Hi Bill, . We should also take in account overlapping spectra as spreading information between symbols under GFSK will affect decoding of the overlapped signals mostly if we are using frequency criterion for candidate ranking and probably due to the inaccurate signal subtraction, assuming all signals on the band are GFSK modulated. Hopefully we can evaluate GFSK benefits/losses via simulation of the signals for overloaded band scenario. . 73, Igor UA3DJY
>Date: Sun, 28 Apr 2019 16:10:16 +0100 >From: Bill Somerville < [email protected] > >To: [email protected] >Subject: Re: [wsjt-devel] GFSK for FT8? >Message-ID: < [email protected] > >Content-Type: text/plain; charset="utf-8"; Format="flowed" >... >Lastly, going back to your initial question. I was probably a bit over >zealous in saying there are only compatibility issues and no sensitivity >issues. For a single signal communication in the presence of noise >alone, using GFSK will reduce sensitivity compared with FSK due to >information being spread between symbols (inter-symbol interference or >ISI for short). The aim is to use an amount of frequency change >smoothing so that, on a busy channel, the overall interference between >signals is reduced, by limiting their bandwidths, such that net >sensitivity is increased. Choosing the exact amount of smoothing to >apply is a complex problem since it depends on the number of signals, >the distribution of signal strengths, the frequency distribution of >signals, as well as the distribution of time-synchronization accuracies. >All of these characteristics vary continuously during real world FT8/4 >communication on the Amateur bands, so all we can really do is carry out >empirical tests and try to pick an optimum filter bandwidth which gives >best results for the whole community. > >73 >Bill >G4WJS. >
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