On 1/19/22 18:31, Bryan Fields wrote:
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.
It only needs to attenuate from 3980 and down to solve this potential
problem.
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.
While the passbands are different in these as they're designed to pass
the C-band satellite signals and reject the radar, C-band filters with
insertion loss in the 1.4 dB range with 60dB rejection 20 MHz down have
been available for quite a while.
https://cdn.shopify.com/s/files/1/0529/5806/8919/t/9/assets/eBPF-C-Spec-Sheet.pdf
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.
Fair enough, but C-band below 4200 has hardly been a desert all of these
years. TD-2 was on mountaintops all over the country pushing a couple of
watts into huge KS-15676 horns with something like 39dB of gain. 4400
and above is also licensed for mobile use.
The C-band satellite operators, having to deal with extremely weak
narrowband signals, only have a 20 MHz guard band from the 5G allocation.
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.
If that's the case, how have they dealt with the signals from other
aircraft in busy airspace that are operating in the same band all of
these years?
The poor filtering on the receiver is obviously the issue. However,
suitable filters have been available for decades, adjacent frequencies
have been in use for decades, and it isn't the FCC's fault nor the
cellular carriers' fault that FAA has certified crappy receivers for use
in mission-critical applications.
Somebody using a crystal set to listen to a 1KW AM station 20 miles away
isn't going to get very far complaining to FCC about a new 5KW signal
100 kHz below it and a couple of miles away. That the FAA would certify
radars with a front-end like a crystal set is the problem.
So can this LTE at C band work? Yes.
Will it require upgrades to avionics and standards? Yep.
If the 5G allocation were shared spectrum with the radar altimeters, I'd
concede your point. However, it's at least 220 MHz away, over 5% of the
actual frequency in use.
All of this talk so far is speculation about potential harmful
interference. Radar altimeters exist. Cell towers exist. Has anyone
gathered any real world data demonstrating actual interference?
--
Jay Hennigan - j...@west.net
Network Engineering - CCIE #7880
503 897-8550 - WB6RDV
