Optisches OFDM – Eine neue Technik für die optischen ... · PDF fileOptisches OFDM – Eine neue Technik für die optischen Datenübertragung Fred Buchali Alcatel-Lucent, Bell Labs

All Rights Reserved © Alcatel-Lucent 2008

Optisches OFDM – Eine neue Technik für die

optischen Datenübertragung

Fred BuchaliAlcatel-Lucent, Bell Labs Germany, Stuttgart, Germany


2

2 | OFC’08, OMU5 | Month 2008

Outline

Einleitung

Grundlagen – OFDM

O-OFDM Systeme und deren Eigenschaften

Realisierungsaspekte

Zusammenfassung


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3 | OFC’08, OMU5 | Month 2008

Einleitung

Typisches optisches Übertragungssystem

INPUT

Tx Rx

OUTPUT

Binary data

- Bitraten: 155 Mb/s … 40 Gb/s

- Bei 10 Gb/s mußte die chromatische Dispersion berücksichtigt werden, teilweise auchPMD

- Bei 40 Gb/s muß die chromatische Dispersion nach Anwendung von Kompensationsfasern berücksichtigt werden, sowie PMD in weiteren Anwendungen

- Verzerrungen (CD und PMD) begrenzen Einsetzbarkeit von Systemen höhererSymbolrate

- Spektrale Effizienz erlaubt keine weitere Erhöhung der Bitrate im 50 GHz Raster

- Empfindlichkeit der Systeme muß stetig verbessert werden


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4 | OFC’08, OMU5 | Month 2008

Einleitung

- Bitraten über 40 Gb/s sind nachgefragt, 100 Gb/s ist nächste Stufe

- Derzeit in Diskussion:

- DQPSK bei 50 Gbaud

- Kohärentes QPSK

- Kohärentes O-OFDM

- Kohärente Systeme ermöglichen die Übertragung der gesamten Feldinformation in die elektrische Ebene

- Dabei wird erstmal massiv DSP in physikalischer Ebene eingesetzt, es sind ADCs (und DACs) erforderlich

- PMD und CD können dann vollständig elektrisch kompensiert werden

- System- und Implementierungsaspekte von kohärentem O-OFDM sind wenigerumfassend untersucht


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Grundlagen von optischem OFDM


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6 | OFC’08, OMU5 | Month 2008

Prinzip von OFDM: Vergleich zwischen Einzelträger und Mehrträgerverfahren (1)

- Einzelträgerverfahren:

- Einzelnes Trägerband welches die gesamte Kanalbandbreite belegt

- Serielle Übertragung der Daten

- Störanfällig für Intersymbolinterferenz

time

freq

uenc

y

band

wid

th

Tsc

Transmitted symbol


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7 | OFC’08, OMU5 | Month 2008

Prinzip von OFDM: Vergleich zwischen Einzelträger und Mehrträgerverfahren (2)

- Mehrträgerverfahren:

- Aufteilung der Kanalbandbreite in Unterkanäle

- Serieller Datenstrom wird parallelisiert und auf Unterkanäle aufgeteilt

- beliebiges Modulationsverfahren für jeden Unterkanal

- Symboldauer verlängert sich um Faktor NC gegenüber Einzelträgerverfahren

weniger anfällig für Störung durch ISI

time

freq

uenc

y

band

wid

th

NCxTsc

Transmitted symbol

∆f=B/NC


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8 | OFC’08, OMU5 | Month 2008

Prinzip von OFDM: Frequency Division Multiplexing

- Schutzbänder zwischen den einzelnen Kanälen nötig

- zur Vermeidung von Kanalübersprechen (Inter Channel Interference, ICI)

- zur Rückgewinnung der Kanäle mittels Filtern

Dadurch schlechte Bandbreiteneffizienz

Abhilfe: Orthogonal Frequency Division Multiplexing


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9 | OFC’08, OMU5 | Month 2008

Prinzip von OFDM

- Mehrträgerverfahren:

- OFDM – Unterträgerbänder überlappen

CN

allzero

1 2 k nn -1... ...

f 1 f 2 f k f n

n x f S

frequency


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10 | OFC’08, OMU5 | Month 2008

Prinzip von OFDM

OFDM modulator/multiplexer

OFDM demodulator/demultiplexer

Basic requirement: use of orthogonal basefunctions

Σ

×Cn,0

tje 0ω

×Cn,N-1

tj Ne 1−ω

s(t)

...× Cn,0

tje 0ω−

× Cn,N-1

tj Ne 1−− ω

s(t)

∫ ⋅ST

dt)(

∫ ⋅ST

dt)(Ts

Ts

...


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11 | OFC’08, OMU5 | Month 2008

OFDM: Sender und Empfänger

DAC

Im

Re

DAC

S/P

Zero

s

iFFT

code

r

P/S

ADC

Im

Re

P/S

Zero

s

FFT de

code

r

S/P

ADC

targeted function is relialized by DFT

OFDM signals are in general complex valued


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12 | OFC’08, OMU5 | Month 2008

Prinzip von OFDM: Guard Interval

- Zyklische Erweiterung des OFDM-Signals

- Im Empfangsfenster muß immer volständige Peride jedes Unterträgers vorhanden sein

- Erweiterung reduziert Nettodatenrate


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13 | OFC’08, OMU5 | Month 2008

Prinzip von OFDM: Subcarrier modulation

DAC

Im

Re

DAC

S/P

Zero

s

iFFTco

der

P/S

QPSK is efficient scheme for high sensitivity


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14 | OFC’08, OMU5 | Month 2008

Subcarrier modulation

Investigated schemesQPSK – 2 bit per subcarrier 10 Gb/s

QAM16 – 4 bit per subcarrier 20 Gb/s

QAM64 – 6 bit per subcarrier 30 Gb/s

Increase of number of bit per subcarrier leads to drastic decrease of distance between adjacent states in the constellation

Reduction of SNR -6.9 dB -13.1 dBConv. schemes, e.g. NRZ -3 dB -4.8 dB


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15 | OFC’08, OMU5 | Month 2008

Subcarrier modulation

Comparison of subcarrier modulation schemes

Makes higher level subcarrier modulation sence?

2.41.60.8Spectral efficiency [bit/Hz]

11.04.1Red. In sensitivity [dB] vs. QPSK

4.83.0Comp. to NRZ [dB]

15.87.1Sensitivity below 10Gb/s [dB]

22.715.07.9 Req. OSNR @ 1E-3 [dB]

302010Data rate [Gb/s]

QAM64QAM16QPSK


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16 | OFC’08, OMU5 | Month 2008

O-OFDM transmitter

IQ-MOD

imag

Electr. OFDM signal

e/o MODOptical OFDMsignal

real

imag

Electr. OFDMsignal

IQe/o MOD

opticalOFDMsignal

real


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17 | OFC’08, OMU5 | Month 2008

O-OFDM receivers

real

opticalOFDMsignal

[⋅]2 IQ-DEMElectr.OFDMsignal

imag

opticalOFDMsignal

LO

realopticalOFDMsignal

90° opt. hybrid

Electr.OFDMsignal

imag

[⋅]2

real

IQ-DEMElectr.OFDMsignal

imag

LO

[⋅]2

[⋅]2

Directdetection

Coherent


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18 | OFC’08, OMU5 | Month 2008

O-OFDM spectra

frequency

Electricalbase bandsignal

I and Q component

amplitude

frequency/wavelength

Opticalsignal

λO

Res. opticalcarrier

frequency

Electricalbase bandsignal

I and Q component

amplitude

nahezu rechteckige Spektren


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19 | OFC’08, OMU5 | Month 2008

O-OFDM Systeme und deren Eigenschaften


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20 | OFC’08, OMU5 | Month 2008

Scaling OFDM towards higher bitrates – first estimation

Data rate Subcarrier modulation DAC/ADC rate DAC/ADC bandw. Resol.10G QPSK 10GSa/s < 4 GHz 6 bit

20G QAM16 10GSa/s < 4 GHz 8 bit



PM 2x50G=100G QAM16 (2x) 25GSa/s < 10 GHz 8 bit

50G QPSK 50GSa/s < 20 GHz 6 bit

PM 2x50G=100G QPSK (2x) 50GSa/s < 20 GHz 6 bit

CO-OFDM transmitter (similar for receiver)

Optical channelData rate Subcarrier modulation Optical bandwidth Spectral efficiency

10G QPSK < 8 GHz 1.4 bit/Hz

20G QAM16 < 8 GHz 2.8 bit/Hz



PM 2x50G=100G QAM16 (2x) < 20 GHz 5.6 bit/Hz

50G QPSK < 40 GHz 1.4 bit/Hz

PM 2x50G=100G QPSK (2x) < 40 GHz 2.8 bit/Hz

req. OSNR [dB](*)

6.0

13.1

16.1

17.1

20.1

13.0

16.0(*) BER 10-3

Interconnect2x60Gb/s

2x80Gb/s

2x160Gb/s

2x200Gb/s

4x200Gb/s

2x300Gb/s

4x300Gb/s


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21 | OFC’08, OMU5 | Month 2008

+100Gb/s OFDM scenario

multiplexing of OFDM sub bands

spectral shaping of sub bands is suitable for narrow wavelength spacingdemux of sub bands using comb lines aligned to each sub bandhigh capacity scenario without WDM gridhighest spectral efficiencies achievable w/o. WDM grid (e.g. 1.7 b/s/Hz for 9x12.5 Gb/s)

OpticalComb Generator

CyclicDeMux

100GPDM O-OFDM

fN x Δλ

Optical Local Comb Oscillator

4 PDseach

ADCDSPCoherent

mixer

N x 100GN x 100G

RxTx

e.g. 1TbE

e.g. 1TbE

Experimental result 9x12.5Gb/s in 7.5GHz spacing113 Gb/s in 67GHz bandwidth

-35

-25

-15

-5

1552.4 1552.5 1552.6 1552.7 1552.8 1552.9 1553 1553.1 1553.2

Wavelength(nm)P

(dB

)

12.5Gb/s37.5Gb/s113Gb/s

5x80km SMF


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22 | OFC’08, OMU5 | Month 2008

-45

-40

-35

-30

-25

-20

-15

1552.4 1552.6 1552.8 1553 1553.2

Lambda(nm)

P a.

u.

Back-2-Back5Spans

S/P

Map

ping

IFFT

Cyc

lic E

xten

sion

Scal

ing

& Q

uant

izat

ion

LD

A/D

Syn

chro

nisa

tion

FFT

Q-, BER-estimation

Error- count

opticalLO

Data

real

Cha

nnel

co

mpe

nsat

ion

Offline D

ecis

ion

Optical Link 5x80km SMF DCF free

Pilo

ts

Zero

s

P/S

TS I

nser

tion

imag

D/A

D/A

I/Q

90°-

Hybrid B-P

D

B-PD

A/D

Dow

ncon

vers

ion

CP

PF-

Rem

ove

Booster

Preamp

SMF

MZM1

MZM2 CombGenerator(9 lines)

OFDMModulation12.57Gb/s

7.46GHz+

22.38GHz

Setup of 113Gb/s OFDM-System

Sampling rateReq. bandwidthReq. resolutionSubcarrier

modulation

Data rate

Experimental parameter:

•Tx: 10 GS/s (3.5GHz bw) 8bit; RX: 20GS/s (6GHz bw) 10bit interp.

•OFDM: 512-FFT, 342 subcarriers, 4-PSK, 3.2ns CPPF

•Total bitrate 9 x 12.57Gb/s = 113.13Gb/s in 67GHz (1.68bit/s/Hz)


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23 | OFC’08, OMU5 | Month 2008

System performance: Back-2-Back

Increase of datarate x9 => 9.5dB penalty (= 10 log 9)

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

4 6 8 10 12 14 16 18 20 22 24 26 28 30

OSNR/0.1nm [dB]

log(

BER

)

12.5Gb/s B-2-B

113Gb/s- B-2-B

9.5dB


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24 | OFC’08, OMU5 | Month 2008

Measurement of Non-linear-threshold

-5

-4

-3

-2

-1

-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

P_launchedlo

g(B

ER)

113Gb/s- 5Spans113Gb/s- (2)+3Spans113Gb/s- (4)+1Spans

-5

-4

-3

-2

-1

-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12

P_launched

log(

BER

)

12.4Gb/s- 5Spans12.4Gb/s- (2)+3Spans12.4Gb/s- (4)+1Spans

Transmission over 5 Spans 80km SMF

12.4Gb/s NLT 1Span: 4dBm

113Gb/s NLT 1Span 8.5dBm


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25 | OFC’08, OMU5 | Month 2008

Signalcharakteristik von OFDM-Signalen: Signal to Average Ratio

-Hohe Peakwerte sind problematisch

- Nichtlineare Effekte sind leistungsabhängig

- Signalverfälschung

-Charakterisierung von Peaks bzw. SAR and PAR

- Amplitude des Peaks wird auf mittlere Amplitude des Signals normiert

- i.d.R. Angabe in dB

)()(

log20)( 11

tsts

tSAR ⋅=


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26 | OFC’08, OMU5 | Month 2008

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

0 2 4 6 8 10 12

20*log(SAR) [dB]

CC

DF

Investigation of signal power distribution

Description of optical signalPAPR: Ratio of max. power to averagebut: ‘density’ of peak values not regarded

CCDF: Complementary Cumulative Distribution Function ⇒ Probability for power (amplitude) exceeds given value

OFDM has statistical distribution of signal amplitudes already at TxStatistics is not influenced by CD

Similar distributions are present in conventional transmission systems w/o. DCF (fiber acts as Fourier transformer)

Multi span systems80km SMF per span

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

-10 -5 0 5 10 15 20

20*log(SAR) [dB]

CC

DF PRBS7

PRBS11PRBS15

FFT 1024


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27 | OFC’08, OMU5 | Month 2008

Transmission on SSMF

Comparison of 10 Gb/s with a 50 Gb/s scenario

Evaluation of peak values from span to span

10G: dispersion is low wrt. bandwidth consumption

•peaks survive over several spans• same peaks meet nonlinear degradation after optical amplification

50G: peak values arise and vanish from span to span

•different peaks meet nonlinear degradation after optical amplification0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16Number of 80km Spans

Am

plitu

de a

.u.

10G t=149.5 ns50G t=20.82 ns50G t=27.6 ns

In 16 span simulations: NLT rises from 11 to 16 dBm integrated power while increasing the bit rate from 10 to 50 Gb/s

OFDM has high and competitive nonlinear tolerances in high bit rate systems without DCMs


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28 | OFC’08, OMU5 | Month 2008

Power budget - nonlinear threshold

Investigation of power budgetMaximum launch powerMaximum link attenuationPreamp noise figureReceiver sensitivity

Tx Rx

ATT NFPlaunch Sens.

dB58+−−= NFATTPOSNR launch

OSNR at the receiver is given by

To achieve high OSNR highest Plaunch is required, which is limited bynonlinear effects


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Nonlinear thresholds

High nonlinear thresholds support OFDM at highest data rates

5

7

9

11

13

15

17

19

0.1 1 10 100Bitrate [Gb/s]

Non

linea

r thr

esho

ld [d

Bm

] 16 spans 2048 FFT1 span 2048 FFT

5.6 times bitrate

2 times bitrate

2.1 dB NLT improvement


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Realisierungsaspekte


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Realization aspects

Investigation of DSP, ADC and DAC

for 100Gb/s 50 GSa/s components are required

Interface rate beyond 1Tb/s

due to high interface rates integrated converter – DSP chips are first choice

Investigation of DSP complexity


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Setup of 113Gb/s OFDM-System

DSP: Tx

– IFFT

DSP: Rx

– synchronization

- Frequency offset compensation

- FFT

- channel equalization

- polarization demux


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33 | OFC’08, OMU5 | Month 2008

CMOS building blocks

ACSU

Flip-Flop Flip-Flop-IP

Comparator

Sample& HoldCircuit

Comparatoractive Peak.

Investigation of CMOS buildingblocks

CMOS S&H as key element for

ADCs

ACS unit as key element for VE

Testchip realized in 90nm CMOS


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Equalizer building blocks: test of S&H resolution

The corresponding

Marker Delta fdelta=230 MHz Pdelta= –28dB

freqData =19.9 GHz PData = 5 dBmfreqclk = 20 GHz Ampclk = 1.2Vpp

bits

dBSINAD

36.402.6

76.1ENOB

=

−=

Pdelta

Beat frequency test: 4.36 bit resolution

BER method @ 10 GHz: 4.0 bit resolution

90nm CMOS is suitable for low resolution 40 GSa/s ADCs


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Feasibility-check for real time system

No dedicated DSPs for O-OFDM available today

Real time systems should base on FPGAs with subsequent DACs or ADCs

Assuming DAC with 10 GSa/s, 6 bit resolution (faster DACs are available)

FPGA

DAC

DAC

6x10Gb/s12x5Gb/s

6x10Gb/s12x5Gb/s

10GSa/s6 bit res.

10GSa/s6 bit res.

FPGAs (e.g. Xilinx) have high speed interconnects with <10 Gb/s rate,

5 Gb/s rate is highest alternative: 24 interconnects are required for setup

Highest complex FPGAs have 24 high speed interconnects

Today no significant increase for base data rates beyond 10Gb/s possible


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36 | OFC’08, OMU5 | Month 2008

Feasibility-check for real time system

OFDM Tx implementation in FPGAData rate 10 Gb/s, QPSK modulation

Input word length: 6 bit

FFT is highest complexity block in Tx

Radix-8 assumed as an efficient realization

Utilization of FPGA resources for 512-FFTFPGA type for investigations: XCE4VFX140

Flip Flops: 8 %

LUTs: 68%!!

Radix-8 FFT

ConclusionsImplementation of OFDM Tx for 10 Gb/s using FPGA is feasible

Increase of data rate or FFT size is limited by FPGA

• in logic resources

• interconnection resources to ADC and DAC


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Potential hardware implementation: FFT

FFT implementation

Loading of data into a parallel structure50GSa, FFT256: 200 MHz processing in a pipelined FFT, 50 MHz @ FFT1024Very suitable for subrate processing, further parallelization or pipelining possibleStructure is same as proposed for frequency domain CD compensation for coh. QPSK, but for whole data stream required (complexity roughly times 4)

FFT is highest effort block within algorithmButterfly is most importantsub-structure


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Potential hardware implementation: FFT

FFT implementation

Each butterfly consists of 1 complex mul and 2 addNumber of stages 128 @ FFT256, 512 @ FFT1024 Number of butterflys 1024 @ FFT256, 5120 @ FFT1024In sum 4x4.7Mgates = 19 Mgates are required, but distributed to Tx and Rx (half each)


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DSP complexity comparison – 1: FFT

Small relative increase in hardware with FFT increaseOFDM: processing of reduced number of samples

Comp. To QPSK: overlap requires higher number of samples, FFT is more complex

(bases on Carlos inputs)

FFT complexity

0

2000

4000

6000

8000

10000

12000

0 500 1000 1500 2000

FFT size

num

ber o

f but

terfl

ies

0

1

2

3

4

5

6

7

8

9

10

num

ber o

f but

terfl

ies

per i

nput

absoluterelativeCoh. QPSK overlap 256OFDM 20% CPPFCoh. QPSK overlap 380

33% reduced


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40 | OFC’08, OMU5 | Month 2008

Potential hardware implementation: Channel equalization

Frequency

…

Nsc+NPSC

Tim

e

…..

…

…..

Frequency Domain Interpolation

Pilot Subc. Data Subc.

Reference Symbol

Tim

e D

om

ain

Ave

ragin

gn

Application of pilot signals within the OFDM signal


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41 | OFC’08, OMU5 | Month 2008

Potential hardware implementation: LO offset estimation and correction

Coherent RX with LO freq. offset + phase noise

Estimation + Compensation of LO offset

))(2( ttfj offe φπ + kTfj este π2−

T

CPFFT

Equ.

channel est.freq. offset est.

LO offset correction is a simple complex multiplication of time domain samples

Yk


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42 | OFC’08, OMU5 | Month 2008


T

receivedNc ,

Na

equalizedNc ,


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43 | OFC’08, OMU5 | Month 2008


Mathmatical operations for OFDM are very simple and very few

Adaptation of system is performed by pilot signals and pilot subcarriers, no blind adaptation required


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44 | OFC’08, OMU5 | Month 2008

Comparison QPSK - OFDM

ECOC 2008, We2E4, B. Spinnler, “Adaptive equalizer complexity in coherent optical receivers”

Similar/comparable approaches for QPSK and OFDM Time domain processing is more complexQPSK: full frequency domain processing challenging

monitor channel advantageous for adaptationIn sum: OFDM has half complexity


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45 | OFC’08, OMU5 | Month 2008

Schlußfolgerung

O-OFDM untersucht

Definition von Szenarios für 100 Gb/s Systeme

Diese haben eine gute Empfindlichkeit und hohe nichtlineare Schwellen, sodass das OSNR Budget ein flexibles Systemdesign erlaubt

Erfordert 50 GSa/s DACs und ADCs, Testchips lieferten ermutigendeErgebnisse

Erfordert DSP in Tx und Rx, Gesamtaufwand ist vergleichsweise gering

O-OFDM ist attraktive Technik für optische Systeme bei höherenDatenraten.


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46 | OFC’08, OMU5 | Month 2008

www.alcatel-lucent.com


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47 | OFC’08, OMU5 | Month 2008

Figure 10: Schematic of a 112-Gb/s PDM-OFDM transceiver architecture. PMDC: PMD compensation. EDC: electronic dispersion compensation. PA-CPEC: pilot-assisted common phase error compensation. DAC: digital-to-analog converter. ADC: analog-to-digital converter.


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0 50 100 150 200 250-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

time [a.u.]

ampl

itude

[a.u

.]

PRBS7

Frequency domainQPSK modulated subcarriers (128 subcarriers)

All have same power

Information is coded in phase

Time domainno constant amplitude (envelope)

critical data pattern leading to high signal peak power (PRBS7 and all “0”)

probability of amplitudes depend on FFT length and data pattern (e.g. PRBS length)

0 50 100 150 200 250-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

time [a.u.]

ampl

itude

[a.u

.] all „0“

very high peak values may occur

OFDM signal characteristic

frequency [a.u.]0 50 100 150 200 2500

0.1

0.2

0.3

0.40.5

0.6

0.70.8

0.91

ampl

itude

[a.u

.]

Documents

Optisches OFDM – Eine neue Technik für die optischen ... · PDF fileOptisches OFDM – Eine neue Technik für die optischen Datenübertragung Fred Buchali Alcatel-Lucent, Bell Labs