----- Original Message -----
Sent: Wednesday, May 19,
2004 9:07 AM
Subject: Re: [10GBT] [SPAM]
[10GBT] symbol rate
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(2.5,650,4,100,6,1,7,2)
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