# Digital Communication, an overview

```Digital communicationstudent version
Dr. Uri Mahlab
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General overview
and Implementation
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MOD
UPCONVERTER
DEMOD
DOWNCONVERTER
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Analog vs. Digital Modulation
AM
FM
PM
DIGITAL
With digital modulation information is in the phase and
amplitude
signal
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The IQ Diagram
Q
magnitude
Vq
phase
I
Vi
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Analog Modulation on the IQ diagram
Q
FM
A
B
I
C
PM
D
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BPSK Timing and State Diagram
BPSK
Reference
Q
Constellation
Diagram
State 1
State 0
0 State
1 State
I
= 0 deg.
= 180 deg.
t
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QPSK Modulation
4 Possible States
Q
01
00
Vq
I
Vi
10
11
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16 QAM State Diagram
Q
0000
0100
0001
0101
0011
0111
0010
0110
I
1100
1000
1101
1111
1110
1001
1011
1010
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Symbol Rate:
“The rate at which the carrier
moves between points in the
constellation”
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Example:
A 16 QAM radio has 4 bit per state (or symbol).
If the radio operates at 16 Mb/s, then the
carrier must change states
16 Mb/s
4 Bits
or
4 million times per second (4 MBaud)
SYMBOL RATE = 4MHz
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Some Typical Modulation Formats
BPSK
QPSK
16QAM
8PSK
64QAM
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QPSK Modulator
BALANCED
SYMBOL RATE:
MODULATION
fs = fb/2
IN-PHASE DATA STREAM
0&deg;
fb
SERIAL TO
PARALLEL
BINARY CONVERTER
I
CARRIER
PHASE
SHIFT
I.F
COMBINER
BPF
90&deg;
NRZ
INPUT
Q
fs = fb/2
01
00
11
10
SIGNAL
BALANCED
MODULATION
COMBINED VECTOR
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STATE DIAGRAM
I, Q, Eye diagram and Constellation
+1
I
-1
+1
Q
-1
EYE
Q
5
3
CONSTELLATION :
I
2
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1,4
QPSK Demodulator
Phase
Demodulation
LPF.
I
Thresh
Comp.
I
fb/2
IF
Input
BPF
Power
Splitter
Car
Rec.
Phase
Splitter
0&deg;
Symbol
timing
rec.(STR)
90&deg;
Parallel
to serial
converter
fb/2
Q
LPF.
Phase
Demodulation
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Thresh
Comp.
Q
Binary
NRZ
fb
16 QAM Modulator
I
2-to-4
level
convert
fb
2
I
fb
4
Premod.
LPF
0&deg;
L.F.
fb
Binary
Data
16 QAM
Phase
split
LO
BPF
NRZ
Data
fb
2
90&deg;
Q
2-to-4
level
convert
fb
4
Premod.
LPF
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Q
Output
16 QAM Demodulator
I
4-Level
Signal
LPF
fb
4
L
O
G
I
C
Vth1
Vth2
fb
2
0&deg;
Vth3
BPF
STR
CR
IF Input
Regeneration
X2 Data
data Out
fb
con
blner
90&deg;
Q
LPF
fb
2
fb
4
4-Level
Signal
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4-to-2 level converter
of Q channel. Same
design as I channel.
Which waveform requires more bandwidth?
A
B
time
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Bandwidth Considerations
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Two random data sequence
time
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frequency
f 0  3FS f 0  3FS
f 0  2FS f 0  FS
f0
f 0  FS f 0  2FS
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f 0  3FS f 0  4FS
f 0  5FS
frequency
U/C
MOD
time
Data is easier to
recover but signal
requires a lot of
bandwidth
D/C
DEMOD
time
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frequency
MOD
U/C
time
Signal requires less
bandwidth but data
is filtered
D/C
DEMOD
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time
Intersymbol Interference
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Nyquist Filtering
Raised Cosine
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Filter Coefficient &amp; Determines Required B.W.
Amplitude
Response
 1
  0.5
Linear Phase
  0.3
 0
(Flat Group Delay)
FS  Symbol _ Rate
FS
2 ‫ד&quot;ר אורי‬
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FS
The Filtering is Distributed in the Radio
MOD
UPCONVERTER
DEMOD
DOWNCONVERTER
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SUMMARY
As the modulation complexity increases,
the radio becomes more spectrally efficient.
However, it also become more susceptible
to errors caused by noise and distortions.
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P(x)
THRESHOLD
TOTAL PROBABILITY
OF NOISE AMPLITUDE
EXCEEDING THIS
THRESHOLD
0.4
0.3
0.2
0.1
-30
-20
-10
10
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20
30
X
How Error Occur
VOLTAGE
BINARY SIGNAL + AMPLTUDE
NOISE FDP
NORMAL
1 VALUE
1
1 ERROR
DECISION
0
0 ERROR
THRESHOLD
NORMAL
0 VALUE
superimposed noise ‫ד&quot;ר אורי מחלב‬
PROB
Gaussian Distribution
PROBABILITY
DENSITY
FUNCTION
P(x)
0.4
0.3
0=RMS VALUE AFTER
SUBTRACTING
DC COMPONENT
0.2
NEVER RECHS
ZEBO
-30
-20
0.1
-10
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10
20
30
X
Meaning of Eye diagram
Threshold
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```