Nonlinearity-Tolerant Modulation Formats for Coherent ...© MERL MITSUBISHI ELECTRIC RESEARCH...

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MITSUBISHI ELECTRIC RESEARCH LABORATORIESCambridge, Massachusetts

Keisuke [email protected]

Mitsubishi Electric Research Laboratories (MERL), Cambridge, MA, USA

02/23/2017 1

Nonlinearity-Tolerant Modulation Formats for Coherent Optical Fiber Communications

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MITSUBISHI ELECTRIC RESEARCH LABORATORIES

Collaborators• MERL

– Toshiaki Koike-Akino (Seminar on Jan 16)– David S. Millar (Seminar on Jan 15)– Kieran Parsons– Milutin Pajovic– Valeria Arlunno (Currently with Acacia Communications)

• Mitsubishi Electric Corp. (Ofuna, Japan)– Takashi Sugihara– Tsuyoshi Yoshida– Keisuke Matsuda– Hiroshi Miura– Keisuke Dohi

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References1. K. Kojima, T. Yoshida, T. Koike-Akino, D. S. Millar, K. Parsons, M. Pajovic, and V. Arlunno, “Nonlinearity-

tolerant four-dimensional 2A8PSK family for 5-7 bits/symbol spectral efficiency,” to be published in IEEE J. Lightwave Technol., 2017 (Invited Paper). DOI:10.1109/JLT.2017.2662942 (available in IEEE Xplore).

Main part of this talk

2. K. Kojima, T. Yoshida, K. Parsons, T. Koike-Akino, D. S. Millar, and K. Matsuda, “Nonlinearity-tolerant time domain hybrid modulation for 4-8 bits/symbol based on 2A8PSK,” to be presented at OFC, paper W4A.5, 2017.

Time-domain hybrid modulation

3. T. Yoshida, K. Matsuda, K. Kojima, H. Miura, K. Dohi, M. Pajovic, T. Koike-Akino, D. S. Millar, K. Parsons, and T. Sugiura, “Hardware-efficient Precise and Flexible Soft-demapping for Multi-Dimensional Complementary APSK Signals,” ECOC, paper Th.2.P2.SC3.27, 2016.

Hardware-efficient LLR calculation and experimental results

4. T. Koike-Akino, K. Kojima, D. S. Millar, K. Parsons, T. Yoshida, and T. Sugihara, “Pareto optimization of adaptive modulation and coding set in nonlinear fiber-optic systems,” to be published in IEEE J. Lightwave Technol., Vol.35, No.3, 2016 (Invited Paper).

Comparison of multiple modulation formats with multiple FEC code rates

5. D. S. Millar, T. Koike-Akino, S. Ö. Arik, K. Kojima, K. Parsons, T. Yoshida, and T. Sugihara, “High-dimensional modulation for coherent optical communications systems,” Optics Express, Vol.22, No.7, pp.8798-8812, 2014.

High-dimensional modulation

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Outline• MERL Overview• Introduction

– Fiber Nonlinearity– Granularity vs Efficiency– GMI (Generalized Mutual Information)

• Modulation Format– 4D-2A8PSK Family (5, 6, and 7 bits/symbol)

• Nonlinear Transmission– Simulation Procedure and Results– Hardware-Efficient Soft-Demapping and Experiment

• Time Domain Hybrid– Using 4D-2A8PSK Family (for 4.5, 5.5, 6.5, 7.5 … bits/symbol)– Simulation Results

• Summary

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MERL Overview

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• 60 leading researchers, including 4 IEEE Fellows and 2 OSA Fellowsà http://www.merl.com/peopleà http://www.merl.com/research

• Open corporate research lab, many publications in leading conferences and journals (~150 publications/year)à http://www.merl.com/publications

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• Time Domain Hybrid– Using 4D-2A8PSK Family (4.5, 5.5, 6.5, 7.5 bits/symbol)

• Simulation Results• Summary

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Optical Fiber Nonlinearity• Self Phase Modulation (SPM)

– Single channel effect• Cross Phase Modulation (XPM)

– Multi-channel effect• Cross Polarization Modulation (XPolM)

– Multi-channel effect

• 2D Constant modulus property is effective to suppress all the above three components

• 4D constant modulus property (constant power for X+Y polarizations) is effective to suppress SPM and XPM

• Legacy fiber plants (including submarine cables) use dispersion managed links (low chromatic dispersion), and they are susceptable to fiber nonlinearity.

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Reducing Fiber Nonlinearity• Polarization managed 8D modulation (2 bit/symbol)

– Shiner et al., Opt. Exp. 22 20366 (2014)• 2D constant modulus + zero DOP over 2 time slots• Coding gain due to block coding• Very effective for 3 bits/symbol and below• Not very effective beyond 3 bit/symbol

• 4D Constant modulus 4D-2A8PSK (6 bit/symbol)– Kojima et al., ECOC 2014, paper P.3.25.

– Note that similar approaches were presented recently, but the details are not described

• Reimer et al., OFC 2016, paper M3A.4.• Turukhin et al., ECOC’16, paper Tu.1.D.3.

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• Time Domain Hybrid– Using 4D-2A8PSK Family (4.5, 5.5, 6.5, 7.5 bits/symbol)– Simulation Results

• Summary

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Effect of Granularity on Efficiency

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Ghobadi et al. (Microsoft), OFC’16, M.2.J.2

Based on the study on Microsoft’s North American optical backbone, 1) Using 100G QPSK as the baseline, having the ability to support 150G 8QAM

and 200G 16QAM increases the available network capacity by up to 70%2) Adding the ability to support 25G granularity increases the available network

capacity by an additional 16%

Finer granularity leads to efficient use of network

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• Summary

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Generalized Mutual Information (GMI)• Maximum achievable bit rate given a channel, modulation format and

demodulator, with any encoding and decoding schemes.• Upper bound for a channel, modulation format and demodulator.• Useful when we want to consider standard bit-interleaved coded modulation

(BICM) systems. (Soft Decision FEC)

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Source Encoder Modulator

Channel

Soft-Decision Bit-Wise

DemodulatorDecoderSink

Fixed Free

GMI

A. Alvarado and E. Agrell, JLT 2015

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Bit Error Rate (BER)• Maximum achievable bit rate given a channel, modulation format and

demodulator, with any encoding and hard-decision decoding schemes.• Upper bound for a channel, modulation format and demodulator.• Useful when we want to consider hard-decision bit-interleaved coded

modulation (BICM) systems. (Hard Decision FEC)

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Source Encoder Modulator

Channel

Hard-Decision DemodulatorDecoderSink

Fixed Free

BER

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Q(BER) vs. GMI

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Normalized GMI0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

Q fr

om p

re-F

EC B

ER (d

B)

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

DP-QPSKDP-Star-8QAM6b4D-2A8PSKDP-16QAM

BER~4e-2

There is ~0.1 dB difference in Qat GMI = 0.85The difference becomes larger when GMI is lower

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• Summary

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6 bit/sym Modulation Formats

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• This fills the gap between DP-QPSK (4 bit/symbol) and DP-16QAM (8 bit/symbol).

• Very important these days when more granularity for distance is required• Most common format is DP-Star-8QAM

• Issues of DP-Star-8QAM• Not constant modulus – susceptible to fiber nonlinearity• Not Gray labeling – BER/GMI performance compromised

• Evaluation of several modulation formats was done by using GMI in the linear region• Rios-Mueller et al., OFC 2015

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6 bit/sym Modulation Formats

02/23/2017

(a)$Star(8QAM$ (b)$Circular(8QAM$

I-1 -0.5 0 0.5 1

Q

-1

-0.5

0

0.5

1 (1 1 1)

(1 0 1)

(0 0 1)

(0 1 1)(0 0 0)

(1 1 0)

(1 0 0)

(0 1 0)

I-1 -0.5 0 0.5 1

Q

-1

-0.5

0

0.5

1

(1 0 1)

(0 1 0)

(0 0 0)

(1 1 0)(1 0 0)

(1 1 1)

(0 1 1)

(0 0 1)

I-1 -0.5 0 0.5 1

Q

-1

-0.5

0

0.5

1

(0 0 0)

(0 0 1)(1 1 1)

(1 0 1)

(1 0 0)

(1 1 0)

(0 1 0)

(0 1 1)

(c)$8PSK$

18

(a)$Star(8QAM$ (b)$Circular(8QAM$

I-1 -0.5 0 0.5 1

Q

-1

-0.5

0

0.5

1 (1 1 1)

(1 0 1)

(0 0 1)

(0 1 1)(0 0 0)

(1 1 0)

(1 0 0)

(0 1 0)

I-1 -0.5 0 0.5 1

Q

-1

-0.5

0

0.5

1

(1 0 1)

(0 1 0)

(0 0 0)

(1 1 0)(1 0 0)

(1 1 1)

(0 1 1)

(0 0 1)

I-1 -0.5 0 0.5 1

Q

-1

-0.5

0

0.5

1

(0 0 0)

(0 0 1)(1 1 1)

(1 0 1)

(1 0 0)

(1 1 0)

(0 1 0)

(0 1 1)

(c)$8PSK$DP-Star-8QAM DP-8PSK

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Stokes Representation of 6 bits/symbol formats

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DP-Star-8QAM DP-8PSK 6b4D-2A8PSK

2D constant modulus 4D constant modulus

Euclidean distance:0.94 – 1.01

Euclidean distance: 0.76Euclidean distance: 0.92

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6b4D-2A8PSK

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I-1 -0.5 0 0.5 1

Q

-1

-0.5

0

0.5

1

x-polarization

(0 1 1)(0 1 0)

(1 1 0)

(1 1 1)

(1 0 1)(1 0 0)

(0 0 0)

(0 0 1)

2, 4, 6, 8

1, 3, 5, 7

I-1 -0.5 0 0.5 1

Q-1

-0.5

0

0.5

1

y-polarization

3

2

1

8

7

6

5

(0 1 1)

(1 1 0)

(1 1 1)(1 0 1)

(1 0 0)

(0 0 0)

(0 0 1)

4

(0 1 0)

• Derived from DP-8PSK.• If the inner circle from x-polarization is chosen, then

outer circle will be chosen from y-polarization• Euclidean distance:

• 1.01 (opt. for BER)• 0.94 (opt. for GMI under fiber nonlinearity)

• Gray labeling (coding)

-1.5 -1 -0.5 0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

1

1.5

r1r2

Ring ratio: r1/r2

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GMI vs. SNR

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GMI = 0.85

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Set-Partitioned 16QAM (4D)

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32SP-16QAM 128SP-16QAM

Renaudier et al., ECOC’12, We.1.C.5

b7 = b0⨁b1⨁b2⨁b3⨁b4⨁b5⨁b6

b0, b1

27 = 12825 = 32

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Generic 2A-8PSK Representation

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-1 0 1XI

-1.5

-1

-0.5

0

0.5

1

1.5

XQ

X-polarization

-1 0 1YI

-1.5

-1

-0.5

0

0.5

1

1.5

YQ

Y-polarizationB[0]B[1]B[2],B[6] B[3]B[4]B[5],B[7]

0-0-0,0-0-0-0,1-

0-0-1,0-0-0-1,1-

0-1-1,0-

0-1-1,1-0-1-0,0-

0-1-0,1-

1-1-0,1-

1-1-0,0-

1-1-1,1-

1-0-1,1-

1-0-0,1-

1-0-0,0-1-0-1,0-

1-1-1,0-

0-0-0,0-0-0-0,1-

0-0-1,0-

0-0-1,1-

0-1-1,0-0-1-1,1-

0-1-0,0-

0-1-0,1-

1-1-0,1-

1-1-0,0-

1-1-1,1-

1-0-1,1-

1-0-0,1-

1-0-0,0-1-0-1,0-

1-1-1,0-

B[0]-B[2]: Phase for X-polarizationB[3]-B[5]: Phase for Y-polarizationB[6]: Amplitude for X-polarizationB[7]: Amplitude for Y-polarization

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Code for 6b4D-2A8PSK

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• 6 bits/symbol 6b4D (64SP)-2A8PSK

B[0] – B[5] are information bitsB[6] – B[7] are parity bits

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Code for 7b4D-2A8PSK

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B[7] = B[6].

• 7 bits/symbol7B4D (128SP)-2A8PSK

B[0] – B[6] are information bitsB[7] is the parity bit

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Codes for 5b4D-2A8PSK

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• 5 bits/symbol5b4D (32SP)-2A8PSK

B[0] – B[4] are information bitsB[5] – B[7] are parity bits

B[5] = B[0] B[1] B[2],B[6] = B[2] B[3] B[4].B[7] = B[6].

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• Summary

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Transmission Simulation Procedures and Parameters• 34GBd, 11 ch with 37.5 GHz spacing, root raised cosine filter (roll-off = 0.1) • Two types of link

– Dispersion managed (DM) link• 25 spans x 80km NZDSF ( = 2,000 km) link• 90% inline dispersion compensation, 50% pre-compensation

– Uncompensated link• 50 spans x 80km SSMF ( = 4,000 km) link• No inline dispersion compensation, no pre-compensation

• EDFA noise loaded just before the receiver• Data-directed LMS equalizer, and minimum distance decision• PMD is assumed to be zero• Calculate the ROSNR that achieves GMI = 0.85 (normalized)• Span loss budget is used to show the margin (NF = 5 dB)

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Simulation Setup

LD Mod.

Pulse

RRCfilter Totalof25spansof80kmfiber

Symbol

OA+DCOA

Transmitter

OA+DC

Link

CoherentDetection

NoiseLoading

RRCfilter

DataDirectedEq.

LLR

ROSNR Receiver

SpanLossBudget

AdjustGMI=0.85?

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5 bits/symbol Modulation Formats

-10 -8 -6 -4 -2 0Launch power (dBm)

17

18

19

20

21

22

23

24

25Sp

an L

oss

Budg

et (d

B)

5b4D-2A8PSK8PolSK-QPSK32SP-16QAM

02/23/2017 30

Chagnon et al., Opt. Exp. 2013.

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15

16

17

18

19

20

21

22

23

Span

Los

s Bu

dget

(dB)

6b4D-2A8PSKDP-8PSKDP-Star-8QAM

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Peak Span Loss Budget vs. Spectral EfficiencyDM Link

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bits/symbol4 5 6 7 8

Span

Los

s Bu

dget

(dB)

14

16

18

20

22

24

26

28

DP-QPSK, 4D-2A8PSK, DP-16QAMOther Modulation Formats

DP-QPSK

DP-16QAM

7b4D-2A8PSK

6b4D-2A8PSK

5b4D-2A8PSK

DP-Star-8QAM

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Q with separated nonlinear effects

AWGN SPM XPM XPolM AllNonlinearity

4

4.5

5

5.5

6

6.5

7

7.5

8

Q fro

m G

MI (d

B)

6b4D-2A8PSKDP-Star-8QAM

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Launch power of -4 dBm and OSNR of 15.2 dBMain benefit of 4D constant modulus format comes from the reduction of SPM and XPM, and not XPolM

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Dispersion Managed vs. Uncompensated Link5 bits/symbol formats

02/23/2017 35


17

18

19

20

21

22

23

24

25

Span

Los

s Bu

dget

(dB)

5b4D-2A8PSK8PolSK-QPSK32SP-16QAM

Dispersion Managed Link

25 x 80km NZDSF90% inline compensation50% pre-compensation

Uncompensated Link

50 x 80km SSMFNo inline compensationNo pre-compensation

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• Time Domain Hybrid– Using 4D-2A8PSK Family (4.5, 5.5, 6.5, 7.5 bits/symbol)

• Simulation Results• Summary

02/23/2017 36

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Hardware-efficient Soft-demapping

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2DLLR

Table(X)

2DLLR

Table(Y)

β0(0)β1(0)β2(0)β6(0)

XI

XQ

YI

YQ

β3(0)β4(0)β5(0)β7(0)

LLRup

date(2SPC

cod

es)

1DLLRTable

(Quantization)

ToSD-FECDecod

er

From

Phase-SlipCom

p.

β0(1)β1(1)β2(1)β3(1)β4(1)β5(1)

β0(2)β1(2)β2(2)β3(2)β4(2)β5(2)

Two 64x64 LUTs

64 levels (in) 16 levels (out)

Yoshida et al., ECOC’16 , paper Th.2.P2.SC3.27

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Hardware-efficient Soft-demapping

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4D-2A8PSK DP-Star-8QAM

TunableLD

s

Mux(A

WG)

Bulk

Mod

ulation

Quad.Parallel

Decorrelation

Pre-CD

comp.

(50%

oftotal)

TransmissionLine

OBP

F

Cohe

rentRx

Tx OfflineDSP

RxOfflineDS

P

50GHz spaced,70channelsLinewidth:<500kHz

32Gbaud

Average span: 70kmTotal length: 1260 kmMixture of NZDSF (local CD of -3ps/nm) and SSMF

Yoshida et al., ECOC’16 , paper Th.2.P2.SC3.27

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Q (from GMI) vs. Launch Power

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4D-2A8PSK(IdealSD)DP-Star-8QAM(IdealSD)4D-2A8PSK(ProposedSD)DP-Star-8QAM(QuantizedSD)

LaunchedPower(dBm/ch)

Q-fa

ctorfrom

GMI(dB

)

9.0

8.0

7.0

6.0

-8 -7 -6 -5 -4 -3 -2 -1 0

8.5

7.5

6.5

5.51

© MERL






• Time Domain Hybrid– Using 4D-2A8PSK Family (for 4.5, 5.5, 6.5, 7.5 … bits/symbol)– Simulation Results

• Summary

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Time Hybrid Modulation

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6b4D5b4D5b4D 5b4D

Benefit of 2A8PSK family- Start with fine granularity (5, 6, 7 bits/symbol)- 4D constant modulus

- power ratio is not compromised by nonlinearity

5.5 bits/symbol

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Proposed and conventional formats (4 – 6 bits/symbol)

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Launch power (dBm)-10 -8 -6 -4 -2 0

Span

Los

s Bu

dget

for G

MI =

0.8

5 (d

B)

16

18

20

22

24

26

28

DP-QPSK (4b/sym)TDH DP-QPSK 5b4D-2A8PSK(4.5b/sym)5b4D-2A8PSK (5b/sym)TDH 5b4D-2A8PSK 6b4D-2A8PSK (5.5b/sym)6b4D-2A8PSK (6 b/sym)TDH DP-QPSK 32SP-QAM (4.5b/sym)TDH SP32-QAM S8QAM (5.5b/sym)

© MERL


Proposed and conventional formats (6.5 – 8 bits/symbol)

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Launch power (dBm)-10 -8 -6 -4 -2 0

Span

Los

s Bu

dget

for G

MI =

0.8

5 (d

B)

10

12

14

16

18

20

22

TDH 6b4D-2A8PSK 7b4D-2A8PSK(6.5b/sym)7b4D-2A8PSK (7b/sym)TDH 7b4D-2A8PSK DP-16QAM (7.5b/sym)DP-16QAM (8b/sym)TDH S8QAM 128SP-QAM (6.5b/sym)TDH 128SP-QAM DP-16QAM (7.5b/sym)

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Span Loss Budget Summary

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4 5 6 7 8bits/symbol

16

18

20

22

24

26

28Pe

ak S

pan

Loss

Bud

get (

dB) TDH based on 2A8PSK

TDH based on conventional formatsDP-QPSK

DP-16QAM

TDH 7b4D-16QAM7b4D-2A8PSK

6b4D-2A8PSK

5b4D-2A8PSKTDH 5b4D-6b4D

TDH QPSK-5b4D

TDH 6b4D-7b4D

© MERL


Summary• Fiber nonlinearity is a limiting factor for dispersion managed links.• Finer granularity is important for higher network utilization.• GMI is used as a metric for transmission quality.• 4D-2A8PSK family is proposed for 5, 6, and 7 bits/symbol spectral

efficiency, corresponding to 25 Gb/s granularity at ~34 GBd.– Linear performance better than most other modulation formats.– 4D constant modulus -> Superb nonlinear performance.– Experimental results of 6b4D-2A8PSK, with HW implementation in

mind, confirmed better performance than DP-Star-8PSK.• Time-domain hybrid modulation using 4D-2A8PSK inherit the excellent

characteristics of the original modulation formats.– Spectral efficiency such as 4.5, 5.5, 6.5, and 7.5 bits/symbol can be

covered (and possibly 4.25, 4.75,…..).

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