Data Communications- Spread Spectrum

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Introduction - Spread SpectrumIntroduction - Spread Spectrum

Important form of encoding for wireless comm.Transmit either analog or digital data Analog signal (transmission)Developed initially for military and intelligence

requirementsSpread data over wider bandwidth

◦ Makes jamming and interception harderTypes of spread spectrum

◦ Frequency hoping Signal broadcast over seemingly random series of

frequencies◦ Direct Sequence

Each bit is represented by multiple bits in transmitted signal

Chipping code

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Spread Spectrum ConceptSpread Spectrum Concept Input fed into channel encoder

◦ Produces narrow bandwidth analog signal around central frequency

Signal modulated using sequence of digits ◦ Spreading code/sequence◦ Typically generated by pseudonoise/pseudorandom

number generator Increases bandwidth significantly

◦ Spreads the spectrumReceiver uses same sequence to demodulate

signalDemodulated signal fed into channel decoder

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General Model of Spread General Model of Spread Spectrum SystemSpectrum System

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GainsGainsImmunity from various noise and

multipath distortion◦ Including jamming

Can hide/encrypt signals◦ Only receiver who knows spreading code

can retrieve signalSeveral users can share same higher

bandwidth with little interference◦ Cellular telephones

Code division multiplexing (CDM) or Code division multiple access (CDMA)

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Pseudorandom NumbersPseudorandom NumbersGenerated by algorithm using initial

seed (initial value)Deterministic algorithm

◦ Not actually random◦ If algorithm good, results pass reasonable

tests of randomness Pseudorandom numbers

Need to know algorithm and seed to predict sequence◦ Only receiver sharing info with transmitter

able to decode the signal successfully.

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Frequency Hopping Spread Frequency Hopping Spread Spectrum (FHSS)Spectrum (FHSS)Signal broadcast over seemingly

random series of frequencies at fixed interval

Receiver hops between frequencies in sync with transmitter to pick msg

Eavesdroppers hear unintelligible blips

Jamming on one frequency affects only a few bits

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Basic OperationBasic OperationTypically 2k carriers frequencies forming

2k channelsChannel frequency spacing corresponds

with bandwidth of input signalEach channel used for fixed interval

◦ Eg:300 ms in IEEE 802.11◦ Some number of bits transmitted using some

encoding scheme May be fractions of bit (see later)

◦ Sequence dictated by spreading codeBoth transmitter & receiver use same

code

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Frequency Hopping Frequency Hopping ExampleExample

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FHSS Performance FHSS Performance ConsiderationsConsiderationsTypically large number of

frequencies used◦Improved resistance to jamming

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Direct Sequence Spread Direct Sequence Spread Spectrum (DSSS)Spectrum (DSSS)Each bit represented by multiple bits

using spreading codeSpreading code spreads signal across

wider frequency band◦ In proportion to number of bits used◦ 10 bit spreading code spreads signal

across 10 times bandwidth of 1 bit codeOne method:

◦ Combine input with spreading code using XOR (see figure)

Performance similar to FHSS

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Direct Sequence Spread Direct Sequence Spread Spectrum ExampleSpectrum Example

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Direct Sequence Spread Direct Sequence Spread Spectrum Using BPSK Spectrum Using BPSK ExampleExample

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Code Division Multiple Code Division Multiple Access (CDMA)Access (CDMA)Multiplexing technique used with spread

spectrumStart with data signal rate D

◦ Called bit data rateBreak each bit into k chips according to fixed

pattern specific to each user◦ User’s code

New channel has chip data rate kD chips per second

E.g. k=6, three users (A,B,C) communicating with base receiver R

Code for A = <1,-1,-1,1,-1,1>Code for B = <1,1,-1,-1,1,1>Code for C = <1,1,-1,1,1,-1>

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CDMA ExampleCDMA Example

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CDMA ExplanationCDMA ExplanationConsider A communicating with baseBase knows A’s codeAssume communication already

synchronizedA wants to send a 1

◦ Send chip pattern <1,-1,-1,1,-1,1> A’s code

A wants to send 0◦ Send chip pattern <-1,1,1,-1,1,-1>

Complement of A’s codeDecoder ignores other sources when using

A’s code to decode◦ Orthogonal codes

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