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RE: Equalization


We don't have "a lot of time to play around" on "interesting research

The TIA FO2.2.1 effort addresses 2 issues in worst case MMF.  1) The
specification of 75% or more of the light within a 15um radius improves the
modal bandwidth.  2) The specification of less than 25% within a 5um radius
limits the effect of defects present in the center of the fiber (much as the
offset patch cord does for 1310nm where the defects have much more severe
effects).  These 2 conditions improve the modal bandwidth of "worst-case"
MMF from 160MHz-km to 385MHz-km with a very high degree of confidence.  Link
simulations indicate that a 385MHz-km modal bandwidth fiber will support a
850nm serial link with a reach of 65-75 meters.  Also, the link simulations
indicate 10Gig serial transmission can go over 100m when the modal bandwidth
reaches ~650-700MHz-km (not 800MHz-km as in Ed's email).  Thus it only
requires about 5dB of equalization gain and the TIA-FO2.2.1-specified launch
to reach 100m over worst-case 62.5um fiber with an 850nm serial link.  This
modest gain is rendered even more reasonable by the fact that the restricted
launch helps to avoid the fiber-center defects.

The 5dB value was determined via the Pisi column in the link simulator.  At
100m for a 385MHz-km fiber, the Pisi was less than 5dB in excess of the
"acceptable" 3.0-3.6dB.  Thus improvement of the Pisi by 5dB brings the isi
back to an acceptable level.

Do we agree on the above as a reasonable means for estimating the
equalization gain?


-----Original Message-----
From: Edward Chang [mailto:edward.chang@xxxxxxxxxxxxxxxx]
Sent: Thursday, July 20, 2000 7:06 AM
To: Jay Hoge
Cc: stds-802-3-hssg@xxxxxxxx
Subject: RE: Equalization


The spot size (affect the flux density) versus bandwidth has been
experimented by TIA FO2.2.1 group and the recommended 15 um radius is an
optimum size.

However the intensity may be improved to further increase the bandwidth to
realistic point. The optimum launch is to increase bandwidth from the OFL
BW, and due to its finite launch area instead of a point, the DMD is
minimized.  The optimum launch is not designed to cure DMD.  Furthermore,
DMD is caused by the defected refractive index profile, which is random; as
a result, it is very hard to characterize DMD for equalization purpose.

However, it could be a interesting research project, if we have a lot of
time to ply around.


Edward S. Chang
NetWorth Technologies, Inc.
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 Jay Hoge
Sent: Thursday, July 20, 2000 3:33 AM
To: edward.chang@xxxxxxxxxxxxxxxx
Cc: stds-802-3-hssg@xxxxxxxx
Subject: RE: Equalization


While reading your comments on line equalization, a
question occured to me. Suppose we could tighten up
the spot size of the 850 solutions. Could we then use
selective launch techniques to get around DMD related
BW limitations to a degree sufficient to allow a
meaningful increase in reach? I have virtually no
experience with selective launch in MMF, but I don't
see why, in principal, it wouldn't work.

Also, has anyone tried selective launch techniques
with spot sizes less than 62.5u, but greater than 10u.
Such a system might speed alignment, but still gain
some BW.

Does any of this make sense as a potential comprimise?


--- Edward Chang <edward.chang@xxxxxxxxxxxxxxxx>
> Comments:
> I have a different view from that of equalizer being
> able to improve the
> installed MM fiber bandwidth, and distance.
> First of all, if all installed 62.5 um MM fibers do
> not have DMD problem,
> then the modal bandwidth will be over 1000 MHz-km at
> 850 nm, which will
> enable CWDM to reach 600 meter, or serial 850 nm to
> reach over 150 meters.
> The reason for the low BW of 62.5 um MM fiber, at
> 160 MHz-km (OFL), or 385
> MHz-km (RML) is the DMD effects, which causes
> bandwidth reduction as the
> resultant effects of the imperfection in the
> refractive-index profile.  The
> off-set patch code, or Vertex launch can improve the
> bandwidth by avoiding
> the bad spots of the fiber's refrective-indrx
> profile (or minimize the DMD
> effect), and restore a major portion of the original
> BW.
> The DMD comes with all kind of IRREGULAR multiple
> pulses propagating in a
> DMD fiber, and there are no predictable bandwidth,
> amplitude, and phase
> relations at all, which are the foundation for
> determining the EQUALIZER
> design parameters.  Some DMD fibers come with
> multiple BWs, showing multiple
> frequency response curves, which is not for an
> equalizers to compensate.
> Furthermore, the DMD effects can change during data
> transferring, if the
> fiber is touched or moved to cause DMD pattern
> changes -- BW changes.
> Although, one can argue to implement a continuously
> correcting equalizer
> such as PLL loop in phase-correction, the range of
> DMD is rather wild, and
> not characterized by any means; as a result, a
> dynamic equalizer can not
> have the target design specification to design for.
> To characterize the DMD
> of all the installed MM fibers is almost impossible.
> To make 850 nm serial transceiver reach 100 meter,
> the EMB of an installed
> fiber has to reach 800 MHz-km, which is near DMD
> free.  Can we use an
> equalizer to make majority of the low bandwidth, or
> DMD fibers to reach 800
> MHz-km?  I will not spend my time in this research
> project.
> The practical way is to use the TIA task FO2.2.1
> "Optimum launch" to avoid
> OFL and minimize DMD affects to achieve high
> bandwidth.  Furthermore, with
> the artificially controlled Vertex launch, the
> optimum bandwidth can be
> further assured.  Nevertheless, to provide 800
> MHz-km BW for all the
> installed 62.5 MM fibers is just too much.
> Regards,
> Edward S. Chang
> NetWorth Technologies, Inc.
> EChang@xxxxxxxxxxxxxxxx
> Tel: (610)292-2870
> Fax: (610)292-2872
> Vipul,
> I agree with you. Equalization can indeed overcome
> DMD with
> an appropriate receiver that can handle the
> multipath problem.
> The Equalizers used in the wireless industry for
> multipath tend
> to be very complex and may not be implementable at
> 10Gbps.
> However, simpler Equalizers can provide adequate
> performance.
> The initialization of the Equalizer can be blind
> like in 1000 Base-T
> i.e. the receiver does not need any training
> sequence. This  prevents
> the need for complex auto-negotiation. Also, the
> Equalizer can
> automatically recalibrate so that it can adapt to
> any time varying
> effects if such conditions should arise.
> Regards,
> -Sudeep
> Vipul Bhatt wrote:
> > Dear colleagues,
> >
> > As we think about 10G on installed MMF, there is
> one issue we
> > haven't discussed - equalization. Perhaps thinking
> about it will
> > throw more light and provide another perspective.
> >
> > In theory at least, equalization looks very
> promising. Even at 850
> > nm, it can permit a Serial PHY to operate over 100
> meters on
> > installed MMF, or a WWDM PHY to operate over 300
> meters on installed
> > MMF. At 1310 nm, longer distances can be achieved.
> (Ignoring DMD for
> > the moment, and using a linear system single pole
> approximation, a
> > 20 dB equalization gain will make a 100 meter
> segment of installed
> > MMF look like a 16 GHz channel at 850 nm.) It can
> be cost
> > effective - a single BiCMOS chip with DSP on CMOS,
> and receiver
> > preamplifier in SiGe. It may even eliminate the
> mode conditioning
> > patch cord.
> >
> > In reality, there are a couple of challenges,
> applicable to both 850
> > nm and 1310 nm cases.
> >
> > 1. DMD: Can equalization overcome DMD? Some have
> suggested that DMD
> > can be modeled as a multipath effect, something
> that the folks in
> > wireless industry know how to deal with. When
> viewed in terms of a
> > transversal filter, the multipath problem boils
> down to having
> > enough taps and setting their coefficients. And if
> we can undo the
> > DMD effect with an IC, we can eliminate the mode
> conditioning patch
> > cord.
> >
> > 2. Initialization: After power on, are a few
> milliseconds of
> > randomized A/K/R enough to initialize the
> equalizer? Can we assume
> > that equalizer will not need to be re-calibrated
> after that? We
> > don't want the tail wagging the dog - equalization
> should not
> > require complex Auto Negotiation.
> >
> > Though new to fiber optics, electrical
> equalization is a
> > tried-and-proven concept. We will see more of it
> as our hunger for
> > bandwidth continues to outpace our ability to
> replace installed
> > low-bandwidth media. Fiber optic folks had the
> luxury of ignoring it
> > because fiber bandwidth was plentiful - until now.
> >
> > I am asking if this idea is worth discussing.
> >
> > Thanks,
> > Vipul
> >
> > vipul.bhatt@xxxxxxxxxxx
> > (408)542-4113

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