introduction to optical ofdm
TRANSCRIPT
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Introduction to Optical OFDM
Richard Schatz
Laboratory of Photonics and Microwave Engineering
KTH
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Outline
• Introduction – why not 100Gb/s On-Off Keying
• Multilevel formats: QPSK, QAM
• Multicarrier formats: SCM, OFDM
• OFDM characteristics
• OFDM experiments
• Conclusions
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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!
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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
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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?
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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
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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...
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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)
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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
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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
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http://www.dspdesignline.com/howto/208801783
for BER=10-5
Spectral Efficiency for Various Modulation Formats
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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!
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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
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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
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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
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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
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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
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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
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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
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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!
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Time signal Frequency spectrum
Discrete Multitone using VCSELs
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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!
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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
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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