Hi Dmitry, Although I may not completely understand S/N ratio, if we can haves some kind of relative comparison between two or more modes, and done consistently, should give us a way to compare in a practical manner. Examples of claims that PSK31 can work down to perhaps-10 dB S/N and Pactor 2 and 3 stop working by -15 dB as do most modes including CW, depending upon the operator.
A practical ham measurement is the hroughput in characters per unit of time at a given S/N and BW,necessary to help determine practical the mode is ... or is not. And the robustness from difficult conditions with ISI, doppler, QRM, QRN, etc. The standard CCIR Channel parameters for Doppler and Multipath are very modest with a "poor" channel being considered to have 1.0 Hz of Doppler and 2.0 ms of delay. In fact, it is not that uncommon to have over 5 ms of multipath delay spread and Doppler spread of 5 Hz. Indeed, under the really difficult conditions of polar paths, the delay can be 10 ms and 50 Hz of Doppler. (Source: Iterative Equalization and Decoding of Stanag 4539 9600 PBS HF Waveforms, John W. Nieto paper). While I don't have any practical way to measure this, do the STANAG modems detect the ISI and doppler spreads? Do they display these parameters? Many of us have used different modes (including PSK and RTTY and the throughput went to zero, even with signals present that one might think should be working. This is where some specialty modes such as DEX (Domino EX) might be able to work well for keyboard speeds due to the very low baud rates possible with multitone modems. Under those really difficult conditions, I suspect that PSK modes, including Pactor, will not work at all. For amateur use, the high baud rate modes are rarely possible to use based upon comments I have received from the digital SSTV operators. They find that 16-QAM is usually the highest constellation that works with the OFDM modems that are commonly used. I found it surprising how rapidly the DSSTV operators moved away from the short lived RDFT waveform, toward OFDM. And this was primarily because of performance of only a few dB improvement. When I look at the required (and even measured computer simulations ... but not real world measurements) S/N for low BER (10 -5) and in a 3 kHz bandwidth with say, STANAG 4539, (functionally the same as MIL-STD 188-110 B), the government expectation is +9 dB for 3200 bps. And much higher of course for faster data rates. Thus, I don't see these data rates as anything very useful for the kinds of things that we hams are involved in on HF. My experience seems to be that we are lucky to have a solid +10 dB S/N, but usually there is still some QSB. Then we can expect signals to deteriorate to -10 dB or even worse. My thinking is that any practical amateur digital modes need to focus on the +5 dB and downward and not be too concerned about faster speeds that are possible with +10 dB or greater signals. Many of the military designs require at least 1000 watts or even much higher power levels plus good antennas on each end of the circuit, something we hams typically don't have available to us. One thing that disappointed me about the STANAG standards is what I view as poor performance at the low bit rates. Even at the slowest data rate of 75 bps, the main standard only has to meet +2 dB S/N. Simulations show some competing products to work close to -5 dB. The STANAG 4415 75 bps "very" robust mode, is designed to work down to around -9 dB and apparently can work down to -11 dB on computer simulations with some products. Also, it is supposed to be able to work down to at least zero dB under extreme 50 Hz Doppler spread and 10 ms Multipath and that may not be possible for most of the amateur modes. I would like to try the slower and more robust STANAG modes when they become available for amateur use. Don't you think that the the most robust modes, often running at the slower speeds will be used on HF amateur frequencies, if they can compete with existing modes? 73, Rick, KV9U dmitry_d2d wrote: > 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 0,3–2.7 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. > > Announce your digital presence via our Interactive Sked Page at http://www.obriensweb.com/sked Check our other Yahoo Groups.... http://groups.yahoo.com/group/dxlist/ http://groups.yahoo.com/group/contesting http://groups.yahoo.com/group/themixwgroup Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! 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