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CS420/520 Axel Krings Sequence 9Page 1
Chapter 9: Spread Spectrum
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Spread Spectrum
Important encoding method for wirelesscommunications
Spread data over wide bandwidth
Makes jamming and interception harder
Frequency hoping
Signal broadcast over seemingly random series offrequencies
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 numbergenerator
Increases bandwidth significantly
Spreads spectrum
Receiver uses same sequence to demodulate signal Demodulated signal fed into channel decoder
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General Model of Spread
Spectrum System
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Spread Spectrum Advantages
Immunity from various noise and multipathdistortion
Including jamming
Can hide/encrypt signalsOnly receiver who knows spreading code can retrieve
signal
Several users can share same higher bandwidthwith little interference
Cellular telephones
Code division multiplexing (CDM)
Code division multiple access (CDMA)
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Pseudorandom Numbers Generated by algorithm using initial seed
Deterministic algorithm
Not actually random
If algorithm good, results pass reasonabletests of randomness
Need to know algorithm and seed to
predict sequence
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Frequency Hopping Spread
Spectrum (FHSS) Signal broadcast over seemingly random
series of frequencies
Receiver hops between frequencies insync with transmitter
Eavesdroppers hear unintelligible blips
Jamming on one frequency affects only a
few bits
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Basic Operation Typically 2kcarriers frequencies forming
2kchannels
Channel spacing corresponds withbandwidth of input
Each channel used for fixed interval
300 ms in IEEE 802.11
Some number of bits transmitted using someencoding scheme
May be fractions of bit (see later)
Sequence dictated by spreading code
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Frequency Hopping Example
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Frequency Hopping Spread
Spectrum System (Transmitter)
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Frequency Hopping Spread
Spectrum System (Receiver)
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Slow and Fast FHSS Frequency shifted every Tcseconds
Duration of signal element is Tsseconds
Slow FHSS has Tc!Ts
Fast FHSS has Tc< Ts Generally fast FHSS gives improved
performance in noise (or jamming)
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Slow Frequency Hop Spread
Spectrum Using MFSK (M=4, k=2)
MFSK = Multiple FSK
M is the number of different signal elements, frequencies to encode data,
2k is the number of channels, each of width Wd
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Fast Frequency Hop Spread
Spectrum Using MFSK (M=4, k=2)
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FHSS Performance
Considerations Typically large number of frequencies
used
Improved resistance to jamming
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Direct Sequence Spread
Spectrum (DSSS)
Each bit is represented by multiple bits usingspreading code
Spreading code spreads signal across widerfrequency bandIn proportion to number of bits used
e.g., 10 bit spreading code spreads signal across 10times 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 Spread
Spectrum Example
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Direct Sequence Spread
Spectrum Transmitter
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Direct Sequence Spread
Spectrum Receiver
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Direct Sequence Spread
Spectrum Using BPSK Example
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Approximate
Spectrum ofDSSS Signal
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Code Division Multiple Access
(CDMA) Multiplexing Technique used with spread spectrum
Start with data signal rate DCalled bit data rate
Break each bit into kchips according to fixed patternspecific to each userUsers code
New channel has chip data rate kDchips 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 1Send chip pattern
As code
A wants to send 0Send chip[ pattern
Complement of As code
Decoder ignores other sources when using Ascode to decode
Orthogonal codes
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CDMA for DSSS nusers 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 Encoding
and Decoding
positive number = 1
negative number = 0
Summary
looked at use of spread spectrum techniques,e.g.,
FHSS
DSSS
CDMA
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