spread spectrum
DESCRIPTION
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:. - PowerPoint PPT PresentationTRANSCRIPT
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SPREAD SPECTRUMSPREAD SPECTRUM
In spread spectrum (SS), we combine signals from In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add achieve these goals, spread spectrum techniques add redundancy.redundancy.
Frequency Hopping Spread Spectrum (FHSS)Direct Sequence Spread Spectrum Synchronous (DSSS)
Topics discussed in this section:Topics discussed in this section:
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Spread SpectrumSpread Spectrum important encoding method for wireless important encoding method for wireless
communicationscommunications analog & digital data with analog signalanalog & digital data with analog signal spreads data over wide bandwidthspreads data over wide bandwidth makes jamming and interception hardermakes jamming and interception harder two approaches, both in use:two approaches, both in use:
Frequency HoppingFrequency Hopping Direct SequenceDirect Sequence
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Figure 6.27 Spread spectrum
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General Model of Spread General Model of Spread Spectrum SystemSpectrum System
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Spread Spectrum AdvantagesSpread Spectrum Advantages immunity from noise and multipath immunity from noise and multipath
distortiondistortion can hide / encrypt signalscan hide / encrypt signals several users can share same higher several users can share same higher
bandwidth with little interferencebandwidth with little interference CDM/CDMA Mobile telephonesCDM/CDMA Mobile telephones
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Pseudorandom NumbersPseudorandom Numbers generated by a deterministic algorithmgenerated by a deterministic algorithm
not actually randomnot actually random but if algorithm good, results pass reasonable but if algorithm good, results pass reasonable
tests of randomnesstests of randomness starting from an initial seedstarting from an initial seed need to know algorithm and seed to need to know algorithm and seed to
predict sequencepredict sequence hence only receiver can decode signalhence only receiver can decode signal
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Frequency Hopping Spread Frequency Hopping Spread Spectrum (FHSS)Spectrum (FHSS)
signal is broadcast over seemingly random signal is broadcast over seemingly random series of frequenciesseries of frequencies
receiver hops between frequencies in sync receiver hops between frequencies in sync with transmitterwith transmitter
eavesdroppers hear unintelligible blipseavesdroppers hear unintelligible blips jamming on one frequency affects only a jamming on one frequency affects only a
few bitsfew bits
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Figure 6.28 Frequency hopping spread spectrum (FHSS)
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Figure 6.29 Frequency selection in FHSS
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Figure 6.30 FHSS cycles
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Figure 6.31 Bandwidth sharing
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Frequency Hopping ExampleFrequency Hopping Example
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FHSS (Transmitter)FHSS (Transmitter)
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Frequency Hopping Spread Frequency Hopping Spread Spectrum System (Receiver)Spectrum System (Receiver)
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Slow and Fast FHSSSlow and Fast FHSS commonly use multiple FSK (MFSK)commonly use multiple FSK (MFSK) have frequency shifted every Thave frequency shifted every Tcc seconds seconds duration of signal element is Tduration of signal element is Tss seconds seconds Slow FHSS has TSlow FHSS has Tcc T Tss
Fast FHSS has TFast FHSS has Tcc < T < Tss
FHSS quite resistant to noise or jammingFHSS quite resistant to noise or jamming with fast FHSS giving better performancewith fast FHSS giving better performance
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Slow MFSK FHSSSlow MFSK FHSS
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Fast MFSK FHSSFast MFSK FHSS
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Direct Sequence Spread Direct Sequence Spread Spectrum (DSSS)Spectrum (DSSS)
each bit is represented by multiple bits each bit is represented by multiple bits using a spreading codeusing a spreading code
this spreads signal across a wider this spreads signal across a wider frequency bandfrequency band
has performance similar to FHSShas performance similar to FHSS
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Figure 6.32 DSSS
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Figure 6.33 DSSS example
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Direct Sequence Spread Direct Sequence Spread Spectrum ExampleSpectrum Example
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Direct Sequence Spread Direct Sequence Spread Spectrum SystemSpectrum System
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DSSS Example Using BPSKDSSS Example Using BPSK
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ApproximateApproximateSpectrum of Spectrum of DSSS SignalDSSS Signal
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Code Division Multiple Code Division Multiple Access (CDMA)Access (CDMA)
a multiplexing technique used with spread a multiplexing technique used with spread spectrumspectrum
given a data signal rate Dgiven a data signal rate D break each bit into break each bit into kk chips according to a chips according to a
fixed chipping code specific to each userfixed chipping code specific to each user resulting new channel has chip data rate resulting new channel has chip data rate kDkD chips per second chips per second
can have multiple channels superimposedcan have multiple channels superimposed
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CDMA ExampleCDMA Example
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CDMA for DSSSCDMA for DSSS
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SummarySummary looked at use of spread spectrum looked at use of spread spectrum
techniques:techniques: FHSSFHSS DSSSDSSS CDMACDMA