spread spectrum communications
(foldoc) | 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://sirius.be/lamarr.htm),
(http://ncafe.com/chris/pat2/).
(2001-08-08)
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