innovative modulation schemes for future optical wdm ... · wr, apoc 2003; slide 1 chair for...

wr, APOC 2003; slide 1

Chair forCommunications

Faculty of EngineeringChristian-Albrechts-University of Kiel

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Innovative Modulation Schemes for Future Optical WDM-Transmission

Werner RosenkranzThanks to: Jochen Leibrich, Murat Serbay, Christoph Wree,

Torsten Wuth, Maike Wichers

University of Kiel, Germany

Chair for CommunicationsKaiserstr. 2, 24143 Kiel, Germany

Email: [email protected]




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Introduction

Innovative modulation formats have impact on transmission

performance of optical networks.

They may:

+ increase bandwidth efficiency (i.e. narrowing of channel

spacing)

+ increase transmission span reach

+ increase system margin and tolerances

– increase system complexity and equipment cost

– not be in compliance with existing standards




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Outline

1. Introduction

2. Modulation of Optical Carriers

3. Options for Alternative Modulation Formats

a. Duobinary Signaling

b. Single Sideband

c. Differential (Q)PSK

4. Comparison of Modulation Formats in WDM Transmission

5. Conclusions




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Modulation of a carrier signal (here: laser light) = mapping of

information into one or more parameters of the carrier, i.e

– Amplitude

– Phase (or frequency as ω=dφ/dt)

– Polarization

Carrier = real-valued band-pass signal (electrical field):

Modulation: Basics

( )( ) ( ) cos[ ( )] Re{ ( ) }Cj tj tCE t a t t t a t e e ωφω φ= + = ⋅

A(t): Complex Envelope

•carries information

•Determines modulation format

( )A t




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Digital transmission of bits b with bit rate rb=1/Tb

M=2m bits collected à one complex symbol d(k), symbol rate rS=1/TS=rb/M

Modulation: Basics

( ) ( ) ( )R Id k d k jd k= +

Parallel/SerialMapping

Binary Bits M-ary Symbols

Complex symbols:

( )( ) ( ) ( ) ( ) j tR IA t A t jA t a t e φ= + =Complex envelope: ( ) ( )S

kd k h t kT= ⋅ −∑

Pulse shape

Transmitted field: ( ) ( ) ( ) cos( ) ( ) ( ) sin( )R S c I S ck k

E t d k h t kT t d k h t kT tω ω

= − − − ∑ ∑

Orthogonal carriers, 2 data streams




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Linear Modulation: Constellations

In the class of linear modulationà format is determined by constellation

Noise performance à Minimum distance between symbols!




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Modulation Formats: Some Options

• Amplitude – RZ/NRZ (2-ASK with different pulse shape)– VSB/SSB – Multilevel (4-ASK, 8-ASK)

• Phase– DPSK– DQPSK– CPM = continuous phase mod.

• Frequency– MSK = minimum shift keying– FSK = frequency shift keying

• Hybrid– CS-RZ, chirped RZ/NRZ, partial response (e.g.

duobinary)




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Optical Modulator: Mach-Zehnder Interferometer

Mach-Zehnder Modulator for arbitraryphase/amplitude modulation

By driving and/or biasing a MZM apropriately, arbitrary constellations in complex plane can be implemented!

Ein,Pin Eout ,Pout

Wave-guide ∆φ1

∆φ2U2

U1

ok_32_modulator.dsf




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Innovative Formats: Duobinary Signaling

• Reduced bandwidth (approx. Factor of 2 against NRZ)• Highly dispersion tolerantà increase uncompensated

transmission reach• Carrier suppressed format: relaxed SBS suppression

requirements • Increased spectral efficiency• Low implementation complexity à upgrade of conventional

IM/DD possible




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Duobinary Signaling - How it Works!

• Optical transmitted signal: (pseudo)-three-level symbols {+1,0,-1}

using 2Vπ MZM drive signal• Coding rule:

– “0”-bit à 0– “1”-bit à +1 or –1, depending on depending on ……??

• Coding rule narrows spectrum of optical transmit signal• IM/DD-RX detects magnitude squared of opt. Signal

– 0-symbol à “0”-bit– +1-symbol and –1-symbol à “1”-bit

• Low implementation complexity – Implementation of coding rule in transmitter – Conventional IM/DD receiver– à simple upgrade of conventional IM/DD schemes

coding rule inverted!




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Duobinary Signaling - Implementation

• Duobinary precoder = differential encoder à IC-solution• Duobinary filter: LP with cut-off frequency of fb/4

datagenerator

data

bias

duobinaryprecoding

MZMpush-pullCW laser

LPfilter three-level

data

clock&datarecovery

Duobinary Encoding




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Duobinary Signaling - Dispersion Tolerance

• Improved dispersion tolerance• High sensitivity to non-linear fibre effects (SPM)

Simulation 10Gb/s Measurement 10Gb/s




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Innovative Formats: SSB/VSB

• Single Side Band (SSB) or Vestigial Side Band (VSB)• Reduced bandwidth (approx. factor 2 against conventional

double-sideband ASK = DSB)• Highly dispersion tolerant• Increased spectral efficiency• Medium to high implementation complexity• Extinction ratio problems! à reduced receiver sensitivity




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SSB/VSB – How it Works!

• Conventional: Double side-band ASK• However: Redundancy à one side-band (either lower or

upper) is redundant (one SB contains full information!)• Eliminate one side-band by

– Filtering in optical domain– Filtering in electrical domain

• SSB/VSB RX requires coherent detection!• However direct (intensity) detection possible, but carrier

component required! • Direct detection results in distortions!




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SSB/VSB – Sideband Suppression

• Optical SSB/VSB filter• Filtering in optical

(band-pass) domain

• Hilbert Transformer = Phase response Filter

• Filtering in electrical (base-band) domain

USBLSB

HSSB(f)

fCfC+fofC+fu f

e.g. 60dB!

nu_9_ESB_AM.dsf

USBLSB

HH(f)

f=0fofu f

nu_9_ESB_AM.dsf

ideal

real

j

- j




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SSB/VSB – Implementation: Optical Filter

• Optical filtering of conventional IM (DSB) signal• Steep filter slope required• Approximation through sideband attenuation• Used for Tb/s-experiments by Alcatel

Pulse PatternGenerator

DataDataRF Bias

RF Amplifier

LaserMach -ZehnderModulator

VSB signaloptical Filter




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SSB/VSB – Implementation: Phase Method

• Phase inversion of one sideband in electrical base-band domain (Hartley)

• Hilbert transformer may be approximated by FIR (finite impulse response) filter

DataData

RF

Mach -ZehnderModulator

Laser

Puls PatternGenerator

RF Amplifier

π/2

coupler


H

LT

TBiasRF

Bias

HilbertTransformator

optical SSB signal

RF Amplifier

Divider




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SSB/VSB – Implementation: Electrical Hilbert Transformer

Reduced complexity compatible SSB with electrical Hilbert Transformer

Phase-modulator

Pulse PatternGenerator

DataDataRF

optical SSB signal

HHilbertTransformer

Laser

RF Amplifier

RF Bias


DSB

RF Amplifier

Bias

m(t) m(t)

Amplitude Modulation Phase Modulation




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SSB/VSB – Implementation: Electrical Hilbert Transformer 2

( ) ( )( ) cos exp

2 2bias

out inm t U m t

E t E jU Uπ π

π π −= ⋅ ⋅ ⋅

AM PM

m(t) - data signalm(t) - Hilbert transformed data signal

transfer function of SSB transmitter

+

( ) [ 1 ( ) ( ) ...]2 22

inout

EE t m t j m t

U Uπ π

π π≈ + ⋅ + ⋅ ⋅ +

Approximation for a linear driven MZ-modulator

=+j·m(t)m(t)1

SSB!




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SSB/VSB: Performance

• Method used: Electrical H.T. • Method requires approx. linear MZM drive conditions for

good side-band suppressionà Extinction ratio reduced!

Transfer characteristic of the Mach-Zehnder modulator for power P and electrical field E

Optical power spectral density (simulation)

0 +1

t

U

P

Et

0

+1P

-30 -20 -10 0 10 20 30-60

-50

-40

-30

-20

-10

0Power Spectral Density

Frequency [GHz]

PS

D [d

B] SSB

DSB




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SSB/VSB: Experimental comparison DB vs. SSB

• 200km uncompensated SSMF, 10Gb/s

• Compare SSB with duobinary

• Linear and non-linear fiber regime

• SSB approx. 6dB penalty, due to reduced extinction ratio 10-10

10-910-810-7

10-6

10-5

10-4

10-3

10-2

Bit

Err

or R

atio

(B

ER

)

receiver input power [dBm]-40 -38 -36 -34 -32 -30 -28 -26 -24 -22 -20

1.7dB

0.8dB

DB0dBm8dBm

SSB0dBm

8dBm




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SSB/VSB: Dispersion Tolerance DSB vs. SSB

• Simulation result, 10Gb/s

• Linear fiber• SSB: Extinction ratio

optimized for maximum reach

• DSB: maximum extinction ratio

0 50 100 150 200 250-2

0

2

4

6

8

10

12

14

SSMF length [km]

Eye

Ope

n P

enal

ty [d

B]

DSB

SSB




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Binary Differential PSK (D-2-PSK)

• Symbols: +1, -1 • NRZ or RZ pulse shape possible

– RZ: +1,-1 identical optical intensity • à optical intensity = periodic function of time• à advantage in XPM environment

• Improved noise performance vs. conventional IM/DD , if balanced receiver used (3dB)

• Differential scheme à avoids coherent detection• Additional DPSK receiver (MZDI) necessary!• Differential precoder necessary!




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Binary DPSK

• DPSK precoder = differential encoder (same as for duobinary)

• Functionality of MZDI receiver. Phase

cos( ( )) cos( ( 1))2

c ct k t kω φ ω α φ+ + − − +

{ }( ) 0,kφ π∈

[ ]{cos ( ) ( 1)0

k kφ φ α− − −=

64444744448... 2cos(...) cos(...) ...= + ⋅ +

( ) ( -1),( ) ( -1)

High if

Low if

k kk k

φ φφ φ

=≠

datagenerator

bias

dataclock

bias

RZ-pulse carving

PSKprecoding

CW laser

delay

Tb

clock&datarecovery

balancedreceiverdelay

interferometer

2VΠ-MZMpush-pull




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Quaternary DPSK

• Our solution: serial DQPSK-transmitter à reduced complexity, relaxed phase control

• 2 parallel data streams (real & imaginary) with speed and bandwidth requirements of single data stream!

• Quaternary differential precoder required

datagenerator

bias

dataclock

bias bias

RZ-pulse carving

QPSKprecoding

MZMpush-pull

phasemodulator

S/P

CW laser

data real

data imag

delay interferometerwith carrier shift π/4

datarealdelay

Tb

balancedreceiver

π/4

dataimagdelay

Tbπ/4




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DPSK: Receiver Sensitivity - Theory

• Ideal coherent/synchronous detection: à both DPSK & DQPSK 3dB sensitivity improvement due to enlarged symbol distance (constellation!) vs. OOK=IM/DD!

• DPSK with MZDI = non-coherent: 0.2dB loss

• DQPSK with MZDI = non-coherent: 2dB loss

10 12 14 16 18

1e-5

1e-6

1e-7

1e-8

1e-9

1e-10

1e-11

Bit

Err

or R

atio

(B

ER

)

SNR [dBm]

DPSK

DQPSK

OOK




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DPSK with RZ pulse shape in WDM: Robustness towards XPM

• RZ: periodic optical intensity àsuppression of XPM-induced phase modulation

• RZ-DPSK extends transmission distance significantly!

Simulation result: 8x10Gb/s WDM, channel spacing: 100GHz,launch power: 9dBm/channelspan length: 100km SSMF, dispersion comp.

5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0- 0 . 5

0

0 . 5

1

1 . 5

2

2 . 5

3

L [km]E

OP[

dB]

RZ-ASK

RZ-DPSK




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Modulation Formats in WDM Transmission

Power spectral density of various modulation formats at40Gb/s

-80 -60 -40 -20 0 20 40 60 80-50

-40

-30

-20

-10

0

10

20

f [GHz]

PS

D[d

B]

NRZ-ASK

SSB

DB RZ-DQPSK

40

SSB

70

NRZ-ASK

5648∆f [GHz]

RZ-DQPSKDB

Two-sided spectral width for a decay of 20dB for DB, SSB, RZ-DQPSK and NRZ-ASK at 40Gb/s




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Modulation Formats and WDM: Intermodulation 0.8b/s/Hz

• Single channel: 4 spans 100km SSMF, Dispersion optimized, 40Gb/s

• RZ-DQPSK most robust against SPM and chrom. Dispersion!

0 1 2 3 4 5 6 7 80

1

2

3

4

5

average input power [dBm]

EO

P [

dB]

NRZ-AS K

SSB

DB

RZ-AS K

RZ-DQPSK

• WDM 8x40Gb/s, ∆f=50GHz: 4 spans 100km SSMF, Dispersion optimized

• RZ-DQPSK most robust against WDM intermodulation(XPM,FWM,SPM, linear cross-talk!)

0 1 2 3 4 5 6 7 80

1

2

3

4

5WDM 50GHz, dispersion optimized

Mean opt. Input power per channel [dBm]

eye

open

ing

pena

lty

[dB

]

RZ-DQPSK

DB

SSB NRZ-ASK100GHz

RZ-AS K 100GHz




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Modulation Formats in WDM: Intermodulation

Sensitivity to nonlinear fibre effects, SSMF, 40Gb/s, ∆f=50GHz, 0.8b/s/Hz

0 1 2 3 4 5 6 7 8

0

1

2

3

4

5

average input power per ch. [dBm]

eye

open

ing

pena

lty

[dB

]

lin. Xtalk+all nonlinear.lin. Xtalk+SPM+XPMlin. Xtalk+SPM

0 1 2 3 4 5 6 7 8

0

1

2

3

4

5


eye

open

ing

pena

tly

[dB

]lin. Xtalk+SPMlin. Xtalk+SPM+XPMlin. Xtalk+all nonlinear.

0 1 2 3 4 5 6 7 8

0

1

2

3

4

5


eye

open

ing

pena

lty

[dB

]

lin. Xtalk+SPMlin. Xtalk+SPM+XPMlin. Xtalk+all nonlinear.

RZ-DQPSK

SSB

DB

FWMXPMSPMLinearcross-talk




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Conclusions

• Duobinary: low complexity, high benefit through spectral narrowing à

promising candidate for system upgrade now!

• SSB/VSB: solution with optical filtering à moderate system improvement

for reasonable additional complexity

• DPSK: medium complexity à high benefit through improved receiver

sensitivity and WDM intermodulation robustness

• DQPSK: medium to high complexity à but doubles channel capacity

• Other advanced formats: (MSK, CPM, M-PSK, M-QAM) worth investigating

in optical environment! à constraints: complexity and chirp!

innovative modulation schemes for future optical wdm ... · wr, apoc 2003; slide 1 chair for...

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