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Theory of Spread-Spectrum Communications



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IEEE TRANSACTIONS ON COMMUNICATIONS VOL NO COM 30 855 5 MAY 1982 Theory of Spread Spectrum Communications A Tutorial RAYMOND L PICKHOLTZ FELLOW I E EDONALD L SCHILLING AND LAURENCE B MILSTEIN SENIOR MEMBER IEEE AbstracrSpread spectrum communications with its inherent illterferenceattenuationcapability hasovertheyearsbecomean increasinglypopulartechniqueforuse inmanydifferentsysteml Applications range from antijam systems to code division multiple access systems to systems designed to combat multipath It is the of the theory c intention of this paper to provide a tutorial treatment spread spectrumcommunications includingadiscussion on the on the properties of commom applications referred to above spreadingsequences and on techniquesthatcanheusedfor a quisition and tracking I INTRODUCTION PREAD spectrum systems have been developed since aboutthe mid 1950 s The initial applications have bee1 to military antijamming tactical communications to guidance systems to experimental ahtimultipath systems and t 3 other applications l A definition of spread spectrurl that adequately reflects the characteristics of this techniqu is as follows S Spread spectrum is a means of transmission in which the signal occupies a bandwidth in excess of the minimum necessary to send the information the band spread is accomplished by means of a code which is independent of the data and a synchronized reception with the code at the receiver is used for despreading and subsequent data recovery Under this definition standard modulation schemes such as FM and PCM which also spread the spectrum of an information signal do not qualify as spread spectrum There are many reasons for spreading the spectrum and i f done properly a multiplicity of benefits can accrue simultaneously Some of these are 0 0 0 0 i 0 Antijamming Antiinterference Low probability of intercept Multiple user random access communications with selective addressing capability High resolution ranging Accurate universal timing Manuscript received December 22 1981 revised February 16 1982 R L Pickholtz is with the Department of Electrical Engineering anc Computer Science George Washington University Washington DC 20052 D L Schilling is with the Department of Electricai Engineering City College of New York New York NY 10031 L B Milstein is with the Department of Electrical Engineering ant Computer Science University of California at San Diego La Jolla CA 92093 FELLOW IEEE Themeansby which thespectnim is spread is crucial Several of the techniques are direct sequence moduldtion in which a fast pseudorandomly generated sequence causes phase transitions in the carrier containipg data frequency hopping in which the carrier iscaused to shift frequency in a pseudorandom way arid time hopping wherein bursts of signalare initiated at pseudorandom times Hybrid combinations of these techniques are frequently used Although the current applications for spread spectrum continue to be primarily for military communications there is a



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