Chapter 4_Part 1-Digital Modulation(w1w2)

7/24/2019 Chapter 4_Part 1-Digital Modulation(w1w2)

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Chapter 4

4.1 : Digital Modulation

4.2 : Digital Transmission

4.3 : Multiple Access Methods


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4.1 Digital Modulation

Outlines

a. Introduction

b. Information capacity, Bits, Bit Rate, Baud,

M-ary Encoding

c. Digital Modulation Techniues

- !"#, $"#, %"#, &!M


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Digital modulation

Is the transmittal of digitally modulated analog signals

et!een to or more points in a communications system.

"an e propagated through #arth$s atmosphere and

used in !ireless communication system % digital radio.

&ffer se'eral outstanding ad'antages o'er traditionalanalog system.#ase of processing#ase of multiple(ing)oise immunity


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Contd...

Applications: *o! speed 'oice and data comm. modems

+igh speed data transmission systems

Digital micro!a'e , satellite comm. systems

-" (personal communication systems)telephone


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Why digital modulation?

The modulation of digital signals !ith analogue carriers

allo!s an impro'ement in signal to noise ratio as

compared to analogue modulating schemes.


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Important Criteria

1. High spectral efficiency

2. High power efficiency

3. Robust to multipath

4. Low cost and ease of implementation

5. Low carrier-to-co channel interference ratio

. Low out-of-band radiation


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Contd

!. "onstant or near constant en#elop

$. %andwidth &fficiency

'bility to accommodate data within a limitedbandwidth

(radeoff between data rate and pulse width

). *ower &fficiency (o preser#e the fidelity of the digital message at

low power le#els.

"an increase noise immunity by increasing signal

power


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Forms of Digital Modulation

)2sin()( += ftVtv

If the amplitude, Vof the carrier is variedproportional to the information signal, a digitalmodulated signal is called Amplitude ShiftKeying (ASK)

If the frequency, fof the carrier is variedproportional to the information signal, a digitalmodulated signal is called Frequency ShiftKeying (FSK)


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Contd

If thephase, of the carrier is varied proportionalto the information signal, a digital modulatedsignal is called Phase Shift Keying (PSK)

If both the amplitude and the phase, of thecarrier are varied proportional to the informationsignal, a digital modulated signal is called

Quadrature Amplitude odulation (QA)


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Contd...


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Example 1

/or the digital message 1101 1100 1010

setch the !a'eform for the follo!ing:

a. A. /

c. -

d. AM


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Block Diagram

Simpli!ed bloc" diagram of a digital modulation system


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Contd

-recoder performs le'el con'ersion , encodesincoming data into group of its that modulate ananalog carrier.

Modulated carrier filtered amplified ,transmitted through transmission medium to 5(.

In 5( the incoming signals filtered amplified ,applied to the demodulator and decoder circuits!hich e(tracts the original source informationfrom modulated carrier.


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Information capacity 6its , 6it 5ate

7 5epresents the numer of independent

symols that can e carried through a systemin a gi'en unit of time.

7 6asic digital symol is the binary digitor bit.

7 #(press the information capacity as a bit rate.


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artleys !a"

tBI 'here

I ( information capacity )bps*

B ( band+idth )*

t ( transmission time )s*

From the equation, Information capacity is a linear

function of bandwidth and transmission time and

directly proportional to both.


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#hannons Formula

)1(log2.)1(log 1!2 NS

NS BIorBI +=+=

'here

I ( information capacity )bps*

B ( band+idth )*

( signal to noise po+er ratio )unitless*

The higher "/ the better the performance and the

higher the information capacity

N

S


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Nyquist Sampling Rate

fsis e


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Example 3

Determine the )y


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M$ary %ncoding

It is often ad'antageous to encode at a le'el higher than

inary !here there are more then t!o conditions

possile.

The numer of its necessary to produce a gi'ennumer of conditions is e(pressed mathematically as

MN 2log=

'here / ( number of bits necessary

M ( number of conditions, le0el or combinations

possible +ith / bits.


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Contd

#ach symol represents n its and has M

signal states !here M 9 2).


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Example 4

/ind the numer of 'oltage le'els !hich

can represent an analog signal !ith

a. ? its per sample . 12 its per sample


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Baud & Minimum BW

6audrefers to the rate of change of a signal on the

transmission medium after encoding and modulation

ha'e occurred.

'here

baud ( symbol rate )symbol per second*

ts ( time of one signaling element 1 symbol

)seconds*

stbaud

1=


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Contd

Minimum 6and!idth7 @sing multile'el signaling the )y


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Contd

baudNf

MfB bb ==

=

2log

here $ is the number of bits encoded intoeach signaling element*

+or necessary to pass M-ary digitally modulatedcarriers


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Amplitude hift eying AB

/re


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'mplitude #hift (eying )'#(*

A inary information signal directly modulates the amplitude of an

analog carrier.

ometimes calledDigital Amplitude Modulation (DAM)

)cos()"(1#)(2

ttvtv cA

mask +=

'here 0as2 )t* ( amplitude shift 2eying +a0e0m)t* ( digital information signal )0olt*

!3 ( unmodulated carrier amplitude )0olt*

+c , analog carrier radian freuency )rads*


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Contd...

=

+==

1)(,$!$logic!

1)(,$1$logic)cos()(

tvfor

tvfortAtv

m

mc

ask

Digital Amplitude Modulation


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Fre+uency #hift (eying )F#(*

"alled as Binary Frequency Shift Keying (BFSK)

The phase shift in carrier fre


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{ }

{ }

=

+=+=

1)(,$!$logic"#2cos

1)(,$1$logic"#2cos)(

tvfortffV

tvfortffVtv

mcc

mcc

fsk

(%&)frequencyspace'marbetweendifferenceabsolute

(%&)deiationfrequency

,

2

=

=

=

sm

sm

ff

f

where

fff


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)(22)()( bbmsbmbs fffffffffB +=+==


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.ont/d***

6inary Input /re


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,hase #hift (eying ),#(*

Another form of angle%modulated constant amplitude

digital modulation.

6inary digital signal input , limited numer of output

phases possile. M%ary digital modulation scheme !ith the numer of

output phases defined y M.

The simplest - is 6inary -hase%hift eying 6-B

7 )9 1 M927 T!o phases possile for carrier !ith one phase for logic 1 and

another phase for logic 0

7 The output carrier shifts et!een t!o angles separated y 1?0


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Contd...

aB Truth Tale B -hasor Diagram cB "onstellation Diagram


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Contd...

BPSK Transmitter


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Contd...

BPSK Receiver


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Band"idth %fficiency

- sed to compare the performance of one digital

modulation techni+ue to another.

6E 9 Transmission it rate psB

Minimum and!idth +;B


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Example 5

/or 1F%- system operating !ith an

information it rate of 32 ps determine:

a. 6aud . Minimum and!idth

c. 6and!idth efficiency


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!N"#S$!N

To decide !hich modulation method shoulde used !e need to mae considerations of

aB 6and!idth

B peed of Modulation

cB "omple(ity of +ard!are


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Documents

Chapter 4_Part 1-Digital Modulation(w1w2)