1. Chapter 3: Digital Modulation and Demodulation

2. Learning Objectives
LO 3.1 – Analyze binary digital modulation techniques such as ASK, FSK and PSK.
LO 3.2 – Describe M-ary digital modulation techniques including QPSK, Offset QPSK, π/4-QPSK, MSK, GMSK, and QAM.
LO 3.3 – Understand coherent and non-coherent including carrier recovery methods.
LO 3.4 – Develop performance-determining criteria in terms of bandwidth efficiency and error probabilities of various digital modulation techniques.

3. 3.1 Binary Digital Modulation Techniques
Amplitude Shift Keying (ASK)
3.1.2 Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Differential Binary Phase Shift Keying (DBPSK)
Differentially Encoded Phase Shift Keying (DEPSK)

4. Types of Digital Modulation Techniques
Digital modulation involves the process of amplitude, frequency, or phase of an analog carrier signal as a function of two or more than two finite, discrete states.
Basic Types
ASK - The amplitude of the carrier signal is varied in proportion to the digital information signal
FSK - The frequency of the carrier signal is varied in proportion to the digital information signal
PSK - The phase angle of the carrier signal is varied in proportion to the digital information signal

5. 3.1.1 Amplitude Shift Keying (ASK)
Binary data
sequence 1
Data Waveform
UP-NRZ(L)
Carrier signal,
sin (2πft)
ASK signal waveform Tb
Figure ASK Signal Waveform

6. ASK Bandwidth and PSD Waveform
Amplitude f fc
fc - fb
fc + fb
BWmax = 2fb
Figure 3.1.2
ASK Signal Bandwidth f
PSD of ASK
fc - fb
fc - 2fb fc
fc + 2fb
fc + fb
Figure 3.1.3
PSD of ASK Signal

7. ASK Modulator Figure 3.1.4 Functional Block Schematic of ASK Modulator
BPF
Balanced Modulator
Carrier signal, sin(2πfct)
Band-limited ASK signal
Binary ASK signal, vASK(t)
Binary digital data (UP-NRZ)
vm(t)
Figure Functional Block Schematic of ASK Modulator

8. Coherent Binary ASK Detection
Balance Modulator & Integrator
Decision Device
Carrier signal
Received binary ASK signal
ASK Demodulated Output
Preset
threshold level
Figure Functional Block Schematic of Coherent ASK Detector

9. Non-Coherent Binary ASK Detection
Envelope Detector (Rectifier + LPF)
Decision Device
Received binary ASK signal
Detected Output
Preset
threshold level
Figure Functional Block Schematic of Non-coherent Binary ASK Detector

10. 3.1.2 Frequency Shift Keying (FSK)
Data Waveform
UP-NRZ (L)
Carrier signal,
sin (2πfct)
BFSK signal waveform Tb f2 f1
Binary data
sequence
Figure Binary FSK Signal Waveform

11. BFSK Bandwidth and PSD Waveform
fc1
fc0
BBFSK = 4fb
PSD of BFSK fc
Figure PSD of a Binary FSK Signal

12. Figure 3.1.9 Functional Block Schematic of FSK Modulator
Polar NRZ Line coder
Binary data
sequence
Carrier Oscillator 2
sin ωc1t
fc1
Balanced modulator M0
Balanced modulator M1
Adder
BFSK signal
Carrier Oscillator 1
sin ωc0t
fc0
Bit Invertor
Figure Functional Block Schematic of FSK Modulator

13. Coherent Binary FSK Detection
BFSK signal
Subtractor
Detected output
Balance modulator M1
Carrier Oscillator 1
cos ω1t +
Balance modulator M2
Carrier Oscillator 2
cos ω2t
Decision Device
Preset
threshold level
Correlator 2
Correlator 1 -
Figure Functional Block Schematic of Coherent Binary FSK Detector

14. Non-coherent Binary FSK Detection
BFSK signal
Comparator
Detected output
Sampling switch 1
(at t = Tb)
Envelope Detector
BPF (fc0)
Sampling switch 2
BPF (fc1)
Figure Functional Block Schematic of Non-coherent Binary FSK Detector

15. FSK Detection using PLL
FSK signal input
Detected output
Amplifier
Phase Comparator
Voltage Controlled Oscillator
dc error voltage dc
PLL
Figure Functional Block Schematic FSK Demodulator using PLL

16. 3.1.3 Phase Shift Keying (PSK)
Binary data 1
Data Waveform
UP-NRZ (L)
Carrier signal,
sin (2πfct)
BFSK signal waveform Tb 0°
180°
Phase shift
Figure Binary PSK Signal Waveform

17. Constellation Diagram of BPSK Singal Phase shift in carrier signal
0° Reference
sin(2πfct)
±180°
Binary level 0
Binary level 1
-sin(2πfct)
cos(2πfct)
-cos(2πfct)
+90°
-90°
Table 3-1
Symbol
Bits in symbol
Phase shift in carrier signal s0
Binary level `0’
180° or π radians S1
Binary level `1’
0° or 0 radians
Figure

18. PSD of a Binary PSK Signal
fc1
fc0
BBFSK = 4fb
PSD of BPSK fc
Figure PSD of a Binary PSK Signal

19. Figure 3.1.16 Functional Block Schematic of a PSK Modulator
Bipolar NRZ Encoder
Binary data sequence
Balance modulator
BPSK signal
Carrier Oscillator
Carrier signal
Figure Functional Block Schematic of a PSK Modulator

20. Coherent Binary PSK Detection
Detected output, d(t)
Synchronous Demodulator (Multiplier)
Recovered Carrier, sin(2πfct)
Decision Device
Correlator
Bit Synchronizer
Sampling instants
Square-law device
Carrier Recovery Circuit
BPF (2fc)
Frequency Divider (/2)
Figure Functional Block Schematic of a Coherent Binary PSK Detector

21. 3.1.4 Differential Binary Phase Shift Keying (DBPSK)
DBPSK is an alternate form of BPSK where the binary data information is contained in the difference between the phases of two successive signaling elements rather than the absolute phase.
Figure Differential Binary PSK Modulator
An Encoder or Logic circuit
Balance Modulator
Carrier Oscillator
Binary data sequence
Delay Tb
Bipolar NRZ line encoder
DBPSK Signal

22. Non-Coherent DBPSK Detection
An Encoder or Logic circuit
DBPSK signal
Delay Tb
BPF (f1)
Decision Device
Preset
threshold level
Detected
output
Balance modulator
Correlator
Figure Functional Block Schematic of Non-Coherent DBPSK Detector

23. Table 3.1.2 Comparison of BPSK and DBPSK
S. No.
Parameter
BPSK
DBPSK 1.
Variable characteristics of analog carrier signal
Phase 2.
Maximum bandwidth
2 fb fb 3.
Probability of error
Low
Higher than BPSK 4.
Noise immunity
Good
Better than BPSK 5.
Bit detection at receiver
Based on single-bit interval
Based on two successive bit intervals 6.
Synchronous carrier at demodulator
Required
Not required 7.
System complexity
Moderate
High

24. 3.1.5 Differentially Encoded Phase Shift Keying (DEPSK)
In DEPSK, the input sequence of binary data is modified such that the next bit depends upon the previous bit.
Figure DEPSK Signal in Time-Domain

25. 3.2 M-ary Digital Modulation Techniques
Quadrature Phase Shift Keying (QPSK)
3.2.2 Offset QPSK (OQPSK)
π/4=Phase Shift Keying QPSK
Minimum Shift Keying (MSK)
Gaussian Minimum Shift Keying (GMSK)
Quadrature Amplitude Modulation (QAM)

26. 3.2.1 Quadrature Phase Shift Keying (QPSK)
QPSK - An M-ary constant-amplitude digital modulation scheme in which number of bits is two and number of signaling elements are four.
SQPSK (t) =

27. Table 3.2.1 Symbols, Bits and Phase Shift Phase shift in QPSK Signal
About QPSK Signal
Table Symbols, Bits and Phase Shift
sin(2πfct)
Symbol 11
-sin(2πfct)
cos(2πfct)
-cos(2πfct)
Symbol 01
Symbol 10
Symbol 00
Symbol
Binary input
Phase shift in QPSK Signal s1
-135° or –3π/4 radians s2
0 1
-45° or –π/4 radians s3
1 0
+135° or 3π/4 radians s4
1 1
+45° or π/4 radians
Figure 3.2.1
Constellation Diagram

28. Figure 3.2.2 PSD of a QPSK Signal
-fc – fb/2
-fc fc
-fc + fb/2
fc – fb/2
fc + fb/2
BQPSK = fb
Figure PSD of a QPSK Signal

29. Figure 3.2.3 Functional Block Schematic of QPSK Modulator

30. QPSK Signal in Time Domain
Figure 3.2.4

31. Coherent QPSK Detection
signal
MUX
Balance modulator, M1
Coherent Quadrature Carrier 1
Correlator 1
Coherent Quadrature Carrier 2
Balance modulator, M2
Correlator 2
Detected
Output
Raised to 4th power
Carrier recovery circuit
BPF (4fc)
Coherent Quadrature carriers
Divide by 4
Power Splitter
Figure Functional Block Schematic of Coherent QPSK Detector

32. Constellation Diagram
Offset QPSK (OQPSK)
OQPSK – A modified form of QPSK where the bit waveform on the I an Q channels are offset in phase from each other by one-half of a bit interval.
Symbol 11
Symbol 01
Symbol 10
Symbol 00
Figure 3.2.6
Constellation Diagram

33. OQPSK Modulator
Binary NRZ encoder and 2-bit
serial-to-parallel converter
Binary input data stream
fb bps
vOQPSK (t)
fb /2 bps
π/2 Σ
Delay Tb
I(t)
Q(t)
Carrier Signal
cos 2πfct +
Balanced Modulator 1
Balanced Modulator 2
Figure Functional Block Schematic of OQPSK Modulator

34. OQPSK Signal in Time Domain
Figure 3.2.8

35. Constellation Diagram
π/4-Phase Shift QPSK
π/4-QPSK – A modified form of QPSK which has carrier phase transitions that are restricted to ±π/4 and ±3π/4 radians. Q I
(11)
(01)
(10)
(00)
Figure 3.2.9
Constellation Diagram

36. π/4-QPSK Signal in Time Domain
Figure

37. Table 3.2.2 Comparison of QPSK, OQPSK, π/4-QPSK
S. No.
Parameter
QPSK
OQPSK
π/4-QPSK 1.
Maximum phase change
±180°
±90°
±135° 2.
Amplitude variations at the instants of abrupt phase changes
large
small
Medium 3.
Simultaneous change of inphase and Quadrature phase
Yes No 4.
Offset between inphase and Quadrature phase
Yes, by Tb seconds
Yes, by Tb/2 seconds 5.
Preferred method of demodulation
Coherent
Coherent or non-coherent 6.
Minimum bandwidth fb 7.
Symbol duration
2 Tb 8.
Receiver design complexity
No in case of non-coherent

38. 3.2.4 Minimum Shift Keying (MSK)
MSK - A special case of binary continuous phase FSK modulation technique in which the change in carrier frequency from symbol 0 to symbol 1 or vice versa is exactly equal to one-half the bit rate of input data signal.
vMSK (t) =

39. About MSK Signal Figure 3.2.12 Figure 3.2.13 MSK Signal at 1200 baud
PSD
-fc – 3fb/4
-fc fc
-fc + 3fb/4
fc – 3fb/4
fc + 3fb/4
BQPSK = 3fb/2
Figure
MSK Signal at 1200 baud
for NRZ data
Figure
PSD of MSK Signal

40. 3.2.5 Gaussian Minimum Shift Keying (GMSK)
GMSK - A special case of MSK in which a pre-modulation low-pass Gaussian filter is used as a pulse shaping filter to reduce the bandwidth of the baseband signal before it is applied to MSK modulator.
Bipolar NRZ Encoder
GMSK signal
Gaussian Filter
Frequency Modulator
Carrier Oscillator cos wct
Figure
GMSK Modulator using Frequency Modulator

41. GMSK Modulator using Phase Modulator
NRZ data
In-phase path
GMSK signal
Gaussian Filter
converter
Parallel To
Serial
Balance modulator, M1
Carrier Oscillator cos wct
Adder + -
Quadrature path
Carrier Oscillator
sin wct
Figure GMSK Modulator using Phase Modulator

42. GMSK Demodulator Figure 3.2.18 GMSK Demodulator In-phase signal, I(t)
GMSK signal
Balance modulator, M1
Carrier Oscillator cos wct
Gaussian Filter
Decision Device
Quadrature-phase signal, Q(t)
Balance modulator, M2
Carrier Oscillator sin wct
Threshold level for binary 0
Threshold level for binary 1
Figure GMSK Demodulator

43. 3.2.6 Quadrature Amplitude Modulation (QAM)
QAM - A form of digital modulation similar to PSK except the digital information is contained in both the amplitude and the phase of the modulated signal
QAM is an efficient way to achieve high data rates with a narrowband channel by increasing the number of bits per symbol, and uses a combination of amplitude and phase modulation.
QAM can either be considered a logical extension of QPSK or a combination of ASK and PSK.

44. QAM Modulator Figure 3.2.21 QAM Modulator vQAM (t) Σ Modulator
2-bit
serial-to-parallel converter
Binary input data stream
fb bps
vQAM (t)
fb /2 bps Σ
d1(t)
d2(t)
Carrier Signal
cos 2πfct
-π/2
sin 2πfct + Ts
D/A converter
Modulator
Figure QAM Modulator

45. QAM Coherent Demodulator
QAM signal
Balance modulator, M1
Correlator 1
Balance modulator, M2
Correlator 2
Detected
Output
Raised to 4th power
Carrier recovery circuit
BPF (4fc)
Divide by 4
Parallel To
Serial
converter A
90° phase shifter
A/D
A/D
Figure QAM Coherent Demodulator

46. 3.3 Coherent and Non-Coherent Detection
Coherent Detection - A synchronous detection in which the digital receiver is phase-locked to the carrier signal of the incoming digitally modulated signal.
Non-Coherent Detection - A non-synchronous detection in which the digital receiver does not require locally-generated receiver carrier signal to be phase-locked with transmitter carrier signal.

47. Coherent Detection Method 1
M-ary PSK signal
Multiplier
Phase-locked loop (PLL)
LPF
VCO
Mth power device
Frequency Divider by M
Phase-shift Network
* * * * *
M-reference output signals
BPF
Figure Mth Power Loop Carrier Recovery

48. Coherent Detection Method 2
BPSK signal
Phase discriminator
Balance modulator, M1
negative
feedback
In-phase path
Balance modulator, M2
Quadrature path
Demodulated
Binary data
LPF
90° phase shifter
VCO + Loop filter
Figure Costas-Loop Carrier Synchronization

49. 3.4 Performance of Digital Modulation Techniques
Bandwidth Efficiency – Number of bits that can be transmitted for each Hz of channel bandwidth, expressed in bps/Hz.
Bit Error Rate (BER) – Ratio of total number of bits received in error and total number of bits received over a large session of information transmission.

50. BER Performance Comparison
10-2
10-3
10-4
10-5 -5
-2.5
2.5
5.0
7.5 10
Eb/N (dB)
BER
10-1
0.5
Non coherent binary FSK
Coherent binary FSK
DPSK
Coherent binary PSK
Coherent QPSK
Coherent MSK
12.5
Figure BER versus Eb/NO curves for BFSK, M-ary PSK, and MSK

51. Table 3.4.6 Performance Comparison of PSK and MSK
Digital Modulation Technique
Spectral Efficiency
Required SNR
BPSK
1 bps/Hz
11.1 dB
QPSK
2 bps/Hz
14.0 dB
16-PSK
4 bps/Hz
26.0 dB
2-MSK
10.6 dB
4-MSK
13.8 dB

52. Table 3.4.7 Spectral Efficiency of QPSK and GMSK
Digital Modulation Technique
Channel Bandwidth, KHz
Data Rate, Kbps
Spectral Efficiency, bps/Hz
Application
π/4-QPSK 30
48.6
1.62
USDC 25
42.0
1.68
JDC
GMSK
(BTb =0.3)
200
270.8
1.35
GSM
100
72.0
0.72
CT-2
(BTb =0.5)
1728
1572.0
0.67
DECT

53. Table 3.4.8 Applications of Digital Modulation Techniques
S. No.
Digital Modulation Technique
Typical Application Areas 1.
Frequency Shift Keying (FSK)
Paging Services, Cordless Telephony 2.
Binary Phase Shift Keying (BPSK)
Telemetry 3.
Quaternary Phase Shift Keying (QPSK)
Cellular Telephony, Satellite, Digital Video Broadcasting 4.
Octal Phase Shift Keying (8-PSK)
Satellite Communications 5.
16- or 32-level Quadrature Amplitude Modulation (16-QAM or 32-QAM)
Microwave Digital Radio Links, Digital Video Broadcasting 6.
64-level Quadrature Amplitude Modulation (64-QAM)
Digital Video Broadcasting, Set Top Boxes, MMDS 7.
Minimum Shift Keying (MSK)
Cellular Telephony

54. About the Author
T. L. Singal graduated from National Institute of Technology, Kurukshetra and post-graduated from Punjab Technical university in Electronics & Communication Engineering. He began his career with Avionics Design Bureau, HAL, Hyderabad in 1981 and worked on Radar Communication Systems. Then he led R&D group in a Telecom company and successfully developed Multi- Access VHF Wireless Communication Systems. He visited Germany during
He executed international assignment as Senior Network Consultant with Flextronics Network Services, Texas, USA during He was associated with Nokia, AT&T, Cingular Wireless and Nortel Networks, for optimization of 2G/3G Cellular Networks in USA. Since 2003, he is in teaching profession in reputed engineering colleges in India. He has number of technical research papers published in the IEEE Proceedings, Journals, and International/National Conferences. He has authored three text-books `Wireless Communications (2010)’, `Analog & Digital Communications (2012)’, and `Digital Communication (2015)’ with internationally renowned publisher McGraw-Hill Education.

55. THANKS!