The document discusses various forms of digital modulation techniques including amplitude shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM). It provides details on the generation and detection of binary amplitude shift keying (BASK), binary frequency shift keying (BFSK), and binary phase shift keying (BPSK). The document also discusses differential phase shift keying (DPSK) and quadrature phase shift keying (QPSK), including their principles of operation and advantages over other modulation techniques.

1. 1
Digital Modulation
Various Forms:
A. Amplitude Shift Keying (ASK) / On-Off Keying (OOK): BASK, MASK
B. Frequency Shift Keying (FSK): BFSK, MSK, MFSK
C. Phase Shift Keying (PSK): BPSK, MPSK, DPSK, QPSK, OQPSK
D. Quadrature Amplitude Modulation (QAM) = ASK+PSK: MQAM
Can also be categorized as:
1. Binary modulation: BASK, BFSK, BPSK
2. M-ary modulation: MASK, MFSK, MPSK, MQAM
Digital bit stream is transmitted using analog carrier (bandpass channel) – Digital-to-
analog conversion

2. 2
BASK, BFSK, BPSK
BASK/
OOK
BPSK
BFSK
Bit
sequence

3. 3
BASK / OOK (1)
Generation:
Detection:
- Both envelope detection and coherent detection (same as AM)
For OOK, A0 = 0
OOK
Envelope detector

4. s(t)
4
BASK / OOK (2)
Advantage: Simplicity
Disadvantage: ASK is very susceptible to noise interference
Amplitude Spectrum:
OOK

5. 5
BFSK (1)
Advantage: FSK is less susceptible to noise than ASK
Disadvantage: Bandwidth twice that of ASK
Generation:
Amplitude Spectrum:
 
 
 



0for,2cos
1for,2cos
0
1
tfA
tfA
ts
C
C



6. s(t)
s(t)
6
BFSK (2)
Envelope (noncoherent) detection
Detection:
- Both envelope detection and coherent detection
Coherent detection

7. 7
BPSK (1)
Generation:
Amplitude Spectrum:
 
 
   




0for,2cos2cos
1for,2cos
tfAtfA
tfA
ts
CCCC
CC


(Polar NRZ)
 tfA CC 2cos

8. 8
BPSK (2)
Detection:
- Only coherent detection
Advantage:
 Less susceptible to errors than ASK requiring the same bandwidth
 Less bandwidth requirement than that of FSK
Disadvantage:
More complex signal detection / recovery process than in ASK and FSK
 tfA CC 2cos

9. 9
DPSK (1)
Generation:
(For this example: 0 means
transition: Logic network
performs XNOR operation)
Detection by Envelope Detector:
Advantage: Envelope detection can be used
 tfA CC 2cos

10. 10
DPSK (2)
Q. Design a DPSK system (transmitter and receiver) assuming ‘1’ means transition
in phase.

11. 11
QPSK: Principle (1)
 
 
 
 
 










11binary,2/32cos
10binary,2cos
01binary,2/2cos
00binary,2cos




tfA
tfA
tfA
tfA
ts
CC
CC
CC
CC
Bits Phase
00 0
01 π/2
10 π
11 3π/2
 
 
 
 
 











10binary,4/72cos
00binary,4/52cos
10binary,4/32cos
11binary,4/2cos




tfA
tfA
tfA
tfA
ts
CC
CC
CC
CC
Bits Phase
11 π/4
01 3π/4
00 5π/4
10 7π/4
11
00
10
01
 QPSK uses phase shifts of 90o ⇒ 4 different signals, each
representing 2 bits
That is 2 bits are mapped onto one signal element (i.e., symbol)
Scheme 1:
Scheme 2:
Constellation diagram

12. 12
QPSK : Principle
 In QPSK, every two
incoming bits are split up
into two streams and each
stream generates own PSK
signal by modulating own
carrier frequency
 Phase difference between
the two carriers is 90o (in
quadrature)
 The two PSK signals are
then added (or subtracted)
to produce one of 4 QPSK
signal elements
 ta2
 ta1
 tcsin
 tccos
5π/4 7π/4 3π/4 π/4
For Scheme 1

13. 13
QPSK Transmitter
 In QPSK, every two incoming bits are split up into two streams and each stream generates
own PSK signal by modulating own carrier frequency
 Phase difference between the two carriers is 90o (in quadrature)
 The two PSK signals are then added to produce one of 4 signal elements, i.e., QPSK signal
 tfA CC 2cos
 tfA CC 2sin

For Scheme 1

14. 14
QPSK Receiver: Coherent Detection
 tfA CC 2cos
 tfA CC 2sin

15. 15
QPSK
Advantage:
Higher data rate than in PSK (2 bits per symbol interval), while bandwidth occupancy
remains the same
• Drawback:
Higher rate PSK schemes are limited by the ability of equipment to distinguish small
differences in phase

16. 16
Noise in Communications (1)
Eb = Energy per bit
N0 = Noise spectral density

17. 17
Noise in Communications (2)
Acceptable BER:
Vocoded speech: 10-2 - 10-3
Data transmission over wireless
channels: 10-5 - 10-6
Video transmission: 10-7 - 10-12
Financial data: 10-11

18. 18
Table of Q-function