[digitalradio] A closer look at ROS]]

Mike O'Dell mo at ccr.org
Fri Feb 26 06:21:58 CST 2010

SS vs forward-error-correction:
2-dimensional redundancy vs 1-dimensional redundancy

another way to think about it is that the
spreading sequence is a forward-error-correction
code which transforms every payload symbol
into a sequence of smaller symbols (chips)
such that receiving slightly more than half of the chip
sequence correctly is sufficient to accurately
identify the original payload symbol.

and if one stopped there, and transmitted the chip
sequence within the same (or less) bandwidth than
would have been occupied by the original payload symbol,
it is forward error correction with maximum-likelihood
decisions at the receiver.

to make it spread-spectrum, the chip sequence is transmitted
using at least circa 10x the bandwidth of the original payload
symbol in order to de-correlate the chip signal with narrow-band
interference. this amounts to "redundant information in the
frequency domain" as well as the chip coding introducing
"redundant information in the time domain".

so a real SS signal carries redundancy in TWO dimensions -
both time and spectrum, whereas forward error correction alone
simply carries redundancy in the time domain.


On 2/25/10 9:50 PM, andre kesteloot wrote:
> Per-Tore Aasestrand wrote:
>> Hmmm... Are you sure this is correct? I thought processing gain was
>> real in case the noise can be considered white over the signal bandwidth.
>> Per-Tore / LA7NO
> Per-Tore,
> You are absolutely correct If, and only IF you mention the improvement
> in S/N ratio.
> But,
> a) people do not necessarily use Spread Spectrum for S/N improvement,
> but implement these technologies to avoid detection, or interference
> from other signals.
> (see the /ARRL SS Sourcebook/ for details. Incidentally, I am not being
> compensated for this free publicity :-)
> b) almost everyone I have met believes --erroneously-- that is is real
> gain, comparable to, for instance, amplifier gain.
> 73
> André N4ICK
> .
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