Transcript
Spread SpectrumCEN 220/CIS 192 Advanced Data Communications and NetworkingData and Computer Communications, W. Stallings 9/E, Chapter 9
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Spread Spectrum“All creative people want to do the unexpected.”
—Ecstasy and Me: My Life as a Woman,
Hedy Lamarr
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Spread Spectrum
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Pseudorandom Numbers
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General Model of Spread Spectrum System
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Spread Spectrum AdvantagesImmunity from noise and multipath distortion
Can hide / encrypt signals
Several users can share same higher bandwidth with little interference
CDM/CDMA Mobile telephones
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Frequency Hopping Spread Spectrum (FHSS)
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Frequency Hopping Example
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FHSS (Transmitter)
Frequency Hopping Spread Spectrum (FHSS)
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FHSS (Receiver)
Frequency Hopping Spread Spectrum (FHSS)
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Slow and Fast FHSSCommonly 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
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Slow MFSK FHSS
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Fast MFSK FHSS
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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
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Direct Sequence Spread Spectrum Example
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Direct Sequence Spread Spectrum System
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DSSS Example Using BPSK
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ApproximateSpectrum of DSSS Signal
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Code Division Multiple Access (CDMA)
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CDMA Example
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CDMA Example
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CDMA for DSSS
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SummaryConcept of spread spectrum
FHSS FHSS using MFSK
DSSS DSSS using BPSK
Code Division Multiple Access CDMA for Direct Sequence Spread Spectrum
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