Digital Modulation Techniques
Contents
Definitions
Coherent binary modulation schemes
Non- coherent binary modulation schemes
M-ary modulation
Performance measure : Bandwidth, Power spectra,
advantages, disadvantages,
Comparison of binary and quaternary modulation
schemes
Hierarchy of Modulation Schemes
Digital Modulation can be classified as
1) According to receiver type :
- Coherent Technique
- Non- Coherent Technique
2) According to signaling schemes :
- Binary Signaling
- M-ary Signaling
3) Hybrid schemes:
Binary M-ary Hybrid
ASK QPSK QAM
FSK M-ary FSK (QASK)
PSK M-ary PSK
Performance measure characteristics of Modulation
schemes :
1) Probability of Error (Pe)
2) Power Spectra
3) Bandwidth Efficiency
ASK (Amplitude Shift Keying)
VASK(t) = d(t) sin(2πfct)
VASK(t) = sin(2πfct) ……. bit 1
VASK(t) = 0 ……… bit 0
ASK Modulator
ASK Demodulator
ASK Restoration
Bandwidth
BW = 2fb
Geometrical representation (Signal Space Diagram)
- Euclidean distance , d = √Eb
- Probability of Error (Pe)
Merits
Demerits
BPSK
VBPSK(t) = A.cos(ωct) ……………Bit 1
= A.cos(ωct + π)
= -A.cos(ωct) ………….Bit 0
BPSK Modulator
Signal Space Representation : Geometrical
representation
- d = 2√Eb
- Importance of Euclidean distance
- As d increases, Pe decreases
- Bandwidth = 2fb
QPSK
Need
Concept
Advantages
- Reliable performance
- Reduces Pe
- Efficient utilisation of BW
QPSK Modulator
Dibit Formation
QPSK Transmitter
QPSK Phasor Diagram
Symbol and corresponding
phase shifts
QPSK Demodulator
Bandwidth of QPSK Signal
BW of QPSK = 2 fb/N = 2 fb /2 = fb
Advantages:
- For the same bit error BW required will be half of
BPSK
- Good Noise Immunity
- Possible to increase bit rate without increasing BW.
M-ary Modulation
Review of Binary and Quad Modulation Schemes
Concept: Send one of the M possible signals, during
each signaling interval of T duration.
No. of possible signals, M = 2^N
Types
1. M-ary PSK
2. M-ary QAM
3. M-ary FSK
1.M-ary PSK
M-ary PSK signal generation-
1. Group N-bits together
2. N-bit symbols are extended over period of NTb.
3. Possible symbols, M.
4. M symbols are represented by sinusoids of
duration 2
M
Ts=N Tb, which is differing in phase by
Mathematical expression of M-ary PSK :
Geometrical Representation
(Signal Space Diagram)
Euclidean Distance
Distance Between Signal Points
- Euclidean Distance , d = 2√Es sin (π / M)
- Substitute different values of M
As d increases error probability decreases.
8-PSK System
Here M=8, therefore N=3
Uses 8 different phase shifts to transmit 8 symbols.
Baud Rate = (1/3) Bit Rate
Bandwidth of M- ary PSK
M-ary PSK Transmitter
Advantages-
- As N increases, BW reduces
- Information is transmitted through signal
phase.
Disadvantages-
- Tra. & Rec. is complex
- Pe increases, as N increases
Conserves BW at the cost of increased transmission
power and error probability.
In digital communication system, the bit rate of NRZ
data stream is 1 Mbps and carrier frequency of
transmission is 100 MHz. Find symbol rate of
transmission and BW requirement of channel for
following techniques –
i) BPSK
ii) QPSK
iii) M-ary PSK
Assume you are required to transmit fb = 90 Mbps
data in authorized BW of 20 MHz. Which modulation
technique you will use.
QASK/ QAM
Amplitude + Phase Shift Keying
BPSK, QPSK, M-ary PSK – Same amplitude
Direct modulation of carriers in quadrature
is involved
QAM- Geometrical Representation
In this Signal space representation : Ts = 4 Tb
M = No.of possible symbols = 16
System must generate 16 distinguishable signals
Here. Each signal point is equallydistant from its
neighbouring point , d = 2a
Consider Ist quad.
- Calculate Avg. normalised energy of Ist quad , Es
- d = 2√0.1Es
Probability of error
BPSK : d = 2√Eb
QPSK : d = 2√Eb
M-ary PSK : 16 –ary PSK : d =
QASK : 16 – ary QASK : d = 2√0.4 Eb
QASK : Low error rate compared to 16 ary PSK
: Higher error rate compared to QPSK
Types of QAM
Scheme Bits/symbol Number of symbols
4-QAM 2 4
8-QAM 3 8
16-QAM 4 16
32-QAM 5 32
64-QAM 6 64
QASK Transmitter
Bit stream b(t) is applied to Serial to Parallel conv
(Clk period = Ts sec)
B(t) are stored by conv and represented in parallel
form.
O/p of DACs Carriers
Ae(t)
Ao(t)
BW of QASK System
Comparison of QPSK & QASK
FSK
VFSK(t) = A.cos(ωct + d(t) Ωt)
VFSK(t) = A. cos(ωc + Ω)t -----------Bit 1
= A. cos(ωc – Ω)t-----------Bit 0
FSK Modulator
BFSK Transmitter
FSK Demodulator
Geometrical representation (Constellation Diagram)
=
- d = √2Eb
Consider following signal space diagram.
- Compare it’s performance with std. 8 level PSK
system
- Which system will give lower Pe?
M-ary FSK Transmitter
M-ary FSK System
PSD of M-ary FSK
BW of M-ary FSK
Advantages:
- Improved Noise Immunity
- Pe reduces as M increases
Disadvantages:
- Correctly tuned filters are required for reception
- Large BW requirements
Performance Comparison of
Digital Modulation Techniques
Parameters :
- Information is transmitted though
- Equation of system
- No. of bits/ symbol
- Symbol Rate
- No. of possible symbols (M)
- Euclidean distance
- Noise Immunity
- Bandwidth
- Probability of Error
Prepare a comparison table
