Electrical Communications Systems 0909'331'01 Spring 2005

1
Electrical Communications Systems0909.331.01
Spring 2005
Lecture 11aApril 19, 2005

• Shreekanth Mandayam
• ECE Department
• Rowan University
• http//engineering.rowan.edu/shreek/spring05/ecom
  ms/

2
Plan

• Digital Bandpass Communications
• Why digital modulation?
• Digital Modulation Introduction (ASK, FSK, PSK)
• Binary Phase Shift Keying (BPSK)
• Quadrature Phase Shift Keying (QPSK)
• M-ary PSK and M-ary QAM

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ECOMMS Topics
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Digital Communications Transceiver
Anti- aliasing Filter
Data Encryption Encoder
Error Control Encoder
Channel/ Line Encoder
Source Encoder
Sampling
Quantization
Modulator
MUX
ADC
Analog i/p
CODEC
MODEM
Multiple access channel
Analog o/p
Data Encryption Decoder
Error Control Decoder
Audio Amp
Source Decoder
Reconstruction/ DAC
Equalization / Decision Circuits
Demod-ulator
DEMUX
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Digital Bandpass Communications

• Why digital modulation?

Reproduced from http//www.tm.agilent.com/tmo/Not
es/pdf/5965-7160E.pdf
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Industry Trends
Reproduced from http//www.tm.agilent.com/tmo/Not
es/pdf/5965-7160E.pdf
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Applications
Reproduced from http//www.tm.agilent.com/tmo/Not
es/pdf/5965-7160E.pdf
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Digital Modulation
Carrier signal Ac cos (2pfct f) Modulation
m(t) Modulated signal Ac
(t) cos (2pfc(t) t f(t))
? m(t) discrete
Vary Vary Vary amplitude
frequency phase
Variations are discrete!!!!!
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Recall Dig Comm Principle
Digital message
1 1
1 0 1 0
0 0
Digital code
Analog message
modulate
1 0 1 0
Sinusoidal carrier
AM
FM
PM
AM PM
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Signal Vector Representation
s(t) Ac(t) cos (2pfct f(t))
fixed!!!
Q
S
Magnitude
Phase
0 degrees
I
I-Q Plane
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Signal ChangesRepresentation in the IQ Plane
I-Q Diagrams or Constellations
Frequency Change
???
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Binary Phase Shift Keying (BPSK)
s(t) Ac cos(2pfct f(t))
IQ Diagram
Q
p radians
f(t)
1
0
0 radians
t
I
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Quadrature Phase Shift Keying(QPSK)
IQ Diagram
s(t) Ac cos(2pfct f(t))
Q
3p/4
01
11
10
f(t)
p/4
00
p/4
t
I
10
-p/4
11
01
00
-3p/4
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M-ary Phase Shift Keying(M-ary PSK)
Q
I
Octophase I-Q Constellation
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M-ary Quadrature Amplitude Modulation(M-ary-QAM)
Q
I
16 QAM I-Q Constellation
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Different ways of looking at a digitally
modulated signal
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Time Frequency Domain View
Reproduced from http//www.tm.agilent.com/tmo/Not
es/pdf/5965-7160E.pdf
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Frequency Power View
Reproduced from http//www.tm.agilent.com/tmo/Not
es/pdf/5965-7160E.pdf
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I-Q Constellation Diagram
Matlab Demo qpsk.m
Reproduced from http//www.tm.agilent.com/tmo/Not
es/pdf/5965-7160E.pdf
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Eye Diagram
Matlab gtgteyediagram
Reproduced from http//www.tm.agilent.com/tmo/Not
es/pdf/5965-7160E.pdf