Dear Rik, dear LF group,
Assuming a QRSS QSO takes 2 hours you can have the same QSO in abt.
38 minutes using DFCW and in abt 22 minutes using 7FSK, leaving the SNR
unchanged.
7FSK is ... abt. 2.5dB superior to DFCW
Implicitly, you raise a very interesting point here: What if we used 1024
freqs to encode 10 bits, would the SNR per symbol still be the same? If so,
could we use infinitely less energy per bit, by spending (exponentially) more
bandwidth?
Spending BW to improve SNR is often done, classical examples are wideband FM
and digital audio. But all these techniques have in common a detection
threshold. If the input SNR is below that minimum, demodulation breaks down
completely. The question for our weak-signal work is not so much how to
decrease the error rate of a strong signal, but rather whether we can
actually lower the threshold bit-energy below the noise energy (kT, equal to
noise-power per bandwidth).
For m-ary FSK, the task of the receiver is simply to select the channel
having the highest energy (or the highest in-phase voltage for coherent
detection). The problem is that with more channels, the statistical
probability of noise in one of the many unwanted channels being stronger than
the signal channel becomes higher. To keep the error rate constant, you have
to spend a little more TX power. For large m, this logarithmic loss in
SNR-efficiency per symbol finally eats up the logarithmic increase of the
number of bits per symbol.
Just have a look at one of the 7FSK screenshots with marginal signal: The
choice of selecting the "right one in a crowd of seven" is more difficult
than for a crowd of two.
Regards
de Markus, DF6NM
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