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spread spectrum communications
communications (Or "spread spectrum") A technique by which a
transmission with a much larger bandwidth
than that 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
and therefore its error rate
, which can be dealt with by
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
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
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
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
and mobile networking.
["Spread Spectrum Communications", Charles E. Cook et al
(Ed.), IEEE Press, New York, 1983. ISBN 0-87942-170-3].