AE4TM has posted a graph based on real world results using groundwave at various distances.
It shows PI starts its dropoff at 10db S/N. It is a gradual dropoff to about a S/N of -5db to 0db. Somewhere in that range things go down very steeply. PII maintains its throughput from +30db S/N all the way down to about a -10db. PIII shows a gradual dropoff from +30db down to 0db where the throughput is down by a factor of about 3.5. From 0db to -10db PIII drops sharply down and it is no faster than PII at -10db. -10db is the big knee for both. Throughput drops off very sharply at that point. tim ab0wr On Sunday 05 March 2006 06:26, Jose Amador wrote: > > Actually, with bad S/N ratios, thruput suffers a lot. > > I have been checking at > http://www.scs-ptc.com/pactor.html before sending this > reply, and there is some info, but I cannot find the > graph > comparing several modes up to P-II, that I think was > published in QST in the late 90s. > > P-I with memory ARQ holds the link down to -5 dB (in a > 3 kHz bandwidth, SNR in pactor´s bandwidth is BETTER > than that). P-II holds the link down to -18 dB, but > that´s all, thruput is almost nil. > > P-III shines at some 20 dB SNR, because with more > complex constellations, the distance between the > constellation points diminishes, and is less noise > tolerant. Interleaving, compression and convolutional > encoding with Viterbi decoding all that helps a lot, > but there is a big distance between "telegraphy" and > "telepathy"...that is, digging for signals down into > the noise. > > After all, the most succesful newer digital modulation > > schemes are using some of the tricks that have made > P-II and P-III successful, but there are physical > limits that cannot be violated. Specially some forms > of FEC, block codes and interleaving are helpful to > avoid signal degradation due to QRM, fading and > multipath, and data recovery without repeats thanks to > block codes. > > But it carries a price tag, the more noise and QRM > resistant a mode is, generally implies more latency, > that is, makes it less "keyboard friendly". > > 73 de Jose, CO2JA > > > > > > > > > __________________________________________________ > Do You Yahoo!? > Tired of spam? Yahoo! Mail has the best spam protection around > http://mail.yahoo.com > > > Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org > > Other areas of interest: > > The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ > DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy > discussion) > > > Yahoo! Groups Links > > > Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
