NTT Technical Review

June 2017 Vol. 15 No. 6
Feature Articles: State-of-the-art Space Division Multiplexing Technologies for FutureHigh-capacity Optical Transport NetworksSpace Division Multiplexing Optical Transmission Technology to Support the Evolution ofHigh-capacity Optical Transport Networks



Dense Space Division Multiplexing (DSDM) Long Distance Optical Fiber Transmission
Technology



Dense Space Division Multiplexing (DSDM) Photonic-node Platform Technology



Optical Amplification Technologies for Space Division MultiplexingResearch and Development of Next Generation Optical Fiber Using Multiple Spatial Channels

Multi-core Fiber Connector Technology for Low-loss Physical-contact Connection

Global Standardization Activities


Activity Report of ITU-T Focus Group on IMT-2020

Practical Field Information about Telecommunication Technologies
Case Studies of Wireless LAN Problems

Information

Event Report: Science Plaza 2016 at NTT Basic Research Laboratories

Short ReportsOne-petabit-per-second Fiber Transmission over a Record Distance of 200 km—Paving the
Way to Realizing 1000-km Inline Optical Amplified Transmission Systems within C Band Only

External Awards/Papers Published in Technical Journals and Conference ProceedingsExternal Awards/Papers Published in Technical Journals and Conference Proceedings

Feature Articles: State-of-the-art Space Division Multiplexing
Technologies for Future High-capacity Optical Transport Networks

Space Division Multiplexing Optical
Transmission Technology to Support
the Evolution of High-capacity
Optical Transport Networks
Yutaka Miyamoto and Ryutaro Kawamura
Abstract

This article describes high-capacity space division multiplexing (SDM) optical transmission technology to support the evolution of broadband networks. A new spatial degree of freedom is introduced in
optical transmission systems, optical node equipment, and optical fibers to overcome the physical limitsof today’s single-mode fiber based systems. Future SDM-based optical networks will achieve highcapacities of over 1 Pbit/s in a single strand of fiber, a 100-fold increase in capacity, and node throughputs of more than 10 Pbit/s.
Keywords: spatially multiplexed optical communication, petabit-per-second optical communication,
multi-core multi-mode fiber

1. Introduction
In recent years, the Internet has become an indispensable infrastructure offering broadband services
such as video streaming and electronic commerce insupport of daily life. Much effort is being devoted in
mobile communications to develop 5G (fifth-generation) services, which will achieve low latency withlarge bandwidths in excess of 10 Gbit/s. The emerging IoT (Internet of Things) technologies based on
the future network infrastructure are expected toenable new services that will be as ubiquitous andessential as the air around us.
The evolution of core and metro optical transport
networks in NTT Group companies is shown in
Fig. 1. The basic transmission medium of these networks has so far been single-mode fiber (SMF),
which has only one waveguide (core) with a single
waveguide mode per strand of optical fiber. In the
NTT laboratories, we have been conducting timely
research and development (R&D) on a state-of-theart optical transport system that economically realiz1

es the required transmission capacity. Our contributions have attained an increase in capacity of almost
5 orders of magnitude over the last 30 years (doubling
every 2 years).
The digital coherent system [1] has been implemented in NTT’s commercial network. Not only does
it improve receiver sensitivity and spectral efficiency,
but it can also greatly improve the compensation
capability of distortion characteristics such as chromatic dispersion and polarization mode dispersion of
optical fibers. Transport systems with 10-Tbit/s-classcapacity transport systems have been installed incommercial networks by using a polarization-division-multiplexed quadrature phase-shift keying(PDM QPSK) modulation format and the typical
50-GHz wavelength division multiplexing (WDM)
spacing.
We have also confirmed the feasibility to upgrade
to optical networks with a capacity of 20 Tbit/s per
fiber and a channel capacity of 400 Gbit/s in the field.
To achieve this, we employed twin-subcarrier multiplexing and high-speed digital signal processing
NTT Technical Review

Feature Articles

nt

me

p
elo

v

&
rch

a

se

Space division multiplexing (SDM)

Transmission capacity per fiber (bit/s)

1P
Fiber-launched power limit
Nonlinear Shannon limit

100T

x

c
er
mm ar
o
C ye
/
1.4

Physicallimit ofsingle-mode
fiber (SMF)

Digital coherent transmission≥ 20 T/fiber

10T
1T

Wavelength divisionmultiplexing (WDM) andoptical amplification

1T
400G – 1 Tbit/s/ch.

100G
10G
1G

tem

ys

s
ial

100G
10G

Time divisionmultiplexing
(TDM)

1G

: Research (with SDM)
: Research (without SDM)
: Commercial system in NTT

100M

Channel capacity (bit/s)

Re

10P

de

100M

10M
1980

1990

2000

2010

2020

2030

Year

Fig. 1. Evolution of high-capacity optical transport network.

techniques based on 16-quadrature amplitude modulation (QAM)*1 [2].
In laboratory experiments, we demonstrated therecord transmission capacity of 102 Tbit/s across asingle SMF by using the subcarrier multiplexed PDM
64-QAM format [1]. Furthermore, we can significantly improve the scalability of optical networks
without electrical signal regeneration by introducing
reconfigurable optical add/drop multiplexer(ROADM) nodes. The above-mentioned 100-Gbit/s
digital coherent channels are implemented in theexisting high-capacity optical networks with multidegree ROADMs, where the 100-Gbit/s channels canbe set at arbitrary wavelengths and arbitrary directions (Fig. 2).
Efforts to increase the capacity of conventional
SMF-based optical communication systems beyond
100 Tbit/s while keeping the same repeater spacing as
the conventional network have reached or exceeded
the ultimate physical limit of SMF (the so-calledcapacity crunch) due to the fiber nonlinear effect*2
and the fiber fuse phenomenon [3, 4]. Furthermore, in
future optical node systems, several fundamental
technologies such as highly integrated optical switchVol. 15 No. 6 June 2017

ing and high-density wiring and connectors will be
required in order to substantially improve nodethroughput in step with the increases in transmission
capacity.
We have conducted extensive R&D on space division multiplexing (SDM) optical communications
that can offset these limiting factors and thus achieve
further increases in both transmission capacity andoptical node throughput [3–5]. In this article, we
review the current status and the future prospects inthis field. We present recent activities in the NTT
laboratories involving SDM optical communications
technologies that demonstrate over 1-Pbit/s long-haultransmission performance, and a potential opticalswitching node system with node throughput of morethan 10 Pbit/s.

*1 QAM: A highly efficient digital modulation scheme whereby the
amplitude and phase of the signal’s electric field are modulated
to multiple levels in order to transmit multi-level symbols.
*2 Nonlinear effects: Phenomena that cause signal distortion such as
the modulation of an optical signal’s phase and frequency.

2

Feature Articles

100P: Examples of commercial systems
from a domestic carrier (NTT)

10P
Node throughput* (bit/s)

Spatially multiplexed photonic node(Throughput: about 10 Pbit/s)

Achievabletarget1P

SDM fiber
SDM fiber

SDM fiber
SDM fiber

SDM fiber

SDM fiber

100T
10T

8-path ROADM

2-path ROADM

1T
100G
1990

2000

2010

2020

2030

Year
* Number of ports × number of wavelengths × bit rate per wavelength

Fig. 2. Enhancing node throughput by means of SDM photonic node technology.

500–1500 km≈ 80 km

SDM node (ROADM function)

SDM node(Amplification function)

SDM optical transmission medium(a) Using multiple conventional SMFs

SDM integrated optical transceiver(b) MCF (multiple cores in single fiber): WDM device(c) MMF (multiplexing of multiple modes): SDM connected device(When using MCF: fan-in/fan-out)

Mode 2Mode 3Mode 1: SDM optical switch device
MCF: multi-core fiber
MMF: multi-mode fiber

Fig. 3. Configuration of high-capacity optical transport network using SDM optical communications technology.

2. Overview and benefits of SDM opticalcommunications system technology
An overview of an SDM optical communications
system is shown in Fig. 3. The principal components
3of this system are the SDM optical transmission
medium, SDM nodes with optical amplification and
ROADM functions, and an SDM integrated opticaltransceiver. Typical candidates for the optical transmission medium of an SDM optical transmission
NTT Technical Review

Feature Articles

Overall transmission capacity

C=BxN

C

+W
DM
M

C

SD

1T
TDM / multi-level

Channel capacity B (bit/s)

10T

100G

WDM

C

00

C

=1

0T

10G
1

=1

10

100

=1

0P

=1

bit

/s

Pb

it/s

Tb

it/s

bit

/s

1000

10,000

Number of optical transceiver circuits for one direction (WDM + SDM): N
NTT commercial systems
Limits of SMF(spectral efficiency: 10 bit/s/Hz; optical amplification bandwidth: 10 THz)

Fig. 4. Possible combination of SDM and WDM in future high-capacity SDM optical transport network.

system are shown on the left side of the figure. A setof conventional SMFs used as the SDM optical transmission medium is shown in Fig. 3(a), and a multicore fiber (MCF) consisting of multiple cores in asingle optical fiber is shown in Fig. 3(b). A multimode fiber (MMF) that uses different modes to transmit different signals across the single core is depicted
in Fig. 3(c).
In high-capacity SDM node systems, spatially multiplexed WDM signals (SDM/WDM signals) are
simultaneously amplified and transmitted to achieve
an economical long-haul transmission system. Anychannel can be sent to an arbitrary wavelength in anarbitrary direction (the typical number of directions iseight) without the need for optical-electrical conversion. The optical node system exemplified by Fig.
3(a) can effectively utilize the existing SMF infrastructure and achieve flexible accommodation of traffic demands in the short term.
The multiplexing structures of existing opticalcommunications systems and future SDM opticalcommunications systems are compared in Fig. 4. In
existing SMF-based commercial systems, the channel capacity has been increased up to 100 Gbit/s bymeans of time division multiplexing (32 Gbaud) andmulti-level coding (QPSK). The typical WDM channel count N is around 100 in today’s SMF-based
systems (i.e., N is the number of multiplexed opticaltransceiver circuits). The WDM signals are simultaVol. 15 No. 6 June 2017

neously amplified and transmitted through several
optical inline amplifiers and optical nodes. Such configurations achieve economical high-capacity longhaul transmission systems with an overall transmission capacity (C = B × N) of 10 Tbit/s.
The available number of channels in a conventional
WDM system is determined by the signal bandwidthof optical amplifiers, roughly 4–5 THz in the C bandor L band, or up to about 10 THz if the two bands areused in parallel (C and L bands). Practical highcapacity systems have been implemented by installing hundreds of high-speed optical transceiver circuits in a limited equipment space without excessive
power consumption, making full use of the available
optoelectronic device performance.
Multi-level modulation can improve the spectral
efficiency (SE) and thus increase the channel capacity. However, we have to consider the trade-off
between SE and transmission distance if we are to
maintain backward compatibility with the existing
system. It is possible to increase the channel capacity
up to 200 Gbit/s by enhancing the SE up to 4 bit/s/Hz
with medium transmission reach through the use of
PDM 16-QAM signals. However, the transmission
distance is severely limited if the SE is increased up
to 10 bit/s/Hz by using a higher-order QAM format[1].
Therefore, when considering channel capacities
beyond 400 Gbit/s, the combination of subcarrier
4

Feature Articles(1) Enhancing the performance,compactness, and energy efficiency of
optical transceivers

SDM opticaltransport
network(2) Increasing the scale and integrationof optical node equipment (opticalamplifier and optical switches)(3) Multiplexing and integrating opticaltransmission media (SDM optical fibers,
fusion splicing of SDM optical fibers,
and optical fiber connectors)

Fig. 5. Key technologies of SDM optical transport network.

multiplexing and/or a high symbol rate is a potential
solution, while keeping the existing order of the
QAM format low so as to ensure long transmission
reach. Since the SE does not increase with this strategy, this naturally requires that the channel bandwidth increase in proportion to the channel rate. As aresult, the number of WDM channels accommodated
in the typical optical amplifier bandwidth naturally
decreases, as shown in Fig. 4. Because the channel
capacity growth rate (increasing roughly tenfold over
20 years) is more gradual than the total capacity
growth rate as shown in Fig. 1, we have to effectively
combine SDM and WDM technologies to ensure thesame number of optical channels as before.
Therefore, if future ultra-high-capacity SDM transmission systems are to have a total capacity C of morethan 1 Pbit/s, the required number of optical transceiver circuits implemented will increase to 10 andeven 100 times that of a conventional SMF system.
Thus, SDM technologies such as scalable integrated
optical switching nodes as well as compact integration of optical transceiver circuits are the key to realizing economical high-capacity SDM optical transport networks.
3. Core technologies of SDM opticalcommunication infrastructure
In the Feature Articles in this issue, we describe thecurrent state of R&D and the future prospects of
SDM optical communications technologies from thefollowing three key aspects (Fig. 5).
(1)	Enhancing the transmission performance,
5compactness, and energy efficiency of opticaltransceivers (transmission equipment)(2)	Increasing the scale and integration of opticalnode equipment (optical amplifier and opticalswitches)(3)	Multiplexing and integrating optical transmission media (SDM optical fibers, fusion splicing of SDM optical fibers, and SDM opticalfiber connectors)
Transmission equipment (1) involves general technologies for both conventional SMF-based opticalcommunications systems and future SDM opticalcommunications systems. In one Feature Article, we
describe the impact of inter-core crosstalk characteristics and efficient digital signal processing in termsof realizing spatially multiplexed long-haul opticalcommunications systems [6]. We also explain thefeasibility of petabit-per-second-class long-haultransmission. In a high-capacity SDM optical transport network, the number of multiplexed opticaltransceiver circuits will be one or two orders of magnitude higher than that of the conventional SMFbased system. Therefore, to realize an economical
high-capacity energy-efficient optical network, it isvery important to increase the transmission reach
without optical-electrical conversion in the opticalnode, since reducing the number of optical transceiver circuits is critical for reducing the total system
power consumption.
With regard to SDM node technology (2), the final
target is to increase the throughput of optical nodes
by over 100 times and to improve the transmission
capacity to more than 1 Pbit/s. For this purpose, we
NTT Technical Review

Feature Articles

Core networks

3. Terrestrial
core/metro
networksSubm
net arineworks

Metro networks

4. Submarine
cable systems

1. Datacenterinterconnections

2. Internal
wiring of relay
facilities
Datacenter networks

Wireless and wireline
access networks

Fig. 6. Application areas of SDM optical communications.

have introduced novel optical node architectures andnovel device integration technologies in SDM opticalamplifiers and SDM integrated optical switches. We
describe the recent R&D activities in this field [7, 8].
Finally, with regard to multiplexing and integrationof optical transmission media (3), we describe theissues and feasibility of optical fiber transmission
media for dense SDM (DSDM) with the SDM levelsof 30 and beyond. We describe the current state of
MCF, MMF, and multi-core multi-mode fiber (a combination of both) [9], as well as the connection
devices between new SDM fibers and existing SMF
and splicing techniques and connector technology for
new SDM fibers [10]. The NTT laboratories havebeen commissioned by organizations including the
Ministry of Internal Affairs and Communications ofJapan and EC Horizon 2020, and the National Institute of Information and Communications Technology
to accelerate our R&D in this field [4, 5].

sider an SDM system that effectively uses existing
installed SMF fiber cables as shown in Fig. 3(a). Furthermore, the use of high-capacity SDM optical fiber
cables is expected to support the implementation of
economical core/metro high-capacity optical networks with a capacity of more than 1 Pbit/s, as it will
promote standardization and practical applications.
In ultra-long-haul submarine cable systems that must
accommodate the traffic growth of international communications, SDM optical communication technology is expected to offset the capacity crunch of conventional SMF-based systems and power supply
constraints (i.e., power crunch), both of which are
critical constraints in ultra-long-haul submarine cablesystems.
References[1]

4. Future prospects
The application areas where the future SDM opticalcommunications system is expected to be applied areshown in Fig. 6. There is strong demand for shortdistance applications such as datacenter interconnections and the internal wiring of SDM network elements in terms of compactness and low-power operation. Such applications have great potential in thisfield.
For medium-term SDM system application in terrestrial core/metro networks, it is promising to conVol. 15 No. 6 June 2017

[2]

[3]
[4]

[5]

Y. Miyamoto, A. Sano, E. Yoshida, and T. Sakano, “Ultrahigh-capacity Digital Coherent Optical Transmission Technology,” NTT Technical Review, Vol. 9, No. 8, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201108fa2.html
Y. Kisaka, M. Tomizawa, and Y. Miyamoto, “Digital Signal Processor(DSP) for Beyond 100G Optical Transport,” NTT Technical Review,Vol. 14, No. 9, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201609fa2.html
T Morioka, “Optical Communication Technologies: Challenges forInnovations,” J. IEICE, Vol. 94, No. 5, pp. 412–416, 2011 (in Japanese).
Y. Miyamoto and H. Takenouchi, “Dense Space-division-multiplexing Optical Communications Technology for Petabit-per-second
Class Transmission,” NTT Technical Review, Vol. 12, No. 12, 2014.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201412ra3.html
Y. Miyamoto, K. Sato, M. Fukutoku, T. Tsuritani, T. Hashimoto, E. de6

Feature Articles

[6]

[7]

[8]

Gabory, K. Nakajima, R. Sugisaki, and R. Nagase, “R&D of SpaceDivision Multiplexing Photonic Node,” IEICE Tech. Rep., Vol. 116,
No. 307, PN2016-40, pp. 87–92, 2016.
T. Mizuno, K. Shibahara, D. Lee, T. Kobayashi, and Y. Miyamoto,“Dense Space Division Multiplexing (DSDM) Long Distance OpticalFiber Transmission Technology,” NTT Technical Review, Vol. 15, No.
6, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201706fa2.html
M. Fukutoku, K. Suzuki, A. Sahara, F. Inuzuka, and Y. Miyamoto,“Dense Space Division Multiplexing (DSDM) Photonic-node Platform Technology,” NTT Technical Review, Vol. 15, No. 6, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201706fa3.html
H. Ono, “Optical Amplification Technologies for Space Division

Yutaka Miyamoto

Senior Distinguished Researcher, Director,
Innovative Photonic Network Research Center,
NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986
and 1988, and a Dr.Eng. in electrical engineeringfrom the University of Tokyo. In 1988, he joined
NTT Transmission Systems Laboratories, where
he engaged in R&D of high-speed optical communications systems including the first 10-Gbit/s
terrestrial optical transmission system (FA-10G)
using EDFA (erbium-doped optical fiber amplifier) inline repeaters. He was with NTT Electronics Technology Corporation from 1995 to 1997,
where he worked on the planning and productdevelopment of high-speed optical modules atdata rates of 10 Gbit/s and beyond. Since 1997,
he has been with NTT Network Innovation Labs,
where he has been researching and developing
optical transport technologies based on
40/100/400-Gbit/s channels and beyond. He has
also been investigating and promoting a scalable
optical transport network with Pbit/s-class capacity based on innovative transport technologies
such as digital signal processing, space divisionmultiplexing, and cutting-edge integrated devices for photonic pre-processing. He currently
serves as Chair of the IEICE technical committeeof Extremely Advanced Optical Transmission(EXAT). He is a member of IEEE (Institute ofElectrical and Electronics Engineers) and a Fellow of IEICE (Institute of Electronics, Information and Communication Engineers).

7

Multiplexing,” NTT Technical Review, Vol. 15, No. 6, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201706fa4.html[9] T. Sakamoto, T. Mori, T. Matsui, T. Yamamoto, and K. Nakajima,“Research and Development for Next Generation Optical Fiber Using
Multiple Spatial Channels,” NTT Technical Review, Vol. 15, No. 6,2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201706fa5.html[10] Y. Abe, K. Shikama, and S. Asakawa, “Multi-core Fiber Connector
Technology for Low-loss Physical-contact Connection,” NTT Technical Review, Vol. 15, No. 6, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201706fa6.html

Ryutaro Kawamura

Director, General Manager, NTT Network
Innovation Laboratories.
He received a B.S. and M.S. in precision engineering and a Ph.D. in electronics and information engineering from Hokkaido University in
1987, 1989, and 1996. He joined NTT Transport
Systems Laboratories in 1989. From 1998 to1999, he was a visiting researcher at Columbia
University, USA. From 2003 to 2015, he was amember of the Board of Directors of OSGi Alliance. He is involved in research on network reliability techniques, network control and management, high-speed computer networks, active
networks, network middleware, and mobile
cloud/edge computing.

NTT Technical Review

Feature Articles: State-of-the-art Space Division Multiplexing
Technologies for Future High-capacity Optical Transport Networks

Dense Space Division Multiplexing(DSDM) Long Distance Optical Fiber
Transmission Technology
Takayuki Mizuno, Kohki Shibahara, Doohwan Lee,
Takayuki Kobayashi, and Yutaka Miyamoto
Abstract

Multi-core and multi-mode space division multiplexing (SDM) technology is being studied as an optical transmission technology targeted for the next generation high-capacity optical communication network. In this article, we describe the latest trends in optical transmission using SDM technology. We
introduce the world’s most advanced ultra-high-capacity long distance optical transmission realized bydense space division multiplexing (DSDM) with a spatial multiplicity above 30, which was achieved injoint global academic and industrial research collaborations.
Keywords: dense space division multiplexing (DSDM), multi-core multi-mode, long distance highcapacity optical fiber transmission

1. Introduction
With the rapid spread of new information and communication services such as cloud computing, wireless communication, and high-definition video communication services, the data traffic flowing throughthe optical network is expected to continue toincrease. Along with this traffic growth, a further
increase in transmission capacity over optical fiber isrequired. The NTT laboratories have been developing
optical transmission technologies over the past 30
years involving time division multiplexing, wavelength division multiplexing, and digital coherent
technology, and have succeeded in realizing a
100-Tbit/s-class high transmission capacity per optical fiber in research, and a 10-Tbit/s-class transmission capacity in commercial large-capacity backbone
optical transmission systems.
To further increase the transmission capacity, it isnecessary to increase the power input to an opticalfiber. However, increasing the power too much will
give rise to nonlinear optical effects and a fiber fusephenomenon. Thus, there is an upper limit to the1

allowable optical power transmitted through a fiber.
The capacity limit due to these physical limits is
known to be around 100 Tbit/s, and we may reach thisupper limit within the next decade in commercial
communication systems.
At NTT Network Innovation Laboratories, we havebeen promoting research on spatial multiplexing
technology since 2009 in cooperation with related
research groups within NTT, and in collaboration
with other companies and universities, in order torealize the next generation ultra-high-capacity opticaltransmission technology.
Space division multiplexing (SDM)*1 is attracting
attention as a state-of-the-art optical transmission
technology that can increase the transmission capacity by several orders of magnitude relative to a conventional single-mode fiber (SMF) by spatially multiplexing optical signals in a transmission line.
Advanced research is being conducted in various*1 SDM: An optical transmission technology that multiplexes andtransmits optical signals using a spatial dimension. Global research and development is progressing towards next generation
high-capacity optical transmission technology.

NTT Technical Review

Feature Articles

MCF
Core 1

Tx 2

Rx 1

Single-mode coreMulti-mode coreRx 2

II. CoupledI. Single-mode
coreIII. Multi-mode

Coupled-coregroup

Multi-core
multi-mode

N cores

N cores x Mgroups

N cores x M modes

Single-mode

Coupled-coreMulti-mode

N cores x 1 group

M modesCore N

DSP

DSP

Tx 1

A

Multi-core

Rx N

Tx N

Multiple spatialchannel groups
MMF
Rx 1

Tx 2

Mode 1Mode 2Mode M

Tx M
Tx: transmitter

Rx 2

B
DSP

DSP

Tx 1

Single spatialchannel group

Rx M
Rx: receiver(a) Schematic of SDM optical transmission system using multi-core
fiber (MCF) and multi-mode fiber (MMF) as a transmission medium

Current DSP

MIMO DSP(b) SDM optical transmission methods

DSP: digital signal processing
MIMO: multiple-input multiple-output

Fig. 1. SDM optical transmission technology.

research institutions around the world. A schematicof an SDM optical transmission system using a multicore fiber (MCF) and a multi-mode fiber (MMF) as atransmission medium is shown as a representative
example in Fig. 1(a). With SDM, we can increase thetransmission capacity by N or M times that of SMFs
currently being used in backbone optical networks,
where N and M are the number of cores and modes,
respectively. Various SDM optical transmission
methods have been reported so far and are depicted ina matrix in Fig. 1(b).
The transmission capacity per optical fiber is plotted as a function of transmission distance in Fig. 2.
These examples have been demonstrated in recent
transmission experiments using SDM technology. In
2012, a transmission experiment reported a 305-Tbit/s
capacity over a 10.1-km 19-core MCF, proving forthe first time that the capacity could exceed thecapacity limit of an SMF by using SDM technology.
In the same year, NTT Network Innovation Laboratories collaborated with optical device research groups
in NTT, an optical fiber manufacturing company, anduniversities in Japan and Europe to demonstrate theworld-first 1-Pbit/s transmission [1] using a one-ring
structured 12-core MCF, which is an order of magnitude larger than the capacity limit of an SMF. The
following year, in 2013, we demonstrated the first
Vol. 15 No. 6 June 2017

capacity distance product exceeding 1 Ebit/s x km by
applying a bi-directional transmission scheme in a
two-ring structured 12-core MCF to reduce inter-corecrosstalk [2].
Although SDM optical transmission technologies
have proven that they can exceed the capacity limit ofa conventional SMF, it is necessary to further increase
spatial multiplicity, that is, the number of cores ormodes multiplexed in an optical fiber, to further
increase capacity. Therefore, developing new technologies for massive spatial multiplexing is the nextchallenge.
2. Towards dense space divisionmultiplexing (DSDM)
We have been working to further increase thecapacity of optical fiber transmission systems using
SDM technology by developing new fundamental
technologies with the goal of realizing dense space
division multiplexing (DSDM)*2 with a spatial multiplicity of 30 or more. To establish DSDM long distance optical transmission using an MCF, we must
*2 DSDM: High density SDM technology with spatial multiplicity
above 30, which we proposed and demonstrated in 2014 for the
first time.

2

Feature Articles

10119-core x 6-mode
KDDI
1

Capacity per optical fiber (Pbit/s)

1

Pb

it/s

xk

m

12-coreNTT

10-modeBell Labs.

7-coreNICT

7-core x 3-mode
Eindhoven Univ.

xkmWorld-first Pbit/s
12-coreNTT

19-core
NICT

Eb

it/s

22-coreNICT

12-coreNTT
7-core7-core
KDDI
KDDI

0.1
19-core x 6-modeKDDI

6 coupled-core

10-modeBell Labs.

0.01

0.001

12-core x 3-modeNTT

12-core x 3-mode
NTT

1

I A: Multi-core

I B: Single-mode

III A: Multi-core
multi-mode

II B: Coupled-core

6-modeBell Labs.

III B: Multi-mode

10

100

Bell Labs.
6 coupled-core
Bell Labs.

3-mode 3 coupled-core
Bell Labs. Bell Labs.
1000

10,000Transmission distance (km)Fig. 2. Transmission capacity per optical fiber as a function of transmission distance.

arrange 30 or more cores in an optical fiber with acladding diameter within 250 μm, taking fiber
strength and reliability into consideration. At thesame time, each core should have an effective area of
80 μm2 or more, which is equivalent to that of a conventional SMF. Since the core arrangement in the
optical fiber becomes dense, crosstalk between cores
increases, which leads to the degradation of transmission quality.
As an example, the inter-core crosstalk after 1000km transmission is shown in Fig. 3 as a function of
spatial multiplicity. The vertical axis indicates theworst inter-core crosstalk in an MCF after 1000-kmtransmission for terrestrial optical communication
systems. The higher a position is on the graph, thelower the crosstalk value is, which means that theeffect of crosstalk from signals in other cores is small
on long distance transmission characteristics.
The dotted lines in the graph are the inter-corecrosstalk values required for each modulation format,
assuming a Q-factor penalty of 0.5 dB. The higher themultilevel degree, the larger the transmission capacity can be with the same resource, but the crosstalkrequirement becomes stricter. For example, it is necessary to suppress the inter-core crosstalk to less than3

–25 dB to apply a polarization division multiplexed
16-quadrature amplitude modulation (16-QAM) *3
format. As shown in Fig. 3, as we increase the numberof cores by 7, 12, and 19, the core arrangement
becomes dense, so the inter-core crosstalk increases.
Therefore, we have set the spatial multiplicity of 30
to 100 and the inter-core crosstalk of < –25 dB as thetarget area for achieving DSDM long distance opticaltransmission.
3. World’s first DSDM optical transmission
As a first approach, we examined the combinationof multi-core and multi-mode optical transmission. In
mode-division multiplexed optical transmission, theapplication of multiple-input multiple-output(MIMO)*4 signal processing, a technique used in*3 16-QAM: A modulation format that associates 16 values of digital signals with 16 types of intensity and phase combinations of
the optical signals in a carrier wave, and transmits 4 bits per
modulation.
*4 MIMO: A digital signal processing method used in practicalwireless communication systems. Application to SDM transmission systems is being considered for the purpose of separating
spatially coupled optical signals.

NTT Technical Review

Feature Articles

N=7Inter-core crosstalk after 1000-kmtransmission (dB)N=12Bi-directional transmission
N=19
Uni-directional
N=16

−30

Crosstalk required foreach modulation format
assuming 0.5-dB Q-factor
penalty

Target area of DSDM long distance
optical transmission(Spatial multiplicity 30–100)−40

36 (N=12, M=3)
N=32

Bi-directional−20

Uni-directional

64-QAM
32-QAM
16-QAM
8-QAM
QPSK

N=22
N=30

−10
Transmission line
Optical fiber
0

0

10

20

30

40

Spatial multiplicity
QAM: quadrature amplitude modulation
QPSK: quadrature phase-shift keying

Fig. 3. Inter-core crosstalk after 1000-km transmission as a function of spatial multiplicity.

commercial wireless communication systems, isbeing considered in order to separate optical signals
between different modes that are coupled during
propagation. The amount of computation required for
MIMO signal processing is proportional to the magnitude of the differential mode delay (DMD)*5. Since
there is a limit to the load that can be tolerated bydigital signal processing (DSP), it is necessary to suppress DMD. In addition, mode dependent loss
(MDL)*6 has a tremendous effect on the transmission
characteristics in mode-division multiplexed transmission.
While conducting research on multi-mode transmission, we found that it was difficult to fully compensate for the degradation caused by MDL with
DSP, and MDL was one of the largest factors limiting
the transmissible distance. As described above,
advanced technology is essential even in mode-division multiplexing itself, and it was extremely difficult
to realize multi-core and multi-mode optical transmission at the same time. Thus, there had been noreports on multi-core multi-mode transmission fromany research institute at that time.
With a view to solving these issues, we proposed anovel parallel MIMO time domain equalization
method to reduce the load of DSP. Also, in cooperation with an optical fiber manufacturer and universities, we developed a low-loss and low-crosstalk
multi-core multi-mode optical fiber. Furthermore, inVol. 15 No. 6 June 2017

cooperation with research groups in NTT studying
optical devices, we developed a multi-core multimode fan-in/fan-out (FI/FO) device for spatial multi/
demultiplexing, a low-loss mode multi/demultiplexer
based on a silica planar lightwave circuit (PLC), andan integrated optical receiver for SDM systems fabricated using commercially available silica PLC technology. We combined these fundamental technologies and in 2014 successfully achieved multi-core
multi-mode DSDM optical transmission for the firsttime in the world, with a spatial multiplicity of 36(12-core multiplexing × 3-mode multiplexing) [3].
The combination of the multi-core and multi-mode
transmission greatly enhanced the spatial multiplicity
because of the multiplication effect of the core andmode multiplexing.
In 2015, we proposed a novel parallel MIMO frequency domain equalization method to further reducethe complexity of DSP and moreover realized agraded-index type multi-core multi-mode opticalfiber with an order of magnitude lower DMD. In
addition, we realized a free-space optics type MDL*5 DMD: Difference in group delay time between modes. It is
known that the DMD can be reduced by using the graded-index
type refractive index distribution. Reducing DMD will reduce theload of digital signal processing in multi-mode transmission.
*6 MDL: Loss difference between multiple modes. It is one of thelargest factors limiting the transmission distance in mode-division multiplexed optical transmission.

4

Feature Articles

Multi-core multi-mode
fiber

Tx 2, λ1
Tx 2, λ20
Tx 3, λ1

MDL equalizer

Multi-mode
FO optical amplifierFI

Rx 1, λ20
Rx 2, λ1
Rx 2, λ20
Rx 3, λ152.7 kmTx 3, λ20

DSP

Mode MUX

DSP

Tx 1, λ20

Rx 1, λ1
Mode DEMUX

Tx 1, λ1

Rx 3, λ20

Wavelength
MUX

Wavelength
DEMUX(a) Schematic diagram of multi-core multi-mode DSDM optical transmission setup

High density
MCF
Tx 1, λ1

Rx 1, λ1FOFI
Tx N, λ151.4 kmMulti-core
opticalamplifierRx 1, λ20
Rx N, λ1

DSP

DSP

Tx 1, λ20

Rx N, λ20

Tx N, λ20
Tx: Transmitter

Rx: Receiver(b) Schematic diagram of multi-core DSDM optical transmission setup
DEMUX: demultiplexer
MUX: multiplexer

Fig. 4. DSDM optical transmission technology.

equalizer and a multi-mode optical amplifier withlow mode dependency in gain, both of which greatly
reduce the MDL in the optical transmission line.
These DMD and MDL suppression technologies
made it possible to achieve long distance multi-mode
transmission, and this enabled us to successfully
demonstrate the world-first long distance multi-core
multi-mode DSDM optical transmission over 527 km[4]. A schematic diagram of the multi-core multimode DSDM optical transmission setup we used in
the experiment is shown in Fig. 4(a).
4. World’s first multi-core DSDM long distance
optical transmission
As another approach, we have also been conducting
studies of high density MCF in a Japanese-European
collaboration [5]. In our first study, we fabricatedhigh density 30-core and 31-core MCFs about 10 kmlong and confirmed good transmission characteristics. However, the crosstalk between cores was large,
5

and thus, these MCFs were not suitable for long distance optical transmission. We improved the MCF
design and fabricated a 32-core high density MCF
51.4 km in length. With this MCF, we succeeded insuppressing the core-to-core crosstalk to less than
–21.6 dB even after 1000-km transmission, and wereached the target area of DSDM transmission shownin Fig. 3 for the first time with a single-mode MCF.
In 2016, we demonstrated the first multi-core
DSDM long distance optical transmission exceeding
1600 km [6] using this low-crosstalk high density
MCF. A schematic diagram of the multi-core DSDM
optical transmission setup we used in the demonstration experiment is shown in Fig. 4(b). The long distance DSDM transmission with a spatial multiplicity
higher than 30 and a transmission distance over 1000km was a world-first achievement and has been the
only successful such demonstration up until now.
To use this multi-core DSDM optical transmission
in a real system, a high density multi-core opticalamplifier is essential. In cooperation with the
NTT Technical Review

Feature Articles

19-core × 6-mode(2016 NTT, 2015 KDDI)

Spatial multiplicity

100

DSDM region: Spatial multiplicity > 30

36-core × 3-mode(2015 NICT)World-first DSDM
12-core × 3-mode(2014 NTT)
19-core22-core30

7-core× 3-modeWorld-first long distance DSDM
12-core × 3-mode(2015 NTT)
32-core(2016 NTT)

12-core15-mode10

10-mode12-coreIII A: Multi-core
multi-mode
I A: Multi-core

6-mode3-modeII B: Coupled-core
III B: Multi-mode

11

7-core6 coupledcore

10

100

3 coupledcore

1000

10,000Transmission distance (km)Fig. 5. Spatial multiplicity versus transmission distance.

members of the EU-Japan project, we also conductedstudies on MCF amplifiers and developed a 32-core
cladding-pumped multi-core erbium/ytterbiumdoped fiber amplifier (MC-EYDFA) for the first time.
Using the 32-core MCFs and the 32-core MC-EYDFA, we have constructed a 111.6-km 32-core inline
amplified DSDM transmission setup and experimentally verified good transmission characteristics overall 32 cores [7].
The spatial multiplicity versus transmission distance in SDM optical transmission reported so far isshown in Fig. 5. At the beginning of our study on
DSDM, the highest spatial multiplicity reported in
multi-core optical transmission was 19. For 1000-kmclass long distance optical transmission, the spatial
multiplicity was even more limited, with 12 being the
maximum. Under these circumstances, we succeededin 2014 in carrying out the first DSDM optical transmission with a spatial multiplicity above 30. Moreover, we extended the transmission distance from 40
km to over 500 km, and then to over 1600 km with
DSDM. More recently, other research institutes havesubsequently studied DSDM, and DSDM with spatial
multiplicity above 100 has been reported.
5. Future directions
In this article, we introduced the latest trends in
SDM optical transmission technology and the DSDM
Vol. 15 No. 6 June 2017

optical transmission system with efforts to further
increase the transmission capacity for the next generation high-capacity optical communication technology. We will continue to promote the research anddevelopment of SDM optical transmission technology as part of efforts to achieve an ultra-high-capacity
long distance optical transmission system as thefoundation for the future optical network.
References
[1]

[2]

[3]

H. Takara, A. Sano, T. Kobayashi, H. Kubota, H. Kawakami, A. Matsu­
ura, Y. Miyamoto, Y. Abe, H. Ono, K. Shikama, Y. Goto, K. Tsujikawa,
Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, M. Koshiba,
and T. Morioka, “1.01-Pb/s (12 SDM/222 WDM/456 Gb/s) Crosstalkmanaged Transmission with 91.4-b/s/Hz Aggregate Spectral Efficiency,” Proc. of ECOC 2012 (the 38th European Conference andExhibition on Optical Communication), Postdeadline paper, Th.3.C.1,
Amsterdam, The Netherlands, Sept. 2012.
T. Kobayashi, H. Takara, A. Sano, T. Mizuno, H. Kawakami, Y. Miyamoto, K. Hiraga, Y. Abe, H. Ono, M. Wada, Y. Sasaki, I. Ishida, K.
Takenaga, S. Matsuo, K. Saitoh, M. Yamada, H. Masuda, and T.
Morioka, “2 × 344 Tb/s Propagation-direction Interleaved Transmission over 1500-km MCF Enhanced by Multicarrier Full Electric-field
Digital Back-propagation,” Proc. of ECOC 2013 (the 39th European
Conference and Exhibition on Optical Communication), Postdeadline
paper, PD3.E.4, London, UK, Sept. 2013.
T. Mizuno, T. Kobayashi, H. Takara, A. Sano, H. Kawakami, T. Nakagawa, Y. Miyamoto, Y. Abe, T. Goh, M. Oguma, T. Sakamoto, Y.
Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, and T. Morioka,“12-core × 3-mode Dense Space Division Multiplexed Transmission
over 40 km Employing Multi-carrier Signals with Parallel MIMO
Equalization,” Proc. of Optical Fiber Communication Conference(OFC) 2014, Postdeadline paper, Th5B.2, San Francisco, CA, USA,
Mar. 2014.

6

Feature Articles

[4]

[5]
[6]

7

K. Shibahara, T. Mizuno, H. Takara, A. Sano, H. Kawakami, D. Lee,
Y. Miyamoto, H. Ono, M. Oguma, Y. Abe, T. Kobayashi, T. Matsui, R.
Fukumoto, Y. Amma, T. Hosokawa, S. Matsuo, K. Saito, H. Nasu, andT. Morioka, “Dense SDM (12-core × 3-mode) Transmission over 527
km with 33.2-ns Mode-dispersion Employing Low-complexity Parallel MIMO Frequency-domain Equalization,” Proc. of OFC 2015,
Postdeadline paper, Th5C.3, Los Angeles, CA, USA, Mar. 2015.
Website of the SAFARI project, http://www.ict-safari.eu/
T. Mizuno, K. Shibahara, H. Ono, Y. Abe, Y. Miyamoto, F. Ye, T.
Morioka, Y. Sasaki, Y. Amma, K. Takenaga, S. Matsuo, K. Aikawa, K.
Saitoh, Y. Jung, D. J. Richardson, K. Pulverer, M. Bohn, and M.
Yamada, “32-core Dense SDM Unidirectional Transmission of PDM-

[7]

16QAM Signals over 1600 km Using Crosstalk-managed Singlemode Heterogeneous Multicore Transmission Line,” Proc. of OFC
2016, Postdeadline paper, Th5C.3, Anaheim, CA, USA, Mar. 2016.
S. Jain, T. Mizuno, Y. Jung, Q. Kang, J. R. Hayes, M. N. Petrovich, G.
Bai, H. Ono, K. Shibahara, A. Sano, A. Isoda, Y. Miyamoto, Y. Sasaki,
Y. Amma, K. Takenaga, K. Aikawa, C. Castro, K. Pulverer, M. Nooruzzaman, T. Morioka, S. U. Alam, and D. J. Richardson, “32-core
Inline Multicore Fiber Amplifier for Dense Space Division Multiplexed Transmission System,” Proc. of ECOC 2016 (the 42nd European Conference and Exhibition on Optical Communication), Postdeadline paper, Th.3.A.1, Düsseldorf, Germany, Sept. 2016.

NTT Technical Review

Feature Articles

Takayuki Mizuno

Senior Research Engineer, Photonic Transport
Network Laboratory, NTT Network Innovation
Laboratories.
He received a B.E. in applied physics, an M.E.
in crystalline materials science, and a Dr.Eng. inquantum engineering from Nagoya University in
1998, 2000, and 2007. In 2000, he joined NTT
Photonics Laboratories, where he was involvedin the research and development of silica planar
lightwave circuit optical waveguide devices,
including arrayed-waveguide gratings, MachZehnder interferometer-based filters and switches, and digital coherent demodulators foradvanced modulation formats. He has been a
Senior Research Engineer at NTT Network Innovation Laboratories since 2013, promoting innovative research on space division multiplexing
transmission technology for ultra-high-capacity
optical transport systems. He was a co-chair of
the 2016 IEEE Photonics Society Summer Topicals Meeting Series: Topicals on Space-Division
Multiplexing and Multimode Photonics (SDMP),
and is currently a technical program committee
member of the Optical Fiber Communication
Conference and Exhibition (OFC) Digital Transmission Systems Subcommittee. He is a memberof the Institute of Electronics, Information andCommunication Engineers (IEICE), the OpticalSociety (OSA), and the Institute of Electrical andElectronics Engineers (IEEE) Photonics Society.
He was awarded the 68th Young Researcher’sAward in 2006 from IEICE.

Kohki Shibahara

Engineer, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received a B.S. in physics and an M.S. ingeophysics from Kyoto University in 2008 and2010. He joined NTT Network Innovation Laboratories in 2010. His current research interests
include spatial division multiplexing transmission systems and advanced multiple-input andmultiple-output signal processing. He is a member of IEICE and the IEEE/Photonics Society. He
received the Tingye Li Innovation Prize from
OSA in 2016 and the Young Researcher’s Award
from IEICE in 2017.

Takayuki Kobayashi

Research Engineer, Photonic Transport Network Laboratory, NTT Network Innovation
Laboratories.
He received a B.E. and M.E. in communications engineering from Waseda University,
Tokyo, in 2004 and 2006. He has been with NTT
Network Innovation Laboratories since 2006.
His current research interests include modulation
formats, coherent detection, and high-speed
fiber-optic communications systems. He is amember of IEICE and the IEEE/Photonics Society.

Yutaka Miyamoto

Senior Distinguished Researcher, Director,
Innovative Photonic Network Research Center,
NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986
and 1988, and a Dr.Eng. in electrical engineeringfrom the University of Tokyo. In 1988, he joined
NTT Transmission Systems Laboratories, where
he engaged in R&D of high-speed optical communications systems including the first 10-Gbit/s
terrestrial optical transmission system (FA-10G)
using EDFA (erbium-doped optical fiber amplifier) inline repeaters. He was with NTT Electronics Technology Corporation from 1995 to 1997,
where he worked on the planning and productdevelopment of high-speed optical modules atdata rates of 10 Gbit/s and beyond. Since 1997,
he has been with NTT Network Innovation Labs,
where he has been researching and developing
optical transport technologies based on
40/100/400-Gbit/s channels and beyond. He has
also been investigating and promoting a scalable
optical transport network with Pbit/s-class capacity based on innovative transport technologies
such as digital signal processing, space divisionmultiplexing, and cutting-edge integrated devices for photonic pre-processing. He currently
serves as Chair of the IEICE technical committeeof Extremely Advanced Optical Transmission(EXAT). He is a member of IEEE and a Fellowof IEICE.

Doohwan Lee

Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received a B.S. (Hons.) and completed an
M.S. course in electrical engineering from Seoul
National University, South Korea, in 2004 and2006. He received a Ph.D. in electrical engineering and information systems from the Universityof Tokyo in 2009. He was with NTT Network
Innovation Labs from 2009 to 2012 and was an
assistant professor at the Research Center for
Advanced Science and Technology, the University of Tokyo, from 2012 to 2014. He has beenwith NTT Network Innovation Labs since 2014and has also been a part-time lecturer at Kanagawa University since 2016. His research interests include compressed sensing, software/cognitive radio, signal processing, and OAM multiplexing. He received the Best Paper Award andBest Technical Exhibition Award from the IEICE
Software Radio in 2011, the IEICE Communications Society Excellent Paper Award in 2012, theIEICE SRW (Short Range Wireless Communications) Young Researcher’s Award in 2016, theBest Technical Exhibition Award from SmartCom 2014, the Best Paper Award from SmartCom 2016, and the Best Paper Award and Special
Technical Award in Smart Radio from the IEICE
Smart Radio in 2017.

Vol. 15 No. 6 June 20178

Feature Articles: State-of-the-art Space Division Multiplexing
Technologies for Future High-capacity Optical Transport Networks

Dense Space Division Multiplexing(DSDM) Photonic-node Platform
Technology
Mitsunori Fukutoku, Kenya Suzuki, Akio Sahara,
Fumikazu Inuzuka, and Yutaka Miyamoto
Abstract

Optical infrastructure networks transporting high-volume traffic have been built using photonic nodetechnologies such as optical signal transmission, optical signal wavelength multiplexing and demultiplexing, and reconfigurable optical add/drop multiplexers. This article gives an overview of photonic
nodes using dense space division multiplexing technology and their potential for implementing highcapacity optical networks.
Keywords: space division multiplexing, optical node architectures, integrated wavelength selective
switching technology

1. Introduction
Communication traffic continues to increase eachyear, and the total Internet traffic in Japan alone
reached 5.4 Tbit/s in 2015, increasing by 50% in thatyear [1]. If we assume a 40% increase per year, we
can predict that the volume will exceed 1 Pbit/s by themid-2020s. The capacity of single-mode fiber (SMF)
is estimated to be 100 Tbit/s. To overcome the capacity limitations of SMF, research is underway on transport technologies using dense space division multiplexing (DSDM) [2]. It is also expected that DSDM
will similarly need to be used to handle high-volume
traffic in the photonic nodes used to build opticaltransport networks [3].
The throughput of a photonic node S is given by the
product of the number of degrees N, the number ofcores M, the spectral efficiency η, and the desired
bandwidth B. The relationships between the numberof degrees, number of cores, spectral efficiency, andattainable throughput are shown in Fig. 1. The twomain bands used for optical communication are the1.55-μm band (C band) and the 1.58-μm band (Lband), which together give a combined 10 THz of1

usable bandwidth. The solid line in the figure shows
the relationship between spectral efficiency and thespace division multiplicity (product of degrees andcores N • M), to implement an optical node with athroughput of 1 Pbit/s, assuming a combined signal
bandwidth of 10 THz for the C band and L band.
Achieving 10-Pbit/s-class throughput using modulation with the multiplicity of polarization-divisionmultiplexed 16-quadrature amplitude modulation(PDM 16-QAM) or greater, which would give a spectral efficiency of 5 bit/s/Hz or better, would give η of
5 to 6 bit/s/Hz. Therefore, with the desired bandwidth
B = 10 THz, space division multiplexing (SDM) photonic nodes with the product of cores and degrees
N • M of 150 to 200 would be needed. Thus, witheight degrees, photonic nodes using DSDM withmore than 20 cores would be necessary. Wavelength
selective switches (WSS) already used to build reconfigurable optical add/drop multiplexers (ROADM)
have around 20 ports, but there are issues in using
them for high-capacity DSDM photonic nodes.
This article gives an overview of the devices andmethods necessary to implement photonic nodes with
DSDM technology for high-capacity optical
NTT Technical Review

Feature Articles

Spectral efficiency η

10Target5

N •PDM 64-QAM
PDM 32-QAM

M

PDM 16-QAM

•

η

•

B

≥1

P
Fo bit/s
rB
of
1

0T

1
10

100

Hz

200

Product of degrees and cores N • M
PDM: polarization division multiplexed
QAM: quadrature amplitude modulation

Fig. 1. Relationship between number of degrees/cores and spectral efficiency.

Table 1. Comparison of joint and independent spatial mode switching.
(a) Spatial mode joint switching(b) Spatial mode independent switching

No
(Low)

Yes
(High)

Switchgranularity

No(All spatial modes together)Yes(In spatial mode units)

Switch degree of
freedom

No(No switching between spatial modes)

No(Switching between spatial modes)

Mode coupling/
No mode coupling

No mode coupling

Switchstructure

Switch function
flexibility

Switched signal
spatial mode
coupling

networks.
2. DSDM photonic node structure and SDM
photonic networks
DSDM photonic nodes introduce a new degree of
freedom in the spatial domain and with it, additional
complexity to the node structure. For this reason, it isa challenge to implement a simple DSDM photonic
node that provides the switching function necessary
for an SDM photonic network.
2.1 Spatial mode switching types
DSDM photonic nodes can be broadly categorized
Vol. 15 No. 6 June 2017

into (a) spatial mode joint switching and (b) spatial
mode independent switching, according to whether
they can switch spatial modes independently or not(Table 1). SDM transmission methods include multicore fiber (MCF), which involves multiple coreswithin a single optical fiber, and multi-mode fiber(MMF), which involves multiple waveguide modes
propagating within a single core. We use spatial
modes to refer to cores within MCF and waveguidemodes within MMF. Spatial mode joint switchingswitches all spatial modes in a single optical fiber atonce, outputting them to the same optical fiber. In
contrast, spatial mode independent switching can
independently select the output optical fiber for each2

Feature Articles

Table 2. Classification by switch granularity and degree of freedom.
(a) Fiber XC

Node structure

Path 1,
Spatial mode 1

Path 1,
Spatial mode 1

Path 1,
Spatial mode K

Path 1,
Spatial mode K

Path M,
Spatial mode 1

MK x MK
Matrix SW

Path M,
Spatial mode K

Function flexibility(b) Wavelength XC(b)-1 No spatial mode switching
MxM
MatrixSW
MxM
MatrixSW

Path M,
Spatial mode 1

MxM
MatrixSW

Path M,
Spatial mode K(b)-2 Spatial mode switching
Path 1,
Spatial mode 1
Path 1,
Spatial mode K
Path M,
Spatial mode 1

MK x
MK
MK x MK
MatrixMatrix SW
SW

Path M,
Spatial mode K

Switchfreedom

Yes(Switching between spatial modes)

No(No switching between spatial modes)

Yes(Switching between spatial modes)

Switchgranularity

No(Spatial mode units)

Yes(Wavelength units)

Yes(Wavelength units)

Signal leveltuningNo(Cannot equalize deviation
between wavelengths)

Yes(Can equalize deviation between
spatial modes, wavelengths)

Yes(Can equalize deviation between
spatial modes, wavelengths)(MK)2W • M2K

W • (MK)2

Switch scope(no. of switch
elements)

M: no. of paths; K: no. of spatial modes; W: wavelength multiplicity
SW: switch
XC: cross-connect

spatial mode.
When MMF is used, the spatial modes (waveguidemodes) of the optical signal are coupled, so multiple
modes are transmitted and received together, and thereceiver must process the signal to separate thesemodes. All waveguide modes input to a single opticalfiber are output from the fiber, so spatial mode joint
switching is adequate. However, if MCF is used, coupling between spatial modes (cores) is weak, so optical signals from different transceiver nodes can betransmitted on each spatial mode. The use of spatial
mode independent switching rather than joint switching for this yields finer switching granularity andmore flexible switching control.
2.2 S
 witching methods according to switching
granularity and degree of freedom
DSDM photonic node structures for spatial mode
independent switching are listed in Table 2. They areclassified according to whether they can switch wavelengths independently (wavelength independence)
and whether they can switch between spatial modes.
With SDM transmission, multiple optical signals
with different wavelengths can be multiplexed andtransmitted in each core or in each waveguide mode.
DSDM photonic node structures can be classified aseither (a) fiber cross-connect (fiber XC), which3

switches all wavelengths of the optical signal in asingle spatial mode at once, or (b) wavelength crossconnect (wavelength XC), which can switch wavelengths independently. Table 2 also lists wavelength
XCs further classified into those that can or cannot
switch signals between spatial modes. In these structures, the switching granularity gets finer movingfrom fiber XC to wavelength XC, and the degree of
freedom in switching increases by enabling switching
between spatial modes, providing greater flexibility
in switching control.
Table 2 also indicates the signal level tuning and thescale (number) of switches needed to implement eachnode structure. The wavelength XC configuration
with spatial mode switching ((b)-2) has very flexible
switching control and is able to adjust levels to reduce
disparity among both optical signal spatial modes andwavelengths in transmission paths and nodes. However, it requires more switch elements, so a simple
node implementation is difficult to achieve.
Network control technologies such as softwaredefined networking (SDN)*1 are expected to advancein the future, and in line with this, network control
techniques providing greater flexibility will be needed.
*1 SDN: A technological approach to control the operation of network devices centrally using software.

NTT Technical Review

Feature Articles

Lens

SBT array

SBT-1
SBT-2
SBT-Com
SBT-3
SBT-4

A
B
C
A
B
C
A
B
C
A
B
C
A
B
C

θ max

SLM
Region A
Region BΩ

Region CSω

SBT: spatial beam transformer
SLM: spatial light modulator

Fig. 2. Multiple-WSS integration on SPOC platform.

Research is underway at the NTT laboratories on
wavelength XC as a DSDM photonic node structurethat will enable more-flexible switching control, as it
can switch independently between spatial modes.
To allocate network resources efficiently in a network using DSDM photonic nodes, the degree of
freedom in the spatial domain must be considered. As
SDM photonic networks are expanded, network
capacity will need to be designed so as to optimize
resource utilization. Wavelength domain resources oncommunication routes will have to be allocated in the
network using conventional wavelength divisionmultiplexing (WDM), but spatial domain resources
will also have to be allocated. The transmission characteristics of the optical fiber medium (MCF or
MMF) and photonic nodes are also conditions of signal capacity. Thus, optical fiber characteristics andphotonic node structures are very important elements
in SDM photonic networks.
3. DSDM photonic node switching devices
Multiple switching devices are used in a DSDM
photonic node. SDM technology is expected to be
used together with WDM communication to increase
throughput, so a WSS able to switch by wavelength isneeded. Thus, a structure with multiple WSS isneeded in an SDM photonic network, and theincreased footprint of node equipment could become
a concern.
We are developing a spatial and planar optical circuit (SPOC) platform [4] combining waveguide optiVol. 15 No. 6 June 2017

cal systems*2 and spatial optical systems*3, which isan original optical system technology integrating
multiple WSS in a single module.
A schematic diagram of an optical system integrating multiple WSS using the SPOC platform is shownin Fig. 2. The spatial beam transformer (SBT) circuit
elements, positioned on optical waveguides, play animportant role in integrating the multiple WSS. The
SBT circuits are composed of a slab waveguide andan array waveguide with uniform lengths. When thelight signal shown with pink arrows is input to the
SBT circuit designated as SBT-Com in Fig. 2, it
diverges in the plane of the slab waveguide and is
output to the spatial optical system through the array
waveguides. Here, the lengths of the array waveguides are the same, so the wavefront of the output
optical signal is planar, with the direction shown by
the pink dotted line. When the light signal is switchedto the green input port, the output optical signal will
be a planar wave in the direction of the green upwardly inclined dotted line. The optical signals arrive at adifferent position on the spatial light modulator(SLM), which is the switching engine, and are independently reflected in different directions. Finally,
the signals are independently switched to any of theoutput ports and optically combined at the output*2 Waveguide optical system: An optical system using optical waveguides, which are optical integrated circuit structures; used to implement highly integrated optical communication devices.
*3 Spatial optical system: An optical system using lenses and diffraction gratings; larger than waveguide optical systems, but capable of extremely high optical performance.

4Feature Articles

Laser inscribed FI/FO
Waveguide frontend

MCF
fiber array

FI/FO: fan-in/fan-out

Fig. 3. Optical waveguide frontend in WSS for MCF.

SBT circuits.
We constructed a WSS for MCF [5] using thismultiple-WSS integrated function. A photograph ofthe optical waveguide system on the SPOC platform
is shown in Fig. 3. The MCF has a two-dimensional(2D) core arrangement, so to combine it with the planar optical waveguide, the two dimensions must be
converted to one dimension. In this case, we fabricated a circuit to convert the core arrangement from
2D to 1D using 3D waveguide technology with anultrashort pulse laser*4 and implemented a direct connection of the MCF to the waveguide circuits.
The switching spectra for each core of a 1 x 4 WSS
for a 7-core MCF are shown in Fig. 4. Here, operationis configured as a flexible grid with different wavelength bands for wavelength channels on the first andseventh cores (see Fig. 4(a)), as two-way switching
and attenuation with a 200-GHz channel width on thesecond (Fig. 4(b)) and third cores (Fig. 4(c)), and as
switching to four different paths with a 50-GHz channel width on cores 4 to 6 (Fig. 4(d)).
Each of these examples implements wavelength
selective switching operation, showing that the SPOC
platform is effective for SDM node switches.
4. Future prospects
The SPOC platform is a key technology for making
DSDM photonic nodes smaller. The NTT laboratories will receive support for contracted research from
NICT (the National Institute of Information andCommunications Technology) and other institutions
and conduct research and development (R&D)
through open innovation [6] to accelerate R&D on
these elemental technologies.

*4 Ultrashort pulse laser: A laser that outputs light pulses withlengths in the range of femtoseconds to nanoseconds.

5

NTT Technical Review

FO

WSS
WSS

FO
FO

Output 1
Output 2
Output 3

Transmittance (dB)

FO

WSS

FI

WSS

Input

WSS

WSS

WSS

Feature Articles

Output 4

−20
−30
−40
−50
−60
−70
1530

1535

1540

1545

1550

1555

1560

1565

Wavelength (nm)

FO

WSS
WSS

FO
FO

Output 1
Output 2
Output 3

Transmittance (dB)

FO

WSS

FI

WSS

Input

WSS

WSS

WSS

(a)

Output 4−20

ATT = 0, 5, 10 dB

ATT = 0, 5 dB

−30
−40
−50
−60
−70
1530

1535

1540

1545

1550

1555

1560

1565

1555

1560

1565

1555

1560

1565

Wavelength (nm)

FO

WSS
WSS

FO
FO

Output 1
Output 2
Output 3

Transmittance (dB)

FO

WSS

FI

WSS

Input

WSS

WSS

WSS

(b)

Output 4−20

ATT = 0, 5, 10 dB

−30
−40
−50
−60
−70
1530

1535

1540

1545

1550

Wavelength (nm)

FO

WSS
WSS

FO
FO

Output 1
Output 2
Output 3
Output 4

Transmittance (dB)

FO

WSS

FI

WSS

Input

WSS

WSS

WSS

(c)

−20
−30
−40
−50
−60
−70
1530

1535

1540

1545

1550

Wavelength (nm)(d)

Fig. 4. Switching spectra examples from WSS for 7-core MCF.

Vol. 15 No. 6 June 20176

Feature Articles

References
[1]

[2]

[3]

Ministry of Internal Affairs and Communications, “Aggregation andProvisional Calculation of Internet Traffic in Japan,” Mar. 2016 (inJapanese).
http://www.soumu.go.jp/menu_news/s-news/01kiban04_02000103.
html
Y. Miyamoto, R. Kawamura, “Space Division Multiplexed Optical
Transmission Technology to Support the Evolution of High-capacity
Optical Transport Networks,” NTT Technical Review, Vol. 15, No. 6,2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201706fa1.html
H. Takara, A. Sano, T. Kobayashi, H. Kubota, H. Kawakami, A. Matsu­
ura, Y. Miyamoto, Y. Abe, H. Ono, K. Shikama, Y. Goto, K. Tsujikawa,
Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, M. Koshiba,
and T. Morioka, “1.01-Pb/s (12 SDM/222 WDM/456 Gb/s) Crosstalk-Mitsunori Fukutoku

[5]

[6]

Fumikazu Inuzuka

Senior Research Engineer, Supervisor, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received a B.S. and M.S. from Tokushima
University in 1989 and 1991. He joined NTT
Transmission Systems Laboratories in 1991,
where he engaged in R&D of optical WDM systems. In 2000, he moved to NTT Communications, where he engaged in WDM network planning. He joined NTT Network Innovation Laboratories in 2009.

Senior Research Engineer, Photonic Transport
Network Laboratory, NTT Network Innovation
Laboratories.
He received a B.E. and M.E. in applied physics
from the Tokyo University of Agriculture andTechnology in 2002 and 2004. He joined NTT
Network Innovation Laboratories in 2004, where
he is researching photonic transport and processing systems and photonic networking systems.
He is a member of IEICE.

Kenya Suzuki

Yutaka Miyamoto

Senior Research Engineer, Supervisor, Optoelectronics Integration Research Group, Photonics-Electronics Convergence Laboratory, NTT
Device Technology Laboratories.
He received a B.E. and M.E. in electrical engineering and a Dr.Eng. in electronic engineeringfrom the University of Tokyo in 1995, 1997, and2000. He joined NTT in 2000. From September
2004 to September 2005, he was a visiting scientist at the Research Laboratory of Electronics(RLE) at the Massachusetts Institute of Technology, USA. From 2008 to 2010, he was with NTT
Electronics Corporation, where he worked on thedevelopment and commercialization of silicabased waveguide devices. He has also been aguest chair professor at the Tokyo Institute ofTechnology since 2014. His research interests
include optical circuit design and optical signal
processing. He received the Young Engineer
Award from the Institute of Electronics, Information and Communication Engineers (IEICE) in2003. He is a member of IEICE, the Institute ofElectrical and Electronics Engineers (IEEE), and
the Physical Society of Japan.

Akio Sahara

Senior Research Engineer, Photonic Transport
Network Laboratory, NTT Network Innovation
Laboratories.
He received a B.S., M.S., and Ph.D. in electrical engineering from Kyoto University in 1992,
1994, and 2003. He joined NTT in 1994 andengaged in research on optical networks. He is amember of IEICE and IEEE.

7[4]managed Transmission with 91.4-b/s/Hz Aggregate Spectral Efficiency,” Proc. of ECOC 2012 (the 38th European Conference andExhibition on Optical Communication), Th.3.C.1., Amsterdam, Netherlands, Sept. 2012.
K. Suzuki, K. Seno, and Y. Ikuma, “Application of Waveguide/Freespace Optics Hybrid to ROADM Device,” IEEE J. Lightw. Technol.,Vol. 35, No. 4, p. 596–606, 2017.
K. Suzuki, M. Nakajima, K. Yamaguchi, G. Takashi, Y. Ikuma, K.
Shikama, Y. Ishii, M. Itoh, M. Fukutoku, T. Hashimoto, and Y. Miyamoto, “Wavelength Selective Switch for Multi-core Fiber BasedSpace Division Multiplexed Network with Core-by-core Switching
Capability,” Proc. of OECC/PS 2016 (the 21th OptoElectronics andCommunications Conference / International Conference on Photonics
in Switching 2016), WF1-2, Niigata, Japan, July 2016.
Y. Miyamoto, K. Saitoh, M. Fukutoku, T. Tsuritani, T. Hashimoto, E.
Le Taillandier de Gabory, K. Nakajima, R. Sugizaki, and R. Nagase,“R&D of Space-division Multiplexing Photonic Node,” IEICE Tech.
Rep., Vol. 116, No. 307, PN2016-40, pp. 87–92, 2016.

Senior Distinguished Researcher, Director,
Innovative Photonic Network Research Center,
NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986
and 1988, and a Dr.Eng in electrical engineeringfrom the University of Tokyo. In 1988, he joined
NTT Transmission Systems Laboratories, where
he engaged in R&D of high-speed optical communications systems including the first 10-Gbit/s
terrestrial optical transmission system (FA-10G)
using EDFA (erbium-doped optical fiber amplifier) inline repeaters. He was with NTT Electronics Technology Corporation from 1995 to 1997,
where he worked on the planning and productdevelopment of high-speed optical modules atdata rates of 10 Gbit/s and beyond. Since 1997,
he has been with NTT Network Innovation Labs,
where he has been researching and developing
optical transport technologies based on
40/100/400-Gbit/s channels and beyond. He has
been investigating and promoting the scalable
optical transport network with Pbit/s-class capacity based on innovative transport technologies
such as digital signal processing, space divisionmultiplexing, and cutting-edge integrated devices for photonic pre-processing. He currently
serves as Chair of the IEICE technical committeeof Extremely Advanced Optical Transmission(EXAT). He is a member of IEEE and a Fellowof IEICE.

NTT Technical Review

Feature Articles: State-of-the-art Space Division Multiplexing
Technologies for Future High-capacity Optical Transport Networks

Optical Amplification Technologies
for Space Division Multiplexing
Hirotaka Ono
Abstract

Technologies that enable simultaneous optical amplification of spatially multiplexed optical signals
are essential for a long-haul space division multiplexing (SDM) transmission system that employs amulti-core fiber and/or few-mode fiber. This article introduces optical amplification technologies thatmake it possible to construct a multi-core erbium-doped fiber amplifier (EDFA) and a few-mode EDFA
for SDM transmission.
Keywords: multi-core fiber amplifier, few-mode fiber amplifier, erbium-doped fiber

1. Introduction
An optical amplifier is necessary for a long-haulspace division multiplexing (SDM) transmission system that employs a multi-core fiber and/or a fewmode fiber as a transmission line. SDM opticalamplifiers utilize an erbium-doped fiber (EDF) as theamplification medium in the same way as the opticalamplifiers used in the current single-core and singlemode fiber transmission system. An important function of SDM optical amplifiers is simultaneous
amplification of spatially multiplexed optical signals.
Two kinds of optical amplifiers have mainly beenstudied in recent years in order to realize such a function. One is a multi-core erbium-doped fiber amplifier (MC-EDFA), which employs a multi-core EDF
that has multiple erbium cores within a single fiber.
The other is a few-mode erbium-doped fiber amplifier (FM-EDFA), which utilizes a few-mode EDF thatis a kind of multi-mode fiber. A few-mode EDF supports several propagation modes used for signaltransmission and restricts unusable higher-ordermodes.
2. MC-EDFA
Table 1 categorizes MC-EDFAs in terms of pumping schemes and active fibers. There are two kinds ofpumping schemes, namely core pumping and clad1

ding pumping.
An MC-EDFA employing core pumping can
employ optical components that are used for a conventional single-core EDFA. It provides high pumping efficiency and can also support conventional
high-speed control to suppress the transient power
caused by a change in the input signal power. The
challenges to be met include integrating the opticalcomponents to reduce the total amplifier size, cost,and power consumption. Cladding pumping has thepotential to achieve both low power consumption anddownsizing by using an uncooled multi-mode pumplaser diode (LD). Challenges include improving thepumping efficiency, developing optical components
for launching the pump and multiple signal lights
simultaneously, and devising a technique for adjusting the gain of several cores to achieve a pump powerwith high-speed control.
Four kinds of active fibers have already beenreported for multi-core amplification: a bundle ofreduced-cladding EDFs, a multi-core EDF with asingle cladding, a multi-core EDF with a double cladding, and a multi-element EDF. The bundle andmulti-element EDFs can utilize conventional mature
fiber fabrication techniques, and the lengths of different EDFs can be adjusted to achieve a uniform gain.
A drawback is the necessity of downsizing the crosssection of the amplification medium. The benefit ofmulti-core EDFs with single and double cladding lies
NTT Technical Review

Feature Articles

Table 1. Categorization of MC-EDFA in terms of pumping scheme and active fiber.
Core pumping

Cladding pumping

Multiplexed pump and signal lights launched into core

All cores pumped by first cladding propagating pump light

BenefitHigh pump efficiency and applicability of components andhigh-speed control used in conventional single-core EDFA

Possibility of reducing size, power consumption, and costby employing uncooled multi-mode pump laser diode (LD).

Challenge

Reducing size, cost, and power consumption

Improving pumping efficiency, developing pump/signal
combiner, and achieving high-speed control

Pumping
schemeFiberSingle-core EDF

Multi-core EDF

Multi-core EDF

Single-core EDF

BundleSingle cladding

Double cladding

Multi-element

Structure

Benefit

Applicability of conventional
fabrication technology andadjustability of EDF length

Size and cost reduced by manufacturing several cores in
one fiber fabrication operation

Applicability of conventional
fabrication technology

Reducing cladding diameter and suppressing crosstalk
Challenge

Developing fiber bundlingtechniqueAchieving uniform gain and noise figure between coresin the reduced cost, which is achieved by manufacturing several cores in one fiber fabrication operation.
Another benefit of multi-core EDFs is that their cladding diameter is small compared with bundled andmulti-element EDFs. Finding a way to achieve a uniform amplification characteristic for all the cores is achallenge for both multi-core and multi-element
EDFs, and finding a way to suppress crosstalk is acommon challenge for all active fibers.
2.1 Core-pumped MC-EDFA
A typical configuration of a core-pumped MCEDFA is shown in Fig. 1(a). Both the pump and signal lights are multiplexed with a wavelength divisionmultiplexing (WDM) coupler and launched into an
erbium-doped core through a fan-in (FI), and theamplified signals are output through a fan-out (FO).
In this amplifier configuration, since the FI and FO
can reverse the propagation direction of the signal
lights, the propagation of the signal light in each corecan be set in any direction. Setting the signal lights in
two adjacent cores to propagate in opposite directions
reduces the intercore crosstalk [1]. An MC-EDFA
was constructed for long-haul transmission through12-core fiber by employing this method. Its configuration is shown in Fig. 1(b). This MC-EDFA utilizes
the outer cores of a dual 7-core EDF. As shown in
Fig. 1(c), a gain of over 11.4 dB and a noise figure ofless than 6.5 dB were achieved across the entire
Vol. 15 No. 6 June 2017

C-band when the signal lights of all the cores propagated in the same direction. The MC-EDFA wasapplied to SDM transmission with a capacity-distance product of 1 Ebit/s, and the results suggest its
feasibility [2]. A bundle of reduced-cladding EDFs
can also be used in this kind of SDM optical amplifier [3].
2.2 Cladding-pumped MC-EDFA
The configuration of a cladding-pumped MCEDFA is shown in Fig. 2(a). To improve pumping
efficiency, we employed double-clad multi-core
erbium/ytterbium-doped fiber (DCMC-EYDF). In
this fiber, the pump absorption is sensitized by transferring energy from the ytterbium to erbium ions andsuppressing the clustering of erbium ions, which resultsin improved pumping efficiency. Twelve erbium/
ytterbium-doped cores were arranged in a hexagon
as shown in the figure. The core pitch is 37.2 μm, andthe first and second claddings and the coating diame­ters are 216, 284, and 356 μm, respectively. The pumpsource was a 976-nm multi-mode LD with a 125-μmdiameter multi-mode fiber pigtail. A schematic of
the pump combiner is also shown in Fig. 2(a). The
pump combiner consists of a multi-mode fiber witha tapered section and the double-clad 12-core fiber,
whose cross-sectional design was the same as thatof the DCMC-EYDF. A short section of the double-clad
12-core fiber was stripped of its low refractive-index
2Feature Articles

Pump LDFI

FO

Pump LDWDM coupler

Pump LD

Isolator
Pump LD
Cross-section image of 7-core EDF(a) Configuration for propagation in the same direction

Pump LD

FI/FO

FI/FO

Pump LD
Pump LD

Crosssection

Pump LD(b) Configuration for propagation-direction interleaving

Gain, noise figure (dB)

20

CH9
CH10
CH11
CH12

CH5
CH6
CH7
CH8

CH1
CH2
CH3
CH4

15

10

Gain
Noise figure

5
Input signal power: +6 dBm
0
1530

1535

1540

1545

1550

1555

1560

1565

Wavelength (nm)(c) Gain and noise figure of MC-EDFA with propagation-direction interleaving configuration for 12-core fiber transmission

Fig. 1. Core-pumped MC-EDFA.

second cladding and coating, and the stripped section
was rounded to form an optical contact with the3tapered multi-mode fiber. The optical contact section
was recoated with a low index polymer. This pump
NTT Technical Review

Feature Articles

Double-clad
multi-core fiber

Multi-core fiber

Double-clad
multi-core EYDF
Core
1st cladding
2nd cladding
Coating
Multi-core fiber

×: Splicing points

Multi-mode LD
Double-clad multi-core fiber(Coating & polymer cladding removed)

Multi-mode fiber

Low index polymer

Tapered section(a) Configuration of amplifier and pump combiner

20Gain, noise figure (dB)

Gain
15

10

5

Noise figure

#1
#2
#3

#4
#5
#6

#7
#8
#9

#10
#11
#12

0
1530 1535 1540 1545 1550 1555 1560 1565

430 × 350 × 132.5 mm(c) Photograph of cladding-pumped 12-core EDFA

Wavelength (nm)(b) Gain and noise figure

Fig. 2. Cladding-pumped MC-EDFA.

combiner was fusion-spliced to the DCMC-EYDF,
which enables the pump light to couple to the firstcladding of the DCMC-EYDF. Twelve-core isolators
were located at the input and output ends of theamplifier to avoid laser oscillation.
The gain and noise figure of the cladding-pumped
MC-EDFA are shown in Fig. 2(b), and a photo of thedevice is shown in Fig. 2(c). The input signal was an
8-channel WDM signal with a power of −14 dBm/ch,
and the pump power was 3.4 W. The optical amplifier
exhibited over 10-dB gain and less than an 8.7-dB
noise figure for all 12 cores at wavelengths longer
than 1534 nm. In this case, the electrical power conVol. 15 No. 6 June 2017

sumption was about 10 W, while the sum of that of 12
conventional EDFAs was estimated to be 20 W at an
ambient temperature of 65ºC. This suggests that the
cladding-pumped MC-EDFA successfully reduced
the power consumption by about half that of the conventional optical amplifier.
The cladding pumping was also adopted in an SDM
optical amplifier for a dense SDM (DSDM) transmission. A 32-core EDFA that employed a DCMCEYDF was used in a DSDM transmission experiment
as an optical amplifier repeater [4].
The gain and noise figure of the cladding-pumped
MC-EDFA with a DCMC-EYDF degraded in the4

Feature Articles

Conventional EDF (circle core)

Ring-core EDF
Clad

Circle core

Ring core

Clad
Ri

Ro

Inner core

Erbium-doping distribution(= refractive index profile)LP01 mode intensity
distribution
LP11 mode intensity
distribution(a) Constriction of EDF cross-section and comparison between erbium-doping and intensity distribution

Gain, noise figure (dB)

25

Gain
6.1 dB (circle-core EDF)

20

4.5-dB improvement in DMG
1.6 dB (ring-core EDF)

15
Ring-core EDF

10Noise figure Circle-core EDF
5

LP01
LP11
LP01
LP11

0
1530 1535 1540 1545 1550 1555 1560 1565
Wavelength (nm)(b) Gain and noise figure of FM-EDFA employing ring-core EDF

Fig. 3. FM-EDFA.

shorter wavelength region because of the strong
absorption of the erbium ions in the EDF. Further
study is necessary to improve the uniformity of thegain and the noise characteristics if we are to use theentire C-band for amplification.
3. FM-EDFA
One issue with FM-EDFAs is the differential modal
gain (DMG) needed to minimize the differences
between the signal-to-noise ratios of all the transmitted signals and thus maintain signal quality. To
reduce the DMG in FM-EDFAs, it is important to
reduce the difference between two overlap integrals,
namely that for the excited erbium ion area and theintensity distribution of the fundamental mode signal
and that for the excited erbium ion area and the intensity profile of higher-order signals. For this purpose,5

the doping of erbium ions with a ring profile and theuse of a reconfigurable pump mode have been reported [5, 6]. Disadvantages of these techniques are thatthe former complicates the EDF fabrication process,and the latter introduces an additional loss for thepump power.
Another approach was taken in an NTT study,
which involves employing a ring-core erbium-doped
fiber (RC-EDF) with a ring-shaped index profile. As
shown in Fig. 3(a), the optical signals of LP01 andLP11 modes at the RC-EDF have a similar intensity
distribution, in which the overlap integral for both theLP01 and LP11 mode signals have similar values,
resulting in a reduction of the DMG. Our approach
has advantages over other approaches in that it maintains a simple fabrication process with uniform erbium doping and eliminates the need for lossy additional pump adjustment. The FM-EDFA with an
NTT Technical Review

Feature Articles

RC-EDF whose parameters were optimized successfully exhibited a small DMG of 1.6 dB, which is 4.5
dB smaller than that for an FM-EDFA with a conventional circular core (Fig. 3(b)). The FM-EDFA with
the RC-EDF was also used for a long-haul modedivision-multiplexing transmission as an opticalamplifier repeater, which confirmed its feasibility [7].
4. Future workIn upcoming research, we will investigate advanced
amplification technologies for gain and output control in SDM optical amplifiers.
This study was undertaken as part of a collaborative
project with Fujikura Ltd., Osaka Prefecture University, Shimane University, and Chitose Institute of
Science and Technology.
References
[1]

[2]

H. Ono, M. Yamada, K. Takenaga, S. Matsuo, Y. Abe, K. Shikama,
and T. Takahashi, “Amplification Method for Crosstalk Reduction ina Multi-core Fibre Amplifer,” Electron. Lett., Vol. 49, No. 2, pp.
138–140, 2013.
T. Kobayashi, H. Takara, A. Sano, T. Mizuno, H. Kawakami, Y. Miyamoto, K. Hiraga, Y. Abe, H. Ono, M. Wada, Y. Sasaki, I. Ishida, K.
Takenaga, S. Matsuo, K. Saitoh, M. Yamada, H. Masuda, and T.

[3]

[4]

[5]

[6]
[7]

Morioka, “2 × 344 Tb/s Propagation-direction Interleaved Transmission over 1500-km MCF Enhanced by Multicarrier Full Electric-field
Digital Back-propagation,” Proc. of ECOC 2013 (the 39th European
Conference and Exhibition on Optical Communication), Postdeadline
paper, PD3.E.4, London, UK, Sept. 2013.
K. Tsujikawa, L. Ma, K. Ichii, S. Matsuo, M. Yamada, N. Hanzawa,and H. Ono, “Optical Fiber Amplifier Employing a Bundle ofReduced Cladding Erbium-doped Fibers for Multi-core Fiber Transmission,” Proc. of 2012 IEEE Photonics Society Summer Topical
Meeting Series, WC3.2, Seattle, WA, USA, July 2012.
S. Jain, T. Mizuno, Y. Jung, Q. Kang, J. R. Hayes, M. N. Petrovich, G.
Bai, H. Ono, K. Shibahara, A. Sano, A. Isoda, Y. Miyamoto, Y. Sasaki,
Y. Amma, K. Takenaga, K. Aikawa, C. Castro, K. Pulver, Md Nooruzzaman, T. Morioka, S. U. Alam, and D. J. Richardson, “32-core Inline
Multicore Fiber Amplifier for Dense Space Division Multiplexed
Transmission Systems,” Proc. of ECOC 2016 (the 42nd European
Conference and Exhibition on Optical Communication), Postdeadline
paper, Th.3.A.1, Düsseldorf, Germany, Sept. 2016.
E. Ip, M.-J. Li, K. Bennett, A. Korolev, K. Koreshkov, W. Wood, C.
Montero, and J. Liñares, “Experimental Characterization of a Ringprofile Few-mode Erbium-doped Fiber Amplifier Enabling Gain
Equalization,” Optical Fiber Communication Conference (OFC)
2013, JTh2A.18, Anaheim, CA, USA, Mar. 2013.
N. Bai, E. Ip, T. Wang, and G. Li, “Multimode Fiber Amplifier with
Tunable Modal Gain Using a Reconfigurable Multimode Pump,” Opt.
Express, Vol. 19, No. 17, pp. 16601–16611, 2011.
K. Shibahara, T. Mizuno, H. Takara, A. Sano, H. Kawakami, D. Lee,
Y. Miyamoto, H. Ono, M. Oguma, Y. Abe, T. Kobayashi, T. Matsui, R.
Fukumoto, Y. Anma, T. Hosokawa, S. Matsuo, K. Saito, H. Nasu, andT. Morioka, “Dense SDM (12-core 3-mode) Transmission over 527
km with 33.2-ns Mode-dispersion Employing Low-complexity Parallel MIMO Frequency-domain Equalization,” Proc. of OFC 2015,
Postdeadline paper, PD.Th5C.3, Los Angeles, CA, USA, Mar. 2015.

Hirotaka Ono

Senior Research Engineer, Photonics-Electronics Convergence Laboratory, NTT Device
Technology Laboratories.
He received a B.S., M.S., and Ph.D. in applied
physics from Tohoku University, Miyagi, in
1993, 1995, and 2004. He joined NTT in 1995.
He was also a Visiting Research Fellow with theOptoelectronics Research Centre (ORC), University of Southampton, UK, from 2005 to 2006.
He has been engaged in research on optical fiber
amplifiers, including L- and S-band erbiumdoped fiber amplifiers. He has also undertaken
research on highly nonlinear fiber devices, photonic crystal fibers, and wavelength-divisionmultiplexing transmission systems. He is now
working on research of multi-core and few-mode
fiber and waveguide devices, including opticalamplifiers, and space-division-multiplexing systems. He received the OECC’97 Best Paper
Award, Electronics Letters Premium in 1997, andYoung Engineers Award from the Institute ofElectronic, Information and Communication
Engineers (IEICE) in 2003. He is a senior member of the Institute of Electrical and Electronics
Engineers (IEEE), The Optical Society (OSA)and IEICE, and a member of the Japan Society ofApplied Physics (JSAP).

Vol. 15 No. 6 June 20176

Feature Articles: State-of-the-art Space Division Multiplexing
Technologies for Future High-capacity Optical Transport Networks

Research and Development of Next
Generation Optical Fiber Using
Multiple Spatial Channels
Taiji Sakamoto, Takayoshi Mori, Takashi Matsui,
Takashi Yamamoto, and Kazuhide Nakajima
Abstract

The capacity of conventional single-mode fiber (SMF) that is widely used in the existing optical communication network is expected to be limited to around 100 Tbit/s. Therefore, next generation opticalfiber such as multi-core and/or multi-mode fiber has been investigated in order to overcome the limited
capacity of SMF and to utilize multiple spatial channels to realize space division multiplexing (SDM).
We introduce in this article the recent progress achieved in SDM fiber.
Keywords: optical fiber, space division multiplexing, multi-mode multi-core

1. Introduction
The capacity of conventional single-mode fiber(SMF) is expected to be around 100 Tbit/s, and therefore, next generation optical fiber that can accommodate traffic at more than 100 Tbit/s is needed for
future optical communications systems. Optical communications systems have been developed in linewith the low loss and wideband characteristics of
SMF and the evolution of transmission technology.
Network capacity has greatly increased because of
wavelength division multiplexing (WDM) technology, which can transmit multiple signals with different
wavelengths in an optical fiber.
Digital coherent transmission using a multi-level
modulation format has recently been studied as a way
to improve spectral efficiency since the operational
wavelength window is limited in terms of the loss
characteristics of SMF and the amplification bandwidth of erbium-doped fiber amplifiers. Although avery high signal-to-noise ratio is required when acomplex modulation format such as quadratureamplitude modulation is used to achieve high spectral
efficiency, the input power into the fiber is severely
limited by nonlinear effects or the fiber fuse phenom1

enon. That is why the capacity of SMF is predicted tobe limited to around 100 Tbit/s for telecom networks.
However, Internet traffic has been increasing at a
rate of 30–50% per year, and innovative technology
will be required in order to accommodate such a large
amount of traffic in the future. One promising solution to this capacity crunch is to use a spatial channel
by developing a space division multiplexing (SDM)
fiber. In this article, we report the recent progressachieved in SDM fiber as the next generation opticaltransmission line for ultra-large-capacity systems.
2. Recent research on SDM fiber
SDM technology involves the transmission of multiple signals in parallel in the same way as WDM
technology. SDM uses spatial channels, for example,
by using multiple cores in multi-core fiber (MCF).
Generally speaking, parallel transmission using multiple SMFs is also considered to be SDM transmission, but we would like to focus on SDM technology
using SDM fiber, where multiple spatial channels
exist in an optical fiber.
The proposed SDM fiber is illustrated in Fig. 1.
SMF has a core in the cross section. In contrast, MCF
NTT Technical Review

Feature Articles

Multi-core fiber (MCF)

Single-mode fiber(SMF)

Multi-mode MCF
Single-mode
coreMulti-mode fiber (MMF)

Multi-mode
cores

Multi-mode
core
Fundamental and higher-order modes

Fig. 1. Schematic diagram of SDM fibers.

has multiple cores, and multiple signals can be transmitted through the multiple cores. Multi-mode fiber(MMF) has a relatively larger core than that of SMF,
and multiple modes can propagate in this core.
Because each propagation mode can be treated as anindividual transmission channel, multiple signals canbe transmitted through the multiple modes. The parallel transmission scheme using multiple modes iscalled mode division multiplexing (MDM). Multimode MCF, which is MCF with multi-mode cores,
has also been investigated as a way to perform MDM
transmission in each core [1–5].
The important parameters that should be taken into
account when designing SDM fiber are listed as follows.
• The number of spatial channels• Optical characteristics of each spatial channel• Mechanical reliability of optical fiber
It is preferable to have as many spatial channels as
possible to increase the capacity, and better (or comparable) optical characteristics for each spatial channel than those of SMF are desired in order to achieve
higher transmission capacity per spatial channel.
In addition to the optical characteristics, mechanical reliability of the fiber is also an important aspect
for telecom networks. The failure probability is one
factor that affects the mechanical reliability of thefiber. This refers to the probability that the fiber will
break if it is stretched or bent. Therefore, the fibershould be properly designed in order to achieve asustainable network infrastructure. In general, thefailure probability increases as the fiber cladding
diameter increases. NTT clarified that 250 μm is the
maximum cladding diameter to provide mechanicalVol. 15 No. 6 June 2017reliability comparable to that of SMF [1] and also
investigated the SDM fiber design to maximize thenumber of spatial channels within the limited crosssection of the fiber. NTT defines DSDM (dense space
division multiplexing) transmission as the systemthat can support more than 30 spatial channels [3].
The design considerations for different kinds of SDM
fiber and fiber arrangements proposed so far are
introduced in the next section.
3. MCF technology
The relationship between the number of cores andthe cladding diameter of the proposed MCFs isshown in Fig. 2. MCF tends to have a larger cladding
diameter than that of SMF (125 μm). This is because
it requires a larger cross section to deploy more cores.
As explained in the previous section, the cladding
diameter is limited to a certain value because of themechanical reliability. Therefore, deploying cores as
closely together as possible inside the fiber is effective. However, there is a lower limit of the core pitchin terms of the inter-core crosstalk value, whichshould be sufficiently low to prevent interference ofthe transmitted signals.
Cross sections of the proposed MCFs are shown in
Fig. 3. Various core arrangements such as hexagonallattice [6–8], circular [9, 10], square lattice [11], or a
combination of these core arrangements [12, 13] havebeen proposed. The trench-assisted refractive index
structure has been used for each core profile in orderto reduce the inter-core crosstalk. The use of a heterogeneous core structure is also considered to be a way
to reduce crosstalk. In the heterogeneous MCF
2

Feature Articles

35[3]
Proposed by NTT

Number of cores

30
25
20
15
10
5
100

150

200

250

300

Cladding diameter (µm)

Fig. 2. Relationship between number of cores and cladding diameter of MCFs.

(a) Hexagonal(b) Circular

[6]

[9]

[7]

[10]

(c) Square lattice[11]

243 µm(d) Others

[12]

[13]

Fig. 3. Cross sections of MCFs.

design, the index profile of the adjacent core isdesigned to be different, which can effectively reducethe crosstalk because the propagation constant of themode in the adjacent core is different. NTT successfully developed a 32-core fiber with a cladding diameter of less than 250 μm by using an MCF design
technique that combined a square lattice core arrangement, trench-assisted core profile, and a heterogeneous MCF design. This work was supported by the
EU-Japan coordinated research and development(R&D) project known as SAFARI (Scalable And
Flexible optical Architecture for Reconfigurable
3

Infrastructure).
Designing MCF with optical properties comparable
to those of the SMF is also an important issue. For
example, investigations have been carried out on
MCF with 125-μm-diameter cladding MCF and with
optical characteristics conforming to ITU-T* recommendations on fiber optics in regard to mode field
diameter, chromatic dispersion, macrobending loss,
and other factors.
*

ITU-T: The Telecommunication Standardization Sector of the International Telecommunication Union.

NTT Technical Review

Feature Articles

Positive DMD fiber

Negative DMD fiber

DMD

Graded core

Distance

Low index trench(a) Trench-assisted graded-index core profile(b) DMD compensated fiber line

Fig. 4. Trench-assisted graded core MMF.

200
C bandC-L band
1550 nm

DMD (ps/km)

150

[16]

Red circles: Proposed by NTT
100

[14]

50

0

2

4

6

8
10
12
Number of modes

14

16

Fig. 5. Relationship between number of modes and DMD value of low DMD fiber.

4. MMF technology
We can increase the number of spatial channels by
increasing the number of modes per core as well as
increasing the number of cores. Reducing the intermodal crosstalk and differential group delay is also
important in the MMF design. However, the propagation modes in multi-mode cores can easily convert toanother mode due to structural perturbation in theevent of a splice point or fiber bending. Optical multiple-input multiple-output (MIMO) has been proposed to deal with this issue, and signal processing
has been investigated in order to compensate for the
crosstalk.
However, the complexity of signal processing
increases as the differential mode delay (DMD)
increases. Low DMD fiber has been proposed toreduce the complexity of MIMO processing, and wehave investigated trench-assisted graded-index coreVol. 15 No. 6 June 2017

MMF to reduce the DMD and control the number of
propagation modes, as shown in Fig. 4(a) [14]. We
successfully adjusted the DMD value by accurately
controlling the index profile of the graded core, andwe designed the optical properties (e.g., bending loss)of all propagation modes to have a comparable value
to that of conventional SMF by optimizing the trench
structure. A DMD compensated fiber line has also
been proposed, where two kinds of fibers with positive and negative DMD values are used (Fig. 4(b)).
This technique is similar to that used in a chromatic
dispersion compensation line; thus, we can flexibly
control the total DMD value by changing the length
ratio of the two kinds of fiber [15].
The relationship between the number of modes andthe DMD value of the proposed low DMD fiber isplotted in Fig. 5. NTT succeeded in developing MMF
with 10 modes and a DMD of less than 100 ps/km.
Although MMF with up to 15 modes has been reported
4

Feature Articles

Number of spatial channels per fiber

140

Single-mode MCF
Multi-mode MCF

120
100

Red circles:
Proposed by NTT
[1]

×2
×2

80

[2]

60
[3]

40
20
0
100

150

200
250
Cladding diameter (µm)

300

350

Fig. 6. Relationship between number of spatial channels as a function of cladding diameter of reported single-mode MCFs
and multi-mode MCFs.

5. Multi-mode MCF technology
The use of either MCF or MMF technology makes
it possible to achieve a few dozen spatial channels,
but many more spatial channels may be needed inorder to accommodate the huge amount of traffic
expected in the future. Multi-mode MCF has been
investigated as a way to greatly increase the numberof spatial modes in the fiber. For example, n-core
fiber with m-mode cores exhibits m × n spatial channels. The number of spatial channels as a function of
the cladding diameter of reported single-mode MCFs
and multi-mode MCFs is shown in Fig. 6. Multimode MCF can realize a larger number of spatialchannels compared to that of single-mode MCF, andmulti-mode MCFs with more than a hundred spatialchannels have been proposed [1, 4, 5]. NTT realized
a 6-mode 19-core fiber with 114 spatial channels anda cladding diameter of less than 250 μm and achieved
the highest reported spatial density while maintaining
comparable mechanical reliability to that of SMF [1].
5

0.55
0.5
0.45
Loss (dB/km)[16], our fiber has been designed to achieve the low
DMD characteristics over a broad wavelength range
from the C to L band. Thus, the current WDM technology can be incorporated with MDM technology
with our fiber. We have also investigated a techniquefor utilizing the higher-order mode in MMF for longhaul transmission by exciting a specific higher-ordermode to reduce the fiber nonlinearity.

0.4

Red circles and red square:
Proposed by NTT

0.35

[2]

0.3
0.25

[1]

0.2

[3]

0.15

0

3-mode MCF
6-mode MCF
1

2

3
4
5
DMD (ns/km)

6

7

8

Fig. 7. R elationship between loss and DMD of proposed
multi-mode MCFs.

The relationship between the loss and DMD of the
proposed multi-mode MCFs is shown in Fig. 7. Our
multi-mode MCF has a low DMD value, which isachieved by employing the trench-assisted gradedindex core profile as we mentioned [1–3]. We alsoachieved loss characteristics comparable to SMF for
all spatial channels. As a result, we successfully
developed SDM fiber with more than a hundred spatialchannels and with optical properties and mechanicalreliability comparable to SMF.

NTT Technical Review

Feature Articles

6. Conclusion
In this article, we briefly reviewed the recent progress in the development of SDM fibers with multiple
spatial channels, for example, MCF and MMF. We
will continue our R&D in order to introduce SDM
fiber and achieve a petabit-per-second transmission
system in the latter half of the 2020s.
References
[1]

[2]

[3]

[4]

[5]

[6]
[7]

T. Sakamoto, T. Matsui, K. Saitoh, S. Saitoh, K. Takenaga, T. Mizuno,
Y. Abe, K. Shibahara, Y. Tobita, S. Matsuo, K. Aikawa, S. Aozasa, K.
Nakajima, and Y. Miyamoto, “Low-loss and Low-DMD Few-mode
Multi-core Fiber with Highest Core Multiplicity Factor,” Proc. of the39th Optical Fiber Communication Conference (OFC 2016), Postdeadline paper, Th5A.2, Anaheim, CA, USA, Mar. 2016.
T. Sakamoto, T. Matsui, K. Saitoh, S. Saitoh, K. Takenaga, S. Matsuo,
Y. Tobita, N. Hanzawa, K. Nakajima, and F. Yamamoto, “Few-mode
Multi-core Fibre with Highest Core Multiplicity Factor,” Proc. of the
41st European Conference and Exhibition on Optical Communication(ECOC 2015), We.1.4.3, Valencia, Spain, Sept. 2015.
K. Shibahara, T. Mizuno, H. Takara, A. Sano, H. Kawakami, D. Lee,
Y. Miyamoto, H. Ono, M. Oguma, Y. Abe, T. Kobayashi, T. Matsui, R.
Fukumoto, Y. Amma, T. Hosokawa, S. Matsuo, K. Saito, H. Nasu, andT. Morioka, “Dense SDM (12-core x 3-mode) Transmission over 527
km with 33.2-ns Mode-dispersion Employing Low-complexity Parallel MIMO Frequency-domain Equalization,” Proc. of the 38th OpticalFiber Communication Conference (OFC 2015), Postdeadline paper,
Th5C.3, Los Angeles, CA, USA, Mar. 2015.
K. Igarashi, D. Souma, Y. Wakayama, K. Takeshima, Y. Kawaguchi,
T. Tsuritani, I. Morita, and M. Suzuki, “114 Space-division-multiplexed Transmission over 9.8-km Weakly-coupled-6-mode Uncoupled-19-core Fibers,” Proc. of OFC 2015, Postdeadline paper,
Th5C.4, Los Angeles, CA, USA, Mar. 2015.
J. Sakaguchi, W. Klaus, J.-M. D. Mendinueta, B. J. Puttnam, R. S.
Luis, Y. Awaji, N. Wada, T. Hayashi, T. Nakanishi, T. Watanabe, Y.
Kokubun, T. Takahata, and T. Kobayashi, “Realizing a 36-core,
3-mode Fiber with 108 Spatial Channels,” Proc. of OFC 2015, Postdeadline paper, Th5C.2, Los Angeles, CA, USA, Mar. 2015.
T. Hayashi, T. Taru, O. Shimakawa, T. Sasaki, and E. Sasaoka,“Uncoupled Multi-core Fiber Enhancing Signal-to-noise Ratio,” Opt.
Express, Vol. 20, No. 26, pp. B94-B103, 2012.
J. Sakaguchi, B. J. Puttnam, W. Klaus, Y. Awaji, N. Wada, A. Kanno,Vol. 15 No. 6 June 2017

T. Kawanishi, K. Imamura, H. Inaba, K. Mukasa, R. Sugisaki, T.
Kobayashi, and M. Watanabe, “305 Tb/s Space Division Multiplexed
Transmission Using Homogeneous 19-core Fiber,” J. Lightw. Technol., Vol. 31, No. 4, pp. 554–562, 2013.
[8] K. Imamura, H. Inaba, K. Mukasa, and R. Sugisaki, “Multi Core Fiberwith Large Aeff of 140 μm2 and Low Crosstalk,” Proc. of ECOC 2012
(the 38th European Conference and Exhibition on Optical Communication), Mo.1.F.2, Amsterdam, Netherlands, Sept. 2012.
[9] S. Matsuo, K. Takenaga, Y. Arakawa, Y. Sasaki, S. Tanigawa, K.
Saitoh, and M. Koshiba, “Large-effective-area Ten-core Fiber withCladding Diameter of about 200 μm,” Opt. Lett., Vol. 36, No. 23, pp.
4626–4628, 2011.
[10] S. Matsuo, Y. Sasaki, T. Akamatsu, I. Ishida, K. Takenaga, K.
Okuyama, K. Saitoh, and M. Koshiba, “12-core Fiber with One Ring
Structure for Extremely Large Capacity Transmission,” Opt. Express,
Vol. 20, No. 27, pp. 28398–28408, 2012.
[11] T. Mizuno, K. Shibahara, H. Ono, Y. Abe, Y. Miyamoto, F. Ye, T.
Morioka, Y. Sasaki, Y. Amma, K. Takenaga, S. Matsuo, K. Aikawa, K.
Saitoh, Y. Jung, D. J. Richardson, K. Pulverer, M. Bohn, and M.
Yamada, “32-core Dense SDM Unidirectional Transmission of PDM16QAM Signals over 1600 km Using Crosstalk-managed Singlemode Heterogeneous Multicore Transmission Line,” Proc. of OFC
2016, Postdeadline paper, Th5C, Anaheim, CA, USA, Mar. 2016.
[12] B. J. Puttnam, R. S. Luis, W. Klaus, J. Sakaguchi, J.-M. D. Mendinueta, Y. Awaji, N. Wada, Y. Tamura, T. Hayashi, M. Hirano, and J.
Marciante, “2.15 Pb/s Transmission Using a 22 Core Homogeneous
Single-mode Multi-core Fiber and Wideband Optical Comb,” Proc. of
ECOC 2015, 1056, Valencia, Spain, Sept. 2015.
[13] T. Kobayashi, H. Takara, A. Sano, T. Mizuno, H. Kawakami, Y. Miyamoto, K. Hiraga, Y. Abe, H. Ono, M. Wada, Y. Sasaki, I. Ishida, K.
Takenaga, S. Matsuo, K. Saitoh, M. Yamada, H. Masuda, and T.
Morioka, “2 × 344 Tb/s Propagation-direction Interleaved Transmission over 1550-km MCF Enhanced by Multicarrier Full Electric-field
Digital Back-propagation,” Proc. of ECOC 2013 (the 39th European
Conference and Exhibition on Optical Communication), PD3-e-4,
London, UK, Sept. 2013.
[14] T. Mori, T. Sakamoto, M. Wada, T. Yamamoto, and F. Yamamoto,“Few-mode Fibers Supporting More Than Two LP Modes for Modedivision-multiplexed Transmission with MIMO DSP,” J. Lightw.
Technol., Vol. 32, No. 14, pp. 2468–2479, 2014.
[15] T. Sakamoto, T. Mori, T. Yamamoto, and S. Tomita, “Differential
Mode Delay Managed Transmission Line for WDM-MIMO System
Using Multi-step Index Fiber,” J. Lightw. Technol., Vol. 30, No. 17,
pp. 2783–2787, 2012.
[16] P. Sillard, D. Molin, M. Bigot-Astruc, K. de Jonph, and F. Achten,“Low-differential-mode-group-delay 9-LP-mode Fiber,” Proc. of
OFC 2015, M2C.2, Los Angeles, CA, USA, Mar. 2015.

6

Feature Articles

Taiji Sakamoto

Research Engineer, NTT Access Network Service Systems Laboratories.
He received a B.E., M.E., and Ph.D. in electrical engineering from Osaka Prefecture University in 2004, 2006, and 2012. In 2006, he joined
NTT Access Network Service Systems Laboratories, where he has been engaged in research on
optical fiber nonlinear effects, low nonlinear
optical fiber, few-mode fiber, and multi-core
fiber for optical MIMO transmission systems. Dr.
Sakamoto is a member of the Institute of Electronics, Information and Communication Engineers (IEICE).

Takayoshi Mori

Engineer, NTT Access Network Service Systems Laboratories.
He received a B.E. from Tokyo University ofAgriculture and Technology in 2007, and an
M.E. and Ph.D. from Tokyo Institute of Technology in 2009 and 2015. Until 2009, he wasengaged in research related to optical signal
processing technology, focusing particularly on atunable laser with a high switching speed, awavelength converter with a semiconductor optical amplifier, and a controller for optical buffering systems. Since 2009, he has been with NTT
Access Network Service Systems Laboratories,
where he has been researching few-mode fibers
for optical MIMO transmission. Dr. Mori is amember of IEICE. He received the Communication Society OCS Young Researchers Award
from IEICE in 2011 and the Communication
Society OFT Young Researchers Award fromIEICE in 2015.

Takashi Yamamoto

Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received a B.E. and M.E. in physics from
Kyushu University, Fukuoka, in 1989 and 1991.
He joined NTT Transmission Systems Laboratories in 1991. Since then, he has been engaged in
R&D of nonlinear fiber optics, ultrafast opticaltime division multiplexed transmission technologies, photonic local oscillators, multi-carrier
light sources, photonic crystal fibers, and multimode fiber transmission technologies. He is amember of IEICE and the Institute of Electricaland Electronics Engineers (IEEE). He received
the Young Investigators Award from IEICE in
1997, the Best Paper Award from IEICE in 2000
and 2002, the Inose Award from IEICE in 2002,
and the Best Paper Award at the International
Topical Meeting on Microwave Photonics in2002.

Kazuhide Nakajima

Senior Research Engineer, Supervisor (SeniorDistinguished Researcher) and Group Leader of
NTT Access Network Service Systems Laboratories.
He received an M.S. and Ph.D. in electricalengineering from Nihon University, Chiba, in
1994 and 2005. In 1994, he joined NTT AccessNetwork Service Systems Laboratories, where
he conducted research on optical fiber design andrelated measurement techniques. He is the Rapporteur of Question 5 in ITU-T Study Group 15.
Dr. Nakajima is a member of IEICE, IEEE, andthe Optical Society (OSA).

Takashi Matsui

Senior Research Engineer, Access Media Project, NTT Access Network Service Systems Laboratories.
He received a B.E., M.E., and Ph.D. in electronic engineering from Hokkaido University in
2001, 2003, and 2008. He also attained the statusof Professional Engineer (P.E.Jp) in electricaland electronic engineering in 2009. In 2003, hejoined NTT Access Network Service Systems
Laboratories. His research interests include optical fiber design techniques. Dr. Matsui is a member of IEICE.

7

NTT Technical Review

Feature Articles: State-of-the-art Space Division Multiplexing
Technologies for Future High-capacity Optical Transport Networks

Multi-core Fiber Connector
Technology for Low-loss Physicalcontact Connection
Yoshiteru Abe, Kota Shikama, and Shuichiro Asakawa
Abstract

The NTT laboratories have been researching and developing connection technology for multi-core
fiber, which is expected to be the transmission medium in future high-capacity transmission systems. In
this article, we introduce a multi-core fiber connector that achieves physical-contact connection with low
loss, and a pluggable fan-in/fan-out device connecting multi-core fiber and single-core fiber.
Keywords: multi-core fiber, optical connector, fan-in/fan-out

1. Introduction
Optical fiber connection technology is essential toconstruct and operate an optical communication network. Fusion splicing and optical connectors are theprevailing methods used for optical fiber connections. Fusion splicing permanently connects the optical fibers by melting them with an arc discharge.
Optical connectors detachably connect the opticalfibers.
Establishment of optical fiber connection technology is necessary for multi-core fiber (MCF), which is
expected to be used as the transmission medium infuture high-capacity transmission systems. To connect MCFs with low loss, it is essential to precisely
match the axis rotation angle of the optical fibers,
which is unnecessary when aligning single-core fiber(SCF). The general-purpose conventional fusion
splicer that aligns the optical fiber by observing itfrom solely a lateral view cannot adjust the rotating
position of the MCF cores. In contrast, another commercially available fusion splicer achieves low-loss
connection of the MCF by showing the fiber end witha mirror, which enables us to adjust the rotating position of the core.
The optical connector needs an alignment mechanism with a repeatable accuracy of less than 1 μm for
an insertion loss of 0.1 dB at the mated core of single1

mode fibers. With the conventional simplex opticalconnector, the external force acting on the opticalconnector does not affect the optical fibers at the connection point because the butt-jointed ferrules arefloated inside the plug housings. The floating mechanism is achieved by enabling the rotation of the optical fiber axis.
However, when we connect the MCFs with the conventional connector, the floating mechanism degrades
the rotational angle alignment. It is therefore difficult
to achieve low-loss connection of MCF with the conventional connector. The NTT laboratories have beenpromoting research and development (R&D) of MCF
connection technology aimed at achieving a level of
performance equivalent to that of the conventional
optical connectors used with SCFs. A fan-in/fan-out(FI/FO) device is also required that couples each coreof the MCF optically with individual SCFs to construct the transmission system with the MCF. In thisarticle, we describe the simplex MCF connector, multiple MCF connector, and FI/FO device.
2. Simplex MCF connector
Precise alignment of the MCF’s rotational angle θz
is required to achieve a low-loss connection of all thecores of the MCF (Fig. 1). Here, conventional simplex connectors such as a single-fiber coupling (SC)NTT Technical Review

Feature Articles

FerruleMCF
θzSplit sleeve

Y axisθ z axis

X axisZ axisFig. 1. Basic structure of simplex MCF connector.

connector have clearance between the flange and theplug, enabling the mated ferrules to be floated inside
the plug housing. The clearance enables us to rotate
the fiber within ± 2 degrees and degrades the rotational angle alignment. Therefore, when we connect
the MCFs with the conventional connector, it is difficult to achieve low-loss connection with good
repeatability for the outer cores of the MCF.
To achieve precise rotational angle alignment for
the MCF, we have studied a simplex MCF connector
that enables us to adjust the rotational angle of theferrule when assembling the connector and to narrow
the rotational angle range of the ferrule when connecting the connector. Our MCF connector components are shown in Fig. 2(a). The plug frame, flange,and stop ring are unique to the MCF connector. The
other parts are the same as those used in the conventional SC connector. The plug frame has two key
guides. The stop ring has guides to rotate the ferrule,and these guides fit the key guide.
A schematic diagram of the rotation structure isshown in Fig. 2(b). As the stop ring rotates around theplug frame, the ferrule housing the MCF also rotatesalong with the stop ring. In this configuration, therotational angle can be aligned precisely by monitoring the MCF facet with a microscope after the MCF
is fixed to the ferrule. Then the stop ring is fixed to theplug housing with an adhesive at an optimum angle.
We tested the repeatability of the connection loss
for the MCF connector using 7-core MCF with a corepitch of 50 μm. The variation between maximum andminimum losses was about 0.1 dB. This result indiVol. 15 No. 6 June 2017

cates that the MCF connector suppresses the variationof the rotation angle to less than 1.1 degrees. We alsomeasured the connection losses of the fabricated
MCF connector at a wavelength of 1.55 μm. The connection loss was sufficiently small at < 0.2 dB with anaverage value of about 0.1 dB. We confirmed that themeasured connection loss values were the same as
those of the conventional simplex SCF connector.
Simplex SCF connectors such as SC or MU (miniature universal coupling) connectors employ a physical-contact (PC) connection that is in direct contact
with the fiber endface, which is done by pressing theferrule with the polished convex spherical end. The
PC connection obtains high return loss by reducing
Fresnel reflection at the connection point and also
makes it possible to achieve high-power durability byeliminating organic substances such as adhesive at
the connection point.
The MCF connector must achieve the PC connection not only at the center core but also at the outside
cores. To do this, the MCF connector has a larger
contact area between the mated fiber endfaces compared to the SCF connector. For the MCF connector,
we clarified the relation between the fiber end shapeand the compression force required for the PC connection. We found that the end shape changes in acomplex manner when applying the compression
force to the ferrule and MCF simultaneously.
We established the design method for the PC connection of MCF using a finite element method simulation with a three-dimensional asymmetric geometry
model that conforms to the actual endface structure2

Feature Articles

Coupling sleeve

Plug frame

FerruleStop ring
SpringBoot

Guide for ferrule rotation
Key guide
Flange(a) Components of simplex MCF connector

MCF

MCF endface

FerruleFlangeStop ring(b) Configuration for rotation alignment of MCF

Fig. 2. Simplex MCF connector.

[1]. We have designed and fabricated 7-core MCF
connectors using our design method. All the cores ofthe fabricated MCF connectors provided a high returnloss of more than 45 dB, which verifies the PC connection.
3. Multiple MCF connector
The MCF enables us to increase the core density ofa fiber cable in a long-haul transmission or to reduce
the number of fibers in a datacenter optical link. To
construct such a system using the MCFs, we need
optical connectors for both single and multiple
MCFs. The conventional multiple SCF connector
achieves the multi-fiber connection by coupling themechanically transferable (MT) ferrule accommodating the multi-fiber array with two guide pins. The
multiple MCF connector also employs the MT ferrulein order to obtain the same performance and operability as the conventional multiple SCF connector.
The basic structure of our devised multiple MCF
connector is shown in Fig. 3(a). The configurationthat mates the MT ferrules using the two pins is thesame as that of the conventional MT connector. The
conventional MT connector fixes the fibers to themicro-hole of the MT ferrule with adhesive. Our multiple MCF connector separates the fixed portion of
the fibers from the MT ferrule. That is, the fibers are
fixed at the fixing component rather than at the ferrule.
We devised the new configuration for PC connec3

tion of multiple MCFs. However, the difference in the
protrusion length of the MCFs from the MT ferruleand the angle error of the polished fiber end make it
difficult to achieve PC connection of multiple MCFs.
We found that the PC connection of the MCF array
could be achieved by using the elastic compression
force of a fiber at the unfixed portion generated whenthe mated MCFs are pressed together [2].
A photograph of the fabricated quadruple-MCF
connector for 7-core MCF is shown in Fig. 3(b). On
the MT ferrule endface, the cores of the four MCFs
are aligned to have the same arrangement. The average insertion loss of the fabricated connectors wasless than 0.3 dB. The result indicates that our multiple
MCF connector has the same insertion loss as themultiple SCF connector. The return losses exceeded
45 dB, which confirmed that all the connection points
achieved PC connections.
4. FI/FO
A FI/FO device that couples each core of the MCF
optically with individual SCFs is essential to construct a transmission system that uses MCF. The NTT
laboratories have studied a bundled fiber FI/FO
device that realizes a closely packed core arrangement of an MCF by utilizing individual SCFs. The
bundled fiber FI/FO device enables us to achieve apluggable connection by accommodating the bundled
fibers in connector housings.
The basic configuration of the FI/FO device is
NTT Technical Review

Feature Articles

MT ferrule

Pin

Fixed component

Compression force

Protrusion
MCF

Unfixed Fixing portion

Clamp spring

Angle error

Compression
force

Difference inprotrusion
length(a) Configuration for PC connection

MT ferrule

MT ferrule endface(b) PhotographFig. 3. Multiplex MCF connector.

shown in Fig. 4. It employs two ferrules and a split
sleeve. The MCF is inserted into the micro-hole ofone of the ferrules. The SCFs required for the MCF
core arrangement are inserted into the micro-hole ofthe other ferrule. The inserted SCFs are then fixed inplace with adhesive. The portion of the SCFs inserted
into the ferrule has the same cladding diameter as thecore pitch of the MCF. The core arrangement of thebundled fibers at the ferrule end is made to correspond to the hexagonal closely packed core arrangement of the MCF by inserting individual SCFs into
the circular hole, as shown in Fig. 4(b). Because the
FI/FO device utilizes the same plug and adapter as thesimplex MCF connector, the rotation of the ferrule
via the stop ring enables us to match the core arrangement of the MCF and SCFs.
The insertion loss measured using the fabricated FI/
Vol. 15 No. 6 June 2017

FO device for 7-core MCF was sufficiently small atless than 0.3 dB, with an average value of 0.12 dB.
The return losses exceeded 45 dB, which confirmedthat all the connection points achieved PC connections. With the configuration of the FI/FO device asshown in Fig. 4(a), we can change the sectional shapeof the ferrule hole to a hexagon (Fig. 4(c)) or a squareto achieve a FI/FO device that corresponds to various
types of MCF with different numbers of cores andcore arrangements from those of the 7-core MCF.
5. Future prospects
In this article, we described connection technologythat includes a simplex connector, multiple connector, and FI/FO device for MCF, which is expected tobe the transmission medium in future high-capacity4Feature Articles

Plug

Adapter

Plug

SCF

MCF

(a) Configuration
Ferrule endface
Ferrule endface
with 7-core MCF with bundled SCFs(c) Photograph of bundled SCFs
for 19-core MCF(b) PhotographFig. 4. FI/FO device.

transmission systems. R&D on MCF connection
technology is steadily progressing and will enable us
to achieve the same performance as the connection
technology for SCF. Meanwhile, further efforts arenecessary to achieve the reliability and mass production required for practical use of MCF connection
technology. We will continue to promote R&D for
future ultra-high-capacity transmission systems.

5

References[1][2]

K. Shikama, Y. Abe, S. Asakawa, S. Yanagi, and T. Takahashi, “Multicore Fiber Connector with Physical-contact Connection,” IEICE
Trans. Electronics, Vol. E99-C, No. 2, pp. 242–249, 2016.
K. Shikama, S. Asakawa, Y. Abe, and T. Takahashi, “Multiple Multicore Fibre Connector with Physical-contact Connection,” Electron.
Lett., Vol. 51, No. 10, pp. 775–777, 2015.

NTT Technical Review

Feature Articles

Yoshiteru Abe

Senior Research Engineer, Media Utilization
Group, Access Media Project, NTT Access Network Service Systems Laboratories.
He received a B.E. in electrical engineering, an
M.E. in electronic device engineering, and aDr.Eng. in electrical engineering from Kyushu
University, Fukuoka, in 1996, 1998, and 2005. In
1998, he joined NTT Opto-electronics Laboratories. His research interests include optical fiberconnectors. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE).

Shuichiro Asakawa

Senior Research Engineer, Electro-Optical
Jisso Group, Product Strategy Planning Project,
NTT Device Innovation Center.
He received a B.E., M.E., and Dr.Eng. in electrical and computer engineering from Yokohama
National University, Kanagawa, in 1990, 1992,
and 1995. He joined NTT Opto-electronics Laboratories in 1995. He has been involved in R&D
of optical connectors and photonic device packaging technologies. He is a member of the Japan
Society of Applied Physics, IEICE, and IEEE.

Kota Shikama

Researcher, Optoelectronic Subsystem
Research Group, Photonics-Electronics Convergence Laboratory, NTT Device Technology
Laboratories.
He received a B.E. and M.E. in materials science from Keio University, Kanagawa, in 2008
and 2010. He joined NTT Photonics Laboratories
in 2010. He has been involved in research on
optical connectors, optical packaging technologies, and optical node modules. He received theYoung Researcher Award from IEEE CPMT
Symposium Japan in 2012, and the Young Engineer Award from IEICE in 2014. He is a memberof IEICE and the Institute of Electrical and Electronics Engineers (IEEE).

Vol. 15 No. 6 June 20176

Global Standardization Activities

Activity Report of ITU-T Focus
Group on IMT-2020
Yoshinori Goto
Abstract

In 2015, the ITU-T (International Telecommunication Union - Telecommunication Standardization
Sector) established the Focus Group on IMT (International Mobile Telecommunication)-2020 (FG IMT2020) to study the non-radio part of IMT-2020. The FG worked until December 2016 and produced key
concepts such as the network slice and network softwarization components constituting the IMT-2020
system. This article presents the activities of FG IMT-2020.
Keywords: network slice, network softwarization, fronthaul

1. Introduction
The International Telecommunication Union Telecommunication Standardization Sector (ITU-T)
established the Focus Group on IMT (International
Mobile Telecommunication)-2020 (FG IMT-2020) in
April 2015 in a decision taken by Study Group 13(SG13), which is the parent group of FG IMT-2020 as
well as the lead study group on the non-radio part of
IMT. The FG carried out its activities until December
2016. The FG was mandated to examine new network
technologies, which were originally studied under the
topic of future networks, for possible application in
future mobile networks. In the latter half of the FG’s
study period, new approaches were investigated suchas prototyping new technologies and collaboratingwith open source communities as a way to promote
standardization activities. This attempt was a uniquefeature of this FG, and it prompted the ITU-T to consider further actions to be taken at higher levels suchas the World Telecommunication Standardization
Assembly (WTSA).
2. History of FG IMT-2020’s activities
Technologies examined in FG IMT-2020 wererooted in the study of future networks, which had
been a topic of study in SG13 since 2009. When study
on future networks began, these networks were a1high-level concept without tangible and implementable technologies. Nevertheless, SG13’s activities resulted in ITU-T Recommendation Y.3001,
which described the objectives and design goals in
2011.
As the concept of IMT-2020 attracted interest fromindustry, the experts working on future networks
began to consider how their achievements might contribute to the realization of IMT-2020. SG13 had atechnical background in future networks and was also
responsible for studying the non-radio part of IMT.
This background and also the increased interest in
IMT-2020 led SG13 to establish FG IMT-2020 at its
meeting in April 2015.
Standards development groups in the private sector
play major roles in the standardization of mobile networks. The 3rd Generation Partnership Project(3GPP) is the most prominent group in this area.
Institutions such as ITU-T that set de jure standards
play a more limited role. This means that this FG may
not be successful by simply taking the conventional
approach to standardization without considering astandardization strategy.
The conventional standardization approach usually
starts with gathering requirements, and then drawing
a high-level architecture and making detailed technical specifications. In contrast, this FG took a new
approach that began with a gap analysis of existing
standardization work to find technical areas where
NTT Technical Review

Global Standardization Activities

SG13 meeting(April 2015)
Establishment of FG

SG13 meeting(December 2015)
Extension of FG

FG IMT-2020 (phase-1)
1st
2nd
meeting
meeting(June 2015) (July 2015)

3rd
4th
meeting
meeting(Sept. 2015) (Oct. 2015)• Analysis of existing standards (gapanalysis)• Identifying potential technologies(network softwarization, FH/BH etc.)

WTSA(October/November 2016)
FG IMT-2020 (phase-2)

1st
2nd
meeting
meeting(Mar. 2016) (May 2016)

3rd
meeting(Sept. 2016)

4th
meeting(Dec. 2016)• Detailed study of the technologies identified
in phase-1 (prototyping including showcase)• Collaboration with open source communitiesand 3GPP

Fig. 1. History of FG IMT-2020.

ITU-T had a competitive advantage. In the more than
two years of work done by the FG, this period (fromJune 2015 to October 2015) is referred to as phase-1.
This author served as vice-chair of SG13 and alongwith other members played a leading role in building
a consensus to create a ToR (Terms of Reference)
document describing the direction of phase-1 of theFG.
The phase-1 FG concluded its activities in October
2015 and produced a deliverable, which is the major
output of an FG as defined in ITU-T Recommendation A.7. The deliverable of this FG describes thecandidate technologies such as network softwarization, FH/BH (fronthaul/backhaul), information centric networking and content centric networking (ICN/
CCN), as well as proposed directions for further
study in ITU-T. With this conclusion of the phase-1
FG, SG13 decided to extend the FG into phase-2,
which was mandated to study details of identified
technologies and to conduct prototyping activities(Fig. 1).
3. Technical study
Studies were carried out on the various technicalelements of IMT-2020. These are described in moredetail in this section.
3.1 Architecture
The network slice is a technology that was newly
introduced in FG IMT-2020 discussions. The firstchallenge of the FG is to determine how to define thearchitecture of this virtual concept of a network slice.
At the beginning of the phase-2 study, the FG tried toVol. 15 No. 6 June 2017draw a typical architectural diagram containing functional blocks and reference points. The efforts continued until the second phase-2 meeting (May 2016), but
the FG did not produce an architectural diagram thatwas satisfactory to the FG experts.
Faced with the slow progress and the difficulty ofdrawing an architectural diagram, the chair and vicechairs considered that once a simplified diagram that
was agreeable to the experts was produced, the discussion on further details could be accelerated.
Therefore, at the meeting in September 2016, themeeting attendees worked on producing a minimal
agreeable diagram.
In this discussion, an idea centered on the conceptof a network slice was proposed. The network slice
architecture consists of two different aspects. One is
the network provider which owns all the network
resources and provides the necessary elements of theresources at the request of the slice users. The other isthe slice instance that is actually produced by the slice
users and offered for actual services to the end users.
Consequently, the different natures of a slice meantthat a specific architecture would be necessary foreach case.
In the FG, the former aspect was called a network
slice blueprint, and the latter was called a network
slice instance (Fig. 2). The network slice blueprintconsists of all resources and functions of data, control, services, and other elements that are used as
foundations of network slice instances. The orchestrator combines the necessary resources and functions to produce a network slice instance. There are
two cases of network slice instances based on whether the application is included in the slice—for example,2

Global Standardization Activities

Application
Management
orchestrator(including
slice
template)

Service
Control
Data

Network slice blueprint

Resource

Application

Instance #1

Control
Data
Resource

Service
Control
DataManagement

Data
Resource

Service

Management

ControlManagement

Service

Application
Network slice
instance

Resource
Instance #n

Instance #2

Fig. 2. Concept of network slice.

an application provided by the slice user—or notincluded—for example, when only connectivity isprovided by the slice user.
There was a general agreement that the network
slice instance does not include its own orchestrator
since the orchestrator does not play any operationrole inside the slice. However, it was pointed out thatmulti-layered slicing is possible, for example, in an
FMC (fixed mobile convergence) slice combining amobile slice and a fixed slice, and that the orchestrator in the slice could play certain roles in such cases.
In contrast to the architectural diagram discussed in
3GPP, which was drawing a physical entity diagram,the diagram drawn in the FG was created from theviewpoint of the hierarchical structure of a network
slice blueprint and instance. There was no contradiction between the two kinds of diagrams; however,further study is needed to clarify the relationshipbetween these two.
3.2 Network softwarization
Network softwarization (Fig. 3) is one of the major
results of FG IMT-2020 for which Japanese experts
actively contributed. This subject is strongly connected to the architecture discussion, and it includes
the major characteristics of the network slice.
The IMT-2020 system consists of a radio accessnetwork, mobile packet core, and other components.
In cases where the network slice provides applica3

tions by itself, IMT-2020 includes the datacenter. All
the elements mentioned above are referred to as
resources in the concept of network softwarization.
The network slice is considered as a collection of
resources. It was discussed whether the user equipment would be considered as a resource consisting ofa slice. This point was not clarified due to a lack ofsufficient use case studies. A future study will reveal
this point as potential use cases are studied in depth.
3.3 Fronthaul
The fronthaul is a link between the baseband unit
(BBU), which generates baseband signals, and theremote radio head (RRH), which emits radio signals.
Fronthaul technology improves the quality of a radio
access network by utilizing multiple base stations ina collaborative manner, although it requires a highlevel of time and frequency synchronization among
the base stations.
The consensus of the experts of the FG at the beginning of phase-1 was to use RoF (radio over fiber)
technology for the fronthaul, which is expensive but
considered to be a promising and technically reliableapproach. However, the Japanese experts proposed apacket based approach as an alternative. Because of
the expected cost effectiveness of a packet based
approach, the majority of the members shifted theirsupport in favor of the Japanese proposal. Among
packet based approaches, Ethernet based methods
NTT Technical Review

Global Standardization Activities

Application

User
equipment

Radio access

Mobile packet
core

Cloud

FH/BH

Core
network

Management
orchestrator

Slice control

Slice (A)Slice (B)
Slice (C)User
equipmentBasestationDatacenter

Physical infrastructure

Fig. 3. Concept of network softwarization.

have been the most intensively developed and discussed. At the FG meeting in May 2016, China
Mobile demonstrated a prototype product based on
its proposed technology to the IEEE* 1914 Working
Group. Additionally, a Chinese manufacturer presented FlexEthernet as a solution to achieve time andfrequency synchronization.
Many researchers participating in this FG had been
studying software-defined networking technology
but were having difficulty in testing their ideas in an
end-to-end network system without technical knowledge or the facilities of a radio access network, which
is an important component of mobile networks. A
solution called OpenAirInterface was introduced as a
useful tool to construct a base station using off-theshelf products. This solution uses open source software to achieve a BBU and RRH on a personal computer. Certain models of commercial user equipment
products can be connected for this purpose, and many
participants of the FG were interested in learning how
to use this solution to set up their own test environment.
3.4 Other technologies
ICN/CCN is network technology that enables more
efficient distribution of content. ICN/CCN caches thecontent and transcodes it adaptively in response to
network conditions. In FG IMT-2020, prototyping
activities and use cases, which were primarily fromVol. 15 No. 6 June 2017

the US, were introduced. The FG considered ICN/
CCN to be a potentially useful technology but
thought that there was room for improvement toincrease the possibility of identifying/caching relevant content before it was selected as an actual solution to be deployed. The FG also noted that one of the
key factors in the development of IMT-2020 is determining how to address the increasing volume of traffic, particularly video traffic. In this sense, potential
use of ICN/CCN technology should be considered.
The application of satellite communications technology was presented at the FG in September 2016.
Satellite communications is relevant for the backhaul
in mobile networks. Many developing countries stilllack fiber backbone networks, so satellite communications could play a bigger role in such cases. Signal
attenuation caused by weather conditions such as rain
and clouds reduces the reliability of satellite communications; therefore, satellite communications plays asupplementary role where the terrestrial fiber network is available. However, some plans are being
made to deploy a number of smaller satellites in therelatively low orbits (about 1000 km above earth)rather than in the conventional geostationary orbit.
This type of satellite network features low latency aswell as global coverage. New use cases will be developed.
*

IEEE: Institute of Electrical and Electronics Engineers

4Global Standardization Activities

Table 1. Structure of SG13 and relationship with FG IMT-2020.
SG13
WP

Title

FG IMT-2020 Working Group
Question

Working Group

Q.6: Quality of service (QoS) aspects including
IMT-2020 networks

Document

QoS
Terms and definitions for ITU-T
IMT-2020

Q.20: IMT-2020: Network requirements andfunctional architecture

Requirements &architecture

Requirements of IMT-2020 fromnetwork perspective
Framework of IMT-2020 networkarchitecture1

IMT-2020 Networks& Systems

Softwarization
Q.21: Software-defined networking, network
slicing and orchestration

Q.22: Upcoming network technologies for
IMT-2020 and future networks

ICN/CCN

Q.23: Fixed-mobile convergence including
IMT-2020

FMC

2

Cloud Computing &Big Data

Q.7 (DPI), Q.17 (cloud requirements), Q.18
(cloud architechture), Q.19 (cloud management)

3

Future Network
Evolution & Trust

Q.1 (innovation services), Q.2 (NGNe), Q.5(developing countries), Q.16 (knowledge & trust)

DPI: deep packet inspection
E2E: end to end

FMC: fixed mobile convergence
NGNe: Next Generation Network evolution

4. Future plan
WTSA-16, which is the highest level meeting in the
ITU-T, was held in October and November 2016.
WTSA recognized that IMT-2020 will be one of themost important subjects over the next four years andtherefore adopted a new resolution to promote IMT2020 related activities based on the outcomes of FG

5

E2E management

Report on the application of network
softwarization to IMT-2020
IMT-2020 network management
requirements
Network management framework for
IMT-2020
Report on the application of ICN to
IMT-2020
Requirements of IMT-2020 fixed andmobile convergence
Unified Network Integrated Cloud on
FMC

WP: Working Party

IMT-2020. IMT related work including IMT-2020 iscoordinated by SG13 as a mission of the lead study
group on this subject. SG13 also organized a dedicated working party to draft questions for particularsubjects of IMT-2020 (Table 1). The FG IMT-2020
produced nine deliverables. These deliverables are
assigned to the relevant questions and will be used asbase documents for further development.

NTT Technical Review

Global Standardization Activities

Yoshinori Goto

Senior Research Engineer, Network Technology Project, NTT Network Technology Laboratories.
He received a B.E. and M.E. in applied physics
from Tohoku University, Miyagi, in 1992 and1994. He joined NTT Basic Research Laboratories in 1994. He has been researching and developing cable television systems, Internet protocol
television (IPTV), and machine-to-machine technology. He has been engaged in the standardization work for IPTV in ITU-T as a member of the
IPTV Focus Group and Global Standards Initiative since 2006. He has also served as Rapporteurof Question 11 of ITU-T SG9, Questions 5 and
25 of ITU-T SG13, and Question 21 of ITU-T
SG16. He has been a vice-chair of ITU-T SG13
since 2013. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE).

Vol. 15 No. 6 June 20176

Practical Field Information about
Telecommunication Technologies
Practical Field Information about Telecommunication Technologies

Case Studies of Wireless LAN
Problems
Abstract

This article describes cases studies of problems occurring in wireless LANs (local area networks).
This is the fortieth article in a series on telecommunication technologies. This contribution is from the
EMC Engineering Group, Technical Assistance and Support Center, Maintenance and Service Operations Department, Network Business Headquarters, NTT EAST.
Keywords: wireless LAN, Wi-Fi, access point

1. Introduction
The increasing popularity of mobile terminals
equipped with wireless local area network (LAN)functions has driven the installation of wireless LAN
access points (APs) in both indoor and outdoor locations and the creation of Internet connection environments in all sorts of places. In line with this trend,
NTT EAST has made its 5th-generation home gateway compliant with the new IEEE802.11ac*1 wireless networking standard and has been expanding its
communication services using wireless LAN such as
by launching FLET’S HIKARI (optical broadband)
services capable of gigabit-class communications.
Services in a variety of formats are also expanding
such as Wi-Fi*2 access for apartment buildings and
Wi-Fi access for corporate use.
However, communication faults can occur as aresult of insufficient signal strength, signal interference, incompatible communication protocols, andother factors. Finding ways of recovering from faults
quickly and improving service quality has consequently become a matter of urgency.
In this article, we introduce recent case studies ofwireless LAN problems handled by the Technical
Assistance and Support Center.
2. Case study 1: Wi-Fi access for anapartment building
In this section, we describe a problem a customer
1

had with Wi-Fi access for an apartment building andexplain how it was rectified.
2.1 Overview and report
Wi-Fi access for apartment buildings is an Internet
connection service via wireless LAN targeting abuilding of 4 to 12 units. In this service, any room inthe building should be able to connect to wireless
LAN via radio signals transmitted from shared APs
installed outdoors. In this case study, a report wasreceived that an AP connection could not be madefrom an indoor location, so an investigation was carried out.
2.2 Results of investigation and causes of fault
We considered the possibility that the signal
strength of wireless LAN was insufficient, so wechecked the manner in which the antennas were
installed. We found some cases in which antennas
were installed at a height midway between the firstand second floors or on the ceilings of balconies, as
shown in Fig. 1, and we surmised that the signals insuch cases were not strong enough to reach indoors.
The indoor signal-strength distribution measured
with a wireless LAN tester [1] (a product developedby the Technical Assistance and Support Center and*1 IEEE802.11ac: A wireless networking standard developed by theInstitute of Electrical and Electronics Engineers (IEEE).
*2 Wi-Fi: A technology for wireless LAN with devices based on theIEEE 802.11 standards. Wi-Fi is a registered trademark of Wi-Fi
Alliance.

NTT Technical Review

Practical Field Information about Telecommunication Technologies

Inappropriate height

Attached to wall

Attached to balcony ceilingFig. 1. Antenna installation formats (examples of inappropriate installation).

Interference
1
Livingroom

9

11 Channel

BalconyFrequency

Entrance

Fig. 3. Channel interference in 2.4-GHz band.
–76 dBm–90

Signal strength (dBm)
–84

–78

–72

Desk
–60

–66

Fig. 2. Signal-strength distribution (before relocating
antenna).

Wall
Window
Antenna(vertically installed)
Transmission into room

awarded a 2016 NTT EAST President’s Commendation) is shown in Fig. 2. It was found that the signal
strength in the living room where the customer wasusing wireless LAN was –76 dBm, which we considered to be insufficient.
Furthermore, considering that wireless LAN channel interference might be occurring, we measured thechannel interference with the wireless LAN testerand found that channels 9 and 11 were indeed interfering with each other as shown in Fig. 3.
We surmised that the causes of the fault were insufficient indoor signal strength due to an inappropriate
antenna installation format and interference of wireless LAN signals due to inappropriate channel allocation to the AP.

Vol. 15 No. 6 June 2017

Antenna(horizontally installed)

Fig. 4. Antenna signal transmission.

2.3 Proposed method
Since radio signals are more able to permeate
through windows than walls, installing an antenna on
the balcony side rather than the entrance side of a unit
would make it easier for indoor terminals to receive
signals. Signals from a Wi-Fi antenna are strongly
transmitted in the direction perpendicular to theantenna direction as shown in Fig. 4. To therefore
transmit radio signals efficiently into a room, the2Practical Field Information about Telecommunication Technologies

Distance: 1 cm

Antenna

Window

Window

Room

Distance: 30 cm
Window Room
Antenna

Wall

Wall

Room

Window RoomFig. 5. Antenna transmission characteristics in 2.4-GHz band (simulation).

Verticallyinstalled

Height:window center
Antenna

Livingroom

Window

Balcony

Entrance

Wall
Installed at a
distance from wall

–61 dBm

Desk–90 Signal strength (dBm) –60–84 –78 –72 –66

Fig. 6. Optimal antenna installation format.

antenna should be vertically installed at a heightaligned with the center of the window frame. Moreover, installing the antenna away from walls cansecure a line-of-sight toward rooms, thereby achieving more efficient transmission of radio signals into
rooms, as shown in Fig. 5.
Therefore, an optimal antenna installation format
would be the one shown in Fig. 6. That is, the antennashould be vertically installed at a height corresponding to the center of the window and at a distance fromthe wall. The indoor signal-strength distribution after
changing the antenna installation format is shown in
Fig. 7. The signal strength in the living room was now–61 dBm, reflecting a 15 dB improvement comparedwith that before antenna relocation.
Finally, allocating wireless LAN channels 1, 6, and11 as shown in Fig. 8 can prevent the occurrence ofinterference.

3

Fig. 7. S ignal-strength distribution (after relocating
antenna).

No interference
1

11 Channel

6Frequency

Fig. 8. Optimal channel allocation in 2.4-GHz band.

3. Case study 2: Wi-Fi access service forcorporate use
The second case study involved a problem a customer had with Wi-Fi access for corporate use. We
describe the problem and the countermeasure in thissection.

NTT Technical Review

Practical Field Information about Telecommunication Technologies

PBX
2.4-GHz band: ch 1
5-GHz band: ch 36

2.4-GHz band: ch 65-GHz band: ch 48

Disconnection

Disconnection

ONU

Router
2.4-GHz band: ch 115-GHz band: ch 40

2.4-GHz band: ch 1
5-GHz band: ch 44

Disconnection

Disconnection

AP

90 smartphones

Fig. 9. Equipment setup.

Signal strength(dBm)
0
No interference–50

No interference
36ch

1ch

–100
2.4

2.5 5.17
Frequency (GHz)
2.4-GHz band

5.33
5-GHz band

Fig. 10. Results of measuring radio-signal condition.

3.1 Overview and report
Wi-Fi access for corporate use is an Internet connection service via wireless LAN targeting corporate
customers. This service delivers AP units to the customer, who then installs them at various indoor locations. There were four AP units (one per room)
installed in the customer’s office with each AP connected to a router, ONU (optical network unit), andPBX (private branch exchange), as shown in Fig. 9.
90 smartphones that could connect to these APs had
been used. The customer reported that disconnections
occurred.
3.2 Results of investigation and causes of fault
We used a spectrum analyzer to measure the radioVol. 15 No. 6 June 2017

signal condition in both the 2.4-GHz and 5-GHz
bands in a room where disconnections had occurred.
The results of these measurements are shown in
Fig. 10. Other than Wi-Fi signals, no interfering signals could be observed.
Next, using AirMagnet (a generally available wireless LAN analysis tool), we measured the protocol on
the radio signal in a room where disconnections had
occurred. The results are shown in Fig. 11. These
results revealed that no Block Ack message (acknowledgment that data were received) was returned fromthe terminal after a data transmission from the AP
when a disconnection had occurred (using smartphone A), which resulted in the resending of Block
Ack Request messages from the AP. To troubleshoot4

Practical Field Information about Telecommunication Technologies

AP

TerminalAP

Terminal

data

data

Block Ack

Block Ack

data

Block Ack Request

Block Ack

Block Ack Request

Normal operation (smartphone B)

Fault (smartphone A)

Fig. 11. Results of measuring protocol (case study 2).

HGW

TabletTablet

HGW

RTS

RTS

CTS

CTS

data

RTS (resend)Ack

RTS (resend)

Normal operation

Fault

Fig. 12. Results of measuring protocol (case study 3).

this result, we used smartphone B and found that the
protocol had no problem in the order of data→Block
Ack→data and so on.
We therefore surmised that the cause of the disconnection when using smartphone A was that Block
Ack messages that should have been transmitted inreply to data packets from the AP were somehow
missing.
3.3 Proposal to the customer
We confirmed that smartphone A was outside thescope of this system’s operational guarantee, so we
proposed to the customer that smartphone A be
replaced with a smartphone unit for which normal
operation was guaranteed.
4. Case study 3: Home gateway (HGW)
equipped with a wireless LAN function
The final case study involves a problem that
occurred in a HGW.

5

4.1 Overview and report
A customer using a tablet terminal and a HGWequipped with a wireless LAN function reported thatan application download would stop 2 to 3 seconds
after starting.
4.2 Results of investigation and causes of fault
We measured the signal strength, channel interference, and interfering signals with a wireless LAN
tester in the room, but found that no signal attenuation, channel interference, and interfering signals
could be observed at the time of the fault.
We then measured the protocol for the radio signals. A download would normally be completed in
the order of RTS (Request To Send)→CTS (Clear toSend)→data→Ack (acknowledgment that data werereceived), as shown in Fig. 12. However, at the timeof the fault, the AP would resend RTS repeatedly,
while no CTS would be returned from the terminal.
We surmised from these results that the non-returnof the CTS message from the terminal to the AP
caused the AP to repeatedly resend RTS messages,
thereby stopping data transmission from the HGW
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Practical Field Information about Telecommunication Technologies

and bringing the download to a halt.
4.3 Countermeasure
We checked the operating system (OS) of the tablet
terminal and found that it was an old version. We
therefore updated the OS to the latest version, andconfirmed that a download was completed.
5. Conclusion
In this article, we introduced recent case studies ofwireless LAN problems handled by the Technical
Assistance and Support Center. With the proliferationof various types of wireless LAN services, the causesof faults in these services are becoming increasingly

Vol. 15 No. 6 June 2017

diverse. The EMC Engineering Group of the Technical Assistance and Support Center endeavors toachieve prompt resolution of faults related to wireless
LAN and to contribute to the smooth provision ofcommunication services. To this end, it is actively
engaged in technology dissemination activities
through technical support, development, and technical seminars.
Reference[1]

K. Okamoto et al., “Introduction of Wireless LAN Tester toward
Smooth Provision of Wi-Fi Services,” IEICE Communications Society Magazine (B-plus), No. 25, pp. 38–43, Summer 2013 (in Japanese).

6Information

Event Report: Science Plaza 2016 at NTT
Basic Research Laboratories
Norio Kumada
Abstract

Science Plaza 2016, an open-house event hosted by NTT Basic Research Laboratories, was held at
NTT Atsugi R&D Center on November 23, 2016. Under the banner “Frontier Science: Open Door to the
Future,” Science Plaza was aimed at disseminating our latest research accomplishments to various
groups of people inside and outside of NTT as well as gathering diverse opinions. This event was held
jointly with NTT Device Innovation Center, NTT Device Technology Laboratories, and NTT Communication Science Laboratories.
Keywords: NTT Basic Research Laboratories, Science Plaza, open-house event

1. Purpose of Science PlazaAt NTT Basic Research Laboratories (BRL), weare engaged in research aimed at creating new principles and concepts that overcome the limitations ofexisting network technology and developing basic
technologies that pave the way to future innovation.
At Science Plaza 2016, our aim was to introduce to awide audience the results of the cutting-edge research
being carried out not only at BRL, but also at NTT
Device Innovation Center (DIC), NTT Device Technology Laboratories (DTL), and NTT Communication Science Laboratories (CSL). This event waschiefly targeted at undergraduate and postgraduate
students and corporate researchers.

Quantum Electronic Optical Properties and Functional Materials (BRL), Groundbreaking New Device
Research (DIC/DTL), and Communication Science
as a Bridge Between Humans and Information (CSL).
After that, Dr. Akira Fujiwara (Senior Distinguished
Scientist, BRL) held a symposium called Ultimate

2. Science Plaza overview
The event included lectures, poster exhibitions, andlaboratory tours, which are briefly introduced here.
2.1 Lectures
At Science Plaza 2016, the lectures were divided
into morning and afternoon sessions. The morning
session started with an opening address delivered by
BRL director, Tetsuomi Sogawa (Photo 1). This wasfollowed by three lectures: Frontier Research in
1

Photo 1. O pening address by BRL Director, Tetsuomi
Sogawa.

NTT Technical Review

Information

Photo 2. S ymposium by Dr. Akira Fujiwara (SeniorDistinguished Scientist, BRL).

Electronics Using Nano-devices (Photo 2). Thissymposium lecture covered the prior development ofsilicon devices, the current state of research into silicon nano-devices at BRL, and the prospects for future
development aimed at practical applications.
In the afternoon session, the 2014 winner of theNobel Prize in Physics, Dr. Hiroshi Amano (Director,
Center for Integrated Research of Future Electronics,
Institute of Materials and Systems for Sustainability,
Nagoya University), was invited to deliver a special
lecture entitled Illuminating the World by LEDs
(Photo 3). In this lecture, Dr. Amano talked about
what happened immediately after the Nobel prize wasannounced, the problems he encountered in the
research and development of blue light emitting
diodes, the factors behind his eventual success, andthe prospects for future development.
The lecture theater was filled to capacity in both themorning and afternoon lecture sessions, and the lectures were followed by lively question and answer
sessions.
2.2 Lab tours
We held lab tours to show the attendees aroundfacilities such as the clean room and laboratories
where our research actually takes place, and to allow
them to experience some of our latest researchachievements (Photo 4). There were a total of ten
tours (three from BRL, six from DIC/DTL, and one
from CSL), each of which was run twice. All the tours
were fully booked (with ten guests each), making a
total of 200 participants in the lab tours. These tours
Vol. 15 No. 6 June 2017

Photo 3. S pecial lecture by Dr. Hiroshi Amano (Nagoya
University).

Photo 4. Lab tour.

were very well received, and the feedback questionnaires included responses such as “I don’t often get
the chance to visit commercial laboratories. This wasa really educational experience.”
2.3 Poster exhibition
We put on an exhibition of posters illustrating 52
research projects, including 13 from the joint research
institutes, and the researchers involved in these projects were introduced directly to the exhibition visitors(Photo 5). The exhibition started with an overview of
each research project and went on to describe in detail2

Information

the originality and impact of the research and its
future prospects, including an in-depth discussion of
the research.
After all the lectures, exhibitions, and announcements were finished, we rounded off the event with an
evening social gathering in the exhibition hall. This
gave us a chance to get to know the visitors better andcontinue with the discussions on our research.

Photo 5. Poster exhibition.

2.4 Conclusion of Science Plaza 2016
Science Plaza 2016 was attended by 198 visitorsfrom universities, research institutes, private companies, NTT Group companies, and other organizations.
We are really grateful to everyone who attended thisevent. The questionnaires filled in by those who
attended the poster exhibition and lab tours will provide valuable feedback for the next Science Plaza.

Norio Kumada
Senior Research Scientist (Distinguished Scientist), Quantum Solid-State Physics ResearchGroup, Physical Science Laboratory, NTT Basic
Research Laboratories.
He received a B.S., M.S., and Ph.D. in physics
from Tohoku University, Miyagi, in 1998, 2000,
and 2003. He joined NTT Basic Research Laboratories in 2003. He has since been engaged in
the study of highly correlated electronic states
formed in semiconductor heterostructures. He
was a visiting researcher at CEA Saclay, France,
during 2013–2014. He was appointed as a Distinguished Scientist of NTT in 2010. He received
the Young Scientist Award of the Physical Society of Japan in 2008 and the Young Scientists’Prize from the Minister of Education, Culture,
Sports, Science and Technology in 2012. He is amember of the Physical Society of Japan.

3

NTT Technical Review

Short Reports

One-petabit-per-second Fiber
Transmission over a Record Distanceof 200 km—Paving the Way to
Realizing 1000-km Inline Optical
Amplified Transmission Systems
within C Band Only

1. Introduction
NTT has demonstrated ultralarge capacity inline
optical amplified transmission of 1 petabit (1000 terabits) per second (Pbit/s) over a 205.6-km length of
32-core optical fiber in collaboration with the Technical University of Denmark, Fujikura Ltd., Hokkaido
University, the University of Southampton, and Coriant GmbH.
This sets a new world record for the transmission
distance of 1-Pbit/s capacity over a single strand of
optical fiber within a single optical amplifier bandwidth (C band), which is half the bandwidth used in
the previous experiment (Fig. 1). The present
achievement indicates that the transmission of 1
Pbit/s—a capacity equivalent to sending 5000 highdefinition television videos each two hours long in asingle second—is potentially possible over 1000 km,
which is approximately the distance between major
cities in both Japan and Europe.
Part of this research utilized results from the EUJapan coordinated research and development projecton Scalable And Flexible optical Architecture for
Reconfigurable Infrastructure (SAFARI) [1] commissioned by the Ministry of Internal Affairs andCommunications of Japan and EC Horizon 2020.
2. Experiment and results
The use of 32-core multi-core fiber (MCF), which
we have successfully prototyped for a long length of1

over 50 km [2], a fan-in/fan-out (FI/FO) device tocouple light into the MCF, and new digital coherent
optical transmission technology made it possible toachieve the high-capacity optical transmission rate of
1 Pbit/s. We accomplished this by exploiting dense
space and wavelength division multiplexing (DSDM
and DWDM) over long distances. The 32-core MCF
used in the experiment utilized a new arrangement ofcores that greatly reduces inter-core light leakage that
otherwise degrades performance [2]. In addition, weused the wave properties of light (phase and polarization) to apply multi-dimensional coding to polarization division multiplexed 16-quadrature amplitude
modulation (PDM 16-QAM) digital coherent technology to improve the long-distance transmission
performance in each core.
The experimental results are shown in Fig. 2. We
achieved 31.3-Tbit/s capacity per core (= 680 Gbit/s
per wavelength x 46 wavelength channels), and using
the 32-core MCF, we recirculated and amplified thesignals over four spans of the 51.4-km fiber, demonstrating that signal transmission of an aggregate
1-Pbit/s capacity was possible over 205.6 km.
The Q-factor indicates the transmission quality of
the PDM 16-QAM signals. Because the Q-factor was
uniform, it showed that high quality transmission
with small variations between cores and low error
was possible. In 2012, we reported on an experimentin which we achieved a world-first 1-Pbit/s capacity
over 52.4 km [3]. In comparison, these new results
demonstrate a distance about four times longer at
NTT Technical Review

Short Reports

This sets a new world record for the transmission distance of 1-Pbit/s capacity over a single strand of opticalfiber within a single optical amplifier bandwidth (C band), which is half of that in previous record-breaking
experiments.

: C band.

: C + L band

MCF

MCF

(2015)

C
C bandCapacity (Pbit/s)(2015)

Simple andcompact configuration.
(2013)

This work

NTT et al. (2012)

C

MCF

MCF

.
L
C + L band
Configuration of optical inline amplifiers

Transmission distance (km)MCF: multi-core fiber

Fig. 1. The achievement in petabit-per-second-class transmission.

- All 1473 channels with a 680-Gbit/s PDM 16-QAM signal format were transmitted over 205.6 km without
eight-dimensional coded modulation.
- Total capacity of 1 Pbit/s/fiber (= 32 cores x 46 wavelengths x 680 Gbit/s) was achieved.
After 205.6 km.

Q-factor (dB).
Threshold of forward
error correction.
Core 1
Core 8Core15
Core22
Core29.

.
1525

Core 2
Core 9Core16
Core23
Core301535

Core 3
Core10
Core17
Core24
Core31

Core 4
Core11
Core18Core25
Core32

Core 5
Core12
Core19Core26

1545
Wavelength (nm)Core 6
Core13
Core20Core271555

Core 7
Core14
Core21
Core281565

Fig. 2. Transmission performance of 1 Pbit/s over 205.6 km.

205.6 km, which is the world’s longest distance forover petabit-per-second capacity transmission.
Moreover, by applying a digital signal processing
technique called multi-dimensional coded modulation, the capacity per wavelength is reduced by 25%
to 510 Gbit/s. Nevertheless, we demonstrated that theVol. 15 No. 6 June 2017transmission distance can be increased to over 1000
km. As a result, with one optical fiber, we showed that
there is a possibility for ultrahigh capacity equivalent
to 0.75 Pbit/s using just the 5 THz bandwidth of the
C band, and 1.5 Pbit/s using the 10 THz bandwidth
provided by the combined C and L bands, with a2

Short Reports

potential transmission distance over 1000 km.
3. Technological features and roles
Here, we describe the technology that was applied
to achieve the 1-Pbit/s transmission over a record
distance.
3.1 Thirty-two-core MCF transmission line
The MCF we used in this experiment was jointly
designed and prototyped by the Technical Universityof Denmark, Fujikura, and Hokkaido University. The
fiber has a new structure (single-mode heterogeneous-core MCF) with 32 cores incorporating several
types of cores, each with different properties [2]. The
characteristic of this fiber is that two kinds of coreswith slightly different refractive indices are arranged
in a square lattice pattern. With this structure, even if
the number of cores is increased to 30 or more, thecrosstalk between adjacent cores can be greatly
reduced [2], making it possible to realize long-distance DSDM transmission [4]. NTT and Coriant
evaluated the long distance characteristics of the
51.4-km MCF transmission line with the 32-core
MCF and FI/FO devices prototyped by Fujikura, theUniversity of Southampton, and NTT. As a result, we
confirmed that all cores had low crosstalk and low
loss characteristics over the entire C band, which is arequirement for a 32-core MCF transmission line
suitable for transmission over a 1000-km distance.
3.2 Multi-dimensional coded 16-QAM technique
In recent large-capacity optical communications,instead of the intensity modulation signal transmittedusing binary states of either ON or OFF, a highly
efficient PDM-QAM digital coherent signal has beenused that realizes a large number of signal states created by using the wave properties (phase and polarization) of light. Such multi-level QAM signals canachieve a highly efficient ultrahigh speed optical signal by associating a plurality of bits of digital signals
with a plurality of optical signal states encoded using
the phase and polarization of light. However, thedrawback is that when we increase transmission efficiency by increasing the number of multi-levels, thetransmission distance sharply decreases. In addition,
signal quality degrades by the crosstalk that arises in3

MCF transmission.
In this case, NTT reduced the number of multilevels of the QAM signal from 32 in the conventional
report [3] to 16 and applied a wideband digital-analog
conversion technique [5] to the digital coherent signal
using highly efficient error correction coding. As aresult, we successfully transmitted a capacity of 680
Gbit/s per wavelength (1 Pbit/s per fiber) over a
205.6-km distance, the longest distance for petabitper-second capacity transmission. Furthermore, byapplying the eight-dimensional encoded 16-QAM
technique [6] and by improving the allocation methodof the digital signal and the optical signal state, thetransmission quality can improve compared with thenormal QAM code. With the same 16-level QAM, thetransmission rate will be reduced to 510 Gbit/s per
wavelength, but by doing this, we showed that it has
the potential to extend the transmission distance topossibly over 1000 km.
References
[1]
[2]

[3]
[4]

[5]
[6]

SAFARI, http://www.ict-safari.eu/
Y. Sasaki, R. Fukumoto, K. Takenaga, K. Aikawa, K. Saitoh, T.
Morioka, and Y. Miyamoto, “Crosstalk-managed Heterogeneous
Single-mode 32-core Fibre,” Proc. of the 42nd European Conference
on Optical Communication (ECOC 2016), Paper W.2.B.2, Düsseldorf, Germany, Sept. 2016.
Press release published by NTT on September 20, 2012.
http://www.ntt.co.jp/news2012/1209e/120920a.html
T. Mizuno, K. Shibahara, F. Ye, Y. Sasaki, Y. Amma, K. Takenaga, Y.
Jung, K. Pulverer, H. Ono, Y. Abe, M. Yamada, K. Saitoh, S. Matsuo,
K. Aikawa, M. Bohn, D. J. Richardson, Y. Miyamoto, and T. Morioka,“Long-haul Dense Space-division Multiplexed Transmission Over
Low-crosstalk Heterogeneous 32-core Transmission Line Using a
Partial Recirculating Loop System,” J. Lightwave Technol., Special
Issue on the Optical Fiber Communication Conference 2016, Vol. 35,
No. 3, pp. 488–498, 2017.
Press release published by NTT on September 23, 2016.
http://www.ntt.co.jp/news2016/1609e/160923b.html
M. Nakamura, F. Hamoka, A. Matsushita, H. Yamazaki, M. Nagatani,
A. Sano, A. Hirano, and Y. Miyamoto, “Coded 8-dimensional QAM
Technique Using Iterative Soft-output Decoding and Its Demonstration in High Baud-rate Transmission,” J. Lightwave Technol., Special
Issue on ECOC 2016, Vol. 35, No. 8, pp. 1369–1375, 2017.

For Inquiries
Public Relations,
NTT Science and Core Technology Laboratory
Group
Email: a-info@lab.ntt.co.jp
http://www.ntt.co.jp/news2017/1703e/170323a.html

NTT Technical Review

External Awards
Postgres PRIZE Award
Winner: Masahiko Sawada, NTT Open Source Software Center(NTT Software Innvation Center)
Date: March 17, 2017
Organization: PgConf.Russia 2017
For “Built-in Sharding Update and Future.”
Published as: M. Sawada, “Built-in Sharding Update and Future,”
PgConf.Russia 2017, Moscow, Russia, Mar. 2017.
Young Researcher’s Award
Winner: Chihiro Kito, NTT Access Network Service Systems Laboratories
Date: March 24, 2017
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE)
For “Loss Distribution Field Measurement of PON Branches with
End-reflection Assisted Brillouin Analysis.”
Published as: C. Kito, H. Takahashi, K. Toge, S. Ohno, and T.
Manabe, “Loss Distribution Field Measurement of PON Brancheswith End-reflection Assisted Brillouin Analysis,” Proc. of the IEICE
General Conference 2016, B-13-15, Fukuoka, Japan, Mar. 2016.
Young Researcher’s Award
Winner: Kohki Shibahara, NTT Network Innovation Laboratories
Date: March 24, 2017
Organization: IEICE
For “Spectrally-efficient Super-Nyquist Transmission by Multistage Successive Interference Cancellation.”
Published as: K. Shibahara, A. Masuda, S. Kawai, and M. Fukutoku,“Spectrally-efficient Super-Nyquist Transmission by Multi-stage
Successive Interference Cancellation,” Proc. of the IEICE General
Conference 2016, B-10-62, Fukuoka, Japan, Mar. 2016.
Young Researcher’s Award
Winner: Hajime Katsuda, NTT Network Innovation Laboratories
Date: March 24, 2017
Organization: IEICE
For “A Study of Weighting Combining for CSI Estimation in SIC
Process.”
Published as: H. Katsuda, S. Ohmori, and K. Akabane, “A Study of
Weighting Combining for CSI Estimation in SIC Process,” Proc. of
the IEICE General Conference 2016, B-5-157, Fukuoka, Japan, Mar.
2016.
Young Researcher’s Award
Winner: Hiroki Kawahara, NTT Network Innovation Laboratories
Date: March 24, 2017
Organization: IEICE
For “Proposal of SDM Node to Enable Path Assignment in Any
Wavelength/Core/Direction Port (1) - Proposal of SDM Node Architecture -.”1

Published as: H. Kawahara, A. Sawara, H. Yamamoto, S. Kawai, M.
Fukutoku, Y. Miyamoto, K. Suzuki, and K. Yamaguchi, “Proposal of
SDM Node to Enable Path Assignment in Any Wavelength/Core/
Direction Port (1) - Proposal of SDM Node Architecture -,” Proc. of
the IEICE Society Conference 2016, B-10-55, Sapporo, Hokkaido,
Japan, Sept. 2016.
Young Researcher’s Award
Winner: Keita Yamaguchi, NTT Device Technology Laboratories
Date: March 24, 2017
Organization: IEICE
For “MxN Optical Switch by Holographic Phase Modulation.”
Published as: K. Yamaguchi, M. Nakajima, Y. Ikuma, K. Suzuki, J.
Yamaguchi, and T. Hashimoto, “MxN Optical Switch by Holographic Phase Modulation,” Proc. of the IEICE General Conference 2016,
C-3-54, Fukuoka, Japan, Mar. 2016.
Young Researcher’s Award
Winner: Yusuke Muranaka, NTT Device Technology Laboratories
Date: March 24, 2017
Organization: IEICE
For “100-Gbps Optical Packet Switching with Ultralow-powerLabel Processor and Optical Switch.”
Published as: Y. Muranaka, T. Segawa, S. Ibrahim, T. Nakahara, H.
Ishikawa, and R. Takahashi, “100-Gbps Optical Packet Switchingwith Ultralow-power Label Processor and Optical Switch,” Proc. of
the IEICE Society Conference 2016, C-4-7, Sapporo, Hokkaido,
Japan, Sept. 2016.
Young Researcher’s Award
Winner: Takahiko Shindo, NTT Device Technology Laboratories
Date: March 24, 2017
Organization: IEICE
For “High Modulated Output Power of 9.0 dBm with L-band SOA
Assisted Extended Reach EADFB Laser (AXEL).”
Published as: T. Shindo, W. Kobayashi, N. Fujiwara, Y. Ohiso, K.
Hasebe, H. Ishii, and M. Itoh, “High Modulated Output Power of 9.0
dBm with L-band SOA Assisted Extended Reach EADFB Laser(AXEL),” Proc. of the IEICE Society Conference 2016, C-4-26, Sapporo, Hokkaido, Japan, Sept. 2016.
Best Presentation Award
Winner: Yoji Yamato, NTT Software Innovation Center
Date: March 31, 2017
Organization: The 5th IIAE International Conference on Industrial
Application Engineering 2017 (ICIAE2017)
For “Proposal of Vital Data Analysis Platform Using Wearable
Sensor.”Published as: A. Watanabe, Y. Matsuo, K. Watanabe, K. Ishibashi,
and R. Kawahara, “Proposal of Vital Data Analysis Platform Using
Wearable Sensor,” Proc. of ICIAE2017, pp. 138–143, Kitakyushu,
Fukuoka, Japan, Mar. 2017.

NTT Technical Review

Papers Published in Technical Journals andConference Proceedings
Entanglement Assisted Classical Communication Simulates “Classical Communication” without Causal OrderS. Akibue, M. Owari, G. Kato, and M. Murao
arXiv:1602.08835 [quant-ph], February 2016.
Phenomena induced by the existence of entanglement, such as
nonlocal correlations, exhibit characteristic properties of quantum
mechanics distinguished from classical theories. When entanglement
is accompanied by classical communication, it enhances the power ofquantum operations jointly performed by two spatially separated parties. Such a power has been analyzed based on the gap between theperformances of joint quantum operations implementable by local
operations at each party connected by classical communication withand without the assistance of entanglement. In this work, we present
a new formulation for joint quantum operations connected by classical communication beyond special relativistic causal order but without entanglement and still within quantum mechanics. Using theformulation, we show that entanglement assisting classical communication necessary for implementing a class of joint quantum operations called separable maps can be interpreted to simulate “classicalcommunication” that does not respect causal order. Our results reveal
a new counter-intuitive aspect of entanglement related to spacetime.

Semi-automated Verification of Security Proofs of Quantum Cryptographic Protocols
T. Kubota, Y. Kakutani, G. Kato, Y. Kawano, and H. Sakurada
Journal of Symbolic Computation, Vol. 73, pp. 192–220, April
2016.
This paper presents a formal framework for semi-automated verification of security proofs of quantum cryptographic protocols. We
simplify the syntax and operational semantics of quantum process
calculus qCCS so that verification of weak bisimilarity of configurations becomes easier. In addition, we generalize qCCS to handle
security parameters and quantum states symbolically. We then prove
the soundness of the proposed framework. A software tool, named theverifier, is implemented and applied to the verification of Shor and
Preskill’s unconditional security proof of BB84. As a result, we succeed in verifying the main part in Shor and Preskill’s unconditional
security proof of BB84 against an unlimited adversary’s attack semiautomatically; i.e., it is automatic except for giving user-defined
equations.

Reducing Dense Virtual Network for Fast Embedding
T. Mano, T. Inoue, K. Mizutani, and O. Akashi
Proc. of INFOCOM 2016 (the 35th Annual IEEE International
Conference on Computer Communications), April 2016.
Virtual network embedding has been intensively studied for a
decade. The time complexity of most conventional methods has been
reduced to the cube of the number of links. Since customers arelikely to request a dense virtual network that connects every node pairdirectly (|E| = O(|V|2)) based on a traffic matrix, the time complexity
is actually O(|E|3 = |V|6). If we were allowed to reduce this dense
network into a sparse one before embedding, the time complexity
could be decreased to O(|V|3); the time gap can be a million times for|V| = 100. The network reduction, however, combines several virtualVol. 15 No. 6 June 2017

links into a broader link, which makes the embedding cost (solution
quality) much worse. This paper analytically and empirically investigates the trade-off between the embedding time and cost for the virtual network reduction. We define two simple reduction algorithms
and analyze them with several interesting theorems. The analysis
indicates that the embedding cost increases only linearly with exponential decay of embedding time. Thorough numerical evaluation
justifies the desirability of the trade-off.

A Mobility-based Mode Selection Technique for Fair Spatial Dissemination of Data in Multi-channel Device-todevice Communication
H. Kuribayashi, K. Suto, H. Nishiyama, N. Kato, K. Mizutani, T.
Inoue, and O. Akashi
Proc. of the 2016 IEEE International Conference on Communications, Kuala Lumpur, Malaysia, May 2016.
Wireless communication devices have spread widely in our society. However, they usually depend heavily on communication infrastructure, leaving them vulnerable to disasters or congestion of basestations. In these situations, a method to send out data without the
support of infrastructure is required. Data transmission by device-todevice (D2D) communication is a reliable method that does not rely
on infrastructure. In this paper, we aim to improve the data dissemination using D2D transmission by applying the concept of assigning“modes” to devices according to their own mobility. In our study, we
assume a multi-channel environment, where devices will be allocateddifferent amounts of frequency channels according to their modes.
We propose a mode selection function that uses velocity informationof the devices to assign modes. By using this function, it is possible
to allocate more frequency channels to devices of high mobility, sothat they can transmit their data to more devices as they move througha wide area. By mathematical analysis, we evaluated the fairness ofdisseminated data density among devices of various velocities, andthe obtained results indicate the effectiveness of the proposed method
for improving the efficiency of data dissemination.

Acceleration of Network Reachability Tests against a
Huge Number of Hypercube Queries
R. Chen, T. Inoue, T. Mano, K. Mizutani, H. Nagata, and O. Akashi
Proc. of the 36th IEEE International Conference on Distributed
Computing Systems, pp. 743–744, Nara, Japan, June 2016.
This paper proposes a novel windowing algorithm for network
verification. Unlike existing windowing algorithms, our algorithm
runs on a compressed data structure because the search space has tobe represented in a compressed form due to the space complexity.

Security of Six-state Quantum Key Distribution Protocol
with Threshold Detectors
G. Kato and K. Tamaki
Scientific Reports, Vol. 6, 30044, July 2016.
The security of quantum key distribution (QKD) is established bya security proof, and the security proof puts some assumptions on the2

Papers Published in Technical Journals and Conference Proceedings

devices consisting of a QKD system. Among such assumptions, security proofs of the six-state protocol assume the use of a photon number resolving (PNR) detector, and as a result, the bit error rate threshold for secure key generation for the six-state protocol is higher thanthat for the BB84 protocol. Unfortunately, however, this type ofdetector is demanding in terms of the technological level comparedto the standard threshold detector, and removing the necessity of sucha detector enhances the feasibility of the implementation of the sixstate protocol. Here, we develop the security proof for the six-state
protocol and show that we can use the threshold detector for the sixstate protocol. Importantly, the bit error rate threshold for the key
generation for the six-state protocol (12.611%) remains almost thesame as the one (12.619%) that is derived from the existing security
proofs assuming the use of PNR detectors. This clearly demonstrates
the feasibility of the six-state protocol with practical devices.

Efficient Virtual Network Optimization across Multiple
Domains without Revealing Private Information
T. Mano, T. Inoue, D. Ikarashi, K. Hamada, K. Mizutani, and O.
AkashiIEEE Transactions on Network and Service Management, Vol. 13,
No. 3, pp. 477–488, September 2016.
Building optimal virtual networks across multiple domains is an
essential technology for offering flexible network services. However,existing research is founded on an unrealistic assumption that providers will share their private information including resource costs.
Providers, as well known, never actually do that so as to remain competitive. Secure multi-party computation, a computational techniquebased on cryptography, can be used to secure optimization, but it is
too time consuming. This paper presents a novel method that can
optimize virtual networks built over multiple domains efficiently
without revealing any private information. Our method employs
secure multi-party computation only for masking sensitive values; it
can optimize virtual networks under limited information without
applying any time-consuming techniques. It is solidly based on thetheory of optimality and is assured of finding reasonably optimal
solutions.
Experiments show that our method is fast and optimal in practice,even though it conceals private information; it finds near optimal
solutions in just a few minutes for large virtual networks with tens ofnodes. This is the first work that can be implemented in practice forbuilding optimal virtual networks across multiple domains.

Towards Low-delay Edge Cloud Computing through a
Combined Communication and Computation Approach
T. G. Rodrigues, K. Suto, H. Nishiyama, N. Kato, K. Mizutani, T.
Inoue, and O. Akashi
Proc. of the 2016 IEEE 84th Vehicular Technology Conference,Montreal, Canada, September 2016.
There are many applications which cannot be executed by mobile
devices due to their limitations in memory, processing, and battery,among others. One solution to this would be offloading heavy tasks
to cloud servers at the edge of the network, in a service model callededge cloud computing. The main Quality of Service requirement of
this model is a low service delay, which can be achieved by loweringtransmission delay and processing delay. Works in literature focus oneither one of those two types of delay. This paper, however, argues
that an approach which combines transmission and processing technologies to lower service delay would be more efficient. This idea isdefended by an analysis of the service model and existing stochastic

3

modeling of the edge cloud computing system. We conclude that adual-focus approach would be the only way of truly minimizing theservice delay, therefore being the desired method to improve Qualityof Service. We conclude by laying the foundation for a future modelthat follows such a concept.

Statistical Estimation of the Names of HTTPS Servers with
Domain Name Graphs
T. Mori, T. Inoue, A. Shimoda, K. Sato, S. Harada, K. Ishibashi,and S. Goto
Computer Communications, Vol. 94, pp. 104–113, November
2016.
This work develops a novel framework called Service-Flow map(SFMap), which estimates names of HTTPS servers by analyzing
precedent domain name graph (DNS) queries/responses in a statistical way. The SFMap framework introduces the domain name graph,
which can characterize the highly dynamic and diverse nature of
DNS mechanisms. Such complexity arises from the recent deployment and implementation of DNS ecosystems, i.e., canonical nametricks used by CDNs (content delivery networks), the dynamic anddiverse nature of DNS TTL (Time To Live) settings, and incomplete
and unpredictable measurements due to the existence of various DNS
caching instances. First, we demonstrate that SFMap establishes
good estimation accuracies and outperforms a state-of-the-art
approach. We also aim to identify the optimized setting of the SFMap
framework. Next, based on the preliminary analysis, we introduce
techniques to make the SFMap framework scalable to large-scaletraffic data. We validate the effectiveness of the approach using largescale Internet traffic.

An Efficient Framework for Data-plane Verification with
Geometric Windowing Queries
T. Inoue, R. Chen, T. Mano, K. Mizutani, H. Nagata, and O. Akashi
Proc. of 2016 IEEE 24th International Conference on Network
Protocols, Singapore, Singapore, November 2016.
This paper presents a novel framework of data-plane verification,
which flexibly checks the inconsistency with great efficiency. For the
purpose of generality, our framework formalizes a verification process with three abstract steps; each step is related to 1) packet behaviors defined by a configuration, 2) operator intentions described in apolicy, and 3) the inspection of their relation. These steps work efficiently with each other on the simple quotient set of packet headers.
This paper also reveals how the second step can be regarded as thewindowing query problem in computational geometry. Two novel
windowing algorithms are proposed with solid theoretical analyses.
Experiments on real network datasets show that our framework withthe windowing algorithms is surprisingly fast even when verifying
the policy compliance; e.g., in a medium-scale network with thousands of switches, our framework reduces the verification time of
all-pairs reachability from ten hours to ten minutes.

Overlapping of /o/ and /u/ in Modern Seoul Korean: Focusing on Speech Rate in Read Speech
T. Igeta, S. Hiroya, and T. Arai
Journal of the Korean Society of Speech Sciences, Vol. 9, No. 1,
pp. 1–7, March 2017.
Previous studies have reported on the overlapping of F1 and F2
distribution for the vowels /o/ and /u/ produced by young Korean

NTT Technical Review

Papers Published in Technical Journals and Conference Proceedings

speakers of the Seoul dialect. However, few studies have examined
whether speech rate influences the overlapping of /o/ and /u/. In thecurrent study, we examined whether speech rates affect overlappingof /o/ and /u/ in read speech by male and female speakers. For female
speakers, discriminant analysis showed that the discriminant rate
became lower as the speech rate increased from slow to fast. Thus,
this indicates that speech rate is one of the factors affecting the overlapping of /o/ and /u/. For male speakers, on the other hand, the discriminant rate was not correlated with speech rate, but the overlapping was larger than that of female speakers in read speech. Moreover, read speech by male speakers was less clear than by female
speakers. This indicates that the overlapping may be related tounclear speech by sociolinguistic reasons for male speakers.

Online MVDR Beamformer Based on Complex Gaussian
Mixture Model with Spatial Prior for Noise Robust ASR
T. Higuchi, N. Ito, S. Araki, T. Yoshioka, M. Delcroix, and T. Nakatani
IEEE/ACM Transactions on Audio, Speech, and Language Processing, Vol. 25, No. 4, pp. 780–793, April 2017.
This paper considers acoustic beamforming for noise robust automatic speech recognition. A beamformer attenuates backgroundnoise by enhancing sound components coming from a direction
specified by a steering vector. Hence, accurate steering vector estimation is paramount for successful noise reduction. Recently, time–frequency masking has been proposed to estimate the steering vectorsthat are used for a beamformer. In particular, we have developed a
new form of this approach, which uses a speech spectral model basedon a complex Gaussian mixture model (CGMM) to estimate thetime–frequency masks needed for steering vector estimation, andextended the CGMM-based beamformer to an online speechenhancement scenario. Our previous experiments showed that the
proposed CGMM-based approach outperforms a recently proposedmask estimator based on a Watson mixture model and the baseline
speech enhancement system of the CHiME-3 challenge. This paper
provides additional experimental results for our online processing,which achieves performance comparable to that of batch processing
with a suitable block-batch size. This online version reduces theCHiME-3 word error rate (WER) on the evaluation set from 8.37%
to 8.06%. Moreover, in this paper, we introduce a probabilistic priorVol. 15 No. 6 June 2017

distribution for a spatial correlation matrix (a CGMM parameter),which enables more stable steering vector estimation in the presenceof interfering speakers. In practice, the performance of the proposed
online beamformer degrades with observations that contain only
noise or/and interference because of the failure of the CGMM parameter estimation. The introduced spatial prior enables the target
speaker’s parameter to avoid overfitting to noise or/and interference.
Experimental results show that the spatial prior reduces the WER
from 38.4% to 29.2% in a conversation recognition task comparedwith the CGMM-based approach without the prior, and outperforms
a conventional online speech enhancement approach.

Human Perception of Sub-resolution Fineness of Dense
Textures Based on Image Intensity Statistcs
M. Sawayama, S. Nishida, and M. Shinya
Journal of Vision, Vol. 17, No. 4, April 2017.
We are surrounded by many textures with fine dense structures,such as human hair and fabrics, whose individual elements are often
finer than the spatial resolution limit of the visual system or that of adigitized image. Here we show that human observers have an ability
to visually estimate subresolution fineness of those textures. We carried out a psychophysical experiment to show that observers could
correctly discriminate differences in the fineness of hair-like denseline textures even when the thinnest line element was much finer than
the resolution limit of the eye or that of the display. The physicalimage analysis of the textures, along with a theoretical analysis basedon the central limit theorem, indicates that as the fineness of texture
increases and the number of texture elements per resolvable unit
increases, the intensity contrast of the texture decreases and the intensity histogram approaches a Gaussian shape. Subsequent psychophysical experiments showed that these image features indeed play
critical roles in fineness perception; i.e., lowering the contrast madeartificial and natural textures look finer, and this effect was mostevident for textures with unimodal Gaussian-like intensity distributions. These findings indicate that the human visual system is able toestimate subresolution texture fineness on the basis of diagnostic
image features correlated with subresolution fineness, such as theintensity contrast and the shape of the intensity histogram.

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