Is C-Band Different In The US?


A confusing collapse in communications which delayed US launches of 5G in C-Band spectrum raised multiple questions, one of the biggest being why the nation’s aviation regulators seem more concerned about interference than counterparts in other countries, writes MARTHA DEGRASSE.

The US Federal Aviation Administration (FAA) is conducting extensive testing in partnership with mobile operators to learn whether 5G in the C-Band can interfere with radio altimeters which measure distance from the ground during some landings.

The FAA claims it is finally getting the data it needs from the mobile industry to conduct its tests, while operators have expressed frustration the agency did not communicate earlier.

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

The US certainly handled the launch of C-Band differently than other countries, with high-profile inter-agency stand-offs and public hearings. But the nation is also unlike others in the way it regulates radio equipment and these differences help explain the kerfuffle.

Unlike other countries, the US does not impose standards on radio receivers. In many cases, manufacturers are not required to include filters, meaning receivers can pick up frequencies from outside their assigned bands.

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

Some older radio altimeters in the US were designed without filters: when they were manufactured, the C-Band was used by satellites.

The 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 most of today’s altimeters “have filters and will not experience any 5G interference problem”.

Roberson was consulted as an impartial technical expert, because his company advises government and commercial clients on spectrum issues.

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

Regional jets and helicopters in the US are the aircraft most impacted by FAA flight restrictions related to 5G, because some of them have older altimeters.

Roberson explained antennas on these legacy devices can be outfitted with ceramic filters, preventing the altimeters from receiving signals originating outside their assigned bands. He described these as a “very low-cost component”, but added “retrofitting and certifying a new radar altimeter in an aircraft is a non-trivial expense in both time and dollars”.

Filters may be inexpensive, but they are parts of larger systems. Earl Lum, president of research outfit EJL Wireless Research, explained the entire unit must be replaced, not only the ceramic filter.

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

CTIA president and CEO Meredith Attwell Baker noted the US government raked in billions of dollars when it auctioned C-Band spectrum.

Powerful arguments

The industry association maintains interference is unlikely to be a problem, noting mobile operators in France, Spain, Denmark, Romania, the Republic of Ireland and Finland are all authorised to transmit 5G in the C-Band at power levels higher than those allowed in the US, and are not fielding reports of interference with flights. Operators in those countries may not always transmit at the highest power levels allowed. “The US has the highest power of operation, other than China”, Lum said. “Europe is typically lower”.

Lum added operators in many other countries are also using a lower part of the C-Band than Verizon and AT&T plan to use, making potential interference with older radio altimeters more likely in the US.

For example, France authorised 5G in the 3.4GHz to 3.8GHz band, but the US operators will eventually be allowed to use 3.7GHz to 3.98GHz once all incumbent satellites have vacated the spectrum.

Radio altimeters use the 4.2GHz to 4.4GHz band.

If any interference is discovered by the FAA’s 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, claiming it is “built from the ground up, to mitigate spurious 5G interference”.

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

Regional Airline Association president and CEO Faye Malarkey Black told the politicians the “deployment of 5G around airports is the cause of potential disruptions and delays, not faulty or defective 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 stand-off could continue if FAA testing finds actual interference between 5G in the C-Band and radio altimeters.

  • DeGrasse is a Freelance tech journalist.
  • Editor’s Note: The editorial views expressed in this article are solely those of the author and will not necessarily reflect the views of the GSMA, its Members or Associate Members.

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