Re: Equalization
Rich,
With the use of the TIA encircled flux and RML bandwidth specifications,
equalization on multimode fiber should be much easier. The two specs
essentially make it a much different type of fiber.
Gair
Rich Taborek wrote:
>
> Jack,
>
> I agree with the gain estimate. However, this only frames the problem.
> We must then find a solution.
>
> I'm assuming that the 5db gain you refer to to counter ISI effects would
> be provided by equalization. Equalization for electrical systems to get
> back 5db is relatively straightforward. The primary difficulty is the
> implementation at 10 Gbps speeds. However, multimode fiber is a
> significantly different beast than an electrical trace. This is the $64K
> question of this equalization thread. Can an equalization system be
> specified to RELIABLY counter the effects of multimode dispersion even
> to the tune of 5db?
>
> I'll note that when I worked at Transcendata on multi-level encoding, we
> did consider equalization. It appeared that equalization would be a slam
> dunk for electrical and single-mode fiber. Multi mode... was another
> story. I agree with Oscar Agazzi that multi-level encoding and
> equalization are orthogonal and both can be used to increase supported
> distances, so the question remains.
>
> Best Regards,
> Rich
>
> --
>
> Jack Jewell wrote:
> >
> > Ed/Jay,
> >
> > We don't have "a lot of time to play around" on "interesting research
> > projects."
> >
> > 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?
> >
> > Regards,
> > Jack
> >
> > -----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
> >
> > Jay:
> >
> > 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.
> >
> > 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 Jay Hoge
> > Sent: Thursday, July 20, 2000 3:33 AM
> > To: edward.chang@xxxxxxxxxxxxxxxx
> > Cc: stds-802-3-hssg@xxxxxxxx
> > Subject: RE: Equalization
> >
> > Ed,
> >
> > 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?
> >
> > Jay
> >
> > --- Edward Chang <edward.chang@xxxxxxxxxxxxxxxx>
> > wrote:
> > >
> > > 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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>
> -------------------------------------------------------
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