Direct-sequence spread spectrum
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In telecommunication, the term direct-sequence spread spectrum has the following meanings:
- A system (a) for generating spread-spectrum transmissions by phase-modulating a sine wave pseudorandomly with a continuous string of pseudonoise code symbols, each of duration much smaller than a bit and (b) that may be time-gated, where the transmitter is keyed periodically or randomly within a specified time interval.
- A signal structuring technique utilizing a digital code sequence having a chip rate much higher than the information signal bit rate. Each information bit of a digital signal is transmitted as a pseudorandom sequence of chips.
Put simply, direct-sequence spread-spectrum transmissions multiply a "noise" signal to the data being transmitted. This noise signal is a pseudorandom sequence of 1 and -1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.
The resulting signal resembles white noise, like an audio recording of "static", except that this noise can be filtered out at the receiving end to recover the original data, by again multiplying the same pseudorandom sequence to the received signal (because 1 × 1 = 1, and -1 × -1 = 1).
As this description suggests, a plot of the transmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission.
By contrast, Frequency-hopping spread spectrum pseudo-randomly retunes the carrier, instead of adding pseudo-random noise to the data, which results in a uniform frequency distribution whose width is determined by the output range of the pseudo-random number generator.
References
- Federal Standard 1037C
- NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management