Digital Modulation Techniques and their comparison is discussed in this PPT

1. RMK COLLEGE OF ENGINEERING & TECHNOLOGY
RSM NAGAR , PUDUVOYAL-601206
EC6501 DIGITAL
COMMUNICATION
DEPARTMENT OF ELECTRONICS AND

2. UNIT I SAMPLING & QUANTIZATION
Low pass sampling – Aliasing- Signal Reconstruction-Quantization -
Uniform & non-uniform quantization - quantization noise - Logarithmic
Companding of speech signal- PCM - TDM
UNIT II WAVEFORM CODING
Prediction filtering and DPCM - Delta Modulation - ADPCM & ADM
principles-Linear Predictive Coding
UNIT III BASEBAND TRANSMISSION
Properties of Line codes- Power Spectral Density of Unipolar / Polar RZ
& NRZ – Bipolar NRZ - Manchester- ISI – Nyquist criterion for
distortionless transmission – Pulse shaping – Correlative coding - Mary
schemes – Eye pattern – Equalization
UNIT IV DIGITAL MODULATION SCHEME
Geometric Representation of signals - Generation, detection, PSD &
BER of Coherent BPSK, BFSK & QPSK - QAM - Carrier Synchronization
- structure of Non-coherent Receivers - Principle of DPSK.
UNIT V ERROR CONTROL CODING
Channel coding theorem - Linear Block codes - Hamming codes - Cyclic
EC6501 DIGITAL COMMUNICATION
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UNIT IV DIGITAL MODULATION SCHEME
Geometric Representation of signals -
Generation, detection, PSD & BER of Coherent
BPSK, BFSK & QPSK - QAM - Carrier
Synchronization - structure of Non-coherent
Receivers - Principle of DPSK.
Department of ECE

4. Course Outcomes
Highest
Cognitive Level
C301.1
Describe the concepts of sampling and
quantization
K2
C301.2
Compare the various source coding
techniques
K2
C301.3
Describe the baseband transmission
schemes
K2
C301.4
Illustrate the different modulation schemes
and equalization techniques
K2
C301.5
Examine the PSD and BER of various
modulation schemes
K3
C301.6 Generate different error control codes K3
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DIGITAL MODULATION -
INTRODUCTION In baseband pulse transmission, the input data is represented
in the form of a discrete PAM signals (line codes). These
signals are transmitted over a low pass channel.
 The baseband signals have an adequately large power at low
frequencies. So they can be transmitted over a pair of wires
or coaxial cables.
 But it is not possible to transmit the baseband signals over
radio links or satellites because impracticably large
antennas would be required to be used.
 Hence, the spectrum of the message signal has to be shifted
to higher frequencies. This is achieved by using the baseband
digital signal to modulate a sinusoidal carrier.
 This is called digital carrier modulation or digital passband
communication.

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DIGITAL MODULATION -
INTRODUCTION
 The data may represent digital computer outputs or PCM
waves generated by digitizing voice or video channels.

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DIGITAL MODULATION -
INTRODUCTION
GSM-900 uses 890 - 915
MHz to send information from
the Mobile Station to the Base
Transceiver Station (uplink)
and 935 - 960 MHz for the other
direction (downlink), providing
124 RF channels (channel
numbers 1 to 124) spaced at 200
kHz. Duplex spacing of 45
MHz is used

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DIGITAL MODULATION -
INTRODUCTIONTo study
 Digital modulation techniques
 Noise performance
 Spectral properties
 Merits
 Limitations
 Applications
 Issues
For the noise analysis, we use signal space approach

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DIGITAL MODULATION -
INTRODUCTIONTwo categories of digital modulation techniques
Coherent Technique - Technique that employs coherent
detection. In coherent detection, the local oscillator generated at
the receiver is phase locked with the carrier at the transmitter.
Thus, the detection is done by correlating received noisy signal
and locally generated carrier. It is a synchronous detection.
Non-coherent Technique – Technique in which the detection
process does not need receiver carrier to be phase locked with
transmitter carrier.
Advantage of Non-coherent Technique - system becomes
simple.
Disadvantage of Non-coherent Technique – error probability
increases (performance is inferior)

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DIGITAL MODULATION
FORMATSTwo types of digital modulation schemes
1. Binary schemes
2. M-ary schemes
 In binary schemes, we send any one of the two possible
signals during each signaling interval of duration Tb.
Examples of binary schemes : ASK, FSK and PSK
 In M-ary systems, we can send any one of the M-possible
signals during each signaling interval Tb. Examples of M-
ary schemes : M-ary PSK, M-ary FSK, QPSK, MSK,
QASK or QAM etc.
 M-ary schemes need less bandwidth as compared to the
binary schemes. But, the error performance of M-ary
schemes is poor as compared to the binary schemes.

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DIGITAL MODULATION -
INTRODUCTION
In transmission and reception of digital carrier signals, we have
to use modulator at the transmitter and a demodulator at the
receiver. Usually both these devices are packed in one unit
called as MODEM for two way communication.

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DIGITAL MODULATION
FORMATS
 In digital communications, the modulating wave consists
of binary data or an M-ary encoded version of it.
 For the carrier, we use sinusoidal wave
 With a sinusoidal carrier, the feature that is used by the
modulator to distinguish one signal from another is a
step change in the amplitude, frequency, or phase of the
carrier.
 The result of this modulation process is amplitude-shift
keying , frequency-shift keying , or phase-shift keying
respectively.
 PSK and FSK signals are much more widely used than
ASK signals

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DIGITAL MODULATION –
USEFUL TERMS Probability of Error (Pe) – The most important goal of
passband data transmission system is to design the receiver
having minimum value of average probability of error in
presence of additive white Gaussian noise (AGWN). The
value of Pe should be as small as possible.
 Power Spectra – It is a graph of power spectral density
plotted on Y axis versus frequency on X axis. It gives us
information about the bandwidth requirement and co-
channel interference.
 Bandwidth efficiency – Ratio of the data rate (bits/sec) to
the effectively utilized channel bandwidth. It is denoted by
 The communication system should be spectrally efficient.
𝝆 =
𝑹 𝒃
𝑩
𝒃𝒊𝒕𝒔/𝑯𝒛

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PASSBAND TRANSMISSION
MODEL

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ASK/ON-OFF KEYING
MODULATIONAmplitude Shift Keying (ASK) is a form of digital bandpass
modulation technique in which the amplitude of the analog
sinusoidal carrier signal is varied to represent the input
binary data. The frequency and phase of the carrier signal
remains unchanged.
Consider the digital baseband signal (modulating signal)
represented by unipolar NRZ, i.e vm(t)=1 for binary “1” and
vm(t)=0 for binary symbol “0” for entire bit duration Tb, in
the form of on-off signal.

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18. GRAM SCHMIDT ORTHOGONALIZATION
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Orthonormal - both
orthogonal and normalized

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ASK, PSK and FSK

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DESIGN GOALS

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BINARY PSK (BPSK)
 PSK is a form of digital modulation technique in
which the phase angle of the analog sinusoidal
carrier signal “fc” is varied to represent the input
digital data.
 However, the amplitude and frequency of the
modulated signal remains constant.
 In binary PSK (BPSK), the phase of the sinusoidal
carrier signal is changed by 0o or 180o (π radians)
corresponding to two different voltage levels of
binary modulating signals (1 and 0).
 This is the reason that BPSK is called bi-phase
modulation or phase reversal keying.

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COHERENT BINARY PSK
 In coherent binary PSK system, the pair of signals,
s1(t) and s2(t), used to represent binary symbols 1
and 0 respectively, are defined by
E=Pt or P=E/t

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COHERENT BINARY PSK

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* The maximum likelihood decision rule is simply to choose the
message point closest to the received signal point
*
COHERENT
BINARY PSK

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COHERENT BINARY PSK

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GEOMETRIC INTERPRETATION OF
SIGNALS
(STATE SPACE DIAGRAM/CONSTELLATION DIAGRAM)
A signal constellation diagram refers to a set of possible message
points

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The separation between two message
points is called Euclidean distance
and as it increases, the isolation
between the symbols in BPSK signal
is more and probability of error
decreases.
COHERENT
BINARY PSK

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COHERENT BINARY PSK

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Pe will decrease as Eb increases
COHERENT BINARY PSK
Noise power spectral density (N0)

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Salient Features of BPSK
 BPSK has a very good noise immunity
 BPSK has a bandwidth which is lower than that of a BFSK
 BPSK has the best performance of all the three digital
modulation techniques in the presence of noise
 It yields the minimum value of probability of error
COHERENT BINARY PSK

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COHERENT BINARY PSK

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COHERENT BINARY FSK(BFSK)
Binary Frequency Shift Keying (BFSK) is a form of digital
modulation technique in which the frequency of the analog
sinusoidal carrier signal “fc” is varied to represent the input digital
data. However the amplitude of the modulated carrier signal
remains constant.

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COHERENT BINARY FSK(BFSK)

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COHERENT BINARY FSK(BFSK)

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COHERENT BINARY FSK(BFSK)
Refer slide 18

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The two message points are defined by the two signal vectors
COHERENT BINARY FSK(BFSK)
S2
S2

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x1 & x2 are elements
of observation vector

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VVI

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COHERENT QUADRATURE-
MODULATION TECHNIQUES
IMPORTANT GOAL IN THE DESIGN OF DIGITAL
COMMUNICATION SYSTEM
 Provision of reliable performance (very low
probability of error)
 Efficient utilization of channel bandwidth
(bandwidth conserving modulation schemes
for the transmission of binary data)

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COHERENT QUADRATURE-
MODULATION TECHNIQUES
Efficient utilization of channel bandwidth
with
Quadrature carrier multiplexing system
1. Quadrature Carrier Signaling Technique
/Quadriphase Shift Keying (QPSK) –
An extension of BPSK
2. Minimum Shift Keying (MSK) –
An extension of BFSK

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COHERENT QUADRATURE-
MODULATION TECHNIQUES
Quadrature carrier multiplexing system
This produces a modulated wave described as follows:
sI(t) - In-phase component of the modulated wave
sQ(t) - Quadrature component of the modulated wave
𝑠 𝑡 = 𝑠𝐼 𝑡 𝑐𝑜𝑠 2𝜋𝑓𝑐 𝑡 − 𝑠 𝑄 𝑡 𝑠𝑖𝑛(2𝜋𝑓𝑐 𝑡)

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QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRIPHASE SHIFT KEYING
(QPSK)

55. DRIPHASE SHIFT KEYING (QPSK)

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QPSK is characterized by having a two-dimensional
constellation (N=2) and four message points (M=4)
QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRIPHASE SHIFT KEYING
(QPSK)Decision Rule
To realize the decision rule for the detection of the
transmitted data sequence, we partition the signal space
into four region as follows:
 The set of points closest to the message point
associated with signal vector s1
 The set of points closest to the message point
associated with signal vector s2
 The set of points closest to the message point
associated with signal vector s3
 The set of points closest to the message point
associated with signal vector s4

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QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRIPHASE SHIFT KEYING
(QPSK)RECEIVED SIGNAL

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QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRIPHASE SHIFT KEYING
(QPSK)
Probability of a
correct decision is
Pc+Pe=1

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QUADRIPHASE SHIFT KEYING
(QPSK)
𝐸 = 2𝐸𝑏

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QUADRIPHASE SHIFT KEYING
(QPSK)
COMPARISON OF PROBABILITY OF ERROR OF
BPSK, BFSK AND QPSK

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QUADRIPHASE SHIFT KEYING
(QPSK)
GENERATION AND DEMODULATION OF QPSK

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QUADRIPHASE SHIFT KEYING
(QPSK)
GENERATION AND DEMODULATION OF QPSK

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QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRIPHASE SHIFT KEYING
(QPSK)
GENERATION AND DEMODULATION OF QPSK

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QUADRIPHASE SHIFT KEYING
(QPSK)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)

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QUADRATURE AMPLITUDE
MODULATION (QAM)