Editing Direct-sequence spread spectrum (section)

==Transmission method==
Direct-sequence spread-spectrum transmissions multiply the symbol sequence being transmitted with a spreading sequence that has a higher rate than the original message rate. Usually, sequences are chosen such that the resulting spectrum is spectrally [[white noise|white]].  Knowledge of the same sequence is used to reconstruct the original data at the receiving end. This is commonly implemented by the element-wise multiplication with the spreading sequence, followed by summation over a message symbol period. This process, ''despreading'', is mathematically a [[correlation]] of the transmitted spreading sequence with the spreading sequence. In an AWGN channel, the despreaded signal's [[signal-to-noise ratio]] is increased by the spreading factor, which is the ratio of the spreading-sequence rate to the data rate.

While a transmitted DSSS signal occupies a wider bandwidth than the direct modulation of the original signal would require, its spectrum can be restricted by conventional [[Pulse shaping|pulse-shape filtering]].

If an undesired transmitter transmits on the same channel but with a different spreading sequence, the despreading process reduces the power of that signal. This effect is the basis for the [[code-division multiple access]] (CDMA) method of multi-user medium access, which allows multiple transmitters to share the same channel within the limits of the [[cross-correlation]] properties of their spreading sequences.