850nm-4WDM-1.25Gbaud
Hello Nariman Yousefi,
I would like to emphasize again what I told you
in the conference call yesterday:
The "850nm-4WDM-1.25Gbaud" proposal needs essentially
only 4-bits ADCs. The low number of levels needed
is due to two facts: 1) we do not need equalization,
and 2) the nice peculiarities of Viterbi decoding.
The first fact is clear: we do not need equalizers
because at 1.25 Gbaud the eye at the input of the
receiver is wide open at the link lengths of interest.
I will explain then the second fact:
All the simulations I presented in Kauai last Nov 99
were obtained using a simple 18-level ADC. This fact
is so important that I dedicated to it a complete separate
section of the presentation (Part IV: Coding Gain).
In Part IV, I compared the BERs of an ideal system with
ideal 6 dB coding gain (which is the maximum coding
again attainable using the Viterbi decoder with an
infinite precision ADC) with the actual BERs obtained
using a simple 18-level ADC (and all the PCS and
Viterbi decoder implemented in Verilog using finite
precision arithmetic, including reallistic adders).
The graphs shown at the end of Part IV indicate
that the actual coding gain (at a BER level of
10^(-12) or lower) is about 5 dB. That is, a simple
18-level ADC is all what is needed to get almost all the
achievable coding gain of an ideal infinite-bit ADC.
This should not come as a surprise for people that
have used Viterbi decoders in the past (and I am sure
that many at Broadcom have).
For example, it is a well known fact that Viterbi decoders
for binary encoded information (PAM-2) need very simple
"soft-slicers" to get most of the coding gain of
the convolutional code. A "soft-slicer" for PAM-2
coding needs only 8 levels to get a performance near
to the ideal Viterbi decoder which uses an infinite-level
soft-slicer (ADC). See, for example,
J. A. Heller and I. M. Jacobs
"Viterbi decoding for satellite and space communications"
IEEE Trans on Commun Tech, vol COM-19, pp 835-848
October 1971
S. B. Wicker
"Error control systems for digital communications and
storage"
Prentice Hall, 1995
(A "hard-slicer" is the standard n-level slicer for PAM-n.
A "soft-slicer" uses more intermediate levels to allow the
Viterbi algorithm to get more accurate decisions).
Nariman,
We have received input in the past from non-PAM5 people
in the Task Force that one of the main reasons they are
not willing to add their support for a PAM-5 proposal is
the present multiplicity of PAM-5 proposals.
The "850nm-4WDM-1.25 Gbaud" proposal has very good
chance to meet the schedule of the 802.3ae. It uses
technology that is available now.
Its PCS/SERDES is complete and implementable in
present mainstraim CMOS technologies. Its architecture
is identical to the one suggested by Howard Frazier
in its presentation "10Gig MII update" (Kauai, Nov 99):
four independent channels, each one with its own PCS
running at 312.5 MHz and SERDES running at 1.25 GHz.
(with Viterbi in the receiver running also at 312.5 MHz).
This is a piece of cake. You could even use HARI on
Copper traces if you wish !
The only remaining open issue is the linearity of the
optical system at 1.25 Gbaud. The needed dynamic range
of its receiver is fairly small (~4-bit ADC, 25 dB).
We have presented data in Albuquerque (see Siva
Yegnanarayanan's presentation "Are lasers linear enough
for PAM-5 over optics ?") that clearly suggest that
oxide-constrained VCSELs have enough linearity to
support a 25 dB dynamic range system using a bandwidth
of 1.25 Gbaud (see the SFDR data on VCSELs in this
presentation, SFDR = Spurious Free Dynamic Range).
Hence, it has good chances to pass this obstacle too.
Economical viability is not an issue either: it will
be much cheaper than other proposals on the table, for
example Agilent's 4-WDM using 1300 nm lasers, expensive
optical multiplexers and non-Silicon photodetectors.
There are important applications that crave for a cheap and
low power solution in the 100 meters installed MMF space.
Nariman,
Let us work together to begin filling the technical
gaps in this proposal and begin talking to people in
the Task Force to gain their support to it.
The success of this proposal to join the privileged
"7-group" is not assured, but a clear support of
Broadcom to it will create the minimum conditions
needed to give it a fair chance.
There is no other realistic way for PAM-5.
Jaime E. Kardontchik