02-Tm2106euo1_eg0001-Basic Concept of Spread Spectrum Tech.

8/12/2019 02-Tm2106euo1_eg0001-Basic Concept of Spread Spectrum Tech.

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Basic Concept of Spread Spectrum Technology Siemens

Basic Concept of Spread SpectrumTechnology

Contents

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22.1

2.1.1

2.1.2

2.2

2.3

2.4

2.5

2.6

2.7

3

3.1

3.2

3.2.1

3.3

3.4

3.5

3.5.1

3.5.2

3.6

3.7

3.

3.!

3.!.1

"istory of C#$%

&hy C#$%'Capacity (ncreases

)o*ering +,-/

0oice %ctiity #etection

(mproed Call uality

Simplified System lanning

+nhanced riacy

(mproed Coerage

(ncreased orta,le Tal Time

Band*idth on #emand

Spread Spectrum Technology

roperties of SS signals

SpreadSpectrum $ultiple %ccess SS$%

#irect Se8uence Spread Spectrum #SSS

%dantages of #SSS9

#isadantages of #SSS9

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SiemensBasic Concept of Spread Spectrum Technology

3.1@

3.113.12

3.13

3.13.1

3.13

Types of Se8uences in C#$%

Correlation ,et*een se8uencesrocess >ain and its Benefits

Spreading Code %c8uisition and Tracing

(nitial Code %c8uisition

Code Tracing

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44

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1 History of CDMA

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C#$% is a form of spreadspectrum a family of digital communications techni8ues thathae ,een used in military applications for years. /riginally there *ere t*o motiations forusing C#$%9 either to resist enemy efforts to am the communications or to hide the factthat communication *as een taing place. The use of C#$% for ciilian mo,ile radioapplications *as proposed 4@ years ago ,ut did not tae place till recently.

(n 1!4! Dohn ierce *rote a technical memorandum *here he descri,ed a multiple?ingsystem in *hich a common medium carries coded signals that need not ,e synchroniEed.This system can ,e classified as a time hopping spread spectrum multiple access system.Claude Shannon and ;o,ert ierce introduced the ,asic ideas of C#$% in 1!4! ,y

descri,ing the interference aeraging effect and the graceful degradation of C#$%. (n1!5@ #e ;osa;ogoff proposed a direct se8uence spread spectrum system andintroduced the processing gain e8uation and noise multiple?ing idea. (n 1!56 rice and>reen filed for the antimultipath F;%G+H patent. Signals arriing oer different propagationpaths can ,e resoled ,y a *ide,and spread spectrum signal and com,ined ,y the ;%G+receier. The near far pro,lem i.e a high interference oer*helming a *eaer spreadspectrum signal *as first mentioned in 1!61 ,y $agnusi. :or cellular application spreadspectrum *as suggested ,y Cooper and ettleton in 1!7. #uring the 1!@s ualcomminestigated #SC#$% techni8ues *hich finally led to the commercialiEation of cellularspread spectrum communications in the form of the narro*,and C#$% (S!5 standard inDuly 1!!3. Commercial operation of (S!5 systems started in 1!!6. $ultiuser detection

$aussian noise %&> channelma?imum lielihood se8uence estimator $)S+. #uring the 1!!@s *ide,and C#$%techni8ues *ith a ,and*idth of 5 $"E or more hae ,een studied intensiely throughoutthe *orld and seeral trial systems hae ,een ,uilt and tested. These include :;%$+S$ultiple %ccess :;%$+S :$%2 in +uropeI Core% in Dapan the +uropean-DapaneseharmoniEed &C#$% scheme cdma2@@@ in the


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08/02 Ericsson, LG Electronics, Lucent Technologies, Motorola, Nortel Networks, Qualcomm, Samsung and ZTE signMemorandum of Understanding to commercially delier !"M#$%%% infrastructure and terminals for $&' G(Zs)ectrum

06/02 !"M#$%%% '*E+"+ -"ata +oice. a))roed /y /oth the Third Generation 0artnershi) 0ro1ect $ -2G00$. andTelecommunications 3ndustry #ssociation -T3#. for )u/lication

05/02 '% million commercial !"M#$%%% '4 su/scri/ers

01/02 S5 Telecom -5orea. launches world6s first !"M#$%%% '*E+"7 network12/01 ''' million !"M# su/scri/ers worldwide

08/01 ' million commercial !"M#$%%% '4 su/scri/ers

06/01 !"M#$%%% '*E+"7 recogni8ed as )art of the 2G 3MT$%%% standard

03/01 !"M#$%%% '*E+"+ successfully demonstrated in la/s

12/00 9%&: million !"M# su/scri/ers worldwide

10/00 S5 Telecom and LG Telecom -5orea. launch world6s first 2G commercial serices using !"M$%%%

10/00 ;irst eer !"M#GSM intero)era/le S3M card introduced to the glo/al market

06/00 !"M#$%%% '*E+ introduced to glo/al market)lace /y the !"G

04/00 T3# a))roes !"M# S3M standard for )u/lication

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2 Why CDMA ?

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&hen implemented in a cellular telephone system C#$% technology offers many ,enefitsto meet $o,ile ;adio ;e8uirements. The follo*ing is an oerie* of the adantages ofC#$%.

11 Capacity !ncreases

Capacity gains in cellular systems can ,e attained in one of t*o *ays9

1. By getting more channels per $"E of spectrum %$S J %$S.

2. By getting more channel reuse per unit of geographic area >S$.

/ne of the ey design principals of cellular telecommunications is the use of the samefre8uencies oer and oer in a particular geographic region. This greatly increases thecommunication carrying capacity of the spectrum used in cellular systemsI ho*eer it isnot possi,le to use eery fre8uency in eery cell site ,ecause of the interference *hich*ould result.

Therefore *ith other cellular technologies it is necessary to plan *hich fre8uencies areused at each cell site in order to minimiEe the interference among cell sites on the same

fre8uency. This re8uirement has led to Kfre8uency reuseH&ith C#$% signals can ,e receied in the presence of high leels of interference %ll userson a carrier share the same ;: spectrum. The same C#$% ;: carrier fre8uency is used ineery cell site and in eery sector of a sector cell site.

(ncreasing capacity in C#$% can ,e done ,y the follo*ing techni8ues9

111 "o#ering $%&'o

+,-oproides a measure of the performance of a C#$% lin ,et*een the mo,ile and thecell. (t is the ratio in dB ,et*een the energy of each information ,it and the noise spectral

density. The noise is a com,ination of ,acground interference and the interference created,y other users on the system.

% decrease in the +,-oratio indicates that the relatie leel of interference as compared tothe leel of the oice information is increasing. This *ill lo*er the oice 8uality of theconersation. &hile all digital cellular systems use error correction coding systems ,asedon narro*,and digital modulation generally use less sophisticated schemes *hich use upless ,and*idth. (n order to eep oice 8uality high therefore the operators of narro*,andsystems re8uire a higher +,-o. This leads to a need to limit the num,er of users on thesystem lo*ering capacity.

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C#$% on the other hand uses adanced for*ard error correction coding as *ell as adigital demodulator lo*ering C#$%Ls re8uired +,-oratio.


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1+ Simplified System ,lanning

%ll users on a C#$% carrier share the same ;: spectrum. This M1-S reuse of fre8uencies*here S M num,er of sectors per cell is one factor *hich gies C#$% its greater capacityoer and also maes no need to perform the detailed fre8uency planning *hich isnecessary in analog and T#$% systems.

1- $nhanced ,ri)acy

C#$% is an F%nti DammingH system soI it is e?tremely difficult for someone to am theC#$% signal. (n addition since the digitiEed frames of information are spread across a*ide slice of spectrum it is unliely that a casual eaesdropper *ill ,e a,le to listen in on aconersation. :urthermore there are future plans to offer digital encryption to proide eengreater leels of security and priacy.

1. !mpro)ed Co)erage

% C#$% cell site has a greater range than a typical analog or digital cell site. Thereforefe*er C#$% cell sites are re8uired to coer the same area. #epending on system loadingand interference the reduction in cells could ,e as much as 5@N *hen compared to >S$O

C#$%Ls greater range is due to the fact that C#$% uses a more sensitie receier thanother technologies.

1/ !ncreased ,orta%le Tal0 Time

Because of precise po*er control and other system characteristics C#$% su,scri,er unitsnormally transmit at only a fraction of the po*er of analog and T#$% phones. This *illena,le porta,les to hae longer tal and stand,y time. This direct comparison assumes ofcourse similar cell siEes ,et*een the C#$% and analog or T#$% systems.

1 Band#idth on Demand

% *ide,and C#$% channel proides a common resource that all mo,iles in a system utiliEe,ased on their o*n specific needs *hether they are transmitting oice data facsimile orother applications. %t any gien time the portion of this K,and*idth poolK that is not used ,ya gien mo,ile is aaila,le for use ,y any other mo,ile. This proides a tremendous amountof fle?i,ility a fle?i,ility that can ,e e?ploited to proide po*erful features such as higherdata rate serices. (n addition ,ecause mo,iles utiliEe the K,and*idth poolK independentlythese features can easily coe?ist on the same C#$% channel.

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+ Spread Spectrum Technology

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The maor concern in &ireless is digital communication is efficient use of Band*idth andpo*er. But there are scenarios *here it is necessary to sacrifice the efficient use for designconsiderations. /ne such scenario is secure communication in hostile enironment. Thisdesign o,ectie is met using a modulation techni8ue called as Spread Spectrum SS.The adantage of using is its a,ility to reect interference .$ay it ,e intentional someamming transmission or unintentional in this techni8ue signal of one user is interferenceto another user.

Defining Spread Spectrum

% complete definition to Spread Spectrum ( feel is the one gien ,y "ayins. ( 8uote him

,elo*."is definition is in t*o parts.

1. Spread Spectrum is a means of transmission in *hich the data se8uences occupy a,and*idth in e?cess of the minimum ,and*idth necessary to send it.

2. Spread Spectrum is accomplished ,efore transmission through the use of a codethat is independent of data se8uences .The same code is used at the receier todespread the receied signal so that the original data se8uence may ,e recoered.

(n C#$% each user is assigned a uni8ue code se8uence it uses to encode its information,earing signal. The receier no*ing the code se8uences of the user decodes a receied

signal after reception and recoers the original data. This is possi,le since thecrosscorrelations ,et*een the code of the desired user and the codes of the other usersare small. Since the ,and*idth of the code signal is chosen to ,e much larger than the,and*idth of the information,earing signal the encoding process enlarges spreads thespectrum of the signal and is therefore also no*n as spreadspectrum modulation. Theresulting signal is also called a spreadspectrum signal and C#$% is often denoted asspreadspectrum multiple access SS$% the spectral spreading of the transmitted signalgies to C#$% its multiple access capa,ility. (t is therefore important to no* thetechni8ues necessary to generate spreadspectrum signals and the properties of thesesignals. % spreadspectrum modulation techni8ue must ,e fulfill t*o criteria9

The transmission ,and*idth must ,e much larger than the information ,and*idth.

The resulting radiofre8uency ,and*idth is determined ,y a function other than theinformation ,eing sent so the ,and*idth is statistically independent of the informationsignal.

This e?cludes modulation techni8ues lie fre8uency modulation :$ and phase modulation$. The ratio of transmitted ,and*idth to information ,and*idth is called the processinggain >p of the spreadspectrum systemI the receier correlates the receied signal *ith asynchronously generated replica of the

spreading code to recoer the original information,earing signal. This implies that thereceier must no* the code used to modulate the data.

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Because of the coding and the resulting enlarged ,and*idth SS signals hae a num,er ofproperties that differ from the properties of narro*,and signals. The most interesting onesfrom the communication systems point of ie* are discussed ,elo*.

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1 ,roperties of SS signals

Multiple ccess Capa!ilit"

(f multiple users transmit a spreadspectrum signal at the same time the receier *ill still,e a,le to distinguish ,et*een the users proided each user has a uni8ue code that has asufficiently lo* crosscorrelation *ith the other codes. Correlating the receied signal *ith acode signal from a certain user *ill then only despread the signal of this user *hile the

other spreadspectrum signals *ill remain spread oer a large ,and*idth. Thus *ithin theinformation ,and*idth the po*er of the desired user *ill ,e larger than the interfering po*erproided there are not too many interferers and the desired signal can ,e e?tracted.

#rotection gainst Multipat$ %nterference

(n a radio channel there is not ust one path ,et*een a transmitter and receier.

#ue to reflections and refractions a signal *ill ,e receied from a num,er of differentpaths. The signals of the different paths are all copies of the same transmitted signal ,ut*ith different amplitudes phases delays and arrial angles. %dding these signals at thereceier *ill ,e constructie at some of the fre8uencies and destructie at others. (n the

time domain this results in a dispersed signal. Spreadspectrum modulation can com,atthis multipath interference.

#ri&ac" ' %nterference (e)ection

The transmitted signal can only ,e despread and the data recoered if the code is no*n tothe receier. Crosscorrelating the code signal *ith a narro*,and signal *ill spread thepo*er of the narro*,and signal there,y reducing the interfering po*er in the information,and*idth.

*+%,-MM%*. C#/%%+

This is more or less the same as interference reection e?cept the interference is no*

*illfully inflicted on the system. (t is this property together *ith the ne?t one that maesspreadspectrum modulation attractie for military applications.

#(//%%+ %*+(C#+%* #%

Because of its lo* po*er density the spreadspectrum signal is difficult to detect andintercept ,y a hostile listener.

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13 Spread4Spectrum Multiple Access 5SS4MA61.9.1 Direct Sequence Spread Spectrum (DS-SS)

(n #SC#$% the modulated information ,earing signal the data signal is directlymodulated ,y a digital discretetime discretealued code signal. The data signal can ,eeither analog or digitalI in most cases it is digital.

(n the case of a digital signal the data modulation is often omitted and the data signal isdirectly multiplied ,y the code signal and the resulting signal modulates the *ide,andcarrier. (t is from this direct multiplication that the direct se8uence C#$% gets its name.

The ,inary data signal modulates a ;: carrier. The modulated carrier is then modulated ,ythe code signal. This code signal consists of a num,er of code ,its called FchipsH that can,e either P1 or Q1. To o,tain the desired spreading of the signal the chip rate of the codesignal must ,e much higher than the chip rate of the information signal. :or the codemodulation arious modulation techni8ues can ,e used ,ut usually some form of phaseshift eying SG lie ,inary phase shift eying BSG differential ,inary phase shifteying #BSG 8uadrature phase shift eying SG or minimum shift eying $SG isemployed. (f *e omit the data modulation and use BSG for the code modulation. The rateof the code signal is called the chip rateI one chip denotes one sym,ol *hen referring tospreading code signals.

%fter transmission of the signal the receier uses coherent demodulation to despread theSS signal using a locally generated code se8uence. To ,e a,le to perform the dispreadingoperation the receier must not only no* the code se8uence used to spread the signal,ut the codes of the receied signal and the locally generated code must also ,esynchroniEed. This synchroniEation must ,e accomplished at the ,eginning of the receptionand maintained until the *hole signal has ,een receied. The codesynchroniEation-tracing ,loc performs this operation. %fter despreading a data modulatedsignal results and after demodulation the original data can ,e recoered.

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Direct-Sequence CDMA

Channel 1

Channel 2

Noise

Low PassFilter

Point-to-Multipoint (Downlink) Transmitter

Channel 1

Receiver for Channel 1

Noise Noise

SNR

0 1 1Code 1

01 0 1 0 1 0 0

0 1 1Code 2

01 0 1 0 1 0 0

0 1 1Code 1

01 0 1 0 1 0 0

1

1 2

2

3 4

3

4

Frequenc! Do"ain Descri#tion

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117Ad)antages of DS4SS8

The generation of the coded signal is easy. (t can ,e performed ,y a simple multiplication.

Since only one carrier fre8uency has to ,e generated the fre8uency synthesiEer carriergenerator is simple.

Coherent demodulation of the #S signal is possi,le.

o synchroniEation among the users is necessary.

111 Disd)antages of DS4SS8(t is difficult to ac8uire and maintain the synchroniEation of the locally generated code signaland the receied signal. SynchroniEation has to ,e ept *ithin a fraction ofthe chip time.

:or correct reception the synchroniEation error of locally generated code se8uence and thereceied code se8uence must ,e ery small a fraction of the chip time. This com,ined *iththe nonaaila,ility of large contiguous fre8uency ,ands practically limits the ,and*idth to1@Q2@$"E.

The po*er receied from users close to the ,ase station is much higher than that receied

from users further a*ay. Since a user continuously transmits oer the *hole ,and*idth auser close to the ,ase *ill constantly create a lot of interference for users far from the ,asestation maing their reception impossi,le. This nearfar effect can ,e soled ,y applying apo*er control algorithm so that all users are receied ,y the ,ase station *ith the sameaerage po*er. "o*eer this control proes to ,e 8uite difficult.

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1129:$*;$'C< H=,,!'> Spread Spectrum 59H4SS6

(n fre8uency hopping C#$% the carrier fre8uency of the modulated information signal isnot constant ,ut changes periodically. #uring time interals T the carrier fre8uency remainsthe same ,ut after each time interal the carrier hops to another or possi,ly the samefre8uency. The hopping pattern is decided ,y the code signal.

The set of aaila,le fre8uencies the carrier can attain is called the hopset. The fre8uencyoccupation of an :"SS system differs considera,ly from a #SSS system. % #S systemoccupies the *hole fre8uency ,and *hen it transmits *hereas an :" system uses only asmall part of the ,and*idth *hen it transmits ,ut the location of this part differs in time.Suppose an :" system is transmitting in fre8uency ,and 2 during the first time period.

% #S system transmitting in the same time period spreads its signal po*er oer the *hole

fre8uency ,and so the po*er transmitted in fre8uency ,and 2 *ill ,e much less than that ofthe :" system. "o*eer the #S system transmits in fre8uency ,and 2 during all timeperiods *hile the :" system only uses this ,and part of the time. /n aerage ,othsystems *ill transmit the same po*er in the fre8uency ,and. The data signal is ,ase,andmodulated.


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1121 Ad)antages of 9H4SS8

SynchroniEation is much easier *ith :"C#$% than *ith #SC#$%. &ith :" C#$%synchroniEation has to ,e *ithin a fraction of the hop time. Since spectral spreading is noto,tained ,y using a ery high hopping fre8uency ,ut ,y using a large hopset the hop time*ill ,e much longer than the chip time of a #SC#$% system. Thus an :"C#$% systemallo*s a larger synchroniEation error.

The different fre8uency ,ands that an :" signal can occupy do not hae to ,e contiguous,ecause *e can mae the fre8uency synthesiEer easily sip oer certain parts of thespectrum. Com,ined *ith the easier synchroniEation this allo*s much higher spreadspectrum ,and*idths.

The pro,a,ility of multiple users transmitting in the same fre8uency ,and at the same timeis small. % user transmitting far from the ,ase station *ill ,e receied ,y it een if usersclose to the ,ase station are transmitting since those users *ill pro,a,ly ,e transmitting atdifferent fre8uencies. Thus the nearfar performance is much ,etter than that of #S.

Because of the larger possi,le ,and*idth a :" system can employ it offers a higherpossi,le reduction of narro*,and interference than a #S system.

1122 Disd)antages of 9H4SS8

% highly sophisticated fre8uency synthesiEer is necessary.

%n a,rupt change of the signal *hen changing fre8uency

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11+T!M$ H=,,!'> Spread Spectrum 5TH4SS6

(n time hopping C#$% the data signal is transmitted in rapid ,ursts at time interalsdetermined ,y the code assigned to the user. The time a?is is diided into frames and eachframe is diided into $ time slots. #uring each frame the user *ill transmit in one of the $time slots. &hich of the $ time slots is transmitted depends on the code signal assigned tothe user. Since a user transmits all of its data in one instead of $ time slots the fre8uencyit needs for its transmission has increased ,y a factor $. % ,loc diagram of a T"C#$%system is gien in :ig. . :igure sho*s the timefre8uency plot of the T"C#$% systems.Comparing :ig. *ith :ig. *e see that the T"C#$% uses the *hole *ide,and spectrumfor short periods instead of parts of the spectrum all of the time. :ollo*ing the sameprocedure as for the preious C#$% schemes *e discuss the properties of T"C#$% *ithrespect to multiple access capa,ility multipath interference reection narro*,and

interference reection and pro,a,ility of interception.R $ultiple access

The multiple access capa,ility of T"SS signals is ac8uired in the same manner as that ofthe :"SS signalsI namely ,y maing the pro,a,ility of users transmissions in the samefre8uency ,and at the same time small. (n the case of time hopping all transmissions are inthe same fre8uency ,and so the pro,a,ility of more than one transmission at the sametime must ,e small. This is again achieed ,y assigning different codes to different users. (fmultiple transmissions do occur errorcorrecting codes ensure that the desired signal canstill ,e recoered. (f there is synchroniEation among the users and the assigned codes aresuch that no more than one user transmits at a particular slot then the T"C#$% reduces to

a T#$% scheme *here the slot in *hich a user transmits is not fi?ed ,ut changes fromframe to frame.

R $ultipath interference

(n the time hopping C#$% a signal is transmitted in reduced time. The signaling ratetherefore increases and dispersion of the signal *ill no* lead to oerlap of adacent ,its.Therefore no adantage is to ,e gained *ith respect to multipath interference reection.

R arro*,and interference

% part from the a,oementioned properties the T"C#$% has a num,er of other specificproperties that *e can diide into adantageous P and disadantageous ,ehaior9

P(mplementation is simpler than that of :"C#$%. P(t is a ery useful method *hen thetransmitter is aerage po*er limited ,ut not peapo*er limited since the data aretransmitted is short ,ursts at high po*er. P%s *ith the :"C#$% the nearfar pro,lem ismuch less of a pro,lem since T"C#$% is an aoidance system so most of the time aterminal far from the ,ase station transmits alone and is not hindered ,y transmissionsfrom stations close ,y.

Q (t taes a long time ,efore the code is synchroniEed and the time in *hich the receierhas to perform the synchroniEation is short.

Q (f multiple transmissions occur a large num,er of data ,its are lost so a good errorcorrecting code and data interleaing are necessary.

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11. ,' Seuences

$at are #* seuences% seudorandom oise se8uence is a se8uence of ,inary num,ers e.g. U1 *hichappears to ,e randomI ,ut is in fact perfectly deterministic. The se8uence appears to ,erandom in the sense that the ,inary alues and groups or runs of the same ,inary alueoccur in the se8uence in the same proportion they *ould if the se8uence *ere ,einggenerated ,ased on a fair Kcoin tossingK e?periment. (n the e?periment each head couldresult in one ,inary alue and a tail the other alue. The se8uence appears to hae,een generated from such an e?periment. % soft*are or hard*are deice designed toproduce a se8uence is called a generator.

seudorandom noise se8uences or se8uences are no*n se8uences that e?hi,it the

properties or characteristics of random se8uences. They can ,e used to logically isolateusers on the same fre8uency channel. They can also ,e used to perform scram,ling as*ell as spreading and despreading functions. The reason *e need to use se8uences isthat if the code se8uences *ere deterministic then eery,ody could access the channel. (fthe code se8uences *ere truly random on the other hand then no,ody including theintended receier *ould ,e a,le to access the channel. Thus using a pseudorandomse8uence maes the signal loo lie random noise to eery,ody e?cept to the transmitterand the intended receier.

$" #* seuence is c$osen as a noise li:e ;a&eform

To no* that *e hae to understand *hat is called F*hite oiseH.

The adectie F*hiteH is used in the sense that *hite light contains e8ual amounts of allfre8uencies *ithin the esi,le ,and of electromagnetic radiation.

(t has po*er spectral density independent of the operating fre8uency. &e e?press thepo*er spectral density of *hite noise ,y

S* f M o-2 ... o M GT@ *atts -"E. *here G is BoltEman contant J

T@is the e8uialent noise temperature.

+8uialent noise temperature of a system FT@H9

(t is the temperature at *hich a noisy resistor has to ,e maintained such that ,y

connecting the resistor to the input of a noiseless ersion of the system it produces thesame aaila,le noise in the actual system it depends only on the parameters of thesystem .

Since the auto correlation function is the inerse :ourier of the po*er spectral density itfollo*s that for *hite noise the auto correlation function of *hite noise consists of a delta

function *eighted ,y the factor o-2 and occurring at M @.

%ccordingly any t*o different samples of *hite noise no matter ho* closely together intime they are taen are uncorrelated. So *e hae to search for a code se8uence has anoise lie *ae or almost has autocorrelation function near that of *hite noise.

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>: ;

@-2

@-2.

:

&hite oise po*er spectral density *hite noise autocorrelation property

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11/ ,' Seuence >eneration

These se8uences are easily generated ,y using an $,it shift register *ith the appropriatefeed,ac taps e.g. as sho*n in :ig. :or $ M 5. &ith the appropriate taps the length ofthe serial ,it stream at the output *ill ,e a ma?imum )ma?9

M )ma?M 2$ 1

The meaning of ,itstream length in this conte?t is the ma?imum length of the ,it se8uence,efore it starts repeating itself. se8uences of ma?imum length are called ma?imal linearcode se8uences ,ut ,ecause nonma?imal se8uences are rarely used in SS systemsF se8uencesH *ill ,e used to denote ma?imal linear code se8uences for this document.%lso F codesH or F code se8uencesH *ill ,e used synonymously *ith F se8uencesH.The feed,ac taps are added modulo2 e?clusie /;ed and fed to the input of the initial

shift register. /nly particular tap connections *ill yield a ma?imum length for a gien shiftregister length. These ma?imal length codes hae the follo*ing properties9

1. Code !alance9

The num,er of ones and the num,er of Eeros differ ,y only 1 i.e. there is 1 more one thanthe num,er of Eeros. This particularly useful *hen the channel is %C coupled no #Ctransmission.

2. utocorrelation9


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11/1 A ,' >enerator $@ample

% generator is typically made of cascaded flipflop circuits and a specially selectedfeed,ac arrangement as sho*n in figure . The flipflop circuits *hen used in this *ay arecalled a shift register since each cloc pulse applied to the flipflops causes the contents ofeach flipflop to ,e shifted to the right. The feed,ac connections proide the input to theleftmost flipflop. &ith ,inary stages the largest num,er of different patterns the shiftregister can hae is 2. The all,inaryEero state ho*eer is not allo*ed ,ecause it *ouldcause all remaining states of the shift register and its outputs to ,e ,inary Eero. The all,inaryones state does not cause a similar pro,lem of repeated ,inary ones proided thenum,er of flipflops input to the module 2 adder is een. The period of the se8uence istherefore 2 1. :or e?ample starting *ith the register in state @@1 as sho*n the ne?t 7states are 1@@ @1@1@1 11@ 111 @11 and then @@1 again and the states continue to

repeat. The output taen from the rightmost flipflop is 1@@1@11 and then repeats. &ith thethreestage shift register sho*n the period is 231 or 7.

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:ig. 13

:ig. 14

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11 Types of ,' Seuences in CDMA

(S!5 uses t*o generators to spread the signal po*er uniformly oer the physical,and*idth of a,out 1.25 $"E. The spreading on the reerse lin also proidesnearorthogonality of and hence minimal interference ,et*een signals from each mo,ile.This allo*s reuse of the ,and of fre8uencies aaila,le *hich is a maor adantage ofC#$%.

(n (S!5 t*o types of ma?imumlength Se8uences or codes are used9 the short code and the long code.

S$ort Code9

The short code is generated ,y a 15stage linear shift register. Therefore the ma?imumlength of the Short Code is 9

) M 21 M 2151 M 32761.

By implementation an e?tra chip is inserted at the end of the se8uence yielding ase8uence of length )M3276 chips. The short code runs at a speed of 122@@ chipsper second. This yields a repetition cycle of 3276-122@@M26.67 ms .

ong Code

02-Tm2106euo1_eg0001-Basic Concept of Spread Spectrum Tech.

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