Thanks for your comments Dimitry as they do help with understanding your approach.
One thing that I wondered about when using the program for our test was the relatively slow speed at the ~ +5 dB S/N. The 600 showed on the screen which now if I understand your comments below, the slowest bps rate for your version of the STANAG modem is 600 bps. For around a +5 dB S/N is that about right for the 600 bps rate? If the program was running at the 600 bps rate, and had a throughput of around 300 wpm, is that about what we should expect? Did you try using the other STANAG modems down to the more robust 75 bps too or did you find that impractically slow? 73, Rick, KV9U dmitry_d2d wrote: > 1. A few words about OFDM and serial tone modem. > Let's find out how the fight between ISI and Doppler shift > takes place in these systems. OFDM uses the great number of low > speed channels so the symbol duration increases. While the > duration of ISI is much smaller than symbol duration everything > goes well. Consequently there is an aim to increase the number > of channels ad infinitum BUT at the same time natural limitation > takes place. It's just a Doppler shift effect. Hence there is > always a compromise between ISI and Doppler shift. Moreover we > should take into consideration a big peak factor which results > in non-effective usage of power of transceiver. There are > methods directed at improvement of peak-factor, but the most > part of them makes the system characteristics worse. > In case of serial tone modulation the fight ISI with Doppler > is provided with adaptive algorithms. The more effective and > faster they are the larger number of Doppler and ISI the modem > can manage. > As for RFSM it should be mentioned that now it includes > rather efficient adaptive algorithms that work properly at a > speed of 600(500) up to 4800(4000) bps (wide/narrow mode). To > work at a speed 6400(5333) - 8000(6666) much more compound > algorithms are needed. In particular using turbo-equalization > will improve noise proof feature at all rates. > Therefore OFDM and serial tone modem can be more efficient > in dependence on channel statement. In my opinion serial tone > modem with effective adaptive algorithms is the most effective. > We'd like to mention that under certain circumstances either > serial tone or OFDM modem can fail to provide connection, for > example, when the Doppler shift is extremely high (polar > communications). In that case one should use the methods of > "spectrum spread" that extending the symbol in time and > frequency. Unfortunately the speed would not be high in this > case. > So the best way out is to measure the channel > characteristics and choose the speed of transmission and > modulation method according to them. The full adaptation of the > all characteristics is required. > > 2. About our users. > The project RFSM-2400/8000 was initially aimed at > organizations (not for HAMs)! (First version had no 0,3-2,7 > band, which is adapted for HAMs). > Its prime value is that high-performance algorithm is used > in it. Consequently only technical specialists of organizations > where data (files, mail etc.) transmission through HF is needed > can estimate the program at its true worth. They need the > following: high speed of connection and data transmission. They > are the FIRS GROUP OF OUR USERS. For example there are > organizations (our users at the moment) who even haven't looked > upon HAM -modems (little speed, instability, absence of files > transmission in spite of excellent chat-exchange). > If you are interested in RFSM as in a program for chat- > exchange (or even for file transmitting but you do not need a > high speed) and runner is not important for you:. You are the > SECOND GROUP OF OUR USERS. $60 may be a pretty penny for this > product for you. > There is also not numerous GROUP OF USERS - THE THIRD ONE > The representatives of this group are specialists in HF- > radiocommunications and radioamateurs at the same time who is > interested in algorithms of a high efficiency - the runner of > the program. May be $60 is rather expensive for them but they > can trial versions for free. They communicate with us suggesting > interesting and moreover useful ideas. We really appreciate > their advices and suggestions. Due to the THIRD GROUP the first > version of RFSM has transformed in the product adopted for HAM. > > 3 . There are several remarks on the open source codes. > a) RFSM-2400 (and all the more RFSM-8000) is not just a > "dumb" modem though such a rate is also possible (it was used in > PSKMail). Our product is an accomplished system of communication > that provides different types of services including > receiving/transmitting e-mail on Internet. > b) Speaking about OFDM it should be pointed out that we have > got experience in such a kind of modulation and can remark that > to construct this modem is incommensurably easier than Serial > Tone Modem. But the modem of this kind doesn't compare with RFSM > characteristics. If we were not be able to realize Mil-STD > correctly and use OFDM in RFSM, we would not be sorry to > distribute source codes. > c) Philosophy. Professional free software is possible > because qualified developer has been grown up by certain > company. The buyers have already paid for software and > progressive developer as well. Then at the same time free > software appears (like RFSM-2400) - like an ad, to create an > image or ease consumers' tasks. The fact that software is free > is a result of successful sales of developer. However free > software is not possible in fact. The bigger the quantity of it > the poorer it's quality. So said "Write on C++ for food ;)" > There is also rather INTERESTING free software that contains > ads in bulk - for example mobile phones or cars or pasta ads. > You don't have to pay for this software!!! BUT you have already > paid for this software as an end-user of these mobile phones, > cars, pasta: As the prices of these goods include the price of > software. > d) You insist that the program should distribute for free. > Ok. RFSM-2400 - a result of years of investigation - is > absolutely free. Now you say that you are not going to use it as > it is not widespread. To sum up what's been said you do not need > this program for free or ... whatever! So we are just making > a conversation... > > Dmitry. > >
