cdma ch1.ppt
TRANSCRIPT
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Modes OF Transmission in wireless
Simplex communication system
Example: Television , radio
TX RXCommunication is possible in one direction only
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Half Duplex communication system Example :
Police radio
TX+RX TX +RX Two-way communication on the same channel.
At any given time a user can only transmit or receive
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Full Duplex communication system Example :
GSM mobile radio
TX+RX TX +RX
simultaneous two-way communication
Two simultaneous but separate channels are
provided for communication to and from the
terminals
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Multiple AccessDiferent multiple access methods use diferentstrategies to:
Divide the radio spectrum into channels (ime or Fre!"#
Allocate those channels amon$ users (%ne channel or more#
&dentify di'erent users on an F carrier (Fixed or dynamic#
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Freuenc! Division Multiple
"ccess #FDM"$
As AMPS and NMT
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Time Division Multiple
"ccess #TDM"$
As GSM and IS-136
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TDM" %s FDM"
FDM" TDM"
"dvantage
s
low cost hardware technology
o need for networ! timing
o restriction regarding the type of base
band "voice or data# or type of modulation
$ermits a fle%ible bit rate
&asy for mobile or base stations toinitiate and e%ecute hands off
Disadvantages
The presence of guard bands
Re'uires (deal R) filtering to
minimi*e adacent channel interference
Re'uires networ!-wide timing
synchroni*ation
Re'uires signal processing for
advancing
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&h! Do 'e need ne' accesstechniues(
Band Saturation
MoreBW efficiency
Capacity(ore t!an " T!e popu#ation of t!e
$#o%e use o%i#e p!ones&
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)hat a*out SDMA+
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,DMA Spread Spectrum
'ri$ina##y dee#oped for i#itary and nai$ation purposes
As C!anne# BW $et sa##er More CapacityN' More necessary
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,DMA Spread Spectrum
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,DMA Spread Spectrum
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,DMA Spread Spectrum)esearc!es started een fro 1*+,s
Ce##u#ar spread-spectru app#ication .as su$$ested %y Cooper and Nett#eton in 1*/0
IS-*+ t!e narro. %and C2MA o%i#e net.or !as %een standardi4ed in 1**3 and
coercia# net.ors .ere introduced in 1**+
3G .ide%and C2MA systes suc! as C2MA5,,, in 7S7 and 8uropean WC2MA
dee#oped fro 1**,sand sti## on$oin$
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,DMA Spread Spectrum
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Si$nal Spreadin$ -revie.&n DS spreadin$/ the incomin$ di$ital speech
si$nal is multiplied *y a di$ital pseudo0noise(-1# code throu$h a process called *it streammultiplication"
T!e resu#t of 2S spreadin$ is an encoded spread spectru C2MA si$na#7
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Si$nal Spreadin$ -revie.Fast fre!uency hoppin$
T!is approac! assues t!at t!e t.o stations !ae9T!is approac! assues t!at t!e t.o stations !ae9
A coon no.#ed$e of t!e fre:uency !oppin$ p#an7A coon no.#ed$e of t!e fre:uency !oppin$ p#an7
Sync!roni4ed c#ocsSync!roni4ed c#ocs
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Si$nal Spreadin$ -revie.ime Hoppin$
Tie !oppin$ assues t!at t!e t.o stations in counication !ae9
A coon no.#ed$e of t!e tie-!oppin$ ;p#an7;
Sync!roni4ed c#ocs
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he Stren$ths of ,DMA&ncreased system capacity"
,ell sites can *e lar$er"
Enhanced privacy due to di$ital codin$ of ,DMA
si$nals"
Enhanced call !uality
2o.er transmit F po.er levels/ lon$er *attery
life/ and increased tal3 time for hand0held units"
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4it Stream Multiplication
,oncepts4it stream multiplication is the process .here
an input *it stream of information (usually adi$ital speech si$nal# called *(t# is multiplied*y a pseudo0noise (-1# code called c(t# toproduce a ne. composite output si$nal calledy(t#"
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4it Stream Multiplication,oncepts
,onsider the di'erence *et.een spreadin$
and scram*lin$:
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4it Stream Multiplication,oncepts
,1,,,1,1111,1,1,1,,1
1,11,1,1,,1,1,1,1,11
2ata
Code
1,11,1,1,,1,1,1,1,11Code
,,,,,1,1,,1,1,1,1,,1Not 2ata
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4it Stream Multiplication
,oncepts*(t# 5 c(t# 6 y(t#
*(t# 5 c(t# 5 c(t# 6 *(t#
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Spreadin$ andDespreadin$emem*er that spreadin$ occurs .hen a
lo.er *it rate input si$nal *(t# is multiplied *ya hi$her chip rate spreadin$ code c(t# assho.n"
Bit
C!ip
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Spreadin$ and Despreadin$
,ont"despreadin$ ta3es y(t# and multiplies a$ain *y
the same spreadin$ -1 code c(t# to recoverthe di$ital information si$nal *(t#"
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Spreadin$ and -o.er
Spectral DensitySpreadin$ does not chan$e total po.er"
Spreadin$ chan$es ho. the po.er isdistri*uted over fre!uency"
he follo.in$ formulas hold true:
F* 678* (the *it rate of the input si$nal#
Fc 678c (the chip rate of the spreadin$ code#
9 (processin$ $ain# 6 Fc8F* 6*8c
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Scram*lin$ andDescram*lin$
Scram*lin$ is often used for privacy" )henscram*lin$ is used for encryption/ then
descram*lin$ is used for decryption"
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)epeated Spreading and
Scrambling,DMA systems use scram*lin$ and spreadin$ in
several .ays for:
Encryption for privacy
,hannel identication
4ase station identication
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-1 ,odesA *inary se!uence .ith xed len$th and has
noise0li3e randomness
1early e!ual num*er of ;eros and ones
2o. correlation *et.een shifted versions of these!uence
2o. correlation .ith other user si$nals(interference# and noise
9ood autocorrelation properties .ith o.n si$nal insynchroni;ation
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,ode 9eneration,ircuitry
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,ode 9eneration,ircuitry
DDFlipFlip
FlopFlop
DDFlipFlip
FlopFlop
DD
FlipFlip
FlopFlop
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)F Transmission Dela! andits Efectadio si$nals typically travel at a rate of a*out vemicroseconds to the mile" Assumin$ that a mo*ile is tenmiles a.ay from the *ase station/ a => microsecond (?s#delay .ould *e the result"
C!ip < 701 usec
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)F Transmission Dela! andits Efect
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2oc3in$ %n to a -articular -1Se!uence ,ont"
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2oc3in$ %n to a -articular -1Se!uence ,ont"
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2oc3in$ %n to a -articular -1Se!uence ,ont"
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Si$nal ,orrelation )ith %'sets 2ess han 7 ime
,hip
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Si$nal ,orrelation )ith %'sets 2ess han 7 ime
,hip
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Si$nal ,orrelation