Digital modulation basics

1. Digital Modulation Basics
N N Maurya

2. Why Digital Modulation ?
 More information capacity
 Compatibility with digital data services
 Higher data security
 Better quality communications
 Digital modulation schemes have greater
capacity to convey large amounts of
information than analog modulation
schemes

3. Constraints ?
 Available bandwidth
 Permissible power
 Inherent noise level of the system

4. Trading off
 There is a fundamental tradeoff in communication
systems
 This tradeoff exists whether communication is
over air or wire, analog or digital
 Spectrally efficient transmission techniques
require more and more complex hardware

5. Trends in the Industry

6. Block Diagram of Digital Communication
System

7. Classification of Communication
System
 Most communication systems can be classified into
one of three different categories:
– Bandwidth efficient
Ability of system to accommodate data
within a prescribed bandwidth
– Power efficient
Reliable sending of data with minimal power
requirements
– Cost efficient
System needs to be affordable in the context
of its use

8. Types of Digital Modulation System
 COHERENT
 NON- COHERENT
 Coherent (synchronous) detection: process receives
signal with a local carrier of same frequency and
phase
 Non coherent (envelope) detection: requires no
reference wave

9. TYPES OF DIGITAL MODULATION
SYSTEM……
 Coherent detection
 Receiver uses the carrier phase to detect signal
 Cross correlate with replica signals at receiver
 Match within threshold to make decision
 Non coherent detection
 Does not exploit phase reference information
 Less complex receiver, but worse performance

10. Hierarchy of digital modulation
technique

11. Digital modulation techniques
 Amplitude shift keying (ASK)
 Frequency shift keying (FSK)
 Phase shift keying (PSK)
 Quadrature phase shift keying (QPSK)
 Quadrature amplitude modulation (QAM)

12. Metrics for Digital Modulation
 Power Efficiency
 Power efficiency is a measure of how much
signal power should be increased to achieve a
particular BER for a given modulation scheme
 Ability of a modulation technique to preserve
the fidelity of the digital message at low power
levels
 Designer can increase noise immunity by
increasing signal power
 Signal energy per bit / noise power spectral
density: Eb / N0

13. Metrics for Digital Modulation…
 Bandwidth Efficiency
 Ability to accommodate data within a limited
bandwidth
 Tradeoff between data rate and pulse width
 Data rate per hertz: R/B bps per Hz
 Shannon Limit: Channel capacity /
bandwidth
 C = B log2(1 + S/N) OR
 C/B = log2(1 + S/N)

14. Considerations in Choice of
Modulation Scheme
 High spectral efficiency
 High power efficiency
 Robust to multipath effects
 Low cost and ease of implementation
 Low carrier-to-cochannel interference
ratio
 Low out-of-band radiation
 Constant or near constant envelope
 Constant: only phase is modulated
 Non-constant: phase and amplitude modulated

15. Amplitude Shift Keying
Digital
information
1 0 1 1 0 0 1 0 1 0
Carrier wave
ASK
modulated
signal
Carrier present Carrier absent
Amplitude varying-
frequency constant

16. ASK Generation
Lower Side band Upper Side band
Band width=2 X Modulating freq.
00
1)2cos(
)(
tfA
ts cc

17. 1 1 1 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 1 0 1 1 1
t
t
tf
tASK
tf tASK
tA cc sin
n
nn awhere
T
nTt
rectatf
0
1
tAtft ccASK sin
0
sin tA
t cc
ASK
(logic 1)
(logic 0)
ASK Generation…

18. ASK Detection

19. Necessity to shape the pulse
A pulse contains infinite number of harmonics and hence bandwidth

20. Frequency shift keying
Digital
information
1 0 1 1 0 0 1
Carrier 1
(frequency #1)
FSK
modulated
signal
Carrier 2
(frequency #2)
Frequency varying-
amplitude constant

21. FSK Generation
0)2cos(
1)2cos(
)(
2
1
btfA
btfA
ts
c
c

22. FSK Detection

23. Minimum Shift Keying
 When the frequency of the
separation becomes lowest it is known
as minimum shift keying (MSK)

24. Phase Shift Keying

25. PSK Generation
1800 shift
0)2cos(
1)2cos(
)(
btfA
btfA
ts
cc
cc

26. PSK Generation…
n
nn awhere
T
nTt
rectatf
0
1
tAtft ccPSK cos
tf tPSK
tA cc cos
1 1 1 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 1 0 1 1 1
t
t
tf
tPSK
~
~
tf02cos
tf02cos

27. Signal Vector Representation
Phase
S
0 degrees
I
Q
I-Q
Plane
s(t) = Ac(t) cos (2 fct + θ(t))
fixed!!!
t =
0
t = t
θ =
90
θ = 0

28. S1
S2
I
Q
Magnitude
Change
S1
S2
I
Q
Phase
Change
S1
S2
I
Q
Magnitude
& Phase
Changes
I-Q Diagrams
or
Constellations
Signal Changes: Representation
in the I-Q plane

29. Constellation

30. QPSK
 The only way to achieve high data rates with a
narrowband channel is to increase the number of
bits/symbol
 The most reliable way to do this is with a combination
of amplitude and phase modulation called quadrature
amplitude modulation (QAM)
 Quadrature Phase Shift Keying is effectively two
independent BPSK
 QPSK systems (I and Q), exhibits the same
performance but twice the bandwidth efficiency of
that of BPSK.
 Large envelope variations occur during phase
transitions, thus requiring linear amplification.

31. QPSK constellation
Basic QPSK constellation QPSK constellation Shifted by 450

32. QPSK……
Carrier phase is changed by 450 ,1350, 2250, 3150
00 10 11 10 01

33. QPSK Generation

34. QPSK Detection

35. Types of QPSK
 Conventional QPSK has transitions through zero
(i.e. 180 phase transition). Highly linear amplifier
required.
 In Offset QPSK, the transitions on the I and Q
channels are staggered.
 Phase transitions are therefore limited to
90degrees.
 In /4-QPSK the set of constellation points are
toggled each symbol, so transitions through zero
cannot occur. This scheme produces the lowest
envelope variations.
 All QPSK schemes require linear power amplifiers.

36. Offset QPSK

37. /4 QPSK

38. Multi-level (M-ary) Phase and
Amplitude Modulation
 Amplitude and phase shift keying can be combined
to transmit several bits per symbol
 These modulation schemes are often referred to
as linear, as they require linear amplification
 Amplitude modulation on both quadrature carriers
 2^n discrete levels, n = 2 same as QPSK
 16-QAM has the largest distance between points,
but requires very linear amplification. 16-PSK has
less stringent linearity requirements, but has less
spacing between constellation points, and is
therefore more affected by noise
 M-ary schemes are more bandwidth efficient, but
more susceptible to noise.

39. 16-QAM

40. 16-QAM Generation

41. 16-QAM Detection

42. 16QAM with different impairments
AWGN
Loss
of
Sync
Interference
Phase
Noise

43. Application

44. Bandwidth efficiency limits

45. Thank You!

46. Actual example
 Here is a 16-level constellation which is
reconstructed in the presence of noise
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Eb/No=5 dB

47. Defining decision regions
 An easy detection method, is to compute “decision
regions” offline. Here are a few examples
decide s1
decide s2
s1s2
measurement
decide s1
decide s2
decide s3 decide s4
s1s2
s3 s4
decide s1
s1

48. On-off keying (OOK)
 Simplest/oldest form of modulation
 Morse code (1837) – developed for telegraphy

49. Eye diagrams