Joseph
–
The main value of the
program you used is that it gets beyond capacity limitations. The
constellation sizes noted in the printout ( PAM-4) are for uncoded.
PAM-5 at 1250 Mbaud is equivalent to coded PAM-4. In order to get the
margin for the PAM-5 system, you can take the PAM4 DFESNR (which is the
Optimal DFE SNR for 1.25 Gbaud PAM, regardless of the number of levels), and
subtract from it the SNR that your coded system requires.
Equivalently the
PAM_2p5 DFESNR is the optimal DFE SNR for a 1 Gbaud system (regardless of the
levels), and the PAM8 DFESNR is for an 833.333 Mbaud system (3 bits/baud/pair
at 10 Gbps) regardless of the number of levels. For coded margins,
these SNRs can be compared to the SNRs listed for the coded systems, properly
adjusted for shaping gain or precoding loss.
For reference, you
mentioned a 1.56 Gbaud PAM-4 system, which I presume has coding overhead, it
would be equivalently (10Gb/sec / 4 pairs / 1.56 Gbaud = 1.60 bits /
baud).
I hope this
helps.
The point of all this
is that it is well-known that increasing the bandwidth of a DFE, DFSE, or TH
Precoded system does NOT always give you the benefit of the increase in the
channel capacity by including more bandwidth. You CAN signal too fast
for the channel. The point of diminishing return is usually where SNR(f)
(SNR at a given frequency f) approaches zero (crossover of signal and noise)
for all frequencies > f_o. For PAM systems, f_o will be the nyquist
rate, half the baud rate. Signalling beyond this rate generally adds
less to the SNR in marginal capacity than it loses due to the additional noise
and shorter baud interval. See slide 25 (1st backup) in my
presentation at the last meeting for the basic math, or go directly to the
reference: J. Salz, “Optimum mean-square decision feedback equalization”, Bell
System technical Journal, pp. 1341-1373, Oct. 1973.
Joseph, on another
topic,I also see that you have had to increase the echo, next and fext
cancellation numbers significantly over the default values. This seems
to fly in the face of your claimed improved nonlinearity tolerance.
While the received equalized signal in the absence of interference will be
more tolerant to nonlinearity, nonlinearities following the point where the
reference for next/fext and echo cancellation are taken will will show up as
residual echo, next, and FEXT, and hence a good proxy for the linearity
requirements is the maximum degree of cancellation required. The signal
that has to be cancelled (echo, next or fext) will have a near Gaussian PAR in
reality, so the linearity of the canceller or cancellation signal is largely
independent of the number of levels used in the line code. As such, you
seem to have required some 50 dB of linearity for FEXT cancellation alone,
(was 30 dB), and upped the echo cancellation to 65 dB. Did you really
require those increases?
-george
-----Original
Message-----
From:
stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On Behalf Of Joseph
Babanezhad
Sent:
Wednesday, May 19,
2004 3:25
AM
To:
STDS-802-3-10GBT@LISTSERV.IEEE.ORG
Subject: Re: [10GBT] [SPAM] [10GBT]
symbol rate
In May of 1998 at the CICC
conference while waiting to present my paper
I was listening to Mehdi Hatamian
of Broadcom, one of the movers & shakers
of IEEE 1000BASE-T standard, give
his tutorial presentation on 802.3ab standard
draft. There was one thing that he
kept repeating it over and over ... and over again;
"Remember the most important
things for Ethernet are power, power and power"
If this was relevant to 1000BASE-T
it definitely is more relative to 10GBASE-T.
With this in mind let me address
your comments:
1-Please do not confuse PAM4 with
PAM5
PAM4 baud-rate=1.56 GB/s
Nyquist-frequency=780 MHz
PAM5 baud-rate=1.25 GB/s
Nyquist-frequency=625 MHz
2-The complete list of PAM5
advantages are:
b-More tolerant to AFE
nonlinearity
c-Significantly lower
power
3-As far as channel's higher IL
& ANEXT at frequencies beyond 500 MHz are
concerned the following are the
capacity simulation results using SolarFlare's
provided program from the
web-site:
Launch Power : 7 dBm (2Vpp
PAM5)
nextcanc=50;
echocanc=65;
fextcanc=50;
solarsep_varlen7a(-10.5,650,4,55,6,1,6,2)
solarsep_varlen7a(4.5,650,4,100,6,1,6,2)
----- Original Message -----
Sent:
Tuesday, May 18, 2004 9:14 PM
Subject: [SPAM]
[10GBT] symbol rate
Looking through some of the
prior presentations proposing 4PAM and 5PAM I see that the
significant benefits claimed for 5PAM or 4PAM
are
a) Lower SNR
requirement.
b) Lower linearity
requirement
Going from 5PAM to 10PAM raises
the SNR required for the same BER by 6dB.
The increase in SNR required is
indeed painful, however the presentations proposing the lower PAM do not
take into account the fact that
a) The higher symbol rate will
result in higher net attenuation since the transmit spectrum extends to
higher frequencies where the attenuation is higher
b) The alien cross talk is
higher at higher frequencies.
Both a and b result in the
available SNR being lower than what you get at symbol rates below 1Gsym/sec
for the Channel models #1 and #3.
For short cable lengths or lower
attenuations, this effect is less severe.
Bottom line is that beyond about
1Gsym/sec, the theoretically achievable system margins drop off
sharply.
The higher PAM (8, 10, 12
etc) will require higher SNR and this implies higher linearity requirements
at the transmitter, lower noise in the receiver than if you targeted a
shorter distance using 5PAM however I don't think this is a choice we have
given our distance objectives.
Please note that this is my
personal opinion and not a directive as editor.
I leave this as a qualitative
argument because quantitative arguments have been made earlier but don't
seem to have been accepted by some. I hope this
helps.
Regards,
Sanjay
Kasturia
sanjay@teranetics.com
cell (650)
704-7686
office (408)
653-2235
fax (408)
844-8187
Teranetics
Inc.
2953 Bunker Hill Lane, Suite
204
Santa Clara, CA
95054