Two weeks ago, AT&T and Verizon reluctantly agreed to delay the launch of 5G on newly acquired C-band spectrum licenses for one month, until January 5, in response to the Federal Aviation Administration’s claim that the new service could interfere with radio altimeters used in airplanes.

Mobile carriers aren’t alone in being frustrated by the delay. Telecom-industry observers point out that the Federal Communications Commission approved use of the C-Band spectrum from 3.7 to 3.98 GHz only after analyzing the aviation industry’s interference claims and finding no evidence to support the claims. The FCC also required a 220 MHz guard band that will remain unused to protect altimeters from interference. That guard band is more than twice as big as the 100 MHz buffer initially suggested by Boeing, the FCC has said.

Moreover, this spectrum is reportedly already being used for 5G in nearly 40 countries without evidence of the problems that US aviation officials are warning of. “Tick, tick, tick… US wireless leadership and national security await ‘resolution’ of unfounded concerns by FAA,” former FCC Commissioner Mike O’Rielly tweeted yesterday.

Verizon and AT&T dominated auction

Verizon and AT&T dominated the C-band auction when the results were announced in February 2021, with Verizon’s winning bids totaling $45.45 billion and AT&T’s adding up to $23.41 billion. T-Mobile spent $9.34 billion on C-band spectrum, but is primarily using 2.5 GHz frequencies for its midband 5G deployment. The FCC issued the C-band licenses in July 2021.

Verizon and AT&T have big plans for the C-band. They expect the 3.7-3.98 GHz spectrum to boost 5G networks with faster speeds than are provided on sub-1 GHz spectrum and larger coverage areas than are possible with millimeter-wave spectrum, which doesn’t perform well with obstacles or long distances.

The radio altimeters used to determine airplane altitudes rely on spectrum from 4.2 GHz to 4.4 GHz. The adjacent C-band was previously allocated to satellite service before the FCC repurposed it for cellular use. The band technically extends from 3.7 to 4.2 GHz, but the FCC limited cellular use to 3.98 GHz and below to create the 220 MHz guard band.

FAA slammed for “new heights of irresponsibility”

The FCC is the expert agency on spectrum interference, and some industry observers say that other US agencies have a history of claiming interference problems without good evidence. The “federal government’s processes for addressing spectrum policy [are] severely broken,” Harold Feld, a long-time telecom attorney and senior VP of consumer-advocacy group Public Knowledge, wrote in a lengthy blog post on the dispute last week. “Unhappy federal agencies that don’t like the outcome of an FCC proceeding respond by undermining the FCC in the press and trying to wage proxy wars through allies in Congress. But the FAA’s actions here take this behavior to new heights of irresponsibility and danger.”

Feld—a frequent critic of how telecom companies treat users—wrote that he sides with the wireless industry on this issue in part because the “FAA had a year to collect information on what altimeters were out there and start collecting data on whether 5G would cause harmful interference to any models, and what interference mitigation might be necessary to avoid any potential for harmful interference… Instead, the aviation industry (with the silent support of FAA) basically went for all or nothing. Every correspondence in the FCC record requests that the FCC prevent activation of 5G networks on any part of the C-Band until the aviation industry was satisfied that there was no potential risk of harmful interference.”

In some of the countries already using the C-band, “5G signals operate in spectrum adjacent to aviation equipment. US airlines fly in and out of these countries every day,” the wireless industry group CTIA’s president and CEO, Meredith Attwell Baker, wrote today. “If interference were possible, we would have seen it long before now. Nevertheless we’ve added a layer of protection in the United States, called a guard band, that is hundreds of times greater than the separation that exists between wireless and other critical spectrum users.”

FAA acknowledged no “proven” interference

The FAA issued a November 2 bulletin that warned of “potential adverse effects on radio altimeters,” but that bulletin acknowledged there have been no “proven reports of harmful interference,” even in countries that allow 5G transmissions above the 3.98 GHz limit set by the FCC.

“Many countries around the world are already deploying wireless networks in the bands from 3300-4200 MHz; some countries have implemented temporary technical, regulatory and operational mitigations, including temporary proximity and power restrictions, on wireless broadband networks operating in bands ranging from 3700-4200 MHz,” the FAA wrote. “There have not yet been proven reports of harmful interference due to wireless broadband operations internationally, although this issue is continuing to be studied.” The US has deployed wireless broadband in 3.65-3.7 GHz since 2007, the FAA noted.

In its February 2020 decision to reallocate C-band spectrum, the FCC said the aviation industry’s research was unrealistic and urged the industry to conduct more testing, saying that “further 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.” About 20 months later, in the bulletin issued this month, the FAA recommended that “radio altimeter manufacturers, aircraft manufacturers, and operators voluntarily provide to federal authorities specific information related to altimeter design and functionality, specifics on deployment and usage of radio altimeters in aircraft, and that they test and assess their equipment in conjunction with federal authorities.”

The FAA’s new warning to the aviation industry also noted that the FCC first sought comment on using the C-band for mobile broadband in 2017.

“The 5G C-band issue has been pending for YEARS… why are they only now looking at better standards for altimeters?” spectrum technology and policy consultant Michael Marcus, an engineer who spent over two decades working for the FCC, wrote on Twitter yesterday. Marcus also pointed to a July 2012 report by a White House advisory council that recommended “methodologies for spectrum management that consider both transmitter and receiver characteristics to enable flexible sharing of spectrum” because “receiver characteristics increasingly constrain effective and flexible spectrum usage.” In other words: receivers should be designed well enough to protect themselves from interference from transmissions in other spectrum bands.

The FAA told the FCC in December 2020 that it “expect[s] that the cost of replacement or retrofit of radar altimeters will be substantial.”

https://arstechnica.com/?p=1814119