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[10GBT] symbol rate

Looking through some of the prior presentations proposing 4PAM and 5PAM I see that the significant benefits claimed for 5PAM or 4PAM are
a) Lower  SNR requirement.
b) Lower linearity requirement
Going from 5PAM to 10PAM raises the SNR required for the same BER by 6dB.
The increase in SNR required is indeed painful, however the presentations proposing the lower PAM do not take into account the fact that
a) The higher symbol rate will result in higher net attenuation since the transmit spectrum extends to higher frequencies where the attenuation is higher
b) The alien cross talk is higher at higher frequencies.

Both a and b result in the available SNR being lower than what you get at symbol rates below 1Gsym/sec for the Channel models #1 and #3.
For short cable lengths or lower attenuations, this effect is less severe.
Bottom line is that beyond about 1Gsym/sec, the theoretically achievable system margins drop off sharply.
The higher PAM  (8, 10, 12 etc) will require higher SNR and this implies higher linearity requirements at the transmitter, lower noise in the receiver than if you targeted a shorter distance using 5PAM however I don't think this is a choice we have given our distance objectives.
Please note that this is my personal opinion and not a directive as editor.
I leave this as a qualitative argument because quantitative arguments have been made earlier but don't seem to have been accepted by some.  I hope this helps.
Sanjay Kasturia
cell (650) 704-7686
office (408) 653-2235
fax (408) 844-8187
Teranetics Inc.
2953 Bunker Hill Lane, Suite 204
Santa Clara, CA 95054