4.

Digital Modulation Techniques

BY:
KIRAN KUMAR. G
ASST.PROFESSOR.

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 Outline

Introduction to digital
modulation

Relevant modulation
schemes

Coherent & Non-Coherent

Detection
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 Modulation & Demodulation

Radio
Modulator Detector
Channel

Baseband Baseband
Signal Signal

Binary Data Binary Data

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 Modulation
• Modulation - process of translating the baseband
message signal into bandpass (modulated carrier)
signal at frequencies that are very high compared
to the baseband frequencies.
• Demodulation is the process of extracting the
baseband message signal back at the receiver
output.
• An information-bearing signal are called baseband
signals because they have low frequency content.
• Ex: Human voice range : 20Hz-20kHz

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 Why Carrier?
• Effective radiation of EM waves requires
antenna dimensions comparable with the
wavelength:
– Antenna for 3 kHz would be ~100 km long
– Antenna for 3 GHz carrier is 10 cm long
• Sharing the access to the telecommunication
channel resources.

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 Analog CW Modulation
Carrier: A sin[t +] • Frequency modulation (FM)
– A = const – A = const
–  = const –  = (t)– carries information
–  = const –  = const
• Amplitude modulation (AM) • Phase modulation (PM)
– A = A(t) – carries information – A = const
–  = const –  = const
–  = const –  = (t) – carries information

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 Digital CW Modulation
Carrier: A sin[t +] • Frequency modulation (FM)
– A = const – A = const
–  = const –  = (t)– carries information
–  = const –  = const
• Amplitude modulation (AM) • Phase modulation (PM)
– A = A(t) – carries information – A = const
–  = const –  = const
–  = const –  = (t) – carries information

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 Amplitude Shift Keying (ASK)
Baseband
Data
1 0 0 1 0
ASK
modulated
signal
Acos(t) Acos(t)

• Pulse shaping can be employed to remove spectral spreading

• ASK demonstrates poor performance, as it is heavily affected by noise,
fading, and interference

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 Frequency Shift Keying (FSK)
Baseband
Data
1 0 0 1
BFSK
modulated
signal
f1 f0 f0 f1
where f0 =Acos(c-)t and f1 =Acos(c+)t

• Example: The ITU-T V.21 modem standard uses FSK

• FSK can be expanded to a M-ary scheme, employing multiple
frequencies as different states
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 Phase Shift Keying (PSK)
Baseband
Data
1 0 0 1
BPSK
modulated
signal
s1 s0 s0 s1
where s0 =-Acos(ct) and s1 =Acos(ct)
• Major drawback – rapid amplitude change between symbols due to phase
discontinuity, which requires infinite bandwidth. Binary Phase Shift Keying
(BPSK) demonstrates better performance than ASK and BFSK
• BPSK can be expanded to a M-ary scheme, employing multiple phases and
amplitudes as different states
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 Differential Modulation
• In the transmitter, each symbol is modulated
relative to the previous symbol and modulating
signal, for instance in BPSK
0 = no change,
1 = +1800
• In the receiver, the current symbol is demodulated
using the previous symbol as a reference. The
previous symbol serves as an estimate of the
channel.

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 DPSK
• DPSK = Differential phase-shift keying: In the
transmitter, each symbol is modulated relative to
(a) the phase of the immediately preceding signal
element and (b) the data being transmitted.

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 Pulse Carrier

• Carrier:
A train of identical
pulses regularly
spaced in time

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Pulse-Amplitude Modulation (PAM)
• Modulation in which
the amplitude of pulses
is varied in accordance
with the modulating
signal.
• Used e.g. in telephone
switching equipment
such as a private
branch exchange
(PBX)
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 Pulse-Duration Modulation (PDM)
Modulation in which
the duration of
pulses is varied in
accordance with the
modulating signal.

Deprecated synonyms:
•pulse-length modulation
•pulse-width modulation

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 Pulse-Position Modulation (PPM)
• Modulation in which
the temporal
positions of the
pulses are varied in
accordance with
some characteristic
of the modulating
signal.

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 Demodulation & Detection
• Demodulation
– Is process of removing the carrier signal to
obtain the original signal waveform
• Detection – extracts the symbols from the
waveform
– Coherent detection
– Non-coherent detection

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 Coherent Detection
• Requires a replica carrier wave of the same
frequency and phase at the receiver.
• The received signal and replica carrier are cross-
correlated using information contained in their
amplitudes and phases.
• Also known as synchronous detection

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 Coherent Detection
• Applicable to
– Phase Shift Keying (PSK)
– Frequency Shift Keying (FSK)
– Amplitude Shift Keying (ASK)
• Carrier recovery methods include
– Pilot Tone (such as Transparent Tone in Band)
• Less power in the information bearing signal, High peak-to-
mean power ratio
– Carrier recovery from the information signal
• E.g. Costas loop

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 Non-Coherent Detection
• Requires no reference wave; does not
exploit phase reference information
(envelope detection)
– Differential Phase Shift Keying (DPSK)
– Frequency Shift Keying (FSK)
– Amplitude Shift Keying (ASK)
• Non coherent detection is less complex than
coherent detection (easier to implement),
but has worse performance.
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 Constellation diagram
Graphical representation of the complex
envelope of each possible symbol state:

– The x-axis represents the in-phase component and

the y-axis the quadrature component of the
complex envelope

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 BPSK Constellation Diagram

I
-Eb Eb

Constellation diagram

Eb  energy per bit;

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 QPSK
• Quadrature Phase Shift Keying (QPSK) can
be interpreted as two independent BPSK
systems (one on the I-channel and other on
Q-channel), and has the same performance
but twice the bandwidth efficiency.
• Large envelope variations occur due to
abrupt phase transitions, thus requiring
linear amplification
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 QPSK

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 PSK Vs QPSK
PSK: {, -} QPSK: {/4, 3/4, 5/4, 7/4}

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For Example:

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 QPSK Constellation Diagram
Q Q

I I

Carrier phases Carrier phases

{0, /2, , 3/2} {/4, 3/4, 5/4, 7/4}

Quadrature Phase Shift Keying has twice the bandwidth efficiency of BPSK since
2 bits are transmitted in a single modulation symbol

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 Multi-level (M-ary) Phase and
Amplitude Modulation

16 QAM
16 PSK

• Amplitude and phase shift keying can be combined to transmit several bits per
symbol.
– Often referred to as linear as they require linear amplification.
– More bandwidth-efficient, but more susceptible to noise.

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