Topband: FT-8 performance

[Paul Ferguson]

Jerry,

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

He says for FT8, the noise bandwidth that impacts FT8 software´s 
ability to decode data is 6.250 Hz. WSJT calculates the total power 
(all signals and noise) in the 2500 Hz bandwith, and uses it as the 
denominator in SNR calculations. 

Jim says "it would make more sense to measure the SNR in the bandwidth 
that's really used by the receiver; but it may be hard to determine or 
define that "true" receive bandwidth. So in short, your eyes and ears 
are not deceiving you. Those JT65/JT9/FT8 signals are very much so well 
above the noise." 

The following shows if you focus on a single FT8 signal in a 6.250 Hz 
bandwidth, the WSJT reported SNR needs to have 26 dB added:

SNRFSKFT8 = SNRreportedFT8 + (10 x LOG (2500 Hz/6.250 Hz))
SNRFSKFT8 = SNRreportedFT8 + (10 x LOG (400))
SNRFSKFT8 = SNRreportedFT8 + 26 dB

73,
Paul K5ESW
Raleigh, NC

> I'm not sure how FT-8 calculates the reported S/N number. I found very
> little information on the subject and what I did find was not easily
> understandable.  What I did was an experiment in which I was able to
> get 
> close to the same number being reported.  According to what I have
> read 
> about FT-8, it does not implement the same method as I was using in my
> testing.
> 
> It was very obvious to me that the number being reported was useless. 
> Example:  How would you be able to report a S/N of -1 dB when the 
> station is S9+40 db on the S meter and the receiver reads S1 when
> tuned 
> to a spot with no stations.  (Actual measurement)
> 
> I made a guess that the number being reported was actually a signal to
> noise plus signal ratio S1(S1 + N), where N is the sum of everything
> else in the passband.  The S9+40 db station in the example would be
> the 
> main contributor to the overall level of the total stuff in the
> passband 
> and that total is just a little more than his signal alone, so -1 dB
> now 
> makes sense.  This seems to work and it works on other FT-8 signals as
> well.
> 
> Jerry, K4SAV
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