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Equalization sounds very promising, and I think this might yield a consensus solution, if it proves viable, for the distance/media objectives it would satisfy.
I've recently been poking at optical folks to try electrical-type equalization on optical systems, but they seem to be making negative noises back at me.... (rate equations not simplifiable, too many parameters to simply model). Since I do not share their pessimism, I'd like to get a bit more information, and since you brought it up, I thought I'd ask you.
- Has optical equalization been tried or deployed before by a system or component vendor? (if so, can you give a reference or two for us to chase down)
- If I wanted to get a good seminal work on optical equalization (if one exists), where would I look (paper/author/journal/year).
To summarize: YES, I believe it's worth discussing and researching.
Photonics Engineer, OPTera Metro Solutions, Nortel Networks
Phone: 613.270.5953 Fax: 613.591.2035
From: Vipul Bhatt [mailto:firstname.lastname@example.org]
Sent: Tuesday, July 18, 2000 9:57 PM
As we think about 10G on installed MMF, there is one issue we
haven't discussed - equalization. Perhaps thinking about it will
throw more light and provide another perspective.
In theory at least, equalization looks very promising. Even at 850
nm, it can permit a Serial PHY to operate over 100 meters on
installed MMF, or a WWDM PHY to operate over 300 meters on installed
MMF. At 1310 nm, longer distances can be achieved. (Ignoring DMD for
the moment, and using a linear system single pole approximation, a
20 dB equalization gain will make a 100 meter segment of installed
MMF look like a 16 GHz channel at 850 nm.) It can be cost
effective - a single BiCMOS chip with DSP on CMOS, and receiver
preamplifier in SiGe. It may even eliminate the mode conditioning
In reality, there are a couple of challenges, applicable to both 850
nm and 1310 nm cases.
1. DMD: Can equalization overcome DMD? Some have suggested that DMD
can be modeled as a multipath effect, something that the folks in
wireless industry know how to deal with. When viewed in terms of a
transversal filter, the multipath problem boils down to having
enough taps and setting their coefficients. And if we can undo the
DMD effect with an IC, we can eliminate the mode conditioning patch
2. Initialization: After power on, are a few milliseconds of
randomized A/K/R enough to initialize the equalizer? Can we assume
that equalizer will not need to be re-calibrated after that? We
don't want the tail wagging the dog - equalization should not
require complex Auto Negotiation.
Though new to fiber optics, electrical equalization is a
tried-and-proven concept. We will see more of it as our hunger for
bandwidth continues to outpace our ability to replace installed
low-bandwidth media. Fiber optic folks had the luxury of ignoring it
because fiber bandwidth was plentiful - until now.
I am asking if this idea is worth discussing.