[hpsdr] source coding, Randomizing, outer FEC, Inner FEC, coding to symbol, modulation of symbol(s)

[Lester Veenstra]

Robert:
     A DSP software engineer, smarter than you and me, can certainly write a
diagnostic that will take the digitized audio, from a sound card A/D, and
attempt to do what we do by ear. Typically such software, which does exist
in other environments, relies on a number of human interventions,  at the
decision points, to classify the signal parameters. 

     In the process of trying to determine what modulation and FEC scheme(s)
are in use for a particular signal, knowing what the input pattern might be
of limited value.   The reason is that in any proper coding scheme, will, as
one of its first steps,  scramble or randomize the incoming data, in order
to provide a uniformly random data stream to the subsequent steps in the
process. These randomizers come in a few well defined forms, so it is not
that hard to derandomize the result , once you have demodulated, and
stripped off the FEC layers. If, as I note below, at this point the random,
data does not appear the consequence of a known randomizing process, you may
be looking at encrypted data.

   So in the end, what we are talking about is a software process that will
try and look at the source encoding (in reverse, as a demod,decode,
process), to see if the transmitted symbols are related to the input (user
information data) by various types of FEC coding, 

(1)   Frequency diversity, in the form of encoding the source to allow it to
be transmitted (as adjacent multiple carriers) on multiple frequencies
simultaneously, is needed to combat the frequency selective fading present
on HF paths. This also can be used to lower the  baud rate of the individual
carrier.

(2)   FEC coding layers, to combat, with one type of FEC, the low signal to
noise ratio  (QRN) inherent in weak signal work, and additional layers of
FEC, of a type appropriate to combat the time carrying interference
environment typical of QRM and atmospheric QRN.

(3)  Time diversity coding, to combat the channels dispersive distortion in
time over HF (short baud bad, long baud good), and frequency selective, but
short duration, fading.  Incidentally the “short baud bad” is one reason why
spreading tends to underperform on real HF circuits compared to a flat white
noise channel in a laboratory environment.

However, in addition to the coding resulting from the input data that I have
summarized in the three steps above, there is an additional data steam
added, at any step in the process, that is not derived from the input data,
and hence, random with respect to the data, and is added at the same symbol
rate as the user derived symbols, you will have a case for encrypting
coding. This certainly  expressly forbidden by the FCC and most national ham
rules and regs.  If, at the addition of the random data, it is done at a
symbol rate higher than the  symbol rate of the user derived symbols, you
have a case of spread spectrum.  The end result, not obvious by the simple
minded analysis allegedly done by the FCC engineering office, is a
transmission here the symbol rate appears much higher that would be expected
from the identified (steps 1-3) coding processes.  

The real answer to the acceptability of a modulation system is not the
result of signal analytics, but an  analysis of the coding specifications,
and hopefully  source code examples, to see how you get from  input data to
modulated waveform.  With this level of knowledge, the use of spread
spectrum will be obvious.

As an aside, the fact that a system uses m-ary FSK or multicarrier PSK, and
the modulation keeps changing transmit symbols, when the input stream is all
ones or all zeros, does not demonstrate the presence of a spread spectrum
process. This is not a sufficient test.
On the other hand, if the sum of the transmit symbol rates is very much
larger than the user data rate, much larger than could be rationally
expected by the FEC expansion, then, spread spectrum could be suspected but
not proven.  

In the end, systems like ROS, Clover, PACKTOR-XXX, etc, where there is not
full published transparency in the encoding process, are not suitable for
legal amateur use, in my humble opinion.

 
   Thanks
     73
       Les

Lester B Veenstra  MØYCM K1YCM
lester at veenstras.com
m0ycm at veenstras.com
k1ycm at veenstras.com
 

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-----Original Message-----
From: notify at yahoogroups.com [mailto:notify at yahoogroups.com] On Behalf Of
g3wku
Sent: Sunday, June 06, 2010 8:24 AM
To: Lester Veenstra
Subject: ROS mode

Good morning Lester

...........................if we take the general case of a digimode
produced through a soundcard, the characteristics of the signal are already
established by the time the sound exits the sound card.

Would it, in concept, be possible to create a simple (?) diagnostic program
which analyses the output from the soundcard in response to a predefined
message sequence entered into the digimode program, and obtain a basic
answer like FHSS, yes/no, MFSK, yes/no?

....................................


Best regards and 73

Robert G3WKU