Re: equalization
Vipal and All:
You did a great initiation in equalization issue. There are more complex
issues than originally thought. Those are very natural development to find
more problems, while we are discussing. Do we have time, or we do not have
time is quite an arbitrary decision depending what is the criteria. In
general, market will not disappear, just because equalization issues will
take more time to come up as enhancement later.
The basic effect of optimum launch is to remove modal BW from OFL to optimize
BW, but nothing to do with curing DMD. Therefore, we should not count on
optimum launch to reduce DMD. However, optimum launch proposed by FOTP-203
happens to concentrate over 75% of flux within 15 um radius, and it can help
to minimize DMD effect. Nevertheless, it was not designed for the irregular
frequency response caused by DMD; therefore, it may not help equalization
objective.
The reasons:
Equalization is required at the corner frequency and the high frequency
attenuation region, but not the flat low frequency region. The accurate
prediction of the corner frequencies (DMD has multiple fc), and the shapes of
attenuation slopes (DMD can have multiple slopes) are required to design
effective equalizers. The optimum launch by FOTP-203 has nothing to do with
all these requirements. We have to spend time to characterize the frequency
responses of the installed MM fibers, which is our task, but not FO2.2.1. It
simply will need time to do it. It can be done, if we have years of time to
do it as an enhancement.
Even if we have time to do equalization as enhancement, we still need 5-PMDs,
but not 2-PMDs, or 3-PMDs. There are time-to-market issue, and variety of
technology options to stimulate competition issues. The serial approach
needs many new technologies, which CWDM/WWDM do not need. The monotonic
serial approach will be lacking challenge from the readily available
CWDM/WWDM approach; as a result, users will suffer, which will affect the 10
GbE market growth.
One thing we do not know yet is what is the price we are going to pay for the
complex equalization, if we get one? The average users will weigh cost,
availability, timel-to-reach market issues. We are dealing the whole
technology and marketing issues. To over simplify the PMD issue as a pure
mathematicaly-met issue is too easy.
Regards,
Ed Chang
NetWorth Technologies, Inc.
Upon reflection, I think it is possible that signal distortion for
multimode fibers will be less troublesome than that created by
wireless multipath. For one thing, by restricting the excitation of
modes to a controlled subset, we may not see the occurrence of
multipath effect. The pulse may not split; it may just spread
according to the classical bandlimiting effect. In that case, we can
use spectral compensation to overcome ISI. Second, even if you see
the multipath effect, it may vary in time so slowly that we can
treat it as time invariant, unlike wireless multipath.
Of course, I agree that before we raise our hopes, we need to
scrutinize this idea.
If more than 25% of us think that there isn't enough time to reach a
judgment about this equalization idea because we need to move on
quickly, I will drop this idea. Before that, I would like us to give
the DSP folks a chance to talk to us. This is a potential solution
that can meet Objectives 1 and 2, and keep the total number of PMDs
to just 2 or 3.
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