How could the 200MHz spacing between the frequency allocations be insufficient? I can't imagine the oscillators in either system are specified for deviations as high as +-100MHz over their lifetime.
So it seems it is basically a question of how wide is the RF input filter bandwidth of RAs on commercial aircraft.
[1] https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf...
3rd ord IMD for non-EE could be described as low power mixers like mixing low power signals and usually the low power signal comes from your intentionally designed radio oscillator, but, whack it with enough incoming RF power and it will go non-linear as its designed to do and start mixing with that incoming signal, mostly making massive levels of interference. You can always improve 3rd order IMD by simply running the oscillator and mixer at higher power levels (which all turns to heat), usually not the best strategy for battery powered gear.
Here is what a $1 ESP wifi dongle has to follow:
https://en.wikipedia.org/wiki/IEEE_802.11
"The mask requires the signal to be attenuated a minimum of 20 dB from its peak amplitude at ±11 MHz from the center frequency"
So 2 dB/MHz filter.
I let you do the math.
FAA is just ridiculous here if they let old junk radio hardware handle safety landings for airplanes, but well after 737 max what do you expect...
n.b. Wifi (and BT and others) got to use 2.4GHz band because of interference from aircraft kitchens causing a push to make 2.4GHz into junk spectrum with less regulation.
Note there's a pretty big difference between "it should be good enough for 99.99% of planes" and "we can guarantee there won't be even one crash even under unlikely conditions".
Aerospace is already VERY comfy doing ADS-B at 1090 and DME/TACAN from 960-1215 and the old GPS L1 allocation at 1575.42 and the military L2 allocation at 1227.60 (admittedly not many civies using that LOL) and someday probably L4 and remember ASR-11 airport radars blast at 25 KW power over 2700-2900 MHz. But, you know, the straw that broke the camels back and all that.
Most things will be fine. A small amount of things will operate degraded, which is the point of the NOTAM. Some things (like very few) will all fail the same time 5G is deployed, which will be a headache. Going forward every installation of every device on the ground or in the plane will be tested against 5G and it'll be just fine. Most likely for a week or a month everyone will have to treat their radar altimeter as potentially broken until proven otherwise, then it'll be fine and they'll drop the notam. Some equipment will indeed fail and connectors will be tightened or even devices replaced, and it'll be fine.
Could you elaborate on how phase noise would cause an issue in this scenario?
N.B. Most of my experience is in optical systems, my wireless knowledge is limited.
Normally you have lots of space to waste at higher frequencies for guard bands and you can allocate compatible uses for next door bands. Still, when you get a powerful transmitter right next door there are often problems. The previous tenants of that spectrum (C band satellite downlinks) used to have a problem with interference from radar altimeters. So I suppose there is some sort of karmic process here where the service that used to cause interference is now the service that is getting hit with interference.
Seriously, just test it.
You have thousands of flights a day. On a day with visual flight rules put a transmitter at 4.0ghz, max power, worst case radiation pattern and land a 737, 787, A320 and A321 etc as a test.
There is something going on here that we don't know about.