spread spectrum communications

    (Or "spread spectrum") A technique by which a
   signal to be transmitted is modulated onto a pseudorandom,
   noise-like, wideband carrier signal, producing a
   transmission with a much larger bandwidth than that of the
   data modulation.

   Reception is accomplished by cross correlation of the
   received wide band signal with a synchronously generated
   replica of the carrier.

   Spread-spectrum communications offers many important benefits:

   Low probability of detection, interception or determination of
   the transmitter's location.  To an observer who does not
   possess information about the carrier, the transmission is
   indistinguishable from other sources of noise.

   High immunity against interference and jamming (intentional
   interference).  The presence of (narrowband) interference
   signals only decreases the channel's signal-to noise ratio
   and therefore its error rate, which can be dealt with by
   using error correcting codes.  A jammer would have to use
   wideband interference signals, which would require very high
   power (again assuming that the jammer does not know the
   characteristics of the carrier).

   High immunity against adverse effects of multipath
   transmission.  In the presence of multiple paths between
   transmitter and receiver (e.g. by reflected signals), signals
   of certain frequencies can be cancelled at certain locations
   when the difference in path delays between multiple
   propagation paths cause the signals to arrive out of phase.
   This effect is particularly troublesome in narrowband mobile
   communications, where it causes "blind spots" - locations
   where no signal can be received.

   Transmitter/receiver pairs using independent random carriers
   can operate in the same frequency range with minimal
   interference.  These are called Code Division Multiple
   Access (CDMA) systems.  Increasing the number of T/R pairs
   again only gradually increases each channel's error rate.  In
   contrast, narrowband systems can only accomodate a fixed
   number of channels determined by available bandwidth and
   channel width (data rate).

   When the data modulation cannot be distinguished from the
   carrier modulation, and the carrier modulation is random to an
   unwanted observer, the spread spectrum system assumes
   cryptographic capabilities, with the carrier modulation taking
   on the function of a key in a cipher system.

   The most important practical modes of spread spectrum coding
   are Direct Sequence (DS) and Frequency-Hopping Spread
   Spectrum (FH).

   In DS, a pseudo random sequence is phase-shift-keyed (PSK)
   onto the carrier.

   Spread Spectrum development began during World War II, with
   the earliest studies dating from the 1920s.  Most papers
   remained classified until the 1980s.

   Direct sequence spread spectrum was invented by Paul Kotowski
   and Kurt Dannehl at Telefunken.

   The technique is used extensively in military communications
   today.  Commercial applications include mobile telephony
   and mobile networking.

   ["Spread Spectrum Communications", Charles E. Cook et al
   (Ed.), IEEE Press, New York, 1983. ISBN 0-87942-170-3].

   <Hedy Lamarr>,
   <http://ncafe.com/chris/pat2/>.

   (2001-08-08)