Source : Free On-Line Dictionary of Computing

spread spectrum communications
     
         (Or "spread spectrum") A technique by which a
        signal to be transmitted is modulated onto a {pseudo-random},
        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 (FH).  In DS, a
        pseudo random sequence is phase-shift-keyed (PSK) onto the
        carrier.  In FH, a {frequency synthesizer} is driven by a
        pseudo random sequence of numbers to generate output
        frequencies that "hop around" in the desired frequency range.
     
        Spread Spectrum development began during World War II, with
        the earliest studies dating from the 1920s.  Most papers
        remained classified until the 1980s.
     
        Frequency hopping spread spectrum was invented by Hedy Lamarr
        ("the most beautiful girl in the world", Samson and Delilah
        etc.) and the composer George Antheil.  They held a patent
        filed in 1942.  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 {cellular 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://www.sirius.be/lamarr.htm)},
        {(http://www.ncafe.com/chris/pat2/)}.
     
        (2001-08-08)