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. 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].

(1996-02-06)