Brenda Ann wrote:

"Frank Dresser" wrote in message
...

Will BPL have the same effect on military radio as it has on radio
hobbyists? Don't they have spread spectrum capability which is highly
resistant to interference?

Frank Dresser


Thing is, BPL is also spread spectrum.. would not two spread spectrum
systems on the same bands interfere with one another??

Yes, but to a limited degree.

One characteristic which distinguishes a spread-spectrum
(whether frequency-hopping, or direct-sequence) system from
a channelized one, is the gradual way the channel
signal-to-noise ratio can degrade as more stations are added
in the same spectrum space. This "graceful degradation" is
in stark contrast to interference in standard
single-frequency applications, where a collision of two
signals means no link.

Think of two different transmitters sharing the same 1000
frequencies. Each "hops" to a new frequency every few
milliseconds at most. If both systems use different channel
sequences, it's just occasionally that both would land on
the same frequency and interfere with each other. And even
in this case, the total amount of time where they're
mutually interfering is perhaps a millisecond or less. So
the extra signal sharing the frequency has mostly the effect
of making the channel a tiny bit noisier for all users, but
not to blot out other signals.

As you add more and more spread-spectrum stations, the
probability of a "channel collision" of course increases.
However, by carefully choosing the sequences and making sure
they don't accidentally "lock horns" in synch for a while
following the same sequence of frequencies, the quality of
the link degrades slowly with each new station.

I suspect this "graceful degradation" property of spread
spectrum is more of a driving force than the potential
security and stealth that this system provides. It allows
more users to share the same small frequency slice, than
would be possible if you just put narrow-band FM equipment
and jammed them together as close as possible.

Funny how often unexpected benefits spin off of basic
research. Here's to renewing science and basic research.
Not everything important to business can be measured in
quarterly profits. Our competitors, who seem to get this
better than a lot of US companies do, will eat our lunch for
us if we don't start thinking long-term again. IBM and HP
"get it". It's Congress that we need to wake up next.

-- Ross