Technologies towards Terabit Transmission Systems - IEEE Xplore

ECOC 2010, 19-23 September, 2010, Torino, Italy

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Technologies towards Terabit Transmission Systems Fred Buchali Alcatel-Lucent, Bell-Labs, Germany, Lorenzstr. 10, D-70435 Stuttgart, Germany, [email protected]

Abstract This tutorial reviews technologies towards terabit transport: Increase of bitrate per channel by higher symbol rate or higher number of bit per symbol, alternatively superchannel application consisting of non-overlapping or overlapping sub-channels contributing to increased spectral efficiency.

Technologies towards Terabit Transmission Systems

Fred Buchali Alcatel-Lucent, Bell Labs, Germany ECOC’10, Torino, Italy

1 F. Buchali: ECOC 2010, +100G technologies

978-1-4244-8535-2/10/$26.00 ©2010 IEEE

All Rights Reserved © Alcatel-Lucent 2006

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Acknowledgements

Roman Dischler Axel Klekamp Andreas Leven Xiang Liu Chandra Chandrasekhar



Technologies towards terabit transmission systems

Outline Introduction State of the art 100 Gb/s systems Technologies for +100 Gb/s transmission +100 Gb/s bitrate Superchannels Summary



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Introduction



Introduction State of the art 5

• 10 Gb/s

Ey

4

• NRZ

3

2

• 1 bit/symbol

Ex

• 10 Gbaud

1

20 GHz 0

-1

• Fits clear into 50 GHz grid • 40 Gb/s

700

800

900

1000

1100

Ey 50 GHz

Ex

• 40 Gbaud

80 GHz -1

700

800

900

1000

1100

1200

• Prefiltering required for WDM, but penalty occurs • Beyond 40 Gb/s new technologies required, if 50 GHz grid is targeted 5 F. Buchali: ECOC 2010, +100G technologies

1300

filter

• DPSK • 1 bit/symbol

1200


1300

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Introduction Technologies for 100G systems • DQPSK with 2 bit per symbol • Compatible with 100 GHz grid Ey 100 GHz

Ex -1

700

800

900

1000

1100

1200

1300

1200

1300

• Polarization multiplex • Compatible with 50 GHz grid TE

TM Ey

Ey 50 GHz

Ex

Ex -1

700


800

900

1000

1100


Introduction Network Interface Rates 1000

Transmission

Interface Rate (Gb/s)

Core Interface 100

IP interface

10

1

0.1

0.01 1980

1990

2000

2010

2020

Year

Tkach, APOC, 2008. 7 F. Buchali: ECOC 2010, +100G technologies


Transmission, Crossconnect, and IP Router interface rates have coalesced. Very soon 100 Gb/s 1 Tb/s rate is projected for 20152020.

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100G transmission systems




Alcatel-Lucent: 100Gb/s transponder



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Coherent dual polarization QPSK Superior performance due to 50 GHz compatibility Single carrier approach Coherent detection enables high CD and PMD robustness Narrow spectra enables high number of ROADM



Technologies for +100G systems



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Technologies for +100G transmission systems

Candidates for +100G transmission: Single carrier and OFDM single carrier

Tx

OFDM I

DATAin

IQ mod

DATAout

DATAin

IQ mod

IFFT DAC

Q 2 polarizations

Rx

I

DAC

DATAout

Q 2 polarizations

Coherent reception for Rx DATAin

+

ADC

DATAout

DSP +

ADC

j

Optical 90° hybrid

LO

2 polarizations 12 F. Buchali: ECOC 2010, +100G technologies



To enable more complex modulation scheme: QPSK x-QAM OFDM

single carrier Tx

I

DATAin

IQ mod

DAC DATAout

DATAin

IQ mod

IFFT DAC

Q

I DATAout

Q

DAC required Tx DAC DATAin

I IQ mod

DAC


• No variation for OFDM Tx for x-QAM DATAout

• Rx is same as for QPSK modulation

Q All Rights Reserved © Alcatel-Lucent 2010

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Single carrier

OFDM

Single carrier: phase and amplitude modulated

Multiple carriers: amplitude and phase-modulated

Quasi-constant amplitude for QPSK in the transmitter

Interference causes large variations in amplitude (PAPR>10 dB)

Blind channel estimation and carrier recovery (no overhead)

Data-aided channel estimation and carrier recovery (overhead required)

Continuous stream transmission and processing

Block transmission and processing

Single carrier and OFDM In addition they have many similar characteristics




Increase of bitrate per channel

Increase of baudrate

Increase of number of bit per symbol

Focusing on Rx: Receivers with ADCs at 500 GSa/s required

QPSK n-QAM modulation

2 bit per symbol 20 bit per symbol, n=220

Superchannels

Inverse multiplexing

Application of state of the art 100 Gb/s technology

10 channels á 100G in a 50 GHz WDM grid

More dense multiplexing techniques

Optical FDM without overlap of sub-channels

FDM with overlapping orthogonal and non-orthogonal sub-channels



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Increase of bitrate per channel



Increase of bitrate per channel Increase of baudrate to 250 Gbaud

Target for Tx: 250 Gbaud for bipolar signals (I and Q)

Demonstrated: 100 Gbaud – 100 Gb/s NRZ

Target for Rx: 500 GSa/s ADC

Demonstrated: 50 GSa/s, Fujitsu: 120 GSa/s is feasible (B. Germann, paper SPTuC2, SPPCom 2010)

1 Tb/s client

1 Tb/s

1 Tb/s

line I IQ mod Q

PD PD PD PD

ADC ADC DSP ADC ADC

Increase of baudrate feasible, but improvement is limited 17 F. Buchali: ECOC 2010, +100G technologies


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Increase of bitrate per channel Increase of number of bit per symbol

n-QAM modulation, today 2 bit per symbol 20 bit per symbol, n=220

QPSK 2 bit/symbol

e.g. 1024-QAM 10 bit/symbol

How far can we go? 18 F. Buchali: ECOC 2010, +100G technologies


Increase of bitrate per channel Constellation diagrams:

QPSK

Theoret. req.OSNR vs QPSK

16-QAM

64-QAM

+6.9 dB

+13.1 dB

(for increasing bitrate) Optical OFDM enables variable constellations: Modulation Format

Spectral Efficiency

Reference

QPSK

theoretical 2 bit/s/Hz

w. 10% overhead 1.7 bit/s/Hz

+ Pol. Mux 3.4 bit/s/Hz

Achieved 3.3 bit/s/Hz

QAM-16

4 bit/s/Hz

3.4 bit/s/Hz

6.8 bit/s/Hz

5.6 bit/s/Hz

QAM-32

5 bit/s/Hz

4.3 bit/s/Hz

8.5 bit/s/Hz

7.0 bit/s/Hz

KDDI - OFC’09

QAM-64

6 bit/s/Hz

5.1 bit/s/Hz

10.2 bit/s/Hz

7.2 bit/s/Hz

HHI – OFC’10

ALU/BL - OFC’09 KDDI - ECOC’08, ALU – OFC’10

Essential OSNR penalties for multi-level modulation formats found Penalties may be reduced using higher overhead (training symbols and pilot tones) Spectral efficiency increase is limited 19 F. Buchali: ECOC 2010, +100G technologies


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Increase of bitrate per channel Electrical back-to-back experiments 11.1-Gb/s QPSK

Q~ 25 dB

22.2-Gb/s 16-QAM

27.8-Gb/s 32-QAM

17 dB

14 dB

33.3-Gb/s 64-QAM

11 dB

Q factor scales as the inverse square of symbol-spacing Intrinsic limitation from digitization noise

Improvement for higher resolution DAC/ADC expected Courtesy of Xiang Liu 20 F. Buchali: ECOC 2010, +100G technologies


Increase of bitrate per channel Optical back-to-back experiments using e/o, o/e Polarization multiplex and coherent reception 22.2-Gb/s QPSK

Q~ 21 dB

44.4-Gb/s 16-QAM

55.5-Gb/s 32-QAM

66.6-Gb/s 64-QAM

10 dB

7 dB

13 dB

Q values ~ 4 dB lower than electrical Q Large implementation penalty due to e.g. laser noise

Are higher constellations feasible? Courtesy of Xiang Liu 21 F. Buchali: ECOC 2010, +100G technologies


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Increase of bitrate per channel Single carrier modulation using higher level constellation: 256-QAM @64 Gb/s Extremely narrow linewidth laser (4 kHz) Optical PLL in receiver for coherent reception Pilot carrier transmission supporting tracking of optical PLL Pilot carrier reduces spectral efficiency Req. OSNR@2E-3 BER: 28.6dB, 5.3 dB penalty for 160 km transmission

B2B

160 km

M. Nakazawa et al., paper OMJ5, OFC‘10

Higher constellations are less feasible even using advanced optical technologies 22 F. Buchali: ECOC 2010, +100G technologies


Increase of bitrate per channel Summary of recent experiments

req. OSNR for 1E-3 @ 107Gb/s

23

OFDM (PDM) No-Guard-Interval OFDM Single Carrier (PDM)

32QAM

PDPB7 KDDI 0.521Tb/S

16QAM 21 8PSK 8QAM

PDPB4 AT&T,NEC RZ-8QAM 32Tb/s

19 QPSK

17

8PSK OFC08 KDDI

PD ECOC06 BL NJ 25.6Tb/s PDPC2 BL Stgt 1.21Tb/s

PDPB5 NTT 13.5Tb/s

PDPB8 BL NJ 1.2Tb/s

ECOC08 KDDI 0.534Tb/s

122Gb/s

PDPC1 Melb 1 Tb/s 107Gb/s

SE / OSNR = const (SE * 2 = OSNR + 3dB)

15 2

Remarks:

3

4

Spectral Efficiency* (bit/s/Hz)

5

6

7

8

*incl. 7%-FEC

Spectral efficiency is determined by modulation (QPSK, n-QAM) Increase of levels increases S/N requirements No differences between single carrier and OFDM 23 F. Buchali: ECOC 2010, +100G technologies


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Increase of bitrate per channel Investigation of OSNR of received signal in a multi span system

OSNR = Plaunch − L − NF − 10 log(N ) + 58dB 30

28 80 km 100 km

rec. OSNR0.1nm [dB]

26

Reduction in maximum transmission distance

24

22

Increase of required OSNR QPSK 16-QAM

20

18

Launch power = 2dBm Attenuation = 0.25 dB/km NF = 6

16 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Number of spans

OSNR at Rx depends on span length, link number and launch power per span and channel 3 dB OSNR increase at halved distance 24 F. Buchali: ECOC 2010, +100G technologies


Increase of bitrate per channel Investigation of OSNR of received signal at optimized launch power

Plaunch = PINT − 10 log(N ) 30

80 km

28

100 km

received OSNR 0.1nm [dB]

26

24

22

Integrated power = 2 dBm Attenuation = 0.25 dB/km NF = 6

20

18

16 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Number of spans

Launch power close to nonlinear threshold is advantageous Assuming 2 dBm integrated power 6 dB increase of OSNR at halved distance 25 F. Buchali: ECOC 2010, +100G technologies


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Increase of bitrate per channel Higher level modulation Increase of bitrate is limited – 2x

QAM 16-QAM

– 3x

QAM 64-QAM

– 4x

QAM 256-QAM

Huge increase in required OSNR Shortens maximum transmission distance

Increase of number of bit per symbol is limited



Superchannels



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Superchannels Inverse multiplexing

10 x 100 Gb/s

...

1 Tb/s

client

line

...

10 channels @100Gb/s in a 50 GHz grid

Bandwidth consumption: 500 GHz

Spectral efficiency: 2 bit/s*Hz



Superchannels Dense multiplex techniques in frequency domain

unused spectral range

...

How dense can we multiplex the channels?



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Superchannels Considering a subdivision of the overall band into 1 Tb/s channels

Starting with 10 x 100 Gb/s scenario 1 Tb/s

…

…

Guard bands for ROADM application

⇒ If 1 Tb/s channel is transmitted as a superchannel the guard bands are not longer required 1 Tb/s

⇒ Alternative WDM grid or grid-less scheme 30 F. Buchali: ECOC 2010, +100G technologies


Superchannels

Dense multiplex of pre-filtered single carrier sub-channels

...

1 Tb/s

DEMU X

MUX

...

e.g. G. Gavioli et al., paper OThD3, OFC‘10

• Filtering prevents overlap of spectra after MUX and cross talk in Rx • Signal processing without cross talk suppression enabled



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Superchannels Dense multiplex of pre-filtered single carrier sub-channels

Back to back and long distance transmission

25 Gbaud

G. Gavioli et al., paper OThD3, OFC‘10

• • • • •

Frequency grid: down to 1.2 x baudrate Low bandwidth filtering causes error floor and penalty at high distance High frequency stability of lasers required to avoid drift towards lower spacing Bandwidth consumption at 10x100 Gb/s: 300 GHz Spectral efficiency: 3.3 bit/s*Hz vs. 2.0 bit/s*Hz for inverse multiplex



Superchannels Dense multiplex of pre-filtered single carrier sub-channels

Rx after optical filtering, o/e and electrical filtering Optical and electrical filter

0

• • • •

frequency

If narrow filtering is applied, there is no cross talk present BWel ≈ 0.6 * baudrate Signal processing comparable with single channel 100 Gb/s Required oversampling in Rx: 1.5 to 2

Increase of spectral efficiency limited by a factor of 1.66



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Superchannels

Narrower sub-channel grid with overlapping spectra 1 Tb/s

1 Tb/s

⇒ Loss less demux of overlapping spectra possible if sub-channels are orthogonal



Superchannels Multiplex of overlapping single carrier sub-channels

1 Tb/s

...

comb

D E MU X

Requirement: baudrate = frequency spacing

MUX

...

cyclic demux

1.0x baudrate 11 x baudrate, 275 Hz • COMB generator required • Orthogonality of sub-channels in Rx required • Processing in Rx with cross talk suppression (delay and add filter, oversampling req.) • Bandwidth consumption: 275 GHz • Spectral efficiency: 3.6 bit/s*Hz, approaches 4 bit/s*Hz for higher #sub-channels 35 F. Buchali: ECOC 2010, +100G technologies


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Can we replace comb generator by independent lasers? Tx

Tx

comb

LD LD LD LD LD

SC

SC LD LD LD

• Transmission without coupled carriers is feasible • Low loss demux requires 4x oversampling • Drift towards lower spacings has to be avoided S. Chandrasekhar, X. Liu, Vol. 17, No. 24, OPTICS EXPRESS 21354 36 F. Buchali: ECOC 2010, +100G technologies



Requirement: Alignment of adjacent channels in time domain (orthogonality)

S. Chandrasekhar, X. Liu, Vol. 17, No. 24, OPTICS EXPRESS 21354

• Delay between adjacent channels < 0.1 x symbol period



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Rx after optical filtering, o/e and electrical filtering Optical and electrical filter

0

frequency

• Signal: BWel ≈ 0.6 * baudrate • Overlapping spectra, cross talk is in-band, high bandwidth required • Delay and add filter required to suppress neighbors Increase of spectral efficiency limited by a factor of 1.8, may approach 2



Superchannels Mulitplexing of orthogonal or non-orthogonal OFDM subchannels

Rx

Tx

OR

comb

LD LD LD LD LD

OFDM

OFDM LD LD LD

Coupled carriers

Uncoupled carriers (free running)

Is orthogonality between subbands required?



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Superchannels Mulitplexing of orthogonal and non-orthogonal OFDM subchannels ∆t Alignment of symbols ∆f

Delay within cyclic extension

20

20

18

18

16

16 Q [dB]

Q [dB]

f

OSNR

14

OSNR 14

12

12

10

10

8 -2

0

2

4

6

∆f/∆fsc [a.u.]

t

8 Targeted operation -100 8 10 FDM OFDM is feasible

-50

0

50

100

subband delay [a.u.]

• Frequency offset > 2⋅⋅subcarrier spacing leads to negligible Q penalty at typical OSNR values • Subband time delay has no impact on performance at relevant OSNR 40 F. Buchali: ECOC 2010, +100G technologies


Superchannels Mulitplexing of orthogonal and non-orthogonal OFDM subchannels: experiments 121Gb/s PDM-OFDM Modulation of 5 coherent cw-lines

MZM 1

CWsource

AWG

∆f, 2∆ ∆f even odd

OFDM I/Q

PDM 5x24Gb/s

f0

∆f

5xmultiband OFDM

1.2Tb/s PDM-OFDM-FDM

PDM-OFDM

Cascading of 10 unlocked (incoherent) lasers CWsource

10 Lasers in 34GHz spacing

CWsource

5 ∆f

1.2Tb/s

CWsource

120Gb/s 120Gb/s 120Gb/s 120Gb/s 120Gb/s

PDMOFDM

CWsource

1.2Tb/s PDM-OFDM-FDM in 10x PDM-OFDM subbands 41 F. Buchali: ECOC 2010, +100G technologies


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Superchannels Mulitplexing of orthogonal and non-orthogonal OFDM subchannels Superchannel

Back-2-back performance

1.2Tb/s within 340 GHz bandwidth

1E-2

-30

24.2Gb/s 121Gb/s

1.21Tb/s

Power (dB)

-35

340GHz

-40

-45

1E-3

7.5dB Back-2-back

-55 1547.5

1548

1548.5

12dB

BER

-50

1549

1549.5

1550

1550.5

1551

Wavelength (nm)

1E-4

24.2 -> 121 Gb/s orthogonal MUX 121-> 1.2 Tb/s non-orthogonal MUX 0.5 dB penalty for orthogonal MUX

1E-5

5

2 dB penalty for non-orthogonal MUX

10

15

20

25

OSNR / 0.1nm (dB)

Bandwidth corresponds to 283 GHz @1 Tb/s 42 F. Buchali: ECOC 2010, +100G technologies


Superchannels Mulitplexing of orthogonal and non-orthogonal OFDM subchannels Rx after optical filtering, o/e and electrical filtering

Optical filtering Electrical filtering Oversampling 1.5 to 2 times Suppression of out of band cross talk without additional signal processing Low penalty demux

Increase of spectral efficiency limited by a factor of 1.7



30

35

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Summary



Technologies towards Terabit Transmission Systems

Starting from 100 Gb/s systems there are several technologies towards 1 Tb/s available They enable increase in bitrate per channel by higher baudrate and increase in number of bit per symbol Bitrates clearly beyond 100 Gb/s are feasible Superchannels applying dense frequency division multiplexed subchannels enable increased spectral efficiency



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www.alcatel-lucent.com

46 F. 46 Buchali: R. Dischler, ECOC 2010, F. Buchali: +100G technologies PDP, OFC 2009
