The issue is that the FCC sale of frequency spectrum up to 3.98GHz doesn't have such limitations, so essentially in 2019 aviation got an unique problem of extra heavy interference in USA
0 - https://www.defensenews.com/2020/12/21/the-military-is-scram...
The FCC really wants to proceed with the 5G rollout. The FAA really wants to make sure aircraft don't crash. They do not agree with each other on this, so they each proceeded in the only way they administratively could...
The FCC continued with the 5G rollout, the the FAA recently banned use of approaches requiring a radio altimeter until each individual model can get recertified or upgraded - and the public will just have to tolerate additional travel delays for the next couple years as a result.
FCC was warned by industry groups but dismissed the warnings.
We’re watching the process play out.
The FCC allocated a specific band for radar altimeters to transmit in. The FCC only specified rules on transmission, and the radar altimeters are 100% compliant with the FCC rules and only transmit within their assigned band.
The FAA published TSO-87C, which is the technical spec for the radar altimeters. But this also just covers transmission and performance. There's nothing in this spec about rejecting external interference. The radar altimeters are compliant with this spec as well.
The problem is on the unregulated receive side. All radio signals have some amount of bleed-over into adjacent bands due to harmonic interference. Historically, the adjacent frequencies were used for low-power satellite communication, and the manufacturers added enough filtering to protect against the harmonic interference from these low-power stations. They did not anticipate a high-power (5G) signal in those adjacent bands, and nothing in the FAA/FCC specifications protected against it either.
So it's not that the radio altimeters are "using" the adjacent bands, but the receive filtering is (in some cases) insufficient to account for the new (louder) users of the adjacent bands.