Understanding Real World MER

8/17/2019 Understanding Real World MER

1/13

Technical Leader

Understanding Real-World MERMeasurements

Ron Hranac Bruce Currivan

Technical Director


2/13

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?


3/13

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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4/13

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


5/13

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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& ! % ( ' ) " * $ &+ && &! &% &( &' &) &" &* &$ !+ !& !! !% !( !' !) !" !*

,-.

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/0123 5

/0123 6

/0123 ,

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/0123 7

0 1 2 7 8 9 :

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6/13

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


7/13

•

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


8/13

• 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


9/13

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


10/13

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).


11/13

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.


12/13

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.


13/13

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.).

Understanding Real World MER

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