128GFC preliminary link budget studies

128GFC preliminary link budget studies
April 2014
T11/14-100v1
Preliminary study of link budget sensitivities

In T11/14-021v0 we identified the following parameters to
be of special interest for 128GFC link budget:

Receiver sensitivity
Tx min and max OMA
Q (forward error correction signal-to-noise, uncorrected)
VCSEL spectral width ∆λ
Start with 32GFC spreadsheet T11/12-376v0
“M5E 850S4500 32GFC 2xCDR” worksheet
Note: T11 Docs doesn’t have a copy of this; need to ask me if
you don’t have a copy from 2012 Boise meeting
Vary each parameter of interest, leaving all the others at nominal
value for 32GFC
Plot impact on link reach

Repeat for 100GBASE-SR4 spreadsheet
2
Link reach vs. receiver sensitivity
3
Rx sensitivity comments

0 dB on the previous plot means the 128GFC
receiver sensitivity is identical to the 32GFC
sensitivity (-10.2 dBm; 10-6 BER; 28.05 Gbd) or to the
100GBASE-SR4 sensitivity (-11.2 dBm; 5x10-5 BER;
25.78 Gbd).

My initial expectation was to see a degradation in
sensitivity for a 4-channel transceiver (whether
QSFP+ or CFP4) because of increased crosstalk
from neighboring lanes.

As we’ll see in later slides, this is not the most critical
parameter for a 4-lane solution.
4
Link reach vs. Tx min OMA

The previous plot was given from the perspective of
impact of receiver sensitivity on link reach.

However, this same analysis also applies to impact of
transmitter min OMA on link reach.

It might be conceivable to compensate for degraded
receiver sensitivity by increasing Tx min OMA on a
128GFC link.

However, ”Case 2” for breakout to individual 32GFC
SFP+ transceivers won’t be compensated. There
would be a link budget discrepancy between “Case 1”
and “Case 2”.
5
Link reach vs. uncorrected signal-to-noise Q
6
Warning: Rx sensitivity not adjusted

This analysis only adjusted the Q parameter box in
the spreadsheet (see above)

Rx sensitivity is dependent upon uncorrected BER
allowed; we will calculate that later in this
presentation

The following analysis did not adjust Rx sensitivity.

With adjustment of Rx sensitivity, the dependence
shown would be even more pronounced.
7
Link reach vs. VCSEL spectral width
8
Comparison of link budgets
Parameter
100GBASE-SR4
FC-PI-6
Tx min OMA
-3.0
-3.2
Rx sensitivity
-11.2
-10.2
Power budget
8.2
7.0
Connector loss
1.50
1.50
Fiber attenuation
0.36
0.36
ISI
3.16
3.14
DJ_center
1.76
0.93
MPN
0.11
0.15
RIN
0.74
0.61
MN
0.13
0.14
Cross
0.44
0.19
8.20
7.02
Total
9
Comparison of transmitter specifications
Parameter
100GBASE-SR4
FC-PI-6
Min Tx wavelength (nm)
840
840
rms spectral width (nm)
0.60
0.57
Min Tx OMA (dBm)
-3.0
-3.2
Tx risetime (20-80, ps)
21.0
20.8
RIN(OMA) (dB/Hz)
-128
-129
DJ & TP4 eye (ps)
21.4
14.15
DCD_DJ (ps)
1.94
0.00
10
Comparison of receiver specifications
Parameter
100GBASE-SR4
FC-PI-6
Nominal sensitivity (dBm)
-11.20
(5x10‐5 BER; 25.78 Gbd)
-10.20
(10‐6 BER; 28.05 Gbd)
Receiver bandwidth (GHz)
18.047
20.0
11
Scaling receiver sensitivity

100GBASE-SR4 sensitivity is better than FC-PI-6
sensitivity mostly because of
Lower signaling rate
50 times higher uncorrected bit error tolerance

Q for 100GBASE-SR4 is 3.89, Q for FC-PI-6 is 4.75
10 log10 (4.75 / 3.89) = 0.867 dB

Signaling rate for 100GBASE-SR4 is 25.781 Gbd,
signaling rate for FC-PI-6 is 28.050 Gbd
10 log10 (28.050 / 25.781) = 0.366 dB
12
Initial search for 128GFC link budget

Option #1: Start with 100GBASE-SR4
Bump up signaling rate
Was 25.781 Gbd
Should be 28.050 Gbd

Option #2: Start with FC-PI-6
Bump up link DJ + CDR allocation
Was 14.15 ps & 0.0 ps
Should be 21.4 ps & 1.94 ps (?)

Results summarized on next page
13
Initial efforts at 128GFC
Parameter
Tx min OMA
Rx sensitivity
Power budget
Connector loss
Fiber
attenuation
ISI
DJ_center
MPN
RIN
MN
Cross
Total
Total - budget
100GBASESR4
FC-PI-6
100GBASESR4
scaled
FC-PI-6
+ more DJ
-3.0
-11.2
8.2
1.50
0.36
-3.2
-10.2
7.0
1.50
0.36
8.2
1.50
0.36
7.0
1.50
0.36
3.16
1.76
0.11
0.74
0.13
0.44
8.20
3.14
0.93
0.15
0.61
0.14
0.19
7.02
3.96
2.30
0.14
1.64
0.13
1.79
11.82
3.65
2.23
0.18
1.82
0.14
0.99
10.83
0.00
0.00
3.62
3.83
14
Summary and next steps
The
single biggest difference for a 4-lane
PHY compared with a single channel PHY is
“DJ” contribution to link penalties.
A
link budget for 100GBASE-SR4 has been
presented.
We
are still struggling to find ways to close
the budget for 128GFC.
15

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