Re: PAM-5 at 5 Gbaud
Oscar,
My questions refer to a presentation you gave a month ago
in Dallas, so they deserve a prompt response. You submitted
your proposal to a strawpoll claiming a support for 500 meters
link length on installed multimode fiber, so I assume that you
were fully aware then of these issues and had answers to them.
Jaime
Jaime E. Kardontchik, Ph.D.
Micro Linear
San Jose, CA 95131
email: kardontchik.jaime@xxxxxxxxxxx
"Oscar Agazzi, Ph.D." wrote:
> Jaime,
> Thank you for raising these issues. We are fully aware of their
> relevance and we plan to give a detailed presentation at the March
> plenary addressing them.
> Thank you again for your interest
> Oscar
>
> *************************************
> Oscar E. Agazzi
> Broadcom Corp.
> 16215 Alton Parkway
> Irvine, CA 92618
> Tel (949) 450-8700
> email oea@xxxxxxxxxxxx
> *************************************
>
> >
> > Oscar,
> >
> > I have some question marks regarding your presentation in Dallas:
> >
> > "10 Gb/s PMD using PAM-5 modulation"
> > by Oscar Agazzi
> > Broadcom
> >
> > a) 5 GHz equalizer
> >
> > You use in your simulations a Decision Feedback Equalizer (DFE) at 5 GHz. You
> > mention, to support your proposal, that DFEs are also used in Fast Ethernet
> > and 1000BASE-T. However, the latter DFEs run at 125 MHz (8 nsec baud period).
> > The DFE that you are proposing must run 40 times faster (200 psec baud
> > period).
> >
> > A DFE has a feedback loop (slide # 15 in your presentation) that consists of
> > at least one adder, a 5-level slicer and the internal delay of one flip-flop.
> > The serial operations in this feedback loop (addition + slicer + internal
> > delay of the flip-flop) have to be completed within one baud period, in this
> > case 200 psec.
> >
> > There was a very heated debate within the 1000BASE-T Task Force two years ago
> > whether the DFE could be implemented at 125 MHz. I remember that during these
> > debates you and Broadcom vehemently sustained that it would be extremely
> > difficult to implement the feedback loop in 8 nsec. Now you propose to
> > implement it in 200 psec.
> >
> > I have doubts whether this DFE could be moved from the world of simulations
> > into a real implemented system. And in CMOS, as slide # 2 of your
> > presentation seems to suggest. Even using parallel processing.
> >
> > For comparison, the architecture I proposed, PAM-5 4-WDM at
> > 1.25 Gbaud, using the 1000BASE-T PCS, (see my presentations
> > in Kauai and Dallas) has two options:
> >
> > 1) Viterbi decoding, with 6 db coding gain
> > 2) symbol-by-symbol decoding, with 3 db coding gain
> >
> > There is already a significant amount of previous work
> > on fast parallel processing of Viterbi decoders that can
> > be found in the open literature. See, for example, Ref. 5
> > in my presentation in Kauai:
> >
> > H. David, G. Fettweis and H. Meyr
> > "A CMOS IC for Gb/s Viterbi decoding: System design
> > and VLSI implementation"
> > IEEE Trans on VLSI Systems, vol 4, pp 17-31, March 96
> >
> > Specifically, following the detailed guidelines of this Ref,
> > the complete Viterbi decoder can be implemented using a
> > 312.5 MHz clock (3.2 nsec clock period). This is also a very
> > handy clock, since we need it anyway in the parallel interface.
> > These 3.2 nsec are enough to implement the path metrics
> > update, which is the bottleneck in fast Viterbi decoders.
> >
> > However, I also suggested to you that we could propose
> > in the 10 GbE Task Force to use the 3-dB coding option
> > of this PCS, if you prefer. The 3-dB coding option does not
> > use Viterbi decoding.
> >
> > The burdens on the receiver analog front end of your proposal are even more
> > daunting.
> >
> > b) 5 GHz ADC
> >
> > The main claim of your proposal is that it can reach 500 meters of installed
> > multimode fiber (500 MHz*km bandwidth)
> >
> > At 5 Gbaud and 1300 nm wavelength the optical eye pattern of PAM-5 is
> > completely closed even before reaching the 200 meters link length.
> >
> > At 500 meters the ISI (Inter Symbol Interference) is as bad or worse than the
> > ISI we get in Fast Ethernet using 100 meters of cat-5 Copper wire. In Fast
> > Ethernet we needed a true 6-bit (64 levels) ADC for the DFE to be able to
> > deal with this strong ISI.
> >
> > Slice # 15 of your presentation shows an ADC.
> >
> > I think that you will have to use at least a 6-bit ADC in your system. This
> > also looks extremely difficult to implement at 5 GHz. For example, in the
> > last International Solid-State Circuits Conference held this month in San
> > Francisco, the maximum sampling rate achieved by a nominal 6-bit CMOS ADC was
> > 800 Msamples/s (only 5-bit effective using a 200 MHz signal). It was
> > fabricated in a 0.25 um process.
> >
> > For comparison, PAM-5 4-WDM at 1.25 Gbaud does not have
> > any ISI up to 400 meters and uses an 18 level "soft slicer".
> > This is barely a 4-bit ADC. And it is sampled at 1.25 Gbaud.
> >
> > All the simulations I presented in Kauai were obtained using
> > this simple 18-level ADC. And, as I showed in Part IV of the
> > presentation, 18 levels are enough to reach an actual coding
> > gain close enough to the ideal.
> >
> > This should not come as a surprise. 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. 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 get more
> > accurate decisions).
> >
> > c) Dynamic range of the Receiver Analog Front End
> >
> > You will need 5 Gbaud Transimpedance Amplifiers (TIA) and AGCs (slice # 15 of
> > your presentation). What should be the needed dynamic range of these blocks ?
> >
> > A 6-bit ADC means about 36 dB dynamic range:
> >
> > 20*log(64) = 36 dB
> >
> > However, you would need to add some margin in your design of the analog front
> > end. This means, you will need TIAs and AGC at 5 Gbaud with a dynamic range
> > of about 41-46 dB. This also looks extremely adventurous to propose in CMOS
> > (and I would add, in any technology).
> >
> > On the other hand, using PAM-5 at 1.25 Gbaud, and
> > remembering that:
> >
> > 20*log(18) = 25 dB
> >
> > we will need TIAs and AGCs at 1.25 Gbaud with a
> > dynamic range of only 30-35 dB.
> >
> > All the above place an interrogation mark on the technical viability of the
> > serial PAM-5 approach at 5 Gbaud.
> >
> > I doubt if the HSSG members were aware of these technicalities when they
> > rushed to a strawpoll in Dallas, specially since you did not post your
> > presentation in the web site before the Dallas meeting for a peer preview.
> > This did not give the HSSG members a fair chance to take a critical look at
> > your proposal.
> >
> > Jaime
> >
> > Jaime E. Kardontchik
> > Micro Linear
> > San Jose, CA 95131
> > email: kardontchik.jaime@xxxxxxxxxxx