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Show CHAPTER 2 OVERVIEW OF SPREAD SPECTRUM COMMUNICATIONS Most communication systems are concerned with the efficiency with which these systems utilize signal energy and band-width[22]. These are legitimate concerns, and in most communication systems are the concerns of paramount importance. When it is necessary, however, for the system to resist external interference, to operate with a low-energy spectral density, to provide multiple-access capability without external control or to make it difficult for unauthorized receivers to observe the message, it may be appropriate to sacrifice the efficiency aspects of the system in order to enhance these other features. Spread-spectrum techniques offer one way to accomplish this objective. Spread-spectrum communications is a digital communication method in which the bandwidth of the message signal is made much larger than would be ordinarily required for transmission. The spectral density of the signal radiated is small at any given frequency and covers a much wider bandwidth than the spectrum of the original message signal, and looks very much like white noise. Upon reaching the receiver, the bandwidth of the signal is collapsed to its original size and the message is recovered. This is called spreading and despreading. There are several methods of spreading and despreading operations used, most notably direct sequence (DS), frequency hopping(FH) and time hopping(TH). For a co1nmunication system to be considered a spread-spectrum, it is necessary that the transmitted signal satisfy two criteria(24]. First, as mentioned above, the bandwidth of the transmitted signal must be much greater than the message bandwidth. This, by itself, is not sufficient because many modulation methods like |
