From: john@anasazi.com (John R. Moore)
Subject: Re: What's the difference.....?
Reply-To: john@anasazi.com
Organization: Anasazi Inc, Phoenix AZ USA
Date: Tue, 15 Sep 1992 16:20:57 GMT

Keywords: 

In article <1992Sep14.215718.7783@rsd0.rsd.dl.nec.com> dave@rsd.dl.nec.com (Dave Rogers) writes:
]In article <1992Sep11.175536.1@novax.llnl.gov>, urbina@novax.llnl.gov writes:
]|> 
]|> 
]|> What's the difference between spread spectrum and frequency hopping ?
]|> Also, what is meant by "BURST" transmissions in the context of covert
]|> communications ?
]|> 
]
]Spread spectrum is a modulation technique in which the data is
]essentially transmitted in paralle on many frequencies simultaneously. 
]
]Frequency hopping involves transmitting on many frequencies on after
]another. 

Actually, frequency hopping is usually called spread spectrum. There
are two primary sorts of spread spectrum: direct sequence, and frequency
hopping. In direct sequence, the transmitted signal is mixed with a
wide band, pseudo-random "noise", which results in it being spread
out across a wide frequency range, with noise (flat) spectral characteristics.
If it is spread out far enough, the resulting signal, look at with
any receiver other than a spread spectrum receiver sync'd to the 
same pseudo-random signal, is invisible and undetectable.
Frequency hopping consists of rapidly moving among a bunch of frequencies.
The transmitter may move relatively slowly (40 times a second) or
very fast (thousands of times a second). The faster it hops (the higher
"chip" rate), the more the signal looks like a direct sequence
system, and the more undetectable it gets.

I once found a spread spectrum transmitter. A local ham repeater was
being keyed up about once a second, and no one could find what was
doing it, so they called me. I started driving around town, and heard
a signal that sounded like a single click about once a second. Using
signal strength, I eventually narrowed in on the area downtown right
next to the building that the repeater sits on. As I got closer, the
signal changed from "click...........click............." to
"CLICKETY-click.......CLICKETY-click......." and then to
"CLICKETY-click-CLICK-click...CLICKETY-click-CLICK-click". I finally
pulled up next to a renta-car with a mag mount antenna on it, and
by this time the receiver was going "CLICKETY-CLIKETY-CLIKETA".

Looking in the car I found a box on the floor that looked industrial
(as opposed to consumer styling), and that was attached to the coax from
the antenna and had a power plug in the cigarette lighter. The signal
was clearly originating from the box.

A little more investigation led me to someone who knew what was
in the car (I am being purposefully vague here). It turned out that
it was a covert spread spectrum transmitter being operated by
a certain national agency. They were quite shocked that I had found it.
They had neglected to check for local two meter repeaters near their
site, and the widget operated in the two meter ham band.

This device was an example of a frequency hop spread spectrum transmitter.
It was a slow hopper, going only about 40 hops per second. The reason
that I heard more "clicks" as I got closer is that each hop generates
a lot of sidebands (splatter) that are quite weak, so the closer I got,
the more of these I heard on my narrowband receiver.

]All of these modes are primarily for digital data and are commonly
]combined to further obscure the transmission. 

Spread spectrum is also used for voice... typically (but not necessarily)
by digitizing the voice before transmitting it.


-- 
John Moore NJ7E, 7525 Clearwater Pkwy, Scottsdale, AZ 85253  (602-951-9326)
john@anasazi.com ncar!noao!asuvax!anasaz!john anasaz!john@asuvax.eas.asu.edu
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