Optical OFDM for Long ---Haul Transport Networks - Sander Lars Jansen

1 © Nokia Siemens Networks

Nokia Siemens Networks, Munich, Germany, email: [email protected]

Optical OFDM for LongOptical OFDM for LongOptical OFDM for LongOptical OFDM for LongOptical OFDM for LongOptical OFDM for LongOptical OFDM for LongOptical OFDM for Long--------Haul Transport Haul Transport Haul Transport Haul Transport Haul Transport Haul Transport Haul Transport Haul Transport NetworksNetworksNetworksNetworksNetworksNetworksNetworksNetworks

Sander Lars Jansen

2 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks

Contributions to this presentation

� KDDI R&D Laboratories

� Itsuro Morita

�Hidenori Takahashi

�Abdullah Al Amin

�Hideaki Tanaka

� Philips

�Tim Schenk

� Uppsala University

�Kamyar Forozesh

� Nokia Siemens Networks

�Dirk van den Borne


Motivation

� Optical OFDM is currently a hot topic in the fiber-optic research community:

But what is OFDM exactly and what are the benefits/challenges? But what is OFDM exactly and what are the benefits/challenges?

OFC 05 ECOC OFC 06 ECOC OFC 07 ECOC OFC 080

20

40

60

Conference

#O

FD

M p

ape

rs


Outline

� Introduction: What is OFDM?

� Generation and detection of OFDM

�Electrical domain

�Optical domain

� Polarization multiplexing: MIMO

� OFDM system design rules and overheads

�Phase noise compensation

�Cyclic prefix overhead

�Training symbol overhead

� Multi-band OFDM

� Latest research results

� Conclusion


Outline




�Optical domain






� Multi-band OFDM


� Conclusion


OFDM transmission concept

timelength of one symbol

f4

f3

f2

f1

Ch #

1 n

am

plitu

de

frequency

Time-domain Frequency-domain

T im e ( a . u . )

Am

plit

ud

e (

a.u

.)

sin(x)/x


How does modulation work for OFDM?


f4

f3

f2

f1

Unmodulated carrier Modulated carrier


f4

f3

f2

f1

The subcarriers of the OFDM signal can be modulated in phase and amplitude

Modulatedconstellation


Constellation diagrams

BPSK

Commonly multi-level modulation formats are used to encode multiple bits per

OFDM symbol

QPSK 8-QAM 16-QAM

1 bit/Symbol 2 bits/Symbol 3 bits/Symbol 4 bits/Symbol

However, increasing the constellation size will reduce the distance between theconstellation points and with that increase the OSNR requirement


How do we generate and detect OFDM subcarriers?

� Many different methods exist to generate subcarriers

Baseband TXs

Mixers

rf source

2x 3x 4x

This method is commonly referred to as

Coherent WDMAll-optical OFDMAnalogue OFDM

For the creation of many subcarriers, a more efficient way is togenerate the subcarriers in the digital domain using an FFT

amplitude

frequency

~

B1B2

B3B4

f 2f 3f 4f


=

−−−

−

−

c

ccc

c

c

NNNN

N

N

y

y

y

y

WWW1

WWW1

WWW1

1111

3

2

1

2)1()1(2)1(

)1(242

)1(21

M

L

MOMMM

L

L

L

x

amplitude

DCf 2f

frequency


Digital OFDM creation/detection: uses FFT and IFFT

)exp(W 2Nc

j π−=

FFT

y1y2y3

yNc

Input #

x1x2x3

xNc

Fyx =

Nc*f




FFT

y1y2y3

yNc

Input #

x1x2x3

xNc

Nc

amplitude

DCf 2f (Nc-1)*f




FFT

y1y2y3

yNc

Input #

x1x2x3

xNc

Sampled signal, thus repetition in the frequency domain after Nc*fNc

amplitude

DCf 2f

frequency

(Nc-1)*f (Nc+1)*f(2Nc-1)*f

Nc*f(-Nc+2)*f

-2Nc*f




FFT

y1y2y3

yNc

Input #

x1x2x3

xNc

Sampled signal, thus repetition in the frequency domain after Nc*fNc

amplitude

DCf 2f

Apply filter to remove aliasing products


Frequency-domain Time-domain

Digital OFDM creation: Example, an 8-size FFT

0 Hz

DC-subcarrier

1 2 3 4 55 6 7 8

IFFT

Input #:

s1s2s3s4s5s6s7s8

TX: IFFT

Pa

ralle

l to

se

rialu1

u2u3u4u5u6u7u8

u1u2u3u4u5u6u7u8

Nyquist-subcarrier

The signal in the frequency domain has negative frequencies

u1u1……u8 is a complex signalu8 is a complex signal

Fyx =RX: FFT

sFu1−

=


Outline




�Optical domain






� Multi-band OFDM


� Conclusion


Fiber-optic OFDM system

OFDM

Baseband

generation

Modulation

Detection

OFDM

Baseband

detection

Transmission line

DACs

ADCs

Digital domain Analogue domain


Baseband modulation

10101100 S

eri

al/P

ar

Chromatic

DispersionIF

FT

Par/

Seri

al

Cycli

c p

refi

x1 0

1 0

1 1

0 0

Map

pin

g

1011

01 00

TRANSMITTER: Digital signal processing

(1-j)

(1+j)(-1+j)

(-1-j)


Baseband modulation

10101100 S

eri

al/P

ar

IFF

T

Par/

Seri

al

Cycli

c p

refi

x1 0

1 0

1 1

0 0

Map

pin

g

Time (a.u.)

Am

plit

ude (

a.u

.)

TRANSMITTER: Digital signal processing


Baseband modulation

10101100 S

eri

al/P

ar

Chromatic

DispersionIF

FT

Par/

Seri

al

Cycli

c p

refi

x1 0

1 0

1 1

0 0

Map

pin

gTRANSMITTER: Digital signal processing


Baseband modulation: training sym.

� At the receiver, the training symbols are used for synchronization and channel estimation

Seri

al/P

ar

IFF

T

Par/

Seri

al1 0

1 0

1 1

0 0

Map

pin


Tra

inin

g S

ym

.

Cycli

c p

refi

xOFDM symbol size

TS TSPayload

TS: Training symbol


Baseband modulation: training sym.

Seri

al/P

ar

IFF

T

Par/

Seri

al1 0

1 0

1 1

0 0

Map

pin


Tra

inin

g S

ym

.

Cycli

c p

refi

xOFDM symbol size

Tektronix AWG7102� Features

� 2 outputs

� 5 GHz bandwidth

Baseband Transmitter

OFDM baseband transmitters reported so far are offline


RECEIVER: Digital signal processing

Baseband detection: Synchronization

10101100

1-T

ap

EQ

Dem

ap

pin

g

Seri

al/P

ar

RX

1 0

1 0

1 1

0 0

FF

T

Par/

Seri

al

CP

rem

oval

Sym

bo

l S

yn

c.

Channel estimation

TS

rem

oval

TS TSPayload

xcorr

Stored TS

abs(.)2 max(.)Signal in

OFDM symbol synchronization can be realized in many different ways. One method is to correlate the incoming data with a stored training symbol



Channel estimation

1-Tap equalizer

10101100

1-T

ap

EQ

Dem

ap

pin

g

Seri

al/P

ar

RX

1 0

1 0

1 1

0 0

FF

T

Par/

Seri

al

CP

rem

oval

Sym

bo

l S

yn

c.

Channel estimation

TS

rem

oval

Channel estimation and 1-tap equalization is required to compensate for linear impairments such as chromatic dispersion


1-Tap equalizer

Preamble Payload Payload

1-Tap equalizer

sent

received

1

2

3

Subcarrier number



Channel estimation

10101100

1-T

ap

EQ

Dem

ap

pin

g

Seri

al/P

ar

RX

1 0

1 0

1 1

0 0

FF

T

Par/

Seri

al

CP

rem

oval

Sym

bo

l S

yn

c.

Channel estimation

TS

rem

oval

Baseband Receiver

Tektronix DPO72004� Features

� 4 inputs

� 16 GHz bandwidth

OFDM baseband receivers reported so far are offline


Fiber-optic OFDM system

OFDM

Baseband

generation

Modulation

Detection

OFDM

Baseband

detection

Transmission line

DACs

ADCs

Digital domain Analogue domain

Features

� 4 inputs� 16 GHz bandwidth

Features

� 2 outputs� 5 GHz bandwidth


B

Fiber-optic OFDM detection methods

B

Optical spectrum Electrical spectrum

B

B

hybrid

B

Optical carrier

B

Direct detected

Optical OFDM

(DDO-OFDM)

LO

Coherent detected

Optical OFDM

(CO-OFDM)


�� Coherent optical (CO) OFDMCoherent optical (CO) OFDM

OFDM detection

�� Direct detected (DD) OFDMDirect detected (DD) OFDM

�� Least components required at the receiver -> most cost-effective solution.

�� Guard band required.

�� At least 50% of power required for optical carrier

-> Inherently 3-dB OSNR penalty.

CO-OFDM -> long-haul transmission systems.

�� Superior transmission performance.

�� Polarization dependent.

�� Most complex setup to realize

(phase noise compensation required)

DD-OFDM -> short reach applications

B

B

Optical carrier

B



Digital OFDM creation: Example, an 8-size FFT

0 Hz

1 2 3 4 55 6 7 8

IFFT

Input #:

s1s2s3s4s5s6s7s8

Input #

Pa

ralle

l to

se

rialu1

u2u3u4u5u6u7u8

u1u2u3u4u5u6u7u8

The signal in the frequency domain has negative frequencies

u1u1……u8 is a complex signalu8 is a complex signal


ττττ

IQ mixer

Transmitter: electrical or optical IQ mixing

Electrical IQ mixer

~~~~Baseband

IMAG

Baseband REAL

Baseband REAL

Baseband IMAG

Optical IQ mixer

To modulate a complex signal IQ mixing is required. This can be realized in the electrical (left) or the optical (right) domain.


ττττ

IQ mixer

Transmitter: electrical or optical IQ mixing

Electrical IQ mixer

~~~~

Electrical carrier

Optical carrier

Baseband IMAG

Baseband REAL

Baseband REAL

Baseband IMAG

Optical IQ mixer

BasebandBasebandReal Imag

Optical carrier


Receiver: Coherent detection methods

LOLO

ττττ

LO

OFDMsignal

LO

OFDMsignal

Realpart

Realpart

Imagpart

Heterodyne reception Homodyne/Intradyne reception(Wireless: direct downconversion)

Electrical equivalent: Electrical equivalent:

(Balanced mixer) (IQ mixer)

Balanced PDs can be used to cancel unwanted mixing of subcarriers,

but with proper filtering result in the same performance

ττττ = 90º

LO

RXj

ADC

ADCIQ

180ºhybrid

90º

hybrid

LO

RXj

ADC

ADC


LO

Coherent detection methods

Heterodyne reception Homodyne/Intradyne reception(Wireless: direct downconversion)

LO

90º

hybrid

LO

RXADC

�� Only one PD required at full bandwidth.

� Common implementation:� 2x2 passive coupler

� Common implementation:� 3x3 passive coupler (2:2:1 ratio)

� free-space (half-mirror and beam splitters)

�� Two PDs required at half the bandwidth.

�� Polarization sensitive -> requires polarization diversity

RXj

ADC

ADCIQ

180ºhybrid j

ADC

ADC

IQ

LO


Outline




�Optical domain






� Multi-band OFDM


� Conclusion


FFT

FFT

Det.

Det.

Polarization division multiplexing

h11

h12

h21

h22

s1

s2

x1

x2

Mod.

Mod.

IFFT

IFFT

( ) ( ) ( ) ( )k k k k= +x H s n

H

� For subcarrier k:

Polarization diverse receiver

NoiseChannel transfer function


MIMO processing

s1

s2~

~

Channel

estimation

FFT

FFT

Det.

Det.

MIMO processing

h11

h12

h21

h22

s1

s2

x1

x2

Mod.

Mod.

IFFT

IFFT

x1

x2

s1

s2

~

~

s1

s2


MIMO processing

s1

s2~

~

TS

Channelestimation

FFT

FFT

Det.

Det.

Channel estimation 1/2

h11

h12

h21

h22

s1

s2

x1

x2

Mod.

Mod.

IFFT

IFFT

TS H~

TSPayload

Tx 1

Tx 2

TS

� TS: Training symbol

� 1 OFDM symbol


TS

Channelestimation

FFT

FFT

Det.

Det.


h11

h12

h21

h22

s1

s2

x1

x2

Mod.

Mod.

IFFT

IFFT

TS H~

Tx 1

Tx 2

TS

Rx 1

Rx 2

TS

s1_t1

s2_t2

x1_t1

x2_t2x2_t1

x1_t2

Transmitter Receiver

MIMO processing

s1

s2~

~


TS

Channelestimation

FFT

FFT

Det.

Det.


h11

h12

h21

h22

s1

s2

x1

x2

Mod.

Mod.

IFFT

IFFT

TS H~

Tx 1

Tx 2

TS

Rx 1

Rx 2

TS

s1_t1

s2_t2

x1_t1

x2_t2x2_t1

x1_t2

s1_t1 x1_t1

x2_t1

h11 = x1_t1 / s1_t1

h12 = x2_t1 / s1_t1

~

~

MIMO processing

s1

s2~

~


TS

Channelestimation

FFT

FFT

Det.

Det.


h11

h12

h21

h22

s1

s2

x1

x2

Mod.

Mod.

IFFT

IFFT

TS H~

Tx 1

Tx 2

TS

Rx 1

Rx 2

TS

s1_t1

s2_t2

x1_t1

x2_t2x2_t1

x1_t2

s2_t2

x1_t2

x2_t2

h21 = x1_t2 / s2_t2

h22 = x2_t2 / s2_t2

~

~

MIMO processing

s1

s2~

~


TS

MIMO processing

Channelestimation

FFT

FFT

Det.

Det.

MIMO processing

h11

h12

h21

h22

s1

s2

x1

x2

Mod.

Mod.

IFFT

IFFT

TS H~

s1

s2~

~

)()(~

)(~ kkk xHs+

=

( ) *1*HHHH

−+=

MIMO processing:

With the pseudo-inverse defined as:


Outline




�Optical domain






� Multi-band OFDM

� Latest research result

� Conclusion


OFDM overheads

� Independent of the type of OFDM system there are some general design rules and

overheads:

� Phase noise compensation technique -> Pilot subcarrier overhead

� Training symbol spacing -> Training symbol overhead

� Guard time -> Cyclic prefix overhead

without compensation

with

compensation


Phase noise compensation method 1: Common phase estimation (CPE)

Time

OFDM symbol:

1 2 3 4 5 6 7 8S

ub

ca

rrie

r

OFDM symbol

1 2 3 4 5 6

Data

Pilot

subcarriers

Per OFDM symbol, one

phase estimate is done by

averaging the phase of the Pilot subcarriers


Phase noise compensation method 1: Common phase estimation (CPE)

TimeSymbol size

�� Per OFDM symbol, one phase estimate

�� During symbol period no large phase deviations allowed

�Requires lasers with a small linewidth

�Requires a small FFT-size

Phase difference Phase estimation


Phase noise compensation method 2: RF-pilot phase noise compensation

R F -P ilo t

frequencyCarrier

frequency

Insertion of RF-pilot tonea

mp

litu

de

frequency frequency

am

pli

tud

eNormal RF-pilot


LPF

Signal in

(.)*

Signal out


R F -P ilo t

Compensation at the receiver

ECL laserDFB laser

The RF-pilot ‘monitors’ the phase difference between the TX

and LO laser. Conjugation of this signal provides the inverse ofthese distortions



TimeSymbol size

Phase difference (=RF pilot tone)

Conjugated RF pilot tone

Phase of compensated signal

Time

Time


Phase noise compensation summary

� Method 1: Common phase estimation

� Well known compensation concept from Wireless

� Compensates for large RF-carrier offsets

� Requires lasers with a narrow linewidth and short OFDM-symbols

� Requires ~10% extra OFDM overhead for subcarrier pilots

� Method 2: RF-pilot phase noise compensation

� Allows for lasers with wide linewidth and long OFDM-symbols

� Does not require additional OFDM overhead

� Computational complexity scales with RF-carrier offset that is to be

compensated for

More information on Method 2 can be found in: S.L. Jansen, et al., JLT, Vol. 26, pp. 6-15, 2008


OFDM overheads


overheads:





with

compensation

TS TSPayload

TS: Training symbol time


Training symbols overhead

� In an OFDM system training symbols are periodically used for channel

estimation.

� The training symbol overhead is dependent:

�Channel dynamics

�Symbol length

� For most systems the training symbol overhead is between 0.2% and 4%

20 40 60 80 1000

0.5

1

1.5

2

Symbol length (ns)

Ove

rhe

ad

(%

)

Training symbol overhead for 1 training period every 10µs


OFDM overheads


overheads:





with

compensation

FFT size Cyclicprefix

TS TSPayload

TS: Training symbol timeTotal symbol

length

Effectivedata


τt = 10ns

τt= 50ns

τt= 100ns

Cyclic prefix overhead

� Dispersion tolerance is dependent on the cyclic prefix, symbol length and data rate

� A large overhead causes a large increase in nominal data rate� For instance: 50% overhead for 100Gb/s -> 160.5Gb/s (inc. FEC)

� In practice, the cyclic prefix overhead varies between 4% and 20%

� A large dispersion tolerance requires long symbol lengths

Symbol lengthDataCyclic prefix

100%

overhead

50% overhead

20% overhead

(For 111Gb/s net data rate)

Cyclic prefixcp


Outline




�Optical domain






� Multi-band OFDM


� Conclusion


Multi-band OFDM: Bandwidth relaxation

� Multi-band OFDM is a well known concept in the wireless community

� In our experiments we introduced multi-band OFDM initially to relax the DAC requirements [1]

[1] S.L. Jansen, et al., proc. OFC 2007, PDP 15.

� A total data rate of 25.8-Gb/s was

realized by multiplexing two 12.9-Gb/s

OFDM bands in the electrical domain.

� The use of two OFDM bands reduced

the bandwidth requirements in this

experiment from ~7GHz to ~3.5GHz


I

Q

DA

CD

AC

LD

Digital data

LPF

LPF

IQ-mixer

AWG

OFDM TX 1

BP

F

LPF

IF LO

90°

Para

llel/S

eria

l

Vbias A

Bia

s-T

Bia

s-T

Vbias BA

dd c

yclic

pre

fix

OFDM TX 2

Ma

p.

Ch. 1 is set to zero

Ma

p.Serial/P

ara

llel

IFF

T

12

Nc

3 to transmission

line

MZ

Virtu

al

subcarr

iers

~~~ ~~~

Multi-band OFDM

After IQ mixer

(RB = 10 MHz)

S.L. Jansen, et al., proc. OFC 2007, PDP 15.

IF 1

After subcarrier multiplexing


Multi-band OFDM: BW Requirement

� 100GbE PDM-OFDM

� With Multi-band OFDM a significant reduction in required DAC/ADC bandwidth can be obtained


Multi-band OFDM: CP overhead (1/2)

Chromatic

Dispersion

Single-band

FFT size

time

fre

qu

en

cy

Cyclicprefix



Chromatic

Dispersion

Chromatic

Dispersion#1

#2

ICI from neighboring OFDM symbolSingle-band

Multi-band


� 100GbE PDM-OFDM

� 2000-km link, two symbol lengths, 10ns and 100ns

� Significant CP overhead reduction can be obtained with Multi-band OFDM

τt = 100ns

τt = 10ns



� Multi-band OFDM:

-10 -5 0 5 10-40

-20

0

20

Relative Frequency (GHz)S

pectr

um

(dB

)

3 4 5 6 7 8-5

0

5

10

15

20

25

30

Relative frequency (GHz)

Spectr

um

(dB

)

~250 MHz

Pro‘s/Con‘s Multi-band OFDM

IQ-mixer

IQ-mixerBaseband

1

Baseband 2

� DAC/ADC bandwidth reduction

� CP Overhead reduction

� More complex modulator/receiver, more DACs/ADCs required


Outline




�Optical domain






� Multi-band OFDM


� Conclusion


Nonlinear tolerance: Influence of SPM

Both Full and 10G exhibit a launch power penalty of 2.6 dB

Ref.

Full

10G

Single channel

-3.1 dBm

-5.8 dBm-5.7 dBm


Nonlinear tolerance: Influence of XPM

Ref: launch power penalty of 0.8dB due to XPM

10G&Full: launch power penalty of 1.9dB, 2.9dB due to XPM

Ref.

Full

10G

OFDM

WDM OFDM

Unlike with single carrier, SPM and XPM are enhanced with OFDM in a periodic dispersion map -> Not suitable for network upgradesNot suitable for network upgrades


192.628 192.648 192.668 192.688 192.708-40

-30

-20

-10

0

Frequency [THz]

Pow

er

[dB

m]

Channel spacing =9GHz Signal bandwidth = 8.4GHz

Odd ch.

Even ch.

PBS

3dB

1 symbol delay

VOA

8x66.8 Gbit/s at 9-GHz channel spacing

AWG #2

Experimental setup (TX)

AWG #1

ch1

ch7 I Q

I Q

ch3

ch5 TX

x3

82 km x4

DGELSPS

These experiments have been conducted at KDDI R&D Laboratories


192.628 192.648 192.668 192.688 192.708-40

-30

-20

-10

0

Frequency [THz]

Pow

er

[dB

m]

Channel spacing =9GHz Signal bandwidth = 8.4GHz

Odd ch.

Even ch.

PBS

3dB

1 symbol delay

VOA

8x66.8 Gbit/s at 9-GHz channel spacing

AWG #2

Experimental setup (TX)

AWG #1

ch1

ch7 I Q

I Q

ch3

ch5 TX

x3

82 km x4

DGELSPS

B

A

Symbol length

TS

TS

data

data

Delay between polarizations

data

data



� Local oscillator (LO):

� 100-kHz linewidth External cavity laser.

� Phase noise compensation

� RF-aided phase noise compensation -> S.L. Jansen, et al., JLT, Vol. 26, pp. 6-15, 2008

Experimental setup (RX)

Offline processing

CP

re

mo

va

l S

eri

al to

pa

ralle

l

FF

T

MIMO processing

Channel estimation

Pa

ralle

l to

se

rial

BE

RT

TS

syn

ch

roniz

atio

n

Real-timeoscilloscope

BPF

~~~~~~

Ph

ase

nois

e c

om

p.

TS

re

mo

va

l

Integrated pol.-diverse hybrid

ADC

ADC

ADC

ADC

LO

90º

hybrid

90º

hybrid~~~

BPF

Single-ended photodiodes



192.628 192.648 192.668 192.688 192.708-40

-30

-20

-10

0

Frequency [THz]

Pow

er

[dB

m]

Optical Spectrum

Optical Spectrum at Rx

after OBPF with 12.5-GHz bandwidth

X Pol. Y Pol.

Constellation

Digital filter



10 15 20 25 305

4

3

2

-lo

g(B

ER

)

OSNR [dB]

Back to Back performance

66.8Gbit/s

16-QAM, 8 WDM

66.8Gbit/s 16-QAMSingle channel

1.1dB

60.9Gbit/s

8-QAM single pol. Single channel

8QAM 16QAM : 3.1dB

60.9 66.8 Gbit/s : 0.4dB

Theoretical OSNR penalty

4.4dB

3.5dB



Transmission Performance

� For all BER points: 5 x 4.25 million bits evaluated per WDM channel

� Average OSNR after transmission was 21.5 dB @640 km

192.63 192.65 192.67 192.69 192.715

4

3

2 -lo

g(B

ER

)

Frequency [THz]

FEC limitAfter 640 km transmissionAfter 320 km transmission



Conclusions

� In this talk OFDM has been discussed for fiber-optic applications

� Typically an OFDM signal consists of many (>50) subcarriers that are

generated in the digital domain using the FFT

� Polarization division multiplexing can be realized combination with MIMO

processing at the receiver

� The FFT size and the OFDM overheads are important design factors for

an OFDM transmission system

� OFDM can be an interesting modulation format for fiber-optic

applications, although we may not see the full benefit until we scale to

high signal constellations


Thank you!

[email protected]

�Questions?

More information: www.SLJansen.com

RF Imperfections in High-rate Wireless SystemsImpact and Digital Compensation

By Tim SchenkISBN: 978-1-4020-6902-4

Optical OFDM for Long ---Haul Transport Networks - Sander Lars Jansen

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Transcript of Optical OFDM for Long ---Haul Transport Networks - Sander Lars Jansen