Thank you for that information, Dimitry, When I read over the MIL-STD/FED-STD/STANAG standards, I could not understand how these slower modes would work all that well by the multi-repeating approach. They repeat multiple times, even if you do not need the repeat (sort of a non automatic ARQ) which seemed very wasteful in terms of throughput. Especially because this was happening at the slower speeds which made them even slower than they might otherwise need to be.
When I saw the charts that compare the throughputs with the S/N ratios, there was some improvement in terms of weak signal capability over the non repeating speeds, but none of the modes seemed to be able to compete with Pactor2 speeds at the low S/N levels. When I have asked those who work daily with STANAG type modems how successful they are, they tell me that they often have difficulty with certain paths. Also, because they use inefficient document types (MS Word .doc files) just to send the shortest possible message can take a very long time. They even have some technologies that have duplex voice and that does not always work either. The reason that they use these technologies is because they have to use them due to the need for interoperability. In fact, this is now mandated more and more with regulations. I often wonder if they may eventually rethink some of the modulation schemes or have a switch over to something that can tolerate severe ISI/doppler/ under weak signal conditions that are common with amateur signals? The STANAG modem standards do allow for OFDM modems as well, but I don't think that anyone switches over to OFDM for part of the time when they are normally using a single tone modem. But perhaps the standard allows it? After seeing which modes seem to work the best under the most difficult conditions, (well below zero dB S/N) which are the conditions we often have with amateur communications, it does seem like a few (two under worst conditions) spaced a moderate distance apart, may have the best compromise on throughput vs. robustness. That is why Pactor 2 (and Pactor 3 when operating in the Level 1 speed) work so well. At this time, it does not appear that there is any other mode that can compete with that modulation technique based on all the comparisons that I have been able to find. Again, thanks for your help on this. 73, Rick, KV9U dmitry_d2d wrote: > Hello Rick. > > As regards the speed that is slower 600 in MIL-STD 188-110A/B. > There are 300, 150, 75. In my opinion speed reduction has been > made nonoptimal, using "dumb" repetition of data in 300 and 150 is > not needed. > The theory of coding says that repetition is the worst way to > improve noise immunity. Speed 75 based on the method of "spectrum > spread" by orthogonal consecution by Walsh. It's rater good but this > speed uses repetition as well. > We consider that the speed 300, 150, 75 allows reaching better > characteristics of noise immunity that the standard MIL-STD 188- > 110A/B allows. > Frankly speaking the standard MIL-STD 188-110A/B has been used our > product to be noticed by customers. But true to say it contains > nonoptimal solutions. > Turning to the point of RFSM we should admit that we have mistaken > making the minimal speed - 600. I hope we improve it in the near > future. > > Dmitry. > >
