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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
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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
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3 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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4 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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5 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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6 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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7 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
How does modulation work for OFDM?
timelength of one symbol
f4
f3
f2
f1
Unmodulated carrier Modulated carrier
timelength of one symbol
f4
f3
f2
f1
The subcarriers of the OFDM signal can be modulated in phase and amplitude
Modulatedconstellation
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8 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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9 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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10 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
=
−−−
−
−
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
Time-domain Frequency-domain
Digital OFDM creation/detection: uses FFT and IFFT
)exp(W 2Nc
j π−=
FFT
y1y2y3
yNc
Input #
x1x2x3
xNc
Fyx =
Nc*f
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11 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Time-domain Frequency-domain
Digital OFDM creation/detection: uses FFT and IFFT
FFT
y1y2y3
yNc
Input #
x1x2x3
xNc
Nc
amplitude
DCf 2f (Nc-1)*f
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12 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Time-domain Frequency-domain
Digital OFDM creation/detection: uses FFT and IFFT
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
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13 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Time-domain Frequency-domain
Digital OFDM creation/detection: uses FFT and IFFT
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
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14 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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−
=
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15 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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16 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Fiber-optic OFDM system
OFDM
Baseband
generation
Modulation
Detection
OFDM
Baseband
detection
Transmission line
DACs
ADCs
Digital domain Analogue domain
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17 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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)
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18 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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19 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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20 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
gTRANSMITTER: Digital signal processing
Tra
inin
g S
ym
.
Cycli
c p
refi
xOFDM symbol size
TS TSPayload
TS: Training symbol
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21 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Baseband modulation: training sym.
Seri
al/P
ar
IFF
T
Par/
Seri
al1 0
1 0
1 1
0 0
Map
pin
gTRANSMITTER: Digital signal processing
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
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22 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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23 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
RECEIVER: Digital signal processing
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
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24 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
1-Tap equalizer
Preamble Payload Payload
1-Tap equalizer
sent
received
1
2
3
Subcarrier number
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25 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
RECEIVER: Digital signal processing
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
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26 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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27 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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)
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28 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
�� 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
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29 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Time-domain Frequency-domain
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
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30 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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.
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31 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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32 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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33 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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34 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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35 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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36 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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37 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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38 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
TS
Channelestimation
FFT
FFT
Det.
Det.
Channel estimation 2/2
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~
~
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39 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
TS
Channelestimation
FFT
FFT
Det.
Det.
Channel estimation 2/2
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~
~
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40 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
TS
Channelestimation
FFT
FFT
Det.
Det.
Channel estimation 2/2
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~
~
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41 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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:
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42 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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 result
� Conclusion
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43 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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44 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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45 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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46 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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47 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
LPF
Signal in
(.)*
Signal out
Phase noise compensation method 2: RF-pilot phase noise compensation
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
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48 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Phase noise compensation method 2: RF-pilot phase noise compensation
TimeSymbol size
Phase difference (=RF pilot tone)
Conjugated RF pilot tone
Phase of compensated signal
Time
Time
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49 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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50 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
TS TSPayload
TS: Training symbol time
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51 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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52 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
FFT size Cyclicprefix
TS TSPayload
TS: Training symbol timeTotal symbol
length
Effectivedata
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53 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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54 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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55 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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56 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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57 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Multi-band OFDM: BW Requirement
� 100GbE PDM-OFDM
� With Multi-band OFDM a significant reduction in required DAC/ADC bandwidth can be obtained
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58 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Multi-band OFDM: CP overhead (1/2)
Chromatic
Dispersion
Single-band
FFT size
time
fre
qu
en
cy
Cyclicprefix
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59 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Multi-band OFDM: CP overhead (1/2)
Chromatic
Dispersion
Chromatic
Dispersion#1
#2
ICI from neighboring OFDM symbolSingle-band
Multi-band
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60 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
� 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: CP overhead (2/2)
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61 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
� 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
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62 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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63 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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64 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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65 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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66 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
These experiments have been conducted at KDDI R&D Laboratories
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67 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
� 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
These experiments have been conducted at KDDI R&D Laboratories
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68 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
These experiments have been conducted at KDDI R&D Laboratories
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69 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
These experiments have been conducted at KDDI R&D Laboratories
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70 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
These experiments have been conducted at KDDI R&D Laboratories
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71 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
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
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72 Sander Jansen, www.SLJansen.com, Leos Annual Meeting 2008 © Nokia Siemens Networks
Thank you!
�Questions?
More information: www.SLJansen.com
RF Imperfections in High-rate Wireless SystemsImpact and Digital Compensation
By Tim SchenkISBN: 978-1-4020-6902-4