Paul,The term for the performance parameter you're asking about is "co-channel rejection" and, as Steve points out, it's very messy to analyze analytically, as it's a very nonlinear function of many variables. Even validating simulation results via experiment can be tricky.Ed N4II. -------- Original message --------From: Paul Kube <[email protected]> Date: 7/29/19 3:31 PM (GMT-05:00) To: WSJT software development <[email protected]> Subject: Re: [wsjt-devel] FT4 Frequency on 40-m TOK, thanks Steve. If I get inspired maybe I'll fire up ft8sim and ft4sim and run some experiments.73, Paul K6POOn Mon, Jul 29, 2019 at 9:35 AM Steven Franke via wsjt-devel <[email protected]> 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 k9anOn Jul 28, 2019, at 8:06 PM, Paul Kube <[email protected]> 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 K6POOn Sun, Jul 28, 2019 at 7:38 AM Steven Franke via wsjt-devel <[email protected]> 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 #47DV2/W8NET in the PhilippinesLicensed since 1974Steve, K9AN_______________________________________________ wsjt-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/wsjt-devel
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