CDMA CH1.ppt

8/10/2019 CDMA CH1.ppt

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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)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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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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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

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CDMA CH1.ppt

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