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Chapter 5 Signal Encoding andChapter 5 Signal Encoding andModulation TechniquesModulation Techniques


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Encoding and Modulation TechniquesEncoding and Modulation Techniques


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Digital Signaling Versus AnalogDigital Signaling Versus AnalogSignalingSignaling

Digital signaling Digital or analog data is encoded into a digital signal Encoding may be chosen to conserve bandwidth or to minimizeerror

Analog Signaling Digital or analog data modulates analog carrier signal The frequency of the carrier fc is chosen to be compatible withthe transmission medium used Modulation: the amplitude, frequency or phase of the carriersignal is varied in accordance with the modulating data signal by using different carrier frequencies, multiple data signals(users) can share the same transmission medium


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Digital SignalingDigital Signaling

Digital data, digital signalimplest encoding scheme: assign one voltage level to binaryone and another voltage level to binary zeroMore comple! encoding schemes: are used to improveperformance (reduce transmission bandwidth and minimize

errors)"E!amples are #$%&', #$% , Manchester, etc"

Analog data, Digital signal nalog data, such as voice and video*ften digitized to be able to use digital transmission facilityE!ample: +ulse ode Modulation (+ M), which involvessampling the analog data periodically and quantizing thesamples


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Analog SignalingAnalog Signaling

Digital data, Analog Signal modem converts digital data to an analog signal so that itcan be transmitted over an analog lineThe digital data modulates the amplitude, frequency, orphase of a carrier analog signalE!amples: mplitude hift -eying ( -), .requency hift-eying (. -), +hase hift -eying (+ -)

Analog data, Analog Signal

nalog data, such as voice and video modulate theamplitude, frequency, or phase of a carrier signal to producean analog signal in a different frequency bandE!amples: mplitude Modulation ( M), .requencyModulation (.M), +hase Modulation (+M)


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Digital Data, Digital SignalDigital Data, Digital Signal

/igital signaldiscrete, discontinuous voltage pulseseach pulse is a signal element

binary data encoded into signal elements


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Interpreting SignalsInterpreting Signals

#eed to 2nowtiming of bits: when they start and endsignal levels: high or low

factors affecting signal interpretation/ata rate: increase data rate increases 5it Error $ate (5E$)ignal to #oise $atio ( #$): increase #$ decrease 5E$5andwidth: increase bandwidth increase data rateencoding scheme: mapping from data bits to signal elements


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Comparison of Encoding Schemes

signal spectrum'ac2 of high frequencies reduces required bandwidth,lac2 of dc component allows ac coupling via transformer,providing isolation,should concentrate power in the middle of the bandwidth

loc2ingsynchronizing transmitter and receiver with a syncmechanism based on suitable encoding

error detectionuseful if can be built in to signal encoding

signal interference and noise immunitycost and comple!ity: increases when increases data rate


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Encoding Schemes


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Non eturn to !ero"#e$el %N !"#&

Two different voltages for 1 and 0 bits8oltage constant during bit interval

no transition, i"e" no return to zero voltagemore often, negative voltage for binary oneand positive voltage for binary zero


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Non eturn to !ero INVE TED %N !I&

#onreturn to zero inverted on onesonstant voltage pulse for duration of bit/ata encoded as presence or absence of signaltransition at beginning of bit time

transition (low to high or high to low) denotes binary 0no transition denotes binary 1

E!ample of differential encoding since have 9 data represented by changes rather than levels 9 more reliable detection of transition rather than level


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Ad$antages and disad$antages ofN !"#, N !I

dvantageseasy to engineer good use ofbandwidth

/isadvantagesdc componentlac2 of synchronization capability

nattractive for signal transmission applications


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Multile$el 'inar('ipolar Alternate Mar) In$ersion %AMI&

se more than two levels (three levels,positive, negative and no line signal)5ipolar& M

zero represented by no line signalone represented by positive or negative pulseone pulses alternate in polarityno loss of sync if a long string of ones

long runs of zeros still a problemno net dc componentlower bandwidtheasy error detection


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Multile$el 'inar(Pseudoternar(

5inary one represented by absence of linesignal

5inary zero represented by alternatingpositive and negative pulses#o advantage or disadvantage over

bipolar& MEach used in some applications


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Multile$el 'inar( Issues

dvantages:#o loss of synchronization if a long string of 0;s occurs, eachintroduce a transition, and the receiver can resynchronize onthat transition#o net dc component, as the 0 signal alternate in voltagefrom negative to positive'ess bandwidth than #$%+ulse alternating provides a simple mean for error detection

/isadvantagesreceiver distinguishes between three levels: 6 , & , 1a < level system could represent log =< 3 0"7> bits

requires appro!"


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Theoretical 'it Error ate %'E & *orVarious Encoding Schemes


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Manchester Encoding

has transition in middle of each bit periodlow to high represents binary onetransition serves as cloc2 and datahigh to low represents binary zeroused by EEE >1="< (Ethernet) ' # standard


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Differential Manchester Encoding

midbit transition is cloc2ing onlytransition at start of bit period representing binary 1no transition at start of bit period representing binary 0

used by EEE >1="7 to2en ring ' #


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Ad$antages and disad$antages ofManchester Encoding

/isadvantages

at least one transition per bit time and possibly twoma!imum modulation rate is twice #$%

requires more bandwidth

dvantagessynchronization on mid bit transition (self cloc2ing codes)has no dc componenthas error detection capability (the absence of an e!pected transition can beused to detect errors)

elements signal per bitsof number L

bps Rate Data R

baud rate Modulation D L R

D

:

][,:

][,:

=


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Modulation ate $ersus Data ate/ata rate (e!pressed in bps)

/ata rate or bit rate $304T b3040?s30MbpsModulation $ate (e!pressed in baud) is the rate at whichsignal elements are generated

Ma!imum modulation ratefor Manchester is/304(1"7T b)3=40?s3=Mbaud


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Scram+ling

se scrambling to replace sequences that wouldproduce constant voltageThese filling sequences must

produce enough transitions to maintain synchronization

be recognized by receiver @ replaced with originalbe same length as original

/esign goals

have no dc componenthave no long sequences of zero level line signalhave no reduction in data rategive error detection capability


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-igh Densit( 'ipolar". 0eros %-D'.&

The scheme replaces strings with 4 zeros by sequences containing one or two pulsesIn each case, the fourth zero is replaced with a code violation (V) successive violations are of alternate polarity


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Digital Data, Analog Signal

Main use is public telephone systemhas freq range of


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Modulation Techniques

mplitude hift -eying( -)

5inary .requency hift-eying (5. -)

5inary +hase hift -eying(5+ -)


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Amplitude Shift 1e(ing %AS1&

n -, the two binary values are represented by todifferent amplitudes of the carrier frequencyThe resulting modulated signal for one bit time is

usceptible to noise

nefficient modulation techniqueused for

up to 0=11bps on voice grade linesvery high speeds over optical fiber

= 0,0 1),2cos()( binarybinaryt f At s c


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'inar( *requenc( Shift 1e(ing %'*S1&

The most common form of . - is 5inary . - (5. -)

Two binary values represented by two different frequencies ( f 1 and f 2 )

less susceptible to noise than -used for

up to 0=11bps on voice grade lineshigh frequency radio (< to


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*ull"Duple2 '*S1 Transmission ona Voice"3rade line

8oice grade lines will pass voice frequencies in the range


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Multiple *S1 %M*S1&

More than two frequencies (M frequencies) are usedMore bandwidth efficient compared to 5. -More susceptible to noise compared to 5. -M. - signal:

element signal per bitsof number Lelements signal different of number M

frequencydifferencethe f frequencycarrier the f

f M i f f here

M it f At s

L

d

c

d ci

ii

=

==

==

+=

=

2

)12(

1),2cos()(


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Multiple *S1 %M*S1&

M. - signal:

+eriod of signal element

Minimum frequency separation

M. - signal bandwidth:

element signal per bitsof number L

elements signal different of number M

f M i f f

here M it f At s

L

d ci

ii

=

==

+=

=

2

)12(

1),2cos()(

d d d Mf f M ! 2)2( ==

period bit " period element signal " L" " b sb s ::,=

)(2/12)/(12/1 ratebit Lf " f L" f " d bd bd s ===


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E2ampleCith f c#2$%&'( , f d #2$&'( , and M#) ( L#3 bits), we have the following frequency assignment for each of the > possible


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E2ample

The following figure shows an e!ample of M. - with M3B" n

input bit stream of =1 bits is encoded =bits at a time, with each ofthe possible =&bit combinations transmitted as a differentfrequency"

d c

d c

d c

d c

d ci

f f f i

f f f i f f f i

f f f i

f M i f f

3411

310201

3100

)12(

4

3

2

1

+==+== ==

==+=


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Phase Shift 1e(ing %PS1&

+hase of carrier signal is shifted to represent data5inary + - (5+ -): two phases represent twobinary digits

0 0 1 1 0 1 0 0 0 1 0

* * % % * 0 * * * % *

1)(),2cos()(

0),2cos(1),2cos(

0),2cos(

1),2cos()(

==

=

+=

t d t f t Ad

binaryt f Abinaryt f A

binaryt f A

binaryt f At s

c

c

c

c

c


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Differential PS1 %DPS1&

n /+ -, the phase shift is with reference to the previous bit transmitted

rather than to some constant reference signal5inary 1:signal burst with the same phase as the previous one5inary 0:signal burst of opposite phase to the preceding one

$


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*our"le$el PS14 5uadrature PS1%5PS1&

+

+

+

=

10)4

2cos(

00)4

32cos(

01)4

32cos(

11)4

2cos(

)(

t f A

t f A

t f A

t f A

t s

c

c

c

c

More efficient use of bandwidth if each signal element representsmore than one bit

eg" shifts of 4= (D1o)each signal element represents two bitssplit input data stream in two @ modulate onto the phase of the carrier

can use > phase angles @ more than one amplitudeD 11bps modem uses 0= phase angles, four of which have two amplitudes:this gives a total of 0 different signal elements

5PS1 and 6ffset 5PS1 %65PS1&


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5PS1 and 6ffset 5PS1 %65PS1&Modulators

)2sin()(2

1)2cos()(

2

1)(:

)2sin()(2

1)2cos()(

21

)(:

t f " t +t f t , t s-+./&

t f t +t f t , t s+./&

cbc

cc

=

=

E2ample of 5PS1 and 65PS1


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E2ample of 5PS1 and 65PS17a$eforms

41101

43

1100

43

1110

41111

:

+./& for

Performance of AS1 *S1 M*S1 PS1


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Performance of AS1, *S1, M*S1, PS1and MPS1

5andwidth Efficiency

-4+ -:

M+ -:

M. -:

10,1

1


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Performance of M*S1 and MPS1

M. -: increasing M decreases 5E$ and decreases bandwidth Efficiency

M+ -: ncreasing M increases 5E$ and increases bandwidth efficiency


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5AM d l


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5AM modulator

A/&

c

A/&

c t f t d t f t d t s+AM )2sin()()2cos()()(: 21 +=

5AM V i


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5AM Variants

Two level - (two different amplitude levels)each of two streams in one of two statesfour state systemessentially F+ -

.our level - (four different amplitude levels)combined stream in one of 0 states

Aave B and =7 state systemsmproved data rate for given bandwidth

but increased potential error rate

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