Download Applications of Optical OFDM: From Automotive to Ultra Long-Haul (Invited Paper) Sebastian Randel

ME3 (Invited)
14.00 - 14.30
Applications of Optical OFDM:
From Automotive to Ultra Long-Haul
(Invited Paper)
Sebastian Randel
Susmita Adhikari
Sander Jansen
Siemens AG
Corporate Technology
Munich, Germany
Email: [email protected]
Chair for Communications
Christian-Albrechts-Universität zu Kiel
Kiel, Germany
Email: [email protected]
Nokia Siemens Networks
DWDM R&D
Munich, Germany
Email: [email protected]
Abstract—Optical OFDM is discussed as a spectrally efficient
modulation format offering the potential to overcome linear and
nonlinear impairments. The applications span from automotive
networking to high-speed transmission over both multi-mode and
single-mode fibers.
I. I NTRODUCTION
During the past decade, optical component technology has
reached a level of maturity and pervasiveness where further
performance increase becomes so costly, that the introduction
of electronic signal processing becomes a viable option for
lowering the overall complexity of optical data transmission
systems. In this scenario, orthogonal frequency division multiplexing (OFDM) has received increased attention as a means
to overcome various limitations of optical communication
systems such as modal dispersion, relative intensity noise,
chromatic dispersion, polarization mode dispersion as well
as self phase modulation. This paper provides an overview
on recent trends in different application fields of optical
OFDM spanning from Gigabit Ethernet transmission in cars
to 40 Gbps and 100 Gbps transmission over multi-mode
fiber as well as single-mode fiber based DWDM long-haul
transmission networks.
Fig. 1.
transmission in the automotive environment is to introduce
polymer cladded silica (PCS) fiber and vertical-cavity surface
emitting laser (VSCEL) based transmitters [3]. However, this
approach requires major changes in the component design, as
well as in installation and handling specifications.
In [4] it has been shown that optical OFDM can be used
to achieve 1 Gbps transmission over 50 m POF with an
LED based transmitter. This is enabled by use of adaptive
modulation with up to 64-QAM constellations. With only
minor adaptation of the electro-optical components and the
introduction of an integrated circuit for digital signal processing Gigabit Ethernet can be introduced in cars without the
need to change fiber type and connectors.
II. O PTICAL E THERNET IN AUTOMOTIVE
The increasing number of electronic control units (ECUs)
within automobiles has lead to a large number of co-existing
networking technologies such as LIN, CAN, Flexray and
MOST [1]. These automotive-specific technologies ensure that
the requirements e.g. in terms of electromagnetic compatibility, robustness, and power consumption are met. The resulting
networks are complex and require large efforts in terms of
development, integration, and maintenance. In this context
the question arises whether Ethernet/IP technology being the
preferred networking technology not only in offices and homes
can reduce these efforts. In Fig. 1 a typical automotive infotainment configuration with 8 ECUs is shown. In MOST these
are connected via a uni-directional ring network based on polymer optical fiber (POF) and operates at 25 Mbps and 150 Mbps
in the next generation. Ethernet based networks operate bidirectionally at either 100 Mbps or 1 Gbps and they support
various topology options [2]. One way to enable 1 Gbps
978-1-4244-3681-1/09/$25.00 ©2009 IEEE
Automotive infotainment configuration with 8 ECUs [2].
III. H IGH -S PEED T RANSMISSION OVER M ULTIMODE
O PTICAL F IBERS
Based on the rising bandwidth demand for interconnects
in data centers and high performance computing, the IEEE
802.3ba task force is currently preparing a standard for
40 Gbps and 100 Gbps Ethernet transmission over at least
100 m of laser-optimized 50 μm multimode fiber. While
the bandwidth of 100 m of OM3 fiber exceeds 20 GHz
the bandwidth of commercial VSCELs still limits them to
10 Gbps NRZ operation [5]. The new standard will thus
specify parallel optics with a line rate of 10 Gbps per fiber
link. A major challenge in this scenario is to meet the joint
reliability requirements of the multiple laser sources and the
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In order to realize 100 Gbps Ethernet long-haul and ultra
long-haul transmission systems it is essential not only to
overcome the linear distortions but also nonlinear impairments
introduced by the Kerr effect. In [10] it has been shown that
in case of periodically compensated dispersion maps OFDM
is particulary sensitive to self-phase modulation (SPM) and
cross-phase modulation (XPM). This reduction in nonlinear
tolerance is especially relevant for mixed data rate transmission scenarios where a high data rate OFDM channel is
installed on an existing 10 Gbps or 40 Gbps network with
periodic dispersion compensation. Du et al. have subsequently
shown that through nonlinear pre- and post-compensation SPM
impairments can be partly compensated for and thereby the
reach of OFDM on periodically compensated dispersion maps
can be increased [11]. However, because of the increased XPM
penalty of OFDM in periodically compensated transmission
systems it is conjectured that optical OFDM will mainly find
its application in uncompensated transmission links where the
reach of optical OFDM [7] is comparable to that of single
carrier modulation [12].
connectors will presumably support redundancy in order to
solve this issue.
In [6] it has thus been proposed to use spectrally efficient
discrete multitone in order to transmit up to 30 Gbps with
a commercial 850 nm VCSEL. The concept behind this
approach is to modulate a large number of sub-carriers with
orthogonal quadrature amplitude modulation (QAM) and to
adapt the transmit signal to the channel using Chow’s rate
adaptive loading algorithm. Fig. 2 shows experimental results
for 30 Gbps discrete multitone transmission over 500 m of
MMF obtained with off-line processing. The sub-carriers are
modulated with up to 128-QAM and the bandwidth up to
6 GHz is used.
SNR (dB)
30
20
10
Norm. Energy
bit
0
8
7
6
5
4
3
2
1
0
0
1
2
3
4
Frequency (GHz)
5
6
V. C ONCLUSION
0
50
100
sub-channel index
150
0
50
100
sub-channel index
150
As discussed for three example applications optical OFDM
can be regarded as an attractive candidate for spectrally
efficient modulation. It provides the possibility to compensate
linear and to a certain degree also nonlinear impairments in
the electrical domain. In automotive networking it allows to
maintain LED based POF systems for Gigabit Ethernet and
in high-speed transmission over MMF it results in relaxed
bandwidth requirements for VCSELs and photo-receivers. The
main advantages of optical OFDM for long-haul transmission
systems are that it allows a reduction of the sampling rate at
the receiver and has negligible linear crosstalk. However, for
all applications OFDM requires high-speed electronic signal
processing which adds complexity to transmitter and receiver.
Therefore, for each application scenario a trade-off exists
between the electrical and optical complexity and as such for
each individual application an optimal balance must be found.
3
2
1
0
Fig. 2. Top: received signal-to-noise ratio per sub-channel in case of unit
energy after transmission over 500m MMF with 850nm VCSEL. Center: bits
allocated to each sub-carrier. Bottom: energy per sub-channel.
IV. O PTICAL OFDM FOR L ONG -H AUL T RANSMISSION
ACKNOWLEDGMENT
In long-haul DWDM transmission coherent optical OFDM
is currently discussed as a candidate for 100 Gbps transmission
within the 50 GHz ITU-T channel grid [7]. It offers the potential to overcome linear effects like chromatic dispersion, polarization mode dispersion and it’s narrow spectrum makes it also
robust versus concatenated filtering in wavelength selective
switches (WSS). Compared to coherent DQPSK transmission
optical OFDM sets reduced requirements on the sampling rates
of analog-to-digital converters. In single-carrier transmission
with polarization multiplex two-times over-sampling resulting
in a ADC sampling rate in the range of 56 GSPS gives the
optimal performance [8]. In OFDM redundancy is introduced
with the cyclic prefix and training symbols. This overhead can
not only be used to overcome inter-symbol interference but it
also simplifies synchronization and the ADC sampling rate
can be reduced to about 1.3 times the baud-rate, i.e., about
35 GSPS [9].
The authors would like to thank Jeffrey Lee and Florian
Breyer. Furthermore, they acknowledge the support of Herbert
Meier from Continental AG.
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