Topband: FT-8 performance

[K4SAV]

K5ESW said:
"The difference in the FT8 reported SNR and how most hams think of SNR
seems explained well by Jim, KC5RUO.
http://www.arrl.org/forum/topics/view/1957
https://tapr.org/pdf/DCC2018-KC5RUO-TheReal-FT8-JT65-JT9=SNR.pdf


Thanks Paul.  I had read one of those articles before. I didn't find the 
more detailed one earlier when I was looking.  He is relating the S/N 
reported by the digital modes to what it really should be. Without 
digging thru all those numbers and just listening to FT8 it was pretty 
obvious that the reported S/N didn't have much meaning for the average 
person.  That was very obvious by considering the example I gave 
previously of a S9+40 dB signal being reported as 1 dB below the noise 
floor when my receiver was reading the noise floor at S1.  That is a 
huge difference.  It just means the reported S/N numbers need to be ignored.

The really important feature that needs to be measured is not the 
reported S/N but the minimum S/N that FT8 will decode.  That's when 
using a real noise number that hams can relate to, like what your 
receiver reads on narrow bandwidth on a frequency with no signals 
present.  (That would be useful for comparing CW to FT8.) That's usually 
atmosphere noise for those that don't have local QRN problems.  I'm 
afraid that FT8 doesn't use that real noise number when describing its 
operation.  It uses something else that most people can't relate to.  So 
what does decoding a signal 24 dB below the noise floor mean?  Who's 
definition of noise floor?  Certainly not mine.

The results of my experiments were that FT8 can decode signals 
approximately 24 dB below the S meter reading of the whole 1.5 KHs band 
(signal plus noise included).  That conclusion was based totally on 
measurements, which I repeated many times.  It was never exactly the 
same number every time (but close)  but this was over the air testing 
with QSB not something in a lab setup.  You won't be able to read S 
meters exactly.

That's not a difficult measurement to make but it helps a lot if your 
receiver has S meters for both the main and sub receivers.  Set the main 
receiver on 1.5 KHz bandwidth and the sub receiver on about 100-200 Hz 
bandwidth and measure the signal strength of the signal being decoded.  
You have to be sure that you isolate the right signal.

 From those measurement you can see that the minimum signal decoded is 
going to be a function of how many stations there are on the band and 
their strength.  I did verify that happening.  I also confirmed that 
when narrowing the bandwidth on the receiver doing the decoding that 
lower level signals can be decoded.  You would think that decoding would 
continue down below the noise floor if there weren't any strong signals 
on the band, however that didn't happen with any testing I did.  FT8 
pooped out at a S/N number of about +15 dB.

Still waiting for someone else to make similar measurements.  I get the 
feeling that I am the only person to ever make these measurements.

Jerry, K4SAV