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Lecture Wireless and mobile computing – Chapter 21: Spread spectrum

The following will be discussed in this chapter: Spread spectrum intro, spread spectrum model, pseudorandom sequences, pn sequence generator, frequency hopping spread spectrum, direct sequence spread spectrum, processing gain, DSSS using BPSK. | Spread Spectrum Lecture 21 Overview Spread Spectrum Intro Spread Spectrum Model Pseudorandom Sequences PN Sequence Generator Frequency Hopping Spread Spectrum Direct Sequence Spread Spectrum Processing Gain DSSS Using BPSK Spread Spectrum Spread spectrum technology has blossomed from a military technology into one of the fundamental building blocks in current and next-generation wireless systems. From cellular to cordless to wireless LAN (WLAN) systems, spectrum is a vital component in the system design process. Spread Spectrum Wideband Modulation Benefits Information Security Interference Resistance Band Sharing Frequency Hopping Spread Spectrum (FHSS) Data are constant Frequencies are randomized Direct Sequence Spread Spectrum (DSSS) Frequency is constant Data are randomized immunity from noise and multipath distortion can hide / encrypt signals several users can share same higher bandwidth with little interference CDM/CDMA Mobile telephones Spread Spectrum Important encoding method for wireless communications Analog & digital data with analog signal Spreads data over wide bandwidth Makes jamming and interception harder Two approaches, both in use: Frequency Hopping Direct Sequence Communication Systems General Model of Spread Spectrum System Pseudorandom Sequences The spread of the “random” sequence of frequencies is determined by a pseudonoise (PN) sequence generator. A PN generator outputs a stream of bits (1s and 0s) that appears random (has no apparent pattern). PN sequence generators are easy to construct using simple logic components: XOR gates, and a shift register, made up of flip flops A PN sequence is not truly random (hence, “pseudo”), but is periodic and repeats at a fixed interval. Pseudorandom Numbers Generated by a deterministic algorithm not actually random but if algorithm good, results pass reasonable tests of randomness Starting from an initial seed Need to know algorithm and seed to predict sequence | Spread Spectrum Lecture 21 Overview Spread Spectrum Intro Spread Spectrum Model Pseudorandom Sequences PN Sequence Generator Frequency Hopping Spread Spectrum Direct Sequence Spread Spectrum Processing Gain DSSS Using BPSK Spread Spectrum Spread spectrum technology has blossomed from a military technology into one of the fundamental building blocks in current and next-generation wireless systems. From cellular to cordless to wireless LAN (WLAN) systems, spectrum is a vital component in the system design process. Spread Spectrum Wideband Modulation Benefits Information Security Interference Resistance Band Sharing Frequency Hopping Spread Spectrum (FHSS) Data are constant Frequencies are randomized Direct Sequence Spread Spectrum (DSSS) Frequency is constant Data are randomized immunity from noise and multipath distortion can hide / encrypt signals several users can share same higher bandwidth with little interference CDM/CDMA Mobile telephones Spread Spectrum Important