Hi Albrecht - see within.
I've copied this onto the reflector.
----- Original Message -----
Sent: Tuesday, April 26, 2005 2:57
AM
Subject: TP3 noise discussion
Hello Tom,
For today's TP3 call,
Jim asked us to look at the noise
calculation again. Therfore, I would like
to share my
thoughts on this topic with you. May be I am
missing
something completely, which is the reason why I am
sharing my
thoughts with you and not sending them
directly to the LRM
reflector.
Our main concern about the TP3 SNR definition
is
that we don't see the relationship
between the link budget
and the SNR calibration as
currently defined by the Qsq ratio
of 11.5 in
the LRM draft.
Regarding
the link budget, the LRM community is assuming
a
reference SNR at the input to the EDC of
30dBe (17+2x6.5).
This value is used for TWDP
calculations and for PIE-D
coverage
work and assumes a reference channel without ISI
but with RIN and MN
penalties.
TL - this particular calculation (for TWDP) does not
include RIN or MN. It is based on a matched filter bound receiver that
provides -13 dBm optical sensitivity to a perfect rectangular waveform. The
min received OMA is -6.5 dBm, so the difference is 6.5 dB (and hence,
the value in the equation).
Having written this, I think the value may have a
problem. Perhaps the SNR without these terms should be 32 dBe. The latest
budgets (see Nov 04) show sensitivities at ~-14 dBm (not -13), required to
accommodate 1 dB of penalties due to RIN and
MN!
For the
TP3 SNR calibration, the value of Qsq is
derived
from 0.9dBo noise power penalty. Even Piers
pointed
out in dawe_1_0504 that the contribution
from
MN of 0.5dB is pessimistic as it was
defined
for GbE where the exact value did not
matter and OFL was
used. We further noticed that
a slight change from 0.9 to 1.0dBo
will have
a large impact on Qsq (11.5 to 12) and the resulting SNR.
TL - the penalties were set at 0.5 and 0.4. They are independent
and assume no other simultaneous noises. These cannot be simply
added when computing penalties. Mathematically, they combine to 1.0 dB.
The math is right, but if the values for the individual terms are too high,
then we should lower them (which will reduce the
combination).
The SNR
corresponding to Qsq=11.5 is 20log11.5 =
21.2dBe
Using Lew's assumption of -3dBo noise
coloring due to IFR,
this improves the SNR at the optical
receiver input to about
27.2dBe. Assuming
a reciever sensitivity of -14dBm and an OMA
of
-6.5dBm, this results in a SNR at the EDC input of about
25.8dBe,
which is significantly below the
30dBe reference SNR derived
from the link
budget. To make the link budget and the Qsq
approaches
match, you would need to
reduce MN+RIN or increase IFR.
Should these match at
all?
TL - I do think there is a problem. For example, the RIN spec
produces Qsq = ~29 at TP2. MN may bring it down further, maybe ~20 - not
11.5. 0.5 and 0.4 came from experience at 802.3ae, where an overall Qsq
on the order of 20 produces penalties in the overall budget of 1 dB due to
interactions with other impairments, mostly ISI closure. We should not be
starting with 1 dB, but ending there. Now, there is the question of whether
any of these are the right values for 802.3aq! Perhaps the 29 factor is fine
for RIN at TP2, and perhaps 0.4 dB for MN is okay (at TP3, not a TP2), but the
interaction with all the dispersion is not well understood. That's the rub
here. Paul Voois did some early spreadsheet work on RIN, and that work
should be revisited. See http://grouper.ieee.org/groups/802/3/aq/public/may04/voois_1_0504.pdf.
I'll try to take some time to look at
this work. Finally, I
wonder why we can ignore jitter during the stress
test,when we needed to apply it for
10G-Base-LR .
TL - if we reduce the
noise, we may need to add some jitter back in, although the added complexity
is not desirable. Currently, the noise is high, so no added jitter is
needed.
--
Best regards,
Albrecht Rommel
Acuid Corporation Ltd.
Phone +49 (0)8141 3555 813
Fax +49 (0)8141 3555 829
Mobile +49 (0)172 607 8039
albrecht.rommel@acuid.de
www.acuid.com