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RE: Equalization


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.


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 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.



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