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RE: Equalization


Not a punishment, just something I've been noodling over and wasn't sure if
we were ready to ask.

Specifically, I have been curious about the potential for the DMD response
(assumes no launch control) to be non-deterministic. In short, what happens
to that thing we used to call "modal noise" in a DMD-sensitive system?

It seems likely that a simple experiment could be created if one had a BERT,
Scope, LX transceiver, and a spool of the "bad DMD fiber" to see if the
general shape of the pulse into the Rx would change under various types of
"abuse." I'm not talking about dynamic changes like wrapping it around a
mandrel or anything that would not be "typical" in an installed
infrastructure. Just fiber blowing in the wind, the temperature of the Tx
changing... stuff like that. 

Now, you could argue that these types of things would be "random" in nature.
Even so, if the response of the system to these changes is significantly
non-linear, then the effect could end up unmanageable. I hope otherwise.


>-----Original Message-----
>From: Vipul Bhatt [mailto:vipul.bhatt@xxxxxxxxxxx]
>Sent: Thursday, July 20, 2000 5:25 PM
>To: Jonathan Thatcher; HSSG
>Subject: RE: Equalization
>If I didn't know better, I would have thought you are punishing me
>for starting this thread!
>Actually, this one is easy, almost elementary. (I suspect you read
>"impulse response" as "impulse", and  the 7th cup of coffee made you
>quickly pull the trigger...)
>Still, here goes. Simple models of communications links assume the
>channel to be linear, time-invariant. "Channel" is copper cable,
>fiber, air, whatever. In time domain, we describe its behavior with
>an impulse response. If this impulse response, the h(t) curve, is
>the same no matter when you look, we call it a time-invariant
>response. The impulse response of a singlemode fiber, for example,
>can be called time-invariant. If not, it's time-variant. (Purists,
>please cut me some slack; I know, nothing is truly time-invariant.)
>If it is time-variant, we have to ask if it varies randomly or
>My contention was that a multimode fiber with DMD can be viewed as a
>channel whose impulse response is changing with time in a random
>fashion. Hence the phrase "randomly time-variant impulse response".
>> -----Original Message-----
>> From: Jonathan Thatcher
>> [mailto:Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx]
>> Sent: Thursday, July 20, 2000 3:07 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