Re: [10GBT] Summary of issues with PAM12
Sanjay,
I specifically addressed Jose's emissions emails in my summary. Here is the
relevant quote:
"In response to this calculation, Jose Tellado pointed out that if we use a
formulaic approach for the transmit filtering (e.g., 3rd order Butterworth
LPF with 3dB point at fs/4 for both proposals), then the “EMI PSD” for PAM8
sticks out at higher frequencies. (Ref.
http://grouper.ieee.org/groups/802/3/10GBT/email/msg00872.html ). This is
correct, but this argument doesn’t apply if we don’t use formulaic
approaches for the transmit filtering for emissions control.
Any transmit low pass filter that is used for emissions control reduces the
SNR margin of the receiver. A reasonable goal is to keep the SNR margin loss
to be around 0.5dB. Let’s say we choose such a transmit filtering scheme for
the PAM12 system. Then, if we use the exact same transmit filter, with the
exact same 3dB point for the PAM8 transmitter, the SNR margin loss for PAM8
will be about 0.6dB. In return for this extra 0.1dB margin loss, the PAM8
EMI PSD will be strictly lower than that for PAM12, by the aforementioned
0.62dB to 1.05dB. "
With respect to EMI susceptibility, this calculation should be combined with
the Emissions penalty and it is only the Total EMI penalty that is
important. This is because one can trade off higher emissions for higher
tolerance to EMI noise. This Total EMI penalty is between 2.6dB (at 100m) to
3.8dB (at 55m) to 4.5dB (at 0m) for PAM12 over PAM8.
Regulatory requirements for EMI susceptibility are always measured with
respect to external noise. It is the SNR margins that are measured RELATIVE
to the internal separation of levels that are not very meaningful.
Finally, I'm not sure that you can extract the extra 3.9dB SNR simply from
the lower symbol rate of PAM12, but this is a receiver issue which is vendor
specific.
Regards,
Sailesh Rao.
------------------------------------------
Dr, Sailesh Krishna Rao, Ph. D.
Phyten Technologies, Inc.,
200 Daniels Way, Suite 110,
Freehold, NJ 07728.
(732)-845-2100
srao@phyten.com
------------------------------------------
>From: Sanjay Kasturia <SKasturia@TERANETICS.COM>
>Reply-To: "IEEE P802.3an" <STDS-802-3-10GBT@listserv.ieee.org>
>To: STDS-802-3-10GBT@listserv.ieee.org
>Subject: Re: [10GBT] Summary of issues with PAM12
>Date: Mon, 23 Aug 2004 12:32:52 -0700
>
>The comment on the decrease in clock speed being 15% is correct.
>
>The emissions penalty numbers quoted are not correct. There are a number
>of earlier email messages from Jose on the subject so I will not repeat
>the discussion.
>
>The way the EMI susceptibility penalty (to come up with numbers ranging
>from 2dB to 3.9dB)is calculated is not very meaningful.
>
>The "extra 3.9dB" claimed to be required by the 12PAM receiver should
>really be ~3.5dB. This number is not at the receiver input but at the
>equalizer output. At the equalizer output this does not need to be found
>"somehow, with more precise analog front ends perhaps"; this comes
>directly from the lower cable loss that the signal sees when you use
>12PAM vs. 8PAM (because of the transmit spectrum being at lower
>frequencies).
>
>Regards,
>
>Sanjay
>
>sanjay@teranetics.com
>cell (650) 704-7686
>office (408) 653-2235
>
>-----Original Message-----
>From: stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On
>Behalf Of sailesh rao
>Sent: Friday, August 20, 2004 9:58 AM
>To: STDS-802-3-10GBT@LISTSERV.IEEE.ORG
>Subject: Re: [10GBT] Summary of issues with PAM12
>
>Sanjay,
>
>The decrease in clock speed is
>
>(952.381/825 - 1)*100% = 15%
>
>However, it is not correct that this clock rate reduction can be done
>"with no significant margin" penalty.
>
>The emissions penalty for this reduction is between 0.62dB and 1.05dB,
>which is real margin that we are losing.
>
>The susceptibility penalty for this reduction is between 2.0dB and 3.9dB
>over all cable lengths, which is also real margin that we are losing.
>
>Finally, even if you are only interested in the SNR margin, the SNR
>requirement for PAM12 is 23.8dB for a 1E-12 BER, vs. 19.9dB for PAM8.
>This requires the PAM12 receiver to find an extra 3.9dB somehow, with
>more precise analog front ends perhaps.
>
>Regards,
>Sailesh Rao.
>srao@phyten.com
>
>
> >From: Sanjay Kasturia <SKasturia@TERANETICS.COM>
> >Reply-To: "IEEE P802.3an" <STDS-802-3-10GBT@listserv.ieee.org>
> >To: STDS-802-3-10GBT@listserv.ieee.org
> >Subject: Re: [10GBT] Summary of issues with PAM12
> >Date: Fri, 20 Aug 2004 09:09:20 -0700
> >
> >Sailesh,
> >
> >I will let others speak for themselves but the main reason I prefer
> >12PAM over 8PAM is the ~10% to 20% lower symbol rate which will reduce
> >the speed requirements for the AFE and the clock speed requirements for
>
> >the digital signal processing.
> >
> >I think the lowering of the symbol rate can be done with no significant
>
> >margin penalty.
> >
> >Sanjay
> >
> >sanjay@teranetics.com
> >cell (650) 704-7686
> >office (408) 653-2235
> >
> >-----Original Message-----
> >From: stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On
> >Behalf Of sailesh rao
> >Sent: Friday, August 20, 2004 8:33 AM
> >To: STDS-802-3-10GBT@LISTSERV.IEEE.ORG
> >Subject: Re: [10GBT] Summary of issues with PAM12
> >
> >Hugh,
> >
> >With regard to issues 1 and 2, I don't believe that there are any
> >disputes about my calculations. I've already addressed these so-called
> >disputes in my summary e-mail and pointed out why my analysis stands.
> >
> >With regard to issue 3, firstly, the main rationale advanced by the
> >PAM12 proponents for their 3bits/baud scheme is that it has 0.7dB
> >higher Salz SNR margin over the 2.5bits/baud scheme used in PAM8, over
> >a 100m cable.
> >However, once you put a hole in the constellation and throw away at
> >least 1.1dB, would you now concede that this rationale is technically
> >invalid, and that the Salz SNR margin advantage also swings to PAM8
> >over
> >PAM12 at ALL cable lengths, from 0m to 100m?
> >
> >Secondly, the hole in the constellation makes the PAM12 transmit PSD a
> >function of the THP coefficients. Therefore, every time a system vendor
>
> >finds that the system violates emissions limits, we can expect him/her
> >to demand to examine the THP coefficients of the PHY, if this task
> >force standardizes PAM12.
> >
> >Thirdly, the hole in the constellation makes the receiver SNR margin a
>
> >function of the THP coefficients of the alien neighbors.
> >
> >With regard to issue 5, please check the details on the PAM8 proposal.
> >You will see that there are NO fixed patterns used anywhere to
> >delineate data frames or anything. Therefore, it is not "necessary" to
> >use fixed patterns for this purpose. These fixed patterns waste the
> >precious Shannon capacity which the cabling community is working hard
> >to provide us, though not on the same scale as the other issues with
> >the PAM12 proposal, and which is why I listed this issue last.
> >
> >Regards,
> >Sailesh Rao.
> >srao@phyten.com
> >
> >_________________________________________________________________
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