Transcript QAM Digital Measurements Seminar

Modulation Error Ratio and Signal-to-Noise
Ratio Demystified
Presented by
Sunrise Telecom Broadband
…a step
ahead
3-14-2002
Introduction
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Viewing the Constellations Demystified seminar prior to
viewing this seminar is recommend.
Modulation Error Ratio (MER) in digital systems is
analogous to Signal-to-Noise or Carrier-to-Noise used in
analog systems.
Determining the MER of a digital signal is a critical part of
determining how much margin the system has before
failure.
Unlike analog systems where you can see degradations in
Carrier to Noise performance, a poor MER is not
noticeable on the picture right up to the point of system
failure.
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Noise Effect Comparison Between
Analog and Digital Systems
Effect of Noise on Analog Systems (Gradually Poorer C/N)
45 dB C/N
35 dB C/N
25 dB C/N
20 dB C/N
Effect of Noise on Digital Systems (Gradually Poorer MER)
34 dB MER
23 dB MER
22.5 dB MER
Noise has very little effect on digital
systems until the system fails completely
22 dB MER
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Analog Carrier-to-Noise
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Carrier-to-Noise ratio in analog systems is simply a
measure of the ratio of peak video carrier power over the
noise in the channel, over the system bandwidth expressed
in dB.
This type of measurement can be done on a digital channel
as well, but unfortunately does not provide the complete
picture.
Analog C/N
Measurement
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Quadrature Modulation
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Unlike analog video, QAM modulation not only uses
Amplitude Modulation, but also phase modulation.
Simply measuring the carrier level relative to the noise
level does not take into account any phase noise that may
also be present on the signal.
Carrier
Amplitude
Modulation
Carrier
Amplitude
Modulation
Analog Video AM
Modulation
Carrier Phase Shift
QAM Modulation
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Other Noise and Interference
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In addition to amplitude and phase noise, other
impairments can also effect the quality of the signal in
similar ways to noise.
Examples of other disturbances that will effect the
performance:
- Implementation Loss due to limitations of A/D and D/A converters
and sampling.
- Reflections not cancelled out by the adaptive equalizer.
- Non-Linear distortions such as laser clipping and amplifier
compression.
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Decision Boundaries
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Each location on the constellation is framed by decision
boundaries.
7
If the signal falls within these boundaries, the correct data
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will be received.
If because of noise or other interference3 it falls 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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Noise Effects on the Constellation
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Because the position on the constellation depends on both
carrier amplitude and carrier phase, both amplitude and phase
noise will affect the location on the constellation.
Amplitude noise will change the distance from the origin and
phase noise will change the rotational position. Other types of
noise and interference can effect the symbol in all directions.
Effect of
Amplitude
Noise
Phase of
Carrier
Amplitude of
Carrier
Effect of
Amplitude
Noise
Effect of
Phase Noise
Other Types of Noise
and Interference
(Effects Symbol in all
Directions)
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Modulation Error Ratio and Error Vector
Magnitude.
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In order to take into account both the amplitude, phase
noise and other impairments on the carrier two different
measurements have been developed, Modulation Error
Ratio, (MER) and Error Vector Magnitude, (EVM).
MER and EVM can be directly correlated with each other
since they are essentially the same measurement, only
performed and specified in different ways.
MER and EVM effectively assign a value to the
“fuzziness” of the constellation clusters. The further the
dots are from their ideal location, the poorer the MER or
EVM.
MER and EVM can be considered a figure of merit for the
QAM signal that includes all types of impairments, not just
noise as in Carrier to Noise in analog systems.
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Constellation Display
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The constellation display gives a visual indication of how far the
noise is moving the signal from its ideal locations on the
constellation.
The further from the ideal locations, the poorer the MER or EVM.
The shape of the cluster gives information on the type of
impairment.
Constellation With
“Good” MER
Constellation With
“Poor” MER
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Modulation Error Ratio
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RMS error magnitude
MER is defined as follows:
10 log
MER is expressed in dB.
average symbol magnitude
RMS Error
Magnitude
Ideal
Symbol
Average
Symbol
Magnitude
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MER and EVM
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Since the MER and EVM measurements are very similar,
they give essentially the same information about the signal.
MER was chosen as the preferred measurement for cable
TV by the SCTE because of it’s similarity to the Analog
Carrier-to-Noise measurement expressed in dB that most
people the cable industry are familiar with.
MER and EVM are sometimes referred to as Signal-toNoise by some manufacturers, but this is not technically
correct.
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What is a Good MER?
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64 QAM set top converters require better than 23 dB MER
to operate. To allow for system degradation a margin of at
least 3 or 4 dB is preferred.
256 QAM set top converters require better than 28 dB
MER to operate. To allow for degradation a margin of at
least 3 dB is preferred.
Typically the maximum MER displayed on portable
analyzers is about 34 to 35 dB.
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Conclusions
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MER is the digital equivalent
of Carrier-to-Noise in analog
systems.
Poor MER does not effect
picture quality right up until
the system begins to fail.
MER takes into account not
only amplitude noise, but also
phase noise.
Determining the MER of a
signal is a key part of
determining the margin from
failure of the channel.
CM1000 Cable
Modem System
Analyzer
AT2000
Spectrum
Analyzer
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