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As was discussed last week, the bottom line performance goal is 1e-12 BER which is determined most directly by the SNR at the decision device ("slicer SNR") by the familiar BER vs SNR curves the task force has been using so far. I'd much rather see results presented in terms of slicer SNR than the more obscure "input-referred RMS noise power". The margin is then simply the dB difference between the "required SNR" and the slicer SNR. Perhaps others could voice their preference.
As was also discussed, the "required SNR" for LDPC codes must be determined by simulation. Error floor and BER slope change issues inherent to many LDPC codes cannot be predicted and simulations must be performed to demonstrate that 1e-12 BER performance is possible from any given code. Extrapolations from 1e-9 or 1e-10 (or even 1e-11) are not always a reliable predictor of required SNR for 1e-12 BER. We have not yet seen results presented that establish the required SNR for the PAM8 case with the proposed LDPC (2048,1723) code as we have for the PAM12 case.
Lastly, we didn't have time to discuss this in detail last week but there is some concern about the applicability of the so called "Crane" noise immunity test for these PHYs. Another bottom line performance goal is for the *PHY + connecting hardware* to pass legally required noise immunity tests. The noise immunity test consists of modulated sinusoidal fields applied to an actual operating PHY in a system. This PHY will still have all other noise sources and will have it's cancellers and equalizers in normal operating mode. As I understand it, the "Crane test" puts the PHY in the unrealistic condition of 1) no other external noise sources and 2) equalizers frozen, not adapting. The Crane test makes the further assumption that the same coding gain predicted for white Gaussian noise will be valid for sinusoidal noise - I don't believe I've seen presentations or literature which backs this assumption up. I don't think we have had enough discussion on the Crane test's advantages/disadvantages, options, and relationship to reality to simply adopt it and use the results to base our PHY architecture decision on.
Dr. Scott Powell
Senior Manager, Ethernet PHYs