On this particular backplane the difference in
attenuation between 2.2 and 4.4 Gbaud is precisely 9 dB.
Therefore, the PAM-4 eye openings at 2.2 Gbaud (=4.4 Gb/s)
are the same as the binary eye openings at 4.4 Gbaud (=4.4 Gb/s).
In the absence of crosstalk or other noise these schemes
would perform the same. When you add crosstalk the difference quickly
becomes apparent - the PAM-4 system, because it works at a lower
bandwidth, is far less susceptible to crosstalk.
Note that PAM-4 enjoys a huge advantage in jitter, since
the PAM-4 eye is twice the width of the binary 4.4 Gbaud eye. The PAM-4
system is less susceptible to random jitter, and since it operates at a
lower bandwidth picks up less noise in the first place.
On this backplane, the overall SNR using PAM-4 in a
realistic multi-transceiver environment would be far better than with
binary at the same delivered data rate.
