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Blog: Is C-Band Different in the US?

A puzzling communications meltdown that delayed US 5G launches in the C-band spectrum has raised multiple questions, one of the biggest being why the country’s aviation regulators seem more concerned about interference than their counterparts in other countries.

The U.S. Federal Aviation Administration (FAA) is conducting extensive testing in partnership with wireless carriers to find out if C-band 5G can interfere with radio altimeters that measure distance to the ground during certain landings.

The FAA says it is finally getting the data it needs from the mobile industry to perform its tests, while carriers have expressed frustration that the agency has not communicated sooner.

Wireless industry association CTIA reports that 40 countries have preceded the US in safely allowing 5G transmissions in the C-band, including the UK, France, Germany, Japan and China.

The United States certainly handled the C-band launch differently than other countries, with high-profile interagency clashes and public hearings. But the nation is also different from others in the way it regulates radio equipment and those differences help explain the kerfuffle.

Unlike other countries, the United States does not impose standards on radio receivers. In many cases, manufacturers are not required to include filters, which means receivers may pick up frequencies outside of their assigned bands.

When designing these devices, engineers will include filters if interference is expected. Their choices are dictated by physics and economics, not regulation.

Some older radio altimeters in the United States were designed without filters: when they were manufactured, the C band was used by satellites.

Altimeters are meant to measure signals reflected from the ground and satellite transmissions were not detected by these instruments. Therefore, filters were not considered a necessary expense.

Earlier this month, Dennis Roberson, president and CEO of consultancy Roberson and Associates, told politicians that most current altimeters “have filters and will not experience any 5G interference issues.”

Roberson was consulted as an unbiased technical expert, as his company advises government and commercial clients on spectrum matters.

Most large commercial aircraft are equipped with modern altimeters, including those operating internationally, and their pilots have not reported interference with 5G C-band radios.

Regional jets and helicopters in the United States are the planes most affected by FAA flight restrictions related to 5G, as some of them have older altimeters.

Roberson explained that the antennas of these legacy devices can be fitted with ceramic filters, preventing the altimeters from receiving signals from outside their assigned bands. He described them as a “very low cost component”, but added that “upgrading and certifying a new radar altimeter in an aircraft is a significant expense of time and dollars”.

Filters can be inexpensive, but they are part of larger systems. Earl Lum, president of the EJL Wireless Research team, explained that the entire unit needs to be replaced, not just the ceramic filter.

The question of who could pay for this came up several times during the politicians’ briefing and no one volunteered.

CTIA President and CEO Meredith Attwell Baker noted that the US government raked in billions of dollars from the C-band spectrum auction.

Powerful arguments
The industry association maintains that interference is unlikely to be a problem, noting that mobile operators in France, Spain, Denmark, Romania, the Republic of Ireland and Finland are all allowed to transmit 5G in C-band at higher power levels than permitted in the United States, and do not report interference with flights.

Carriers in these countries do not always transmit at the highest power levels allowed. “The United States has the greatest operating power, apart from China,” Lum said. “Europe is generally lower.”

Lum added that carriers in many other countries also use a lower portion of the C-band that Verizon and AT&T plan to use, which in the United States makes potential interference with older radio altimeters more likely.

For example, France has authorized 5G in the 3.4 GHz to 3.8 GHz band, but US operators will eventually be authorized to use 3.7 GHz to 3.98 GHz once all the satellites in place have freed up. spectrum.

Radio altimeters use the 4.2 GHz to 4.4 GHz band.

If interference is discovered by FAA testing, manufacturers are sure to be ready with solutions for regional aircraft. Altimeter maker FreeFlight Systems is already promoting a new device on its website, saying it’s “built from the ground up, to mitigate spurious 5G interference.”

But for now, some aircraft owners are resisting the idea of ​​upgrading their equipment.

Regional Airline Association President and CEO Faye Malarkey Black told politicians that “5G deployment around airports is the cause of potential disruption and delays, not faulty or faulty radio altimeters”.

“These radio altimeters meet current regulatory and certification standards established by the FAA.”

Clearly, the US is in a unique position when it comes to 5G and aviation, and the stalemate could continue if FAA testing uncovers actual interference between C-band 5G and aviation. radio altimeters.

The editorial opinions expressed in this article are solely those of the author and do not necessarily reflect the views of the GSMA, its members or associate members.

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