spread spectrum communications

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 (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://ncafe.com/chris/pat2/.
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