Re: [10GBT] Validity of comparing PSDs to determine electromagnetic emissions
Hi All,
Regulatory EMI tests are not conducted with worst-case alien NEXT
environments set up in the range.
Since we are comparing two TH-precoded (pre-equalized) PAM systems, and not
grossly dissimilar systems, it is also sufficient to compare the amplitude
of the pulse responses at the receiver input to understand which PAM system
will be more robust in the EMI susceptibility tests. Therefore, there is no
question that PAM8 will be 2.0dB to 3.9dB better than PAM12 in the 0-100m
range. This advantage is critical in the installed base, where this
regulatory test will be especially difficult to pass. In the installed base,
this advantage ranges from 3.2dB to 3.9dB.
With respect to the PAM12 constellation, please note that changing the
constellation is tantamount to doing a heart bypass surgery in a data
communications system. With respect to the holes, to draft an analogy, Swiss
cheese is Swiss cheese any way we slice it, and it won't suddenly become
American. After the two PAM12 hole repositioining schemes that I have
examined only in the past few days, isn't it time we converged on the PAM8
proposal instead of looking for other ways to patch up the holes in the
PAM12 constellation?
Furthermore, I've found another issue with the PAM12 proposal. I leave you
now with a description of this Issue #6.
Regards,
Sailesh Rao.
srao@phyten.com
Issue #6: LDPC Code
The (1024,833) LDPC code in the PAM12 proposal is constructed using
Djurdjevic's method by placing blocks of parity check matrices in a
systematic manner. In the course of this construction, as more and more
blocks are used, the rate of the code decreases and the lower bound on the
Hamming distance increases.
The parity check matrix for the (1024,833) LDPC code used in the July PAM12
proposal contains the parity check matrix for the (1024,845) LDPC code that
the PAM12 proponents were using in the May presentation. Indeed, the parity
check matrix for the (1024,833) code is obtained by adding 64 rows to the
parity check matrix for the (1024,845) code. However, of the 64 rows added,
52 of them are dependent (do-nothing) rows and only 12 of them are
independent rows. This is why the number of information bits in the code
only reduced from 845 to 833.
It was reported in seki_1_0704.pdf that the (1024,845) code has a BER slope
change in the 1E-9 BER range, which made it unacceptable. However, it was
also reported that the (1024,833) code has no such BER slope change until
the 1E-12 BER.
There is only so much that 12 parity check equations can do, that the
previous 179 parity check equations could not. Therefore, as an engineer, I
wouldn't bet on the BER slope change for the (1024,833) code to occur well
beyond the 1E-12 BER point.
Please note that a 1E-12 BER in a 10GBASE-T system corresponds to 1 error
every 1.6minutes. If we work very hard on the receiver and squeeze out a 3dB
SNR margin to this 1E-12 BER, it would be truly tragic if that implied 1
error every hour or so for our customers, due to this property of the LDPC
code used in the system.
>From: George Zimmerman <gzimmerman@SOLARFLARE.COM>
>Reply-To: "IEEE P802.3an" <STDS-802-3-10GBT@listserv.ieee.org>
>To: STDS-802-3-10GBT@listserv.ieee.org
>Subject: [10GBT] Validity of comparing PSDs to determine electromagnetic
>emissions
>Date: Tue, 31 Aug 2004 14:33:24 -0700
>
>Scott -
>Thank you for pointing out a simple fact that has made for amusing
>reading while I've been busy with other things. Much of the discussion
>here of PAM-12 vs. PAM-8 is well within the bounds of measurement
>tolerances, and, as will be discussed below, can be flip-flopped by
>design trades in these measurement tolerances. This covers more than
>just EMI, so here are some of the summary points:
>
>1) EMI emissions - We're way beyond 20 log (f) approximations. We've
>seen measured results in numerous presentations by multiple sources (see
>cohen_1_0903.pdf for methodology and also a presentation by Scott
>Powell). Scott's right. Differences in the PSDs are TINY when compared
>with differences test setups, cabling configurations, and measurement
>equipment can make - if a design is so close that the small difference
>in the PSD is going to make it fail, it will fail for a dozen larger
>reasons. I'd be interested in hearing from Thuyen Dinh which proposal
>looks easier to build magnetics for, keeping in mind both emissions and
>insertion loss.
>
>2) Coded margin - we're asking for accuracy way beyond assuming the same
>coding gain, or even running AWGN simulations on non-TH Precoded
>constellations. In my experience this can cloud things by 1 dB.
>
>3) Component and noise budgeting accuracy - The solarsep simulation code
>with Salz-bounds provides an optimal DFE basis which can be used as a
>line/environment limited bound on how well you can do. It does NOT
>provide a basis for evaluating sensitivity to noise which enters the
>receiver at different points in the receive chain, hence it should not
>be used by varying the AWGN level to try to measure sensitivity to
>receiver noise, or for accurately tweaking the transmit and receive
>filtering. It can be adjusted for that, but for that you need to become
>architecture-specific to a particular vendor, and add additional linear
>transfer functions (such as magnetics, location of gain stages, sources
>of noise and hybrid structure). Net-net, the degradation from optimal
>becomes significantly vendor-architecture dependent and therefore less
>capable of multiple vendors tracking the results.
>
>4) EMI susceptibility - we're also way beyond looking at susceptibility
>just as the partition separation of a sine wave on a single pair in the
>absence of the consider ANEXT noise (e.g., Crane's test), because in the
>presence of noise the cumulative coded errors will cause errors first.
>Real EMI happens on all 4 pairs at once (to differing levels), generally
>in the presence of noise. In this situation, generally, the code breaks
>down way before the partition difference (at least for 1e-12 BER). As
>such, the EMI susceptibility must be measured in the presence of full
>ANEXT. This will require LDPC code simulations in the presence of EMI
>ingress and ANEXT. It must also respect the difference in receive gain
>required by the PAM 12 and PAM 8 systems. The answer is not obvious,
>but I'm pretty sure I haven't seen it discussed on this forum amidst all
>of the heated arguments, AND, it really requires selection of a code to
>get it done.
>
>Long and short -
>We need to get this matter decided already. However, in order to
>finalize this decision, we need not only PAM levels on the table, but
>code and modulation rates nailed. We also need to consider impact on
>non-silicon components (cabling and magnetics, both of which are
>stressed for bandwidth). There are only a small number of points that
>have been made that I think are unambiguous:
>
>1) The PAM-8 baud rate is faster, so for a fixed cancellation ratio, it
>will require more digital processing. If it, in fact, has higher
>margins, these may be traded for implementation loss to ease the burden.
>
>2) The PAM-12 constellation with a hole in it is inefficient. This fact
>is responsible for more than one of Sailesh's complaints. The cross
>constellation just moves the inefficiency around to a more familiar
>form, but still suffers precoding losses. A more efficient mapping can
>be adapted.
>
>3) The framing proposed in the PAM-12 is complex and possibly can be
>improved.
>
>4) The choice of transmit filter will govern the emissions more than the
>number of PAM levels.
>
>5) The sensitivity of margin to baud rate is broad enough that the
>ANEXT-limited performance differences in an optimal signal processing
>sense are small.
>
>-george zimmerman
>---
>
>-----Original Message-----
>From: stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On
>Behalf Of Scott Powell
>Sent: Tuesday, August 31, 2004 11:29 AM
>To: STDS-802-3-10GBT@LISTSERV.IEEE.ORG
>Subject: Re: [10GBT] Validity of comparing PSDs to determine
>electromagnetic emissions
>
>I agree with Kishore's point that there is not a significant EMI
>difference
>between the two proposals. It's been shown a couple of times on the
>reflector that one proposal is slightly better over certain frequencies
>and
>the same proposal slightly worse over others. In response to a recent
>posting on the importance of "slight" PSD differences:
>
> > "saliesh rao" wrote
> >
> > With regard to issue #1, I disagree that a 1dB difference in the
>transmit
>PSD is "well below the
> > measurements and setup errors" for the emissions profile. Anyone who
>has
>spent hours and days
> > meeting the FCC/CISPR emissions profile in the range will know that
>1dB
>will make a huge difference
> > between a pass and fail, since the likelihood of a fail would be
>exponentially determined by this single dB.
> >
>
>I would respectfully request that you take a look at ANSI standard C63.4
>before relying too heavily on the fractional dB emissions you think you
>are
>measuring. This standard document can be ordered from the IEEE and
>describes the measurement accuracy that certified EMI test ranges must
>comply to. As you can see in section 5.4.6.1 on page 20, the
>"Acceptability
>Criterion for Emission Test Range" is +/- 4dB. Your EMI range may not
>always "volunteer" this information but should if you ask.
>
>ANSI C63.4 standard means that if the same 10GBASE-T transceiver (+
>cabling/connectors) is measured in 2 different *ANSI Qualified* EMI test
>ranges and results differ but are within +/- 4dB of each other, the
>measurements are both considered valid - even though they differ by +/-
>4dB.
>Susceptibility accuracy requirements for qualified ranges are even
>worse.
>These permitted inaccuracies are in addition to the inherent set-up
>inaccuracies of finding the "worst case" antenna height, antenna
>polarity,
>and cable configuration/orientation for each frequency in the scan.
>
>I don't believe the small PSD differences shown so far between the two
>proposals are significant enough to warrant choosing one over the other
>based on expected emissions. Again repeating my original message: "it
>has
>been clearly shown that the PAM-8 and PAM-12 proposals have similar
>performance (slight edge for PAM-12), but the PAM-12 proposal offers the
>advantage of a reduced operating frequency resulting in reduced power
>and
>reduced implementation difficulties."
>
>Regards,
> - Scott
>
>-----Original Message-----
>From: stds-802-3-10gbt@ieee.org [mailto:stds-802-3-10gbt@ieee.org] On
>Behalf
>Of Kishore Kota
>Sent: Tuesday, August 31, 2004 7:21 AM
>To: STDS-802-3-10GBT@listserv.ieee.org
>Subject: [10GBT] Validity of comparing PSDs to determine electromagnetic
>emissions
>
>
>I would like to question the validity of comparing the power spectra
>(PSD)
>of various proposals to determine electromagnetic emissions. In earlier
>postings on this reflector, such comparisons have been used to claim
>superior emissions performance for PAM8. There are several unknowns
>here:
>
>a) As a taskforce we haven't agreed to a model for how differential
>signal
>gets converted to common mode, and
>b) we do not have a model for how the common mode signal then gets
>converted
>to electromagnetic emissions.
>
>Simply comparing PSDs without applying such models would lead to
>nonsensical
>conclusions. For instance, in the lower frequency band 1000BASE-T
>spectrum
>is atleast 8db higher than either of the 10GBT proposals (because the
>same
>transmit power is spread over a smaller frequency band). However, this
>does
>not mean 1000BASE-T is tougher than 10GBT for emissions compliance
>testing.
>
>Some comparisons of PAM12 and PAM8 spectra have been posted to the
>reflector
>which include a 20logf model. Although this is a crude approximation, it
>does seem to have some consensus in the group. By playing games with
>this
>model and lowpass filters, either proposal can be made to look slightly
>better than the other. This suggests that there are no significant
>differences between these proposals as far as emissions are concerned.
>Obsessing over these differences just keeps us from answering the more
>difficult question of whether either proposal is sufficient from an EMI
>perspective.
>
> regards,
> Kishore
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