on figure of merit in pam4 optical transmitter evaluation ...€¦ · on figure of merit in pam4...

On Figure of Merit in PAM4 Optical Transmitter Evaluation, Particularly TDECQ

§ Pavel Zivny, Tektronix

V1.0

2017/09

On figure of merit for 50 Gb/s and 100 Gb/s per lane PAM4 standards2017/09

On Figure of Merit in PAM4 Optical Transmitter Evaluation, Particularly TDECQ§ A brief presentation assessing current state of the art in the evaluation

of optical transmitters for PAM2 and PAM4 links. Focus on transition from TDP and Mask Test to TDEC and TDECQ, some comments on alternatives.

§ Practical outcomes from 100G and 400G systems with 28GBd and 56GBd signaling in optical Ethernet, 112 GBd outlook

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How did we get to TDECQ: Mask test

§ Test such as Mask test, TDECQ, etc. are of little interest to the end user; the end-user only cares about BER

§ For a number of reasons that’s rarely directly measured in production. A quality-of-transmitter measurement tool developed in the 1950’s was an eye mask (made with a wax pencil on a CRT front-plate). (Bell Labs idea?)

§ When real-time digitizing enables standard mask-test, a completely automated method of mask test becomes possible. Unfortunately the statistical nature of an high waveform-rate acquiring CRT disappears… it takes a while for everyone to realize that the new, PkPk nature of the mask test is not better than the ‘relative intensity’ of a wax-pencil and an analog screen

§ This digital mask test has a notoriously unclear relationship to the BER. Likely was better in the original test on analog oscilloscope….

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§ From: mask test to TDEC to TDECQ: Mask test with Hit Ratio

§ One improvement to the Mask test is the hit-ratio; comes close to the original analog.

§ Mask testing used to be a 0-hit allowed§ Current standards allow a small fraction of hits around 1E-5§ Often there also is a minimum number of hits recommended

or requiredSo1. Why is this “ratio” a good idea?2. Why you should NOT test to 0 hits3. What happens when you test with more hits …

or if you test with less hits?


Mask test without and with hit ratio

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Note: the green line shifts relative to the blue line. That means that you need a different mask for “ratio mask” than for “old-style” mask (to get the same test)

§ The Orange curve: IDEAL (perfectly separates good quality devices from bad quality devices

§ Blue: traditional margin test: only POORLY separates bad from good. Many FAILED good, many PASSING bad.

§ Green: hit ratio mask test. Better than Blue, but not as good as Ideal.


Mask test: you should not test to 0 hits…

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Because:§ It is a worse tool for separating good from bad§ Also:

More waveforms à more fails. That’s a bad test.The test you want is: More waveforms àhigher accuracy… so –use the Mask test with hit ratio.

§ Those are the two reasons why you should NOT test to 0 hits.


Recap: 1 of 2: TDECQ: a measurement methodology for PAM4 optical signals§ TDECQ measures the merit of an optical transmitter DUT as follows:§ For a given TX DUT, imagine a perfect TX with the same OMAouter at the RX

(i.e. after the channel) as the TX DUT§ The TDECQ is a ratio of:

- certain vertical eye opening measure of an (simulated) perfect TXto

- the same vertical eye opening measure of an physical DUT TX as it performs through its worst case optical channel (max. allowed reflection at the worst polarization, worst dispersion allowed)

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Mask test final thoughts

§ Mask test still doesn’t correlate well to the user’s figure of merit (BER)§ Mask Margin not related to link performance§ It doesn’t accommodate equalization well… so

… it figures that it would be replaced !– 100GBASE-SR4 defines TDEC– PAM4 standards (“400G”) define TDECQ

§ Reason to replace mask test:– Its correlation to BER (bit error rate) (or for PAM4 to SER, symbol error

rate) is not direct enough– Margin is not in terms usable to the link designer

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100GBASE-SR4 and TDEC (TDEC: Transmitter and Dispersion Eye Closure penalty)

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• TDEC replacing both mask test and TDP, Transmitter and Dispersion Penalty

(TDEC: Transmitter and Dispersion Eye Closure penalty)

• Some comparisons to 10GBASE-LRM, an early equalized optical standards

• TDEC replacing both mask test and TDP, Transmitter and Dispersion Penalty

Figure: IEEE 802.3bm


TDEC compares the vertical bathtub height… between your DUT and an ideal transmitter.

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§ Relatively complicated processing of four histograms to find the eye closure to BER of interest

This accumulates the histograms (PDFs, probability density functions) into a CDF, cumulative density function)… equivalent to a BER contour, not to a waveform density


TDECQ measurement

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OMA,outer= amplitude of PAM4 signalSmaller TDECQ is better (penalty). More acceptable receiver noise is better.


TDECQ measures the eye opening by adding noise till closure ( @BER)… same for TDECQ§ Note on the vertical eye opening measure: how much added Gaussian noise

can be added to close the eye to a given symbol error rate.– Since the ideal TX is simulated, it’s not really physically measured in the method,

just accounted for. Removal of the (difficult to obtain) ideal TX is a big advantage over the predecessor method (TDP, Transmitter Dispersion Penalty).

– Since the ideal TX noise is just accounted for, and since the vertical eye opening measure is ‘how much noise can be added…’, the ratio reduces to an expression with noise in the denominator ( ‘R’ ) plus constants and spectra width related values.

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§ The oscilloscope receiver includes a 5-tap, T-spaced FFE optimized for minimum mean square error. (7 taps for 100Gb/s lanes)


TDECQ measurement setup

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O/E

After 802.3bs D2.1 Figure 121–4—TDECQ conformance test block diagram

CRU

Polarization rotator

TestfiberPMD

Tx

Variablereflector

Opticalsplitter

Oscilloscopewith B-T f resp.

and pattertn trigger

DUT Optical channel Optical oscilloscope with SW

Reference EqualizerTDECQ algorithm


Reference Equalizer Optimizes TDECQ result

Before EQ After EQ


Current problems

§ TDECQ measurement is ok,but

§ The Equalization is an issue.§ See previous slide – the equalization to a large degree compensates

for dispersion.With some license it might be ok to say that TDECQ w/o equalization is ok for system that don’t suffer large dispersion (narrow lasers, and/or short links)§ Currently Tek can disable the IEEE equalization.

This might be a path to a subset of the 802.3bs test if one is needed.

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Concerns re TDECQ:TDECQ measures TDECQ. What do we optimize?§ The calculation of TDECQ requires a set of FFE tap coefficients. The method

for determining the taps is to minimize the mean square error (MMSE) of the equalized waveform.

§ Unfortunately, sets of taps with relatively small differences in MSE can have relatively large differences in TDECQ. Also, the MMSE optimized taps do not necessarily produce the minimum TDECQ. Both of these problems causes a repeatability issues with TDECQ.

§ Having found many sets of optimized taps, where each set was optimized using MMSE, the next task is to find the overall optimal set of taps. The most obvious method would be to pick the set with the smallest MSE.

§ Plotting the TDECQ and MSE for many sets of taps shows (see next page) that small changes in MSE can produce large changes in TDECQ and that the minimal MSE point doesn’t always correspond to the minimal TDECQ.

§ This sensitivity and the difference in minimal points is one of the reasons for the repeatability issues seen in the TDECQ measurements.

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…. TDECQ optimized MMSE, but it measures TDECQ- this has been changed§ Here is an

example of a partially convergent MMSE –TDECQ; note that several near-optimal MMSE solutions yield rather disparate TDECQ results

§ Again this is a penalty, so smaller is better

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Figure 1: MSE, TDECQ with different phase and cursor positions

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sample MSEE and TDECQ correlation near the minimal MSEE

TDECQ MSEE


TDECQ measurement vs oscilloscope BW

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§ Tek’sresultaresimilarasexperimentsdoneon56GBdtransmitteratCisco.HereshowingtheCiscoresults,asperIEEEmazzini_01_0317_smf;byMarcoMazzini.(traceHighlightbyprz)

§ AllTDECQplotsshow similartrends byvaryingRX BW.Longerequalizers thanreference (>5T- spaced,> 7T/2) have<0.5dBdeltaTDECQ.

NOTE:sufficientoscilloscopeBWfor26+GBdhasbeenavailable(e.g.Tek80C15).BWfor53GBd:onlysolution:useTek80C10Cultra-fastmodule


Measurement bandwidth going from 0.75 *BW_elto 0.5 * BW_el

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Note the difference between 25 Gb/s NRZ lanes and the 25 GBd PAM3 lanes…


The end

§ Summary:TDECQ is here to stay. It might be less intuitive than the Mask test but it’s sufficiently similar… and it’s an improvement for the whole process.

§ Thank youany questions?

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