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RE: Equalization


You have really caught my attention on this one. What exactly is a "
time-variant impulse?"


>-----Original Message-----
>From: Vipul Bhatt [mailto:vipul.bhatt@xxxxxxxxxxx]
>Sent: Thursday, July 20, 2000 1:46 PM
>Subject: RE: Equalization
>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
>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.