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### PMD and PMD compensationNOBEL WP5 results

- Yu Rong Zhou
- Alfons Schinabeck
- Stefano Santoni
- Andrew Lord
- Bernd Bollenz
- Thomas Fischer

NOBEL_WP5_PMD_summary_draft_4_extended.ppt

PMD > threshold

OSNR> threshold

Curve:

Q-penalty vs PMD

Table:

Q-penalty vs PMD / Q

Representationin model

PMD / PMDC models OverviewPMD / Q dependency

- Receiver (NRZ, CS-RZ, DB)
- Equalisers (NRZ, CS-RZ, DB) (FFE+DFE, MLSE=VE)

PMD: 1st order

1st+2nd

multi-order

PMD orders

- Equaliser S
- (FFE + DFE)

- Receiver
- 1stage/2stage PMDC

- Receiver (AT)
- In-line compensator S E

S: NW simulation

E: Experiment

PMD > threshold

OSNR> threshold

Curve:

Q-penalty vs PMD

Table:

Q-penalty vs PMD / Q

Representationin model

PMD/Receiver modelsPMD / Q dependency

- Receiver (NRZ, CS-RZ, DB) see mitigation
- Equalisers (NRZ, CS-RZ, DB) (FFE+DFE, MLSE=VE)

PMD: 1st order

1st+2nd

multi-order

PMD orders

- Equaliser (FFE + DFE)

- Receiver
- 1stage/2stage PMDC

- Receiver (AT)
- In-line compensator

NRZ 10Gb/s

PMD penalties (all-orders) evaluation- EOP evaluation modelling the fiber with the wave plates approach (all-orders PMD) and numerical simulation (Split-Step Fourier method)
- EOP vs. instantaneous DGD (100,000 realisations) for RZ and NRZ
- EOP pdf for NRZ and RZ

NRZ 10Gb/s

PMD penalties (all-orders) evaluationResult (table representation)

- Q penalty vs. baseline Q for different meanDGD (OP = 10-5 , 10Gb/s NRZ signal)

RZ

NRZ

RZ

PMD penalties (all-orders) evaluation Impact of fiber non-linearity- Comparison with simulations including non linear (Kerr) effects (0dBm)
- G.652 (top) and G.655 (bottom) cases
- NRZ (left) and RZ 30 ps FWHM (right)
- Comparing EOP due to non linear effects and to PMD (blue line) with EOP due to PMD only (red line):
- EOP due to non linear effects and to PMD can be linearly added for NRZ signals
- Cumulative EOP due to non linear effects and to PMD is less than the linear sum of the two independent components, for RZ signals

Comparison of different PMD modelling approaches

- Analytical model for 1st order PMD:
- Q-factor penalty as a function
- of PMD and outage probability (OP):
- A: pulse factor, B: bit rate,

: mean DGD

- Comparisonof approaches:
- Wave plate approach (all order PMD)
- Analytical (1st order PMD)
- split-step (1st order PMD)
- Good agreement of all different approaches for 10Gb/s NRZ signal
- Analytic model giving efficient calculation with sufficient accuracy for baseline Q value relevant to system applications

PMD > threshold

OSNR> threshold

Curve:

Q-penalty vs PMD

Table:

Q-penalty vs PMD / Q

Representationin model

PMD mitigationPMD / Q dependency

- Receiver (NRZ, CS-RZ, DB)
- Equalisers (NRZ, CS-RZ, DB)(FFE+DFE, MLSE=VE)

PMD: 1st order

1st+2nd

multi-order

PMD orders

- Equaliser
- (FFE + DFE)

- Receiver
- 1stage/2stage PMDC

- Receiver (AT)
- In-line compensator

PMD mitigationInvestigated approaches

- optical PMDC
- 1stage
- 2stage
- in-line(distributed
- el. Equalizer
- FFE+DFE
- MLSE (=VE)

PMD-, Q-thresholds from literatur

details on next pages

Q-penalty vs. PMD curves andQ-p. vs. (Q,PMD) for FFE+DFE

details on next pages

Q threshold

(referenced to ATC receiver)

PMD rules with optical compensators (1stage, 2stage)- PMD thresholds are based on literature values (10-5 outage) (multi-order PMD simulations)
- Near-optimum feedback signal (eye monitor) for 2 stage device
- Q threshold referenced to ATC receiver (w/o. PMD)

DOP [dB]

EOP [dB]

PMD

PMD + CD + NL

DOP

PMD in-line mitigation /1- In-line PMD mitigation (optical, bit-rate independent approach)
- Simulation on EOP and DOP correlation
- DOP < 0.9 is a condition to limit the penalty below around 2 dB

EOP [dB]

PMD in-line mitigation /2

- EOP – DOP correlation
- Possible behaviour of DOP along the link
- Pulses depolarisation can be caused by both first and second order PMD (in this cases, first order is dominant)

PMD

PMD + CD + NL

EOP [dB]

PMD in-line mitigation /35 x 100 Km

Compensation at receiver

- DOP degrades along the link. The energy causing ISI can be no longer discriminated from the energy within the bit slot based on the polarisation. As a consequence, the performance improvement is limited.

In-line compensation

- DOP is maintained high (> 0.9) along the link and pulses are confined in the bit slot

5 x 100 Km

Physical terminal designElectronic equalisation / Receiver

- Dynamic electronic signal processing in receiver
- for PMD / distortion mitigation
- ensures maximum length optical paths under dynamically changing path conditions in dynamic optical networks
- Most-likely equalisation schemes identified
- Feed-forward + decision feedback equal. (FFE+DFE) analog processing
- Viterbi equaliser (VE, also referred to as MLSE) digital processing

FFE + DFE

Viterbi equaliser (MLSD)

FFE+DFE

ATC

ATC

NRZ

VE

duobinary

VE1

VE2

VE

CSRZ

PMD rules without and with mitigation by electronic equalisers- Performance analysed: Q-penalty vs. DGD
- equalisers: FFE+DFE, VEas reference: Receiver w. adaptive threshold control (ATC)
- modulation formats: NRZ, duobinary, CSRZ

CM-DML

NRZ

PMD rules with mitigation by MLSE- MLSE model for network simulation
- VE with 4 states and 3 ADC bits for 10.7 Gb/s
- Assumption: PMD 1st order is the dominant effect for NRZ, ODB, CM-DML
- Figures show PMD penalty afterMLSE related to b-t-b with equaliser for each modulation format
- Parameter DGD; Chromatic dispersion: 0 ps/nm

pdf of 1st + 2nd order PMD

CP (BER>limit)

BER=

limit

SOPMD/PMD2

0

4

DGD/PMD

PMD rules of FFE+DFE equaliserbased on refined PMD model- In detail: FFE+DFE equaliser model for 10Gb/s NRZ
- BER limit trace in 1st and 2nd order PMD plane; given PMD, OSNR
- Integration of outage probability OP; Iteration: OP=10-5 by OSNR variation
- Table quantifies: PMD improvement by equaliser / margin

PMD rules for FFE+DFE PMD equaliser

PMD > threshold

OSNR> threshold

Curve:

Q-penalty vs PMD

Table:

Q-penalty vs PMD / Q

Representationin model

Experimental evaluation of PMDCPMD / Q dependency

- Receiver (NRZ, CS-RZ, DB)
- Equalisers (NRZ, CS-RZ, DB) (FFE+DFE, MLSE=VE)

PMD: 1st order

1st+2nd

multi-order

PMD orders

- Equaliser
- (FFE + DFE)

- Receiver
- 1stage/2stage PMDC

- Receiver (AT)
- In-line compensator

PMD-C Measurement results

Measurement setup:

Measurement results:

System parameters:

- Modulation format NRZ
- Bitrate 10.7 Gbit
- BER w/o FEC 1e-6

Conclusion:

- Compensation possible with the polarizer approach at 10 Gbit
- Can compensate 4.7 ps mean PMD (2 dB OSNR penalty)

PMD > threshold

OSNR> threshold

Curve:

Q-penalty vs PMD

Table:

Q-penalty vs PMD / Q

Representationin model

Network simulations w. PMDPMD / Q dependency

- Receiver (NRZ, CS-RZ, DB)
- Equalisers (NRZ, CS-RZ, DB) (FFE+DFE, MLSE=VE)

PMD: 1st order

1st+2nd

multi-order

PMD orders

- Equaliser
- (FFE + DFE)

- Receiver
- 1stage/2stage PMDC

- Receiver (AT)
- In-line compensator

Network simulation (RWA)

Modeled for standard receivers and for equalisers FFE5+DFE1

Static dimensioning simulations on DT-17nodes network, with random fiber PMD coefficient distribution:

Avg. links load with standard Rx: 72%

Avg. links load with equaliser FFE5+DFE1: 79%

And consequently the network dimensioning with equaliser results in less node relations

Equaliser provides more flexibility in the route selection (more routing options with Q-factor higher than the accepted threshold) thus enabling a more efficient network optimisation.

Possible approach for impairments-aware RWA including PMDC, Equalisers and 3R

Network simulations with scaled PMD and with different PMD distribution (and other some pessimistic assumptions) on DT-17 network show blocking due to impairments (no transparent routing possible)

Possible extension of RWA to overcome blocking events due to physical impairments

Modified Dijkstra algorithm including information on different mitigation methods, together with a strategy to properly assign resources (regenerators, equalisers, PMDCs), trying to maintain transparent routing in a cost-effective way

Algorithm description

- In its iterative process the Dijkstra routing algorithm starts from a node that is reachable and tries to move to adjacent nodes: in the impairments-aware routing, to consider a node reachable the Q factor shall remain over the selected threshold. Comparison between Q and Q-threshold refers to the total Q-factor of the lightpath segment or to a single component of the Q factor, e.g. Q-penalty due to PMD greater than few dBs).
- If Q is below the threshold a blocking occurs and, in this case, instead of discarding the path routing under analysis (as in D26 simulations), a possible solution can be selected, among:
- Putting an equaliser (or any other compensation technique) at the receiver
- Inserting an in-line PMDC
- Inserting a Regenerator (no transparent routing, last choice)
- It should be noted that all the rest of the network is unknown at this point (from the algorithm point of view) and decisions on how to solve possible Q-related blocking is not optimal, since cannot be based on the knowledge of the whole path/network
- In case all of the listed solutions can solve the blocking, the iterative Dijkstra process can continue. In order not to select just one solution (that in few next steps can become apparently the ‘worst’ one) all the three possible solutions are kept and independently ‘propagated’ with proper ‘labelling’ in order to keep trace of them
- For each kind of label (Regen, PMDC, EQUAL), in case a certain node is reached with different paths the path with the best Q-factor is selected. Since this procedure applies independently for each label, a certain node can be reached from a subpath ‘x’ adopting Regen and with subpath ‘y’ adopting PMDC
- The multiple labelling is kept and propagated till another blocking event occurs: in this case a single solution to solve the previous blocking has to be selected (otherwise the alternatives can grow exponentially), according to a predefined rule (e.g. minimum cost) and the same process applies.
- As a final result a single lightpath can be routed, for instance, with a regenerator and a couple of in-line PMDCs, or with any other combination. At this point a post-analysis can be performed in order to optimise Regenerators/PMDCs placement

A

A

A

Z

Z

Z

Z

Network example /1Comparison between Q and Qthreshold could refer to the total Qfactor of the lightpath segment or to a single component of the Q factor (e.g. Qpenalty due to PMD greater than few dBs)

Q > Qthreshold

Q < Qthreshold

Label type

Node not yet reached

Q > Qthreshold

Q < Qthreshold

lightpath

a

i

j

To overcome the blocking, one among the three approaches below can be chosen

Equaliser at RX

b

i

j

In-line PMDC

g

i

j

Regenerator

i

j

d

A

A

Z

Z

Z

Network example /2In case some solutions experience a further block while others are still valid, they are no longer considered. When only one solution remains, it is ‘promoted’ to permanent

Q < Qthreshold

Equaliser at RX

b

j

k

i

Q < Qthreshold

In-line PMDC

g

j

i

k

Regenerator

d

j

i

k

‘promoted’ to permanent solution

Network example /4

x

y

i

in case more than one subpath with the same label reach a node, the subpath associated to the best Q will be selected and further propagated

w

Possible approach for impairments-aware RWA including PMDC, Equalisers and 3R

Preliminary considerations

- This procedure only applies to static network dimensioning. Extension to dynamic dimensioning requires further evaluations
- This method fits with heuristic approaches as the multi-layer graph RWA adopted in D26
- The method does not reach an optimum solution, but a post-elaboration with optimisation is possible (once the preferred path has been chosen, actual placement of PMDC/Regenerator can be optimised and some over-dimensioning removed)
- Mixing up different mitigation techniques, some configurations resulting form the algorithm could need further analysis
- Modelling of some components (e.g. PMDC) not yet available for implementation of the RWA algorithm. Moreover the computation of Qpenalty due to PMD in the different configurations should take into account other mitigation techniques possibly applied on the path
- In principle more solutions can be included (different mitigation solution at receiver can be computed, provided that a proper modelling in term of Q-factor is available)

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