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[10GBT] Issues with solarsep_varlen7a.m



George/Carlos,

I would like to bring up some issues with the solarsep_varlen7a.m matlab
code that is found on

http://grouper.ieee.org/groups/802/3/an/public/material/index.html

I believe that these issues alter the conclusions that one could derive from
this code.

1. Frequency Base:
The frequency base for the DFE margin calculations is being inferred from
the first column in the Insertion Loss files found in this directory. As a
result, the frequency base ranges from 0 to 650MHz in all cases. However,
the average power P is being distributed evenly over (line 391)

bwmhz=1250MHz for the 2-bit PAM system,

bwmhz=1000MHz for the 2.5-bit PAM system,

bwmhz=833MHz for the 3-bit PAM system

This means that 48% of the input power is unused in the 2-bit system, 35% is
unused in the 2.5-bit system and only 22% is unused in the 3-bit system.
This tips the scale in favor of the 3-bit PAM system.

2. Folded SNR calculation:

The folded SNR calculations in lines 443, 453 and 463 are not right. If f1
and f2 are mirror frequencies about fs/2, the formula being used is

S/N = ABS(S1/N1) + ABS(S2/N2) ;

However, the actual SNR at the folded frequency would be

S/N = ABS(S1+S2)/ABS(N1+N2)

where S1, S2, N1, and N2 are complex phasors. Therefore, in the context of
folding, the actual PSD of the signal becomes relevant, whereas the original
Salz formula for the optimum DFE SNR is independent of the PSD.

As done in the matlab code, this calculation also tips the scale in favor of
the 3-bit PAM system since signal power is assumed to be zero for f>650MHz
due to (1) above.

3. SNR Requirement for 1E-12 BER

The SNR requirement for 2.5bit PAM is 3.01dB worse than the SNR requirement
for 2-bit PAM.
The SNR requirement for 3bit PAM is 6.23dB worse than the SNR requirement
for 2-bit PAM.

The code is using 2.95dB and 5.94dB respectively in lines 446, 456 and 466,
which also tips the scale in favor of the 3-bit PAM system.

Regards,
Sailesh Rao.

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