[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
Lower SNR requirement.
b) Lower linearity
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
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
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