Transcript BER and MER sem

Why Do You Need to Measure Both BER
and MER on Digital Signals
Presented by
UNAOHM
is a trademark of START S.p.a
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Introduction
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Most digital analysers measure Bit Error Ratio
(BER) and Modulation Error Ratio (MER).
BER and MER each have their limitations.
This seminar explains why it is important to
measure both BER and MER and tells you what
types of impairments will be missed if you only
measure one or the other.
Viewing of the Constellation Made Easy
Seminar and the Modulation Error Ratio Made
Easy Seminar is recommended prior to viewing
this seminar.
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Modulation Error Ratio
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MER is defined as follows: 10 log
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MER is expressed in dB.
RMS error magnitude
average symbol magnitude
RMS error
magnitude
Ideal
symbol
Average symbol
magnitude
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MER
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MER effectively assigns a value to the fuzziness of the
symbol cluster.
The larger or fuzzier the cluster, the poorer the MER.
The further from the ideal locations, the poorer the
MER.
Constellation with
“good” MER
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Constellation with “poor”
MER
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How Errors Occur
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Each symbol on the constellation is
framed by decision boundaries.
When the signal falls inside the
decision boundaries the information
is transmitted error-3
-1
-7
-5
free.
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3
1
1
-1
3
5
7
-3
Correct locations fall
within decision
boundaries
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Locations in error fall outside
decision boundaries
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Comparison Between Three
Error Free Constellations
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-5
All constellations below have a perfect BER with no errors, because
the cluster always falls within the decision boundaries.
7
7
The constellations
to the right, however,
have significantly better7 MER
with less noise.
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5
5
When the
cluster falls within the decision boundaries, BER is not an
effective measurement of quality because the BER is perfect.
-3
-1
3
3
3
1
1
1
1
-1
5
7 -7
-5
-3
-1
1
-1
3
5
-77
-5
-3
-1
1
-1
-3
-3
-3
-5
-5
-5
-7
-7
-7
Poor MER
Perfect BER
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3
Good MER
Perfect BER
3
5
Best MER
Perfect BER
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Theoretical MER vs BER With
Only Gaussian Noise
Impairing a 64 QAM Signal
BER
Note there are no errors
in this range of MER.
No
errors
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
35 34 33 32 31 30 29 28 27 26 25 24 23 22 21
1.00E-02
MER
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In practice errors will tend to occur at higher MERs due to other
forms of impairments besides Gaussian Noise.
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Why Measure BER?
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Since MER can quantify signal quality when no
errors exist the question can be raised, why
measure BER at all if MER will do?
The major limitation of MER is the inability of
the measurement to capture fast intermittent
transients.
A signal can have a very good MER, but poor
BER due to intermittent interference.
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Examples of Intermittent
Interference That Cause
Poor BER But Good MER
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Loose Connections
- Corroded or loose connections.
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Sweep System Interference
- Sweep pulses from a sweep system set up to sweep empty
spectrum.
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Laser Clipping
- Occasional overload of the laser due to analogue sync pulses
lining up.
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Microphonics
- Vibration of digital origination equipment can cause
intermittent errors.
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Troubleshooting By Measuring
Both MER and BER
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One way to determine if you have intermittent
problems is to measure both MER and BER.
If the MER is high, but you still see errors, then the
errors are probably caused by an intermittent
problem.
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Intermittent Errors on a
Constellation Display
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Intermittent errors will show up on a
constellation display as lone dots away from the
main cluster.
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BER vs MER vs C/R
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Q&A
Q. What is MER?
A. MER is a measurement of the modulation impairment that
affects the ability of a digital receiver to recover data bits.
It accounts for modulation problems such as non linearity,
group-delay and flatness variations, filter mismatching, and
ingress.
Q. When is MER useful considering?
A. When considering digital video system margin or SNR issues.
Q. What is the difference between MER and C/N?
A. A good C/N accounts for the effect of noise only while MER
accounts for modulation problems.
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BER vs MER vs C/R
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Q&A
Q. Can we calculate MER by measuring C/N (for instance using a
spectrum analyser) and then applying the relationship between
C/N and MER(SNR)?
A. No, because C/N accounts for noise ONLY while MER accounts
for ALL kinds of signal impairments —noise, signal leakage, IQ
level and quadrature imbalance etc —.
Q. What minimum value of MER is measurable?
A. To all practical purposes, down to 18 dB for DVB-C systems,
and down to 5dB for DVB-S systems. This is due to the fact that a
test equipment must be able to demodulate the signal before
MER measurement. If the signal is so poor that this process fails,
the measurement result cannot be trusted.
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BER vs MER vs C/R
Q&A
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Q. What are we going to use for faultfinding when MER is below
18 dB (DVB-C) or 5 dB (DVB-S)?
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A. At this point C/N (measured as channel power/noise power)
can be used instead of no (MER) measurement. Noise only will
be accounted for this way, but it is better than nothing.
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BER vs MER vs C/R
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Q&A
Q. What is the difference between BER and MER?
A. BER reflects noise and modulation impairments severe
enough to cause bit errors, remaining insensitive to subtle
trends in the digital modulation. MER allows the installer to
detect modulation impairments being concealed by the system’s
equalisation and error correction (before they become errors). In
other words BER starts counting when problems surface as
errors. MER is useful to detect impairments underneath the
surface that is, while impairments are still within the reach of
the correcting capabilities of the system.
Q. What does a good BER mean?
A. It indicated proper service delivery.
Q. What does a bad BER mean?
It highlights impaired service, but does not identify the cause of
the problem.
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BER vs MER vs C/R
Comparison Table
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Conclusions
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In order to see the effects of
all types of impairments on a
QAM signal you need to
measure both BER and MER.
MER can quantify the quality
of a digital signal that does
not have any errors.
MER has the limitation of not
being able to see
intermittent errors so a
signal can have a good MER
but a poor BER (Bit Error
Ratio).
The most common type of
intermittent errors are
caused by laser clipping.
UNAOHM EP3000
Digital signal analyzer with
COFDM/QAM/QPSK testing
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