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
Does any of this make sense as a potential comprimise?
--- Edward Chang <edward.chang@xxxxxxxxxxxxxxxx>
> 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
> 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
> 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.
> Edward S. Chang
> NetWorth Technologies, Inc.
> Tel: (610)292-2870
> Fax: (610)292-2872
> 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
> However, simpler Equalizers can provide adequate
> 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.
> 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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