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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