Chapter 4.1 Yani

8/17/2019 Chapter 4.1 Yani

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Chapter 4

1. Digital Modulation

2. Digital Transmission

3. Multiple Access Methods


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Modulation ---A process of changing

one or more properties of the analogcarrier in proportion with theinformation signal.

Digital Modulation --- the transmittalof digitally modulated analog signalsbetween to or more points in acommunications system.

• Can be propagated through Earth’satmosphere and used in wirelesscommunication system - digital radio.


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Traditional electronic

communications’s system.

Mostly replace with modern digitalmodulation


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transportation


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W y Digital Modulation?

The modulation of digital signals withanalogue carriers allows animprovement in signal to noise ratio ascompared to analogue modulating

schemes.


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Factor inuence the choice of a

digital modulation


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Forms o Digital Modulation

• ! the amplitude" # o the carrier is $aried proportional to

the inormation signal" a digital modulated signal is called

Amplitude %hit &eying 'A%&(• ! the re)uency" o the carrier is $aried proportional to

the inormation signal" a digital modulated signal is called

Fre)uency %hit &eying 'F%&(

• ! the phase" * o the carrier is $aried proportional to the

inormation signal" a digital modulated signal is called+hase %hit &eying '+%&(

• ! ,oth the amplitude and the phase" * o the carrier are

$aried proportional to the inormation signal" a digital

modulated signal is called -uadrature Amplitude

Modulation '-AM(

)2sin()( θ π += ft V t v


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loc/ Diagram%impliied ,loc/ diagram o a digital modulation system


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artleys 5a6

Where!7 inormation capacity ',ps(

7 ,and6idth '8(

t 7 transmission time 's(

From the e)uation" !normation

capacity is a linear unction o

,and6idth and transmission time and

directly proportional to ,oth.

t B I ×∝


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%hannons Formula

•W ere

! 7 inormation capacity ',ps(

7 ,and6idt '38(

7 signal to noise po6er ratio 'unitless(

•T e ig er %9: t e ,etter t e perormance

and t e ig er t e inormation capacity

)1(log32.3)1(log 102 N S

N

S

B I or B I +=+=

N S


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E$ample

• 'sing the (hannon’s formula) what isthe theoretical ma$imum data rate%information capacity& that can be

supported in *++ , channels for(/" 0

i& 1+ d#

ii& 2+ d#

3hat you can conclude from bothresults4


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Mary ncoding• !t is often advantageous to encode at

a level higher than binary wherethere are more then two conditionspossible.

• The number of bits necessary toproduce a given number ofconditions is e$pressed

mathematically asWhere N = number of bits necessaryM = number of conditions, level or combinations

possible with N bits.

• Each symbol represents n bits, andhas M signal states, where M = 2N

M N 2log=


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Baud & Minimum BW

• #aud refers to the rate of change of asignal on the transmission mediumafter encoding and modulation have

occurred.

!herebaud = symbol rate "symbol per

second#

ts = time of one signaling element

$ symbol "seconds#

st

baud 1=


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The :y)uist ormulation or channel

capacity

!here

f b= channel capacity

"bps#

B = minimum Ny%uist

bandwidth "&'#

M = number of discretesignal

M B f b 2log2=

0 th ti t l th i W


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0earrange the e)uation to sol$e or the min. W

necessary to pass digitally modulated carriers

3here / is thenumber of bits

encoded into eachsignaling element.

• 3ith digital modulation ) the baud and the idealmin. /y5uist #3 have the same and are e5ual tothe bit rate divided by the number of bit encoded.

• This statement holds true for all forms of digital

modulation e$cept fre5uency 6shift 7eying.

baud N

f

M

f B

bb

==

=

2log


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Amplitude %hit &eying 'A%&(

;also called as Digital amplitide modulation

'DAM(• The simplest digital modulation techni)ue

• A ,inary inormation signal directly

modulates the amplitude o an analog carrier.

!here (as) "t# = amplitude shift

)eying wa(e

(m"t# = digital information signal

"(olt#

*+2 = unmodulated carrieram litude (olt

)cos()](1[)(2

t t vt v c A

mask ω +=


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Digital *mplitude Modulation

−=

+==

1)(,'0'logic0

1)(,'1'logic)cos()(

t v for

t v for t At v

m

mc

ask

ω


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Fre)uency %hit &eying 'F%&(

• Called as .inary Fre)uency %hit &eying

'.F%&(

• The phase shit in carrier re)uency '


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{ }

{ }

−=∆−

+=∆+=

1)(,'0'logic][2cos

1)(,'1'logic][2cos)(

t v for t f f V

t v for t f f V t v

mcc

mcc

fsk π

π

(Hz)frequencyspace&mar !e"#een$ifferencea!solu"e

(Hz)$e%ia"ionfrequency

,

2

=−

=∆

−=∆

sm

sm

f f

f

where

f f f


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)(22)()( bbm sbmb s f f f f f f f f f B +∆=+−=−−−=


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Binary Input Frequency Output

0 Space (f s)

1 Mark (f m)


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+hase %hit &eying '+%&(

• Another orm o anglemodulated" constant

amplitude digital modulation.

• inary digital signal input = limited num,er o

output phases possi,le.

• Mary digital modulation scheme 6ith the

num,er o output phases deined ,y M.• The simplest +%& is inary +hase%hit

&eying '+%&(

– :7 1" M72

– T6o phases possi,le or carrier 6ith onephase or logic 1 and another phase or

logic >

– The output carrier shits ,et6een t6o

angles separated ,y 1?>@


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a) Truth Tabe b) !has"r #ia$ram c) %"nsteati"n #ia$ram


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Cont’d...

BPSK Transmitter


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BPSK Receiver


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• Combine amplitude and phase-shift7eying.

• Method of voice band datatransmission.

• 8AM 9 :-;(,

-uadrature Amplitude

Modulation '-AM(


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•Amplitude and p ase s it /eying can ,e com,ined

to transmit se$eral ,its per sym,ol.

–ten reerred to as linear as t ey re)uire linear

ampliication.

–More ,and6idt eicient" ,ut more suscepti,le

to noise.

•For M 7 4" 1B-AM as t e largest distance

,et6een points" ,ut re)uires $ery linear

ampliication. 1B+%& as less stringent linearity

re)uirements" ,ut as less spacing ,et6een

constellation points" and is t ereore more

aected ,y noise.

•3ig le$el Mary sc emes 'suc as B4-AM( are

$ery ,and6idt eicient ,ut more suscepti,le to

noise and re)uire linear ampliication


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and6idt 2iciency

– Used to compare the performance of one digital

modulation technique to another.

#< 9 Transmission bit rate %bps&

Minimum bandwidth %&


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8ui

For =-;(, system) operating with aninformation bit rate of 2+ 7bps)determine0

a. #aud

b. Minimum bandwidth

c. #andwidth e>ciency

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Chapter 4.1 Yani