8/17/2019 Understanding Real World MER
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Technical Leader
Understanding Real-World MERMeasurements
Ron Hranac Bruce Currivan
Technical Director
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MER can be confusing!
• Just what is MER?
• Do different instruments report different values on the same
signal under identical conditions, and if so, why?
•
When developing requirements for MER at various pointsin the network, should one specify whether the
measurement is equalized or unequalized, the make/model
of test equipment used, and basic measurementconditions?
•
What can be done to ensure more consistent andmeaningful MER measurements?
• How can the MER in a plant be improved?
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What is MER?
MER is an abbreviation for modulation error ratio
MER is the ratio of average signal constellation power to averageconstellation error power — that is, digital complex baseband signal-
to-noise ratio (SNR). Indeed, MER is often called SNR.
MER = 10log10(average symbol power/average error power )
I
In effect, MER isa measure of how
“fuzzy” the symbolpoints in a
constellation are.
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Major factors that affect
reported MER valuesQAM receiver MER measurement
implementation:
•
Statistical variation
• Unequal occurrence of symbols
•
Linkage of carrier loop bandwidth to
capture length
• Implementation loss MER ceiling
•
Symbol error MER floor
• Analog front-end noise
•
Adaptive-equalizer type and length
Operational:
• TX and RX phase noise
• CNR, CNIR, CCN
•
Nonlinear distortions such as CTB,
CSO, XMOD, and CPD
• Linear distortions such as micro-
reflections, amplitude ripple and tilt, and
group delay
• In-channel ingress
•
Laser clipping
•
Burst noise•
Data collisions
•
Suboptimal modulation profile settings
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Lab and field test summary A QAM signal was measured on
8 different MER test sets, atseveral different points in the lab
and in the field.
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Lab test configurations #1-5
•
Test configuration #1: QAM modulator
direct output, single channel (>60 dB
CNR)
• Test configuration #2/3:
Headend combiner output, 154
active QAM channels (lower
and upper adjacent channelsturned off for config #3)
• Test configuration #4/5 : Optical receiver
output, 154 active QAM channels (lower
and upper adjacent channels turned offfor config #5)
MER range (dB)
Config -10 dBmV 0 dBmV +10 dBmV
1 30.2 ~ 39.4 33.5 ~ 45.0 34.4 ~ 46.3
2 29.6 ~ 38.4 33.3 ~ 44.3 33.2 ~ 45.6
3 29.5 ~ 39.3 32.4 ~ 44.5 33.0 ~ 45.8
4 29.7 ~ 37.6 31.9 ~ 41.1 32.5 ~ 41.6
5 29.6 ~ 38.1 32.2 ~ 41.1 32.6 ~ 41.6
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•
Test configuration #6 : Optical receiver + 3 amplifiers, 154active QAM channels
• Test configuration #7 : Optical receiver + 3 amplifiers, 152 active QAM
channels (lower and upper adjacent channels turned off), injected AWGN for
nominal 35 dB CNR (measured ACP with R&S EFA: -35.2 dBc)
Lab test configurations #6-7
Config #6 -10 dBmV 0 dBmV +10 dBmV
Range of reported
MER values (dB)
28.5 ~ 37.2 30.7 ~ 40.6 31.0 ~ 41.1
Config #7 Equalized MER
at 0 dBmV
Unequalized MER
at 0 dBmV
Range of reported
MER values (dB)
30.1 ~ 35.1 28.8 ~ 32.6
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• Optical receiver + 3 amplifiers, micro-reflection, measured on Ch. 2
(57 MHz center frequency), approx. 35.7 dB CNR
Config #8 Equalized MER
at 0 dBmV
Range of reported
MER values (dB)
33.2 ~ 35.4
Lab test configuration #8
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Field measurement points #1-3
• Measurements performed on Ch. 71 (256-QAM) at downstream
laser test point, node downstream output, end-of-line (N + 6)
89:;9
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Lab and field test results:
Limitations of real-world MER
measurements•
Different makes/models of test equipment report different equalized
and unequalized MER values on the same signal under identical
conditions. The variations from instrument to instrument when
measuring a given signal under identical conditions ranged from as
little as a few tenths of a dB to more than 12 dB.
•
The vintage of the equipment also was important, with the first-
generation QAM analyzer consistently providing lower MER than all
of the newer analyzers.
• Measurement conditions clearly affect the reported MER value. Low
RF input level (e.g., -10 dBmV) in every case produced lowerreported MER than higher RF input levels (0 dBmV and +10 dBmV).
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Lab and field test results:
Limitations of real-world MER
measurements (cont’d)•
Some instruments reported slightly different MER when adjacent
channels were present, likely related to receiver selectivity.
• With nominal CNR = 35 dB (ACP = -35.2 dBc) and the same
impairments from the signal source, combiner, optical link, andamplifier cascade, the reported equalized MER ranged from 30.1 -
35.3 dB. This illustrates variations in receiver implementation loss,
adaptive equalizer performance, etc.
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Guidelines to ensure more
consistent and meaningful MER
measurements! The type of MER measurement—equalized, unequalized, or both—must be
stated when defining MER performance metrics.
! The same make/model of test equipment should be used when comparing
measurements.
!
Apply approximately the same signal level at the test equipment input when
comparing measurements.
! Ensure that the total signal power at the equipment or device input does not
exceed manufacturer’s recommended value.
! If reverse tilt is a concern, a subscriber drop equalizer may be used to
flatten the signal amplitudes across the spectrum.
!
Ensure that the CNR or CNIR at the point of measurement meets or
exceeds the SCTE-40 and/or DOCSIS minimum values.
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Wrapping Up:
Tips to improve MER•
Maximizing downstream and upstream MER is fairly
straightforward, and often involves little more than “Cable
101.”
•
The most common causes of low reported MER aretypically the things that should be done correctly in the
first place.
•
Ensure proper headend, forward path, and return path
alignment; identify and troubleshoot problems such aslinear distortions; and adhere to top-notch installation
and maintenance practices (sweep, leakage/ingress,
etc.).
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