Nasa Seminar Lecture 3

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




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




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




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




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




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




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




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




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




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)




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




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)




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)




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)




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




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




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




mContinuous Phase FSK:

mDemodulation of FSK:No Phase Discontinuities

1 1 1 100

θ θ0 1=

Coherent Noncoherent

Envelop Detection




mPSD of CPFSK:

Sunde's FSK




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:




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




Quadrature PSK

E

10

01

11

00s0

s1

s2 s3




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

Simultaneous transition of Q and I channels




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




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




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:




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)




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 /




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




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




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




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




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




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

P M QME

EsNo

( ) sin≅ FH IK2 2 πBit/Symbol Error Rate

Symbol Error Rate




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




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?”




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

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Nasa Seminar Lecture 3