrum boundaries and as a result they allow transmitted energy far outside their band into the receiver. For decades this was not an issue given their quiet neighborhood, but with new neighbors now moving in (AT&T and Verizon), the spectral space that they were allowing into the receiver is now a potential problem.
Roberson said the current 400 MHz separation between altimeters and 5G “is very, very large,” pointing out that the entire FM radio band is 20 MHz wide. The FCC “determined that there shouldn’t be an issue because of the vast separation between the 5G cellular use of the new spectrum and the altimeter spectrum allocation,” Roberson said. “Unfortunately, this is not the case for old, technically ‘wide open’ altimeters. These radar altimeters may send out a signal and be unable to discern the reflected signal because of energy from the faraway 5G towers entering the receiver, causing the radar altimeter to either fail to function or possibly provide a false reading.”
Making matters worse, altimeters today are “highly integrated into the avionics for modern aircraft,” he said. “If, for instance, the altimeter says the aircraft is still in the air when it has actually landed, it will cause the reverse thrusters and spoilers that normally create a rapid reduction in the airplane’s speed on the ground to not operate.”
During that hearing on February 3, FAA Administrator Steve Dickson told Congress that “we are confident we will work through this issue safely with minimal disruptions, but we acknowledge that some altimeters—especially older models used by certain segments of the aviation industry—may not receive approval as being safe in the presence of 5G emissions and interference and may need to be replaced.” Dickson subsequently announced his resignation.