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

Thu Jan 20 02:31:56 UTC 2022

On 1/18/22 9:03 PM, Brandon Martin wrote:
> One thing the FCC could potentially do to wipe some egg of their 
> collective faces, here, is mandate that transmitters operating in this 
> newly allocated wireless band face additional scrutiny for spurious 
> emissions in the radio altimeter band as well as the guard band between 
> the two services and a similar bandwidth above the radio altimeter band. 

The issue is not one of out of band emissions, but rather close but strong
signals near the receiver pass band.  This can cause compression of the first
RF amplifier stage and de-sensitize the receiver so it cannot hear the
intended signal.  I won't get into the physics, but it is difficult to realize
an effective filter that will permit 4200-4400 with low loss and attenuate
everything else starting at 4200 MHz and down.  The narrower the filter is,
the higher the loss is. The greater the stopband attenuation is, the more
elements required and more ripple is present in the pass band.  Now granted
for avionics, this is doable in the thousands of dollars, but older radar
altimeters will not have this level of filtering, nor can you slap a filter on
avionics without manufacturer support.

Further complicating this, radar altimeters in the 4200-4400 MHz band are
frequency modulating continuous wave transmitters.   In this configuration the
frequency is not closed loop controlled, it can be anywhere in the 200 MHz
band, as it's modulating a free running VCO nominally at 4300 MHz. This is a
non-issue as the transmitter is used for the receiver reference, so they are
locked to the same free-running oscillator.

Only in recent avionics has the receiver been improved via DSP circuits and
FFT to do real time spectral analysis and pick out the right receive signal.
The older altimeters out there use simple zero crossing counting to determine
the frequency of the strongest signal.  This leaves them open to potential
interference by strong near band signals. Exasperating this is the poor
filtering on the RF receiver in 99% of altimeters when dealing with wide band
signals.

So can this LTE at C band work? Yes.
Will it require upgrades to avionics and standards? Yep.

Last time this sort of change out was needed Sprint/Nextel bought every major
public safety agency new radios.  One could plot the decline of Sprint stock
to an uptick in Motorola stock.

This reminds me of the Lightsquared case where they were using adjacent
spectrum to GPS for low speed data from satellites, and wanted to add in
repeaters on the ground, or an ATC/ancillary terrestrial component.  Sirrus XM
does this, in tunnels and such and it's just the rather low power repeater of
the same signal from the satellite. Lightsquared wanted this the be a high
power LTE signal, which wouldn't "fill in" their satellite signal but make an
LTE network they would sell access on.  Do to the proximity to the GPS bands
and the rather poor selectivity of the GPS receiver, it would have
dramatically limited GPS performance.

The issue here is that Lightsquared was too small.  The establishment wireless
carriers know that commissioners don't work at the FCC for life, and have paid
lobbyists crawling all over capital hill.
-- 
Bryan Fields

727-409-1194 - Voice
http://bryanfields.net