Paul, I don’t know the answer to your question(s).
In addition to frequency separation and signal strength difference, one would have to consider overall signal strength (not just difference), the DT difference between the two signals, and the delay and Doppler spread on each of the two channels that are involved. There are too many dimensions in that parameter space! Steve k9an > On Jul 28, 2019, at 8:06 PM, Paul Kube <paul.k...@gmail.com> wrote: > > Steve -- > > Related to this, and to another recent thread on replying to CQ's on the > caller's frequency: > > What is the decoding probability a FT8 (or FT4) signal when being interfered > with by another FT8 (or FT4) signal, as a function of frequency separation > and signal strength difference? Seems clear that it would not be appropriate > to model the interfering signal as additive Gaussian noise, so is this even > something that you can solve or nicely approximate analytically? I'd be > interested to know. > > 73, Paul K6PO > > On Sun, Jul 28, 2019 at 7:38 AM Steven Franke via wsjt-devel > <wsjt-devel@lists.sourceforge.net <mailto:wsjt-devel@lists.sourceforge.net>> > wrote: > Hi Gene, > >> FT8 is WAY MORE sensitive! (~8db) > > That number is not right. On the additive white Gaussian noise (AWGN) > channel, the 50% decode probability of FT8 occurs at SNR=-20.8 dB and the 50% > decode probability of FT4 occurs at SNR=-17.5 dB. The sensitivity difference > is therefore 3.3 dB. > > On channels with severe Doppler spreading, the threshold SNR is higher for > both modes, and the difference between FT8 and FT4 will decrease somewhat > because FT4’s larger tone separation tends to give it an advantage in those > cases. > > It might be of interest to some to consider that FT8 uses symbols with > duration 160 ms to send 3 bits apiece. FT4 uses symbols with duration 48 ms > to send 2 bits apiece. For a given average transmitter power, the energy that > is transmitted per bit for FT8 is larger than the energy transmitted per bit > for FT4 by the factor (160/3)/(48/2) = 2.22. Thus, the theoretical > sensitivity difference (ignoring any differences in signal detection, > synchronization or LDPC decoding efficiency) is 10*log10(2.22) = 3.46 dB, > very close to the actual difference of 3.3 dB that I quoted above. > > I have no strong feelings one way or the other about your main point, but I > think that it’s preferable to base the discussion on accurate numbers. > >> >> FT4 is awesome for MORE contacts (i.e. contests). >> >> I’m sticking with FT8 for QUALITY. >> >> 73 de W8NET Miles / “Gene” >> Secretary, Portage County Amateur Radio Service (PCARS) >> 3905 Century Club - Master #47 >> DV2/W8NET in the Philippines >> Licensed since 1974 > > Steve, K9AN > > > _______________________________________________ > wsjt-devel mailing list > wsjt-devel@lists.sourceforge.net <mailto:wsjt-devel@lists.sourceforge.net> > https://lists.sourceforge.net/lists/listinfo/wsjt-devel > <https://lists.sourceforge.net/lists/listinfo/wsjt-devel>
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