From experimental tests to link
engineering: an operator perspective
ECOC 2014
Orange Labs & Networks
JeanJean-Luc Augé
Tests protocol
Determination of the linear and
nonlinear parameters:
Transponder back to back
Anl parameter
Introduction
Why testing transponders?
•
Performance comparison of different vendors solution, beyond the marketing story.
However, transponder performance is only a part of the system performance!
•
Build an engineering expertise independent of vendor design tools:
- “what if ?” analysis, network evolution anticipation, specific use cases…
- Design verification
- Alien wavelengths engineering: there is little support from vendors on alien engineering and
performance prediction : with good in house expertise, the operator can save on field trials.
From experiment to simulations:
We will focus on the exploitation of experimental results with the GNLI model
•
Determination of the non linear parameter Anl
•
Discussion on the relative importance of Anl and OSNR sensibility
Test bed
Orange Lab generic testbed: not vendor
specific, up to 20x100km DCU free
transmission
35 x 100G@ 28Gbd 35 x 100G@ 28Gbd
100 km
100G under
evaluation
100 km
100 km
100 km
100 km
100 km
100 km
100 km
100 km
100 km 100 km
OSA
OSA
100 km
Tx
DGE
DGE
WSS
100G channels
comb
100 km 100 km
100 km
100 km 100 km
100 km
100 km
100 km
Rx
Transponder under
test in its shelf and
OADM termination
Client tester
ASE
Vendor system manager
BER / Q reading
Vendor independent
post FEC errors checking
Stress the system to noise and non linear limits: non linear measurements
•
@ different amplifier powers (1dB steps)
•
@ different OSNR (noise loading in 1dB steps)
•
Record the resulting BER (Q) / OSNR vs Pch abacus
Results exploitation with the GNLI model
Consider the NLI noise as gaussian and additive
PNLI = Anl ⋅ Pch3
or
ε
PNLI = anl ⋅ N span
Pch3
SNR =
Pch
PASE + PNLI
P. Poggiolini et al., “Analytical modeling of non-linear propagation in uncompensated optical transmission links”, PTL 2011
Anl & ε are determined from experimental results
Anl = f ( Pchamplifier , OSNR OSA , QvendorTRX )
amplifier tuning
Anl (dB) for 10, 15 & 20 spans
10 spans
Pch=0.2dBm
Pch=1.2dBm
Pch=2.2dBm
Pch=3.3dBm
2
2
1,1dB
0
1,9dB
-2
-4
0
5
10
relative OSNR (dB)
Anl (dB)
15 spans
Anl vs span count
1,0dB
relative Anl (dB)
20 spans
measured
+noise loading
0
ε ~ 1,4
-2
vs GNLI in Nyquist WDM:
ε~1
-4
10
11
12
N spans (dB)
Anl uncertainty and parameters correction: see backup slides
 SNRlin

∆SNRdB ≈ 
− 1 ⋅ ∆AnldB
 OSNRlin 
Mo.4.3. modeling & system design session: P. Poggiolini & al, Impact
of Low-OSNR Operation on the Performance of Advanced Coherent
Optical Transmission Systems
13
Tests results exploitation:
what parameters can we
trust?
-Anl measurement uncertainty
-little Anl variation between
transponders and in the line (mixed
transponders)
⇒ Is
Anl a good performance indicator
to compare transponders ?
Anl vs Non Linear Threshold (NLT) and amplifier tuning
20 spans, Q vs Pch at different OSNR levels
Q (dB)
11,0
Q for 21dB SL
10,0
Q for 24dB SL
9,0
PchNLT is defined as the optimum channel
power at a fixed number of spans N
8,0
PchNLT
dependency
with OSNR
7,0
6,0
5,0
-3
-2
-1
0
1
2
3
PchNLT =
4
1
2 ⋅ Anl ⋅ OSNR0
5
Pch (dBm)
⇒ OSNR0 (B2B transponder response) is as much as Anl contributing to the NLT
⇒ What about amplifiers tuning? Amplifiers are not tuned to PchNLT but to the power required for
maximum reach at a given (fixed) span loss and can be derived from a simple calculation:
We introduce the following variables and consider anl dependency with span count
•
•
•
•
Anl = anl ⋅ N ε
(OSNR0,Q0) defined as the system minimum Q for the operator: Q0 = FEClimit+operatormargin. Q0 is also used to define
the transponder NLT.
∆margin are the system margins
OSNRT is the system OSNR target: OSNRT=OSNR0+∆margin
span ASE contribution Aspan = NF ⋅ SL ⋅ hυ∆υ
We look at the power engineering Pch required to achieve maximum reach:
2+ε
N max =
Pch
2 +ε
ε∈ ~[1 - 1,5]
1
1
⋅
2anl ⋅ OSNR03 (Aspan ⋅ ∆ m arg in )2
⇒ weak dependency of the reach with anl
OSNR0ε −1
ε
=
⋅ (Aspan ⋅ ∆ m arg in )
2anl
⇒ weak dependency of the power
settings with OSNR sensibility
Illustration over an example: 100G SDFEC vs HDFEC
Example of two imaginary transponders with
•
•
•
HDFEC & SDFEC back top back
Q (dB)
12
11
10
•
Q B2B HDFEC
9
8
⇒ Same Anl but different NLT
⇒ Performance difference almost equal to the OSNR
difference !
⇒ Anl is only part of the story…
HDFEC
limit
Q B2B SDFEC
7
SDFEC
limit
6
12
13
14
15
16
OSNR (dB)
17
18
19
the same back to back Q/OSNR response
same Anl and ε = 1,3 (not withstanding baud rate difference)
but different FEC limit (OSNR sensitivity) : 8,5dB Q (HDFEC) & 6,5dB Q
(SDFEC).
we consider operation at 1,5dB Q margin (just a use case)
N spans NLT (given by PchNLT)
maximum span loss for a given reach => not an
engineering comparison
Large NLT improvement ( ½ ∆OSNRlimit)
fixed 20 spans reach (variable SL)
Q (dB)
line amplifiers tuning (Pch) (previous calculation)
maximum reach for the same span loss => real
engineering comparison
Similar Pch: can be mixed with the same engineering!
fixed 21dB span loss (variable reach)
Q HDFEC 21dB SL
11,0
Q (dB)
11,0
Q HDFEC 20 spans
10,0
Q SDFEC 30 spans
Q SDFEC 24dB SL
10,0
Q B2B HDFEC
9,0
HDFEC
limit
Q B2B SDFEC
8,0
SDFEC
limit
HDFEC
limit
1dB ∆P
vs
2dB Q
7,0
6,0
almost same
power
8,0 requirement
7,0
SDFEC
limit
6,0
5,0
5,0
-3
-2
-1
Q SDFEC 20 spans,
24dB SL
9,0
0
1
2
Pch (dBm)
3
4
5
-3
-2
-1
0
1
2
Pch (dBm)
3
4
5
wrap up & discussion
A set of (Anl, ε) constants may be determined by experiments and used to compare
transponders from different vendors in the same conditions
Is it a key differentiator? considering:
•
•
similar anl between modulation formats (BPSK, QPSK, 16QAM…) at same baud rate
anl baud rate dependency but today transponders share similar baud rate (from 28Gbd to 32Gbd for
100Gb/s) and are mixed over the same line, averaging the differences (XPM from all transponders)
Reach & power settings:
N
2 +ε
Pch
1
1
=
⋅
2anl ⋅ OSNR03 (Aspan ⋅ ∆ m arg in )2
2 +ε
ε −1
OSNR0
ε
=
⋅ (Aspan ⋅ ∆ m arg in )
2anl
N3 =
if ε ~> 1 (Nyquist WDM?)
1
1
⋅
2anl ⋅ OSNR03 (Aspan ⋅ ∆ m arg in )2
Pch3 =
1
⋅ Aspan ⋅ ∆ m arg in
2anl
no Pch dependency with OSNR0 (transponder B2B):
⇒ Same power settings for all 100G QPSK / 200G 16QAM transponders in the network
Strong dependency of maximum reach to OSNR0, whereas Anl is a weaker contributor
⇒ The linear back to back response is a powerful indicator to compare the same type
of transponders from different vendors
Not withstanding different fiber types or transponder with significant baud rate difference
Non linear post compensation (back propagation) could also change this statement
BACKUP
Anl uncertainty
Pch correction
Can we trust the results?
Q factor
Chose a unique value for Anl to cover all noise and power conditions for a given span count
Fit over N spans
⇒ good fit with |∆Q|<0.2dB for 1dB Anl spread
Q response over N spans and fit
⇒ a quick uncertainty calculation confirms that
the need for Anl precision is moderate
B2B
13,5
Pch=0.2dBm
Pch=1.2dBm
linearly:
Pch=2.2dBm
dSNR  SNR
 dAnl
=
− 1 ⋅
SNR  OSNR  Anl
Pch=3.3dBm
B2B fit
0.2dBm NLI fit
1.2dBm NLI fit
At non linear threshold (dSNR/dP = 0), SNR/OSNR = 0,66,
hence in dB (at low dAnl/Anl values):
2.2dBm NLI fit
5
3.3dBm NLI fit
10
15,7
21,4
OSNR
∆SNR ≈ −0.33 ⋅ ∆Anl
∆Q(dB) ≈ −0.33 ⋅ ∆Anl (dB)
In return, measurement points at SNR~OSNR (low Pch or span #) are less precise for
the determination of Anl: for example ∆Anl > 3∆Q below the non linear threshold.
Weight each contributions (Pch, Q, OSNR measurement triplet) by its SNR/OSNR ratio
and do not use data with OSNR/SNR < 1,3 (reminder OSNR/SNR=1,5 at NLT)
⇒ measure Anl at high power (better accuracy)
⇒ Use the model below NLT (less impact of Anl errors)
Pch correction: Rx noise loading & power mode amplifiers
We use power mode amplifiers in our labs PTOT = Psig+PASE = Cte, whereas in the field, smart
gain mode amplifiers are used: ASE noise build up power and may contribute to excess NL
We use Rx noise loading : little impact with power mode amplifiers, unlike if we were using gain
mode amplifiers
Pch include in line ASE noise and NLI: correction is needed
• B is the signal bandwidth ratio to 0,1nm (~3) because NLI
noise is mostly contained in signal bandwidth
• B’ is calculated for a 90 channels amplifier bandwidth with
only 60 channels in use (=90/60*0,4/0,1)
Pch = PTOTamplifier / 60
Psig = Pch − B'⋅PASEligne − B ⋅ PNLI
Consider Pch for NLI contribution because ASE and NLI noise contribute
PNLI = Anl ⋅ Pch3
OSNRTOTAL =
SNR =
Mo.4.3. modeling & system design session: P. Poggiolini & al, Impact of Low-OSNR
Operation on the Performance of Advanced Coherent Optical Transmission Systems
Psig + B ⋅ PNLI
The measured OSNR with OSA include some PNLI noise!
PASEtotal
Psig
PASEtotal + Anl ⋅ Pch3
correcting factor at low Q factor
and low line OSNR
Anl =
1
Pch 2
 1

1

⋅ 
−
 SNR OSNRTOTAL 


B'
1 + B +

 SNR OSNR 
ligne 
