Lec 7 - Spread Spectrum

7/24/2019 Lec 7 - Spread Spectrum

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Background of Wireless

Communication

StudentPresentations andProjects

Wireless Communication

Technology

Wireless Networking and

Mobile IP

Wireless Local Area

Networks

Wireless Communication

Spread SpectrumSpread Spectrum


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Chapter 7


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Spread Spectrum

Input is fed into a channel encoder

Produces analog signal with narrow bandwidth

Signal is further modulated using sequence of digits Spreading code or spreading sequence

Generated by pseudonoise, or pseudo-random number

generator

ffect of modulation is to increase bandwidth ofsignal to be transmitted


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Spread Spectrum

!n recei"ing end, digit sequence is used to

demodulate the spread spectrum signal

Signal is fed into a channel decoder to reco"er data


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Spread Spectrum


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Spread Spectrum

What can be gained from apparent waste of

spectrum#

Immunity from "arious $inds of noise and

multipath distortion

Can be used for hiding and encrypting signals

Se"eral users can independently use the same

higher bandwidth with "ery little interference


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%requency &oping Spread Spectrum

'%&SS( Signal is broadcast o"er seemingly random series of

radio frequencies ) number of channels allocated for the %& signal

Width of each channel corresponds to bandwidth of inputsignal

Signal hops from frequency to frequency at fi*edinter"als +ransmitter operates in one channel at a time its are transmitted using some encoding scheme

)t each successi"e inter"al, a new carrier frequency isselected


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Channel sequence dictated by spreading code

ecei"er, hopping between frequencies in

synchroni.ation with transmitter, pic$s up message

)d"antages

a"esdroppers hear only unintelligible blips

)ttempts to /am signal on one frequency succeed only at

$noc$ing out a few bits


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%&SS 0sing 1%S2

1%S2 signal is translated to a new frequency e"ery

Tcseconds by modulating the 1%S2 signal with the

%&SS carrier signal

%or data rate ofR:

duration of a bit3 T 4 56Rseconds

duration of signal element3 Ts4LT seconds

TcTs- slow-frequency-hop spread spectrum

Tc Ts- fast-frequency-hop spread spectrum


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%&SS Performance Considerations

8arge number of frequencies used

esults in a system that is quite resistant to /amming

9ammer must /am all frequencies With fi*ed power, this reduces the /amming power

in any one frequency band


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:irect Sequence Spread Spectrum ':SSS(

ach bit in original signal is represented by multiple

bits in the transmitted signal

Spreading code spreads signal across a wider

frequency band

Spread is in direct proportion to number of bits used

!ne technique combines digital information stream

with the spreading code bit stream using e*clusi"e-! '%igure 7;


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:irect Sequence Spread Spectrum ':SSS(


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:SSS 0sing PS2

1ultiply PS2 signal,

sd't( 4A d't( cos'=fct(

by c't(>ta$es "alues ?5, -5@ to gets't( 4A d't(c't( cos'=fct( A 4 amplitude of signal

fc4 carrier frequency

d't( 4 discrete function >?5, -5@

)t recei"er, incoming signal multiplied by c't(

Since, c't( * c't(=5, incoming signal is reco"ered


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:SSS 0sing PS2


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Code-:i"ision 1ultiple )ccess 'C:1)(

asic Principles of C:1)

D4 rate of data signal

rea$ each bit into kchips Chips are a user-specific fi*ed pattern

Chip data rate of new channel 4 kD


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C:1) *ample

If k4< and code is a sequence of 5s and -5s %or a A5B bit, ) sends code as chip pattern

c5, c=, c, cD, cE, c


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C:1) *ample

0ser ) code 4 5, H5, H5, 5, H5, 5F +o send a 5 bit 4 5, H5, H5, 5, H5, 5F

+o send a bit 4 H5, 5, 5, H5, 5, H5F

0ser code 4 5, 5, H5, H 5, 5, 5F +o send a 5 bit 4 5, 5, H5, H5, 5, 5F

ecei"er recei"ing with )Bs code

')Bs code( * 'recei"ed chip pattern(

0ser ) A5B bit3 < -F 50ser ) AB bit3 -< -F

0ser A5B bit3 -F unwanted signal ignored


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C:1) for :irect Sequence Spread

Spectrum


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Categories of Spreading Sequences

Spreading Sequence Categories P sequences

!rthogonal codes

%or %&SS systems P sequences most common

%or :SSS systems not employing C:1) P sequences most common

%or :SSS C:1) systems P sequences

!rthogonal codes


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P Sequences

P generator produces periodic sequence thatappears to be random

P Sequences Generated by an algorithm using initial seed

Sequence isnBt statistically random but will pass many testof randomness

Sequences referred to as pseudorandom numbers or

pseudonoise sequences 0nless algorithm and seed are $nown, the sequence is

impractical to predict


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Important P Properties

andomness

0niform distribution

alance property un property

Independence

Correlation property

0npredictability


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8inear %eedbac$ Shift egister

Implementation


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Properties of 1-Sequences

Property 53 &as =n-5 ones and =n-5-5 .eros

Property =3 %or a window of length nslid along output forN'4=n-5(

shifts, each n-tuple appears once, e*cept for the all .erossequence

Property 3 Sequence contains one run of ones, length n

!ne run of .eros, length n-5

!ne run of ones and one run of .eros, length n-=

+wo runs of ones and two runs of .eros, length n-

=n-runs of ones and =n- runs of .eros, length 5


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Properties of 1-Sequences

Property D3

+he periodic autocorrelation of a J5 m-

sequence is

( )otherwise

;;;=I,I,G,

55 =

=

N

R


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:efinitions

Correlation +he concept of determining how much similarity one set of

data has with another

ange between H5 and 5 5 +he second sequence matches the first sequence

+here is no relation at all between the two sequences

-5 +he two sequences are mirror images

Cross correlation +he comparison between two sequences from different

sources rather than a shifted copy of a sequence with itself


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)d"antages of Cross Correlation

+he cross correlation between an m-sequence and

noise is low

+his property is useful to the recei"er in filtering out noise

+he cross correlation between two different m-

sequences is low

+his property is useful for C:1) applications

nables a recei"er to discriminate among spread spectrumsignals generated by different m-sequences


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Gold Sequences

Gold sequences constructed by the K! of two m-sequences with the same cloc$ing

Codes ha"e well-defined cross correlation properties

!nly simple circuitry needed to generate largenumber of unique codes

In following e*ample '%igure 7;5


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Gold Sequences


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!rthogonal Codes

!rthogonal codes )ll pairwise cross correlations are .ero

%i*ed- and "ariable-length codes used in C:1) systems

%or C:1) application, each mobile user uses onesequence in the set as a spreading code Pro"ides .ero cross correlation among all users

+ypes

Welsh codes Lariable-8ength !rthogonal codes


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Walsh Codes

Set of Walsh codes of length n consists of the n

rows of an n M n Walsh matri*3

W54 '(

n4 dimension of the matri*

"ery row is orthogonal to e"ery other row and tothe logical not of e"ery other row

equires tight synchroni.ation

Cross correlation between different shifts of Walsh

sequences is not .ero

= nnnn

nWWWWW ==


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+ypical 1ultiple Spreading )pproach

Spread data rate by an orthogonal code

'channeli.ation code(

Pro"ides mutual orthogonality among all users in

the same cell

%urther spread result by a P sequence 'scrambling

code(

Pro"ides mutual randomness 'low crosscorrelation( between users in different cells


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NO)

#

Lec 7 - Spread Spectrum

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