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CS420/520 Axel Krings Sequence 11Page 1
Spread Spectrum Spread data over wide bandwidth Makes jamming and interception harder 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 Concept Input fed into channel encoder
Produces narrow bandwidth analog signal around centralfrequency
Signal modulated using sequence of digits Spreading code/sequence Typically generated by pseudonoise/pseudorandom number
generator Increases bandwidth significantly
Spreads spectrum
Receiver uses same sequence to demodulate signal Demodulated signal fed into channel decoder
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CS420/520 Axel Krings Sequence 11Page 3
General Model of SpreadSpectrum System
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Gains Immunity from various noise and multipath
distortion Including jamming
Can hide/encrypt signals Only receiver who knows spreading code can retrieve
signal Several users can share same higher bandwidth
with little interference Cellular telephones Code division multiplexing (CDM) Code division multiple access (CDMA)
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CS420/520 Axel Krings Sequence 11Page 5
Pseudorandom Numbers Generated by algorithm using initial seed Deterministic algorithm
Not actually random If algorithm good, results pass reasonable
tests of randomness
Need to know algorithm and seed topredict sequence
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Frequency Hopping SpreadSpectrum (FHSS) Signal 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
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CS420/520 Axel Krings Sequence 11Page 7
Basic Operation Typically 2 k carriers frequencies forming
2k channels Channel spacing corresponds with
bandwidth of input Each channel used for fixed interval
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 code
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Frequency Hopping Example
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CS420/520 Axel Krings Sequence 11Page 9
Frequency Hopping SpreadSpectrum System (Transmitter)
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Frequency Hopping SpreadSpectrum System (Receiver)
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CS420/520 Axel Krings Sequence 11Page 11
Slow and Fast FHSS Frequency shifted every T c seconds Duration of signal element is T s seconds Slow FHSS has T c Ts Fast FHSS has T c < T s Generally fast FHSS gives improved
performance in noise (or jamming)
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Slow Frequency Hop SpreadSpectrum Using MFSK (M=4, k=2)
MFSK = Multiple FSK
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CS420/520 Axel Krings Sequence 11Page 13
Fast Frequency Hop SpreadSpectrum Using MFSK (M=4, k=2)
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FHSS PerformanceConsiderations Typically large number of frequencies
used Improved resistance to jamming
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CS420/520 Axel Krings Sequence 11Page 15
Direct Sequence SpreadSpectrum (DSSS) Each bit is represented by multiple bits using
spreading code Spreading code spreads signal across wider
frequency band In proportion to number of bits used e.g., 10 bit spreading code spreads signal across 10
times bandwidth of 1 bit code One method:
Combine input with spreading code using XOR Input bit 1 inverts spreading code bit Input zero bit doesnt alter spreading code bit
Data rate equal to original spreading code Performance similar to FHSS
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Direct Sequence SpreadSpectrum Example
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Direct Sequence SpreadSpectrum Transmitter
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Direct Sequence SpreadSpectrum Transmitter
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CS420/520 Axel Krings Sequence 11Page 19
Direct Sequence SpreadSpectrum Using BPSK Example
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ApproximateSpectrum of DSSS Signal
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CS420/520 Axel Krings Sequence 11Page 21
Code Division Multiple Access(CDMA) Multiplexing Technique used with spread spectrum Start with data signal rate D
Called bit data rate Break each bit into k chips according to fixed pattern
specific to each user Users code
New channel has chip data rate kD chips per second
E.g. k =6, three users (A,B,C) communicating with basereceiver R Code for A =
Code for B =
Code for C =
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CDMA Example
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CDMA Explanation Consider A communicating with base Base knows As code Assume communication already synchronized A wants to send a 1
Send chip pattern As code
A wants to send 0 Send chip[ pattern
Complement of As code
Decoder ignores other sources when using Ascode to decode Orthogonal codes
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CDMA for DSSS n users each using different orthogonal
PN sequence Modulate each users data stream
Using BPSK
Multiply by spreading code of user
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CDMA in a DSSS Environment
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Seven Channel CDMA Encodingand Decoding
positive number = 1negative number = 0