[Laser] Troposcatter and Information Theory-2

[stjohn at ocsnet.net]

try http://aintel.bi.ehu.es/psk31.html

"PSK31 is a new digital mode:

designed by Peter G3PLX 
that betters SLOWBPSK, an idea and implementation of Pawel SP9VRC 
based on the RTTY mode of operation, 
useful for live keyboard to keyboard QSO 
that works at 31.25 bauds, 
that uses varicode character coding what gives 50wpm, 
easy to use and monitor, 
that give very good copy under low Eb/No numbers and is thus suitable
for
          QRP, 
that instead of using FSK or on/off keying uses BPSK or QPSK with a
          Viterbi decoder, 
that is available for free for many platforms, including Windows (c)
with
          SoundBlaster type Soundcard, 
and uses advanced DSP and narrow band (31 Hz!!) techniques."


PSK31, also known as "warbler", is very narrow band, and is used by the
QRP (Reduced Power) Hams to talk amazing distances (1500 miles) on much
less
than a tenth of a watt of radio frequency power.  

It is quite able to be used on telephone freqs.  It
is easily detected below the noise threshold of receivers.  

Should we not be looking at it in addition to long duration dits and
dahs?

Tom Upton  AD6N


TWOSIG at aol.com wrote:

> I am still trying to absorb the ideas on posted on this topic (and recovering
> from Field Day).
>
> I see the benefit of slow transmission rate at 136 KHz, it is obviously a
> bandwidth limited channel.  Similarly a phone modem cannot transmit fast data.
>
> What is not clear to me, is the reason why very long closely spaced tones
> have an advantage sending information through air on a laser beam, which is not a
> bandwidth limited channel.  It seems to me sending the same message with
> shorter tones, but repeating the message, so long as the total throughput is
> constant, should provide the same probability that the message is correctly
> received.
>
> I think that part of the reason I am not understanding is that things are
> described in radio communications terms or techniques that have been used for
> weak signal radio reception and Very Low Frequency work.  I know that there are
> similarities, but there are also hugh differences when we are talking about
> light beams.
>
> Here is an example of a difference.  The laser diodes that I have been
> working with are notoriously non-linear.  I use a 555 audio frequency square wave
> generator to modulate it, and with CW, I start and stop the generator.  If I did
> that on any RF frequency, I would get terrible key clicks.  Are they on my
> laser signal?  Probably.  Do they interfere with my communication or with anyone
> else's communication?  No.  So I probably do not need to find a way to
> suppress the key clicks.
>
> Another example.  In theory, I could key a crystal oscillator with the
> control line of my Ramsey laser transmitter.  It would be a simple on-off keying of
> the oscillator at about 18 kHz, pulse width modulated.  I could then tune a
> receiver to the crystal frequency and using AM mode, listen to the sounds picked
> up by the microphone.  For that matter, at close range, I could use an
> un-tuned wire for an antenna, a chunk of galena, a cat-whisker, and high impedance
> headphone.  Would it work?  Yes.  Should you use it on HF?  No.
>
> My point is that the differences between laser work and VLF, MF, HF, VHF,
> UHF, SHF, weak signal, and all the rest, are not being communicated, and maybe
> are not being considered.  If a technique works for VLF weak signal, another
> technique might suit laser weak signal better.
>
> James
> N5GUI
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