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Page 1Communication Systems Seminar, Summer 2000

Glenn Research Center University of Akron

Modulation and Demodulation

Communication Systems Seminar

Lecture 3


Techniques in Communication Systems

Dr. Oke C. Ugweje

Department of Electrical & Computer Engineering

The University of Akron

Akron, OH 44325-3904

Wednesday June 28, 2000


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Outline of Presentation

FModulation and Demodulation (MODEM)

FClassification of Modulation Techniques

FBaseband versus Bandpass Communications

FWhy Modulate?

FDefinition of Modulation

FAnalog Modulation Techniques

FDigital Modulation Techniques (Sample)

FDetection Detection Techniques

FDigital MODEM Examples

mASK, FSK, PSK, QPSK, OQPSK, DPSK, QAM

F Factors Affecting Choice of Modulation

FComparisons of Digital MODEM

FReferences


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Modulation and Demodulation (MODEM)

Format MultiplexChannel

Encoder

Source

EncoderSpread

Format DemultiplexChannelDecoder

SourceDecoder

Despread

Bits or

Symbol

To otherdestinations

From other

sourcesDigitalinput

Digital

outputSource

bits

Sourcebits

Channelbits

Carrier & symbolsynchronization

Channelbits

$mil q

mil q MultipleAccess

Waveforms

MultipleAccess

Tx

Rx

PerformanceMeasure

$Pe

Modulate

Demodulate&Detect


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Classification of Modulation Techniques

mModulation Techniques can be broadly classified as follows:lDigital versus Analog Modulation

lBaseband versus Bandpass (Passband) Modulation

lBinary versus M-ary Modulation

lMemoryless Modulation versus Modulation with memory

l Linear versus Nonlinear Modulation

lConstant envelope versus Non-constant envelope Modulation

l Power efficient versus Bandwidth efficient Modulation


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Baseband versus Bandpass Communications

mBaseband (Lowpass):lA signal whose frequency content (i.e. its spectrum) is in the

vicinity of zero (i.e.,f = 0 or dc) is said to be a baseband signal

wOriginal source signal are sometimes said to be baseband

lBaseband systems transmit baseband signals

l This is usually not an effective means of communication. Why?

mBandpass (Passband or Narrowband):

lBandpass signal spectrum is nonzero in some band of frequency

with BW = 2B centered aboutf = fc, wherefc >> 0

mEffective transmission of signal usually requires bandpass signal

X(f)

-B2-B1 -fc 0B2B1 ffc

X(fc)

2B2B

fc is carrier frequency


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mBandpass transmission involves some translation of the basebandsignal to some band of frequency centered aroundfc

mBandpass Transmitter:

lCarrier (high frequency pure sinusoidal generated by the local

oscillator) is altered in response to a given low frequency signal

(message signal) generated by the source

ModulatorFrequency

Translation

Power

Amplifier

LocalOscillator

Source

MessageSignal

RF CarrierModulatedCarrier

Carrier for

ModulationCarrier for

TranslationWire


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Why Modulate?

mCoupling EM wave into space - antenna size wavelength

l For speech signalf = 3 kHz = 105m

lAntenna size without modulation = 105m = 60 miles

l Practically unrealizable

lHence, efficient antenna of realistic physical size is needed for

radio communication system

m Information signal must conform to the limitation of its channel

(channel matching)mReduce the effect of interference, e.g. Spread Spectrum

m Place signals at desired frequency band for signal processing purposes

such as filtering, amplification, multiplexing

mUsed to map digital information sequence into waveforms

= cf


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Definition of Modulation

mThe technique of superimposing the message signalon the carrierisknown as modulation

mThat is, modulation is the process by which a property or parameter of

one signal (in this case the carrier) is varied in proportion to the

second signal (in this case the message signal)

mModulation is performed at the transmitter, and the reverse operation(demodulation/detection) is performed at the receiving end

mLet m(t) = message (or information) signal

c(t) = carrier signal

s(t) = modulated signal (transmitted signal)

Modulatorm(t) s(t)

c(t)

Modulating

Carrier

Modulated


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lThe carrierc(t) is a pure sinusoidal signal generally given as

whereAc = Amplitude,fc= Frequency, c(t) = Phase

l Examination ofc(t) indicate that there are 3 parameters which may

be varied:

1. The amplitudeAc,

2. Thefrequencyfc, and

3. Thephase c(t)

l These parameters can be varied in Analog or Digital form

lWhen varied in Digital form, it is referred to as Shifting &

Keying

c t Ac

fct

ct( ) cos( ( ))= +2


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Analog Modulation Techniques

mUsing the message signal m(t) to varyAc,fc, c(t) leads to 3 basictypes of analog modulation schemes respectively known as

1. Amplitude Modulation

2. Frequency Modulation and

3. Phase Modulation

mThese types of modulation are carrier/continuous wave modulation

m In this case, theIntermediate Frequency (IF) or theRadio

Frequency (RF) is modulated

m Frequency & Phase Modulation are also known asAngle ModulationmAmplitude Modulation (AM) is used whenever a shift in the

frequency components of a given signal is desired

l E.g., transmitting voice signal (3 kHz) via EM wave requires that

3 kHz be raised several orders of magnitude before transmission

AmplitudeModulator

m(t) s(t)

c(t)


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m There are 4 kinds of Amplitude Modulation techniques, namely:

1) Conventional Amplitude Modulation

Carrier + Upper Sideband + Lower Sideband

2) Double Sideband (DSB) Suppressed Carrier (SC) AM

Upper Sideband + Lower Sideband

3) Single Sideband (SSB) AM

Only one Sideband (Upper Sideband or Lower Sideband)

4) Vestigial Sideband (VSB) AM

Upper Sideband + portions of the Lower Sideband

fm fm0

M f( )

fc fm +fc fmfc fc fm fc fm+fc

M f fc( )

USBUSB

M f fc( )+

aAc2

S fam

( )

M( )0

LSB LSB

f

f

Ac2

Ac2


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Digital Modulation Techniques (Sample)

FThe purpose of digital modulation is to convert an information-bearing discrete-time symbol into a continuous-time waveform

FBasic Techniques (Binary, M = 2):

mCommon binary modulation schemes include

lAmplitude Shift Keying (BASK)

lFrequency Shift Keying (BFSK)

lPhase Shift Keying (BPSK)

lDifferential Phase Shift Keying (DPSK)

FFor M > 2, many variations of the above techniques exist usually

classified as M-ary modulation

mM-ary modulation schemes include

lPhase Shift Keying (MPSK)

w Quadrature Phase Shift Keying (QPSK)


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w Offset QPSK (Staggered QPSK) (OQPSK/SQPSK)

w /4 Differential QPSK (no carrier) (/4 DQPSK)

w /4 Differential QPSK (with carrier) (/4 QPSK)

w Differential MPSK (no carrier recovery) (DMPSK)

lContinuous-Phase Frequency Shift Keying (CPFSK)

lSinusoidal Frequency Shift Keying (SFSK)

lMinimum Shift Keying (MSK)

w Differential MSK (DMSK)

w Gaussian MSK (GMSK)

lAmplitude Phase Keying (MAPK)

lQuadrature Amplitude Modulation (MQAM)

w Superposed QAM

lQuadrature Partial Response Signaling (QPRS)


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Digital Detection Techniques

MODEM

NONCOHERENTCOHERENT

BINARY M-ary HYBRID BINARY M-ary HYBRID

ASK(OOK)

FSK

(MSK)

PSK

ASK

FSK

PSK(QPSK,OQPSK)

APK(QAM) ASK

FSK

DPSK

CPM

ASK(OOK)

FSK

DPSK

CPM

(Phase inforequired)

(No Phase inforequired)


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Digital MODEM Examples

FAmplitude Shift Keying (ASK)

mModulation Process:

wAmplitude of the carrier is switched between two (or more)

levels according to the digital data

xm t( )

A tocos( )

s t( )

Baseband Data Modulated bandpass SignalOOK Modulator

Product modulator or

ON-OFF switch

0 T 3T


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mDetectors for ASK:

mPower Spectral Density:

2Tb f Rc b+

f Rc b

+ 2

impulse

B RT

bb

= =2 2

l Bandwidth

w Null-to-null bandwidth


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Frequency Shift Keying (FSK)


l InFSK, the instantaneous frequency of the carrier is switched

between 2 or more levels according to the baseband digital data

mWaveform:

mDiscontinuous Phase FSK:

f1

f2

s t A t o c( ) cos( )= + 1 1 s t A t c1 2 2( ) cos( )= +

1 2 Phase Discontinuities

1 1 1 100


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mContinuous Phase FSK:

mDemodulation of FSK:

No Phase Discontinuities

1 1 1 100

0 1=

Coherent Noncoherent

Envelop

Detection


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mPSD of CPFSK:

Sunde's FSK


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Phase Shift Keying (PSK)


l InPSK, the phase of the carrier signal is switched between 2 or

more values in response to the baseband digital data

mWaveform:

mPSK Generation:


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mReceiver (Demodulator) for PSK:

?There is no non-coherent detection equivalent for PSK. Why?

mPower Spectral Density of PSK:

l Similar to that of ASK


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Quadrature PSK

E

10

01

11

00

s0s

1

s2

s3


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m In QPSK, the bit transition in I- & Q-channels occur simultaneously

Simultaneous

transition of Qand I channels


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Offset QPSK

m In OQPSK, I-channel (or Q-channel) bit stream is offset by one bit

period w.r.t. the Q-channel (or I-channel) prior to multiplication by

the carrierNotice that the Q and I channels are

not aligned and only one phase

transition can occur once every Ts =

Tb sec with a max at 90o

I-channel: even bits

Q-channel: odd bitsPhase Diagrams


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Differential PSK (DPSK)

mDPSK is regarded as the noncoherent version of binary PSK

DelayTs

dk

dk1

dd a

d akk k

k k

===

RST

1

1

0

1

,

,ak ak dk dk1

0 0 1

0 1 0

1 0 0

1 1 1

M_ary Case


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Quadrature Amplitude Modulation (QAM)

mMost commonly used combination ofamplitude andphase signaling

is the Quadrature Amplitude Modulation (QAM)

mMQAM Modulator:

mM-ary QAM Demodulation:


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mQAM Constellation:Q

II I

QQ

Type I Type II Type III

16 QAM (8, 8) 16 QAM (4, 12) 16 QAM (4, 8, 4)


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Factors Affecting Choice of Modulation

m Signal-to-noise ratio (SNR)

m Probability of error or Bit Error Rate (BER)

m Power Efficiency, p

l Power efficiency is a measure of how much received power is

needed to achieve a specified BER (inversely proportional to BER

lAs BER increases, p decreases since transmitted power is

wasted on more bad data

mBandwidth Efficiency (or Spectral Efficiency), B

lDefined as the ratio of the bit rate to the channel bandwidth

w IfR is data rate and B is the RF signal bandwidth, then

wThe capacity of a digital system is directly related to B

BR

B BT M bps Hz = =

12log /


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wThe max possible bandwidth efficiency is

?Note: Binary systems are more Power Efficient, but less Spectral

Efficient than M-ary systems

m Performance in multipath environment

l Envelope fluctuations and channel non-linearity

m Implementation cost and complexity

?No modulation scheme possesses all the above characteristics; hence,

trade-off are made when selecting modulation/demodulation schemes

BC

B

S

Nbps Hz

maxlog / = = +FH

IK2 1


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m For example, in wireless communications, it is important to select

MODEM based on the following requirements

lHigh Spectral Efficiency

lHigh Power Efficiency

lHigh Fading Immunity

FPractical Modulation Schemes

mFM AMPS

mMSK CT2

mGMSK GSM, DCS 1800, CT3, DECT

mQPSK NADC (CDMA) - base transmitter

mOQPSK NADC (CDMA) - mobile transmitter

m4-DQPSK NADC (TDMA), PDC (Japan), PHP (Japan)

mMPSK (some wireless LANs)

w These factors are affected

by baseband pulse shape

and phase transition

characteristics of the signal


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Error Performance Comparison

Modulation Type PM(coherent) P b (coherent) P b (noncoherent)

m Baseband Systems

l Unipolar

l Polar

l Bipolar

m Bandpass Systems

l BASK (OOK)

l BFSK

l BPSK

l QPSK

l OQPSK

l DPSK

QEsNo

e j

QEbN

2

0e j

QEb

N

2

0e j

12

2

8exp ANo

e j

12 2exp

Eb

Noe j

QEbN0

e j

32

0

QEbNe j

QEbN0

e j

QEbN0

e j

QEbN0

e j

22

0

QEs

Ne j

12 exp

EbN

o

e j

22

0

QEs

N Msine j

Requires coherent detection

QEb

N

2

0e j Requires coherent detection

QEsN0

e j

QEs

No

2e j

Not used in practice

22

12

0 0Q

EN Q

EN

b bFHIK

FH

IK

L

NM

O

QP


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mError Performance of BPSK/QPSK:

P QE

NQ

A T

Nb

b

o

b

o

=FHG

IKJ

FHG

IKJ

22

2

2

P QE

Nerfc

E

Ne

b

o

b

o

=FHG

IKJ

=FHG

IKJ

22 1

2

Bit Error Rate

Symbol Error Rate


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mError Performance of BPSK/QPSK/DPSK/DQPSK/MQAM:

P M QME

EsNo

( ) sin FHIK2

2

Bit/Symbol Error Rate

Symbol Error Rate


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mOther Performance Comparison

24.3 dB40.33Rb18.3 dB16-PSK

18.8 dB30.33Rb14.0 dB8-PSK

13.6 dB20.5Rb10.6 dBQPSK

10.6 dB1Rb10.6 dBBPSK

Required

CNR

Max B(bits/s/Hz)

Min Channel B for

ISI free signaling

Required

Eb/No

Modulation

SchemePb = 10

-6

Null-to-Null

2/3

1.0

1.0

0.5

Bandwidth Efficiency, B

d (complex)A (best)N/A9.6 dBMSK

cB2.09.6 dBOQPSK

aC2.09.6 dBQPSK

a (simple)D (worst)1.09.6 dBBPSK

Implementation

Complexity

Immunity to

NonlinearityNyquist

Eb/No

(dB)

Modulation

Scheme

Pb = 10-5


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mComplexity

Complexity High

APK

M-ary PSKQPR

CPFSK - optimal detection

MSK

OQPSK

QAM, QPSK

BPSK

Low

OOK - envelope detection

DQPSK

DPSK

CPFSK -discriminator detection

FSK - noncoherent detection

Ref: IEEE Communications Magazine 1988?


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References

1. O. C. Ugweje, Class Handouts on Communications and Signal Processing, Digital

Communications, Wireless Communications, University of Akron, Akron Ohiohttp://www.ecgf.uakron.edu/ugweje/web/home.html

2. B. Sklar,Digital Communications Fundamentals and Application, Prentice-Hall,

Englewood Cliffs, NJ, 1988.

3. A. Bateman,Digital Communications Design for the Real World, Addison-

Wesley, 19884. J. G. Proakis,Digital Communications, 3rd Edition, McGraw-Hill, 1994.

5. J. G. Proakis and Masoud Salehi, Communication Systems Engineering, Prentice-

Hall, 1994

6. A. Ambardar,Analog and Digital Signal Processing, PWS Publishing Company,

MA, 19957. K. Feher, Digital Communications: Satellite/Earth Station Engineering,

Prentice-Hall, Inc., New Jersey, 1983

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