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Refining TDECQ
Piers Dawe
Mellanox
Introduction• A simple reference receiver will reduce cost in
measurement (search time for TDECQ) but also in some real receiver implementations, as explained in sun_3cd_01a_0118, which showed that more than two precursor taps is not necessary
• This presentation looks at whether 0, 1 and 2 precursors are all desirable in a reference equalizer
• Also, starts to consider how to ensure transmitter quality
P802.3cd March 2018 Refining TDECQ 2
Slides 1 to 10 are the almost the
same as dawe_022818_3cd_adhoc
Slowest time-symmetric SMF signal• A simulated signal is created with a fourth-order Bessel-
Thomson filter, bandwidth chosen to set SECQ to 3.4 dB (the highest spec limit for any SMF PMD in 802.3bs or P802.3cd)
• No noise, jitter, distortion or emphasis. Any reasonable signal must be faster than this to make room for noise, jitter and distortion. This includes 100G/s/lane signals, relative to the unit interval
• If this were a 100GBASE-LR4 signal, its TDP would be 3.2 dB: too slow (spec is 2.2 dB for 100GBASE-LR4, 2.5 dB for 100GBASE-ER4)– So we expect that real 50G/s/lane signals will be faster than this
anyway
• Most other possible responses (filter types) would have a relatively faster attack and slower decay than the time-symmetric signal– See later for discussion of chromatic and modal dispersion
P802.3cd March 2018 Refining TDECQ 3
•
1 1.5 2 2.5 33
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
4.8
5Estimate of SECQ (dBo)
Cursor position
Est
imate
of
SE
CQ
(d
Bo
)
Refining TDECQ
Slow, symmetrical
0 1 2 3 4 5 6-0.2
0
0.2
0.4
0.6
0.8
1
After Tx and fb/2 BT4 filter
Time (UI)
Resp
on
se
, u
nit p
uls
e
For this extremely slow signal,
positions 2 and 3 have almost the
same SECQ => don't need 2nd
precursor for this signal if we have 3rd
postcursor
-2 -1 0 1 2 3 4-1
-0.5
0
0.5
1
1.5
2Tap weights for different cursor positions
Tap position relative to cursor (UI) (direction?)
Tap
weig
ht
Same transmitter in 25G PAM2 mode, 19.34 GHz BT4
UI at 25.78125 GBd
0 0.2 0.4 0.6 0.8 1 1.2 1.4
0
0.2
0.4
0.6
0.8
1
After Tx and fb/2 BT4 filter
0 0.2 0.4 0.6 0.8 1 1.2 1.4
0
0.2
0.4
0.6
0.8
1
This waveform needs
one precursor and
three postcursors
2nd precursor would be about as strong as 3rd postcursor
BT4 filter as Tx, as slow as allowed for SMF
P802.3cd March 2018 4
Slow, nearly symmetrical
Brown red orange
TDP would be 3.2
Cursor is >1.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Eyes after reference equalizer
Cursor at position 2 Cursor at position 3
Real signals are faster than this and not so clean
P802.3cd March 2018 Refining TDECQ 5
Corrected histogram windows; showing thresholds
Other worst-case waveforms
• A first order filter (faster attack than decay) gave the same conclusion
• Even slower waveforms with moderate 2-tap Tx FFE – same conclusion
• Would any of these waveforms have been acceptable with the original T/2-spaced equalizer?
P802.3cd March 2018 Refining TDECQ 6
Chromatic dispersion?
• Could a signal be that slow AND have enough chirp on some edges (not necessarily rising vs. falling), enough to make it significantly asymmetric after the fb/2 BT4 filter? A DML?
• High chirp goes with fast edges, so such a transmitter would have a high chromatic dispersion penalty if used in a PAM2, non-equalised link
• Reasonable, or a corner case the standard and the receivers don't need to go out of their way to support?
P802.3cd March 2018 Refining TDECQ 7
What about MMF, with its higher TDECQ limit?
• Also, modal dispersion– Contained by the fibre and modal launch specs
– Modal bandwidth is significantly more than the reference bandwidth in the receiver
• Not addressed here – for further study
P802.3cd March 2018 Refining TDECQ 8
What about the opposite: fast but "dirty" signals?
• While (OMA-TDECQ) controls the net useful signal strength,
• TDECQ doesn't control the net signal quality
• Conceptually – TDECQ with Ceq fixed to a constant, would
• We need something to ensure that the small opening in the eye is a reasonable proportion of the signal size – to do the job of the VEC spec in C2M
• There is a related problem with strongly over-emphasised signals that would require "inverting" FFE settings that no copper equalizer would need
• A simple way to mitigate this problem is a minimum cursor tap weight spec, e.g. 0.9
P802.3cd March 2018 Refining TDECQ 9
Conclusion so far
• The reference equalizer for SMF should not include the case with two precursor taps (cursor in third position) because it would be expensive to provide in some real equalizer architectures, would add search time to TDECQ measurement, and does not benefit reasonable waveforms– There might be some super-slow waveforms
(which would have failed e.g. 100GBASE-LR4) that might get slightly worse TDECQ; marking them down will help the standard and real receivers
P802.3cd March 2018 Refining TDECQ 10
Other maximum-TDECQ signals
• Next, looking at the variety of bad signals and considering where the limits of compliance should be
P802.3cd March 2018 Refining TDECQ 11
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
-1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
Ro
ugh
nes
s an
d s
ign
al's
no
ise
p
en
alti
es (
dB
o)
Slowness penalty (dBo)
SMF TDECQ limit
No patterning ornoise
Dirty
SRS 1/2 from filtering
Construction line
SRS signals
MMF TDECQ limit
TDECQ map
Different kinds of bad signal 12
TDECQ (dBo)
Ideal waveform Half the SECQ
from filtering
Slowest, as on
slides 2 to 5
P802.3cd March 2018
<- fast slow ->
<-
open
a
fter
FF
E c
losed -
>
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
-1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
Ro
ugh
nes
s an
d s
ign
al's
no
ise
p
en
alti
es (
dB
o)
Slowness penalty (dBo)
SMF TDECQ limit
No patterning ornoise
Dirty
SRS 1/2 from filtering
Construction line
SRS signals
MMF TDECQ limit
Mismatch between SRS and real signals?
Different kinds of bad signal 13
TDECQ (dBo)
Ideal waveform Half the SECQ
from filtering
Slowest, as on
slides 2 to 5
So SRS signal must be
somewhere in this range
Where will real poor
signals be? Here?
P802.3cd March 2018
<- fast slow ->
<-
open
a
fter
FF
E c
losed -
>
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
-1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
Ro
ugh
nes
s an
d s
ign
al's
no
ise
p
en
alti
es (
dB
o)
Slowness penalty (dBo)
SMF TDECQ limit
No patterning ornoise
Dirty
SRS 1/2 from filtering
Construction line
SRS signals
MMF TDECQ limit
Don't support unrealistic bad scenarios
Different kinds of bad signal 14
TDECQ (dBo)
Ideal waveform Half the SECQ
from filtering
Slowest, as on
slides 2 to 5
So SRS signal must be
somewhere in this range
A region like this
should be excluded
because it requires
strong tap weights not
useful in practice
A region like this should
be excluded because
the eye after FFE is very
closed, and small
amounts of e.g.
nonlinearity would cause
big additional penalties
(cliff edge)
Like VEC issue in C2M
<-
open
a
fter
FF
E c
losed -
>
P802.3cd March 2018
<- fast slow ->
Where will real poor
signals be? Here?
A region like this
should be
excluded
because it
requires
significant tap
weights of the
opposite sign to
normal
"Exclusion" could be by giving signals in the red boxes
worse TDECQ scores, or by "hard" pass-fail rules
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
-1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
Ro
ugh
nes
s an
d s
ign
al's
no
ise
p
en
alti
es (
dB
o)
Slowness penalty (dBo)
SMF TDECQ limit
No patterning ornoiseDirty
SRS 1/2 from filtering
Construction line
SRS signals
MMF TDECQ limit
Bad ISI
Don't support unrealistic bad scenarios
Different kinds of bad signal 15
TDECQ (dBo)
Ideal waveform Half the SECQ
from filtering
Slowest, as on
slides 2 to 5
So SRS signal must be
somewhere in this range
A region like this
should be excluded
because it requires
strong tap weights not
useful in practice
A region like this should
be excluded because
the eye after FFE is very
closed, and small
amounts of e.g.
nonlinearity would cause
big additional penalties
(cliff edge)
Like VEC issue in C2M
<-
open
a
fter
FF
E c
losed -
>
P802.3cd March 2018
<- fast slow ->
Where will real poor
signals be? Here?
A region like this
should be
excluded
because it
requires
significant tap
weights of the
opposite sign to
normal
Example of a fast but
bad signal on next slide
"Exclusion" could be by giving signals in the red boxes
worse TDECQ scores, or by "hard" pass-fail rules
0 0.2 0.4 0.6 0.8 1 1.2 1.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
A tidy-looking fast bad eye after reference equalizer
• Compare slide 5 – vertical eye opening is half as much here, although both signals have same SECQ, 3.4 dB
• Worse signals are allowed by Draft 3.1 (up and to the left on the map), even for SMF
• Worse again for MMF where the draft TDECQ limit is 4.9 dB
P802.3cd March 2018 Refining TDECQ 16
Like the earlier bad signal, this one needs only one precursor tap