Page 1 Communication Systems Seminar, Summer 2000 Glenn Research Center University of Akron Modulation and Demodulation Communication Systems Seminar Lecture 3 Modulation and Demodulation 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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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
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:
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
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