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Page 1Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Communication Systems Seminar
Lecture 3
Modulation and DemodulationTechniques in Communication Systems
Dr. Oke C. Ugweje
Department of Electrical & Computer EngineeringThe University of AkronAkron, OH 44325-3904
Wednesday June 28, 2000
Page 2Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Outline of Presentation
FModulation and Demodulation (MODEM)FClassification of Modulation TechniquesFBaseband versus Bandpass CommunicationsFWhy Modulate?FDefinition of ModulationFAnalog Modulation TechniquesFDigital Modulation Techniques (Sample)FDetection Detection TechniquesFDigital MODEM ExamplesmASK, FSK, PSK, QPSK, OQPSK, DPSK, QAM
F Factors Affecting Choice of ModulationFComparisons of Digital MODEMFReferences
Page 3Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Modulation and Demodulation (MODEM)
Format MultiplexChannelEncoder
SourceEncoder
Spread
Format DemultiplexChannelDecoder
SourceDecoder
Despread
Bits orSymbol
To otherdestinations
From othersources
Digitalinput
Digitaloutput
Sourcebits
Sourcebits
Channelbits
Carrier & symbolsynchronization
Channelbits
$mil q
mil q MultipleAccess
Waveforms
MultipleAccess
Tx
Rx
PerformanceMeasure
$Pe
Modulate
Demodulate&
Detect
Page 4Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Classification of Modulation Techniques
mModulation Techniques can be broadly classified as follows:lDigital versus Analog ModulationlBaseband versus Bandpass (Passband) ModulationlBinary versus M-ary ModulationlMemoryless Modulation versus Modulation with memoryl Linear versus Nonlinear ModulationlConstant envelope versus Non-constant envelope Modulationl Power efficient versus Bandwidth efficient Modulation
Page 5Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
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 signalwOriginal source signal are sometimes said to be baseband
lBaseband systems transmit baseband signalsl 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 about f = ±fc, where fc >> 0
mEffective transmission of signal usually requires bandpass signal
X(f)
-B2-B1 -fc 0 B2B1 ffc
X(fc)
2B2B
fc is carrier frequency
Page 6Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
mBandpass transmission involves some translation of the baseband signal to some band of frequency centered around fc
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
ModulatorFrequencyTranslation
PowerAmplifier
LocalOscillator
Source
MessageSignal RF Carrier
ModulatedCarrier
Carrier forModulation
Carrier forTranslation
Wire
Page 7Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Why Modulate?
mCoupling EM wave into space - antenna size α wavelength λ
l For speech signal f = 3 kHz ð λ = 105m
lAntenna size without modulation ≅ λ = 105m = 60 milesl Practically unrealizablelHence, efficient antenna of realistic physical size is needed for
radio communication systemm Information signal must conform to the limitation of its channel
(channel matching)mReduce the effect of interference, e.g. Spread Spectrumm Place signals at desired frequency band for signal processing purposes
such as filtering, amplification, multiplexingmUsed to map digital information sequence into waveforms
λ = cf
Page 8Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Definition of Modulation
mThe technique of superimposing the message signal on the carrier is known 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) signalc(t) = carrier signals(t) = modulated signal (transmitted signal)
Modulatorm(t) s(t)
c(t)
Modulating
Carrier
Modulated
Page 9Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
l The carrier c(t) is a pure sinusoidal signal generally given as
where Ac = Amplitude, fc= Frequency, θc(t) = Phasel Examination of c(t) indicate that there are 3 parameters which may
be varied: 1. The amplitude Ac,
2. The frequency fc, and
3. The phase θc(t)l These parameters can be varied in Analog or Digital formlWhen varied in Digital form, it is referred to as “Shifting &
Keying”
c t Ac fct c t( ) cos( ( ))= +2π θ
Page 10Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Analog Modulation Techniques
mUsing the message signal m(t) to vary Ac, fc, θc(t) leads to 3 basic types of analog modulation schemes respectively known as1. Amplitude Modulation
2. Frequency Modulation and 3. Phase Modulation
mThese types of modulation are carrier/continuous wave modulation
m In this case, the Intermediate Frequency (IF) or the Radio Frequency (RF) is modulated
m Frequency & Phase Modulation are also known as Angle Modulation
mAmplitude Modulation (AM) is used whenever a shift in the frequency components of a given signal is desiredl E.g., transmitting voice signal (3 kHz) via EM wave requires that
3 kHz be raised several orders of magnitude before transmission
AmplitudeModulatorm(t) s(t)
c(t)
Page 11Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
m There are 4 kinds of Amplitude Modulation techniques, namely:
1) Conventional Amplitude Modulation ððCarrier + Upper Sideband + Lower Sideband
2) Double Sideband (DSB) Suppressed Carrier (SC) AM
ðð Upper Sideband + Lower Sideband
3) Single Sideband (SSB) AMðð Only one Sideband (Upper Sideband or Lower Sideband)
4) Vestigial Sideband (VSB) AMðð Upper Sideband + portions of the Lower Sideband
− fm fm0
M f( )
− −fc fm − +fc fm− fc fc fm− fc fm+fc
M f fc( )−
USBUSB
M f fc( )+
aAc2
S fam
( )
M( )0
LSB LSB
f
f
Ac
2
Ac
2
Page 12Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
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 includelAmplitude 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 usuallyclassified as M-ary modulation
mM-ary modulation schemes includelPhase Shift Keying (MPSK)w Quadrature Phase Shift Keying (QPSK)
Page 13Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
w Offset QPSK (Staggered QPSK) (OQPSK/SQPSK)
w π/4 Differential QPSK (no carrier) (π/4 DQPSK)w π/4 Differential QPSK (with carrier) (π/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)
Page 14Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
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)
Page 15Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
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( )ω
s t( )
Baseband Data Modulated bandpass SignalOOK Modulator
Product modulator orON-OFF switch
0 T 3T
Page 16Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
mDetectors for ASK:
mPower Spectral Density:
2Tb f Rc b+
f Rc b+ 2
impulse
B RTb
b
= =2 2l Bandwidth
w Null-to-null bandwidth
Page 17Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Frequency Shift Keying (FSK)
mModulation Process:l In FSK, the instantaneous frequency of the carrier is switched
between 2 or more levels according to the baseband digital datamWaveform:
mDiscontinuous Phase FSK:
f1 f2
s t A to c( ) cos( )= +ω θ1 1 s t A tc1 2 2( ) cos( )= +ω θ
θ θ1 2≠ Phase Discontinuities
1 1 1 100
Page 18Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
mContinuous Phase FSK:
mDemodulation of FSK:No Phase Discontinuities
1 1 1 100
θ θ0 1=
Coherent Noncoherent
Envelop Detection
Page 19Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
mPSD of CPFSK:
Sunde's FSK
Page 20Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Phase Shift Keying (PSK)
mModulation Process:l In PSK, the phase of the carrier signal is switched between 2 or
more values in response to the baseband digital datamWaveform:
mPSK Generation:
Page 21Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
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
Page 22Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Quadrature PSK
E
10
01
11
00s0
s1
s2 s3
Page 23Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
m In QPSK, the bit transition in I- & Q-channels occur simultaneously
Simultaneous transition of Q and I channels
Page 24Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
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 carrier
Notice 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 bits Phase Diagrams
Page 25Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Differential PSK (DPSK)
mDPSK is regarded as the noncoherent version of binary PSK
DelayTs
dk
dk−1
dd ad ak
k k
k k= =
=RST
−
−
1
1
01
,,
akak dk dk−10 0 1
0 1 0
1 0 0
1 1 1
M_ary Case
Page 26Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Quadrature Amplitude Modulation (QAM)
mMost commonly used combination of amplitude and phase signaling is the Quadrature Amplitude Modulation (QAM)
mMQAM Modulator:
mM-ary QAM Demodulation:
Page 27Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
mQAM Constellation: Q
II I
Type I Type II Type III
16 QAM (8, 8) 16 QAM (4, 12) 16 QAM (4, 8, 4)
Page 28Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Factors Affecting Choice of Modulation
m Signal-to-noise ratio (SNR)m Probability of error or Bit Error Rate (BER)
m Power Efficiency, ηηp
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, ηηp decreases since transmitted power is “wasted” on more bad data
mBandwidth Efficiency (or Spectral Efficiency), ηηB
lDefined as the ratio of the bit rate to the channel bandwidthw If R is data rate and B is the RF signal bandwidth, then
wThe capacity of a digital system is directly related to ηηB
η BRB BT
M bps Hz= =1
2log /
Page 29Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
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 environmentl 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
ηBCB
SN
bps Hzmax
log /= = +FH IK2 1
Page 30Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
m For example, in wireless communications, it is important to select MODEM based on the following requirementslHigh Spectral EfficiencylHigh Power Efficiency lHigh Fading Immunity
FPractical Modulation SchemesmFM ⇒ AMPSmMSK ⇒ CT2
mGMSK ⇒ GSM, DCS 1800, CT3, DECTmQPSK ⇒ NADC (CDMA) - base transmittermOQPSK ⇒ NADC (CDMA) - mobile transmitter
m 4-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
Page 31Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Comparisons of Digital MODEM
m For practical application, the choice of digital MODEM depends on:l bandwidth efficiency
l power efficiency
l error performance
l Complexity of implementation, and Cost
mProbability of symbol error or Probability of bit error is related to:l Power efficiencyl Bandwidth efficiency (spectral efficiency)
mUsually transmitted power and complexity increases with increase in bandwidth efficiency
mThe linear or nonlinear nature of the channel also affect the choice of digital MODEM
mLastly, but not the least, government regulations also affect the choice of digital MODEM
Page 32Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
Error Performance Comparison
Modulation Type PM (coherent) Pb (coherent) Pb (noncoherent)m Baseband Systemsl Unipolar
l Polar
l Bipolar
m Bandpass Systemsl BASK (OOK)
l BFSK
l BPSK
l QPSK
l OQPSK
l DPSK
Q EsNo
e j
Q EbN
2
0e j
Q EbN
2
0e j
12
28exp − A
Noe j
12 2exp −
EbNo
e j
Q EbN0
e j
32
0Q Eb
Ne j
Q EbN0
e j
Q EbN0
e jQ Eb
N0e j
2 2
0Q Es
Ne j
12 exp −
EbNo
e j≈ 2 2
0Q Es
N Msin πe j
Requires coherent detection
Q EbN
2
0e j Requires coherent detection
Q EsN0
e j
Q EsNo
2e j
Not used in practice
22
1 20 0
QE
N Q EN
b bFH
IK − F
HIK
LNM
OQP
Page 33Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
mError Performance of BPSK/QPSK:
P QE
NQ
A TNb
b
o
b
o
= FHG
IKJ ≈
FHG
IKJ
22
2
2
P QE
Nerfc
ENe
b
o
b
o
= FHG
IKJ = F
HGIKJ2
2 12
Bit Error RateSymbol Error Rate
Page 34Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
mError Performance of BPSK/QPSK/DPSK/DQPSK/MQAM:
P M QME
EsNo
( ) sin≅ FH IK2 2 πBit/Symbol Error Rate
Symbol Error Rate
Page 35Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
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 ηB(bits/s/Hz)
Min Channel B for ISI free signaling
Required Eb/No
Modulation Scheme
Pb = 10-6
Null-to-Null
2/3
1.0
1.0
0.5
Bandwidth Efficiency, ηB
d (complex)A (best)N/A9.6 dBMSK
cB2.09.6 dBOQPSK
aC2.09.6 dBQPSK
a (simple)D (worst)1.09.6 dBBPSK
ImplementationComplexity
Immunity to Nonlinearity
Nyquist
Eb/No
(dB)Modulation
Scheme
Pb = 10-5
Page 36Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
mComplexity
Complexity High
APKM-ary PSK
QPR
CPFSK - optimal detectionMSK
OQPSKQAM, QPSK
BPSK
Low
OOK - envelope detection
DQPSKDPSK
CPFSK -discriminator detectionFSK - noncoherent detection
Ref: IEEE Communications Magazine “1988?”
Page 37Communication Systems Seminar, Summer 2000
Glenn Research Center University of Akron
Modulation and Demodulation
References
1. O. C. Ugweje, Class Handouts on Communications and Signal Processing, Digital Communications, Wireless Communications, University of Akron, Akron Ohio http://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, 1988
4. J. G. Proakis, Digital Communications, 3rd Edition, McGraw-Hill, 1994.5. J. G. Proakis and Masoud Salehi, Communication Systems Engineering, Prentice-
Hall, 19946. A. Ambardar, Analog and Digital Signal Processing, PWS Publishing Company,
MA, 1995
7. K. Feher, “Digital Communications: Satellite/Earth Station Engineering,”Prentice-Hall, Inc., New Jersey, 1983