Transcript QAM Digital Measurements Seminar

Constellations Demystified
Presented by
Sunrise Telecom Broadband …a
step ahead
2-14-2002
Introduction
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Understanding the constellation display and how it is
derived will help you understand QAM Modulation and
how the digital signal is transmitted.
The Constellation Display on a test instrument is can be an
extremely valuable tool for determining the health of a
digital signal.
Understanding how the various impairments can affect the
constellation display will help to troubleshoot the source of
the problem.
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Bi-Phase Shift Keying (BPSK)
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BPSK is the simplest method of digital transmission.
Data is transmitted by reversing the phase of the carrier.
The amplitude of the carrier remains constant.
Is a very robust transmission method but consumes significant
bandwidth.
Data
Amplitude

1
1
In Phase
0
180° Out
of Phase
0
-1
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Bits and Symbols
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A higher data rate can be
achieved by adding amplitude
modulation to the carrier in
addition to the phase
modulation.
By having multiple levels of
amplitude, groups of bits can be
transmitted.
A group of bits that is
represented by a particular level
and phase of the carrier is called
a symbol.
Bits
Amplitude

Symbol
(Group of Bits)
3
[10]
1
11
In Phase
-1
01
180° Out
of Phase
-3
00
0
Two Levels of Amplitude Modulation and
Bi-Phase Modulation Makes Four Possible
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Symbols
Bits and Symbols
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Bits are grouped into pairs or symbols and the appropriate
phase and amplitude is transmitted.
A negative amplitude indicates a phase reversal of the
carrier.
Symbol
10 01 10 11 00 01
Amplitude
+3 -1 +3 +1 -3 -1
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Quadrature Modulation
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Quadrature Modulation is a method of amplitude
modulation that allows two channels to be carried at the
same frequency effectively doubling the bandwidth that
can be carried.
A form of Quadrature Modulation has been used for many
years in analog television to carry the two components of
the color subcarrier.
By modulating two carriers at exactly the same frequency
but shifted by 90° both the amplitude and phase of the
carrier is modulated.
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Quadrature Amplitude Modulation (QAM)
Eight levels of modulation are achieved on both the I and
Q channels by using four AM modulation levels and by
reversing the phase 180°.
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Two Levels of Amplitude Modulation
(Carrier On or Off)
0 1
0 1 0 1
0
Four Levels of Amplitude Modulation
With 180° Carrier Phase Reversal
Transmits Eight Levels
Data
1 or 0
000 100
001 101
010 110
011 111
Data
000 to 111
Carrier Off = 0
Carrier On = 1
180° Phase Reversal of Carrier
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Quadrature Amplitude Modulation (QAM)
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Both the I and Q channels are AM modulated at the same
frequency on carriers shifted by 90°.
The two signals are combined to make up the 64 QAM
signal.
8 Level AM
Modulator
I Component
Bit Stream
101 010
Local Osc
Combiner
64 QAM
Signal
Oscillator
Shifted 90°
8 Level AM
Modulator
Q Component
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Quadrature Modulation
I Channel
Carrier
Phase
+ =
Q Channel
Carrier
Phase 90°
Shifted
Carrier
Amplitude
Carrier Phase Shift
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Quadrature Modulation
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The phase and amplitude of the carrier at any given time
determine the location on the Constellation and the I and Q
channels can be derived from this information.
Location On
Constellation
Phase of
Carrier
Amplitude
of Carrier
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Constellation
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The location on the Constellation determines the I and Q
components amplitude.
The amplitudes of the I and Q channel are derived from the
rectangular coordinates of the carriers amplitude and
phase.
Q Channel
I Channel
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Gray Coding
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The symbol which each location on the constellation represents
is chosen using a technique known as Gray Coding.
Gray coding insures that any adjacent location will only be one
bit different, reducing the effect of an error.
10
11
00
01
Adjacent Locations are only different by
one bit.
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64 and 256 QAM Constellations
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By adding more levels to the I and Q channels, higher data
rates can be carried.
The higher the number of levels, the more effect there will
be from noise or interference.
64 QAM uses 8 levels in the I direction and 8 levels in the
Q direction for a total of 8 squared or 64 symbols.
256 QAM uses 16 levels in the I direction and 16 levels in
the Q direction for a total of 16 squared or 256 symbols.
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64 and 256 QAM Constellations
256 QAM Constellation
64 QAM Constellation
Q Amplitude
Q Amplitude
15
13
11
9
7
5
3
1
-7
-5
-3
-1
1
3
5
-1
-3
7
I Amplitude
7
5
3
1
-15 -13 -11 -9 -7 -5 -3 -1 1 3 5 7 9 11 13 15
-1
-3
-5
-5
-7
-9
-7
-11
-13
-15
64 Possible
Combinations of I and Q
I Amplitude
256 Possible
Combinations of I and Q
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Decision Boundaries
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Each location on the constellation is framed by decision
boundaries.
If the signal falls within these boundaries,7 the correct data
will be received.
5
If because of noise or other interference it3falls in an
adjacent area the data will be in error.
1
-7
-5
-3
-1
1
-1
3
5
7
-3
Correct Locations Fall
Within Decision
Boundaries
-5
-7
Locations in Error Fall Outside
Decision Boundaries
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Constellation Buildup
7
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On a test instrument the locations on the constellation build
5 can tell you
up over time and the shape and distribution
much about the health of the signal and any problems that
3
it has.
1
-7
-5
-3
-1
1
-1
3
5
7
-3
Buildup of
Dots
-5
-7
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Clean Constellation Display
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Example of a relatively good 64 QAM constellation.
Dots are reasonably well defined and positioned in a
square, indicating good gain, phase noise and Modulation
Error Ratio.
Well Defined
and away from
the decision
boundaries
Well Positioned
Dots in a Square
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System Noise
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A constellation displaying significant noise.
Dots are spread out indicating high noise and most likely
significant errors.
Dots are
spread out
causing
errors to
occur
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Phase Noise
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A display that appear to be rotating a the extremes
indicates excessive phase noise.
Phase noise can be caused by headend down/up converters.
Rotation
Constellation With Phase Noise
Rotation
Zoomed Constellation With Phase Noise
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Coherent Interference
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If build up takes on a circular look, the problem is coherent
interference.
Examples of coherent interference are CTB, CSO, spurs
and ingress.
Circular
Cluster
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Intermittent Interference
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Isolated dots away from the main cluster indicate
interference that is intermittent.
Examples of intermittent interference are intermittent
ingress and laser clipping.
Random
Dots Away
from the
Cluster
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Gain Compression
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It the outer dots are pulled into the center while the middle
ones are not effected, the signal has gain compression.
Gain Compression can be caused by IF and RF amplifiers
and filters, up/down converters and IF equalizers.
Outer Dots
Pulled In
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I Q Imbalance
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I Q Imbalance is caused by a difference between the gain
of the I and Q channels.
The display is taller than wide.
This indicates a problem with headend baseband amplifiers
or filters.
Taller
than Wide
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Automatic Constellation Diagnosis*
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Analyzer automatically
analyzes the constellation
and displays the type of
distortion present
Minimizes training and
eliminates guess work.
By quickly type of
distortion you can quickly
find the source of the
problem.
Analysis of distortion
*Patent Pending
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Conclusions
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Understanding the
constellation display will go
along way towards helping you
understand QAM Modulation.
The constellation display can
tell you many things about the
health of the signal and be a
valuable tool for locating and
troubleshooting problems.
CM1000 Cable
Modem System
Analyzer
AT2000
Spectrum
Analyzer
2-14-2002