Jim Phillips asks:

>Okay, I'll ask the obvious question:  What good is a radio transmission that
>is undetectable?  I mean, isn't that the point of a radio transmission - to
>be detected?

The idea is for the signal to be undetectable to anyone who doesn't know
where and how to look for it. Many new digital wireless devices,
including cellular phones and wireless LAN connections, use this in order
to share a channel among multiple users. Each channel user has a unique
code which enables detection of transmissions to/from that user. Other
users won't even "see" those transmissions.

Most of today's spread-spectrum implementations use a technique called
direct-sequence spread spectrum, which is different from Hedy Lamarr's
invention. Her invention was frequency-hopping spread-spectrum, in which
the sender and receiver agree to switch frequencies at a rapid rate in a
prearranged random or pseudo-random sequence. An eavesdropper who doesn't
know the sequence can't "see" more than a tiny fraction of the
transmission.

There's lots more to this, of course; for an introduction, see the
December issue of MacTech magazine (which covers spread-spectrum use in
wireless LAN connections), or the Spread Spectrum Sourcebook, published
by ARRL (<http://www.arrl.org>).

-- Bruce


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