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http://Vada.skuu.ac.kr Kdream@nature.skku.ac.kr Spread spectrum modulation Chapter 9
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http://Vada.skuu.ac.kr Kdream@nature.skku.ac.kr 9.1 Introduction Digital communication Bandwidth(secure communication), power efficiency Spread spectrum modulation jamming, multipath-rejection, multiple-access communications Bandwidth in excess of the minimum bandwidth necessary A code that is independent of the data sequence(operating in synchronism with the transmitter) Direct-sequence spread spectrum Modulate a wide-band code Second modulation using a phase-shift keying Frequency happing pseudo-random or pseudo-noise sequence
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http://Vada.skuu.ac.kr Kdream@nature.skku.ac.kr 9.2 pseudo-noise sequences Pseudo-noise Periodic binary sequence with a noiselike waveform that is generated by means of a feedback shift register . 2 m -1 period Said to be linear when the feedback logic consists entirely of modulo-2 adder (not permitted zero state)
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http://Vada.skuu.ac.kr Kdream@nature.skku.ac.kr 9.2 pseudo-noise sequences Properties of maximum-length sequence have many of the properties possessed by a truly random binary sequence Balance property : the number of 1s is always one more than the number of 0s Run property : 1 run(1/2), 2 run(1/4), 3 run(1/8) total number of run = (N+1)/2 (when N=2 m -1) Correlation property :autocorrelation function of maximum-length sequence is periodic and binary valued. Because of the maximum-length sequence, R c( ) is similar to tat of a random binary wave Same envelope sinc 2 (fT) for their power spectral density Random binary sequence: continuous spectral density Maximum-length sequence:delta function spaced 1/NT c Hz
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http://Vada.skuu.ac.kr Kdream@nature.skku.ac.kr 9.2 pseudo-noise sequences Choosing a Maximum-Length Sequence How find the feedback logic : error control code Table 9.1: shift-register length & feedback taps, image 9.3 A notion of spread spectrum Attribute of spread- spectrum modulation Jamming(fairly powerful broadband noise, multi-tone waveform) Noise-like appearance so as to blend into the background) Spread spectrum modulation
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http://Vada.skuu.ac.kr Kdream@nature.skku.ac.kr 9.4 direct-sequence spread spectrum with coherent binary phase-shift keying Passband transmission over a satellite channel Mode for analysis Synchronization Acquisition and tracking
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