1

Introduction to Optical OFDM

Richard Schatz

Laboratory of Photonics and Microwave Engineering

KTH

2

Outline

• Introduction – why not 100Gb/s On-Off Keying

• Multilevel formats: QPSK, QAM

• Multicarrier formats: SCM, OFDM

• OFDM characteristics

• OFDM experiments

• Conclusions

3

Source:Great Wall Broadband Network

> 100 km, 10-40 Gbit/s

DFB with Integrated

or External Modulator

> 5 km, 2.5-10 Gbit/s

Directly Modulated

DFB or DBR Laser

< 5 km , 0.1-2.5 Gbit/s

Directly Modulated

VCSEL

Network Structure

P2P filesharing: 20% of internet traffic

Online video (e.g. Youtube): 27% of internet traffic

Next step 100 Gbit/s!

4

112 Gb/s Complete On-off-keying ETDM System

Successful 42 km transmission over

dispersion compensated fiber in

ground demonstrated by

ACREO+KTH

Transmitter made by

KTH+Syntune+Svedice

J. Li, et al.”112 Gb/s Field Trial of Complete ETDM System Based

on Monolithically Integrated Transmitter & Receiver Modules for

Use in 100GbE”, ECOC 2010

5

The problem: fiber dispersion

Different wavelength components travel with different velocity

Dispersive fiber

Distance 1/(Bitrate)2

10 Gbit/s: 65 km

40 Gbit/s: 4 km

100 Gbit/s: 650 m!

Adaptive dispersion

compensation needed but

still difficult to reach e.g. 65

km with 100 Gbit/s!

The solution?

6

1998

DVBT

OFDM

with QAM

1991

GSM

GMSK

Gaussian

Minimum

Shift

Keying

1995

DVBS

QPSK

Quadrature

phase shift

keying

2001

UMTS (3G)

W-CDMA

1994

GPS

CDMA

Code

Division

Multiple

Access

1998

ADSL

DMT

Discrete

multitone

1991

WiFi

OFDM or

CCK

Orthogonal

frequency-division

multiplexing

Next generation optical transmission systems will be

advanced digital radio systems at optical frequencies

Radio systems today are the future for photonics

7

Why advanced modulation formats?

•Better tolerance to fiber dispersion

•More wavelength channels per fiber (or higher bitrate for

same channel grid)

•Lower bandwidth demands of electronics and photonics

Higher spectral efficiency!

( lower modulation bandwidth for same bitrate)

DQPSK, QPSK, QAM, OFDM, SCM, SSB...

8

OOK

1bit/symbolEb/N0=10dB(for BER=10-3)

PSK

1bit/symbolEb/N0=7dB(for BER=10-3)

QPSK

2bits/symbolEb/N0=7dB(for BER=10-3 Gray Coding)

16-QAM

4bits/symbolEb/N0=11dB(for BER=10-3 Gray Coding)

Different carrier modulation formats

+ polarisation multiplex!

cos(2t)

sin(2t)

9

SNRBC 1log 2max

Spectral Efficiency Limit

The maximum capacity Cmax (correctly transmitted bits/s) of a communication channel

with additive white Gaussian noise and bandwidth B is given by the Shannon-Hartley law:

Spectral efficiency proportional

to the SNR in dBIf SNR>>0dB: bps/Hz 33.0log

)2(log

1dB10

10

max SNRSNRB

C

Spectral efficiency is proportional

to the SNR in linear unitsbps/Hz 44.1

)2ln(

)1ln(max SNRSNR

B

C

If SNR<<0dB:

If SNR=0dB: bps/Hz 1max B

C

10

http://www.dspdesignline.com/howto/208801783

for BER=10-5

Spectral Efficiency for Various Modulation Formats

11

Multilevel modulation

50 GHz

Radio Carrier

200 THz50 GHz

Optical Carrier

Optical Carrier

200 THz

Intensity modulation

Direct or coh. detect.

Opt. IQ modulation

Opt. Coherent detection

Baseband

B

El. IQ modulation

El. Coherent detection

Direct or Coherent Detection System

Baseband

Coherent Detection System

Optical Carrier not needed if coherent detection!

12

Optical QPSK system with Polarization Multiplex

Complex integrated optical

transmitters & receivers will be

needed for low cost!

(one channel in a WDM system)

Coherent receiver enables

electrical dispersion

compensation, pol-tracking

and phase locking

LO-phase noise potential

problem

13

Linewidth values necessary for 10 Gbaud

DPSK: <30MHz

DQPSK: <3MHz

8PSK: <300kHz

J. Kahn and K. Ho, \Spectral Efficiency Limits and Modulation/Detection

Techniques for DWDM Systems," IEEE J. Select. Topics Quantum Electron., vol.

10, pp. 259-272, 2004.

Evgeny Vanin and Gunnar Jacobsen, Analytical estimation of laser phase noise induced

BER floor in coherent receiver with digital signal processing, Optics Express, Vol. 18, Issue

5, pp. 4246-4259 (2010)

S. Savory and A. Hadjifotiou, ”Laser linewidth requirements for optical DQPSK systems,"

IEEE Photonics Technol. Lett., vol. 16, pp. 930 - 932, 2004.

D.-S. Ly-Gagnon, K. Katoh, and K. Kikuchi, \Coherent Demodulation of Differential 8-

Phase-Shift Keying with Optical Phase Diversity and Digital Signal Processing," in

Lasers and Electro-Optics Society (LEOS), annual meeting, 2004

These values assume white frequency noise=Lorentzian linewidth.

Non-flat frequency noise will affect the system differently

14

Multicarrier modulation formats

200 THzk.10 GHz

Optical Carrier

SCMSubcarrier modulation

1-20 subcarriers, RF-generated

OFDMOrthogonal Frequency

Division Multiplex

100-10000 subcarriers,

digitally generated.

Carriers orthogonal over

symbol slot 200 THzk.100 MHz

More subcarriers Less bitrate per subcarrier Longer symbol

length T

15

Optical SCM System

• Compare with ADSL modem for high speed data over telephone line

• High demands on linearity of modulator and detector

• Integrated optical components needed for low cost

16

Orthogonal Frequency Division Multiplex

•Used for e.g. Wifi, DVBT, DAB and Digital Radio

Mondial

•Many (50-10000) digitally generated subcarriers and

long symbol slot time T

•Subcarrier frequencies are spaced 1/T from each other

so they are orthogonal over a symbol slot independent of

their phase and amplitude. Hence each subcarrier can be

QPSK or QAM modulated

•The minimum symbol length T is determined by

dispersion, the maximum determined by phase noise of

laser

•With an separate RF pilot carrier as phase noise

reference, it is possible to use up to 100 ns symbol length

for 100 GbE

Spectrum

17

OFDM system

Pictures from http://www.wirelesscommunication.nl/reference/chaptr05/ofdm/ofdmmath.htm

To compensate for distortion in

receiver a temporal guard band

(cyclical prefix) with a length of

the impulse response of fiber is

needed

18

Comparison single carrier QPSK vs OFDM

QPSK OFDM - QPSK

OSNR requirements 13 dB 13 dB

Spectral Efficiency 4 bit/Hz (with PolMux) 4 bit/Hz (with PolMux)

Nonlinear tolerance + - (high peak-average ratio)

Filtering tolerance + + (+ known which bits are lost)

Dispersion tolerance + ++

Transmitter complexity + - (DSP+DAC needed)

Receiver complexity - -

System complexity - - -

S. L. Jansen , B. Spinnler, I. Morita, S. Randel and H. Tanaka, ”100GbE: QPSK versus OFDM” Optical Fiber

Technology,Vol. 15, Issues 5-6, Oct.-Dec. 2009, Pages 407-413

19

Optimum modulation format for a power limited system!

It is possible to reach the

ultimate power limit

CNP 0)2ln(

by increasing the number

of orthogonal functions,

i.e. OFDM with many

subcarriers

But the power

consumption in DA/AD

converters and DSP-

units is several orders of

magnitude larger than

the transmitted power ln(2)

Shot noise limited

N0 = h and Rb = C =100

Gbit/s gives P=9.2nW!

20

21

Time signal Frequency spectrum

Discrete Multitone using VCSELs

22

PDPD1 • 6:00 p.m.

Field Trial of a Real Time, Single Wavelength, Coherent 100 Gbit/s PM, QPSK Channel Upgrade of an

Installed 1800km Link, AT&T Labs, Optical Systems Res., USA, Opnext, Inc., USA, Cisco Systems Inc,

Canada. We demonstrate a real time, single wavelength, coherent 100G PM QPSK upgrade of a field

system. Performance sufficient for error free operation after forward error correction was achieved over

installed 900km and 1800km links, proving the viability of seamless 100Gb/s upgrades. (using 4 ADCs and

FPGA-array based real time signal processing from Opnext),

PDPD4 • 6:36 p.m.

End to End Native IP Data 100G Single Carrier Real Time DSP Coherent Detection Transport over

1520 km Field Deployed Fiber, Verizon, USA, NEC Corp., Japan, NEC Corp. of America, USA, Juniper

Networks, USA, Finisar Corp., USA. The first end-to-end 100G transport of native IP packet traffic over

1520‐km field deployed fiber is realized with multi suppliers 112 Gb/s single carrier real time coherent DP-

QPSK DWDM transponder, 100GE router cards, and 100G CFP interfaces. (NEC single-carrier 112 Gb/s

prototype transponder at 28 Gbaud)

PDPD9 • 7:36 p.m.

41.25 Gb/s Real Time OFDM Receiver for Variable Rate WDM OFDMA PON Transmission, NEC Labs

America, USA. The first real time, record speed 41.25Gb/s 8QAM OFDM receiver is demonstrated using

FPGA based DSP. Superior upstream WDM OFDMA PON performance with variable rate transmission is

exhibited over 20km SSMF and a 1:32 split.

OFC Postdeadline Papers

The future is already here!

23

Two intensity modulated wavelengths to transmit I and Q of OFDM

signal separately with 22 subcarriers on each

27 Gb/s QPSK (B=6.9GHz), 41Gb/s 8-QAM (B=6.9GHz) or 30Gb/s

16-QAM (B=3.75GHz) over 20 km fiber

0 km 20 km

24

Conclusions

Future high speed photonic transmission systems will be advanced

radio systems at optical frequencies and utilize:

• multilevel modulation waveforms: QPSK, QAM

• polarization multiplex

• frequency division multiplex: WDM(+SCM/OFDM)

• integrated photonic frontends

• advanced electronic signal processing

OFDM combined with n-QAM is in many ways the ultimate

modulation format (channel tolerant, adaptive, minimum energy

per bit) but is difficult to realize today above 10 Gb/s speed