[Lf] Wolf]

[Andre' Kesteloot]

Talbot Andrew wrote:

> Replying to Stuart's comments re Wolf signalling .
> This is quite a long EMail so if you have no interest in real radio
> comms engineering switch off now :-(
>
> > I was thinking some more about James' talk on Wolf yesterday,
> > and I have a few basic questions.
> > Wolf consists of a data encoding system layered over an
> > error recover system, layered over a bit synchronisation system
> > finally layered over bpsk. The upper layers woud work fine
> > over any transmission layer ie dfcw. Is BPSK actually the most
> > optimal bit transmission system?
>
> How is convolutional coding going to work with a fuzzy mode signalling
> waveform such as DFCW ?. By Fuzzy Mode, I mean it requires human
> intervention to decode the signal.   If you mean Frequency Shift Keying,
> see below.
>
> In an environment where the distortion mechanism is purely additive
> noise and interference - such as seems to be the case at LF, with no
> significant multipath or coherent time delayed interference - BPSK will
> nearly always be the optimum solution for the signalling waveform.  This
> is due to its bipolar nature ie +/-1 as compared with any orthogonal
> solution such as FSK where the descision is being made between 0/1 - ie
> there is a 3dB advantage immediately.   Practical matters such as clock
> recovery and frequency error can make the descision less clear cut, but
> on a purely mathematical basis BPSK will always win in a noise + QRM
> only environment
>
> > Am I correct in thinking that BPSK is so good because you are
> > actually repeating the bit on every carrier cycle and integrating
> > the result?
>
> Integrating over multiple bits is nothing to do with it being BPSK -
> that technique could be used for any signalling waveform.  The 16 times
> integration - if it is applied to a coherent waveform, ie. voltage
> summing, can give a 16^2 times improvement or 24dB in S/N which is the
> case for Wolf   If the summing is incoherent, ie using Power (which is
> VERY unlikely to be the case for any BPSK demodulator, but would be the
> case for incoherent wqveforms such as FSK) the improvement would only be
> 16 times, ie 12dB.
>
> > I was also thinking that you could send wolf on the divide down
> > CW transmitters that some folks use. By injecting the audio
> > tone from a PC into a 13.6MHz transciever operating SSB and
> > then dividing down by 100 you should generate an equivelent
> > signal to the linear translation approach save for the envelope
>
> To generate a 180 degree  phase shift by dividing down cannot be done.
> Phase shift scales with division ratio, and if dividing by 100, this
> would require 180 * 100 = 18000 degrees phase shift at the fundamental.
> This, of course, is a multiple of 360 degrees so would not give any
> phase shift at all.   To generate the shift would still require a
> separate modulator.    The converse is also true of course.   To
> generate 180 deg on a mutiplied signal requires a lower shift at the
> fundemental.  eg to generate BPSK at 1296MHz would require just a 0 / 15
> degree phase shift to be applied to a 108MHz drive signal.   This is the
> reason why old style VHF FM transmitters (before synthesizers were in
> use) always started with a low frequency crystal and multiplied up. The
> phase shift - and hence FM - could be generated in a simple circuit and
> increased by multiplication.
>
> It would probably work if you used an odd division ratio though.
>
> > shaping. However by keying the carrier off-on during the transition
> > the normal CW wave shaping should clean up the signal.
> > Does this work? Note that the error in the carrier will be the
> > dominant term, and not the error in the the modulation, which may
> > have some advantages.
> >
> As we tried to say at the meeting, it is NOT just a case of switching
> the carrier off at the phase transition point then back on.   The
> amplitude has to be slowly ramped down, the phase switched, then the
> amplitude ramped back up again.   The width of the sidebands is directly
> related to the speed of this ramp.   Therefore, a ramp of 1ms from full
> carrier to zero would result in sidebands 1/(2.pi * 1ms) Hz wide -
> approx 160 Hz at some defined level.   A 2ms  ramp 80 Hz at this same
> level and pro-rata. The shape of the ramp dictates how fast the higher
> order sidebands roll off.  A linear ramp is poor, the high order
> sidebands ones roll off slowly, whatever their initial level may be.
> However, a ramp based on the shape of half a sinewave (the so-call
> raised cosine shape) gives ones of the best roll offs of all.
>
> I suggest you fully read the article on PSK31 By G3PLX that appeared in
> RadCom a few years ago.  PSK31 is the ultimate case of waveform shaping
> where the complete bit interval is a half sine wave and a 0/1 repeat
> cycle gives two single tones separated by half the baud rate - and
> nothing else.   But in any PSK mode, there will always be a trade off of
> bandwidth vs. signalling efficiency.   PSK31 throws away several dB of
> Signal / Noise performance to achieve a very narrow bandwidth.   The
> VE2IQ system on the other hand works best if no shaping at all is
> employed.
>
> Where waveform shaping is used, any system works best at optimum Signal
> / Noise  when the receiver is exactly matched to the transmitted
> waveform - a so called matched filter technique - even if this means the
> Rx appears to take in the signal over a very wide bandwidth.   It is
> collecting as much of the signal as it can and processing this correctly
> to give the best Signal to Noise ratio possible - Whatever the bandwidth
> of the signal may end up as.   It could well be that the energy of a
> signal keyed at  1Hz is spread over 1MHz bandwidth for very sharp BPSK.
> BUT if the receiver takes in every 1Hz whisker over this entire bandidth
> it will give better decoding and optimise S/N than if the signal were
> filtered to 1 or 2Hz bandwidth before demodulation.
>
> For data communications you need always to take a holistic approach and
> not just consider bandwidth, filtering, modulation type data rate etc.
> as separate entities.   All are closely related and it may be necessary
> to separate out or sacrifice one parameter for the sake of optimising
> other factors such as bandwidth, resistance to interference etc.   For
> example, at HF the dominant interference is often not noise but
> multipath.  Here parameters such as data rate need to be optimised to
> counter the several milliseconds of multipath, and often the best HF
> waeforms are those that take up a whole 3kHz bandwidth and are
> subsequently reduced by coding and repetition to allow data rates that
> can be a slow as 70 Bits / s.   Needless to say these are not favoured
> by radio Amateurs - but can often be heard all over HF these days
> sounding a bit like a diesel engine chuntering away.   The repetition
> rate is the repeat length needed to test and measure the multipath and
> repeat data if necessary.
>
> Whew.........!
>
> Andy  G4JNT
> >
>
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