For the Echo, NEXT and FEXT canceller filter orders @ 625 and 833 MHz
pls refer to p.9:
I believe, the figures for NEXT and FEXT could be an overkill, however, I did not
attempt further optimization at this time.
Vivek Telang wrote:
My question was specifically about the Echo
and NEXT cancelers, since these are filters that are relatively simple
and well-understood, and not about the FFE-DFE, for which there are
many different implementation approaches. I appreciate your detailed
answer on the FFE-DFE complexity, but on my specific EC-NC complexity
question, you left me hanging with a vague "reasonably below 6.7". And by the way, the complexity factor is 45x, not 6.7x,
because of the symbol rate increase of 6.7x. So what you really mean is
"reasonably below 45x".
But now that we're talking about the FFE/DFE
complexity, let's use your numbers. If
you need a 24-tap EQ @ 833MHz for 10GBASE-T, compared to a 12-tap EQ @
125MHz for 1000BASE-T, by my calculations that's a 2x6.7 ~= 13x
increase in complexity for the FFE/DFE.
It is not possible for me know the actual number you may have
used as a reference in 1000BASE-T to obtain the x6.7 metric for
the respective filter order increase in 10GBASE-T.
I simply noticed that the x6.7 figure got somehow caught on in
the SG, and had been used so far as an indicator for the filter
order growth in 833 MHz sys. I see this figure as overly conservative.
If, for example, we assume a 12-tap FFE-DFE in 1000B-T, the 6.7
factor would result in 80-tap FFE-DFE in 10GB-T @ 833 MHz.
A very reasonable ISI cancellation could be achieved with just
24-tap EQ. So, a factor of 2 vs 6.7 would make a substantial
difference for implementation. (Coincidentally, a large, although
quoted as non-optimized, 80-tap FFE-DFE appears in the joint Jul.
vendor presentation.) Simulation shows, that in the case of Echo,
NEXT and FEXT the orders do not scale as favorably as 2, nevertheless
the respective factors still remain reasonably below 6.7.
Vivek Telang wrote:
I have to disagree. I think the 45x
complexity (again, I said this is a first-order approximation) *does*
apply to the cancellers. You are assuming that the "system resource
sharing and advanced DSP techniques" are not already being used in the
1000BASE-T PHYs that are being shipped today. Unless I missed
something, the techniques that I saw in the presentations are not
unique to the 10GBASE-T system. Considering that today's 1000BASE-T
PHYs have power numbers that are an order of magnitude lower than those
in '99, you should assume that many of these techniques are already
being employed today.
If there was something in the techniques
that was specific to the 10GBASE-T proposal, please point me to the
I don't dispute that many good techniques employed in 1000BASE-T would be
applicable to 10GBASE-T. In fact, this is what I see in my analysis as well.
DSP feasibility is an important point that would need to be properly
addressed. However, the DSP filter orders do not necessarily follow
the x6.7 rule.
There have been at least two presentations, which showed that with a
relatively simple processing in the analog front end, the DSP burden
could be substantially reduced. Further, there would be savings from
the system resource sharing and the use of advance DSP techniques.
Vivek Telang wrote:
To re-iterate the point that Dan made
yesterday, I would add to the feasibility list:
DSP feasibility for the above configurations.
Note that going from 100BASE-TX to
1000BASE-T, the symbol rate (125MBaud) didn't change, and the DSP
processing period was a relatively healthy 8ns. In the proposed
transition from 1000BASE-T to 10GBASE-T, the symbol rate goes up from
125MBaud to 833MBaud, which means the DSP processing period is reduced
from 8ns to 1.2ns (a factor of 6.7). Also, note the double whammy
in that the number of cancellation filter taps (Echo and NEXT) for the
same coverage goes *up* by the same factor (x6.7). So one could argue
that to the first order the canceller complexity is ~45x the canceller
complexity of 1000BASE-T. Of course, the processes that will be used
for 10GBASE-T will be faster than those used for 1000BASE-T, but it's
an issue that requires some thought and discussion before we're all