On Mon, Feb 22, 2010 at 08:30:29PM -0500, KH6TY wrote:
> It will be spread spectrum if the tone frequencies are controlled by a
> code as explained in the ROS documentation:
>
> "A system is defined to be a spread-spectrum system if it fulfills the
> following requirements:
> 1. The signal occupies a bandwidth much in excess of the minimum
> bandwidth necessary to send the information.
> 2. Spreading is accomplished by means of a spreading signal, often
> called a code signal, which is independent of the data.
> 3. At the receiver, despreading (recovering the original data) is
> accomplished by the correlation of the received spread signal with a
> synchronized replica of the spreading signal used to spread the information.
> Standard modulation schemes as frequency modulation and pulse code
> modulation also spread the spectrum of an information signal, but they
> do not qualify as spread-spectrum systems since they do not satisfy all
> the conditions outlined above.
>
> Note that all three conditions must be met to be considered spread
> spectrum. I don;t know if it would be possible to send the data in less
> bandwidth, but, for example, PSK31 accomplishes the same typing speed in
> a bandwidth of 31 Hz, instead of in 2000 Hz, so ROS is probably truly
> spread-spectrum.
>
> Remember that spread spectrum was conceived as a way of coding
> transmissions so they could not be intercepted and decoded. In fact
> actress Hedy Lamarr invented spread spectrum, and you can read that
> here: http://en.wikipedia.org/wiki/Hedy_Lamarr. The difference is the
> use of a code to spread the data and signals to avoid detection and
> monitoring by those without the same code.
She invented FHSS as a torpedo control technique; most folks don't know
that she had an EE degree. DSSS came about later, as a classified technique
called "Phantom", to permit transmissions with a low probability of
interception ("LPI"). With a typical 3 KHz bandwidth receiver, or even a 50
KHz wide panadaptor, you won't see all the spectrum from a wideband (say,
100 KHz spreading code) DSSS transmission. You may notice only a slightly
raised noise floor.
But that's only part of the deal with DSSS. The correlation and despreading
produces a really nice gain in noise immunity, as well.
--
Mike Andrews, W5EGO
[email protected]
Tired old sysadmin
