34 Spread spectrum techniques
A spread spectrum (SS) communications system is one in which the transmitted frequency spectrum is much wider than absolutely necessary. Amplitude modulation (AM) is usually considered to be a minimum bandwidth technique, thus by comparison, wideband frequency modulation (FM) represents an analogue example of a spread spectrum system. FM can be shown to have a signal to noise (S/N) ratio advantage over AM of 3β2F, where β is the deviation ratio and F is the ratio of peak deviation to baseband width.
Similarly the transmission bandwidth of a digital signal can be expanded so that many bit periods are necessary to represent one bit at baseband. Such a system has a processing gain expressed as the ratio of transmission ...
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