Data and Computer Communications. Chapter 9 – Spread Spectrum. Ninth Edition by William Stallings. Spread Spectrum. All creative people want to do the unexpected . —Ecstasy and Me: My Life as a Woman , Hedy Lamarr. Spread Spectrum.
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Data and Computer Communications Chapter 9 – Spread Spectrum Ninth Edition by William Stallings Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011
Spread Spectrum All creative people want to do the unexpected. —Ecstasy and Me: My Life as a Woman, Hedy Lamarr
Pseudorandom Numbershttp://www.math.utah.edu/~pa/Random/Random.htmlseealsorand() and srand() functions
Spread Spectrum Advantages • immunity from noise and multipath distortion • can hide / encrypt signals • several users can share same higher bandwidth with little interference • CDM/CDMA Mobile telephones
Slow and Fast FHSS • commonly use multiple FSK (MFSK) • have frequency shifted every Tc seconds • duration of signal element is Ts seconds • Slow FHSS has Tc Ts • Fast FHSS has Tc < Ts • FHSS quite resistant to noise or jamming • with fast FHSS giving better performance
Direct Sequence Spread Spectrum (DSSS) • each bit is represented by multiple bits using a spreading code • this spreads signal across a wider frequency band • has performance similar to FHSS
Code Division Multiple Access (CDMA) Allocatingfrequencychannelson-demand in a multi-users environnement is inefficient (needcomplexcircuitry and protocol). More efficient to giveeach user all the bandwith and givehim a « key » to encode his transmission.
CDMA “chip” Orthogonality : http://en.wikipedia.org/wiki/Chip_(CDMA) Or not…
Summary • concept of spread spectrum • FHSS • FHSS using MFSK • DSSS • DSSS using BPSK • Code Division Multiple Access • CDMA for Direct Sequence Spread Spectrum