UNIT 4 Spread Spectrum

8/17/2019 UNIT 4 Spread Spectrum

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UNIT 7SPREAD SPECTRUM SIGNALSFOR DIGITAL COMMUNICATION:


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What is Spread Spectrusi!"a#$ I" c%u"icati%" ti## "%& &e had

discussed h%& t% uti#i'e the cha""e# (We)cie"t#* + h%& t% i"ii'e the a%u"t

%, tra"sitted p%&er$ (ut -eside this s%e %ther pr%-#es are

there the* are as ,%##%&s:

./ Securit*0/ 1a the tra"sissi%" -* %thers

2/ Sae cha""e# is used -* %thers


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Reed*:

“SPREAD SPECTRUM MODULATION”

It uses s%e 3i"d %, c%di"! 4spreadi"! attra"sitter + despreadi"! at the recei5er%-tai"ed &ith the c%de &%rd

C%de &%rd is i"depe"de"t %, the

i",%rati%" carried -* the si!"a# SS si!"a# is 6pseud%ra"d% i" "ature

Specia##* desi!"ed recei5er is re8uired


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Ad5a"ta!es +Disad5a"ta!es:


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C#assi9cati%":


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M%de# %, Spread Spectru Di!ita#C%u"icati%":

Cha""e#

E"c%de

r

Pseud%ra"d% patter"!e"erat%r

M%du#at%r

Cha""e#

Pseud%ra"d% patter"!e"erat%r

De%du#at%r

Cha""e#

Dec%de

r

I",%rati%" se8 Outp

Transmitter

Receiver

Spreadspectrumsignal


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Direct Se8ue"ce:


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Tie %pped spreadspectru:


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Fre8ue"c* %ppi"! spread spectru:


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Idea#i'ed %de# %, -ase-a"d spreadspectru s*ste:


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Wa5e,%rs i" the tra"sitter %, DSSS


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P%&er Spectra# De"sit*:


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PSD a,ter Despreadi"!:


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DSSS &ith C%here"t (PS;:


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Fre8ue"c* %p Spread Spectru S*ste:

Data is used t% %du#ate a carrier

The data %du#ated carrier is the"ra"d%#* h%pped ,r% %"e ,re8ue"c* t%

%ther Due t% this spectru %, tra"sitted

si!"a# is spread se8ue"tia##* rather tha"

i"sta"ta"e%us#* M=ar* ,s3 is 3"%&" as F?MFS;

(ased %" rate %, h%ppi"! F?MFS;s*ste has -ee" c#assi9ed i"t% t&%cate!%ries


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./ S#%& ,re8ue"c* h%ppi"!:

Each frequency hop=several symbols

0/ Fast ,re8ue"c* h%ppi"!:

Each symbol Transmission=Severalfrequency hop

.Fre8ue"c* shi,ted e5er* Tc sec%"ds

.Durati%" %, si!"a# e#ee"t is Ts sec%"ds

.S#%& FSS has Tc ≥ Ts.Fast FSS has Tc @ Ts.Ge"era##* ,ast FSS !i5es ipr%5ed per,%ra"ce

i" "%ise


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(asic Operati%":

T*pica##* 03 carriers ,re8ue"cies ,%ri"!03 cha""e#s

Cha""e# spaci"! c%rresp%"ds &ith

-a"d&idth %, i"put


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FSS EBap#e:


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FSS Tra"sitter:


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FSS Recei5er:


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S#%& Fre8ue"c* %p Spread SpectruUsi"! MFS; :


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Fast Fre8ue"c* %p Spread SpectruUsi"! MFS; :


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Chip Rate:

I"di5idua# t%"e %, sh%rtest durati%" isde9"ed as a 6chip i" FSS

RcaB

&here Rh=h%p rate

Rs=s*-%# rate

Rc

is hi!her tha" Rh

%r Rs

,%r s#%&

,re8ue"c* h%ppi"!

Rc Rs Rh

Where ;#%!0 %r M0;


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Pr%cess Gai":

PG(W?(W

(W0",s

(W,s

PG0"

" is "% %, -its at the %utput %, PN c%de

!e"erat%r


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DiHere"ce:

Slow FrequencyHopping

Fast FrequencyHopping

.> M%re tha" %"e s*-%#s are

tra"sitted per ,re8ue"c* h%p

.> M%re tha" %"e ,re8ue"c* h%ps

are re8uired t% tra"sit %"es*-%#

0> Chip rate is e8ua# t% s*-%#rate

0> Chip rate is e8ua# t% h%p rate

2> S*-%# rate is hi!her tha" h%p

rate

2> %p rate is hi!her tha" s*-%#

rate> Sae carrier ,re8ue"c* is usedt% tra"sit %"e %r %re s*-%#s

> O"e s*-%# is tra"sitted%5er u#tip#e carriers i" diHere"th%ps

> A aer ca" detect thissi!"a# i, the carrier ,re8ue"c* i"%"e h%p is 3"%&"

> A aer ca"Jt detect thissi!"a# -ecause %"e s*-%# istra"sitted usi"! %re tha" %"e

UNIT 4 Spread Spectrum

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