Jose,
If you were going to design a mode that filled 2200 Hz, but did not use
SS, and was as sensitive as possible in that bandwidth, how would you do
it? It would have to be highly resistant to fast Doppler shift also, but
minimum S/N would be the most important parameter, as it would be used
at UHF. So far, Olivia 16-500 seems to be the best compromise between
minimum S/N and Doppler shift survival at UHF. The more narrow Olivia
modes, even though more sensitive, do not decode as well if there is
noticeable fast Doppler shift, and sometimes, not at all. DominoEx is
completely destroyed by the Doppler shift and MFSK16 is not tolerant
enough to drift to be usable at UHF. MT63-2000 covers 2000 Hz, has
highly redundant FEC, but the minimum S/N is only -2 dB, so that is not
an alternative.
What I am looking for is a mode that will copy under the visible and
audible noise on UHF during deep fades, but survives fast Doppler shift.
Olivia 16-500 makes it down to the noise, but not under, during deep
fades. CW by ear is just slightly better than Olivia 16-500, and the
note is very raspy sounding - much like Aurora communications.
Another observation - most stations I copy on ROS 16 are reading a
metric of -12 dB or greater. Only once have I copied a station (using 1
baud ROS) that was measuring a metric under -25 dB. Is the ROS "metric"
supposed to correlate with the path S/N? I ask this because even the
weakest ROS tones at 1 baud are still visible on the waterfall, whereas
weak Olivia 32-1000 signals with a -12 dB minimum S/N stop decoding just
about the time the tones become hard to see in the noise, but still can
be heard faintly. It is a long way from even -25 dB S/N to -12 dB S/N,
so I would expect if the metric is just another way to say S/N, I would
not be able to see the tones, yet I can, and not only on the ROS
waterfall, but on the DigiPan waterfall as well.
73 - Skip KH6TY
Jose A. Amador wrote:
El 10/03/2010 7:57, g4ilo escribió:
> What does ROS gain by using SS over another mode that carries the
same amount of data at the same speed using the same bandwidth and the
same number of tones but uses an entirely predictable method of
modulation?
Processing gain. Signals correlated with the hopping sequence add up,
non correlated signals do not add up.
It does not mean that SS is not a predictable modulation method, you
just need to know the key, in the USA, the key must be one of a few
specific codes, and if you don't have the key, "security by obscurity"
applies.
73,
Jose, CO2JA
