Some background here, if it's useful. This has been ongoing for quite some time, and has a lot of nuance to it, IMO. The following is my summary as a layperson. I've tried to quote sources wherever possible.
- The FCC, which is the government agency that has ultimate control over permitting what goes on in the nation's airwaves, proposed to make additional frequencies available for mobile phone use in 2018: https://www.fcc.gov/document/37-42-ghz-public-notice-opening...
- This was going to be done by "repacking" satellite communications in the C-band (3.7 GHz to 4.2 GHz) freeing up spectrum to be used by cell carriers.
- The FCC implemented a 220MHz "guard band" based on studies with radar altimeters, leaving more than double the guard Boeing requested. This was based on studies to evaluate any interference using this frequency (https://arstechnica.com/tech-policy/2021/11/faa-forced-delay... is a good summary, there is a lot to read here).
- The FCC auctioned these 280MHz of airwaves (3.7 - 3.98 GHz) last year in FCC Auction 107, garnering $81,168,677,645 in gross winning bids. As there is a delay for the incumbent spectrum users to clear their usage, ATT and Verizon were only granted the use of a combined 120MHz of spectrum in limited areas, spanning the 3.7GHz to 3.82GHz range.
- Verizon and AT&T, the largest winners, have sought to deploy the spectrum they paid for in concert with the FCC rules. Specifically, this is the "lower" part of the overall winnings, which leave a minimum of ~400MHz between the top end of the broadcast frequency and the bottom end of altimeter operation.
- Airline carriers and the FAA came in at the 11th hour and asked the carriers and the FCC to halt their deployment, asking for more time to be able to study whether this was safe. After a lot of back and forth, Verizon and AT&T have agreed to some power reductions around airports and runways, and voluntarily delayed the rollout of broadcasting on these frequencies twice - most recently at the beginning of the year.
Where we are now is a bit of a battle of regulators, airlines, and the cell operators that spent a significant amount of money on spectrum and capital expenditure (radios, antennas, etc) to deploy that spectrum.
The FCC has cleared the use of the airwaves, but the FAA insists that not all altimeters can avoid being interfered with, and are now issuing requirements that may ban the use of these instruments in cases where they otherwise would have been necessary. This may result in landings that may not be able to happen, causing diversions, etc.
The carriers and the FCC insist that over 40 countries are using this spectrum (which appears to be true) with various restrictions. In Japan, for example, there is a significantly smaller guard band of 100MHz, though the transmit power level is much lower than it will be in the USA. The carriers have voluntarily agreed to adopting restrictions around airports that roughly mirror that of France.
At this point, it's unclear what happens next. I am sure I am missing a lot here (why are altimeters sensitive to broadcasts happening 400MHz away from them? are they?), and there seem to be valid concerns and issues on both sides. Ultimately, this is largely looks like embarrassing game of cat/mouse between the FCC, FAA, cell carriers and airlines.
It seems like Boeing evaluated the altimeters they use and determined that with the filtering they have only a 100MHz guard band was needed.
Apparently though there exist radio altimeters with basically no filtering at all, which worked properly in practice, since sat downlinks had nowhere near the power level needed to interfere. But now we are adding more powerful transmission closer to the altimeter band than was used previously, and altimeters designed without using meaningful amounts of filtering would be affected.
That claim that 400MHz of guard band (as this initial rollout still provides) might be insufficient for some altimeters is really something. Ideally the FAA can determine which ones they are, and deem them no longer airworthy.
I'm also wondering if in fact it is just that the standards for altimeters are far looser than what all altimeters actually implement. The FAA concerns seem to be based on the signals potentially interfering with an altimeter built to just barely meet the testing specifications, but they might all actually greatly outperform those specifications.
- The FCC, which is the government agency that has ultimate control over permitting what goes on in the nation's airwaves, proposed to make additional frequencies available for mobile phone use in 2018: https://www.fcc.gov/document/37-42-ghz-public-notice-opening...
- This was going to be done by "repacking" satellite communications in the C-band (3.7 GHz to 4.2 GHz) freeing up spectrum to be used by cell carriers.
- Boeing encouraged this in late 2018, mentioning that the FCC should give a 100MHz "guard band" to prevent interference with radar altimeters, which operate in the 4.2-4.4 GHz range: https://ecfsapi.fcc.gov/file/121184623679/Boeing%20C-band%20...
- The FCC implemented a 220MHz "guard band" based on studies with radar altimeters, leaving more than double the guard Boeing requested. This was based on studies to evaluate any interference using this frequency (https://arstechnica.com/tech-policy/2021/11/faa-forced-delay... is a good summary, there is a lot to read here).
- The FCC auctioned these 280MHz of airwaves (3.7 - 3.98 GHz) last year in FCC Auction 107, garnering $81,168,677,645 in gross winning bids. As there is a delay for the incumbent spectrum users to clear their usage, ATT and Verizon were only granted the use of a combined 120MHz of spectrum in limited areas, spanning the 3.7GHz to 3.82GHz range.
- Verizon and AT&T, the largest winners, have sought to deploy the spectrum they paid for in concert with the FCC rules. Specifically, this is the "lower" part of the overall winnings, which leave a minimum of ~400MHz between the top end of the broadcast frequency and the bottom end of altimeter operation.
- Airline carriers and the FAA came in at the 11th hour and asked the carriers and the FCC to halt their deployment, asking for more time to be able to study whether this was safe. After a lot of back and forth, Verizon and AT&T have agreed to some power reductions around airports and runways, and voluntarily delayed the rollout of broadcasting on these frequencies twice - most recently at the beginning of the year.
- The FAA has so far cleared the altimeters in ~45% of the commercial fleet: https://www.cnbc.com/2022/01/17/us-faa-clears-45percent-of-c...
Where we are now is a bit of a battle of regulators, airlines, and the cell operators that spent a significant amount of money on spectrum and capital expenditure (radios, antennas, etc) to deploy that spectrum.
The FCC has cleared the use of the airwaves, but the FAA insists that not all altimeters can avoid being interfered with, and are now issuing requirements that may ban the use of these instruments in cases where they otherwise would have been necessary. This may result in landings that may not be able to happen, causing diversions, etc.
The carriers and the FCC insist that over 40 countries are using this spectrum (which appears to be true) with various restrictions. In Japan, for example, there is a significantly smaller guard band of 100MHz, though the transmit power level is much lower than it will be in the USA. The carriers have voluntarily agreed to adopting restrictions around airports that roughly mirror that of France.
At this point, it's unclear what happens next. I am sure I am missing a lot here (why are altimeters sensitive to broadcasts happening 400MHz away from them? are they?), and there seem to be valid concerns and issues on both sides. Ultimately, this is largely looks like embarrassing game of cat/mouse between the FCC, FAA, cell carriers and airlines.