Hello Rick. Let's turn our attention to the "astonishing" robustness of Pactor-2/3. Surprisingly low signal to noise ratio can be reached due to using low-speed rate of the low band signal ONLY. How does it work? First of all low signal to noise ratio declared by the producer are not measured in the small band, used by Pactor signal, but in the whole band of the standard telephone channel (0,3-3,4 kHz). So the noise is assigned evenly to the whole band. Then the process of treatment is as follows: as the "useful" signal is concentrated in a certain definitely appointed frequency area the preliminary digital filtration is used. As a result the noise that does not get to the "useful" signals would be filtered consequently the signal to noise ratio will be much better in fact. So the question arises "it still works" where is dirty trick? In fact it works if the noise is assigned evenly. But it would fail because of any interference in "useful" Pactor signal, even if the signal to noise ratio in the whole band is more than 30 dB. Furthermore if considerable staggering of transceiver or Doppler shift occurs the system will work badly or even stop working as the part of "useful" Pactor signal will be cut by the filters. The experience shows that the situations of that kind are widespread. Ok I believe that the further development of this technology can be in using much more low band signals like CW. Especially if transmit symbols at a rather low speed..... The duration of sending will be high and signal to noise ratio (for the WHOLE band) will be really impressive... don't you think that it is rather amazing and this way carries to the point of absurdity... If use the decision like this using low band signal, one should provide a frequency hopping in band 0,32.7 êÃö. In that case it will work in lager number of occasions, but still not in every one. The technologies like this are going out of use in professional radio-communication. As for serial tone modems with adaptive algorithms they are of far-reaching importance. In particular using the antenna diversity method allows improving the signal to noise ratio and filter out the noise from other directions. Speaking about changing the modulation in MIL-STD modems it should be remarked that in supplement to the standard the 39-tone OFDM modem is really described. But I'm afraid that standard software (like STANAG-5066) will fail to switch rapidly from one rate to another due to rather simple reason. To choose the optimal rate one should always estimate the channel characteristics. Only this information can help to make a good choice. Such modems do not estimate this and however can not give them to the application software. In RFSM uninterrupted estimation of channel state takes place and this very estimation allows choosing the optimal speed of data transmission. So if the state improves RFSM will choose the maximum speed that is possible. For example in Pactor-2/3 increasing/decreasing the speed carries stepwise. Like the method "now I feel better so I can transmit faster" or vise versa "I feel bad so I should decrease the speed at one level" :) About using MIL-STD 188-110A/B technology. The modems used in MIL- STD are "dumb", and can provide just non-guaranteed simple txt-files transmission. In that case even the speed of transmission must be chosen by the user. The software that realizes data link protocol and different user's services provided additionally. The RFSM has not only a "dumb" modem, but also data link protocol optimized for this standard, and different users services.
Dmitry.
