Direct-sequence spread spectrum: Difference between revisions

Revision as of 00:11, 23 July 2003

Revision as of 00:24, 12 September 2003

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<b>2.</b> 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 addmultiply pseudorandoma "noise" signal to the data being transmitted, which is then removed by the receiver. TheThis resultingnoise signal resembles the result ofis a senderpseudorandom transmittingsequence anof audio<code>1</code> recordingand of<code>-1</code> staticvalues, exceptat thata thefrequency "static"much canhigher bethan filteredthat out at the receiving end to recoverof the original data. As this description suggestssignal, athereby plot ofspreading the transmittedenergy waveform has a roughly bell-shaped envelope centered onof the carrieroriginal frequency,signal just likeinto a normal [[Amplitude modulation|AM]] transmission, except that the added noise causes the distribution to be much wider thanband. that of an AM transmission.

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 x 1 = 1, and -1 x -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 [[Amplitude modulation|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|pseudo-random]] number generator.