SPREAD SPECTRUM COMMUNICATIONS
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Definitions of SPREAD SPECTRUM COMMUNICATIONS
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(Or "spread spectrum") A technique by which asignal to be transmitted is modulated onto a pseudorandom,noise-like, wideband carrier signal, producing atransmission with a much larger bandwidth than that of thedata modulation.Reception is accomplished by cross correlation of thereceived wide band signal with a synchronously generatedreplica of the carrier.Spread-spectrum communications offers many important benefits:Low probability of detection, interception or determination ofthe transmitter's location. To an observer who does notpossess information about the carrier, the transmission isindistinguishable from other sources of noise.High immunity against interference and jamming (intentionalinterference). The presence of (narrowband) interferencesignals only decreases the channel's signal-to noise ratioand therefore its error rate, which can be dealt with byusing error correcting codes. A jammer would have to usewideband interference signals, which would require very highpower (again assuming that the jammer does not know thecharacteristics of the carrier).High immunity against adverse effects of multipathtransmission. In the presence of multiple paths betweentransmitter and receiver (e.g. by reflected signals), signalsof certain frequencies can be cancelled at certain locationswhen the difference in path delays between multiplepropagation paths cause the signals to arrive out of phase.This effect is particularly troublesome in narrowband mobilecommunications, where it causes "blind spots" - locationswhere no signal can be received.Transmitter/receiver pairs using independent random carrierscan operate in the same frequency range with minimalinterference. These are called Code Division MultipleAccess (CDMA) systems. Increasing the number of T/R pairsagain only gradually increases each channel's error rate. Incontrast, narrowband systems can only accomodate a fixednumber of channels determined by available bandwidth andchannel width (data rate).When the data modulation cannot be distinguished from thecarrier modulation, and the carrier modulation is random to anunwanted observer, the spread spectrum system assumescryptographic capabilities, with the carrier modulation takingon the function of a key in a cipher system.The most important practical modes of spread spectrum codingare Direct Sequence (DS) and Frequency-Hopping SpreadSpectrum (FH).In DS, a pseudo random sequence is phase-shift-keyed (PSK)onto the carrier.Spread Spectrum development began during World War II, withthe earliest studies dating from the 1920s. Most papersremained classified until the 1980s.Direct sequence spread spectrum was invented by Paul Kotowskiand Kurt Dannehl at Telefunken.The technique is used extensively in military communicationstoday. Commercial applications include cellular telephonyand 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/).
By Denis Howe