What do you think about this airline vs 5G brouhaha?

Brandon Martin lists.nanog at monmotha.net
Thu Jan 20 21:41:53 UTC 2022

On 01/19/2022 03:47, Masataka Ohta wrote:
> That's not saturation.
> Saturation means a receiver does not have adequate dynamic range.
> With digital processing under saturation, effective number of bits
> is reduced. That is, some necessary bits are lost, which is not
> "everything that's there".

I think we're saying the same thing, but with a different focus.

Yeah, front-end overload will always be a problem if the overload is 
caused by an unwanted signal or if the overload is so severe that it 
causes distortion going into the next stage even if it's just from a 
desired signal.

But even a moderately powerful signal that's outside your band of 
interest by as much as the entire width of the interested band shouldn't 
easily overload your frontend if you designed it reasonably, IMO. 
Obviously the separation at which point you say it's a receiver issue 
vs. a physics issue is something of a judgement call.  Obviously it's my 
problem if my 2.4GHz Wi-Fi receiver is overloaded by the 500kHz 1MW AM 
transmitter next door, but who's fault is it if my low-band cell phone's 
650MHz receiver is overloaded by the 50kW 400MHz UHF TV transmitter half 
a mile away?

IF you had enough dynamic range to receive both your radar reflections 
and the 5G signal as attenuated by your front-end band-pass filter, you 
could use digital tricks (I would think, I Am Not A Radar Engineer, 
though do engineer RF comm systems from time to time) similar to spread 
spectrum to effectively get processing gain on your radar reflection vs. 
that "white noise" 5G signal, but of course none of these devices 
probably do that mostly because they're so old that they predate the 
concept or at least commercial deployment of such techniques which 
didn't become common until around the turn of the century.

 From the sound of it, at least some of these altimeters were designed 
around the (probably poor) assumption that there would be essentially no 
RF power within half a GHz of them, and that assumption is no longer 
going to be true.  Was that a good design decision?  Probably not, but 
we need to figure out what to do about it.  This is more of an FAA 
problem than an FCC problem since it involves functional device 
performance rather than emissions.

The FCC can (and should) attempt to balance the needs of existing users, 
including practical performance of their equipment as deployed, with the 
public good in terms of what has become spectrum that is very valuable 
(not just in $$$s but also practicality) bandwidth for wireless 
communications.  That does, to some degree, involve nudging existing 
users to migrate to semi-modern best practices in order to more 
efficiently use their allocation.  They've done this before with e.g. 
reducing bandwidth limits on FM voice in the VHF/UHF "business bands".

Brandon Martin

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