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Vivek,
Why
not have a "Dove test" which is just like the Crane test except it uses randomly
spaced transient impulses of a finite amplitude? :)
Back
in 1000BASE-T days, I provided a presentation where I measured noise on CAT-5
installations and one site in particular was located next to an 89.9MHz radio
transmission tower. I really expected to see a lot of 89.9MHz carrier on the
wire, but much to my surprise, found transient pulses due to what was likely an
air-conditioner that were much worse in amplitude. The amplitude of these pulses
varied from a few mV (which is approximately the Vpp of the 89.9MHz carrier on
the wire) to as much as 19mVpk.
The
model for such a noise source could be readily defined as randomly distributed
between 50 and 70Hz and from 1mV to 20mV. One could shape the distribution
if they wanted to. Also, the pulse spectra could be defined. I don't have the
data, but I believe I can dig up that presentation and perform some FFT to get
it if it would be useful.
Dan
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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