TOK, thanks Steve. If I get inspired maybe I'll fire up ft8sim and ft4sim and run some experiments.
73, Paul K6PO On Mon, Jul 29, 2019 at 9:35 AM Steven Franke via wsjt-devel < wsjt-devel@lists.sourceforge.net> wrote: > 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> 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 >> https://lists.sourceforge.net/lists/listinfo/wsjt-devel >> > > _______________________________________________ > wsjt-devel mailing list > wsjt-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/wsjt-devel >
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