RE: PAM-5 at 5 Gbaud
Jaime:
The analyses and data you (and some of Oscar's) presented are very
entertaining, and are very solid engineering work. I really enjoyed going
through all of them. I believe it is a viable technology.
However, to implement the PAM-5 coding in an optical link is a new
application, which I am not quite accustomed to it. The straightforward
digital coding is much more familiar to me. As a result, there are several
questions in my mind, which I like to address. Furthermore, I do not see
many discussions on reflector concerning PAM-5 over an optical link, and I
like to initiate one to further enhence its viability.
In the past, the multiple voltage-level coding was adopted by two LAN
standard, ATM and Ethernet. Both of them were twisted-pair applications,
and the BER were 10^-10. I proposed BER of 10^-12 in both working groups to
be consistent with LAN optical links' BER; however, for some reason, they
remain 10^-10, officially in both standards.
The BER of 10^-12 by a multi-level coding technique has not been proved in a
real application, or adopted by any standard yet. We need a vigorous
effort to prove it.
Worst, when the data rate increases from 1.0 Gbps to 10 Gbps, the BER should
improve to 10^-13 to maintain the 10x advantage in throughput.
The BER analysis in your presentation is based on the S/N ratio at the TIA
input. The system BER will include all other components in a link. If the
system BER is 10^-12, the component BER, at least, for the development
analysis, should be better, or 10^-13 and less.
It seems, the advantage of the multilevel coding is that the baud rate is
one half of the digital coding, while maintain the same data rate for both.
However, there is an extra price to pay. Not only the "timing jitter", but
also the "amplitude jitter" will affect the link reliability, or BER.
For the part of the amplitude jitter, there is no specification existing to
guide component vendors and system designers to develop a reliable products
to meet the specified BER.
The overshoot and ringing of a laser are not characterized, nor specified in
a specification. They are wide variety of amplitudes and waveforms. The
maximum allowed external noises (power noise, cross-talk, etc) are not
specified. The tolerance of the signal amplitude is not specified. In a
straightforward digital coding, only timing information is important, and it
does not require amplitude information. The minimum amplitudes are for
component functionality only, but not data bit information.
A commonly used equalizer is used to minimize the timing errors caused by
insufficient bandwidth, which is predictable. For a random, non-
characterized amplitude distortion, it will be hard to compensate.
The laser output power tolerance is around 8 dB in GbE specification for a
low cost source. To tighten it output tolerance will increase cost.
Another uncomfortable reality is that the extinction ratio of the narrowest
pulse is quite larger (more dc component) than the widest pulse (less dc
component) - near 1 dB in many cases, which is pattern dependent. This will
be a very expensive problem to resolve for a level detecting coding.
For the timing part, the PAM-5 coding has maximum of three transitions in
period defined by its baud rate. For example, at the 1.25 G Baud data rate
(800 ps period), within an 800 period, there are maximum transitions of
every 266 ps, which is equivalent of 3.7 G Baud of a NRZ coding. These
pulses are so crowed together to be near sine waves. The advantage is that
the rise time is slowed down to a sine waveform, which requires a lower
bandwidth than a pulse shape. However, it added additional disturbing
factor "Crowding Effect". When two pulses of opposite polarity with
different amplitude, or width crowded together (super impose each other),
the timing (the location of the peak) and amplitude will be altered. The
larger one will dominate the smaller one. This is neither the ISI caused by
bandwidth deficiency, nor the amplitude jitter discussed in the above
paragraph. It is a third disturbing factor can be minimized by
pre-compensation requiring advanced knowledge of the data pattern.
I believe, the channel-to-channel skew will be generated not only from fiber
due to the wavelength difference, but mostly from chips, and PC board.
Therefore, we will need
de-skewing any way, and the fiber length will be more likely constrained by
the fiber bandwidth.
One last comment: I believe the bandwidth of an optical link is determined
by the system rise time equation, 0.8 T = (tT^2 + tF^2 + tR^2)^0.5 to assure
a reliable performance. Although the Nyquist theorem defines a filter
bandwidth related to the bit rate of a NRZ data, for the PAM-5 is not NRZ
data, which has a maximum of three transitions per a period, the Nyquist
bandwidth can be adjusted to 3/2T.
There is another way to achieve an 850 nm, 150 meter installed MM fiber
solution without those issues mentioned above by using 8B/10B-4WDM method.
TIA FO 2.2.1 is introducing an improved launch method and RML fiber
bandwidth to achieve MM fiber of 380 MHz-km. Those techniques will not
increase the transceiver cost (or negligible increase). At the 8B/10B data
rate of 3.125 Gbps, the required fiber minimum bandwidth is about 2.5 GHz.
It is 80% of the bit rate, if all other bandwidth is scaled from the GbE
specification.
Therefore, the fiber length L is:
L = (380x1000)/2500 = 152 meter.
The 2.5 Gbps VCSEL transceivers are already available from Fibre Channel
products, and 4 .0 Gbps VCSEL transceivers are coming some time.
The 8B/10B coding technique is a field proven optical link coding technique
with multiple vendors supplying all parts to drive the cost down and to
maintain availability high. We can have a cost-effective 8B/10B-4WDM link
today, if market is ready.
Regards,
Edward S. Chang
NetWorth Technologies, Inc.
EChang@xxxxxxxxxxxxxxxx
Tel: (610)292-2870
Fax: (610)292-2872
-----Original Message-----
From: owner-stds-802-3-hssg@xxxxxxxx
[mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Jaime Kardontchik
Sent: Wednesday, February 16, 2000 12:41 PM
To: stds-802-3-hssg@xxxxxxxx
Subject: 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