Re: Equalization
Dear colleagues,
(Vipul - I hope to answer some of your questions).
The 802.3 committee is very comfortable with Equalization techniques
being used to extend the bandwidth of the copper media
(100 Base-TX, 1000 Base-T). Similar techniques can be used to
extend the distance on MMF.
Using a first order linear model for fiber, we find that about 6-9 dB
of gain at 1.6GHz extends the 160 Km-Mhz MMF fiber to 300 meters
for the 4WDM solution at 850nm.
A similar extension of distance will also be observed at
1310 nm. This is implementable in standard CMOS technologies, which
makes it a very low cost and attractive addition to the overall
solution.
Considering the effects of DMD make the fiber media a higher order
system. Based on some initial data we have seen, equalization
techniques can be successfully used to equalize the effects of DMD
on MMF.
It will be very useful to define the worst-case fiber characteristics
in terms of attenuation, order of roll-off, nulls, template, etc. Has
this
been specified and is it available for analysis. Please let me know if
this information is available.
Equalization can be done at the transmitter (pre-emphasis) which
will require sending the coefficients of the fiber link from the
receiver to the transmitter during initialization. While this has other
complications, it also complicates the process on initialization/
auto-negotiation.
The preferred approach would be to do Equalization at the receiver
(as in 100 Base-TX and 1000 Base-T). The receiver initializes itself
during start-up and configures its coeffients commensurate to the
attached link. This keeps the initialization/auto-negotiation relatively
simple. The receiver can be made to be continuously adaptive
so as to track out slow variations in the overall link.
Equalization could be the way of meeting all the distance objectives
and offering higher performance to the end-user.
Best Regards,
Abhijit Phanse
Principal Engineer, Network Products,
National Semiconductor
408-721-8074, phanse@xxxxxxxxxxx
Vipul Bhatt wrote:
> Dear colleagues,
>
> 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
> patch cord.
>
> 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
> cord.
>
> 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.
>
> Thanks,
> Vipul
>
> vipul.bhatt@xxxxxxxxxxx
> (408)542-4113