Alan,
What happens, for example, if 100 DSSS stations are all on at the same
time, on the same beginning and ending frequencies, because everyone
assumes his presence at any one frequency is too short to be noticed?
Will they interfere with each other, or will they collectively interfere
with other users of the frequency, such as SSB stations?
When you say "multiple" how many would that be with a spreading factor
of 100?
It seems to me that enough chips randomly spread over the band (by
enough multiple stations) could also raise the general noise level, even
if they were very weak. This was a concern of weak signal operators.
For example, suppose it was decided to let multiple DSSS stations span
the whole length of the 20m phone band so there was sufficient
spreading. How many on the air at one time would it take to create
noticeable QRM to SSB phone stations, or raise the noise background if
they were on VHF?
I ask this because I believe that the question arose several years ago
regarding allowing hi-speed multimedia to operate over 20 kHz on 20m,
which may be OK for one station, but what happens if there are 100
stations doing the same thing?
If there are enough randomly dispersed chips, won't they eventually fill
the entire area with if there are enough of them?
I studied communications theory and auto-correlation functions, etc., 50
years ago in college, but unfortunately I don't remember much of it at all!
73, Skip KH6TY
On 7/13/2010 8:15 PM, Alan Barrow wrote:
W2XJ wrote:
>
>
> It is generally accepted that 10 times bandwidth is the minimum
> necessary to achieve enough processing gain to make the use of SS
> worthwhile.
Not only is it not worth doing, it also increased chances of
interference. I'm not aware of any weak signal DSSS using spreading
factors of less than 100. The lowest I've seen is 16 for consumer strong
signal wide band stuff. And that's just due to economics, not for
performance.
Take that same psk'ish data rate, use a more conventional spreading
factor of 128, and you could see decent weak signal performance due to
processor gain, and most likely not impact strong signal legacy modes in
the same band segment.
Of course, you could not do this with an audio SSB approach. But you
could certainly decode it with SDR, which is why we should not throw out
the baby with the bathwater.
Remember, ROS somewhat sucked because it's spreading was so small there
was a large likelihood of any given bit interfering with another weak
signal.
Spread that out, and it's only the individual "chips" (fraction of a
data bit) that is on any given frequency at any given time.
Put another way, you could probably run multiple DSSS signals at psk
data rates in the SSB (voice) sub-bands with minimal impact to existing
qso's if spread like conventional DSSS. You could see the impact on a
properly setup monitor, but realistically the SSB stations would not
detect the chips in their slot.
Not that I'm proposing we do so, just that we need to fully understand
the technology, it's potential advantages & impacts before we throw it
out.
All that said, I'm not expecting to see any SS on HF by hams in the next
decade or two. I view it as a lost cause and we'll just learn to deal
with the beeps & bloops from advance digital modes from non-amateur
services on our shared bands.
Have fun,
Alan
km4ba
