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SPREAD SPECTRUM

SPREAD SPECTRUM. In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy. Topics discussed in this section:.

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SPREAD SPECTRUM

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  1. SPREAD SPECTRUM In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy. Topics discussed in this section: Frequency Hopping Spread Spectrum (FHSS)Direct Sequence Spread Spectrum Synchronous (DSSS)

  2. 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

  3. Figure 6.27 Spread spectrum

  4. General Model of Spread Spectrum System

  5. 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

  6. 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 • hence only receiver can decode signal

  7. Frequency Hopping Spread Spectrum (FHSS) • signal is broadcast over seemingly random series of frequencies • receiver hops between frequencies in sync with transmitter • eavesdroppers hear unintelligible blips • jamming on one frequency affects only a few bits

  8. Figure 6.28 Frequency hopping spread spectrum (FHSS)

  9. Figure 6.29 Frequency selection in FHSS

  10. Figure 6.30 FHSS cycles

  11. Figure 6.31 Bandwidth sharing

  12. Frequency Hopping Example

  13. FHSS (Transmitter)

  14. Frequency Hopping Spread Spectrum System (Receiver)

  15. 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

  16. Slow MFSK FHSS

  17. Fast MFSK FHSS

  18. 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

  19. Figure 6.32 DSSS

  20. Figure 6.33 DSSS example

  21. Direct Sequence Spread Spectrum Example

  22. Direct Sequence Spread Spectrum System

  23. DSSS Example Using BPSK

  24. ApproximateSpectrum of DSSS Signal

  25. Code Division Multiple Access (CDMA) • a multiplexing technique used with spread spectrum • given a data signal rate D • break each bit into k chips according to a fixed chipping code specific to each user • resulting new channel has chip data rate kD chips per second • can have multiple channels superimposed

  26. CDMA Example

  27. CDMA for DSSS

  28. Summary • looked at use of spread spectrum techniques: • FHSS • DSSS • CDMA

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