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Re: PAM-5 at 1.25 Gbaud


My basic disagreement with your spreadsheet is that you seem to be simply
summing optical intensity penalties, with little regard for how the penalties
interact. In addition, some of these penalties are not quite accurate.

In general, the factor limiting the distance of a gigabit MMF link is ISI, not
optical intensity. I believe that the HP/Agilent model developed by Del Hanson,
et. al. does a good job of assessing the coupling between different link
penalties. It does not do so in a straight additive fashion as your spreadsheet

For example, for PAM5 loss, you note a penalty of 6 db optical. My understanding
is that this penalty is an SNR penalty whose value is more correctly 4.1 db
optical. The "swing" of a PAM5 signal at the receiver is of approximately the
same amplitude from the lowest PAM5 level to the highest as it is for a PAM2
(binary signaling) 0 -> 1 transition. It is incorrect to simply subtract even
4.1 db from the link budget due to PAM5 loss, especially in light of coding gain
which can significantly increase effective SNR.

In York (September '99), Del Hanson proposed modifications to the GbE link model
to extend the model to be used for 10 GbE PMD development. I also presented some
issue modeling issues. I believe that it's in the best interest of the HSSG to
use a common, well understood and proven link model for 10 GbE PMD development.
I suggest that you obtain a copy of Del's model and use or modify it to get a
better apples to apples comparison of the various PMD proposals including

Best Regards,


Jaime Kardontchik wrote:
> Hello 10Giga'ers,
> I have received some private emails saying that
> I might be underestimating the performance of
> my own proposal ...
> May be that I am to blame for this in the way
> I constructed the spreadsheet for the installed
> MMF.
> In the entry corresponding to the "electrical
> signal power @ Rx", the PAM-5 at 1.25 Gbaud
> proposal would appear to have much less signal
> power compared to the other on-off n-WDM
> proposals (n=4,5)
> In practice, the input power at the Rx of my proposal
> does not differ from the input power of the other
> on-off n-WDM proposals. What happens is that,
> for the purposes of calculating the SNR, what is
> important is the *difference* between adjacent levels.
> For a  5-level approach this difference is much
> smaller. And it is this difference between adjacent
> levels that appears under the entry "electrical
> signal power @ Rx".
> Of course, this smaller "differential power"
> between levels is  compensated by the smaller
> noise power due to use of a smaller receiver
> bandwidth.
> Jaime
> Jaime E. Kardontchik
> Micro Linear
> San Jose, CA 95131
> email: kardontchik.jaime@xxxxxxxxxxx
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