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Hi
Scott,
On the
subject of the Crane test, it is just a simple way of evaluating system
robustness against noise. There is no reason why it cannot be evaluated with
worst case impairments ON, as well as OFF. But I believe the
cancellers/equalizers do need to be frozen, because the intent is to capture the
effect on the system of a transient noise event, that is not long enough to
allow the system to adapt to it. I agree with you that the coding gain validity
is in question, but since we are (fortunately) comparing very similar systems
(both PAM, and both using LDPC), I'm assuming that this will affect both systems
equally.
Regards,
Vivek
Hi
Sailesh,
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.
Regards,
- Scott
Dr. Scott Powell
Senior Manager, Ethernet PHYs
Broadcom Corp.
(949)926-5105
spowell@broadcom.com
Sailesh,
Please, define
default cancellation parameters and necessary parameters to create transmit
PSDs for both PAM8 and 12.
Hiroshi
KeyEye
From:
stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On Behalf Of Sailesh Rao
Sent: Friday, July 16, 2004 9:01
PM
To:
STDS-802-3-10GBT@listserv.ieee.org
Subject: [10GBT] Proposed PAM8 vs.
PAM12 resolution process
All,
I would like to propose the
following process for resolving the robustness of PAM8 vs. PAM12 towards
external noise.
1. Compute the Optimum DFE SNR
Margin for PAM8 and PAM12 using solarsep_varlen7a.m for Models 1 and 3 using
default cancellation parameters and -150dBm/Hz background
noise.
2. Compute the input-referred
RMS noise power at the slicer by integrating the residual noise in the
Optimum DFE solution. I volunteer to add this code to solarsep_varlen7a.m
unless someone else wants to do so.
3. Compute the input-referred
external noise power that can be tolerated for a BER of 1E-12 for both
systems using the results from (1) and (2) above. I volunteer to add this
code to solarsep_varlen7a.m unless someone else wants to do
so.
Regards,
Sailesh
Rao.
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