For ARQ there is an additional criterion, viz. the value to look for is the thin border line between 95% and 100% copy. 95% copy is generally not enough for efficient ARQ operation. That is why pskmail does not use S/N levels as the only parameter for switching modes, additionally we use block size adaptation and ARQ success. Using this the MFSK and THOR modes get their fair share of the cake under NVIS conditions.
Rein PA0R >Gary, > >Thank you for the feedback. Using ARQ systems like you did we have also >noticed that faster PSK modes show better performance as in less repeat >request than slower ones. > >Your first theory is the one we believe is the most likely contributor for the >ARQ links in general benefiting from faster psk speed (up to a certain point >of course). > >But like you we are a bit puzzled by the apparent robustness of these modes in >NVIS conditions. > >If you are so inclined, could you please perform a comparison between psk250 >and psk250R as available in the beta version 3.20 of Fldigi? > >The FEC of course reduces the speed by almost half but the error rare (ARQ >repeats) appears to be lower than even the half psk speed (i.e. psk250r seems >more robust than psk125) as Rein reported in his post on this subject. > >Another perspective would be appreciated. > >73s, > >John > > >--- In digitalradio@yahoogroups.com, "Gary" wrote: >> >> Your question is one that I have also. In our recent NVIS testing with >> fldigi/flarq we found BPSK250 provided better throughput than other modes we >> tested, and most notably MFSK32 which we thought would be our safe, robust >> mode. >> >> This was with a variety of band conditions including strong signal, weak >> signal, selective fading, lightning QRN, grungy power line noise, and all >> the normal stuff we experience here in the Midwest. Path distances varied >> from 40 miles to 150 miles which were the distances we were interested in. >> >> There were a couple ideas we kicked around as possible reasons why BPSK250 >> worked so much better than we expected. One was that when the signal took a >> hit from something like a lightning burst, BPSK250 recovered and >> resynchronized very fast. The second was even more speculative in that >> maybe the higher phase modulation rate (250 times per second) was faster >> than Doppler path modulation allowing the BPSK decoder to ride through. >> >> Anyway, we expected BPSK250 to be useless on NVIS but every time we have >> tried it, it has worked. (and better than most modes.) Maybe there is >> something that the path simulators are missing. >> >> Dunno... Just throwing some ideas out. >> >> Gary - N0GW >> > > > > >------------------------------------ > >http://www.obriensweb.com/digispotter.html >Chat, Skeds, and "spots" all in one (resize to suit)Yahoo! Groups Links > > >
