Topband: FT8 - How it really works

[K4SAV]

Thanks to the folks commenting on how FT8 works.

VE3KI said:
"The noise floor the wsjt-x signal is referenced to is the noise within 
the bandpass during the two-second period when no-one is transmitting, 
not the signal level when people are transmitting."

That was what I originally thought might be a possibility because that 
would result in a real S/N number.  However that doesn't seem to be the 
case.  That S9+40 dB signal I referenced would result in a huge S/N 
number, probably greater than 50 dB.  FT8 gives a report of -1 dB.  
Doing it that way would also have some problems produced by people 
transmitting at the wrong time and other out of band stations, however 
it seems that FT8 doesn't make that measurement.

Thanks to Arunas, LY2IJ .  Your comments agree 100% with what I 
measured.  As to your question of can FT8 decode signals below the noise 
floor and below the level that can be decoded by CW.  My experiment of 
adding audio noise which covered up the signals and the software still 
being able to decode signals says that under some conditions FT8 can 
decode signals below the noise floor.  Of course that experiment was 
done at audio levels, not at RF.  If you use CW you get the benefit of a 
much narrowed passband,  I can't run that test using audio mixing.

In a condition of only one weak signal on the band, I haven't run a test 
that says whether FT8 decodes better than CW or not.  NN4T said that 
using FT8 on 6 meter sporadic E that he observes signals being decoded 
with no audio in the receiver.  That is probably with a wide bandwidth, 
and it would be interesting to know if the signals would be audible with 
a narrow bandwidth.

In the case of a crowded band it becomes obvious that CW is much 
superior to decoding a weak signal because all those strong signals 
limit the ability of FT8 to decode a weak signal.  That was the basis 
for my conclusion that FT8 didn't seem to be useful for working weak 
signal DX because most of the bands are very crowded. However there may 
be a case where FT8 can beat CW, that is if you are tying to decode a 
signal on an essentially dead band.  Since FT8 seems to be able to 
decode below the noise floor, the noise floor in that case would just be 
real noise, not signals.  You could improve FT8's ability to decode by 
narrowing the bandwidth, although that's not normally done.  That would 
cut into CW's advantage obtained by using a narrow passband. I don't 
have a measurement with the results of that showdown of CW versus FT8 in 
dead band conditions but the answer would be interesting to know.

Jerry, K4SAV