Optical Networks Group, Electronic and Electrical Engineering,
University College London. r.killey@ee.ucl.ac.uk

• Introduction to Optical Networks Group and UCL
• Collaboration with Intel and CMU
• Optical OFDM concept
• Demonstration of 21.4 GS/s OFDM with real-time DSP
• Future work towards 100 Gb/s OFDM
• Planned 100 Gb/s OFDM experimental demonstrator

Senior Lecturer at University College London, U.K.
PhD from the University of Oxford in 1998.
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Research interests: nonlinear fiber effects in WDM transmission systems optical interconnects for computer networks wavelength-routed optical networks high speed digital signal processing.
Associate Editor of the IEEE/OSA Journal of Optical Communications and Networks
Member of technical program committees of ECOC and OECC conferences, and IEEE Photonics Society meetings.
Author of approx. 120 journal and conference papers.

• UCL founded in 1826, oldest university in London
• Consistently ranked within the top 5 universities in the UK (together with
Cambridge, Oxford, Imperial College London and the London School of
Economics)
• Charles Kao (Nobel Prize winner 2010 for pioneering work in optical fiber communications) was a PhD student in the Dept. Electronic Engineering

Academics: Polina Bayvel, Robert Killey, Seb Savory, Benn Thomsen
Research Fellows: Yannis Benlachtar, Steve Desbruslais
Doctoral students: 10 students (including Rachid Bouziane), working on:
• Fibre transmission systems
• Optical interconnects
• Optical burst and packet switching - hardware and network modeling
• Plasmonics

Future network architectures
Recirculating fibre loop testbed
Ultra-low crosstalk free-space grating wavelength router

Funding and collaborations
UK Engineering and Physical Sciences Research Council
•
Fibre Nonlinearity and Dispersion Compensation using High-Speed Digital Signal
Processing, 2005-2008
• Putting the Quantum into Information Technology, 2003-2007
• Terabit WDM Optical Network Testbed (TWON) for Transmission and Optical
Networks Research, 2001-2005
EU Framework funding
• Broadband Optical Networks for Europe ‘BONE’, 2009-2011
• Integrated Project ‘NOBEL’, 2004-2005
• Network of Excellence ‘e-Photon/ONe’, 2004-2006
Industrial funding and collaborations: Agilent, Azea Networks, Bookham
(Caswell), BT, Ericsson, Huawei Technologies, Intel, JDSU (San Jose, CA),
Lucent Technologies – Bell Labs (Crawford Hill, NJ), Nortel Networks, Siemens,
Sierra Monolithics, T-Systems (Berlin), Xtera

Collaborative research with Intel since 2004, and with CMU since 2008
 Developed novel DSP-based transmitters for optical interconnects to
 Compensate for signal distortion
 Maximize data rate and spectral efficiency
 Assessed the feasibility of optical transmitter designs employing realtime DSP
 Investigated the effects of hardware constraints on the performance of such systems:


Limited DSP resolution
Latency
 Limited DAC resolution and bandwidth
 Developed efficient algorithms for the generation of electronically predistorted (EPD) signals and optical OFDM signals

Collaborative research with Intel and CMU
Intel-funded PhD studentship 2007-2010 (Ramanan Thiruneelakandan)
Three interns from UCL worked at Intel labs (Phil Watts, Robert
Waegemans, Rachid Bouziane)
Jointly published numerous papers in journals and at international conferences.

Future collaborative research with Intel and CMU
Investigate the feasibility of 100 Gb/s per wavelength optical interconnects, with the following goals:
• Use of directly modulated VCSELs with modulation bandwidth < 20 GHz
• Use of high-order modulation formats (QPSK, 16-QAM, 64-QAM)
• Error-free transmission over up to 300 m of multi-mode fiber
• Total link power consumption below 20 mW/Gbit/s
• Low cost – low complexity optics
• Transceiver electronics implemented using conventional CMOS technology
• Suitable for coarse wavelength division multiplexing to achieve interconnect capacity > 1 Tbit/s per fiber
• Compatible with optically switched and hybrid networks for data center and parallel computing applications
Orthogonal frequency division multiplexing (OFDM) is a promising candidate to achieve these.

Introduction to OFDM
• Conventional optical communication systems use modulation (amplitude or phase) of a single optical carrier wave
• suffers from distortion at high data rates
• requires complex optics to recover phase modulation
• Orthogonal frequency division multiplexing (OFDM) concept - divide the bandwidth of the channel into many non-interfering (orthogonal) subchannels running at lower bitrates
• Reduces impact of fibre dispersion and filtering effects etc
• Allows increased spectral efficiency through high order formats, with phase and amplitude modulation, e.g. quadrature amplitude modulation
(QAM), carrying more than one bit per symbol.

Introduction to OFDM
• OFDM is a multi-carrier modulation scheme, in which adjacent sub-channels are separated in frequency by 1/T, where T is the symbol period.
• All sub-channels are orthogonal to each other, and can be detected with low inter-channel interference
Spectrum of an OFDM subcannel
OFDM spectrum frequency

OFDM transmission system
A. J. Lowery and J. Armstrong, ’10 Gb/s multimode fiber link using powerefficient orthogonal frequency division multiplexing’, Optics Express 13, 25,
2005, pp. 10003-10009

OFDM transmission system
Significant interest in optical OFDM over the last 3-4 years.
Most experiments have used offline DSP to encode and decode the signals
Fiber
DSP
PC Arbitrary waveform generator
E/O O/E DSP
Real-time sampling oscilloscope
PC
The next major challenge - to verify the performance using practical real-time
DSP
• Limited resources for FFT/IFFT
• Fixed point arithmetic
• Limited resolution

(I)FFT
Res.
f s
(GS/s)
Bitrate
(Gb/s)
Distance
(km)
FPGA
Type
Reference
Rx 16-bit 2.5
5 Stratix II Qi Yang et al., Proc.
OFC’09, PDPC5
Tx
Rx
2 - 4 1.5 - 3 0.5 MMF
75 SMF
Tx 10-bit 21.4
8.34
Tx 10-bit 10 12.1
Stratix II R. P. Giddings et al.
ECOC 09, Opt. Exp. 17
1600 SMF Virtex 4 Y. Benlachtar et al.
ECOC ‘09
400 SMF Virtex 5 F. Buchali et al.
ECOC ‘09

DSP-based optical transmitter
The transmitter configuration, based on a Cartesian Mach-Zehnder modulator, is shown below
Applications –
• electronic predistortion
• high order modulation formats
• optical orthogonal frequency division multiplexing

DSP-based optical transmitter
VIRTEX-4
EVALUATION
BOARD
(V real
DSP)
16
DAC
V real
BIAS
+
167.2 MHz
CLOCK
PATTERN
SYNCH.
10.7 GHz
CLOCK
LASER
V real
MZM
V imag
VIRTEX-4
EVALUATION
BOARD
(V imag
DSP)
16
DAC +
V imag
BIAS
(a)
Top level Tx design
PHASE
SHIFTERS
P
P
P
P
4:1
MUX
P
P
P
P
P
P
P
P
P
P
P
P
ATTENUATORS
0dB
6dB
12dB
18dB
MSB
+
LSB
(b)
21.4 GSamples/s 4-bit DAC

DSP-based optical transmitter
Transmitter testbed constructed in UCL lab
FPGA board multiplexers
4:1 RF combiner
RF phase shifter amp laser
+ mod amp RF phase shifter
FPGA board multiplexers
4:1 RF combiner

167.2 MHz
CLOCK
CLIP UPDATE
PATTERN
COUNTER
PATTERN
MEMORY
50
25 x
10
128 x
10
128 x
4
MANUAL DELAY
CONTROL
32
MGT0
32
MGT1
32
MGT2
16 x
32
2 15 DeBruijn Sequence
32
MGT15
DAC interface

 IFFT cores generated using SPIRAL tool*
 128-point 10bit resolution using two’s complement fixed-point format
 The core has a throughput of 21.4 GS/s with a latency = 18 clock cycles
(107.6ns)
 Utilises a mix of radix 8 and 16 algorithm
 Requires 84% of the FPGA’s slices (reconfigurable logic elements), and all
160 of the Virtex4’s embedded arithmetic units (DSP48 slices)
* P. A. Milder et al., Design Automation Conference (DAC), 385-390 (2008) http://www.SPIRAL.net

 1.28 oversampling
 Effective bandwidth = 8.36 GHz
 OFDM band 4.18 – 8.36 GHz
Unwanted tones
Equalised
5dB



1-1.5 dB improvement with 10-bit resolution
Variation in EVM across sub-channels due to DAC roll-off

BIAS
L PF
+
VIRTEX-4
EVALUATION
BOARD
16
167.2 MHz
CLOCK
10.7 GHz
CLOCK
LASER MZM
OPTICAL
FILTER

Our work has shown that OFDM signal generation and transmission at >20 GS/s is feasible
How can we prove that OFDM will allow the realization of low cost, low power 100 Gb/s per wavelength optical interconnects?

Year 1
• ASIC design for the transceiver and DSP power consumption calculations through circuit simulations
• Power consumption calculations of signal converters, analog electronics and optical sub-systems
• Interface IFFT/FFT cores and Micram D/A and A/D converters using
FPGA multi-gigabit transceivers
• Real-time receiver synchronization circuits
Year 2
• Development of I/Q mixing circuits to achieve multi-band operation
• MMF transmission properties modeled and used in interconnect simulations – dealing with frequency-dependent attenuation

Years 3 and 4
• Simulations and experiments assessing the impact of non-ideal directly/externally modulated laser (chirp, relaxation oscillations) on the performance of OFDM interconnects
• Investigate methods to achieve peak-to-average power ratio
(PAPR) of optical OFDM.
• Predictions of the future potential of optical OFDM technology over a 10 year time frame, including expected data rate (single channel and WDM), power consumption (power per Gbit/s), latency, and suitability for optically switched networks

VCSEL
T +
Bit-pattern generator
Virtex5
FPGA
Micram
DAC25 amp sin f
SC t a. Transmitter
Photodiode amp sin f
SC t
Micram
ADC30 b. Receiver
Virtex5
FPGA
Error detector

Year 1
• Construction of real-time transmitter based on Xilinx Virtex 5 FPGA and
Micram 25 GSa/s 6-bit DAC
• up to 62 sub-channels, QAM-16 format,
• achieving 50 Gbit/s (receiver based on real-time scope and offline DSP)
Year 2
• Construction of real-time receiver based on Xilinx Virtex 5 FPGA and
Micram 30 GSa/s 6-bit ADC
• Transmitter and receiver pair operating together up to 62 sub-channels,
QAM-16 format, total capacity 50 Gbit/s
• Demonstrate operation over 100 m MMF

Year 3
Upgrade transceiver to 112 Gbit/s (124 sub-channels) with 2-band
OFDM using I/Q mixing
Demonstrate 112 Gb/s single wavelength interconnect over 300 m
MMF
Demonstrate 10 x 112 Gbit/s (1.12 Tbit/s) CWDM optical interconnect
Year 4
Testing of 112 Gbit/s channels in University of Columbia optically switched network testbed

 FPGAs enable the investigation of real-time multi-gigabit per second optical OFDM
 We have demonstrated 21.4 GS/s real-time OFDM transmitter using
10-bit IFFT core designed at CMU
 Key steps have been identified to assess the suitability of OFDM for
100 Gb/s per wavelength optical interconnects, including
• Power consumption calculations of DSP and analog circuits
• Interfacing FPGAs to D/A and A/D converters
• Analog circuit design for multi-band operation
• Impact of MMF dispersion
• Non-ideal characteristics of the directly/externally modulated laser
• Methods to reduce peak-to-average power ratio
 We propose to construct a 100 Gb/s experimental demonstrator, and test it in Columbia’s optical network testbed