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
> >
>