Skip:
Spectral analysis cannot differentiate between a high rate FEC
operating after, as it invariably must, a randomizer, and a true spread
spectrum system. And a spread spectrum system does not need to employ
frequency hopping. And a signal that frequency hops is not necessarily a
spread spectrum signal. I refer you to the old favorite of the UK
Diplomatic service, the Piccolo.
As I advocated in an earlier post, the way to end this endless discussion
would be for the inventor to disclose the block diagram of the various
steps in his encoding/modulation system. In fact I was rash enough to
suggest that IMHO, all of these systems being played with by hams, should
be open sourced, so that, the end user can have some confidence in what he
is using, and the state of the art can be mutually advanced. We started
with this philosophy with the TTL MAINLINER-II, and continue it today with
many of the DSPR systems out there, including the primary commercial
company. Their disclosure does not seem to have slowed them down at all.
Thanks 73
Les
Attached is a repeat (edited a bit) of my previous semi random thoughts on
this to a UK member of the reflector.
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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 is 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 where 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 is 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
amateur use, in my humble opinion. I am not suggesting this as a point of
law, but rather as a point of good amateur practice, to be encouraged.
Obviously there are some, with commercial market hopes for non-ham HF radio
networks, (This is a very small and already well served market today) would
not be willing to open source, but publishing the encoding details does not
give away the company store. That good stuff is in the decode/demod section.
-----------------------------------------------------------------
Lester B Veenstra MØYCM K1YCM
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From: [email protected] [mailto:[email protected]] On
Behalf Of KH6TY
Sent: Monday, July 12, 2010 5:17 PM
To: [email protected]
Subject: Re: [digitalradio] Random data vs Spread Spectrum
Lester, I really appreciate your mannerly characterization of what I wrote
as "BS". :-(
>From the Wikipedia, a more exact definition of spread spectrum is,
"Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio
signals by rapidly switching a carrier among many frequency channels, using
a pseudorandom sequence known to both transmitter and receiver. "
Please post here what your own spectral analysis finds, and state if you are
willing to testify to the FCC whether or not ROS is really FHSS.
Thanks.
73, Skip KH6TY
(No BS at this QTH!)
