Topband: FT-8 performance

[Richard (Rick) Karlquist]

It is instructive to calculate the Shannon
maximum theoretical data rate (power limited case)
(refer to wikipedia page for Shannon-Hartley theorem).

If S/N ratio (BW=2,500 Hz) = -24 dB,
then S/N ratio (BW=1Hz) = -24 + 10 log 2,500 = -24 +34
= +10 dB. 10 dB converted to a dimensionless ratio is 10.

Now, channel capacity = C <= 1.44 X 10 = 14.4 bits/second.

This rate is a little more than twice the FT-8 rate.

Now a days, achieving 1/2 of the Shannon limit is
possible for AWGN.  If your noise isn't AWGN, well
then that is another source of error.

Thus the claim of -24 dB sensitivity seems plausible,
where the S/N is the true signal vs AWGN, as opposed
to whatever random number FT-8 reports.

So I think the beef with FT-8 is in the way it
calculates the displayed S/N.  We used to call
those "marketing specs".

It is also notable that FT-8 uses at least twice
the average power compared to CW.  If you compared
them on an average power basis (vs PEP) the FT-8
advantage, if any, would drop 3 dB.

On CW, you could send your call many times
in 15 seconds for "error correction" and take advantage
of QSB peaks.  That tends to level the playing field.
More playing field leveling is using Super Check Partial
analogous to what FT-8 does.

Rick N6RK