Hi Bill, . Many thanks for the detailed answer. Absolutely agree with you on the need to increase FT8 band capacity, we will try to implement FT8 GFSK support too when it is done in WSJT-X. . 73, Igor UA3DJY Re: [wsjt-devel] GFSK for FT8? From: Bill Somerville <g4wjs@cl...> - 2019-04-27 13:37:57 On 27/04/2019 13:01, Игорь Ч via wsjt-devel wrote: > Hello Joe, > . > Is there any possible losses, e.g. sensitivity or compatibility, at > pushing FT8 in software towards GFSK modulation? > . > 73, > Igor UA3DJY
Hi Igor, the simple answer is yes. But only related to compatibility, for example if we elect to use more smoothing of frequency shift discontinuities then a decoder designed to be optimal for that will loose sensitivity when decoding old style FSK signals. We have some choices if we switch to GFSK for FT8, some more practical than others. We can go all out for best GFSK performance by matching the decoder to the new GFSK shaped signal and accept a loss of sensitivity when decoding old style FSK signals, we can try and detect the signal shape and adjust dynamically, and so on. One important area is multi-pass multi-signal decoding with subtraction of previously decoded signals, subtraction is most effective if the synthesized signal facsimiles to be subtracted are generated in exactly the same way that the original transmitted signal was. I.e. it is not optimal to generate a GFSK facsimile of a decoded message for the purpose of subtraction if the original was actually an FSK signal. The approach we propose is to restrict the amount of smoothing applied to make a GFSK modulated signal to a relatively low amount and to perhaps apply an even smaller amount of smoothing to facsimile signals used for subtraction, so as to minimize the overall loss of sensitivity in a mixed FSK/GFSK environment. Obviously the intent is for all stations to move to GFSK modulation eventually as it has clear benefits despite the extra complexity of implementation, so any interim measures to limit sensitivity loss when decoding FSK modulated signals will be biased towards best performance for GFSK decoding. If we take that approach I assume the intention will be to move towards decoding purely optimized for GFSK modulation over time as users migrate. This in itself is not a compatibility issue, rather a quality of implementation issue. Fortunately the Gaussian function convoluted with a raw FSK signal to make GFSK is continuously variable so we can pick one that exactly suits our needs and vary it over releases as needed. The final choice of Gaussian smoothig functions for transmission and for signal subtraction has been made by doing empirical tests using multiple simulated FT8 signals and using sample data gathered on air. For example measuring how many potential decodes are lost decoding real-World FSK modulated signals using a decoder optimized for GFSK modulated signals. Fortunately for the levels of smoothing we propose this loss of sensitivity is relatively small as other effects like noise mixed with signals tend to dominate. For FT4 none of this applies as we are using GFSK modulation from the get go and it is simply a case of selecting the amount of smoothing that achieves the wanted signal bandwidth characteristics. Note also that using GFSK modulation has an impact on transmission when changing a message on the fly which is considerably more complex as the whole waveform must be pre-calculated whereas with FSK modulation only the input symbols need be changed as the synthesized signal is generated instantaneously on the fly. There are also implications for ramp up and down of the transmission boundaries since the GFSK filtering will naturally do this without extra implementation complications. What is important is that moving to GFSK modulation *will increase overall channel capacity* so we hope any temporary loss of sensitivity will be outweighed by more potential concurrent decodes per bandwidth slot. This should be an attribute that is welcomed by HF users when occupancy is often approaching capacity at peak traffic times and at other times there will be no loss of performance once GFSK modulation is 100% adopted. 73 Bill G4WJS. >Суббота, 27 апреля 2019, 15:01 +03:00 от Игорь Ч <[email protected]>: > >Hello Joe, >. >Is there any possible losses, e.g. sensitivity or compatibility, at pushing >FT8 in software towards GFSK modulation? >. >73, >Igor UA3DJY > -- Игорь Ч
_______________________________________________ wsjt-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/wsjt-devel
