[Lf] LF: <TECH>Transatlantic modes - what next?]

[Andre' Kesteloot]

James Moritz wrote:

> Dear LF Group,
>
> The winter has seen a fair amount of success in the transatlantic
> tests, and now that the "season" is probably near to an end, it is
> worth thinking about what the next steps could be, in good time for
> next winter. The following ramblings were the result of thinking what
> I could do next, in the way of technical development of my LF
> station, in particular regarding what would be the best mode to
> work on:
> -------------------------------------------------------------------------------------
> So far, all reported trans-atlantic signals have used some type of
> QRSS. The variable parameter is the dot length - longer dots mean
> better signal-to-noise ratio, with the penalty of slower message
> transmission. Recognisable callsigns have been produced with 60s
> dots or longer, which requires more than an hour to transmit one
> callsign. ON7YD's DFCW roughly halves the time required, at the
> expense of doubling the bandwidth occupied by the transmitted
> signal and being slightly more complex to implement, which seems
> to be a good trade-off.
>
> The maximum radiated power available to several stations has
> reached the 1W ERP level in Europe, and somewhat more than
> this for the stations active in Canada. The ERP that is feasible
> depends mostly on how big an antenna can be put up, but in any
> case is limited by regulation to 1W in Europe, which eliminates
> further improvements. ERPs of a few 100 milliwatts have been
> sufficient to cross the Atlantic on several occasions.
>
> Under optimum conditions, with favorably sited and equipped
> stations, 3s per dot QRSS signals have been passed both ways
> across the Atlantic, but this is the exception rather than the rule.
> The problem with longer duration dots is that any kind of 2-way
> contact takes an excessively long time, running into several hours.
> This problem is compounded by the fact that propagation "lifts" on
> LF only last an hour or so. So it would seem we are reaching the
> practical limits with QRSS.
>
> What is required for a practical 2 way LF DX mode, capable of
> operating at the extremes of distance and SNR? A while ago,
> G4JNT posted an estimate of what might be theoretically possible
> using different techniques; another way is to look at what is
> needed to serve our purposes. I would suggest the following "wish
> list":
>
> 1)Be able to complete a minimal QSO (about 50 characters) in one
> hour. This would give the lowest rate of signalling capable of using
> the propagation lifts to complete a QSO "in one sitting".
>
> 2)Be able to transmit/receive all alphanumeric characters and
> essential punctuation/procedure signs, in order to be generally
> usable by any station without special arrangements.
>
> 3)Occupy a bandwidth of less than 10Hz - this is neccesary
> because of the very limited spectrum available, and the fact that
> several stations will be operating simultaneously.
>
> The QRSS modes easily meet 2 and 3; in order to meet 1, a dot
> length of about 7 seconds maximum would be required. With the
> best possible conditions, I guess several stations might manage
> transatlantic QSOs with these dot lengths. However, it would
> probably not be enough to reach the more inland parts of Canada,
> or the USA and further afield. By the way, I reckon about 6dB SNR
> is needed to see a QRSS signal on a spectrogram under
> favourable conditions; if there is much QRN, 10dB is probably
> required. It is possible to see a trace of signal with 0dB or less
> SNR. All this is fairly subjective, however.
>
> One way to use longer dot lengths without increasing overall QSO
> time is to use multiple frequencies - for example, DFCW, VA3LK's
> 7-tone scheme, and VK2ZTO's one-tone-per character VFSKCW.
> Taking this to it's logical extreme, it would be possible to assign
> different frequencies to all possible combinations of callsigns and
> signal reports, so each over of a QSO would just consist of a
> single tone. However, it would then be difficult to meet condition 2.
> I guess there must be an optimum trade off between number of
> tones, difficulty of encoding and decoding, redundancy and so on. I
> suspect it might be 2 tone DFCW, but I don't know.
>
> Then there are the "digital" modes, specifically BPSK. Currently,
> most effort has been expended on the MS100, 10 bits per second
> variety of BPSK. This easily meets conditions 1 and 2. However,
> for the same signal levels, QRSS seems to do better with
> acceptable, if much slower, speed. Also, the bandwidth occupied is
> roughly 40Hz, too wide for condition 3. But with the 16 bits per
> character coding scheme normally used for BPSK, 2250
> characters per hour can be transmitted, far higher than is actually
> required. So the bit rate could be greatly reduced, and/or the
> coding altered to a greatly increased number of bits per character,
> hopefully improving the readability of the signal. Reducing the
> overall speed by a factor as much as 45 would still meet condition
> 1. To fit into a 10Hz bandwidth, the bit rate would have to be 2.5
> bits/sec (MS400) or less, so you could encode each character with
> up to 180 bits if you wanted to. Or, sticking with 16 bit codes, 0.22
> bits/second (MS4500) would still be OK. What we want is the best
> trade off between bit rate and encoding for very poor signal to
> noise ratio. I don't know a great deal about this subject, but I
> expect some readers of this reflector already know the answer.
>
> Beacon signals are a bit different; here, the only requirement is to
> positively identify the signal, and make some estimate of the signal
> level. An on-off keyed carrier with a simple repeating pattern and a
> well defined frequency is easily identified with simple equipment,
> and has the advantage of flexibility at the receiving end - you can
> make the bandwidth arbitrarily narrow, or perhaps take advantage
> of the coherent nature of the signal, to improve detection
> capability. You can also monitor several signals at once.
>
> So any suggestions/comments would be welcome -     well, almost
> any! By the way, I now have BPSK at up to 1200W PEP from my
> Decca TX, if anyone would like a sked/tests, etc.
>
> Cheers, Jim Moritz
> 73 de M0BMU