Modulation Documentation

7/24/2019 Modulation Documentation

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Hyder Khoja Page 1 2/22/2016

DIGITAL MODULATION TECHNIQUES

1. MODULATION

- Modulation is the process of changing some characteristics of a carrier wae inproportion to the signal to !e transmitted"

- # general e$uation for a sine wae is%&2'&' QtfSinEte CC += Equation 1.

(here &'te is instantaneous amplitude of the sine wae as a function of time"

CE % pea) amplitude of the sine wae"

Cf % fre$uency of the sine wae in hert*"

t % time in seconds"Q % phase in radians"

- +$uation one suggests that there are only , ways the sine wae can !e changed%a" .he amplitude CE "

!" .he fre$uency Cf "

c" .he phase Q "

- t is also possi!le to change more than one of these $uantities simultaneously"

- n digital communications it is common practice to change !oth the amplitude andthe phase angle to o!tain higher data rates"

- t should !e noted that once a carrier is modulated it !ecomes a complete waeformcontaining more than one fre$uency components and therefore would re$uire an

appropriate channel that can carry all fre$uency components of this comple

modulated signal"

- .his gies use to the concept of !andwidth" .he signal now occupies a ( and the

channel must hae sufficient ("

2. INFORMATION CAPACITY

- Hartleys 3aw states that%BWkI +=

(here I is the information capacity in !its"k % constant that depends upon the modulation scheme and the signal to

noise ratio"BW % andwidth of the system in hert*"

- 4ote that Hartleys law is an etremely important law and applies to the operation ofall communication systems"


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3. ANALOG MODULATION

- (hen the modulating signal 'the information to !e transmitted& is analog in nature itis called analog modulation" #nalog modulation is characteri*ed !y continuous

changes in the modulated signal"

- #nalog modulation is of , types as suggested !y e$uation 1%a" Amplitud M!dul"ti!# $AM%% n #M the amplitude of the carrier is aried in

accordance to the instantaneous changes in the amplitude of the modulating signal

as shown in the figure !elow"

!" F&'u#() M!dul"ti!# $FM%% n 5M the fre$uency of the carrier is changed in

proportion to the amplitude of the modulating signal as shown n the figure page0 H"

c" P*"+ M!dul"ti!# $PM%% n PM the phase of the carrier is changed in proportion

to the instantaneous amplitude of the modulating signal as shown in the figure

page 0 H"

,. DIGITAL MODULATION

- (hen the modulating signal is a digital signal it is called digital modulation"

- 7igital modulation results in discrete changes in the modulated signal"- .he receier eamines the signal at specified times only and the state of the signal at

each such time is called a sym!ol"

- .he eact data rate depends upon the num!er of signal changes per second"

- -it R"t% s the num!er of !its transmitted per second which is the data rate"- -"ud R"t% s defined as the num!er of signal changes per second or num!er of

sym!ols per seconds that the line eperiences or senses the changes in signal states" t

M!dul"ti#

+i#"l

C"&&i&

M!dul"td

+i#"l

/

tim


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is possi!le to carry seeral !its per signal change giing a higher data rate than the

!aud rate"

- .here is a theoretical limit to the maimum data rate that can !e transmitted with agien ("

- 8hannon-Hartleys 3aw states that%

&'log2 2 mBC= Equation 2.(here C is the information capacity of the channel in !its/secB % andwidth"m % num!er of possi!le states per sym!ol"

- 4oise also puts limits to the information capacity"

- 8hannons 3aw states that%

+=

N

SBC 1log2 Equation 3.

9 can also !e epressed as%

&'log2 mSC=(here S is the !aud rate in sym!ols per second"

m % num!er of sym!ols"

0. DIGITAL MODULATION SCHEMES

- 5igure page ,2: H shows arious digital modulation schemes used in arious

communication systems"

- Amplitud S*it )i# $AS%% #s seen in the figure in #8K the Cf is turned on

and off in response to the digital data" # !inary ;1< is represented !y the presence of

the carrier and a ;0< !y the a!sence" ell 202 modem uses #8K for error control"

- F&'u#() S*it )i# $FS%% n this case the fre$uency of the carrier is shifted inresponse to a !inary 1 or 0" 5or eample ell 10,/11, modems use 58K to transmit

digital data"

=riginate Mode% Cf > 11?0 H* ;1< > 12?0 H* ;0< > 10?0 H*"

#nswer Mode% Cf > 212 H* ;1< > 222 ;0< > 202 H*"

- =ne method to generate 58K is to fre$uency modulate a single carrier oscillator using

the message signal"- G"u++i"# Mi#imum S*it )i# $GMS%% n @M8K the ;1< and ;0< fre$uencies

are separated !y half the !it rate""0= SM ff !it rate where Mf is the mas) fre$uency and Sf is the space

fre$uency" .hus the deiation from the center fre$uency of the carrier will !ebitrate2"0= "

.hus the modulate inde will !e% 2"02"0 ===bitrate

bitratef

mfM

"

.hus @M8K uses less ( than conentional 58K"

+ample%.he @8M cellular radio system uses @M8K in a 200KH* channel with a channel

data rata of 2?0"A,, K!ps" 9alculate%


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a" 5re$uency shift !etween mas) and space"

!" f the carrier fre$uency is AA0 MH* calculate the transmitted fre$uency"

c" .he ( efficiency of the system in !/s/H*"8olution%

a" 5re$uency shift >

KHzKHzbitrateff SM :16"1,A,,"2?0"0"0 === "!" .he shift each way for the carrier fre$uency will !e half of

MHzf

MHzf

B,22B"A?BA,,"2?02"0AA0

06??"AA0A,,"2?02"0AA0

min

ma

==

=+=

c" ( efficiency > Hzsb //,"1200A,,"2?0 = "

.he word ;@aussian< refers to the shape of the filter that is used !efore the modulated@aussian filter reduces the ( of the transmitted signal !y allowing to change

gradually from mas) to space rather than instantaneously as is the case with regular

58K"

- P*"+ S*it )i# $PS%% n this case the phase of the carrier is charged in

accordance with the data while )eeping the fre$uency constant and when only twophases are used it is called P8K"P8K re$uires changing the phase of the carrier with respect to a reference phase"

# ;0< represent a 0 degree reference phase and ;1< represents a carrier shift of 1A0C

as shown !elow on the constellation diagram"

;< stands fro n-phase and ;D< stands for Duadrature"

5or P8K one !it causes one phase shift in the carrier phase as one signal change/!it"- Qu"d&"tu& P*"+ S*it )i# $QPS%% DP8K implements the concept of phase

shift further" .he carrier can !e made to under go : phase shifts thus can represent :

!inary !it patterns of data effectiely EEE the data rate as shown !elow%

00 > 0C 00 > :C

01 > B0C or 01 > 1,C

10 > 1A0C 10 > -:C11 > 2?0C 11 > -1,C

Q

I

D"t" -it

1

4

5

165

78718

1

1

11

1

1

11 111

11

11

1

111

1


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- GPRSuses A-P8K as shown a!oe effectiely tripling the data rate as we are nowtransmitting , !its per signal change" 8ince it is difficult to maintain a *ero degree

reference phase changes are measured with respect to preious phase"

.he figure !elow shows how way to generate a di-!it P8K output"

- Qu"d&"tu& Amplitud M!dul"ti!# $QAM%% .he only way to o!tain higher data

rates with a narrow !and channel is to increase the num!er of !its per signal change"D#M uses a com!ination of amplitude and phase modulation for eample the ta!le

!elow shows two messages of amplitude and : possi!le phase shifts to transmit , !its

per signal change"

- Mathematically there is no limit to the data rate that may !e supported !y a gien

!aud rate howeer practically it is limited !y the presence of noise when theadjacent states !ecome too small to !e detected relia!ly"

11

11

111

1

11

2/

165

1/

165

1/

5

2/

5

1

2/9 :5

1/9 :5

1

1

11

1

11

11

111

1/

2/

1/

2/

1/

2/

1/

2/

5

5

:5

:5

165

165

2;5

2;5

Amp P*"+

5unction@enerator

O+(ill!+(!p

1 2I < Q =

,05

I = 5


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- =n the scope D#M signal has noise appearance as !lurred data Page 12: Page

,,2 ,,, ,, ,,? H Page 16: 16 #gen"

- 16 D#M inoles applying the signal with 12 different pahses and , differentamplitudes for a total of 16 different possi!le alues each encoding : !its"

16 D#M is used in microwae digital radio digital ideo !roadcasting - ca!le and

high speed modes"

>. SPREAD SPECTRUM MODULATION TECHNIQUES

- 8pread 8pectrum Modulation techni$ue was deeloped !y F"8" military some 0years ago to preent or minimi*e jamming of F"8" radio signals !y enemy forces"

- .oday in its adanced form it is !eing widely implemented in 97M# systems as

well as A02"11 wireless 3#4"

- 4ot only that some ariation of 97M# is epected to !e deployed in all future ,@systems including @8M/4#-.7M# wireless networ)s"

- .he 88 modulation techni$ue consists of spreading a narrow!and signal oer a ery

large ( so that only a small portion of the signal is transmitted at a time" .hus onlya small portion of the signal is transmitted at a time"

- .hus only a small portion of the signal is affected !y the presence of interference and

noise and it also improes security !y ma)ing eaesdropping irtually impossi!le"

TYPICAL QPS TRANSMITTER

? :5 p*"+ S*it @ -PFQPS

!utput

Si#$B(t%-i#"&)

it+


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- .his is ery similar to haing a 12-lane highway" f an accident occurs in one lane the

traffic can still moe along the other 11 lanes"

- ( spreading is accomplished !efore the transmission through the use of a code thatis independent of the transmitted data"

- .he same code is used to demodulate the data at the receiing end"

- .he figure !elow shows spreading of the data signal &'ts !y the code signal &'t creating a modulated message &'tm %

&'&'&' ttstm =

- .here are !asically 2 fre$uency spread techni$ues%a" .he 5re$uency Hopping 8pread 8pectrum '5H88&"

!" 7irect 8e$uence 8pread 8pectrum '7888&"

>.1. F&'u#() H!ppi# Sp&"d Sp(t&um $FHSS%

- 5H88 uses a range of fre$uencies to send a signal rather than just one fre$uency"

- # pseudorandom se$uence is used to change the radio signal fre$uency across a

!road fre$uency !and in a fashion that appears to !e random to an unintended

receier"- Howeer the intended receier is preprogrammed to de-hop the receied !and of

fre$uencies to recoer the original signal"- .he techni$ue consists of transmitting a short !urst of the signal at one fre$uency

then another short !urst at another fre$uency and so on until the entire signal has

!een sent as shown in the figure !elow%

($t%

T&"#+mitt&

Diit"l

+i#"l

+$t%

Sp&"d

+i#"l

m$t%

P!B&

P!B&

H!ppi#

P"ddl+

T&"#+mitt&

Diit"l

+i#"l

D?H!ppi#

R(i&

Diit"l

+i#"l

Sp&"d

+i#"l

P!B&

P!B&


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- .he receied signal is de-hopped at the receier using a fre$uency synthesi*er

controlled !y a pseudo random se$uence generator synchroni*ed to the

transmitters pseudo random se$uence generator"- Multiple users can transmit simultaneously as long as they are using a different

hopping se$uence or if they are using the same hopping se$uence at different

times"- 9onsider the figure shown 'Page :1 9iampa&"

>.2. Di&(t S'u#( Sp&"d Sp(t&um $DSSS%

- n 7888 the radio signal is multiplied !y a pseudorandom se$uence whose

!andwidth is much greater than of the signal itself"

- +ffectiely 7888 transmits a lot more !its than the data !its there!y spreads thesignal"

- +rrors or interference or fading will affect only a small portion of the original

signal"

- .hus all su!scri!ers can use the entire allocated fre$uency spectrum as long asthey are using a different modulating code"

- 97M# uses 7888 techni$ue"- n 7888 a pseudo random se$uence directly phase modulates a data modulated

carrier as shown in the figure !elow%

- .he pseudo random code is called a chipping code"

- 9onsider the figure in the hand out Page :: 9iampa"

- , !its are !eing transmitted 101"- Howeer 788 su!stitutes these , original !its with 'mied with the chipping

code& 1001 0110 1001"- 4otice that for 1 it transmits 1001 and for a 0 it transmits 0110"

- Most chipping odes are actually 11 !its long"

- 4et it com!ines the chipping code with the data !it as follows%a" 5or a data !it of ;1


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!" 5or a data !it of ;0


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- .here are 2 ways to implement this scheme%

a" 5irst multiply 'E-=G& data !its with the P49 and then perform P8K" .his

was done in fig ?"6 a!oe"!" .he second implementation would !e to first perform P8K on the data

stream and then com!ine the result of P8K with the P49" .his is shown in

fig ?"A page 16B ("8" 4otice that in this figure ;1< and ;-1< hae !een used'instead of ;1< and ;0 6"- #ll , users are communicating with the same !ase station receier"

- .he user # data !its are 1101"

- .he figure shows how the 8 receier recoers user #s data !its when synchroni*edwith #s transmission which rejects and 9 transmission"

- 9hipping code for #9 are%

1-1-1-1-1-1

1-1-1-1-1-1

1-1-1-1-1-1

++++=

++++=

+++=

C

B

!

- 5or data !its ;1< the chipping code is sent as it is"

- 5or a data !it ;0


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66::,,2211###### +++++

(here d represents the receied data !its and c represents the chipping code for

the user" .he receier is trying to decode as shown"- 5or user #%

5or data !its ;1


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- 5igure shows 6: !it (alsh codes"

;.3. Implm#ti# t* Sp&"d Sp(t&um H"&dB"&

- #s mentioned earlier the spreading se$uence 'the code c't&& is a se$uence of

!inary digits shared !y !oth the transmitter and the receier"- 8preading consists of multiplying 'E-=Ging& the input data !y the spreading

se$uence where the !it rate of the spreading se$uence is much higher than the

input data rate"- (hen the signal is receied the spreading is remoed !y multiplying 'E-=Ging&

again with the same spreading code eactly synchroni*ed with the receied

signal"

- 97M# uses !oth pseudorandom and orthogonal codes"

- # pseudorandom generator produces a periodic se$uence that eentually repeats!ut that appears to !e random"

- .he period of a se$uence is the length of the se$uence !efore it starts repeating"

- P4 se$uences are generated !y an algorithm using some initial alue called the;seed


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c" .he presence or a!sence of an +E-=G gate corresponds to the

presence of a term in the generator polynomial @'& >

#0#1#22O#n-1

n-1n ecluding the nterm"- Geferring !ac) to figure ?"1, and ta!le ?"2"

- .a!le ?"2 shows the output se$uence generated !y the circuit of figure ?"1,'a& for

different initial states of the shift register"- .a!le ?", '("8& tests a generator polynomial that produces an m-se$uence for the

shift register of arious si*es"

- 8-B uses 2 short P4 codes and one long P4 code"a" .wo short codes% 21-1 > ,2"?6A"

!" =ne long code% 2:2-1 > ::00 !illion"

- # ery important property of P4 code is that ;time shifted ersions of the same

P4 code se$uence hae ery little correlation with each other


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1100?

11106

11110011:

1101,

00102

00011

11000

,21 out$ut%%%&u'seC'ok

- P4 codes are m-se$uences or maimum length codes"

- n general if there are 4 flip-flops the length of the P4 code is 24-1" 5or

eample% in the a!oe eample 4>, so the length of the P4 se$uence > 2,-1>?"

- .he system )eeps generating the se$uence 100101110010111001011 withcontinuous cloc)ing"

;.0. M"+i#

- Mas)ing proides shift in time for P4 codes"

- 7ifferent mas)s will proide different time shifts"- n 97M# '8-B& standard +84 '+lectronic 8erial 4um!er& are used as mas)s

for users on the traffic code channels"

- 5igure !elow shows Dualcomm mas)ing scheme 'page 6 ? K&"

- .here figures show how a mas) generates the same original signal code !utshifted in time"

- .he , digit mas) in this diagram determines the offset of the se$uence"

- 3ong code offsets are used to separate channels in the reerse lin)"

- .he offsets in the short code are to uni$uely identify the forward channels ofindiidual sectors or cells"

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Modulation Documentation