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Data and Computer Communications

Data and Computer Communications

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Data and Computer Communications

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  1. 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

  2. Spread Spectrum All creative people want to do the unexpected. —Ecstasy and Me: My Life as a Woman, Hedy Lamarr

  3. Spread Spectrum

  4. Pseudorandom Numbershttp://www.math.utah.edu/~pa/Random/Random.htmlseealsorand() and srand() functions

  5. General Model of Spread Spectrum System

  6. 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

  7. Frequency Hopping Spread Spectrum (FHSS)

  8. Frequency Hopping Example

  9. FHSS (Transmitter)

  10. Frequency Hopping Spread Spectrum System (Receiver)

  11. 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

  12. Slow MFSK FHSS(PN: pseudo noise)

  13. Fast MFSK FHSS(PN: pseudo noise)

  14. 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

  15. Direct Sequence Spread Spectrum Example

  16. Direct Sequence Spread Spectrum System

  17. DSSS Example Using BPSK

  18. ApproximateSpectrum of DSSS Signal

  19. 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.

  20. Code Division Multiple Access (CDMA)

  21. CDMA Example

  22. CDMA “chip” Orthogonality : http://en.wikipedia.org/wiki/Chip_(CDMA) Or not…

  23. CDMA Example

  24. CDMA for DSSS

  25. Summary • concept of spread spectrum • FHSS • FHSS using MFSK • DSSS • DSSS using BPSK • Code Division Multiple Access • CDMA for Direct Sequence Spread Spectrum