2-14-2002




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.
2-14-2002





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
1
In Phase
1
0
-1 0
180° Out of Phase
2-14-2002




A higher data rate can be achieved by adding amplitude modulation to the carrier in addition to the phase modulation.
3
Bits
[10]
Symbol
(Group of Bits)
By having multiple levels of amplitude, groups of bits can be transmitted.
1
0
11
In Phase
A group of bits that is represented by a particular level and phase of the carrier is called a symbol.
-1
-3
01
00
180° Out of Phase
Two Levels of Amplitude Modulation and
Bi-Phase Modulation Makes Four Possible
Symbols
2-14-2002



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
2-14-2002




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.
2-14-2002

 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°.
Two Levels of Amplitude Modulation
(Carrier On or Off)
0 1 0 1 0 1 0
Data
1 or 0
Four Levels of Amplitude Modulation
With 180° Carrier Phase Reversal
Transmits Eight Levels
011 111
010 110
001 101
000 100
Data
000 to 111
Carrier Off = 0 Carrier On = 1
180° Phase Reversal of Carrier
2-14-2002



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
Combiner
64 QAM
Signal
101 010
Local Osc
Oscillator
Shifted 90°
8 Level AM
Modulator
Q Component
2-14-2002

I Channel
Carrier
Phase
Q Channel
Carrier
Phase 90°
Shifted
Carrier + =
Amplitude
Carrier Phase Shift
2-14-2002

 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
2-14-2002



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
2-14-2002



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.
2-14-2002





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.
2-14-2002

64 QAM Constellation
Q Amplitude
7
5
3
-7 -5 -3 -1
1
-1
1
-3
3 5
-5
-7
64 Possible
Combinations of I and Q
7 I Amplitude
256 QAM Constellation
-15 -13 -11 -9
Q Amplitude
15
13
11
9
-7 -5 -3
3
1
-1 1
-1
-3
-5
7
5
-7
-9
3 5 7 9 11 13 15
-11
-13
-15
256 Possible
Combinations of I and Q
I Amplitude
2-14-2002



 Each location on the constellation is framed by decision boundaries.
will be received.
5 adjacent area the data will be in error.
-7 -5 -3 -1
1
-1
1 3 5 7
-3
Correct Locations Fall
Within Decision
Boundaries
-5
-7
Locations in Error Fall Outside
Decision Boundaries
2-14-2002


7
On a test instrument the locations on the constellation build much about the health of the signal and any problems that it has.
3
-7 -5 -3 -1
1
-1
1 3 5 7
-3
Buildup of
Dots
-5
-7
2-14-2002



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
2-14-2002



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
2-14-2002



Rotation
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 Zoomed Constellation With Phase Noise
2-14-2002



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
2-14-2002



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
2-14-2002



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
2-14-2002



 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
2-14-2002




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.
*Patent Pending
Analysis of distortion
2-14-2002



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