Re: [STDS-802-3-400G] FW: Few questions on cole_02_0814_smf & clarification on updates from cole_3bs_01a_0714

[Chris Cole <chris.cole@xxxxxxxxxxx>]

Hi Ian,

 

The DMT Electrical BTB measurements on page 15 (page 14 footer numbering) show the combined effect of DAC and ADC quantization. This is an elegant measurement that has been used for the past 30 years to show the total electrical SNDR (Signal-to-Noise Distortion Ratio) in communications systems due to DAC quantization in TX and ADC quantization in RX. When the quantization contribution is uncorrelated with the signal, as is the case for the PMD proposals in 802.3bs, the contribution can be approximated as an additive noise term Nq’ in SNR at the decision slicer. The Wiki reference discusses this nicely.

 

SNR (w/o quantization) = S/N

SNR (w/ quantization) = ~S/(N + Nq’)

 

The reason for the prime term is that this is a filtered version of the quantization noise that is measured by the electrical BTB measurement. Some DSP algorithms, for example for sigma delta converters, dramatically reduce the quantization term.

 

For finite quantization:

Nq’ > 0

Therefore:

                S/N > S/(N + Nq)

 

Since Nq reduces or degrades the SNR, it is a penalty. As pointed out in an earlier email, the tricky part is that the is an additive term, whereas penalties are multiplicative (dB). To convert Nq effect on SNR to a penalty, it has to be calculated together with all other impairments, i.e. a full end to end link simulation. The penalty is the difference between SNR with finite quantization and SNR with no quantization. This is similar to how TDP is calculated.

 

For the 2km LAN-WDM DMT proposal in 802.3bs, we calculated the SNR penalty with quantization (6 ENOB DAC and 6 ENOB ADC) as 3dB (excluding other RX design limitations) of which we allocated 2.5dB to TDP and 0.5dB as RX quantization penalty. The latter is in rough agreement with your 10% estimate, which corresponds to a 0.4dB SNR penalty. We also agree that this is not the dominant term. The dominant term is the modulation penalty including PAPR effects which is 9dB.

The SNR penalty for PAM-4 is less than DMT because there is no quantization clipping effect. We are tweaking this penalty and will have slightly revised numbers in our September presentation.

 

Chris

 

 

From: Dedic, Ian [mailto:Ian.Dedic@xxxxxxxxxxxxxx]
Sent: Tuesday, August 26, 2014 2:49 AM
To: STDS-802-3-400G@xxxxxxxxxxxxxxxxx
Subject: Re: [STDS-802-3-400G] FW: Few questions on cole_02_0814_smf & clarification on updates from cole_3bs_01a_0714

 

Just to make it clear, the page referred to does not really show the effects of quantisation noise as such – it shows the effect on bit rate of using better ADCs and DACs with higher ENOB (Effective Number Of Bits), which includes everything causing noise and distortion (including jitter).

 

At these sample rates ADC/DAC performance is not limited by quantisation noise in either case, other causes of noise and distortion dominate – for example if an 8 bit ADC or DAC has ENOB=6.3 (a good figure!) the ideal quantisation noise (ENOB=8.0) is 10dB below the overall noise, so only contributes 10% with 90% coming from other sources.

 

I guess American page numbering has the title slide labelled as page 1, where European numbering has the first content slide labelled as page 1, just like a book J

 

Ian

 

Ian Dedic

Chief Engineer

Communications Business Unit

Fujitsu Semiconductor Europe GmbH

3 Concorde Park, Concorde Road

Maidenhead SL6 4FJ, UK

Tel : +44 (0) 1628 504 711

Mob : +44 (0) 7795 534 253  *** NEW ***

 

From: Chris Cole [mailto:chris.cole@xxxxxxxxxxx]
Sent: 20 August 2014 22:02
To: STDS-802-3-400G@xxxxxxxxxxxxxxxxx
Subject: Re: [STDS-802-3-400G] FW: Few questions on cole_02_0814_smf & clarification on updates from cole_3bs_01a_0714

 

While the technical results in Ian’s July presentation are impressive, his page numbering is off. Or we have the same problem as with how floors are numbered in a building, with Ian following the European convention.  

 

Below is the correct link to the page showing the additive effects of TX and RX quantization:

 

http://www.ieee802.org/3/bs/public/14_07/dedic_3bs_01a_0714.pdf#page=15

 

 

Chris

 

From: Chris Cole [mailto:chris.cole@xxxxxxxxxxx]
Sent: Wednesday, August 20, 2014 7:43 AM
To: STDS-802-3-400G@xxxxxxxxxxxxxxxxx
Subject: Re: [STDS-802-3-400G] FW: Few questions on cole_02_0814_smf & clarification on updates from cole_3bs_01a_0714

 

Hi Sudeep,

 

An elegant measurement showing the additive effects of TX and RX quantization is shown in Ian’s July presentation:

 

http://www.ieee802.org/3/bs/public/14_07/dedic_3bs_01a_0714.pdf#page=14

 

Chris

 

From: Chris Cole
Sent: Tuesday, August 19, 2014 10:31 PM
To: 'Sudeep Bhoja'
Cc: STDS-802-3-400G@xxxxxxxxxxxxxxxxx
Subject: RE: [STDS-802-3-400G] FW: Few questions on cole_02_0814_smf & clarification on updates from cole_3bs_01a_0714

 

Hi Sudeep,

 

Here is a nice discussion on the  subject:

 

http://en.wikipedia.org/wiki/Quantization_(signal_processing)

 

Chris

 

From: Sudeep Bhoja [mailto:sbhoja@xxxxxxxxx]
Sent: Tuesday, August 19, 2014 10:25 PM
To: Chris Cole
Cc: STDS-802-3-400G@xxxxxxxxxxxxxxxxx
Subject: Re: [STDS-802-3-400G] FW: Few questions on cole_02_0814_smf & clarification on updates from cole_3bs_01a_0714

 

Chris, 

 

I am still struggling to understand what you mean by quantization noise penalty in TX and RX. Can you please explain? 

 

Thanks,

 

Sudeep 

On Aug 19, 2014, at 9:23 PM, Chris Cole <chris.cole@xxxxxxxxxxx> wrote:

Hi Matt,

 

Thank you for the questions and comments. Please see detailed responses inserted in the text of your email below. 

 

More generally, the PMD specifications presented on today’s SMF Ad Hoc call are idealized and scrupulously apples-to-apples. As we quantify more penalties, the specifications will get harder not easier. Before fractions of a dB are challenged piecemeal, we should first have a comprehensive set of understood penalties and design limitations, which can then be looked at in total. A detailed contribution from you moving us in this direction would be welcome.

 

Chris

 

From: Matt Traverso (mattrave) [mailto:mattrave@xxxxxxxxx]
Sent: Tuesday, August 19, 2014 11:12 AM
To: Chris Cole
Cc: STDS-802-3-400G@xxxxxxxxxxxxxxxxx; Marco Mazzini (mmazzini)
Subject: Few questions on cole_02_0814_smf & clarification on updates from cole_3bs_01a_0714

 

Hi Chris,

 

Thanks for the contributions.  You may have addressed some of these questions at the microphone (on cole_3bs_01a_0714) or verbally, yet as someone who has been unable to attend the calls/meetings, I’d appreciate your patience in addressing the questions.

 

1)      Quantization (*) noise: I count three penalties in your draft budget containing quantization noise.  I am concerned that you are overestimating by putting it into three line items.  (a) modulation penalty <adding 0.2dB for “exact levels”>; (b) TDP- on slide 4 of cole_02_0814_smf, you indicate that TDP includes quantization <unclear constituent pieces of TDP>; (c) TIA & Quantization penalties on the RX.  These are all reasonable sources of penalty to further investigate, yet I’m concerned that your comparison table probably mixes some cells which are well known with others that have more rigorous experimental basis such that the reader may inadvertently  draw conclusions which are not beneficial for the industry.  In particular, what is the basis of the RX Quantization penalty?  Do you have a reference?

 

*** The total of all quantization related penalties in the TX and RX specifications is ~1 dB which is not an overestimation.

RX quantization and other penalties were calculated through simulations. Quantization effects are well understood and extensively published on. Estimating this penalty is complicated by dependence on relative contribution of all other noise sources. We are open to refining it, although any update is likely to be small.

 

2)      FEC Coding Gain on KP4 vs. KR4: (this refers to cole_3bs_01a_0714): I believe you used the 1e-15 input BER for the KP4 and the 1e-12 input BER for KR4 (leveraging zhai_400_01_0713).  Also, I noted on on slide 8, you took a 0.5dB OMA relaxation to accommodate a ~3% rate increase in KP4 vs. KR4 – this seems excessive.  I didn’t notice KR4 mentioned in cole_02_0814_smf, is there a reason that you are selecting KP vs. KR?

 

*** All FEC gains are referenced to BER = 1e-12 so that a comparison can be made to LR4.

The penalty in the presentation for 3% KP4 FEC overhead is not 0.5dB but 0.064dB (rounded to nearest 1/10 of a dB). It is equally applied to all PMDs.

KP4 was used because that was proposed in bhat_3bs_01a_0714, of which you are a supporter, and is minimum required for higher order modulation PMDs. KR4 was used for some 50Gb/s lane PMD alternatives in the July presentation, but changed to KP4 to enable a fair comparison. KP4 could not be used for DMT because it requires a stronger FEC, as was noted during the SMF Ad Hoc call.

 

3)      TDP: On slide 8 of cole_3bs_01a_0714, you have a 2.0 dB penalty for PAM4 2x λs DML KP4 FEC, yet on slide 5 of cole_02_0814_smf, this has shifted to a 1.5dB penalty.  Why did the TDP change?  Conversely, the “4x100G KP4 FEC PAM4 MOD” column in your ad hoc preso has 2.5dB versus a 2dB penalty in the July presentation – why the change?  Any references you can point to?  (seems strange that the 8x got easier yet the 4x got harder?)  As you can tell from mazzini_01a_0814_smf, we are trying to understand the behavior & best characterize penalties.

 

*** Generally, we have refined our analysis, and will continue to refine our analysis, so numbers changed and will change. In this iteration, we added jitter to 50G and 100G alternatives, and quantization to latter.

Specifically, 50Gb/s NRZ TDP decreased because of use of stronger FEC (KP4 in Aug. vs. KR4 FEC in July). 100Gb/s PAM-4 TDP increased (“4x got harder”) because of use of a weaker FEC (KP4 in Aug. vs. BCH FEC in July). TDP decreases as pre-FEC operating BER increases because BER curves converge. The comment that “8x got easier” is incorrect; TDP for one 8x increased, for one 8x stayed the same, and for one 8x decreased. Coincidently, although arrived at differently, the 2.5dB TDP for 100G PAM-4 in today’s SMD Ad Hoc presentation happens to be the same as in bhat_3bs_01a_0714, which you are supporting. The suggestion of bias is unfortunate.

Marco’s presentation is great. I appreciate his accuracy in describing the measurement environment and its limitations, and the fidelity of the data. Please support this effort so that we can better understand and quantify all penalties and design limitations.

 

4)      FEC Optical Gain: Based on verbal conversations with IC folks, I do not believe the halving of NCG (page 6, row “FEC Optical Gain v. 1e-12 BER” cole_02_0814_smf) is appropriate.  I am trying to find a better approximation.  If you have a good reference (beyond the generic intensity vs. E-field) I’d appreciate seeing it.

 

*** A factor of approximately half is appropriate for going from electrical to optical domain. Exact values have been extensively discussed in 802.3bm, for example on the reflector with the subject line: “[802.3_100GNGOPTX] FEC and error ratios.”

Detailed derivations can be found in:

http://www.ieee802.org/3/bm/public/mmfadhoc/meetings/nov29_12/anslow_01a_1112_mmf.pdf

http://www.ieee802.org/3/bm/public/jan13/anslow_03_0113_optx.pdf

We have used 2.6dB and 3.2dB as the optical gain for KR4 and KP4, respectively, as these have been generally accepted as correct. We are open to refining these gains, but any update is likely to be small.

 

5)      OMA Eye v. EyeSNR: Suggestion that you move to using Eye SNR (see slide 7 of mazzini_01a_0814_smf).  As we are moving into links that leverage FEC/DSP, I think it is appropriate that we adopt SNR terminology rather than eye openings.

 

*** Since 802.3bs is chartered with writing optical specifications and many participants have an optics background, using optics terminology is the most helpful in promoting a broader understanding of different proposals. Many participants have a long history of using optics terminology for FEC, and more recently DSP, in telecom and ITU-T applications. Optics terminology was used to include FEC in 802.3bm 100GBASE-SR4 specifications, so we have a clear example of how to do this successfully in the IEEE.

 

thanks

--matt traverso