Digital communicationstudent version
Dr. Uri Mahlab
‫ד"ר אורי מחלב‬
General overview
Digital Radio Theory
and Implementation
-How a Digital Radio Works
‫ד"ר אורי מחלב‬
Digital Radio Block Diagram
MOD
UPCONVERTER
DEMOD
DOWNCONVERTER
‫ד"ר אורי מחלב‬
Analog vs. Digital Modulation
AM
FM
PM
DIGITAL
With digital modulation information is in the phase and
amplitude
signal
‫ מחלב‬of
‫ר אורי‬the
"‫ד‬
The IQ Diagram
Q
magnitude
Vq
phase
I
Vi
‫ד"ר אורי מחלב‬
Analog Modulation on the IQ diagram
Q
FM
A
B
I
C
PM
D
‫ד"ר אורי מחלב‬
BPSK Timing and State Diagram
BPSK
Reference
Q
Constellation
Diagram
State 1
State 0
0 State
1 State
I
= 0 deg.
= 180 deg.
t
‫ד"ר אורי מחלב‬
QPSK Modulation
4 Possible States
Q
01
00
Vq
I
Vi
10
11
‫ד"ר אורי מחלב‬
16 QAM State Diagram
Q
0000
0100
0001
0101
0011
0111
0010
0110
I
1100
1000
1101
1111
1110
1001
1011
1010
‫ד"ר אורי מחלב‬
Symbol Rate:
“The rate at which the carrier
moves between points in the
constellation”
‫ד"ר אורי מחלב‬
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
‫ד"ר אורי מחלב‬
Some Typical Modulation Formats
BPSK
QPSK
16QAM
8PSK
64QAM
‫ד"ר אורי מחלב‬
QPSK Modulator
BALANCED
SYMBOL RATE:
MODULATION
fs = fb/2
IN-PHASE DATA STREAM
0°
fb
SERIAL TO
PARALLEL
BINARY CONVERTER
I
CARRIER
PHASE
SHIFT
I.F
COMBINER
BPF
90°
NRZ
INPUT
Q
fs = fb/2
01
00
11
10
SIGNAL
QUADRATURE DATA STREAM
BALANCED
MODULATION
COMBINED VECTOR
‫ד"ר אורי מחלב‬
STATE DIAGRAM
I, Q, Eye diagram and Constellation
+1
I
-1
+1
Q
-1
EYE
Q
5
3
CONSTELLATION :
I
2
‫ד"ר אורי מחלב‬
1,4
QPSK Demodulator
Phase
Demodulation
LPF.
I
Thresh
Comp.
I
fb/2
IF
Input
BPF
Power
Splitter
Car
Rec.
Phase
Splitter
0°
Symbol
timing
rec.(STR)
90°
Parallel
to serial
converter
fb/2
Q
LPF.
Phase
Demodulation
‫ד"ר אורי מחלב‬
Thresh
Comp.
Q
Binary
NRZ
fb
16 QAM Modulator
I
2-to-4
level
convert
fb
2
I
fb
4
Premod.
LPF
0°
L.F.
fb
Binary
Data
16 QAM
Phase
split
LO
BPF
NRZ
Data
fb
2
90°
Q
2-to-4
level
convert
fb
4
Premod.
LPF
‫ד"ר אורי מחלב‬
Q
Output
16 QAM Demodulator
I
4-Level
Signal
LPF
fb
4
L
O
G
I
C
Vth1
Vth2
fb
2
0°
Vth3
BPF
STR
CR
IF Input
Regeneration
X2 Data
data Out
fb
con
blner
90°
Q
LPF
fb
2
fb
4
4-Level
Signal
‫ד"ר אורי מחלב‬
4-to-2 level converter
of Q channel. Same
design as I channel.
Which waveform requires more bandwidth?
A
B
time
‫ד"ר אורי מחלב‬
Bandwidth Considerations
‫ד"ר אורי מחלב‬
Two random data sequence
time
‫ד"ר אורי מחלב‬
frequency
Unfiltered Digital Radio Spectrum
f 0  3FS f 0  3FS
f 0  2FS f 0  FS
f0
f 0  FS f 0  2FS
‫ד"ר אורי מחלב‬
f 0  3FS f 0  4FS
f 0  5FS
An UNFILTERD Radio
frequency
U/C
MOD
time
Data is easier to
recover but signal
requires a lot of
bandwidth
D/C
DEMOD
time
‫ד"ר אורי מחלב‬
A FILTERED Radio
frequency
MOD
U/C
time
Signal requires less
bandwidth but data
is filtered
D/C
DEMOD
‫ד"ר אורי מחלב‬
time
Intersymbol Interference
‫ד"ר אורי מחלב‬
Nyquist Filtering
Raised Cosine
‫ד"ר אורי מחלב‬
Filter Coefficient & Determines Required B.W.
Amplitude
Response
 1
  0.5
Linear Phase
  0.3
 0
(Flat Group Delay)
FS  Symbol _ Rate
FS
2 ‫ד"ר אורי‬
‫מחלב‬
FS
The Filtering is Distributed in the Radio
MOD
UPCONVERTER
DEMOD
DOWNCONVERTER
‫ד"ר אורי מחלב‬
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.
‫ד"ר אורי מחלב‬
P(x)
THRESHOLD
TOTAL PROBABILITY
OF NOISE AMPLITUDE
EXCEEDING THIS
THRESHOLD
0.4
0.3
0.2
0.1
-30
-20
-10
10
‫ד"ר אורי מחלב‬
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
Received signal with
superimposed noise ‫ד"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
‫ד"ר אורי מחלב‬
10
20
30
X
Meaning of Eye diagram
Threshold
‫ד"ר אורי מחלב‬