ECOC 2010, 19-23 September, 2010, Torino, Italy
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Technologies towards Terabit Transmission Systems Fred Buchali Alcatel-Lucent, Bell-Labs, Germany, Lorenzstr. 10, D-70435 Stuttgart, Germany,
[email protected]
Abstract This tutorial reviews technologies towards terabit transport: Increase of bitrate per channel by higher symbol rate or higher number of bit per symbol, alternatively superchannel application consisting of non-overlapping or overlapping sub-channels contributing to increased spectral efficiency.
Technologies towards Terabit Transmission Systems
Fred Buchali Alcatel-Lucent, Bell Labs, Germany ECOC’10, Torino, Italy
1 F. Buchali: ECOC 2010, +100G technologies
978-1-4244-8535-2/10/$26.00 ©2010 IEEE
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Acknowledgements
Roman Dischler Axel Klekamp Andreas Leven Xiang Liu Chandra Chandrasekhar
2 F. Buchali: ECOC 2010, +100G technologies
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Technologies towards terabit transmission systems
Outline Introduction State of the art 100 Gb/s systems Technologies for +100 Gb/s transmission +100 Gb/s bitrate Superchannels Summary
3 F. Buchali: ECOC 2010, +100G technologies
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Introduction
4 F. Buchali: ECOC 2010, +100G technologies
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Introduction State of the art 5
• 10 Gb/s
Ey
4
• NRZ
3
2
• 1 bit/symbol
Ex
• 10 Gbaud
1
20 GHz 0
-1
• Fits clear into 50 GHz grid • 40 Gb/s
700
800
900
1000
1100
Ey 50 GHz
Ex
• 40 Gbaud
80 GHz -1
700
800
900
1000
1100
1200
• Prefiltering required for WDM, but penalty occurs • Beyond 40 Gb/s new technologies required, if 50 GHz grid is targeted 5 F. Buchali: ECOC 2010, +100G technologies
1300
filter
• DPSK • 1 bit/symbol
1200
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1300
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Introduction Technologies for 100G systems • DQPSK with 2 bit per symbol • Compatible with 100 GHz grid Ey 100 GHz
Ex -1
700
800
900
1000
1100
1200
1300
1200
1300
• Polarization multiplex • Compatible with 50 GHz grid TE
TM Ey
Ey 50 GHz
Ex
Ex -1
700
6 F. Buchali: ECOC 2010, +100G technologies
800
900
1000
1100
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Introduction Network Interface Rates 1000
Transmission
Interface Rate (Gb/s)
Core Interface 100
IP interface
10
1
0.1
0.01 1980
1990
2000
2010
2020
Year
Tkach, APOC, 2008. 7 F. Buchali: ECOC 2010, +100G technologies
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Transmission, Crossconnect, and IP Router interface rates have coalesced. Very soon 100 Gb/s 1 Tb/s rate is projected for 20152020.
We.6.C.1
100G transmission systems
8 F. Buchali: ECOC 2010, +100G technologies
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100G transmission systems
Alcatel-Lucent: 100Gb/s transponder
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100G transmission systems
Coherent dual polarization QPSK Superior performance due to 50 GHz compatibility Single carrier approach Coherent detection enables high CD and PMD robustness Narrow spectra enables high number of ROADM
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Technologies for +100G systems
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Technologies for +100G transmission systems
Candidates for +100G transmission: Single carrier and OFDM single carrier
Tx
OFDM I
DATAin
IQ mod
DATAout
DATAin
IQ mod
IFFT DAC
Q 2 polarizations
Rx
I
DAC
DATAout
Q 2 polarizations
Coherent reception for Rx DATAin
+
ADC
DATAout
DSP +
ADC
j
Optical 90° hybrid
LO
2 polarizations 12 F. Buchali: ECOC 2010, +100G technologies
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Technologies for +100G transmission systems
To enable more complex modulation scheme: QPSK x-QAM OFDM
single carrier Tx
I
DATAin
IQ mod
DAC DATAout
DATAin
IQ mod
IFFT DAC
Q
I DATAout
Q
DAC required Tx DAC DATAin
I IQ mod
DAC
13 F. Buchali: ECOC 2010, +100G technologies
• No variation for OFDM Tx for x-QAM DATAout
• Rx is same as for QPSK modulation
Q All Rights Reserved © Alcatel-Lucent 2010
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Technologies for +100G transmission systems
Single carrier
OFDM
Single carrier: phase and amplitude modulated
Multiple carriers: amplitude and phase-modulated
Quasi-constant amplitude for QPSK in the transmitter
Interference causes large variations in amplitude (PAPR>10 dB)
Blind channel estimation and carrier recovery (no overhead)
Data-aided channel estimation and carrier recovery (overhead required)
Continuous stream transmission and processing
Block transmission and processing
Single carrier and OFDM In addition they have many similar characteristics
14 F. Buchali: ECOC 2010, +100G technologies
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Technologies for +100G transmission systems
Increase of bitrate per channel
Increase of baudrate
Increase of number of bit per symbol
Focusing on Rx: Receivers with ADCs at 500 GSa/s required
QPSK n-QAM modulation
2 bit per symbol 20 bit per symbol, n=220
Superchannels
Inverse multiplexing
Application of state of the art 100 Gb/s technology
10 channels á 100G in a 50 GHz WDM grid
More dense multiplexing techniques
Optical FDM without overlap of sub-channels
FDM with overlapping orthogonal and non-orthogonal sub-channels
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Increase of bitrate per channel
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Increase of bitrate per channel Increase of baudrate to 250 Gbaud
Target for Tx: 250 Gbaud for bipolar signals (I and Q)
Demonstrated: 100 Gbaud – 100 Gb/s NRZ
Target for Rx: 500 GSa/s ADC
Demonstrated: 50 GSa/s, Fujitsu: 120 GSa/s is feasible (B. Germann, paper SPTuC2, SPPCom 2010)
1 Tb/s client
1 Tb/s
1 Tb/s
line I IQ mod Q
PD PD PD PD
ADC ADC DSP ADC ADC
Increase of baudrate feasible, but improvement is limited 17 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Increase of number of bit per symbol
n-QAM modulation, today 2 bit per symbol 20 bit per symbol, n=220
QPSK 2 bit/symbol
e.g. 1024-QAM 10 bit/symbol
How far can we go? 18 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Constellation diagrams:
QPSK
Theoret. req.OSNR vs QPSK
16-QAM
64-QAM
+6.9 dB
+13.1 dB
(for increasing bitrate) Optical OFDM enables variable constellations: Modulation Format
Spectral Efficiency
Reference
QPSK
theoretical 2 bit/s/Hz
w. 10% overhead 1.7 bit/s/Hz
+ Pol. Mux 3.4 bit/s/Hz
Achieved 3.3 bit/s/Hz
QAM-16
4 bit/s/Hz
3.4 bit/s/Hz
6.8 bit/s/Hz
5.6 bit/s/Hz
QAM-32
5 bit/s/Hz
4.3 bit/s/Hz
8.5 bit/s/Hz
7.0 bit/s/Hz
KDDI - OFC’09
QAM-64
6 bit/s/Hz
5.1 bit/s/Hz
10.2 bit/s/Hz
7.2 bit/s/Hz
HHI – OFC’10
ALU/BL - OFC’09 KDDI - ECOC’08, ALU – OFC’10
Essential OSNR penalties for multi-level modulation formats found Penalties may be reduced using higher overhead (training symbols and pilot tones) Spectral efficiency increase is limited 19 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Electrical back-to-back experiments 11.1-Gb/s QPSK
Q~ 25 dB
22.2-Gb/s 16-QAM
27.8-Gb/s 32-QAM
17 dB
14 dB
33.3-Gb/s 64-QAM
11 dB
Q factor scales as the inverse square of symbol-spacing Intrinsic limitation from digitization noise
Improvement for higher resolution DAC/ADC expected Courtesy of Xiang Liu 20 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Optical back-to-back experiments using e/o, o/e Polarization multiplex and coherent reception 22.2-Gb/s QPSK
Q~ 21 dB
44.4-Gb/s 16-QAM
55.5-Gb/s 32-QAM
66.6-Gb/s 64-QAM
10 dB
7 dB
13 dB
Q values ~ 4 dB lower than electrical Q Large implementation penalty due to e.g. laser noise
Are higher constellations feasible? Courtesy of Xiang Liu 21 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Single carrier modulation using higher level constellation: 256-QAM @64 Gb/s Extremely narrow linewidth laser (4 kHz) Optical PLL in receiver for coherent reception Pilot carrier transmission supporting tracking of optical PLL Pilot carrier reduces spectral efficiency Req. OSNR@2E-3 BER: 28.6dB, 5.3 dB penalty for 160 km transmission
B2B
160 km
M. Nakazawa et al., paper OMJ5, OFC‘10
Higher constellations are less feasible even using advanced optical technologies 22 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Summary of recent experiments
req. OSNR for 1E-3 @ 107Gb/s
23
OFDM (PDM) No-Guard-Interval OFDM Single Carrier (PDM)
32QAM
PDPB7 KDDI 0.521Tb/S
16QAM 21 8PSK 8QAM
PDPB4 AT&T,NEC RZ-8QAM 32Tb/s
19 QPSK
17
8PSK OFC08 KDDI
PD ECOC06 BL NJ 25.6Tb/s PDPC2 BL Stgt 1.21Tb/s
PDPB5 NTT 13.5Tb/s
PDPB8 BL NJ 1.2Tb/s
ECOC08 KDDI 0.534Tb/s
122Gb/s
PDPC1 Melb 1 Tb/s 107Gb/s
SE / OSNR = const (SE * 2 = OSNR + 3dB)
15 2
Remarks:
3
4
Spectral Efficiency* (bit/s/Hz)
5
6
7
8
*incl. 7%-FEC
Spectral efficiency is determined by modulation (QPSK, n-QAM) Increase of levels increases S/N requirements No differences between single carrier and OFDM 23 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Investigation of OSNR of received signal in a multi span system
OSNR = Plaunch − L − NF − 10 log(N ) + 58dB 30
28 80 km 100 km
rec. OSNR0.1nm [dB]
26
Reduction in maximum transmission distance
24
22
Increase of required OSNR QPSK 16-QAM
20
18
Launch power = 2dBm Attenuation = 0.25 dB/km NF = 6
16 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Number of spans
OSNR at Rx depends on span length, link number and launch power per span and channel 3 dB OSNR increase at halved distance 24 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Investigation of OSNR of received signal at optimized launch power
Plaunch = PINT − 10 log(N ) 30
80 km
28
100 km
received OSNR 0.1nm [dB]
26
24
22
Integrated power = 2 dBm Attenuation = 0.25 dB/km NF = 6
20
18
16 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Number of spans
Launch power close to nonlinear threshold is advantageous Assuming 2 dBm integrated power 6 dB increase of OSNR at halved distance 25 F. Buchali: ECOC 2010, +100G technologies
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Increase of bitrate per channel Higher level modulation Increase of bitrate is limited – 2x
QAM 16-QAM
– 3x
QAM 64-QAM
– 4x
QAM 256-QAM
Huge increase in required OSNR Shortens maximum transmission distance
Increase of number of bit per symbol is limited
26 F. Buchali: ECOC 2010, +100G technologies
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Superchannels
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Superchannels Inverse multiplexing
10 x 100 Gb/s
...
1 Tb/s
client
line
...
10 channels @100Gb/s in a 50 GHz grid
Bandwidth consumption: 500 GHz
Spectral efficiency: 2 bit/s*Hz
28 F. Buchali: ECOC 2010, +100G technologies
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Superchannels Dense multiplex techniques in frequency domain
unused spectral range
...
How dense can we multiplex the channels?
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Superchannels Considering a subdivision of the overall band into 1 Tb/s channels
Starting with 10 x 100 Gb/s scenario 1 Tb/s
…
…
Guard bands for ROADM application
⇒ If 1 Tb/s channel is transmitted as a superchannel the guard bands are not longer required 1 Tb/s
⇒ Alternative WDM grid or grid-less scheme 30 F. Buchali: ECOC 2010, +100G technologies
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Superchannels
Dense multiplex of pre-filtered single carrier sub-channels
...
1 Tb/s
DEMU X
MUX
...
e.g. G. Gavioli et al., paper OThD3, OFC‘10
• Filtering prevents overlap of spectra after MUX and cross talk in Rx • Signal processing without cross talk suppression enabled
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Superchannels Dense multiplex of pre-filtered single carrier sub-channels
Back to back and long distance transmission
25 Gbaud
G. Gavioli et al., paper OThD3, OFC‘10
• • • • •
Frequency grid: down to 1.2 x baudrate Low bandwidth filtering causes error floor and penalty at high distance High frequency stability of lasers required to avoid drift towards lower spacing Bandwidth consumption at 10x100 Gb/s: 300 GHz Spectral efficiency: 3.3 bit/s*Hz vs. 2.0 bit/s*Hz for inverse multiplex
32 F. Buchali: ECOC 2010, +100G technologies
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Superchannels Dense multiplex of pre-filtered single carrier sub-channels
Rx after optical filtering, o/e and electrical filtering Optical and electrical filter
0
• • • •
frequency
If narrow filtering is applied, there is no cross talk present BWel ≈ 0.6 * baudrate Signal processing comparable with single channel 100 Gb/s Required oversampling in Rx: 1.5 to 2
Increase of spectral efficiency limited by a factor of 1.66
33 F. Buchali: ECOC 2010, +100G technologies
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Superchannels
Narrower sub-channel grid with overlapping spectra 1 Tb/s
1 Tb/s
⇒ Loss less demux of overlapping spectra possible if sub-channels are orthogonal
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Superchannels Multiplex of overlapping single carrier sub-channels
1 Tb/s
...
comb
D E MU X
Requirement: baudrate = frequency spacing
MUX
...
cyclic demux
1.0x baudrate 11 x baudrate, 275 Hz • COMB generator required • Orthogonality of sub-channels in Rx required • Processing in Rx with cross talk suppression (delay and add filter, oversampling req.) • Bandwidth consumption: 275 GHz • Spectral efficiency: 3.6 bit/s*Hz, approaches 4 bit/s*Hz for higher #sub-channels 35 F. Buchali: ECOC 2010, +100G technologies
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Superchannels Multiplex of overlapping single carrier sub-channels
Can we replace comb generator by independent lasers? Tx
Tx
comb
LD LD LD LD LD
SC
SC LD LD LD
• Transmission without coupled carriers is feasible • Low loss demux requires 4x oversampling • Drift towards lower spacings has to be avoided S. Chandrasekhar, X. Liu, Vol. 17, No. 24, OPTICS EXPRESS 21354 36 F. Buchali: ECOC 2010, +100G technologies
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Superchannels Multiplex of overlapping single carrier sub-channels
Requirement: Alignment of adjacent channels in time domain (orthogonality)
S. Chandrasekhar, X. Liu, Vol. 17, No. 24, OPTICS EXPRESS 21354
• Delay between adjacent channels < 0.1 x symbol period
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Superchannels Multiplex of overlapping single carrier sub-channels
Rx after optical filtering, o/e and electrical filtering Optical and electrical filter
0
frequency
• Signal: BWel ≈ 0.6 * baudrate • Overlapping spectra, cross talk is in-band, high bandwidth required • Delay and add filter required to suppress neighbors Increase of spectral efficiency limited by a factor of 1.8, may approach 2
38 F. Buchali: ECOC 2010, +100G technologies
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Superchannels Mulitplexing of orthogonal or non-orthogonal OFDM subchannels
Rx
Tx
OR
comb
LD LD LD LD LD
OFDM
OFDM LD LD LD
Coupled carriers
Uncoupled carriers (free running)
Is orthogonality between subbands required?
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Superchannels Mulitplexing of orthogonal and non-orthogonal OFDM subchannels ∆t Alignment of symbols ∆f
Delay within cyclic extension
20
20
18
18
16
16 Q [dB]
Q [dB]
f
OSNR
14
OSNR 14
12
12
10
10
8 -2
0
2
4
6
∆f/∆fsc [a.u.]
t
8 Targeted operation -100 8 10 FDM OFDM is feasible
-50
0
50
100
subband delay [a.u.]
• Frequency offset > 2⋅⋅subcarrier spacing leads to negligible Q penalty at typical OSNR values • Subband time delay has no impact on performance at relevant OSNR 40 F. Buchali: ECOC 2010, +100G technologies
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Superchannels Mulitplexing of orthogonal and non-orthogonal OFDM subchannels: experiments 121Gb/s PDM-OFDM Modulation of 5 coherent cw-lines
MZM 1
CWsource
AWG
∆f, 2∆ ∆f even odd
OFDM I/Q
PDM 5x24Gb/s
f0
∆f
5xmultiband OFDM
1.2Tb/s PDM-OFDM-FDM
PDM-OFDM
Cascading of 10 unlocked (incoherent) lasers CWsource
10 Lasers in 34GHz spacing
CWsource
5 ∆f
1.2Tb/s
CWsource
120Gb/s 120Gb/s 120Gb/s 120Gb/s 120Gb/s
PDMOFDM
CWsource
1.2Tb/s PDM-OFDM-FDM in 10x PDM-OFDM subbands 41 F. Buchali: ECOC 2010, +100G technologies
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Superchannels Mulitplexing of orthogonal and non-orthogonal OFDM subchannels Superchannel
Back-2-back performance
1.2Tb/s within 340 GHz bandwidth
1E-2
-30
24.2Gb/s 121Gb/s
1.21Tb/s
Power (dB)
-35
340GHz
-40
-45
1E-3
7.5dB Back-2-back
-55 1547.5
1548
1548.5
12dB
BER
-50
1549
1549.5
1550
1550.5
1551
Wavelength (nm)
1E-4
24.2 -> 121 Gb/s orthogonal MUX 121-> 1.2 Tb/s non-orthogonal MUX 0.5 dB penalty for orthogonal MUX
1E-5
5
2 dB penalty for non-orthogonal MUX
10
15
20
25
OSNR / 0.1nm (dB)
Bandwidth corresponds to 283 GHz @1 Tb/s 42 F. Buchali: ECOC 2010, +100G technologies
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Superchannels Mulitplexing of orthogonal and non-orthogonal OFDM subchannels Rx after optical filtering, o/e and electrical filtering
Optical filtering Electrical filtering Oversampling 1.5 to 2 times Suppression of out of band cross talk without additional signal processing Low penalty demux
Increase of spectral efficiency limited by a factor of 1.7
43 F. Buchali: ECOC 2010, +100G technologies
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30
35
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Summary
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Technologies towards Terabit Transmission Systems
Starting from 100 Gb/s systems there are several technologies towards 1 Tb/s available They enable increase in bitrate per channel by higher baudrate and increase in number of bit per symbol Bitrates clearly beyond 100 Gb/s are feasible Superchannels applying dense frequency division multiplexed subchannels enable increased spectral efficiency
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www.alcatel-lucent.com
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