Re: Equalization
Ed,
The characterization has already been done by FO-2.2. I believe Mike
Hackert's working group has a ton of data on the frequency response of
RML compliant fibers.
Gair
Edward Chang wrote:
>
> Rohit:
>
> It is interesting. You ,may like to share your results with us?
>
> However, fiber dispersion and DMD is not exactly the same thing.
> The linear dispersion proportional to cable length is straight forward, the
> dispersion related to the non-linear DMD is different. We need
> characterization.
>
> 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 Rohit Sharma
> Sent: Thursday, July 20, 2000 9:21 PM
> To: HSSG
> Subject: RE: Equalization
>
> i believe DMD can be (has been) modelled as a Rayleigh multipath fading
> channel and fast Decision Feedback Equalization (DFE) techniques with RLS
> (Recursive Least Squares) or LMS (least mean squares) algorithms applied for
> rapid convergence with a short training sequence (# of bits roughly equal to
> the # of taps). J. Winters and cohorts in IEEE Transaction (1990 - will dig
> out exact ref. tomorrow) and Stanford ISL with John Cioffi did a bunch of
> work in this area in 1991-95 timeframe.
>
> part of my grad studies demonstrated an 11 (goes to 11!) tap adaptive
> optoelectronic filter to equalize dispersion in mmf for 1 and 2.5 Gbps IMDD
> signals. Similar results were shown by other researdch groups using
> electrical equalization with bipolar taps and DFE structures. some basic
> simulation results are available (and were published in Optics Letters) and
> if anyone is interested, send me email and that will prompt me to dig
> through any relevant results i have...
>
> -rohit
>
> Rohit Sharma
> ONI Systems.
>
> > -----Original Message-----
> > From: Jonathan Thatcher
> > [mailto:Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx]
> > Sent: Thursday, July 20, 2000 4:07 PM
> > To: 'vipul.bhatt@xxxxxxxxxxx'; HSSG
> > Subject: RE: Equalization
> >
> >
> >
> > Vipul,
> >
> > You have really caught my attention on this one. What exactly is a "
> > time-variant impulse?"
> >
> > jonathan
> >
> > >-----Original Message-----
> > >From: Vipul Bhatt [mailto:vipul.bhatt@xxxxxxxxxxx]
> > >Sent: Thursday, July 20, 2000 1:46 PM
> > >To: HSSG
> > >Subject: RE: Equalization
> > >
> > >
> > >
> > >Rich,
> > >
> > >I was skeptical, like you, about the ability of equalization to
> > >overcome DMD. I still am. But what made me more open minded is my
> > >willingness to examine two key propositions: DMD behavior can be
> > >approximated as that of a fading multipath channel. And equalization
> > >can overcome the effect of a fading multipath channel.
> > >
> > >Why fading multipath? Consider this. A multimode fiber with DMD is
> > >seen by an optical signal as a channel that propagates various
> > >portions of its energy (modes) through a refractive index profile
> > >that is sharply different (at places) than intended. The propagation
> > >velocity of a mode depends on refractive index. Over distance, on
> > >average, some modes will have a cumulative average of a low (lower
> > >than intended) refractive index path, thereby abnormally decreasing
> > >their path delay, while others may not. In some bad cases, a bit
> > >arriving at the receiver is almost split in two or three replicas.
> > >And the amplitude of each replica is changing dynamically. Guess
> > >what, that is not far from the behavior of a fading multipath
> > >channel. Equalizers that deal with this phenomenon are used in TV
> > >de-ghosting circuits and digital radio.
> > >
> > >To an equalization expert, DMD may not look so challenging - it's
> > >just another channel with randomly time-variant impulse
> > >response...:-)
> > >
> > >But then, we both are putting the cart before the horse. Let's wait
> > >to hear some presentations from equalization experts. Even if it can
> > >be done, I will want to know whether it can be done cost-effectively
> > >and on time. Our discussion should serve as a guide to them about
> > >what we would like to hear.
> > >
> > >Thanks,
> > >Vipul
> > >
> >
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