Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

re: missing MAS......


Some additional thoughts based on your latest.
>yes, in a strict sense it is true that one cannot correctly use "Baud" and 
>"Hertz" interchangeably for multi-level AM.  but it is frequently convenient 
>to refer to a fundamental line rate in Hertz to illustrate a point of
>efficiency.  i believe that is what Rich intended in his earlier comment.

I believe you are right, but this fundamental line rate
would only be 1/2 the baud rate in the trivial case of
a repeating 10101010101010101 sequence which has been
filtered to remove all but the fundamental.  A signal
such as this would be of little use for transmission
over multimode fiber.  Therefore, to emphasize this
"fundamental" frequency ignores the fundamentals of optical
fiber transmission.

>nevertheless if one can pack more information into the same symbol
>space that is consumed by a two-level line code, you realize a net reduction 
>in the minimum half-power frequency that is needed to transmit error-free 
>(error-reduced, if you prefer) information.  you may quibble over the
>of "true" fundamental frequency if you like -- perhaps through debate of 
>coding scheme, maximum allowed run length, and so on -- but the net 
>effect stands.

My intent in pointing out this error was not to quibble,
but to remove a possible source of misunderstanding.  It
is clear to me the necessary transmitter bandwidth for the
PAM5 MAS proposal is ~ 5.5GHz.  See Rich Taborek's recent
e-mails to this effect.  Part of the increase over the 
theoretical minimum of .5 is to offset the dispersive nature 
of the fiber media.
>i don't understand why you believe that would be true, but i suspect the
>only way we can make progress on this is to compare power spectra for
>both 2-PAM (which is OOK) and 5-PAM, say, with a 512-bit length PRBS
>or the like.  i suspect someone in the HSSG studio audience will have
>done that by now.

See above for explanation.

>now it is -my- turn to quibble -- "baud" means "symbol rate", so by 
>extension "baud rate" is "symbol rate rate", which is either redundant 
>or really does refer to a rate of a rate.  i don't understand the latter.

Guilty as charged, but I meant well.

>otherwise, FEC is meant to be implemented in what i call the 'back
>end' section of the PHY, where most signal processing can be done
>in parallel at a rate which isn't nearly as high as the line rate.  there
>is no 'free lunch' in this choice -- FEC will restrict the number of 
>possible 'next states' (if you like) in a code sequence -- but with it we 
>realize lower signal processing burden and higher effective SNR.
>i see this as a significant advantage over 2-PAM at 10Gbaud.

Yes, FEC has some compelling advantages
over simpler schemes such as 8B/10B.

>i don't know where you get your numbers, but it is my understanding
>that edge-emitters must be assembled (connectorized?) before they 
>can be tested.  until they are tested, you don't know whether they
>pass a minimum design spec.  for those that don't pass, you throw
>away laser dice -and- a package, -after- you've spent the time in 
>finished-part test.  VCSELs can be tested at wafer sort and you 
>only use good dice.  are you saying that you know of a better wafer
>sort test or better process control for side emitters (or both)?

Not at all.  I am only making a point with
regard to the advantages of volume which one
must consider when selecting components.

The advantages in processing of surface emitters
are as you have named them.  At present, their
volumes are not sufficient to make them less costly
than their edge emitting counterparts. 

>to the extent that my view is correct, VCSELs are cheaper than 
>side emitters.  otherwise, my claim stands -- the cost of parallel
>data processing in back-end, mainstream digital silicon is cheap
>relative to the cost of high-performance optics and fiber.  and that
>is the path everyone wants.  

Yes.  Everyone wants it cheap.  Cost effective
sounds so much better, though.  One component
which will meet your cost targets you might want 
to consider is a 1300nm VCSEL.

There is even work ongoing to integrate an external 
absorption modulator positioned as a cap layer(s)
on the surface of the 1300nm VCSEL. 

There is no reason to link 1300nm transmission with
edge emitting lasers when considering future 10Gb
standards.  The literature is replete with examples.

>on top of that, fiber folks will tell you that us datacom types have 
>been wasting bandwidth for far too long.

I think the potential benefits of bandwidth reducing
techniques such as MAS are very exciting, yet I also
know there will be difficulties.  I also look forward
to witnessing a demonstration of a 10Gbit capable MAS
transmitter/receiver in the near future.  This would 
go a long way towards dispelling some the doubts I have
about laser linearity, etc.

>my point is this:  implementation of MAS, in whatever kind of line
>code you like, does not depend on wavelength.  therefore, if one can
>implement (C or D)WDM at all, and if one can implement MAS at all, 
>then one can certainly consider implementing MAS on (C or D)WDM.  
>if one can think of any means to implement CDR for more than one 
>serial lane of data traffic on the copper side of a PHY, one can certainly 
>consider applying that method to the optic side, with MAS, through 
>(C or D)WDM.  for what we know right now, choice of more than one 
>wavelength does not preclude consideration of one or another choice 
>of modulation for all.

I may have failed to express a point clearly.
The difference between MAS and WDM lies in the
need to skew compensate the individual WDM channels
because of differential delays in the fiber due to
chromatic dispersion, etc.  After the skew is removed,
only they may then be reassembled into a single channel.

This need is not present where MAS is used to increase
channel bandwidth, due to the fact only a single laser 
is used to transmit the entire signal.  This is a big
plus for MAS relative to WDM in my opinion.

>the moment that one first says "it cannot be done" is frequently the 
>same moment one is proven wrong.
Agreed.  Didn't someone once claim the "Titanic" 
was unsinkable?

Best Wishes to all and a Happy New Year,

Pat Gilliland