NTT Technical Review

November 2021 Vol. 19 No. 11
View from the Top Hideaki Ozawa, Chief Operations Officer and Chief Technology Officer, NTT Research, Inc.
Front-line ResearchersShiro Saito, Senior Distinguished Researcher, NTT Basic Research Laboratories

Rising Researchers

Ryo Ishii, Distinguished Researcher, NTT Human Informatics Laboratories

Feature Articles: NTT DOCOMO’s Initiatives on 5G evolution & 6G
Trends and Target Implementations for 5G evolution & 6G



Research of Transparent RIS Technology toward 5G evolution & 6G



Research on NTN Technology for 5G evolution & 6G



Improving Communication Performance in High-mobility Environments by Millimeter-wave
Base Station Cooperation for 5G evolution

Regular Articles
High-gain Optical Parametric Amplifier Module by Using a PPLN Waveguide and Its
Application to Quadrature SqueezingDevelopment of Beyond 100G Optical Cross Connect (B100G-OXC) System

Global Standardization Activities
Standardization Activities in the Asia-Pacific Region—New Management Structure Approvedat the 33rd Asia-Pacific Telecommunity

Information

Event Report: NTT Communication Science Laboratories Open House 2021

External Awards/Papers Published in Technical Journals and Conference Proceedings

View from the Top

Try It Yourself and Pursue the Truth
Hideaki Ozawa
Chief Operations Officer and Chief
Technology Officer, NTT Research, Inc.
Overview

NTT Research focuses on cutting-edge basic research in the fields ofquantum computing, cryptography, blockchain, and medical/healthinformatics with partners around the world. Two years after its establishment, it has won several prestigious awards, including the International Association for Cryptologic Research (IACR) Test-of-TimeAward, SPIE Maiman Laser Award, and IACR Crypto 2020 Best Paper
Award. The institute’s achievements also include solving the 20-yearold problem of program obfuscation. We interviewed Hideaki Ozawa,
chief operations officer and chief technology officer of NTT Research,about the progress of research and the art of management.
Keywords: basic research, NTT Research, Inc., R&D management

Finding the seeds of new technologies—Two years have passed since NTT Research wasestablished. Could you give us an overview of theinstitute and the progress of research?
NTT Research was established in July 2019 in
Silicon Valley, USA, to focus on basic research andadvance technologies to bring positive change tohuman life by assembling scientists and experts
around the world. It is composed of three laboratories: the Physics & Informatics Laboratories (PHILab), whose main research area is quantum physics,
including quantum computers based on optical technology, which will form the basis of the novel technologies required in the future beyond the era of theInnovative Optical and Wireless Network (IOWN);
the Cryptography & Information Security Laboratories(CIS Lab), whose main research area is informationand mathematics theory, including cryptography necessary to promote the safe use of various data such as
personal information; and the Medical & Health
Informatics Laboratories (MEI Lab), whose main
research area is medical and health information,
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which will be required to individualize and revolutionize healthcare services.
These three laboratories aim to create a new technological direction toward business creation over the
next 10 to 20 years by combining the basic technology fields that NTT’s research and development(R&D) labs have cultivated over many years in Japan
with the wisdom of scientists and experts around theworld. From April 2020, our 68 papers were acceptedand published, and 16 patents (PHI Lab: 4; CIS Lab:
11; and MEI Lab: 1) were filed.
PHI Lab aims to develop optimal systems by combining the advantages of parallel quantum computing
with those of classical digital computing. It is developing the world’s most-advanced quantum computer,
centered on a Coherent Ising Machine, which canoperate at room temperature by using the quantum
phenomenon of light. It has formed a joint researchteam with 14 research institutes in the US, including
universities (Stanford University, Massachusetts
Institute of Technology (MIT), Cornell University,
Harvard University, etc.) and national research institutes such as National Aeronautics and Space
Administration (NASA). In 2020, PHI Lab took up a1

View from the Top

new research theme, the “quantum approximate optimization algorithm,” and began joint research on itwith MIT, the University of Chicago, and the
University of Waterloo.
CIS Lab researches (i) strong cryptography that canmaintain security even if a quantum computer that
overrides the security of existing cryptography is created, (ii) high-functionality and flexibility encryption
technology that ensures the security of encrypted
information, and (iii) highly secure blockchains. In
the last two years, NTT has become the organization
with the highest number of accepted papers at Crypto
and EuroCrypt, the world’s most prestigious conferences in the field of cryptography. CIS Lab accounts
for about 70% of the papers accepted as NTT. Dr.
Brent Waters, who invented “attributed-based
encryption,” received the Test-of-Time Award fromthe International Association for Cryptologic
Research (IACR). Similarly, Dr. Mark Zhandry and
Dr. Brent Waters were awarded the Best Paper Awardat EuroCrypt 2019 and Crypto 2020, respectively.
MEI Lab aims to develop physician-support systems and autonomous therapeutic systems by constructing bio digital twins, which apply Digital Twin
Computing to the human body, as well as developing
micro/nano devices that can be implanted in thehuman body. MEI Lab has started joint research with
the Technical University of Munich on electrodes thatcan extract electrical signals from the human bodyand have experimentally implanted the electrodes inlaboratory animals (insects, etc.) to verify their effecNTT Technical Review

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tiveness.
—You have achieved a lot in a short period.
As chief operations officer and chief technology
officer, I oversee the entire staff of the laboratories,
and for the past two years, I’ve been focusing on
improving our basic R&D capabilities, technologicalcapabilities of the entire group, and corporate valueof the group. One of our most urgent tasks was toestablish the organization in regard to, for example,
recruitment of human resources and conclusion of
joint-research agreements with many universities andother institutions. The directors of PHI, CIS, and MEI
Labs are world-renowned and who have made significant achievements in their respective fields. By
using their human networks, we have invited outstanding researchers worldwide to work in researchareas that support the unique technologies that NTT
has accumulated and formed research teams that collaborate with researchers in the United States andabroad. The number of researchers has grown from adozen or so at the time of establishment to 38. Of
those researchers, 35 have doctorates, and two arelicensed physicians. We have strengthened diversity
over the past two years by hiring an increasing number of non-Japanese. We now have 12 from North
America, 2 from Europe, and 24 from Asia-Oceania,
half of whom are from India, Singapore, Australia,
and South Korea, and the other half are Japanese. We
have also established a branch in Munich, Germany,2View from the Top

to carry out joint research with the Technical
University of Munich.
We have set the following three key performanceindicators (KPIs): (i) improvement of our basic R&D
capabilities, (ii) improvement of the technical capabilities of the entire group, and (iii) improvement of
the corporate value of the group. Regarding (i), we
have set a target for the average number of papers
accepted per researcher per year as one and average
impact factor of each paper as 5.0 or more. Regarding(ii), we have set a target of applying for more than ten
patents per year and developing human resources bytaking on one to three people per year. Regarding(iii), we have set a basic-R&D-capability indicator
based on an average number of citations of 6.0 or
more for at least five years after the publication of apaper and holding a global R&D workshop once ayear. As a result of our efforts, I believe we have met
all three KPIs: (i) we have achieved a mean impact
factor of 6.3, (ii) accepted more than 25 interns fromuniversities in the US and abroad, and (iii) in
September 2021, we held a two-day global R&D
workshop that was a hybrid meeting of online andon-site, attracting a total of about 2000 views online
and about 130 participants on-site.
I believe that one of the main reasons we have beenable to achieve these results is because talented
researchers assemble and inspire each other centeredon world-class researchers having a unifying cohesive power. However, it is not enough to simply havethe world’s top-class group of researchers. It is also
necessary to have researchers who support the topclass researchers in each research field. In the quantum and medical fields, we need people who can
support experimental research. Therefore, we are also

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hiring young researchers such as those who have justcompleted their doctoral course. In light of these factors, I believe that we have created an almost ideal
environment for basic research.
From the perspective of using the results of basic
research for the benefit of society, applied research isnecessary; however, since NTT Research focuses onbasic research, we will consider how to transfer the
results of basic research to applied research.
Take a bird’s-eye view and find the excesses
and deficiencies—It is said that the location of work has become nolonger an issue during the coronavirus pandemic.
Under such circumstances, what is the significance ofNTT having research bases abroad?
NTT’s business has changed dramatically over the
past 20 years. Currently, NTT Group’s revenue isabout 12 trillion yen, of which about 2 trillion yen is
generated outside Japan as our business expands
globally, mainly through mergers and acquisitions(M&As).
When I was working at an NTT operating company
in Japan and approached overseas companies for
global business collaboration, they would ask,
“Where are you doing your R&D, Japan or the US?”At that time, I realized the importance of not only
taking the results of Japanese R&D overseas but also
conducting R&D and turning it into business at and in
the style of the location where the business would berolled out. In this sense, I believe that expanding ourglobal business through M&As and creating the technologies necessary for our business on a global scale
is an appropriate way for NTT to grow its global business. As you said, remote work has become widespread, and it is now possible to conduct businesswithout visiting the site. Business is fundamentally
based on communication between people, and fromthat viewpoint, as long as seamless communication isachieved, it might not matter whether business isdone face-to-face or remotely.
For R&D, however, the situation is a little different
from that of business. An important factor in conducting the best research is to have an environment withthe best resources (i.e., people and equipment).
Silicon Valley offers exactly such an environment,
and it is one of the reasons that NTT Research chose
to locate in Silicon Valley. Although there has beenmuch talk about the spread of remote work in thewake of the coronavirus pandemic, researchers have3

View from the Top

been using email as well as online discussions andother activities long before the pandemic. However,research activities require experimentation and verification. Accordingly, a research base should be established where the resources for experiments and verification are located. Silicon Valley has the best
resources needed for research at PHI, CIS, and MEI
Labs. When we talk about the necessity of establishing overseas bases, I believe whether the location isoverseas or in Japan is unimportant; instead, it isimportant to determine the essence of the research
activity and where to conduct such activity.
—Expectations concerning what NTT Research has
to offer are high. Do you have any difficulties in yourposition as a uniter of the world’s top researchers?
For me, life in the US is a good fit, and I’m happy
working here. I’ve had a rather unique career inNTT’s R&D labs. I first engaged in R&D as a
researcher for about nine years, then in business at an
operating company for about 15 years, and recently in
R&D management. When I switched from R&D tobusiness then to R&D management, I felt that everything I experienced in my previous job was useful in
my current job, and I was gradually understanding thetruth of things as I gained more experience. I feel thatthrough interaction with a variety of professions andpeople, I was able to recognize turning points andunderstand how to use experiences, and such accumulation of knowledge would lead to success. Of
course, there are some failures and disappointments,
but I am trying to focus on positive aspects of myexperiences.
From my experience, I’d say that the job of top
management is to create an environment in whichpeople can work happily, define the concept of operations, create a roadmap, and build a team by acquiring
human resources. Moreover, I think the most important role of management is to look at the whole picture from a bird’s-eye view and find out the excesses
and deficiencies from there. It is also important tochange the way of managing the organization
depending on the organization, mission, and qualityof work. For example, one time, I was seconded to a
subsidiary of Kadokawa Shoten Publishing Co. Ltd.,
whose mission was to develop and operate web services to ensure that work was carried out efficiently
to meet release dates for the weekly local guidemagazine and website, so a certain degree of strong
management was necessary. When I was the head of
NTT Media Intelligence Laboratories, however, it
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was important for the laboratory staff to come up withnew added value on their own, so strong management
was not necessary. Here at NTT Research, I don’t
think strong management is necessary because toomuch control can reduce the originality of researchers and their ability to advance R&D. I think as longas the direction of research does not deviate significantly, I’ll only focus on the results of the research.
NTT Research has been operating for only two
years, so I think we still have some omissions andshortcomings in the management of the organization.
I’d like to improve and revitalize our institute with amore comprehensive and bird’s-eye view of thewhole picture.
Management that makes everyone involved
feel confident—So management style differs completely dependingon the subject, right? What matters most to you in
business or management?
What matters most for me is to be of service to others, and that fundamental part of my life will neverchange. I think that work is what humans do, andwhat humans plan and do to achieve what they want
is work. I believe that even though our positions androles differ, work is about producing some kind of
result and having an impact on society.
With that in mind, what I aim to do is to try it
myself. I want to create an environment in which mycolleagues can all feel confident in their work.
Therefore, I’ll continue to challenge myself to do newthings while making myself, team, and customers all
happy through their work. To meet that challenge, I
first challenge myself and show them what I can do
4View from the Top

myself to some extent then ask them to challengethemselves to go beyond that. For example, when anyof my staff or teams were about to give up and say, “I
can’t do this,” I’d first try to work on the problem
myself and show them that they can solve it if they
try. Then I’d step back and let them to work on it.
—Do you have a message for researchers and engineers inside and outside the company?
The first is to challenge yourself every day. There
are many talented people in the world, and I want you
to aim to take on challenges with them. For example,
I heard that Sakichi Toyoda, the founder of Toyota
Industries Corp., the predecessor of Toyota Motor
Corporation, faced a lot of concerns from within thecompany when he decided to expand his businessoverseas. On hearing those concerns, he said, “Open
your mind, and look at the great world outside.” I
want many researchers and engineers to see the wide
world.
The common quality among world-class researchers is a strong spirit of inquiry. For example, I don’t
think Einstein started his research on relativity
because it would have a high impact factor, as wewould say today, and I’ve also heard that his papers
on relativity were first published in journals that
didn’t attract much attention. I encourage researchers
to conduct their research activities not simply to sub-NTT Technical Review

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mit papers to journals with high impact factor but toinvestigate the essence of their research problems. I
believe that what is most important in any job is topursue the truth.

Interviewee profile
Career highlights
Hideaki Ozawa joined Nippon Telegraph andTelephone Corporation (NTT) in 1991. After
engaging in research and practical application ofmultimedia processing technologies at NTT
Human Interface Laboratories and Cyber Solution
Laboratories, he joined NTT WEST (seconded toWalkerplus, Inc.) in 2000, where he was involvedin the provision of local multimedia information.
He then joined NTT Resonant Inc. in 2004, where
he worked on the development and managementof “goo” internet services including its searchengine and the establishment of mobile-searchbusiness, and became head of the Search Business
Division in 2011. After serving concurrently aspresident of NTT Resonant Technology, Inc.
from 2013, he became vice president, head of
NTT Media Intelligence Laboratories in 2015and head of the Global Business Promotion
Office at NTT TechnoCross Corporation in 2018.
He assumed his current position in June 2019.

5

Front-line Researchers

Where There Is Will, There Is a Way.
Researchers, Be Long-term Optimists
Shiro Saito
Senior Distinguished Researcher,
NTT Basic Research Laboratories
Overview
Quantum information technology is attracting attention as asource of technological innovation that will bring about majorchanges in the future economy and society. Countries around theworld, including Japan, the United States, Europe, and China, arepositioning research and development of quantum information
technology as one of their national strategies. We interviewed
Shiro Saito, a senior distinguished researcher at NTT Basic
Research Laboratories and who is making a significant contribution to the development of quantum information technology,
about the progress of his research and his attitude as a researcher.
Keywords: quantum information technology, superconducting flux qubit, macroscopic quantum phenomena

Research on new devices that contributes to thedevelopment of quantum information technology—Please tell us about your current research.
Quantum mechanics is applied in various devices—
ranging from semiconductor devices to lasers andmagnetic resonance imaging systems—and I am
researching quantum information technology using
superconducting quantum circuits. Quantum information technology provides high-performance functions unattainable with conventional technology byskillfully controlling quantum states, which are vulnerable to external noise and have a short lifetime.
Superconducting quantum circuits, which are composed of superconducting elements with quantum
properties, have been extensively researched with theaim of developing quantum computers. As a result of
the improvement in quantum-state-control techNTT Technical Review

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niques, applications of superconducting quantum
circuits to quantum devices other than quantum computers, such as quantum sensors, have begun to be
explored.
The origin of research on superconducting quantum
circuits is the question, “Does the violation of realismthat occurs in the world of quantum mechanics occur
in the macroscopic world we live in?” Clarifying thequestion will lead us to the creation of quantum computers that enable massively parallel computing without consuming high energy and quantum sensors thatenable highly sensitive sensing that exceeds the limitsof the past.
Realism or violation of realism means, for example,
if you roll a die under a paper cup, the die’s dots are
determined even if you do not observe them from theoutside, as shown in Fig. 1, and this phenomenon iscalled realism. However, in the world of quantum
mechanics, violation of realism occurs, that is, the6Front-line Researchers(a) Realism in the macroscopic world (seemingly obvious)
The dots of the die are determined even if you do not look at them.
Observation

Readout(b) Violation of realism in the macroscopic world
The dots of the die are not determined until you look at them.
Observation

Readout(c) Violation of realism in the
microscopic world(Quantum behavior)Electrone−
It is impossible to say which slit
the electron goes through when
it is not being observed.
(The origin of quantum
computation and communication)

Electron passing
through double-slit
L

R

+ R

L

Observation

Fig. 1. Realism and violation of realism in the macroscopic world and violation of realism in the microscopic world.

dots on the die under the cup are in a superposition
state and determined only when they are observedwith the eye by lifting the cup.
In an experiment we conducted, a single electron
was sent through a double-slit one by one. When we
took a number of measurements of the position of thesingle electron by using a detector placed at a certain
position behind the slit, the position of the electron
came together to form a wave pattern (i.e., interference fringes). This phenomenon can be explained asfollows. Each time the single electron passes througha double-slit, a state called “superposition” occurs,
namely, the electron is superposed between one
through the upper slit and the other through the lower
slit every time it is sent, and the place where the electrons gather is determined by the interference effect.
The interference fringe is a proof of the superposition
state, namely violation of realism. However, when we
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try to measure which slit the electron passed through,the interference fringe disappears. The travel direction of the electron is determined at the moment its
passage through the slit is measured, and the electron
does not travel in the direction to form the interference fringe. This is the consequence of realism.
Violation of realism has been confirmed in thenanometer-sized microscopic world of electrons,
atoms, etc., and the origin of research on superconducting quantum circuits has been whether such phenomenon occurs in the macroscopic world.
—I heard that you have achieved pioneering results.
Please tell us about your recent achievements.
In 2016, in collaboration with the University of
Illinois, USA, we successfully demonstrated violation of realism concerning superconducting currents
7Front-line Researchers(a) Experimental results
Difference due to incompleteness of observation
Main experimentControl experimentddg
Violation of realism

−0.10

−0.08

−0.06

−0.04

de
Realism

−0.02

0.00

Violation of realism

0.02

0.04

Difference in states due to the presence or absence of observation(b) Quantum superposition of current states
2 µm

2 µm

+

 =
+1

−1

Fig. 2. Experimental results concerning quantum superposition of current states.

using a superconducting flux qubit (Fig. 2). As I
mentioned with the earlier question, if the macroscopic world follows quantum mechanics, violationof realism should also occur. The question of whether
this violation occurs in the macroscopic world or
whether it is not owing to the limitations of quantum
mechanics (i.e., the macroscopic realism problem)has been an unsolved problem since the dawn ofquantum mechanics. A current of 170 nA (equivalent
to 1012 electrons flowing per second) flows through asuperconducting flux qubit. Our recent results indicate that realism is violated in this current, and ourdemonstration that quantum mechanics is applicableon the macroscopic scale, i.e., current states, will
significantly contribute to the field of fundamental
physics. It also confirms that superconducting flux
qubits can work as quantum devices that use true
quantum properties. The results were published in the
online edition of the British scientific journal Nature
Communications [1].
In 2019, we demonstrated electron-spin resonanceusing a superconducting flux qubit, which makes it
possible to analyze a small-volume sample containing several dozens of electron spins (Fig. 3).
This research on quantum sensing using superconducting qubits is part of the “Creation of Innovative
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Quantum Technology Platform Based on Advanced
Control of Quantum States” project supported by theJapan Science and Technology Agency’s Core
Research for Evolutional Science and Technology(CREST) program [2]. Electron-spin resonance spectroscopy is one analytical method for investigating
the properties of electron spins in materials and is
widely used for analysis of molecular structures.
However, analysis with a current electron-spin-resonance spectrometer requires a sample containing alarge number of electron spins, namely, about 1013,
and a sample volume of several milliliters. As a result,the samples that can be analyzed are limited. A superconducting flux qubit functions as a highly sensitive
magnetometer. We have demonstrated electron-spin
resonance spectroscopy by using this magnetometerto detect electron spins, which have the properties ofa tiny magnet. We succeeded in detecting about 400
electron spins in a 0.05-pL sample volume. The
development of a new electron-spin-resonance spectroscopy method that can be used to analyze a material sample containing a small number of spins in aminute volume is considered to contribute to a wide
range of fields from basic science to material characterization, bioanalysis, and medical applications.
This achievement was published in 2019 in the online
8

Front-line Researchers

Qubit-control
magnetic field

Qubit excitation

it

b
Qu

Spin-polarization
magnetic field

e

in
el

ate
av
str
ow
b
r
u
c
Mi
ns
ico
Sil
Spin excitation

50

0.5
0.0
−0.5
−1.0

45

−1.5
±1

−2.0
−2.5

Switching probability (%)

Qubit-readout
circuit

Magnetic-flux shift (m0)Spin sample

0
40

−3.0
2.7

2.8

2.9

3.0

3.1

Spin-excitation frequency (GHz)(a) Local-electron-spin-resonance spectrometer using flux qubit(b) Electron-spin-resonance spectra due to thenitrogen-vacancy (NV) center in diamond. The
inset shows the energy levels of the NV center.

Fig. 3. Demonstration of electron-spin resonance.

version of the British scientific journal Communications Physics [3]. Then, in 2020, the detection
volume reached 0.006 pL, and the detection sensitivity reached 20 electron spins [4].
We are currently working on the “Development ofIntegration Technologies for Superconducting Quantum Circuits” project as part of the national Moonshot Research & Development Program launched in
December 2020 entitled “Realization of a fault-tolerant universal quantum computer that will revolutionize economy, industry, and security by 2050” [5, 6].
Curiosity toward macroscopic
quantum phenomena—How do you feel about the expansion and fulfillment of your research field?
Looking at the field of superconducting quantum
circuits as a whole from the perspective of a pioneer,
I feel that this field is evolving ever more rapidly. In
the beginning, the activities were small scale andslow, but as the potential of this field was revealed,researchers from academia and industry, as well as
outstanding researchers in a wide variety of fields
other than superconductivity, joined the field. Thissituation has created an interesting trend; for example, when high-temperature superconductivity was invogue, researchers in the field of quantum circuits
studied high-temperature superconductivity as a primary theme then returned to quantum circuits toadvance them dramatically.
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Starting from pursuing the question of violation ofrealism by creating macroscopic superposition, in
2016, we were able to answer that question not only
phenomenologically but also mathematically. Duringthat time, quantum technology had matured, and thequantum field had become a focus of public attention.
Twenty years ago, only a handful of researchers were
working on superconducting quantum circuits; however, a huge number of groups are now working onthem, and new papers are being published every day.
On reading those papers, I feel a difference between
the volume of resources we have and those of theUSA, Europe, and China. In the field of quantum
computing, the USA—led by Google, IBM, and others—is investing heavily and seems to have the upper
hand.
Under these circumstances, I believe that Japanese
research institutes must join forces in the field of
quantum research. If each research lab were to workon its own, it would be left behind by the rest of theworld; therefore, it is necessary for each lab to takecharge of a research theme it has expertise in andcombine them to compete with the rest of the world.
Having played a pioneering role in this field, I try
to conduct research that is one step ahead and will be
useful in the future. My participation in the Moonshot
project on quantum error correction is one example ofsuch research. Although our resources in terms ofboth funding and human resources may be an order ofmagnitude less than those overseas, I believe that myrole is to draw a picture that attracts outstanding
researchers by effectively promoting new initiatives
9

Front-line Researchers

and using the achievements as material.
In drawing such a picture, I feel that it was very
meaningful to participate in the new projects of
CREST and Moonshot that I mentioned earlier.
Quantum technology is a hot topic attracting muchattention, and I think it is important to demonstrate
the unique characteristics of NTT.
I think that this field is now in the phase of thinking
about what can be done with new ideas. When considering the overall development of the field, I believethat we shouldn’t put too much pressure on researchers; instead, we should give them a certain degree of
freedom. If we put excessive pressure on them, theirideas will be meager, and that situation will affect thedevelopment of the field as a whole.

lems can be solved, I was unable to achieve successfor a while after joining the company. It was aroundthe time when the qubit I created had a short lifetimeand could not be superposed. Still believing that
“Where there is a will, there is a way” and that if I
make improvements steadily, the lifetime of the qubit
will be extended, I continued to improve the samples
and measurement system. Then, in the middle of thenight at the laboratory, I saw Rabi oscillations in thequbit signal indicating superposition. I was reallyexcited when I was able to confirm the data for the
first time; I even jumped for joy in our large, empty
gymnasium-like laboratory alone. I think experiencing that kind of success makes us expect to achieve
further success.

—What has supported you in your pursuit of quantum
technology for the past 20 years since you joinedNTT?

Cherish your connections with people

My starting point was curiosity about macroscopic
quantum phenomena. When I was at university, I
studied superconductivity; however, neither the environment nor the technology needed to investigate
macroscopic quantum phenomena was available, so I
joined NTT because I admired its environment andsystem. Researchers should be motivated to work on
something because they are interested in or because
there is an unknown in a particular field.
For example, the main reason that I’m devoted toquantum technology is that when I was a university
student, I admired the research of Prof. Yasunobu
Nakamura (who is currently at the University ofTokyo), who was the first person in the world to demonstrate quantum superposition in a superconducting
circuit. I was hooked by the wonder of creating superpositions in the macroscopic realm.
Prof. Nakamura is now in a position to coordinate
research on quantum computers in Japan, and I’m
now able to tackle such research from a different
angle. One time, Prof. Nakamura said to me, “Iremember a student who asked me questions in particular detail during a special lecture, and it turned out
it was you, Prof. Saito.” That explains how much I
admired Prof. Nakamura’s research. With that memory in mind, I try to convey to students who come to
NTT for training that research is an interesting world.
Perhaps the way of thinking behind the saying“Where there is a will, there is a way” can support our
research life. I think researchers need to be “longterm optimists.” Although I have always believed inthat saying and have always believed that any probNTT Technical Review

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—What do you keep in mind when determining a
research theme?
When determining a new theme, I first analyze thecurrent situation then select a theme that will allow
me to find a new method or novelty. For example, it
is important to (i) understand your strengths and thetrends being followed by other researchers in yourresearch area by comparing their strengths with yours
and (ii) narrow down the theme you should pursue
while considering the direction in which you cancome up with something new.
Although it depends on the research phase andfield, the current field of superconducting circuits, forexample, is quite broad, so it is necessary to survey
how far each area within it has progressed. The number of papers published in the field is considerable, so
it may be difficult to keep track of the overall
research. Even so, if you do not keep track properly,
you may go in a wasteful direction or even if you
produce results, they might not receive proper
appraisal.
I also tried my hand at creating macroscopic superposition by trial and error. However, time is not infinite, so I try to get a general idea of the direction of
the field of superconducting quantum circuits byreading papers, including references, published bygroups who are making progress in the field. The
references are particularly very useful for obtaining
an overview of the field since they contain most of therelevant papers in the field.
You should select the theme you are interested in.
Regarding macroscopic superposition, it is necessary
to extend the lifetime of the qubit. Therefore, I was
10

Front-line Researchers

excited to see that the lifetime of the qubit had recently begun to extend again, and that excitement confirmed that I do really like studying macroscopic
superposition.
In addition to being aware of this emotion, it is also
important to be interested in what is right and new.
For example, I’m exploring a new direction called“bosonic codes” and am applying my accumulated
knowledge of qubits to build a new quantum system
based on bosonic codes. I want to make the most of
the foundations that we have built but also want tolook for new things.
—What would you like to say to our young researchers?
Young researchers, cherish your connections withpeople. I say that because connecting with people
gives us new knowledge, and collaboration withresearchers in completely different fields can bringnew perspectives. For example, when we were trying
to develop a bosonic code in the Moonshot project, I
was looking to collaborate with researchers fabricating a high-Q (quality factor) cavity for storing bosonic qubit information and was able to connect withcertain collaborators as a result of casual conversation that took place in a symposium.
As another example, I was considering using diamond as a material that might improve the limit on
coherence time (i.e., the timescale over which thequantum information stored in the qubit is lost) of thesuperconducting qubit. Coincidentally, around thattime, a superior of mine casually started a conversation with a researcher sitting next to him at lunch

during a conference, and it turned out that the
researcher was an expert on diamond. Following theadvice given by that expert led us to achieve outstanding results. These experiences made me recognize theimportance of showing up at places where researchers gather and connecting with them. Therefore, I
hope that you will value your interactions with others—both inside and outside NTT.
During your research activities, you don’t have to
be bound by the purpose of your research. A researchgoal can have derivation and detours. There are manydifferent ways to achieve goals, and I think it is goodto have a variety of offshoots. If you put 90% of yourenergy into your research theme and the remaining
10% into something completely unrelated, you will
be able to broaden the scope your research. Convey topeople what you want to do. Opportunities even lie incasual conversations in daily life. I encourage you topursue a research theme that you are interested inwhile maintaining your curiosity.
References
[1]

[2]

[3]

[4]

[5]
[6]

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Vol. 19 No. 11 Nov. 2021

NTT R&D website, “Experimental Test of Macroscopic Realism
Problem Using a Superconducting Flux Qubit,” Nov. 2016.
https://www.rd.ntt/e/brl/latesttopics/2016/11/latest_topics_
201611042019.html
Website of CREST on “Creation of an innovative quantum technology
platform based on the advanced control of quantum states,”
https://www.jst.go.jp/kisoken/crest/en/research_area/ongoing/bunyah28-2.html
NTT R&D website, “Electron Spin Resonance Spectroscopy Using a
Superconducting Flux Qubit Featuring High Sensitivity and Spatial
Resolution,” Mar. 2019.
https://www.rd.ntt/e/brl/latesttopics/2019/03/latest_topics_
201903291552.html
R. P. Budoyo, K. Kakuyanagi, H. Toida, Y. Matsuzaki, and S. Saito,“Electron Spin Resonance with up to 20 Spin Sensitivity Measured
Using a Superconducting Flux Qubit,” Appl. Phys. Lett., Vol. 116, No.
19, 194001, 2020.
Website of Moonshot Goal 6: Realization of a fault-tolerant universal
quantum computer that will revolutionize economy, industry, andsecurity by 2050, https://www.jst.go.jp/moonshot/en/program/goal6/
Website of the Development of Integration Technologies for Superconducting Quantum Circuits project, https://ms-iscqc.jp/en/

11Front-line Researchers

Interviewee profile
Shiro Saito
Senior Distinguished Researcher and Group
Leader of Superconducting Quantum Circuits
Research Group, NTT Basic ResearchLaboratories.
He received a B.E., M.E., and Dr.Eng. inapplied physics from the University of Tokyo in
1995, 1997, and 2000. He joined NTT Basic
Research Laboratories in 2000. Since then he has
been engaged in quantum information processing
using superconducting circuits. He was a guest
researcher at Delft University of Technology
from 2005 to 2006 and a guest associate professor
at Tokyo University of Science from 2012 to2020, where he is currently a guest professor. He
was appointed as a distinguished researcher of
NTT in 2012 and senior distinguished researcher
in 2021. He is a member of the Physical Societyof Japan and the Japan Society of Applied
Physics.

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12

Rising Researchers

Studying Multimodal Interactions:
Understanding Dialogue Mechanisms
through Combined Observation of
Speech, Language, and Body
Movement
Ryo Ishii
Distinguished Researcher, NTT Human
Informatics Laboratories
Overview
Digital Twin Computing is one of the three key fields of technology in the IOWN (Innovative Optical and Wireless Network)
vision. In order to build a digital-world representation of a personthat includes not only their external aspects, but also their internal
aspects, such as consciousness and thought, it is essential tounderstand and model the mechanisms of human communication.
In this article, we speak to Distinguished Researcher Ryo Ishii,
who works with multimodal information including speech, language, and body movement with the aim of understanding themechanisms of human communication, for example in how aperson conveys their mental state.
Keywords: human communication, multimodal interaction, dialogue system

Communication is born of the interaction
between multiple modalities such as speechand gestures—What kind of research field is multimodal interaction?
When people talk with others, they communicate
using more than just their voice and language. Theyuse multiple modalities, such as gaze, facial expression, and gestures, communicating what we call
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“multimodal information.” These modalities are usedin combination, and the modalities influence eachother as people use them to communicate information. These interactions using multimodal information are called “multimodal interactions.” What’s
particularly important about these interactions is thatthe transmitted multimodal information is dealt withas a whole. For example, during a conversation someone might say, “Don’t joke about that.” Looking at
the written words alone, you can’t tell whether the
speaker is angry or joking. But when you listen to the13

Rising ResearchersA) Multimodal expression
generationB) Multimodal mental
state estimation

C) Recipient’s cognitive
state estimation

SpeechLanguage

Mental state(mindset/intention)

Body movement
SenderRecipientD) Methods supporting
communicationof mental stateFig. 1. The field of multimodal interaction.

tone of their voice and look at their facial expression,
you know that if they are speaking softly with a big
smile, they’re probably not serious. One major
research topic is comprehensively processing thismultimodal information to understand the intentions
and messages that people convey, and understanding
and modeling the mechanisms for how this information is transmitted between people. Another major
research topic in multimodal interactions is using thisunderstanding and these models to support communication between people and enable smooth communication between people and dialogue systems.
In this research area, I am carrying out a multifaceted study of mechanisms for expressing and recognizing multimodal information (Fig. 1). This
involves A) “multimodal expression generation,”
which is researching and modeling the mechanisms
for generating speech, language, and physical motion
to transmit the current mental state; B) “multimodal
mental state estimation,” which is estimating themental state of the sender through their speech, language, and body movement; and C) “recipient’s cognitive state estimation,” which is looking at how therecipient interprets the sender’s mental state. In addition, I am carrying out practical research into D)“methods supporting communication of mental
state,” which involves recognizing discrepancies
between the sender’s state of mind and their state ofmind as perceived by the recipient, and enabling thesender’s state of mind to be correctly conveyed to therecipient.
—What are you researching specifically?
As an example, one topic I worked on at NTT Communication Science Laboratories was predicting the
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next speaker and timing of speech. In the future, dialogue systems will need to quickly predict who
should talk next, enabling them to speak at the appropriate time during conversations with people. So, webuilt a model that predicts who will speak next andwhen, based on subtle behavior like people’s eye
movements, head movements, and breaths taken
before speaking. Since there were no existing studies
that measured people’s respiration during a conversation and analyzed it in relation to speech behavior,
our paper presenting this research was given an Outstanding Paper Award at the ACM International Conference on Multimodal Interaction, the leading conference in this research field.
In addition, we work closely with internal andexternal researchers to conduct a variety of research
related to multimodal interactions. This includes
work on the “MoPaCo Window Interface,” which creates three-dimensional video from video conferencing systems to encourage users to interact using natural gaze and body movement, “body movement generation technology,” which automatically generates
gestures based on the speaker’s speech and language,and “personal characteristic/skill estimation technology,” which accurately estimates the personal characteristics and communication skills of participants
based on speech, language, and image informationfrom their conversation.
—What are the current challenges you’re facing?
One useful approach is applying machine learningtechnology, which is already widely studied andused, to the field of multimodal interactions. This canhelp us understand and model how humans’ intentions are conveyed, and the mechanisms of how we
14

Rising Researcherstransmit information to each other. Machine learningtechnology generally requires a large amount of data
anyway, but when dealing with human communication, there’s a massive diversity of communication
methods due to the large number of variables, such as
different situations, number of people, cultures, relationships, and locations. This makes collecting thenecessary data a difficult task that requires a hugeamount of work.
For example, when gathering data from a conversation, the first step is to record the content of the
speech by listening to it and manually writing up thetiming and what was said. For example, “XX was
said between XX seconds and XX seconds” (althoughspeech recognition technology can be used for preprocessing). To some extent, we can automaticallygather data on facial expressions, gaze, and posturefrom images of the speakers. However, for somemodalities this method isn’t accurate enough, so each
image must be checked manually, by checking who’s
looking at who and labeling the eye movement, forexample. A one-second video usually consists ofabout 30 still images, which makes the manual process time consuming and cumbersome. For thesereasons, it takes a lot of work to construct corpus data
(in this case, a data group for dialogue research) containing multimodal information for human conversations, and in many cases, we’re unable to collect all of
the data and we have to conduct our research using
only a small amount of data from a particular situation.
Researchers nowadays say that once the data arecollected, the research is 70% or 80% done. It may be
a bit of an exaggeration, but collecting data is a very
important and very expensive task. Being able to efficiently collect large amounts of data from humanconversation is a major challenge, but if it is overcome, I think research in this field will take a hugeleap forward.
Aiming for an all-encompassing model
for communication—What are the plans for future research?
As a new approach, we are carrying out research
into modeling all aspects of human communication inorder to gain a deep understanding of the communication mechanisms. Current communication modeling technology defines a human behavior as input X
and a single state such as a human emotion, a personal characteristic, or a location as output Y, and
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only works to understand the relationship between
the two. These individual studies are very enlightening and interesting, but from the perspective of understanding and modeling the entire mechanism of
human communication, they only cover a tiny number of phenomena. I have started to work on forming
an integrated understanding and model of the relationships between the various phenomena that occur
in human communication.
As an example, a person may express (or “transmit”) multimodal information such as speech, language, and body movement through their actual
behavior, but before they get to that point, they will
have a certain internal mindset and intention. People
will also have individual characteristics, such as apersonality and values. In addition, the dialogue will
be influenced by personal relationships, roles, and theatmosphere during the conversation. Figure 2 showsa simple example of this relationship. We have modeled the entirety of natural communication in four
different layers: a “high-order layer” that holds people’s relationships, roles, and the atmosphere of theconversation; an “actual-behavior layer” for multimodal information transmitted and received throughactual behaviors such as speech, language, and body
movement; a “mental layer” that holds internal information such as people’s mindsets and intentions; anda “personal-characteristics layer” that holds information such as personality and values.
As mentioned above, there have been many studies
carried out in the past on things like using one behavior to predict the next, and estimating individual personal characteristics and relationships from behavior,
but essentially, communication can be thought of as amodel in which the four layers are interrelated and act
in combination with each other. In addition, Fig. 2
shows the communication status at one point in time,
but this model will change over time.
The current goal is to create an ultimate, all-encompassing model of communication by looking at therelationships between these layers and the changes
over time.
—What kinds of possibilities will this technology
unlock?
If we can create this ultimate, all-encompassing
model of communication, the system will not only beable to accurately understand the state of the conversation, but will also be able to predict and simulate
the future state. We expect this to enable three mainthings.
15

Rising Researchers

Personal
relationships

High-order layer

Atmosphere of
the conversation
Roles

B

A
Actual-behavior layerA
Mental layer

Personal-characteristics layerC

Speech/language/bodymovement, facial
expressions

B

C

Mindset/intention

Mindset/intention

Mindset/intention

A
Personal
characteristics

B
Personal
characteristics

C
Personal
characteristics

Fig. 2. Communication model devised by Dr. Ishii.

The first is the provision of real-time support forhuman communication. During a conversation, forexample, if someone is a little upset, the system canfollow up with them, or if one person is talking all thetime it can ask what others think to try and switch the
speaker and facilitate conversation, making the situation more harmonious and encouraging communication.
The second is the creation of the ultimate dialoguesystem. This dialogue system will be able to understand its conversation partner and the communication
situation, and communicate appropriately just like aperson.
The third is training people’s communication skills.
We are currently working on research into estimating
how well we compliment people and automating
training for this skill. Complimenting people is an
important skill, and many people are concerned about
how to do it best. With the all-encompassing communication model, it will be possible to evaluate how
effective a person is at complimenting their conversation partner based on a wide range of perspectives
such as their mindset and intention, their personal
characteristics, the circumstances of the conversation, and the personal characteristics of their partner.
Based on these evaluations, the system will give
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advice like, “A conversation partner with XX personality has XX intentions in XX situation, so complimenting XX more will help you build a better relationship.” The aim is to use this ability to create
applications that can improve people’s communication skills.
—Do you have any messages for people aiming tobecome researchers?
What I’m aiming toward and what I want to achieve
hasn’t changed much since I was a student. I think mycurrent research topic is my life’s work, and I’mpretty invested in it. I think it’s really important tospend your life in pursuit of what you want to achieve,and not to give up. There are opportunities forresearch and development in universities, companies,
and various other places, but I don’t think there’s any
environment where you will be 100% satisfied. At
times, you may have to work on research topics that
don’t match your own aims. I think it’s a real waste if
you let yourself get dissatisfied in that kind of situation, and lose your motivation for research and yourappetite for developing your personal skills. Whatever situation you find yourself in, you should think
hard about what you can learn to help you reach your16

Rising ResearchersThat means a single area of expertise alone is notenough for the creation of good technology, and awide range of expertise is required. It’s not easy togain expertise across such a broad area, and this is
still a new field that’s under development. In order tobreak new ground in this field and succeed as a
researcher, you need a strong desire to understand themechanisms of human communication and to change
the world through these new interaction technologies.
You may not have the necessary expertise at the start,
but I think if you have a strong drive to get stuck into
the research, you’ll be able to succeed in this field.
final goal, how to achieve results efficiently, and how
to make the most of your own abilities to succeed.
And I think it’s very important to build up yourachievements one by one. In doing so, I believe that
opportunities to pursue the research topics you’reinterested in will come around, and you will also haveopportunities to start up the research yourself.
I also think it’s important to involve other people. I
am currently conducting four joint research projects
with researchers from outside the company, and I
have lots of other talented colleagues outside thecompany who I can work together with to help further
my research. There are real limits to what you canachieve on your own. I think it’s very important thatyou build up a group of colleagues who can work
together to help each other achieve their goals.
I think multimodal interaction is a great researcharea for those who like people and are interested incommunication. Multimodal interaction is an interdisciplinary field that involves a wide range of academic areas, including humanities and engineering.

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Vol. 19 No. 11 Nov. 2021

Interviewee profile
Ryo Ishii
Distinguished Researcher, NTT Human
Informatics Laboratories.
He received a B.S. and M.S. in computer andinformation sciences from Tokyo University ofAgriculture and Technology in 2006 and 2008,
and a Ph.D. in informatics from Kyoto University
in 2013. Since joining Nippon Telegraph andTelephone Corporation (NTT) in 2008, heworked for NTT Cyber Space Laboratories(2008–2012), NTT Communication Science
Laboratories (2012–2016), NTT MediaIntelligence Laboratories (2016–2021, including
as a visiting researcher at Carnegie Mellon
University from 2019–2020). He has been withNTT Human Informatics Laboratories since July
2021 and with NTT Digital Twin Computing
Research Center since January 2021.

17

Feature Articles: NTT DOCOMO’s Initiatives on 5G
evolution & 6G

Trends and Target Implementationsfor 5G evolution & 6G
Yoshihisa Kishiyama, Satoshi Suyama,
and Satoshi Nagata
Abstract

In Japan, commercial 5th-generation mobile communication systems (5G) services first became available in March 2020. Studies of the next generation of communication services (6G) and the telecommunication technology of the 2030s are now gathering momentum. This article provides a summary ofthe domestic and international trends and schedule prospects for 6G research and development, and the5G evolution & 6G concept proposed in the DOCOMO 6G White Paper.
Keywords: 5G evolution & 6G, requirements, wireless technology

1. Introduction
Mobile communication systems have been continuously developing and evolving, with a new generationof systems coming out roughly once every decade. In
the 1980s and 1990s, the 1st and 2nd generations
mobile communication systems (1G and 2G) mostly
supported voice calls, with some support for simple
messaging functions. With the arrival of 3G in the
2000s, it became possible for anyone to access multimedia content such as photos, music, and video. And
from 2010, the launch of 4G with Long Term Evolution (LTE) technology capable of speeds in excess of
100 Mbit/s supported the explosive spread of smartphones. Then in March 2020, Japan’s first 5G services were launched, offering maximum transmission
speeds of over 4 Gbit/s.
5G includes technical advances such as high speed/large capacity communication, low latency, and theability to connect to multiple terminals simultaneously. It not only improves on the multimedia communication capabilities of 4G, but is also expected tocreate new value as an infrastructure technology forbusiness and society in the fields of artificial intelligence (AI) and the Internet of Things (IoT). In particular, the combination of 5G and AI technology is
expected to lead to the creation of new services andsolutions in diverse industrial fields by enhancing
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“cyber-physical fusion”*1 whereby the real world isrecreated in cyberspace to facilitate future predictions
and the acquisition of new knowledge. Since thistrend is likely to continue until the 2030s, it is necessary to promote research and development so that 5G
evolution and 6G can provide the fundamental technological infrastructure for industry and society inthe 2030s. This article presents a summary of thedomestic and international trends in 6G technology,
and the expected schedules for the introduction ofthis technology. It also discusses the 5G evolution &6G concept proposed in the DOCOMO 6G White
Paper [1].
2. 6G trends and schedules
Figure 1 shows the development history of 5G andthe schedule for the introduction of 6G. Following thelaunch of 4G LTE services in 2010, NTT DOCOMO
began studying 5G with the aim of implementing
services by around 2020. In our 5G White Paper of2014, we announced the start of 5G demonstration
experiments in cooperation with major global vendors. Discussions on the international standardization*1 Cyber-physical fusion: Services and systems for realizing a betterand more advanced society by collecting information in real
space (physical space) from various sensors, etc. and linking it tovirtual space (cyberspace).

18

Feature Articles

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

DOCOMO 5G White Paper published
DOCOMO 5G (FRA)
Presentation at an early
concept conference

5G

METIS launched(Europe)

5G demonstration experiments

DOCOMO 5G
Start of experiments
announced

ARIB 20B AH
launched

3GPP standardization

5GMF
launched

System development

5G
commercialization

Basic concept
Study project launched
Demonstration experiments

Standardization
System development

2017

2018

2019

DOCOMO B5G
Presentation at an early
concept conference
6Genesislaunched (Europe)

2020

2021

2022

VersionVersion Version
1.0
3.0
2.0
DOCOMO 6G White Paper published
Next G Alliance (USA)

Promotion of Beyond 5G
Strategy meeting

Beyond 5G Promotion
Consortium

Basic concept

2023

2024

2025

2026

2027

WRC23

2028

2029

2030

WRC27

6G

Commercial 6G
services?

Study project launched
Demonstration experiments – Standardization – System development

ARIB 20B AH: Association of Radio Industries and Businesses 2020 and Beyond Ad Hoc
5GMF: The Fifth Generation Mobile Communication Promotion Forum
FRA: Future Radio Access
METIS: Mobile and wireless communications Enablers for the Twenty-twenty Information Society
WRC: World Radiocommunication Conference

Fig. 1. 5G development history and 6G schedule.

of 5G began at the 3rd Generation Partnership Project(3GPP) in around 2015, and the first commercial 5G
services were launched overseas in 2019 based on the
Release 15 specification (the first international 5G
standard) [2].
In contrast, worldwide discussions on the standardization of 6G for the 2030s tended to start earlier. Thiscan be attributed to the impact of global competitionin the development of 5G. Projects studying 5G inJapan and overseas gradually took shape from around2012, which preceded the launch of 5G by about 8
years. On the other hand, discussions related to 6G
started in around 2018, which precedes the expectedlaunch by 12 years. These include the 6Genesis Project led by Oulu University in Finland, and the efforts
being made in the United States following thenPresident Trump’s call for stronger efforts in thedevelopment of 6G in 2019 and the decision by theFederal Communications Commission to make terahertz waves*2 available for research purposes [3]. In
Japan, the Ministry of Internal Affairs and Communications (MIC) launched the Beyond 5G Promotion
Strategy Roundtable in January 2020 to formulate acomprehensive strategy for Beyond 5G*3, and pubNTT Technical Review

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lished a roadmap outlining the expectations for telecommunications infrastructure in the 2030s and the
direction of policies to achieve these targets [4]. Thenin December 2020, the Beyond 5G Promotion Consortium was established to promote strong and proactive collaboration between industry, academia, andgovernment on 6G technology [5].
At NTT DOCOMO, we have been studying Beyond
5G since around 2017 [6], and released the first version of the DOCOMO 6G White Paper in January
2020. This is currently being updated to version 3.0[1]. In addition, research institutes and major vendors
in Japan and other countries have also released a slewof white papers related to Beyond 5G and 6G. Compared to the launch of 5G studies, it can be seen that
there is a more positive trend towards 6G around theworld.
In the future, it is expected that demonstration*2 Terahertz waves: Electromagnetic waves with a frequency ofaround 1 THz. Often used to refer to frequencies ranging from
100 GHz to 10 THz.
*3 Beyond 5G: A term that is widely used to describe wireless communication systems that emerge after 5G. It is almost synonymous with “6G.”19Feature Articles

6G

Extreme-high-speed andhigh-capacity communication(Section 3.1)

Extreme low latency(Section 3.4)
• Extreme low E2E latency of 1 ms or less• Consistently stable low latency• Improvement of communication speed:
Peak speeds of over 100 Gbit/s• 100 times greater capacity (bit/s/m2)• Ultra-large uplink capacity
eMBB

5G

Extreme coverage extension(Section 3.2)
• 100% land (area) coverage• Also seek to provide coverage in the sky(up to 10,000 m altitude), at sea (up to 200
nautical miles offshore) and in space
Extreme low power
consumption and cost
reduction (Section 3.3)
• Further reduction of cost per bit• Extreme low power devices that do not
require recharging

URLLC

mMTC

Combination ofrequirements with newuse cases

Extreme-reliablecommunication (Section 3.5)
• Quality assurance for a wide range ofuse cases (reliability improved to sevennines)• High level of security and safety
Extreme massive
connectivity & sensing(Section 3.6)
• 10 million devices per square kilometer• High-precision positioning and sensing(<1 cm)Fig. 2. Requirements for 6G.

experiments and international standardization efforts
will be promoted with the aim of introducing 6G
commercial services by 2030.
3. 5G evolution & 6G target implementations
In the following, we will describe the concept of 5G
evolution & 6G as proposed in the DOCOMO 6G
White Paper. Figure 2 shows the six requirementsthat we aim to achieve in 6G based on 5G evolution.
These include the requirement for further performance enhancements compared with 5G, and are
expected to cover a wider range of features in new
areas that are not supported by 5G or earlier systems.
These requirements and their expected use cases are
summarized below.
3.1 E
 xtreme-high-speed and high-capacity communication
Communication systems with higher communication speeds and larger communication capacities areuniversal requirements for all generations of mobile
communication systems. With 6G, it should be possible to combine extremely high speed and ultra-large
capacity so that large numbers of users can enjoy
services at the same time. As communication speeds
approach the speed at which information is processed
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by the human brain, it should become possible torealize not only video (visual and auditory) transmission, but also sensory communication that conveys asense of reality by involving the five senses, and“multi-sensory communication” that includes othersensations such as atmosphere and a sense of security.
In order to implement these unprecedented extremehigh-speed and high-capacity communication services, user interfaces will have to exceed what can be
implemented on a smartphone. For example, it isexpected that new interface technologies will evolveto support features such as three-dimensional holographic playback and wearable devices such as eyeglass-type terminals. It is also expected to be possible
to share these new sensory-type services among
multiple users in real time through the use of ultrahigh capacity communication, thereby facilitating
new synchronized applications such as shared experiences and cooperative work in cyberspace. Furthermore, if industrial use cases and trends such as cyberphysical fusion are taken into consideration, it isparticularly important to achieve much higher speeds
and capacities in the uplink*4 because it will be necessary to transmit diverse real-world information in realtime to the cloud and AI processes that constitute the*4 Uplink: The flow of information from terminals to the network.

20

Feature Articles“brains” of the network.
3.2 Extreme coverage extension
In the future, communications will become as ubiquitous as the air, and will provide a lifeline of equal oreven greater importance than electricity and watersupplies. 6G should therefore aim to provide themaximum possible service area so that mobile communications services are available everywhere. For
this reason, the aim is to provide 100% coverage over
all the world’s lands. With the establishment of communication areas in other environments and the commercial development of space, there are also plans toextend this coverage to include air, sea, and spaceenvironments where existing mobile communication
systems do not operate. As a result, we can expect
further expansion of the environment for activities
involving people and things, and the creation of new
industries. Promising examples include logistics
applications such as drone home delivery, and the useof unmanned and/or more sophisticated technology
in primary industries such as agriculture, forestry,
and fisheries. In the 2030s, it could also be applied tomore futuristic use cases such as flying cars, spacetravel, and underwater travel.
3.3 E
 xtreme low power consumption and cost
reduction
Reducing the power consumption and cost of networks and terminals in mobile communication systems is an important challenge for realizing the sustainable society that the world needs in order toaddress global environmental issues. Assuming that
network traffic will continue to increase in the future,
we aim to significantly reduce the per-bit power consumption and cost of communication. For example, if
communication traffic increases a hundredfold, theper-bit capital investment and operating costs should
be reduced to less than 1/100th to achieve both highperformance and economy.
Furthermore, there are also expectations that theterminal devices of the future will not require charging due to the development of power supply technology using wireless signals and technology for reducing the power consumption of devices. The need forthis technology will become even greater if (asexpected) the number of terminals such as sensor
devices grows due to the increased sophistication of
cyber-physical fusion and the growth of use cases
involving wearable user interfaces.

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3.4 Extreme low latency
In cyber-physical fusion, the wireless communication that connects between AI processes and devices
is equivalent to the nerves that transmit informationin the human body. To implement more advanced
remote services based on real-time interactive AI, abasic requirement is end-to-end (E2E) communication with low latency that is always stable. Our goal
is to achieve E2E with extremely low latency of 1 ms
or less. This will enable lag-free services to provide
immediate feedback from cyberspace, and will allow
robots and other devices that are remotely controlled
by AI to approach or exceed the capabilities ofhumans in terms of performing agile movements and/or understanding subtle cues. For example, a robot
controlled remotely by AI may be able to instantly
determine a user’s needs based on cues such as theuser’s tone of voice and facial expression, allowing it
to respond with at least the same level of consideration as a human would be able to achieve. This could
be particularly important in the post-coronavirus era,
when extreme low-latency communication will beessential in such fields as teleworking, remote control, telemedicine, and distance learning.
3.5 Extreme-reliable communication
When radio communication is used for industrial
and lifeline applications, reliability is a key requirement. In particular, in some industrial use cases, suchas the remote control of industrial equipment andfactory automation, the quality and availability ofcommunication have a significant impact on safety
and productivity. Extreme-reliable communication istherefore an important prerequisite for ensuring therequired levels of performance and safety, and 6G is
expected to surpass 5G in terms of reliability andsecurity. For ultra-reliable and low latency communications (URLLC), researchers are studying how toachieve a “six nines” level of reliability (99.9999%)
in 5G. For 6G, a target of “seven nines” (99.99999%)
is assumed.
Attention is also being drawn to non-public networks that are specialized for industrial use anddepart from the best-effort services of public networks such as private 5G. URLLC technology ismainly being considered for limited areas such as
factories. On the other hand, in the future, as robots
and drones become more widespread and wirelesscoverage expands to the air, sea, and space, it will benecessary to provide reliable communication over a
wider area.

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Feature Articles

Performance

6G (= ?)
5G (= eLTE + NR)

NR
4G
3G
2G
1G

TDMA

W-CDMA

OFDM-based
MIMO
Turbo coding
IoT

FDMA

FutureRAT?
eNR?

OFDM-based and/or new waveform
cmW & mmW & THz
OFDM-based
Extreme coverage
cmW & mmW
New network topology
Massive MIMO
Further enhanced Massive MIMOLDPC/Polar coding
Enhanced URLLC/mMTC
URLLC/mMTC
AI for everywhere
eLTE

Generation

FDMA: frequency-division multiple access
LDPC: low-density parity-check coding
mMTC: massive machine type communications
TDMA: time-division multiple access
W-CDMA: Wideband Code Division Multiple Access

Satisfy 6G requirements and use cases bycombining more wireless technologies

Fig. 3. Technological development of mobile communication systems towards 6G.

3.6 Extreme massive connectivity & sensing
With advances in cyber-physical fusion, there is
expected to be a massive proliferation of communication-related devices used by people and things, and it
is considered that 6G will have to support ten times
the connectivity of 5G (i.e., 10 million devices perkm2). For human users, there are expected to be use
cases in which cyberspace provides real-time support
for people’s thoughts and actions via wearable devices and micro-devices attached to the body. It is also
expected that cyberspace will provide links between
all manner of things, including cars and other vehicles, construction machinery, machine tools, surveillance cameras, and diverse sensors. This will make itpossible to realize a world where cyberspace supports
industry and transportation, provides solutions tosocial issues, and helps people to enjoy safe, secure,and affluent lifestyles.
Furthermore, the wireless communication network
will itself be equipped with functions for sensing thereal world, such as using radio waves to measure theposition of terminals and detect surrounding objects.
These position measurements are expected to achieveultra-high accuracy with an error of no more than a
few centimeters in some environments. In wirelesssensing, it is expected that the combined use of radio
waves and AI technology will be able to support
object identification and behavior recognition in
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Vol. 19 No. 11 Nov. 2021

addition to highly accurate object detection.
4. Development of wireless technology in 5G
evolution & 6G
Figure 3 illustrates the development of technology
from previous mobile communication generations to
6G. Earlier generations had a single representativetechnology (RAT: radio access technology)*5 thatthey used for radio access, but since 4G, mobile communication has used multiple technologies based onorthogonal frequency division multiplexing(OFDM).*6 As a result, RAT now comprises a mixture of wireless technologies, resulting in extended
technological development. This is because OFDMbased radio technology has already achieved frequency utilization efficiency*7 close to the Shannon
limit*8, while the requirements for mobile communication systems, frequency bands, and use cases are*5 RAT: Radio access technologies such as New Radio, LTE, WCDMA, and GSM (Global System for Mobile Communications).
*6 OFDM: A digital modulation method where information is divided into multiple orthogonal carrier waves and sent in parallel
making for high spectral efficiency in transmission.
*7 Frequency utilization efficiency: The number of bits of information that can be sent per unit time and unit bandwidth.
*8 Shannon limit: The theoretical maximum rate at which information can be transmitted through a communication channel of agiven bandwidth and signal-to-noise ratio.

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Feature Articles

Distributed network enhancementin the spatial domain(New Radio Network Topology)

Further enlargement of the frequency
domain and advanced technology
for frequency utilization

Enhancement of coverage
including non-terrestrial networks

Further advancement of Massive
MIMO technology and wirelesstransmission technology

Extension of URLLC andindustrial networks

Multi-functionalization of wirelesscommunication systems and useof AI technology in all areas

Integration of other wireless communication
technologies besides
mobile communication

Network architecture

1G

3G 10G 30G 100G 300G

Fig. 4. Technology areas that need to be considered for 5G evolution & 6G.

continuously being expanded.
Therefore, 6G will require the combination of evenmore radio technologies after 5G evolution, and it
will also be necessary to consider how these combinations can be extended to include technologies otherthan mobile communication in order to fulfil theabove requirements and various use cases. In addition, whereas 5G was defined by the combination of
LTE enhancements and New Radio (NR)*9, the NR
aspect of 5G is designed to be highly expandable toaccommodate the future introduction of new technologies. It will therefore be necessary to discuss thedefinition of RAT in 6G.
The technology areas that need to be considered for5G evolution & 6G are shown in Fig. 4 [1]. With anadvanced spatially distributed network technology(New Radio Network Topology), communication
will be performed via the shortest possible distanceand by line of sight (the path of least loss) wherever
possible. Also, as many communication paths as possible will be created to provide a broader selection ofpaths (greater redundancy). In this way, we will pursue wireless communication with extreme highspeed, high capacity (especially in uplinks) and
improved reliability. To achieve this, we need to figure out how to economically implement a distributed
antenna deployment to build a distributed wirelessnetwork topology*10 in the spatial domain.
This extended coverage technology will include
non-terrestrial networks (NTNs) by incorporating
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Vol. 19 No. 11 Nov. 2021

geostationary satellites, low-orbit satellites, and highaltitude platform stations (HAPSs), allowing it toprovide coverage in remote mountainous areas, out atsea, and even in outer space. 3GPP has already begun
studying the use of satellite and HAPS systems to
expend NR to NTNs.
For further expansion in the frequency domain andtechnology for making more advanced use of frequency resources, we will establish wireless technology for 6G that is capable of working with millimeterwaves and terahertz waves in the 100 to 300 GHz
range (above the frequency bands used by 5G). To
study these frequency bands, we will also need toclarify their radio wave propagation characteristics,
build propagation models, and address any technicalissues that arise in devices using these frequencies.
With the multifunctional use of wireless communication systems and the pervasive use of AI technology, it will be possible to analyze not only information
obtained by radio waves, but also video pictures andinformation obtained by diverse forms of sensing,
thereby facilitating various benefits, including
advanced control of wireless communication, highprecision measurements of positions and distances,
object detection and wireless charging systems.
*9 NR: A radio system standard formulated for 5G. Compared with
4G, it enables faster communication by utilizing high frequency
bands (e.g., 3.7 GHz and 28 GHz bands), and low latency andhighly reliable communication for achieving advanced IoT.
*10 Topology: The location and network configuration of devices.

23

Feature Articles

We will also need to carry out a fundamental reviewof the 6G network architecture in order to accommodate additional future requirements and keep abreastof changes in the market, while considering how to
optimize the deployment of functions and the generalization of equipment across the entire network, so
there are still many network design issues that need tobe resolved.
Although this article has skimmed over the detailsof each technical area, discussions of related researchand development activities can be found elsewhere inthis issue [7–9].
5. Conclusion

References
[1]
[2]
[3]

[4]

[5]
[6]
[7]

In this article, we have presented an outline ofdomestic and international trends and schedule prospects for 5G evolution & 6G, and the concepts proposed in the DOCOMO 6G White Paper. At present,
studies are being vigorously pursued by the Beyond
5G Promotion Consortium and 6G-related projects inJapan and overseas, and we hope to continue contributing to discussions of 6G among various stakeholders from industry, academia, and government.
This article is the reproduction of the special article
published in NTT DOCOMO Technical Journal (Vol.
23, No. 2, Oct. 2021).

[8]

[9]

NTT DOCOMO, “White Paper: 5G evolution & 6G,” Jan. 2020.
https://www.nttdocomo.co.jp/english/corporate/technology/
whitepaper_6g/
Y. Kishiyama and S. Suyama, “History of 5G Initiatives,” NTT
DOCOMO Technical Journal, Vol. 22, No. 2, pp. 4–12, Oct. 2020.
MIC, “White Paper on Information and Communications in Japan
2020,” Aug. 2020.
h t t p s : / / w w w. s o u m u . g o . j p / m a i n _ s o s i k i / j o h o _ t s u s i n / e n g /whitepaper/2020/index.html
MIC, Official announcement, “Beyond 5G Promotion Strategy;
Roadmap towards 6G,” June 2020.
https://www.soumu.go.jp/main_sosiki/joho_tsusin/eng/presentation/
pdf/Beyond_5G_Promotion_Strategy-Roadmap_towards_6G-.pdf
Website of the Beyond 5G Promotion Consortium, https://b5g.jp/en/
Y. Kishiyama, “Initial Considerations for Beyond 5G Radio AccessTechnology,” IEICE Society Conference, BS-2-2, Sept. 2017.
D. Kitayama, Y. Hama, K. Miyachi, and Y. Kishiyama, “Research ofTransparent RIS Technology toward 5G evolution & 6G,” NTT Technical Review, Vol. 19, No. 11, pp. 26–34, Nov. 2021.
h t t p s : / / n t t - r ev i ew. j p / a r c h ive / n t t t e c h n i c a l . p h p ? c o n t e n t s =ntr202111fa2.html
Y. Hokazono, Y. Kishiyama, and T. Asai, “Research on NTN Technology for 5G evolution & 6G,” NTT Technical Review, Vol. 19, No. 11,
pp. 35–43, Nov. 2021.
h t t p s : / / n t t - r ev i ew. j p / a r c h ive / n t t t e c h n i c a l . p h p ? c o n t e n t s =ntr202111fa3.html
T. Okuyama, S. Suyama, N. Nonaka, and T. Asai, “Improving Communication Performance in High-mobility Environments by Millimeter-wave Base Station Cooperation for 5G evolution,” NTT Technical
Review, Vol. 19, No. 11, pp. 44–51, Nov. 2021.
h t t p s : / / n t t - r ev i ew. j p / a r c h ive / n t t t e c h n i c a l . p h p ? c o n t e n t s =ntr202111fa4.html

Trademark notes

All company names or names of products, software, and services appearing
in this article are trademarks or registered trademarks of their respective
owners.

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Vol. 19 No. 11 Nov. 2021

24

Feature Articles

Yoshihisa Kishiyama

Manager, 6G-IOWN Promotion Department,
NTT DOCOMO, INC.
He received a B.E., M.E., and Ph.D. from Hokkaido University in 1998, 2000, and 2010. Sincejoining NTT DOCOMO in 2000, he has been
involved in the research and development of 4G
and 5G radio access technologies, including
concept development, standardization, and trials.
He is currently working on “5G evolution & 6G”
and is the main author of “DOCOMO 6G White
Paper.” He has more than 400 granted patents inthe area of mobile communications. In 2012, hereceived the ITU Association of Japan Award forhis global contributions to LTE.

Satoshi Suyama

Satoshi Nagata

Senior Manager, 6G-IOWN Promotion Department, NTT DOCOMO, INC.
He received a B.E. and M.E. from Tokyo Institute of Technology in 2001 and 2003. In 2003, hejoined NTT DOCOMO, where he was engaged
in research and development of wireless accesstechnologies for LTE, LTE-Advanced, and 5G.
He is currently a manager working for 5G and
3GPP standardization. He has been contributing
to 3GPP over 10 years. He served as a vice chairof 3GPP TSG-RAN WG1 from November 2011
to August 2013, and a chair from August 2013 toAugust 2017. He also served as a vice chair of
3GPP TSG-RAN from March 2017 to March
2021. He has been a vice chair of 3GPP TSG-SA
since March 2021.

Manager, 6G-IOWN Promotion Department,
NTT DOCOMO, INC.
He received a B.E., M.E., and Dr.Eng. from
Tokyo Institute of Technology in 1999, 2001, and2010. From 2001 to 2013, he was an assistantprofessor at Tokyo Institute of Technology. Sincejoining NTT DOCOMO in 2013, he has been
involved in research and development of 5G/6G
radio access technologies. He received the Best
Paper Prize from the European Wireless Technology Conference (EuWiT) in 2009, the Best Paper
Award from the International Symposium on
Personal, Indoor and Mobile Radio Communications (PIMRC) in 2016, and the Paper Award
from the Institute of Electronics, Information andCommunication Engineers (IEICE) in 2012.

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25

Feature Articles: NTT DOCOMO’s Initiatives on 5G
evolution & 6G

Research of Transparent RIS
Technology toward 5G evolution &6G
Daisuke Kitayama, Yuto Hama, Kensuke Miyachi,
and Yoshihisa Kishiyama
Abstract

The introduction of the 5th-generation mobile communications system (5G) commercial services has
begun throughout the world, and at present, research toward the further development of 5G (5G evolution) and the development of 6G as the next-generation mobile communications system is being actively pursued. In this article, we describe the intelligent radio environment (IRE), an important concept in“New Radio Network Topology” now under discussion on the road to 5G evolution & 6G. We also
describe NTT DOCOMO’s initiatives toward the reconfigurable intelligent surface (RIS), an importantcomponent of IRE, and metamaterial/metasurface technologies as elemental technologies of RIS.
Keywords: 5G evolution, millimeter wave, metasurface

1. Introduction
In Japan, NTT DOCOMO launched commercial
services of the 5th-generation mobile communications system (5G) in March 2020. This launch has
raised expectations for the application of 5G technology to XR*1 such as virtual reality (VR), augmented
reality (AR), and mixed reality (MR) and to a variety
of fields as in industry/infrastructure enhancement
through Internet of Things (IoT) devices [1]. Against
this background, NTT DOCOMO has come to demonstrate through field experiments the potential of 5G
for achieving high-speed/high-capacity, low-latency,
and high-reliability communications using radio signals in the millimeter-wave*2 frequency band [2, 3].
However, it became clear through these field experiments that problems existed in the effective use of
millimeter waves in radio communications of a cellular system. With millimeter waves, radio signals
have a strong tendency to propagate in a straight line
much like light, so their ability to wrap around shielding objects (diffraction) is small. It can therefore be
said that the key to using millimeter waves in radio
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communications of a cellular system is determining
how to expand the area covered by a base-station
antenna to non-line-of-sight locations.
In this article, we first present the concept of theintelligent radio environment (IRE) that is now
attracting attention as a useful approach to forming
coverage areas in the millimeter-wave band, an
important issue in achieving 5G evolution & 6G. We
then turn to NTT DOCOMO initiatives surrounding
the reconfigurable intelligent surface (RIS) as key toachieving IRE and describe metamaterial*3/metasurface*4 technologies, the elemental technologies of RIS.
*1 XR: Generic name for technology that provides new experiencesby merging virtual space and real space as in VR, AR, and MR.
*2 Millimeter waves: Radio signals in the frequency band from 30
GHz to 300 GHz as well as the 28 GHz band targeted by 5G allof which are customarily called millimeter waves.
*3 Metamaterial: An artificial material that causes electromagnetic
waves to behave in ways that they do not in natural materials.
*4 Metasurface: Artificial surface technology. As a type of artificial
medium (metamaterial (see *3)), it can achieve an arbitrary dielectric constant and magnetic permeability through a two-dimensional periodic arrangement of structures each smaller than
the wavelength of the propagating wave.

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Tx

Propagation channel (H)

Rx

: RIS

Propagation channel (H)Tx

Estimation
Control

Rx

Control

Optimization

Achieved by RIS

Fig. 1. Conceptual diagram of IRE.

2. IRE and RIS
2.1 IRE
Research toward IRE has been quite active in recent
years with the aim of adaptively and dynamicallycontrolling the radio environment to achieve nonline-of-sight coverage in the millimeter-wave band[4]. In a white paper [1] issued by NTT DOCOMO
toward 6G, a New Radio Network Topology was proposed to increase the number of connection paths tothe network including discussions on controlling theradio environment. In this regard, it is difficult tosolve the problem of radio waves being blocked byshielding objects simply through the evolution oftransmitter/receiver technology, so the need is felt for
constructing a new radio network system.
Against this background, means of achieving a dramatic jump in radio network performance are being
vigorously studied by breaking with the traditional
assumption that the radio environment is uncontrollable and treating it instead as a controllable entity.
This type of approach has come to be called the intelligent radio environment (IRE) or smart radio environment (SRE) to emphasize its conceptual difference with the conventional radio network system. A
conceptual diagram of IRE is shown in Fig. 1. Givenan environment with shielding objects, IRE can
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secure propagation routes that make detours aroundshielding objects by optimally controlling not only
the transmitter (Tx) and receiver (Rx) but the propagation channel*5 (H) as well.
2.2 RISRIS is essential technology for achieving IRE
described above. A RIS consists of multiple elementsthat scatter electromagnetic waves. Metamaterial/
metasurface technologies that can design and control
the distribution of these scattering characteristics arecommonly used to achieve RIS. A metasurface has athin flat shape, and depending on the base materials
selected, it is even possible to manufacture one in the
shape of a flexible sheet that can be installed along
the side of a structure. This makes it possible to control the scattering characteristics of radio waves while
maintaining the shape of existing structures. Within
the Tx – propagation channel – Rx sequence, thismeans controlling the propagation channel. In general, RIS can be expected to adaptively control theradio environment by periodically repeating the*5 Propagation channel: An individual communication path in radio
communications. Here, a communication path between a transmitting antenna and receiving antenna. The characteristics of acommunication path are expressed in terms of its transfer function H.

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following operations.
•	Estimate the radio channel characteristics
required for determining RIS operation.
•	Control the scattering intensity and phase distribution on the RIS based on the above estimatedinformation to obtain the desired propagation
channel.
Various approaches to implementing this procedure
have been researched. Furthermore, as a term referring to the same technology as RIS, there is the large
intelligent surface (LIS), and there is also the intelligent reflecting surface (IRS) that focuses only on
controlling reflected waves.
2.2.1 Effect of RIS size
Given a non-line-of-sight situation between a basestation (BS) and mobile station (MS) in which areflector or repeater*6 is used to relay radio signals(BS – reflector/repeater – MS), path loss*7 occurs twotimes, once in the BS – reflector/repeater interval andagain in the reflector/repeater – MS interval. The
drop in energy density of a wave due to propagationis larger at positions closer to the wave source. Thus,
even for the same total path length, path loss is generally greater for the BS – reflector/repeater – MS paththan the BS – MS path leading to a drop in received
power. This tendency of path loss to increase applies
to RIS as well.
Path loss via RIS is affected by surface size thesame as an ordinary metal reflector, but in the case of
RIS, path loss depends not only on size but also on themethod of phase control. With this in mind, we firstexamine how RIS size affects received power andintroduce the results of analyzing the relationshipbetween path loss that occurs twice via RIS (hereinafter referred to as “double path loss”) and path lossof a direct wave for the same path length [5]. In thisanalysis, calculations were performed for the following two cases.
•	Receiving a direct wave with a BS – MS of
200 m•	Receiving radio waves with a BS – RIS distanceof 100 m and a RIS – MS distance of 100 m andno paths other than the path via RIS (total path
length = 200 m)
The relationship between RIS size and path loss foreach of the above paths in the 28 GHz band as determined by computer simulation is shown in Fig. 2. In
this computation, the RIS is square-shaped, reflectivity is 100%, and BS – RIS – MS is assumed to lie
along a straight line. In actuality, RIS can be expected
to lie at an angle with respect to BS/MS and not along
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a straight line, in which case its effective area
decreases by the amount of that angle.
Focusing our attention on controlling only the
propagation direction with no special phase control(blue line in Fig. 2) and considering the case that RIS
is smaller than a certain size ((1) in the figure), double
path loss via RIS is considerably greater than pathloss of a direct wave for the same path length. However, as RIS size becomes larger than that of (1), it canbe seen that double path loss asymptotically approaches path loss of a direct wave for the same path length.
Furthermore, though described in more detail later,
double path loss when subjecting RIS to optimized
control to maximize received power at MS takes on
the values shown by the red line in the figure. Now,
by analytically determining the RIS size for which
this optimally controlled double path loss is equal todirect-wave path loss at (1) in the figure, the length of
one side of this RIS turns out to be the Fresnel radius*8. It can also be seen from the figure that, even
without optimized control of RIS, the RIS size at
which path loss is equal to that of the direct wave
nearly agrees with the Fresnel radius, and if the RIS
becomes larger than that size, an amount of power
equivalent to that of the direct wave for the same path
length can be received (blue line in the figure).
In addition, the reason for the alternating increase/decrease in double path loss as it asymptoticallyapproaches path loss of the direct wave with increase
in RIS size is the canceling out of signals via the oddnumbered and even-numbered Fresnel zones*9 at thereceiving point.
2.2.2 Effect of phase control by RIS
On breaking down the BS – RIS – MS propagation
path by each element making up the RIS, we consider
that the received power of the BS – RIS – MS pathcan be maximized by performing RIS phase control
so that the waves on each path have the same phase at*6 Repeater: Relay equipment on the physical layer that amplifies
the power of a signal from a BS or MS and forwards the signal toa MS or BS.
*7 Path loss: Propagation path loss estimated from the difference
between the transmitted power and received power.
*8 Fresnel radius: Radius of the first Fresnel zone (see *9).
*9 Fresnel zone: Given the shortest distance between the transmitting/receiving points (here, the shortest distance via RIS), therange in which the difference in path lengths is less than half awavelength, that is, in which the difference in signal phase is lessthan π (rad), is called the first Fresnel zone. Furthermore, therange in which the difference in path lengths is greater than n – 1
but less than n half-wavelengths is called the nth Fresnel zone.
Signals within the same Fresnel zone strengthen each other andsynthesize.

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Cross section of Fresnel zone at RIS position

One side marks Fresnel radius(1)
1st Fresnel zone

(2)

2nd Fresnel zone

(3)

3rd Fresnel zone
(1)

180
170
160
150
Path loss (dB)

(2)

(3)
Fresnel shift
Optimized control
Control of only
propagation direction

Fresnel radius

190

Path loss via RIS

140
130

Direct-wave path loss for distance of 200 m

120
110
100
90
80
70
0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Length of one side of RIS (square-shaped) (m)Fig. 2. Relationship between RIS size and path loss.

the receiving point (red line in Fig. 2). If optimized
control is performed in this way, the received powerends up having a positive correlation with RIS size
instead of converging to the value of the direct wave
path. Furthermore, if RIS size should be less than the
Fresnel radius, practically no difference can beobserved between conventional control of only the
propagation direction (blue line in the figure) and
optimized phase control, so direction-only control issufficient in this interval.
When performing optimized control of a RIS fromthe viewpoint of received power, the amount of phasechange in each element of the RIS must be set to an
optimal value as described above. This, however, may
complicate the control process. To simplify this process, we introduce a technique that can significantly
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improve received power by adding phase compensation consisting of only two values—0 and π (rad)—todirection-only control. As shown in Fig. 2 for the caseof direction-only control, path loss increases anddecreases repeatedly as RIS size becomes larger.
Here, received power can be significantly improvedby shifting the wave phase by π (rad) via the evennumbered Fresnel zones, the cause of this increasedecrease behavior (green line in the figure). This
technique (hereinafter referred to as “Fresnel shift”)
can improve received power depending on RIS size
the same as optimized control even for users positioned at any distance. Details of this technique areprovided in another paper [5].

29Feature Articles

Conventional reflector

Metasurface reflector

RIS with dynamic metasurface
Controlsignals
Controls beam shape,
direction, etc.
e
tw
en
id

In
c

tw
en
id
In
c

e
av
w

av

av

e
av
tw
en
id

ct
ed

ed
ct

In
c

le
ef
w
e
av

R
ef
le

R

ed
ct
le
ef ve
R wa

e

DriverFig. 3. Conceptual diagram of RIS using metasurface technology.

3. Study of metasurface technology toward
transparent RIS
As described above, metamaterial/metasurface
technologies are commonly used to achieve RIS. A
metamaterial features a periodic arrangement ofstructures each sufficiently smaller than the wavelength of an electromagnetic wave. This artificial
periodic structure can, in effect, behave as a material
having a negative refractive index, so it can be used toobtain characteristics that cannot be achieved by ordinary material. As a consequence, metamaterial technology has been vigorously researched since 2000
[6]. Demonstrations of metamaterial technology were
initially conducted in the microwave band (5 GHz),but studies in the millimeter-wave and terahertzwave*10 bands have been active since 2010 [7]. Thisis thought to be due to various factors. For example,
the elemental structures making up metamaterial areof a size on the order of mm – µm that can be easily
fabricated by existing manufacturing processes. In
addition, these are frequency bands for which theelectrical resistance of metals can be nearly ignored,
the same as for frequency bands used in conventional
mobile communications.
Moreover, while a metamaterial is a three-dimensional artificial periodic structure, a two-dimensional
artificial periodic structure is often called a metasurface. Controlling the reflection phase distribution on
the metasurface enables the propagation of reflected
waves to be controlled (Fig. 3).
At present, metamaterial/metasurface technologies
are described as important technologies in 6G-oriented white papers issued by a variety of research institutions. The following describes NTT DOCOMO
initiatives surrounding metasurface technology
toward the practical implementation of RIS.
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3.1 Transparent dynamic metasurface
In 2018, researchers at NTT DOCOMO began
studying static metasurface reflectors that do not provide dynamic control with an eye to expanding millimeter-wave coverage to non-line-of-sight locations[8]. However, for the metasurface reflector studied
here, it was necessary to design a reflection phase
distribution according to incident/reflected angles
calculated from the installation location, base station
position, and location targeted for reception. Anotherproblem was that the area behind the reflector becamea new non-line-of-sight location. It was also desirable
that the reflector has a design or look that blends inwith the city landscape.
In response to the above issues, we have beendeveloping and studying a “transparent dynamic
metasurface” in cooperation with AGC Inc. as a RIS
prototype that can dynamically control the reflection
and penetration of radio waves while maintaining
high transparency (Fig. 4) [9]. The metasurface substrate is covered with a transparent glass substrate to
make it transparent, and moving the glass substrateslightly enables dynamic control of radio waves inthree modes: full penetration of incident waves, partial penetration and partial reflection of radio waves,and reflection of all radio waves.
The mainstream method for achieving dynamic
metamaterials and metasurfaces in the past was to use
semiconductors to control the resistance and electrical-capacitance components of metallic patterns. Our
new method for achieving transparent dynamic metasurfaces is superior to the conventional techniqueusing semiconductor devices by virtue of “enabling*10 Terahertz wave: Electromagnetic waves with a frequency ofaround 1 THz. Often used to refer to frequencies ranging from
100 GHz to 10 THz.

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Prototype of transparent dynamic metasurface

Maketransparent
Approx. 2 mm

Operating principle

Inter-substrate distance
Trials results

Movable substrate
Metasurface substrate

Inter-substrate distance
Reflection mode>200 µm
0 µmPenetration mode

26.8 GHz 26.9 GHz 27.0 GHz 27.1 GHz 27.2 GHz 27.3 GHz

Frequency
Transmittance

Penetration −0.3 dB
mode−1.4 dB−0.8 dB−1.2 dB −1.2 dB−0.3 dB

Reflectionmode−14 dB−10 dB−11 dB−10 dB−15 dB−11 dB

Fig. 4. Transparent dynamic metasurface initiative.

dynamic control while maintaining transparency”
and “making it easy to enlarge the substrate.” Use ofa transparent dynamic metasurface also minimizes
impact on the surrounding environment and on
existing designs on installation.
A trial performed to assess the performance of the
prototype RIS showed that it could achieve transmittance of approximately –1.4 dB or greater in penetration mode and approximately –10 dB or less in reflection mode (calculated as reflectivity of –1 dB orgreater) in 400 MHz or higher bands (Fig. 4).
Going forward, we plan to study further functional
enhancements such as functions for controlling penetration/reflection directions with the aim of achieving a practical transparent RIS.
3.2 M
 aking windows into radio-wave lenses bytransparent metasurface lensCompared with radio waves in the frequency bands
currently used in LTE (Long Term Evolution) and theSub-6*11 band, radio waves in the millimeter-wave
band have high straight-line propagation properties
and attenuate easily. For this reason, radio waves
emitted from outdoor base station antennas attenuate
before arriving at the glass window of a building, and
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Vol. 19 No. 11 Nov. 2021

in addition, attenuated and weak radio waves penetrate an indoor space without spreading out. It istherefore difficult to make such an indoor space intoa coverage area using outdoor base station antennas.
In response to these issues, we developed in cooperation with AGC Inc. a “transparent metasurfacelens” that can guide millimeter waves passing
through a glass window to a specific indoor location(referred to below as a “focal point”). This is a filmlike lens that can be affixed to a glass window fromthe indoor side [10, 11]. Focusing weak radio waves
that pass through the entire surface of a glass window
at a focal point in this way can increase power, so
installing an area improvement tool such as a repeater, reflector*12, or RIS at the focal point position
should make it possible to extend area coverage fromoutdoor base station antennas to the inside of a building (Figs. 5 and 6). In a trial using this metasurface
lens, we found that the received power at the focal
point could be improved by 24 dB or more comparedwith the case of an ordinary transparent glass.
*11 Sub-6: A specific range of frequencies. Radio signals having frequencies from 3.6 GHz to 6 GHz.
*12 Reflector: In this article, reflectors include conventional metal reflectors and metasurface reflectors.

31

Feature Articles

Features of dynamic metasurface lensConventional dynamic metasurface

Concept of metasurface lensMetasurface lens
+ repeater

Ordinary window

Metasurface withuniform structure

Metasurface lens + repeaters for
focal-point control

Matrix control ofeach device

×

Indoors

Repeater

New metasurface lensDynamic switchingof focal points

Metasurface with distribution offour types of structures

Uniform control*

Outdoors

×

Metasurface lens prototype
Static metasurface lensDynamic metasurface lens
*In this prototype, distance with adjoining glass
substrate is controlled.

50cm

80 cm

Trial results
80 cm

50cm

Static metasurface lensReceived power at
focal point(1) Float glass(2) (1) + metasurface lens

0 dB

>24 dB*Values when using as reference the received power at focal point with only float glass

Dynamic metasurface lens*Shows photos before transparency processing so that metasurface patterns are visible

Received
power atfocal point

Focal-point 1

Focal-point 2

Single focal point mode

11 dB

0 dB

Double focal point mode

6 dB

6 dB

*Values when using as reference the received power at focal-point 2 in single focal point mode

Fig. 5. Concept of making windows into radio-wave lenses by transparent metasurface lens.

Metasurface lensRepeater, etc.
Out

doo

r ba

se s

tatio
n

Fig. 6. Example of using a transparent metasurface lens.

We also tested a function for dynamically controlling focal points. In this regard, the conventional
approach to controlling the penetration/reflection
direction of radio waves was to configure a metasurface with a uniform arrangement of identical devices
and to apply different control signals to each device.
In contrast, our new dynamic metasurface lens appropriately arranges devices with four different types of
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Vol. 19 No. 11 Nov. 2021

structures so that focal point positions can beswitched (presently between a single focal point andtwo focal points) even when applying the same control signal to all devices (Fig. 5). In single focal point
mode, the received power at focal-point 1 was shown
to be 11 dB higher than that at focal-point 2 indicating that only focal-point 1 was functioning as a focal
point. Meanwhile, in double focal point mode,32

Feature Articles

received power at both focal-point 1 and focal-point
2 was shown to be 6 dB higher than that at focal-point
2 in single focal point mode indicating the formationof two focal points. Simplifying the control process inthis way raises the possibility of dynamically controlling focal points even with a large-area metasurfacelens.
4. Conclusion
In this article, we presented the concept of IRE that
attempts to adaptively control the radio environment,
which is deemed to be a useful approach to achieving
non-line-of-sight coverage in the millimeter-wave
band, an important issue in achieving 5G evolution &6G. We also described RIS as an elemental technology of IRE and metamaterial/metasurface technologies that form the foundation of RIS. When influencing the propagation environment by a reflector or RIS
of limited area, controllability increases the larger isthat area with respect to that wavelength, i.e., thehigher is the frequency of the propagating wave. The
technology described here is expected to become abase technology for constructing coverage areas not
only for 5G but also for 6G and beyond radio systems
whose frequencies are expected to be even higher.
Going forward, we plan to assess the effectivenessof the RIS technology described in this article inactual environments and to study the application ofeven higher frequencies with RIS toward 6G.
This article is the reproduction of the special article
published in NTT DOCOMO Technical Journal (Vol.
23, No. 2, Oct. 2021).
References[1]

NTT DOCOMO, “White Paper: 5G evolution & 6G,” Version 3.0,
Feb. 2021.

NTT Technical Review

Vol. 19 No. 11 Nov. 2021

https://www.nttdocomo.co.jp/english/binary/pdf/corporate/technology/whitepaper_6g/DOCOMO_6G_White_PaperEN_v3.0.pdf[2] D. Kitayama, K. Tateishi, D. Kurita, A. Harada, M. Inomata, T. Imai,
Y. Kishiyama, H. Murai, S. Itoh, A. Simonsson, and P. Okvist, “High
Speed Mobility Experiments on Distributed MIMO Beamforming for
5G Radio Access in 28-GHz Band,” IEICE Trans. Commun., Vol.
E102-B, No. 8, Aug. 2019.
[3] D. Kurita, K. Tateishi, D. Kitayama, A. Harada, Y. Kishiyama, H.
Murai, S. Itoh, A. Simonsson, and P. Okvist, “Indoor and Field
Experiments on 5G Radio Access for 28-GHz Band Using Distributed
MIMO and Beamforming,” IEICE Trans. Commun., Vol. E102-B, No.
8, Aug. 2019.
[4] M. D. Renzo, A. Zappone, M. Debbah, M. S. Alouini, C. Yuen, J. de
Rosny, and S. Tretyakov, “Smart Radio Environments Empowered by
Reconfigurable Intelligent Surfaces: How It Works, State of Research,
and The Road Ahead,” IEEE Journal on Selected Areas in Commun,
Vol. 38, No. 11, pp. 2450–2525, Nov. 2020.
[5] Y. Hama and D. Kitayama, “Reconfigurable Intelligent Surface Aided
Coverage Enhancement towards 5G evolution and 6G,” IEICE Tech.
Rep., Vol. 120, No. 406, SRW2020-81, pp. 98–103, Mar. 2021 (inJapanese).
[6] D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S.
Schultz, “Composite Medium with Simultaneously Negative Permeability and Permittivity,” Phys. Rev. Lett., Vol. 84, No. 18, pp. 4184–
4187, May 2000.
[7] D. Kitayama, M. Yaita, and H.-J. Song, “Laminated Metamaterial Flat
Lens at Millimeter-wave Frequencies,” Opt. Express, Vol. 23, pp.
23348–23356, Sept. 2015.
[8] D. Kitayama, D. Kurita, K. Miyachi, Y. Kishiyama, S. Itoh, and T.
Tachizawa, “5G Radio Access Experiments on Coverage Expansion
Using Metasurface Reflector at 28 GHz,” 2019 IEEE Asia-PacificConference on Microwave (APMC), pp. 435–437, Singapore, Dec.
2019.
[9] NTT DOCOMO Press Release, “DOCOMO Conducts World’s First
Successful Trial of Transparent Dynamic Metasurface—Dynamic
wave manipulation and high transparency expected to optimize 5G
network construction—,” Jan. 2020.
[10] NTT DOCOMO Press Release, “DOCOMO and AGC Use Metasurface Lens to Enhance Radio Signal Reception Indoors—New technology efficiently guides millimeter waves to target locations indoors—,”
Jan. 2021.
[11] D. Kitayama, Y. Hama, K. Goto, K. Miyachi, T. Motegi, and O.
Kagaya, “Transparent Dynamic Metasurface for Visually Unaffected
Reconfigurable Intelligent Surface: Controlling Transmission/Reflection and Making a Window into an RF Lens,” Opt. Express, Vol. 29,
No. 18, pp. 29292–29307, Aug. 2021.

Trademark notes

All company names or names of products, software, and services appearing
in this article are trademarks or registered trademarks of their respective
owners.

33

Feature Articles

Daisuke Kitayama

Kensuke Miyachi

Senior Researcher, NTT Device Technology
Laboratories, NTT Corporation.
He received a B.S. in electrical and electronicengineering and an M.S. in electronics andapplied physics from Tokyo Institute of Technology in 2010 and 2012. He joined NTT Device
Technology Laboratories in 2012. He has been
engaged in research on metamaterials for controlling terahertz waves. From 2017 to April
2021, he was a researcher at 6G Laboratories,
NTT DOCOMO, where he was involved inresearch on 5G and 6G. Since May 2021, he has
been with NTT Device Technology Laboratories
and engaged in research on terahertz metasurfaces for RIS.

Manager, the 6G-IOWN Promotion Department, NTT DOCOMO, INC.
He received a B.S. and M.S. in electrical andelectronic engineering from Tokyo Institute ofTechnology in 1999 and 2002. Since joining
NTT DOCOMO, he has been engaged in the
research of radio propagation and the systemdesign of radio access networks.

Yuto HamaManager, 6G-IOWN Promotion Department,
NTT DOCOMO, INC.
He received a B.E., M.E., and Ph.D. from Hokkaido University in 1998, 2000, and 2010. Sincejoining NTT DOCOMO in 2000, he has been
involved in the research and development of 4G
and 5G radio access technologies, including
concept development, standardization, and trials.
He is currently working on “5G evolution & 6G”
and is the main author of “DOCOMO 6G White
Paper.” He has more than 400 granted patents inthe area of mobile communications. In 2012, hereceived the ITU Association of Japan Award forhis global contributions to LTE.

Yoshihisa Kishiyama

Ph.D. student, Yokohama National University(JSPS Research Fellow).
He received a B.E. and M.E. in information andcommunication engineering from Yokohama
National University, Kanagawa, in 2016 and2018, where he is currently pursuing a Ph.D.
degree. From 2018 to 2021, he was a researcher
at 6G Laboratories, NTT DOCOMO. Since April
2021, he has been a research fellow of Japan
Society for the Promotion of Science (JSPS). His
current research interests include communication
theory and coded modulation for MIMO systems.

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Vol. 19 No. 11 Nov. 2021

34

Feature Articles: NTT DOCOMO’s Initiatives on 5G
evolution & 6G

Research on NTN Technology for 5G
evolution & 6G
Yuki Hokazono, Yoshihisa Kishiyama, and Takahiro Asai
Abstract

In the 5th-generation mobile communication system (5G) evolution & 6G, extreme coverage extension is being studied for use cases in all places including air, sea, and space. Non-terrestrial networks
using geostationary satellites, low earth orbit satellites, and high-altitude platform stations are promisingtools for providing high-quality communication services in areas that cannot be covered by the conventional mobile communication network. In this article, we describe these technologies and the details of
a 39-GHz band airborne propagation measurement experiment performed using a small airplane at analtitude of about 3 km.
Keywords: NTN, HAPS, extreme coverage extension

1. Introduction
While the 5th-generation mobile communication
system (5G) is expected to be an important technology for regional development and solving regional
issues, a key issue for the 5G evolution and 6G era is

expected to be expanding the communication area toany place where its benefits can be enjoyed [1]. As
shown in Fig. 1, NTT DOCOMO is conducting
research and development aimed at achieving
extreme coverage extension whereby mobile communication can be made available in all locations,

New wireless technology
extends the communication
environment to air, sea,
and space

Extreme coverage extension

Fig. 1. Extreme coverage extension.

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35

Feature Articles(1) GEO
Altitude: approx. 36,000 km
Ground area radius: approx. 1000 km(2) LEO
Altitude: from a few hundred to approx. 2000 km
Ground area radius: several hundred km(3) HAPS
Altitude: approx. 20 km
Ground area radius: approx. 50 km

Space station
Passenger spaceliner

AircraftDrone

MobileBackhaulDisaster countermeasures

Ship

Mountainous IoT
and remote areas

Remote island
GEO

Coverage image
LEO
HAPS

Fig. 2. Illustration of how satellites and HAPS can be used to extend coverage to the sky, sea, and space.

including the air, sea, and space, that are not adequately covered by conventional mobile communications networks, thereby extending coverage to drones,
flying cars, ships, and even space stations.
To achieve this extreme coverage extension, we arefocusing on non-terrestrial network (NTN)*1 technology using satellites and high-altitude platform stations (HAPS)*2. This technology is able to provide
communication coverage in mountainous and remoteareas, at sea, and even in outer space by employing
satellites and HAPS systems that are free of geographical restrictions.
This article describes extreme coverage extension,
which is one of the key issues for the realization of
5G evolution & 6G. Specifically, we describe theconcept of NTN technology, which has been attracting attention as a promising approach, the use cases
and technical issues of wireless system technology
using HAPS, and our work on measuring the propagation of radio waves using a small aircraft.
2. Extreme coverage extension and NTN
technology for 5G evolution & 6G
Extreme coverage extension supports use cases inany location, including air, sea, and space. This will
extend coverage to users that cannot be covered by
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Vol. 19 No. 11 Nov. 2021

conventional mobile communication networks,
including drones, flying cars, ships, and space stations. To achieve extreme coverage extension, it will
be necessary to develop technologies that facilitate
highly efficient long-range wireless transmission
over at least several tens of kilometers.
As shown in Fig. 2, by considering the use of (1)geostationary (GEO) satellites, (2) low earth orbit(LEO) satellites, and (3) HAPS, we will be able tocover mountainous and remote areas, the sea, the sky,and even outer space, and to provide communication
services to these areas [2].
(1) GEO satellite
A GEO satellite orbits the Earth at an altitude ofabout 36,000 km. Although the one-way radio wave
propagation time between a GEO satellite and aground station antenna is relatively long (about 120ms), just three or four GEO satellites can provide theentire planet with constant coverage. Even today,
GEO satellites are used to complement terrestrial*1 NTN: Any network in which the communication area is not limited to the ground but extended to other places such as the air,sea, and space through the use of non-terrestrial equipment suchas satellites and HAPS.
*2 HAPS: An airborne platform that is designed to operate in thestratosphere on board a vehicle such as a solar-powered aircraft
or airship.

36

Feature Articles

networks by providing a mobile backhaul*3. Since
additional network capacity will be required in the
6G era, very high throughput satellites are being studied with the aim of improving system capacity by
optimizing the allocation of power and frequency
across multiple beams [3].
(2) LEO satellite
An LEO satellite orbits the Earth at an altitude
ranging from a few hundred km to about 2000 km.
Unlike a GEO satellite, it has a much lower orbit anda much smaller propagation delay (just a few ms fora one-way trip). LEO satellites are currently used forsatellite mobile phone systems and satellite sensing*4.
They are also expected to be used for the expansionof communication capacity through the reduction ofsatellite fabrication costs and the use of multiple
input, multiple output (MIMO)*5 technology, and forhigh-capacity, low-latency backhauls in satellite constellations that form networks through cooperation
between multiple satellites [4].
(3) HAPS
HAPS have recently attracted renewed attention
due to their ability to parked at an altitude of about 20
km in fixed locations, allowing them to provide services across terrestrial cells*6 with a radius of approximately 50 km or more [5]. Since their altitude islower than that of LEO satellites, it is possible toachieve a one-way radio wave propagation time ofabout 0.1 ms, depending on the cell radius. As aresult, they are considered to be an effective way of
deploying services not only in regions that have been
hit by natural disasters but also in many of the industrial use cases envisioned for 5G evolution & 6G.
The 3rd Generation Partnership Project (3GPP) has
begun studying how satellites and HAPS can be used
to extend New Radio (NR)*7 to NTNs [6].
3. HAPS use cases and network configuration/
control techniques
NTT DOCOMO is working on the research anddevelopment of communication methods and network architectures that can flexibly link 5G networks
and other terrestrial networks with stratospheric
HAPS networks [7, 8]. In addition to providing flexible support for a wide range of future use cases asenvisioned in 5G evolution & 6G, this project is conducting studies aimed at the implementation of communication systems that use realistic HAPS in termsof development and operation costs.

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3.1 HAPS use cases
As shown in Fig. 3, for the 5G evolution & 6G era,
it is expected that various use cases will involve using
HAPS to relay radio waves or emit radio waves as abase station. These use cases include fixed systems
that provide services for backhaul applications, andmobile systems that provide services to terminals
either directly or by via repeaters and relays. In particular, the use of broadband millimeter wave*8 radio
signals is expected to enable the timely provision ofhigh-speed, large-capacity, low-latency lines requiredfor various applications including industry and public
events, regardless of whether or not optical fibers orother wired networks are available, and in any location, including at sea, in the air, or in remote areas.
The requirements of HAPS systems can vary widely from one use case to the next. As shown in Fig. 4,
different use cases require different communication
speeds and different bandwidths. There is a need forflexible communication methods and systems thatcan support all use cases of fixed and mobile systems.
For example, it is considered that the communication
speed for backhaul applications to 5G base stations
will have to be at least 1 Gbit/s per service link*9.
Furthermore, to provide multiple simultaneous service links, the feeder link*10 will have to be capableof even faster communication speeds (several Gbit/s
to several tens of Gbit/s) and must operate as stably
as possible regardless of weather-related effects.
It is also necessary to flexibly control lines so that
*3 Backhaul: In a mobile communication network, a backhaul is a
fixed line that supports high-speed, high-capacity transmission of
information between a large number of wireless base stations andthe core network.
*4 Satellite sensing: Observing the state of the atmosphere and theEarth’s surface from space by means of instruments carried on
board satellites.
*5 MIMO: A signaling technique whereby multiple transmit and receive antennas are used to transmit signals simultaneously and at
the same frequency to improve communication quality and theefficiency of frequency utilization.
*6 Cell: The unit of area division that makes up the service area of amobile communications network.
*7 NR: A radio system standard formulated for 5G. Compared with
4G, it enables faster communication by utilizing high frequency
bands (e.g., 3.7 GHz and 28 GHz bands), and low latency andhighly reliable communication for achieving advanced Internet ofThings (IoT).
*8 Millimeter waves: Radio signals in the frequency band from 30
GHz to 300 GHz as well as the 28 GHz band targeted by 5G allof which are customarily called “millimeter waves.”*9 Service link: A communication path between a satellite or HAPS
and a terminal in an NTN communication system.
*10 Feeder link: A communication path between a satellite or HAPS
and a terrestrial base station (gateway) in an NTN communication system.

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Feature Articles

Broadband foraircraft

HAPS

Broadband for ships
and railways
Coverage inremote areas
Public safety

Marine & airborne
coverage

Industrial networks(temporary)

Backhaul tobase stations

Wide-area
IoT

Repeaters &relaysPortablebasestations
Natural disasters

Sea/air
Mountains, forests,
and farmland

Remote islands, etc.

EventsConstruction sites

Fixed line (providing services as backhaul to terrestrial networks, etc.)
Mobile system (providing services directly to terminals or via repeaters and relays)

Fig. 3. Various use cases expected for HAPS.

Gbit/s class~10 Gbit/s

Communication speed~1 Gbit/s

GHz class
bandwidth

Broadband for Broadband foraircraftships and railways
Industrial networks (temporary)~100 Mbit/s

100 MHz classbandwidth~10 Mbit/s
Marine & airborne Coverage incoverage
remote areas
Wide-area IoT

Disastercountermeasures

10 MHz classbandwidth

Mbit/s class
Preferably uses flexible communication methods and systems that can handle all fixed and mobile use cases

Fig. 4. Requirements for each HAPS use case.

they can be adapted from normal business applications to public safety*11 applications in the event of adisaster. Current disaster countermeasures are geared
towards providing basic communication services
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*11 Public safety: A generic term for services that ensure the safetyof the public, including disaster prevention, police, fire, and firstaid services.

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Feature Articles

such as voice calls and short message services, but inthe future, it may also be necessary to consider use
cases that require faster communication speeds, suchas remote control of equipment at disaster sites, video
transmission, and communication via drones. For
disaster countermeasures, it will also be necessary to
study network configurations and control techniquesthat assume the ability of a system to operate even if
some devices become unavailable.
3.2 T
 echnology for network configuration andcontrol in conjunction with 5G networks
3.2.1 Classification of HAPS-mounted stations
In the network configuration and control technology used when implementing backhauls to 5G basestations via HAPS, we are focusing on the categorization of HAPS-mounted stations. They can be roughly
divided into two types: (1) relay stations, which
receive signals from ground stations and relay themback to other ground stations after performing necessary processes such as frequency conversion, and (2)base stations, which are made by installing 5G network base station equipment (or at least part of it) ina HAPS.
(1) Relay station typeThe relay type is effective when the number of onboard devices is relatively small and the size, weight,
and power consumption of the HAPS-mounted station are strictly limited.
(2) Base station typeThe base station type is formed by equipping a
HAPS with an antenna device, together with manybase station functions. The more of these functions it
includes, the greater the amount of control that can be
performed within the HAPS, making it possible toreduce the amount of feeder link information. On theother hand, installing more functions results in a station that is larger, heavier, and consumes more power.
In general, implementing more of the base station
functions on the ground network side has the advantages of lower development costs and ease of operation, but implementing these functions on the HAPS
results in greater resilience to natural disasters. In
terms of performance, a HAPS-mounted station
should at least implement some functions, such as
beam control when using millimeter waves. It is also
necessary to comprehensively study a wide range ofrequirements to be considered when incorporating
HAPS systems into a 5G network. These include thesize, weight, and power consumption of HAPSequipped stations, their development and operation
costs, the ability of these HAPS platforms can be
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shared by fixed-line and mobile communication systems, and their ability to cooperate with GEO/LEO
satellites.
3.2.2 E xamples of network configuration in conjunction with the 5G network
An example of a HAPS base station in a network
configuration linked to the 5G network is shown in
Fig. 5. Here, the distributed unit (DU)*12 and radio
unit (RU)*13 of the 5G base station are mounted on
the HAPS in accordance with Open RAN (O-RAN)*14
Alliance specifications [9]. In this configuration,
availability is ensured by installing a centralized unit
(CU)*15 at a disaster-resistant point on the ground.
Information received by the HAPS from the CU in
the feeder link is transmitted via 5G radio to a smallterrestrial base station device (relay station) in theservice link, thereby enabling the use of portable 5Gbase stations without having to use a wired backhaul.
In this configuration, it is also possible to provide
direct communication from the HAPS to 5G terminals without the need for intervening relay stations.
As a further extension, site diversity*16 can be implemented by using multiple CUs on the ground side to
reduce the impact of bad weather and natural disasters, and mobility support*17 can be implemented byswitching the communication target to a different
HAPS when the terminal moves from one communication area to another.
In addition to the configuration shown in Fig. 5, weare also considering other promising configurations:
one in which a HAPS is used to carry a standalone*18
5G base station, and another with a relay-type configuration where a 5G radio repeater is installed in a
HAPS. For each configuration, it is necessary to conduct a comprehensive study that takes account of*12 DU: A functional unit of a wireless base station that performs realtime data link layer control and other functions.
*13 RU: The radio unit of a wireless base station.
*14 O-RAN: An open, intelligent radio access network aimed at efficiently providing a variety of services in the 5G era.
*15 CU: An aggregating node that implements functions such as nonreal-time Layer 2 functions and RRC (radio resource control)
functions in a radio base station.
*16 Site diversity: A technique for improving communication quality
by switching between multiple ground stations when radio waves
are highly attenuated due to rain or obstacles.
*17 Mobility support: Technology that allows communication to continue when a terminal moves across a communication area by
switching it to a different base station before communication isinterrupted.
*18 Standalone: A deployment scenario using only NR, in contrast
with non-standalone operation which uses Long Term Evolution(LTE)-NR Dual Connectivity to coordinate existing LTE/LTEAdvanced and NR.

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Feature Articles

Base station equipment
mounted on
HAPS
O-DU

HAPS
Core network

O-RU

5G wirelessBase station equipment
Backhaul

EPC

Repeateror IAB

CU

Core
buildings, etc.

5G wireless

Small base station equipment (relay station)
CU

Site diversity (for bad weather
and as a disaster countermeasure)

Disaster-resistant locations wherebase stations can be installed*.
*Stoppages caused by disasters are rareThis configuration
can also support
terminals directly.

EPC: evolved packet core
IAB: integrated access and backhaul

Fig. 5. Example of cooperative configuration when HAPS is used for backhaul.

various attributes such as mobility support, site diversity technology and frequency sharing technology*19,
as well as HAPS installation requirements such as
links with GEO/LEO satellites, the equipment weightand power consumption.
4. Experimental 39 GHz band propagation
measurements using a small aircraft
To implement a communication area from an airborne station in 5G evolution & 6G, we conducted anexperimental demonstration of radio wave propagation measurements in an urban area (Odawara City,
Kanagawa Prefecture), a mountain forest (Tanzawa),
and a remote island (Izu Oshima) using a small aircraft (February 15–26, 2021) [10]. Before using theactual HAPS system, we performed an initial experiment to compare the propagation of millimeter wave
(39 GHz band) radio signals, which are suitable for5G high-speed communication, and signals at a lower
frequency (2 GHz band), which propagate more easily than millimeter wave signals. These signals weresent from the ground to a receiver mounted on a small
aircraft about 3 km above ground level. In the urban
environment, we measured the effects of obstacles
such as buildings and reflected waves. In the forest,
we measured the effects of terrain and trees. And in
the remote island, we measured the effects of clouds
and low elevation angles above the sea. Our results
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show that radio wave propagation in the 39 GHz and
2 GHz bands depends on various environmental factors, and changes when the airplane turns.
4.1 M easurement environment and measured
items
Figure 6 shows an illustration of the airborne
propagation measurement test. The radio wave transmission points were located in an urban area
(Odawara City in Kanagawa Prefecture), a forest(Tanzawa), and a remote island (Izu Oshima), and thereception point was a small aircraft circling with aradius of 1 to 2 km. The elevation angle of the small
aircraft (receiving point) from the transmitting point
was determined to be equivalent to the use case of a
HAPS circling at an altitude of 20 km. Specifically,
we assumed a coverage radius of 50 km for an altitude of 20 km in the urban and forest use cases (elevation angle: 21.8°), and a coverage radius of 200 km
for an altitude of 20 km in the remote island use case(elevation angle: 5.7–11.5°). In addition to the lineof-sight environment, we also measured the received
power with intervening obstacles such as buildings*19 Frequency sharing technology: Technology that makes it possible
to share frequencies by suppressing the interference effects that
occur when two systems use the same frequency at the same location. In this article, we are mostly concerned with frequency
sharing between HAPS systems and terrestrial mobile communication systems.

40

Feature Articles

AircraftReceiverTurning radius:
1.0–2.0 km

Elevation angle of transmission point andreception point (center of turning):• 21.8° (Odawara City, Tanzawa)• 5.7–11.5° (Izu Oshima)Transmitter(39 GHz band or 2 GHz band)

Altitude:
0.5–3.8 km

Transmission point:

Urban area
(Odawara City)

Forest(Tanzawa)

Remote island(Izu Oshima)

Fig. 6. Illustration of the airborne propagation measurement test.

(a) Buildings in the urban test area(b) Trees in the forest test area(c) Mountains in the forest test area(d) Clouds in the remote island test area

Fig. 7. Test environments with various obstacles.

and trees in each use case, as shown in Fig. 7.
For the transmitting antenna, we selected a productthat was able to cover the turning range of the aircraftwithin the beam width*20, and we fixed the directionof this antenna toward the aircraft’s turning center.
The receiving antenna was housed in a mounting
frame, and a 3-mm-thick polycarbonate radome*21
designed to cause almost no propagation loss*22 wassecured to its base. We also performed an initial
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Vol. 19 No. 11 Nov. 2021

experiment in which neither of the transmitting or*20 Beam width: The antenna radiation angle at which the beam isradiated with gain of −3dB or less from the maximum antennagain.
*21 Radome: An enclosure to protect an antenna. These are made ofmaterials that are transparent to radio waves.
*22 Propagation loss: The amount of attenuation in the power of asignal emitted from a transmitting station until it arrives at a reception point.

41

Free space loss (dB), path loss (dB)

Feature Articles

130
140
150
160
170
180
190
200
9:35

9:45

9:55

10:05
Time

10:15

10:25

Free space loss (dB)

10:35
Path loss (dB)

Fig. 8. Propagation loss time series data in the 39 GHz band at Odawara City (line of sight)/2 km turning diameter.

receiving antennas were provided with tracking functions, and the receiving antenna was fixed directly tothe underside of the aircraft.
4.2 Evaluation
4.2.1 Overview
From this experiment, we found that the maximum
reception sensitivity in the unobstructed line-of-sightenvironments was almost identical to the value
obtained by desktop calculations, while the loss ofreceived power in the 39 GHz band was relatively
large when there were intervening buildings andtrees. To reduce the influence of obstacles of this sort,
it will be necessary to consider adopting measures
such as site diversity. We also confirmed that the
influence of clouds was relatively small in theabsence of rainfall.
Furthermore, since signals were transmitted andreceived in this experiment by means of antennas
with the same directivity pattern regardless of theaircraft’s position and flight attitude, it also became
clear that the received power varies greatly withchanges in the angle of the antenna caused by turningof the aircraft. For the practical use of HAPS in the
future, these results confirm the importance of using
control technology to suppress the effects of turning
the aircraft and maintain a constant received power.
4.2.2 Experimental results in Odawara
As a concrete example of the measurement results,
this section describes an experiment that models an
urban use case. For the line-of-sight environment inOdawara City, Kanagawa Prefecture with the aircraft
circling with a turning diameter of 2 km, Figure 8
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Vol. 19 No. 11 Nov. 2021

shows the time series data of losses in the 39 GHz
band. When the maximum receiver sensitivity in the
39 GHz band is calculated from the free space loss
shown in Fig. 8, the results generally match the figures obtained in desktop calculations with an error of
only about 1.2 dB. On the other hand, the time-series
data fluctuated greatly when the aircraft turned, withthe average received power decreasing by about 17
dB from the maximum value and the median decreasing by about 14 dB. It is thought that these reductions
were mainly caused by fluctuations in the directive
gain of the transmitting and receiving antennas,
polarization losses*23, and the obstruction of radio
waves by the aircraft itself when the underside of theaircraft was facing away from the transmission point.
In the building-occluded environment, the 39 GHz
propagation loss has a mean value about 17 dB larger
and a median value about 19 dB larger compared withthe line-of-sight environment. Although obstruction
by buildings also affects the 2 GHz band, the losses
are relatively small—about 8 dB in the average value
and about 9 dB in the median value—compared withthe line-of-sight environment. Furthermore, when theunderside of the aircraft is tilted towards the transmitting point, the received power is at least 40 dB greaterthan when the aircraft is tilted in the other direction.
5. Conclusion
In this article, as part of our efforts toward implementing extreme coverage extension, which is one of*23 Polarization loss: A loss of power received by a receiving antennathat occurs due to the vibration direction (polarization) of theelectric field when radio waves propagate in space.

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Feature Articles

the important issues for 5G evolution & 6G, we havedescribed NTN technology, especially HAPS use
cases and network configuration/control technology,
and we have shown the results of airborne radio
propagation tests performed using a small aircraftassuming HAPS use cases. NTT DOCOMO will continue developing NTN technology aimed at achieving
extreme coverage extension and technology for realizing HAPS networks, and promoting demonstration
experiments and standardization activities.
Finally, part of this research and development iscarried out by the Ministry of Internal Affairs andCommunications (Research and Development for
Expansion of Radio Resources; JPJ000254).
This article is the reproduction of the special article
published in NTT DOCOMO Technical Journal (Vol.
23, No. 2, Oct. 2021).
References
[1]

[2]

NTT DOCOMO, “White Paper: 5G evolution and 6G,” Version 3.0,
Feb. 2021.
https://www.nttdocomo.co.jp/english/binary/pdf/corporate/technology/whitepaper_6g/DOCOMO_6G_White_PaperEN_v3.0.pdf
T. Onizawa, T. Tatsuta, N. Kita, and F. Yamashita, “Recent Researchand Development in Fixed Wireless and Satellite Communication
Systems,” IEICE Technical Report, Vol. 119, No. 31, RCS2019-32,
pp. 53–58, May 2019.

Yuki Hokazono

Research Engineer, 6G-IOWN Promotion
Department, NTT DOCOMO, INC.
He received a B.E. and M.E. from Graduate
School of Engineering, the University of Tokyo,
in 2017 and 2019. Since joining NTT DOCOMO
in 2019, he has been involved in research anddevelopment activities related to radio frequency
wireless technologies for 5G evolution & 6G,
especially non-terrestrial networks. He also contributed to the 3GPP TSG-RAN WG1 as a delegate from NTT DOCOMO.

[3]

J. Bejarano, C. Nieto, and F. Piñar, “MF-TDMA Scheduling Algorithm for Multi-Spot Beam Satellite Systems Based on Co-channel
Interference Evaluation,” IEEE Access, Vol. 7, pp. 4391–4399, Dec.
2018.
[4] B. Di, H. Zhang, L. Song, Y. Li, and G.Y. Li, “Ultra-dense LEO: Integrating Terrestrial-satellite networks into 5G and Beyond for Data
Offloading,” IEEE Trans. Wireless Commun., Vol. 18, No. 1, pp.
47–62, Dec. 2018.
[5] HAPS Alliance, “Introducing the HAPS Alliance.”https://hapsalliance.org/[6] 3GPP, RP-193234: “Solutions for NR to Support Non-terrestrial Networks (NTN),” Dec. 2019.
[7] J. Suzuki, H. Kitanozono, T. Takamori, Y. Morita, Y. Kishiyama, T.
Sotoyama, and R. Miura, “Development of a 38-GHz Band WirelessCommunication System with High-altitude Platform Stations (HAPS)
in Cooperation with 5G Networks: A Study of High-speed Highcapacity Backhaul Lines for 5G Networks,” Proc. of 2021 IEICE
General Conference, B-3-1, Mar. 2021.
[8] Y. Hokazono, Y. Kishiyama, T. Asai, J. Suzuki, and H. Kitanozono,“Development of a 38-GHz Band Wireless Communication Systemwith high-altitude Platform Stations (HAPS) in Cooperation with 5G
Networks: A Study of HAPS Network Configuration and Control
Technology,” Proc. of 2021 IEICE General Conference, B-3-2, Mar.
2021.
[9] A. Umesh, T. Yajima, T. Uchino, and S. Okuyama, “Overview of
O-RAN Fronthaul Specifications,” NTT DOCOMO Technical Journal, Vol. 21, No. 1, pp. 46–59, July 2019.
[10] NTT DOCOMO Press Release, “Trial Measurements of 39 GHz Band
Radio Propagation with a View to Serving Communication Areas with
Airborne Systems,” Mar. 2021.

Trademark notes

All company names or names of products, software, and services appearing
in this article are trademarks or registered trademarks of their respective
owners.

Takahiro Asai

Deputy General Manager, 6G-IOWN Promotion Department, NTT DOCOMO, INC.
He received a B.E. and M.E. in electrical andelectronics engineering, and Ph.D in communications and computer engineering from Kyoto
University in 1995, 1997, and 2008. In 1997, hejoined NTT Mobile Communications Network,
Inc. (currently, NTT DOCOMO, INC.). Since
then, he has been engaged in the research of signal processing for mobile radio communication.

Yoshihisa Kishiyama

Manager, 6G-IOWN Promotion Department,
NTT DOCOMO, INC.
He received a B.E., M.E., and Ph.D. from Hokkaido University in 1998, 2000, and 2010. Sincejoining NTT DOCOMO in 2000, he has been
involved in the research and development of 4G
and 5G radio access technologies, including
concept development, standardization, and trials.
He is currently working on “5G evolution & 6G”
and is the main author of “DOCOMO 6G White
Paper.” He has more than 400 granted patents inthe area of mobile communications. In 2012, hereceived the ITU Association of Japan Award forhis global contributions to LTE.

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43

Feature Articles: NTT DOCOMO’s Initiatives on 5G
evolution & 6G

Improving Communication
Performance in High-mobility
Environments by Millimeter-wave
Base Station Cooperation for 5G
evolution
Tatsuki Okuyama, Satoshi Suyama, Nobuhide Nonaka,
and Takahiro Asai
Abstract

Although the 5th-generation mobile communications system (5G) commercial services are now being
provided, further evolution of 5G is needed to meet a variety of future demands. As part of these studies,
we consider the provision of millimeter-wave high-speed communications over a wide area to multiple
mobile stations traveling at high speed. In a high-mobility environment, area construction must be performed over a wide area through the cooperation of multiple base stations. Additionally, to achieve
simultaneous communications with multiple mobile stations, interference must be suppressed so thatsignals transmitted to each mobile station do not interfere with each other. To deal with these issues, wedeveloped millimeter-wave base station cooperation technology to enable multiple base stations to cooperate with each other while suppressing inter-mobile-station interference by applying digital beamforming to base stations to generate and control beams by digital signal processing. We showed throughoutdoor experimental trials that high communication speeds could be achieved over a wide area.
Keywords: digital beamforming, base station cooperation, outdoor experimental trials

1. Introduction
In the 5th-generation mobile communications system (5G), high-speed communications is being pursued using frequency bands below 6 GHz and theso-called 28 GHz millimeter-wave*1 band. Here, tocompensate for the large path loss*2 of high-frequency bands such as the millimeter-wave band, beamforming (BF)*3 technology using Massive multipleinput multiple-output (Massive MIMO)*4 has cometo be researched as a 5G radio access technology.
NTT DOCOMO is rolling out 5G commercial services using the 3.7, 4.5, and 28 GHz bands. Here, theavailable bandwidth in the 28 GHz band is wider than
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that in the 3.7 and 4.5 GHz bands, so high-speed communications can be expected. On the other hand,strong straight-line propagation and large path loss of*1 Millimeter waves: Radio signals in the frequency band from 30
GHz to 300 GHz as well as the 28 GHz band targeted by 5G allof which are customarily called “millimeter waves.”*2 Path loss: The amount of attenuation in the power of a signal
emitted from a transmitting station until it arrives at a reception
point.
*3 BF: Technology for increasing/decreasing signal power in a particular direction by giving directionality to the transmission signal. It includes analog beamforming that forms directionality byphase control of multiple antenna elements (radio-frequency
equipment) and digital BF that performs phase control in the
baseband section.

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Feature Articles

millimeter waves means that technical issues still
remain in the provision of stable high-speed communications over a wide area. Nevertheless, the use of
the millimeter-wave band is essential to further theevolution of 5G [1].
In this article, we show by outdoor experimental
trials that high-speed communications can be provided over a wide area by (1) using multiple 28 GHz
band experimental base stations equipped with digital
BF to perform BF by digital signal processing and (2)having those base stations cooperate with each otherwhile suppressing the interference generated between
multiple mobile stations traveling at high speed. We
present and discuss the experimental results.
2. Overview of experimental equipment
equipped with millimeter-wave base stationcooperation technology by digital BF
2.1 E
 ffects of applying digital BF and base stationcooperation to high-mobility environments
The following problems must be solved to achieveeven higher communication speeds using millimeterwaves:(1)	While higher communication speeds can be
expected for each base station through simultaneous communications with multiple mobile
stations (by achieving multi-user MIMO),
interference generated between mobile stations must be suppressed.
(2)	For example, when a base station is communicating with mobile stations within vehicles
traveling on an expressway at 100 km/h, cooperation among multiple base stations is necessary to provide wide-area communications.
A variety of methods have been proposed to suppress inter-mobile-station interference such as transmitting signals to mobile stations using a different
beam for each. Nevertheless, interference can still
occur between beams, so this method is not necessarily able to sufficiently suppress interference. Furthermore, given the large path loss of millimeter waves,the service area of each base station cannot be easily
expanded. In environments with mobile stations traveling at high speeds, this means that the time during
which a mobile station is present in that area is short,which in turn means frequent switching between basestations.
Based on the above, an important requirement forproviding high-speed communications for multiple
mobile stations traveling at high speeds is to maintainstable high-speed communications even during a swiNTT Technical Review

Vol. 19 No. 11 Nov. 2021

tchover between base stations while suppressing
inter-mobile-station interference. To this end, we herereport on the development of Massive MIMO experimental equipment using digital BF in the millimeterwave band as opposed to implementing BF using
analog circuits (hereinafter referred to as “analog
BF”) as a base station function.
In general, analog BF operates by having the basestation select the beam to be used from a set of beamcandidates determined beforehand. The advantage
here is that information only on beam direction is sufficient thereby simplifying equipment structure. On
the other hand, the beam is not optimized to radio
wave propagation conditions.
In contrast, digital BF performs communication by
calculating the optimal beam shape (number of individual beams and directions) according to radio wave
propagation conditions. In this way, an improvement
in communication quality can be expected, but otherproblems arise; that is, information on the propagation channel must be estimated, but tracking of wildly
fluctuating radio wave propagation conditions in anenvironment with mobile stations traveling at highspeeds is difficult. However, the fact that digital BF isachieved by digital signal processing means that suppression of inter-mobile-station interference can beincorporated in beam generation and control and that
optimal BF can be performed even in an environment
with multiple mobile stations, all of which makedigital BF a key technology for enhancing communication performance in the future. Additionally, as
examples of base station cooperation, information(received power, degree of spatial multiplexing available for transmission, etc.) obtained from channel
information used for digital BF can be used for
instantaneous switching to the base station best suitedfor communication, or multiple base stations can becontrolled to perform simultaneous transmissions.
Such base station cooperation can achieve stable andhigh-speed communications within an area.
2.2 Overview of digital BF
In contrast to analog BF, digital BF generates andcontrols beams by digital signal processing. Analog*4 Massive MIMO: MIMO systems transmit radio signals overlapping in space by using multiple antenna elements for transmission and reception. Massive MIMO systems aim to achieve highspeed data communications with greater numbers of simultaneous streaming transmissions while securing service areas. They
achieve that aim by using antenna elements consisting of super
multi-element arrays to create sharply formed radio beams tocompensate for the radio path losses that accompany high-frequency band usage.

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Feature Articles

Massive MIMO
antenna

Massive MIMO
antennaInterference suppression

Mobile station

Interference(a) Before suppression

Mobile station(b) After suppression

Fig. 1. Conceptual diagram of interference suppression by digital BF.

BF uses a phase shifter*5 and amplifier*6 connected toeach antenna element to achieve strong radio wave
directionality in a particular direction by superimposing radio waves emitted from each of those elements.
The problem here, however, is that the superimposingof radio waves always results in the same beam shapein whatever the direction. Digital BF, on the other
hand, generates beams by using channel information
between the base station and mobile station, calculating the weighting factors to obtain maximum received
power, and multiplying the transmission signal by
those factors by digital signal processing. In this way,
digital BF can deal with a situation in which themobile station’s peripheral environment is fluctuating
though the mobile station itself may be stationary byreforming an optimal beam according to that fluctuation. Digital signal processing also enables highaccuracy MIMO multiplexing of multiple signals andsuppression of inter-mobile-station interference. A
conceptual diagram of interference suppression isshown in Fig. 1. In analog BF, when a beam faces aparticular direction as shown in Fig. 1(a), the shape ofthat beam always has a fixed pattern. As a result,interference can occur between mobile stations
depending on the environment and communication
performance can greatly deteriorate if the effect ofthat interference is large. Digital BF, though, can perform beam shaping correctly according to the peripheral environment. In Fig. 1(b), the beam shape isformed so as to suppress inter-mobile-station interference, which can improve communication quality
compared with no interference suppression.
However, channel information between the basestation and mobile station must be determined indetail to achieve digital BF. This channel information
is estimated from the channel matrix, whose number
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of rows is equal to the number of antenna elements
and number of columns is equal to the number of basestations. In 5G, a base station turns out to be a massive-element antenna, so it is necessary to estimate amatrix of enormous size. Additionally, if the temporalfluctuation of the mobile station and peripheral environment is gentle, it should be possible to estimate
channel information, but if this temporal fluctuation
is intense as in a high-mobility environment, a discrepancy will emerge in channel information between
the time of estimation and the time at which the signal
is actually transmitted. As a result, an optimal beamcannot necessarily be formed.
With the aim, therefore, of exploiting the benefitsof digital BF while contracting the size of the matrix
to be estimated, we adopted technology that firstforms multiple beams in predetermined directions
using a many-element antenna and then estimateschannel information between those multiple beams
and the mobile station [2]. With this approach, thesize of the matrix to be estimated can be reduced fromnumber of mobile station elements × number of basestation elements to number of mobile station elements × number of beams. In this way, by performing
digital signal processing using a matrix of a size equal
to number of mobile station elements × number ofbeams, it becomes possible to estimate channel information in a relatively short time while minimizing
quality degradation from the use of all elements. With
this approach, digital signal processing as in forming
and controlling beams and suppressing inter-mobilestation interference can still be performed thereby*5 Phase shifter: A circuit that can change the phase going to eachantenna element.
*6 Amplifier: A circuit that amplifies the signal.

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Feature Articles

Massive MIMO
antenna

BBU

Antenna
array

BBU
CU
RE

RE

(a) Base station(b) Mobile station

Fig. 2. Experimental equipment.

enabling transmission by digital BF even in a highmobility environment.

3. Overview and results of millimeter-wave
outdoor experimental trials

2.3 Overview of experimental equipment
External views of base-station and mobile-station
experimental equipment that we developed are shownin Fig. 2. The base station features a 240-element
Massive MIMO antenna connected to a baseband unit
(BBU)*7 that performs digital BF weight calculations. In addition, the central unit (CU)*8 connects tomultiple BBUs with the role of performing a control
function in multi-base-station cooperation.
The mobile station, meanwhile, features four antenna arrays*9 each having 15 vertical elements. Amongthese four arrays, only two panels for a total of 30
elements are used for transmitting a reference signal*10, but in reception, all four panels are used toincrease receive gain. These antenna arrays connect
to the BBU via radio equipment (RE)*11.
With this experimental equipment, the base station
uses the reference signal periodically transmitted by
the mobile station to estimate channel information. It
then generates digital BF weights based on the resultsof that estimation and transmits a maximum to two
streams per mobile station. A mobile station canachieve a maximum throughput of 705 Mbit/s with a
total of two streams. Additionally, once the mobile
station begins to receive signal streams, it calculatesa receive filter*12 at its BBU, detects transmitted signals, and measures throughput.

3.1 Experimental environment
In outdoor experimental trials targeting multiple
mobile stations traveling at high speeds, we performed transmission experiments to evaluate throughput under base station cooperation [3]. The experimental setup is shown in Fig. 3. In these experiments,
we used three base stations each temporarily installedin the bed of a truck. We also used two mobile stations each installed in a vehicle and had them travel athigh speed. At this time, antenna height was set at 2.2
m for both the base stations and mobile stations.
Experimental configuration is shown in Fig. 4. In
the experiments, two mobile stations each pass by
three base stations while traveling at a uniform speedof 90 km/h. These three base stations are positioned

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*7 BBU: One component of base station equipment performing digital signal processing of transmit/receive information when communicating with a mobile station.
*8 CU: Equipment that connects to a baseband unit and performs
radio resource control.
*9 Antenna array: An arrangement of multiple antenna elements or
panels forming an antenna group.
*10 Reference signal: A known signal from base stations, configured
in user equipment.
*11 RE: The equipment that connects with the baseband processor
via the fronthaul.
*12 Receive filter: In MIMO communications, transmitting/receiving
by multiple antennas enables the transmission of multiple streams
and an improvement in received power of the desired signal. On
the other hand, the information in multiple streams through transmitting/receiving by multiple antennas is received in a complicated overlapping state, so a filter is used to mitigate that overlapping and make it easier to estimate the desired signal.

47

Feature ArticlesBasestation #1

Mobilestation #2
Mobilestation #1

Base stationBasestation #3

RE

Base
station #2

Mobile station
Antenna
section

Massiveelement
antennaRE

Fig. 3. Experimental setup.

1st stream

Mobile station #1
Mobile station #2

Traveling at
90 km/hBasestation #1

0m

2nd stream

Basestation #2

200 m

2nd stream

Basestation #3

400 m

Fig. 4. Experimental configuration.

at distances of 0, 200, and 400 m, respectively. At thistime, it is assumed that two streams are transmitted toeach mobile station and that distributed MIMO technology is applied to transmit each stream from a different base station. Specifically, base station #1 transmits the 1st stream of each mobile station continuously while base station #2 and base station #3
transmit the 2nd stream. Transmitting different
streams from multiple base stations in this way
causes the correlation of channel information fromeach base station to drop making it easy to separate
MIMO spatially multiplexed streams. In addition,
two methods of base station cooperation were implemented here: high-speed switching to base station #2
and base station #3 and simultaneous transmission
from base station #2 and base station #3. In theseexperiments, we evaluated downlink throughput formobile stations traveling at a high speed of 90 km/h.
3.2 Experimental results
3.2.1 Base station cooperation experiment
To test the effects of base station cooperation, we
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compared the case of not using base station #2 in Fig.
4 (no base station cooperation) and the case of using
all base stations (cooperation between base station #2
and base station #3). To examine only the effects ofbase station cooperation, we used only mobile station#2 in this experiment traveling at a speed of 90 km/h.
Here, base station #1 transmits the 1st stream whilebase station #2 or base station #3 transmits the 2nd
stream, and when performing base station cooperation, the base station from among base station #2 andbase station #3 that CU judges most capable ofimproving communication quality based on channel
information will transmit the 2nd stream.
Throughput versus mobile station position from 0
to 400 m is shown in Fig. 5. It can be seen from theseresults that throughput deteriorated at the 100 m position when not performing base station cooperation.
However, communications could be achieved without
this drop in throughput when performing base stationcooperation. This was the effect of installing base station #2 at the 200 m position and performing basestation cooperation (base station high-speed switching)48

Feature Articles

700

Mobile station #2 throughput (Mbit/s)

600
500
400
300
200
100
0

0

100
200
Mobile station position (m)

300

400

Base station cooperation
No base station cooperation

Fig. 5. Effects of base station cooperation.

so that communications could be continued from adifferent base station even in an environment or at a
mobile station position where the communication
performance of a certain base station had deteriorated. This result demonstrates that base station cooperation can achieve high-speed communications overa wide area within the coverage area.
3.2.2 B ase station cooperation experiment (basestation high-speed switching) during twomobile-station multiplexing
Figure 6 shows throughput for a total of two
streams transmitted to each of two mobile stations
during high-speed switching between base station #2
and base station #3 depending on communication
quality. Since the antenna array of base station #2 isfacing to the left in Fig. 4, we consider that a mobile
station will switch from base station #2 to base station #3 near the 200 m position. Consequently, on
checking throughput near 200 m, no major deterioration in throughput can be observed for either mobile
station #1 or mobile station #2 showing that basestation switching could be achieved in a relatively
stable manner.
From these results, it can be seen that stable andhigh throughput can be achieved within the coveragearea through base station switching while achieving
simultaneous communications with two mobile stations by suppressing inter-mobile-station interference
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through digital BF. This holds even in an environment
in which two mobile stations are traveling at a highspeed of 90 km/h.
3.2.3 B ase station cooperation experiment (basestation simultaneous transmission) during
two-mobile-station multiplexing
Throughput when having base station #2 and basestation #3 perform simultaneous transmission isshown in Fig. 7. Basic conditions are the same as
those of Fig. 6, but with respect to the 2nd stream,base station #2 and base station #3 transmit the same
signal simultaneously, so an improvement in received
power can be expected at the mobile stations.
Examining throughput in the range of 0–200 m, it
can be seen that simultaneous transmission operated
without a problem. Furthermore, on comparing theseresults with those of Fig. 6, it can be seen thatthroughput was improved if only slightly in the range,for example, of 0–50 m. We offer two reasons for this,
one that received power increased by simultaneoustransmission, and another that deterioration in communication quality could be suppressed overall even
if the quality of the signal from one of the base stations deteriorated due to the peripheral environment
since there was also a signal from the other base station. However, at positions from 50 to 200 m,
throughput in Fig. 7 could not necessarily maintainhigher values than throughput in Fig. 6. This is49

Feature Articles

700

Throughput (Mbit/s)

600
500
400
300
200
100
0

0

100

200

300

400

Mobile station position (m)Mobile station #1
Mobile station #2

Fig. 6. User throughput for base station switching at 90 km/h.

700

Throughput (Mbit/s)

600
500
400
300
200
100
0

0

100

200

300

400

Mobile station position (m)Mobile station #1
Mobile station #2

Fig. 7. User throughput for simultaneous base station transmission at 90 km/h.

because, in this experimental environment, line-ofsight waves are dominant and radio waves on which
signals of base station #2 and base station #3 aresuperimposed cancel each other out at some positions. Nevertheless, we consider that throughput in ahigh-mobility environment can be made stable
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through simultaneous transmission.
The results of this experiment demonstrate theeffectiveness of simultaneous transmission as a formof base station cooperation.

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4. Conclusion
This article described research and development for
5G evolution with the aim of providing stablethroughput with wide coverage by applying base station cooperation technology while suppressing interference between multiple mobile stations by applying
digital BF in the millimeter-wave band. On developing millimeter-wave band experimental equipment
incorporating base station cooperation technology bydigital BF and performing outdoor experimental trials, it was shown that high-speed switching among
three base stations could be achieved while suppressing interference between two mobile stations moving
at a high speed of 90 km/h. It was also shown thatstable and high throughput could be provided over awide area by performing simultaneous base-stationtransmissions through base station cooperation. In
future research, we plan to test the effects of base station cooperation technology with a variety of basestation arrangement methods.
This article includes a portion of the results of the“R&D for Expansion of Radio Wave Resources(JPJ000254)” project commissioned by The Ministry

Tatsuki Okuyama

of Internal Affairs and Communications, Japan.
This article is the reproduction of the special article
published in NTT DOCOMO Technical Journal (Vol.
23, No. 2, Oct. 2021).
References
[1]

[2]

[3]

NTT DOCOMO, “White Paper: 5G evolution and 6G,” Version 3.0,
Feb. 2021.
https://www.nttdocomo.co.jp/english/binary/pdf/corporate/technology/whitepaper_6g/DOCOMO_6G_White_PaperEN_v3.0.pdf
S. Yoshioka, S. Suyama, T. Okuyama, J. Mashino and Y. Okumura,“Digital Beamforming Algorithm for 5G Low-SHF Band Massive
MIMO,” IEICE Trans. on Commun., Vol. E102-B, No. 8, pp. 1371–
1381, Aug. 2019.
NTT DOCOMO, “Successful Experiment in Stable High-speed 5G
Communications using Millimeter Waves with Mobile Stations
Mounted on Multiple Vehicles Traveling at High Speeds of 90 km/h
or Faster,” Mar. 2021 (in Japanese).
https://www.nttdocomo.co.jp/binary/pdf/info/news_release/
topics_210330_00.pdf(URL of experiment video: https://youtu.be/q86dRrs0rSw)

Trademark notes

All company names or names of products, software, and services appearing
in this article are trademarks or registered trademarks of their respective
owners.

Nobuhide Nonaka

Research Engineer, 6G-IOWN Promotion
Department, NTT DOCOMO, INC.
He received a B.E. in electrical, electronic andinformation engineering, an M.E. and Ph.D. in
information and communications technology
from Osaka University in 2012, 2014, and 2019.
Since joining NTT DOCOMO in 2014, he has
been involved in research of radio access technologies for 5G and 6G. He received the Young
Researchers’ Award from the Institute of Electronics, Information and Communication Engineers (IEICE) in 2017.

Research Engineer, 6G-IOWN Promotion
Department, NTT DOCOMO, INC.
He received a B.S. in electrical engineering andan M.S. in electronic engineering from the Tokyo
University of Science in 2013 and 2015. He
joined NTT DOCOMO in April 2015. Since
April 2018, he has been engaged in the researchof next-generation radio access technologies. He
received the Young Researchers’ Award fromIEICE in 2019.

Satoshi Suyama

Takahiro AsaiManager, 6G-IOWN Promotion Department,
NTT DOCOMO, INC.
He received a B.E., M.E., and Dr.Eng. from
Tokyo Institute of Technology in 1999, 2001, and2010. From 2001 to 2013, he was an assistantprofessor at Tokyo Institute of Technology. Sincejoining NTT DOCOMO in 2013, he has been
involved in research and development of 5G/6G
radio access technologies. He received the Best
Paper Prize from the European Wireless Technology Conference (EuWiT) in 2009, the Best Paper
Award from the International Symposium on
Personal, Indoor and Mobile Radio Communications (PIMRC) in 2016, and the Paper Award
from IEICE in 2012.

NTT Technical Review

Vol. 19 No. 11 Nov. 2021

Deputy General Manager, 6G-IOWN Promotion Department, NTT DOCOMO, INC.
He received a B.E. and M.E. in electrical andelectronics engineering, and Ph.D. in communications and computer engineering from Kyoto
University in 1995, 1997, and 2008. In 1997, hejoined NTT Mobile Communications Network,
Inc. (currently, NTT DOCOMO, INC.). Since
then, he has been engaged in the research of signal processing for mobile radio communication.

51

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High-gain Optical Parametric
Amplifier Module by Using a PPLN
Waveguide and Its Application to
Quadrature Squeezing
Takahiro Kashiwazaki, Takeshi Umeki, Takushi Kazama,
Koji Enbutsu, Osamu Tadanaga, and Kei Watanabe
Abstract

Optical parametric amplification is expected as a key technique for high-speed optical fiber
communication and optical quantum computing. We developed a high-gain and broadband opticalparametric amplifier (OPA) module by using a periodically poled lithium niobate (PPLN) waveguide,
the single-mode core of which is directly bonded to a lithium tantalate substrate. The OPA is assembled
as a pump-combiner-integrated module with four optical-fiber input/output ports. By improving the
fabrication processes of the PPLN waveguide and the assembly techniques of OPA modules, we achieved
over-30-dB phase sensitive amplification with a continuous-wave pump. By using the PPLN waveguide,
we successfully obtained over-6-dB squeezed light, the spectral bandwidth of which was 2.5 THz. This
high-gain broadband OPA module will help build an advanced information society.
Keywords: periodically poled lithium niobate, optical parametric amplifier, squeezed light

1. Introduction
The explosive spread of information and communication technologies makes it possible to build an
intelligent social and economic system that intricately combines various technologies, such as artificial
intelligence and robotics technologies. These information technologies will not only improve our lives
but will also promote the sustainable development of
the global environment. To ensure this prediction,higher-capacity lower-latency communication andhigher-speed lower-power-consumption computation
are needed as the technological infrastructure for a
future society. However, these techniques areapproaching several limits. In optical fiber communication, we are facing limits such as the standard quantum limit for optical linear amplifiers [1] and nonlinear Shannon limit, which determines the practical
NTT Technical Review

Vol. 19 No. 11 Nov. 2021

capacity of optical fiber communication [2]. In thecomputing field, we are facing the limit of Moore’s
law for semiconductor microfabrication [3].
To break these limitations, we conducted research
on nonlinear optical devices and their applications to
optical fiber communication and optical signal processing. Nonlinear optics describe various nonlinear
responses of properties such as frequency and phaseof light with strong interaction in nonlinear media[4]. By handling a wide variety of these nonlinear
optical phenomena, it is possible to break through thelimitations in optical communication and computation mentioned above. For example, by using phasesensitive amplification (PSA) with an optical parametric amplifier (OPA), we can break the standard
quantum limit of conventional optical linear amplifiers on the basis of stimulated emission in long-haulfiber communication [5]. Optical phase conjugation
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by using an OPA can compensate for the nonlinear
distortions in optical fibers, which is one factor causing the nonlinear Shannon limit [6]. Wideband optical wavelength conversion also contributes to the
expansion of communication resources of wavelengths without reconverting to electrical signals [7].
Regarding computation, OPAs play key roles in nonvon Neumann computers, such as a coherent-Isingmachine (CIM) [8] and optical quantum computers[9]. In a CIM, an OPA is located in an optical fiber
loop and generates artificial spins that have a 0 or π
relative phase of light. With the use of time-domain
multiplexing and measurement feedforward techniques, the CIM can find solutions for NP (nondeterministic polynomial time)-hard problems such as
maximum cut problems. OPAs are also used forquantum light sources for various types of opticalquantum computing (QC). For discrete-variable QC,
OPAs can generate entangled photon pairs by spontaneous parametric down conversion processes [10].
OPAs also play an important role in continuous-variable (CV) QC because they generate a broadband
squeezed vacuum state of light, which is an essential
state to generate CV quantum entanglement [11].
Studies have shown the possibility of using OPAs inquantum measurement without electrical circuits[12]. This will be a key technique to achieve ultra-fast
QC.
With the demand of OPAs for such applications, wedeveloped OPA modules that are based on a periodically poled lithium niobate (PPLN) waveguide [13].
Thanks to its second-ordered nonlinearity, a PPLN
waveguide has advantages as an OPA media compared with third-ordered nonlinear media such as
high-nonlinear optical fibers. A PPLN-based OPA
operates at room temperature without Brillouin scattering under high-power-pump injection, which often
becomes a problem with third-ordered nonlinear
media, and has the capability of integration in opticalcircuits. In this article, we introduce our PPLN-waveguide-based OPA module and its application for PSA
and optical quadrature squeezing. Our PPLN-waveguide-based OPA module has high pump-power
durability, low insertion loss, and high optical parametric gain. The module will be key for future opticalcommunication and computation.
2. PPLN-waveguide-based OPA module
For various applications, an OPA should have afiber-pigtailed configuration with pump combiners.
Fiber-pigtailed devices can be used with various reliNTT Technical Review

Vol. 19 No. 11 Nov. 2021

able optical components, which are sophisticated in
optical communication technologies. The configuration promises easy installation into an optical communication system and various quantum technologies for out-of-lab application.
2.1 Waveguide fabrication
For achieving high-gain optical parametric amplification, a PPLN waveguide should have a small coreand high durability for a high-power pump and maintain the phase matching condition along it. Figure 1(a)shows the fabrication process of our PPLN waveguide. First, a periodically poled structure is formedby the electrical poling process [14] for an LN wafer.
The poling period is about 18 µm. We used a zincoxide (ZnO)-doped LN wafer because of its highresistance to photorefractive damage [15]. After poling, the wafer was directly bonded to a lithium tantalate wafer. This directly bonded structure also allows
us to use high-power light to obtain high nonlinearity[16]. The upper side of the PPLN layer was polishedinto a thin layer with a thickness of about 5.0 µm.
Then, by using photolithography and a dry etching
technique, the waveguide structure was obtained.
Figure 1(b) shows a cross-sectional view of a fabricated waveguide taken using a scanning electronbeam microscope. To avoid several problems due tothe existence of higher-order modes in the waveguide, our waveguide is designed as a single-mode
waveguide for light in the communication wavelength band. The waveguide has a length of about 45
mm, as shown in Fig. 1(c). The second-harmonic
generation (SHG) efficiency at the phase-matchedwavelength is 1200%/W, as shown in Fig. 1(d),which was acquired under 3-mW light injection witha wavelength of 1550 nm. Thanks to the fine processing, the phase matching curve is close to the ideal
sinc-squared shape. This means the waveguide’s core
has a uniform shape.
2.2 Module assembly
We assembled the fabricated PPLN waveguide as a
pump-combiner-integrated OPA module with four
optical-fiber input/output (I/O) ports [17]. Figure 2(a)shows an overview of the module. The module houses the fabricated PPLN waveguide, six lenses, four
dichroic mirrors, and four pigtailed optical fibers, as
shown in Fig. 2(b). To suppress optical loss between
the waveguide and optical fibers, we designed a spotsize-convertor at both input and output end regions of
the waveguide. Thanks to the optimum coupling ofboth signal and pump lights with the use of two
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(a)

(b)
Electrical poling

PPLN waveguide

ElectrodeZnO-doped LN

10 µm

LiTaO3

Direct bonding
Lithium tantalate(LiTaO3)(c)

Grinding & polishing
PPLN waveguide

45 mm
Photolithography

SH conversion efficiency (%/W)(d)

Patterned resist

Dry etching
Waveguide
45 mm

1400
1200
1000
800
600
400
200
0
1551.01551.51552.01552.5
Wavelength of input light (nm)1553.0Fig. 1. ( a) Fabrication process of our PPLN waveguide by using direct bonding and dry etching techniques. (b) Schematic
image of PPLN waveguide taken using scanning electron microscope. (c) Photograph of fabricated PPLN
waveguide. (d) SH conversion efficiency [9].

lenses, we obtained coupling losses of 0.5 dB at 1550
nm and 0.7 dB at 775 nm on each side. The total module insertion losses are 2.0 and 2.3 dB for the wavelengths of 1550 and 775 nm, respectively, which
include waveguide losses. To maintain the phasematching condition at any environmental temperature, we placed a Peltier device under the waveguide.
3. Application
Our PPLN-waveguide-based OPA module can beused for various applications. In this article, we focus
on degenerate PSA for long-haul optical fiber communication and optical quadrature squeezing for CV
optical QC.

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Vol. 19 No. 11 Nov. 2021

3.1 Over-30-dB PSA
Figure 3(a) shows the experimental setup fordegenerate PSA [18]. We used a continuous wave at1551.6 nm as a signal and pump source. The pumplight was amplified using an erbium doped fiber
amplifier (EDFA) and launched into a PPLN module
for SHG. The generated SH pump was launched into
the four-port PPLN module with the signal light tocause optical parametric amplification. The relative
phase between the signal and pump was locked using
a lead zirconate titanate (PZT)-based fiber stretcher
in accordance with the error signal from an opticalphase-lock-loop. The output signal light from the
PPLN device was detected using an optical spectrum
analyzer. Figure 3(b) shows the input and output
spectra of degenerate PSA. The difference between
the input spectrum and output spectrum without
54

Regular Articles(a)Communicationband light

SH-pump light(b)

Control pin

Peltier device

PPLN waveguide

Lens

Polarizationmaintaining fiber

Dichroic mirror

Fig. 2. ( a) Overview of fabricated PPLN-waveguide-based OPA module, which has four pigtailed optical fibers as I/O ports.
(b) Schematic of inside the fabricated module. The PPLN-waveguide length is 45 mm. The waveguide temperature
is controlled with a Peltier device mounted under it.

pump corresponds to the insertion loss of 2.0 dB. The
input spectrum is amplified with the external gain of
32.3 dB when the injected pump power is 1.35 W.
Figure 3(c) shows the external gains as a function of
the input pump power. The PSA gains are exponentially increased due to the increase in the SH-pump
power without any degradation due to photorefractive
damage. This means that our module showed highdurability against the high-power SH pump to operatewith a continuous-wave-pump power of over 1 W.
3.2 Optical quadrature squeezing
A squeezed vacuum state of light is a non-classicalstate, which has a reduced quantum uncertainty in a
specific amplitude quadrature compared with that ofthe vacuum state [19], as shown in Fig. 4(a). This
state should be used as an essential resource for various optical quantum technologies. For example, byusing squeezed light, optical beamsplitters, and optical delay line interferometers, we can generate multipartite optical entanglement, namely an optical cluster state, that is multiplexed in the time domain [20].
This is one of the most important calculation resourcNTT Technical Review

Vol. 19 No. 11 Nov. 2021

es in measurement-based QC, which has potential forhigh-speed large-scale fault-tolerant operation [21].
For achieving this large-scale universal and fault-tolerant QC, a highly squeezed state of light is desired
because it results in high-fidelity CV quantum gatesand high-dimensional quantum entanglement. For
high-speed QC, broadband squeezed light is desired
because the maximum clock rate of QC is limited by
the bandwidth of the quantum resource. A squeezed
light is generated by pumping an OPA and is typically characterized by measuring the variances of theelectrical field with balanced homodyne detection[22]. Figure 4(b) shows the measured noise intensities of squeezed light and the vacuum state of light. In
accordance with the scanning of the relative phasebetween the local oscillator and squeezed light,
squeezed and anti-squeezed noise are shown one after
the other. However, the noise of the vacuum state,
namely shot noise, shows a constant value despite thephase scanning. This is because, in the coherent vacuum state, quantum noise equally exists in two optical quadratures. The squeezed level shows over-6-dB
noise reduction compared with the vacuum noise.
55

Regular Articles(a)1551.6 nm
EOM

Laser

PPLN (SHG)

Pump

SignalFG

EDFA
Opticalspectrum
analyzer

4-port PPLN
(PSA)

PZT

PD
PID
controller

100Intensity (dBm)

−10

LPF

(c)

40

Output w/ pump
Input
Output w/o pump

35
30

−20

25

−30

Gain (dB)

(b)

−40
−50

20
15

−60

10

−70

5

−80

0

−90
1551.1DBM

Coaxial cable
PM fiber for 1.5-µm wavelength
PM fiber for 0.78-µm wavelength
DBM: double balanced mixer
FG: function generator
EOM: electro-optic modulator
LPF: low pass filter
PD: photodiodePID controller: proportional integral
differential controller
PM fiber: polarization-maintaining fiber
PZT: PZT-based fiber stretcher−51551.31551.51551.71551.91552.1

Wavelength (nm)

0

0.2

0.4
0.6
0.8
1
SH-pump power (W)

1.2

1.4

Fig. 3. ( a) Experimental setup for degenerate PSA. (b) Measured spectrum of amplified light [18]. (c) External gain of PSA
as a function of the input pump power [18].

This value exceeds 4.5 dB, which is required for thegeneration of a two-dimensional optical cluster state.
The squeezed light also shows 2.5-THz spectral
bandwidth, as shown in Fig. 4(c). This broad bandwidth indicates the possibility of THz-order clockrate QC. The squeezed light will lead to the development of a high-speed on-chip quantum processor
using time-domain multiplexing with a centimeterorder optical delay line.
4. Conclusion
We introduced our broadband high-gain OPA module assembled with a ZnO-doped PPLN waveguide.
The PPLN waveguide was fabricated by direct bonding and photolithography. Thanks to the fine process,we can obtain a uniform waveguide and SHG efficiency of 1200%/W. Its directly bonded core enabled
us to use over-1-W of pump light to achieve a highgain optical parametric process without pumpinduced degradation. The waveguide is assembled
NTT Technical Review

Vol. 19 No. 11 Nov. 2021

into a pump-combiner-integrated OPA module withfour optical-fiber I/O ports. By using this OPA module, we demonstrated over-30-dB PSA even with acontinuous-wave pump. This value is sufficient for
the amplification gain in long-haul optical communication. We also used the waveguide as an opticalquadrature squeezer and obtained over-6-dB squeezed
light. This value is sufficient for generating a twodimensional optical cluster state, which enables
multi-qubit computation. The squeezed light also has
a 2.5-THz spectral bandwidth. This broad bandwidth
indicates the possibility of THz-order clock-rate QC,
shortens the length of a flying qumode into sub-millimeter, and enables the implementation of opticaldelay line interferometers into an integrated opticalchip. This broadband high-gain OPA module will
break the limits in optical communication and computation.

56

Regular Articles(a)Vacuum state

Squeezed state
Quadraturephase

Quadraturephase
Squeezing
1/ G

Squeezing

In-phaseG
Anti-squeezing(b)

Noise of one quadrature is reduced
while the other is amplified.
(c)Noise power (dBm)−10
Noise of squeezed light

−15
−20
−25
−30

Shot noise

−35
−40

0

0.2

0.4
0.6
Sweep time (s)0.81Normalized light intensity (arb. unit)

Quantum noise equally exists for two
quadratures.

In-phase

1.2
1.0
0.8
0.6
0.4
0.2

283 mW
141 mW
0 mW

0.0
−0.2
190.4 191.4 192.4 193.4 194.4 195.4 196.4
Frequency (THz)

Fig. 4. ( a) Schematics of vacuum noise and squeezed vacuum noise. (b) Noise intensities of a squeezed state of light andvacuum state measured using a spectrum analyzer with balanced homodyne detection [8]. (c) Optical spectra forsqueezed light with various pump power [8].

References
[1]
[2]
[3]

[4]
[5]

[6]

[7]
[8]

C. M. Caves, “Quantum Limits on Noise in Linear Amplifiers,” Phys.
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R.-J. Essiambre, G. Kramer, P. J. Winzer, G. J. Foschini, and B.
Goebel, “Capacity Limits of Optical Fiber Networks,” J. Lightw.
Technol., Vol. 28, No. 4, pp. 662–701, 2010.
Semiconductor Industry Association, “International Technology
Roadmap for Semiconductors (ITRS) 2.0,” 2015 Edition, Executive
Report, https://www.semiconductors.org/resources/2015international-technology-roadmap-for-semiconductors-itrs/
J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan,“Interactions between Light Waves in a Nonlinear Dielectric,” Phys.
Rev., Vol. 127, No. 6, 1918, 1962.
M. Asobe, T. Umeki, and O. Tadanaga, “Phase Sensitive Amplification
with Noise Figure below the 3 dB Quantum Limit Using CW Pumped
PPLN Waveguide,” Opt. Express, Vol. 20, No. 12, pp. 13164–13172,
2012.
T. Umeki, T. Kazama, A. Sano, K. Shibahara, K. Suzuki, M. Abe, H.
Takenouchi, and Y. Miyamoto, “Simultaneous Nonlinearity
Mitigation in 92 × 180-Gbit/s PDM-16QAM Transmission over 3840
km Using PPLN-based Guard-band-less Optical Phase Conjugation,”Opt. Express, Vol. 24, No. 15, pp. 16945–16951, 2016.
S. J. B. Yoo, “Wavelength Conversion Technologies for WDM
Network Applications,” J. Lightw. Technol., Vol. 14, No. 6, pp. 955–
966, 1996.
T. Inagaki, Y. Haribara, K. Igarashi, T. Sonobe, S. Tamate, T. Honjo,
A. Marandi, P. L. McMahon, T. Umeki, K. Enbutsu, O. Tadanaga, H.

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Takenouchi, K. Aihara, K. Kawarabayashi, K. Inoue, S. Utsunomiya,and H. Takesue, “A Coherent Ising Machine for 2000-node
Optimization Problems,” Science, Vol. 354, No. 6312, pp. 603–606,
2016.
[9] T. Kashiwazaki, N. Takanashi, T. Yamashima, T. Kazama, K. Enbutsu,
R. Kasahara, T. Umeki, and A. Furusawa, “Continuous-wave
6-dB-squeezed Light with 2.5-THz-bandwidth from Single-mode
PPLN Waveguide,” APL Photonics, Vol. 5, No. 3, 036104, 2020.
[10] C. Hong and L. Mandel, “Experimental Realization of a Localized
One-photon State,” Phys. Rev. Lett., Vol. 56, No. 1, 58, 1986.
[11] H. Yonezawa, S. L. Braunstein, and A. Furusawa, “Experimental
Demonstration of Quantum Teleportation of Broadband Squeezing,”
Phys. Rev. Lett., Vol. 99, No. 11, 110503, 2007.
[12] N. Takanashi, A. Inoue, T. Kashiwazaki, T. Kazama, K. Enbutsu, R.
Kasahara, T. Umeki, and A. Furusawa, “All-optical Phase-sensitive
Detection for Ultra-fast Quantum Computation,” Opt. Express, Vol.
28, No. 23, pp. 34916–34926, 2020.
[13] Y. Nishida, H. Miyazawa, M. Asobe, O. Tadanaga, and H. Suzuki,“Direct-bonded QPM-LN Ridge Waveguide with High Damage
Resistance at Room Temperature,” Electron. Lett., Vol. 39, No. 7, pp.
609–611, 2003.
[14] L. E. Myers, G. D. Miller, R. C. Eckardt, M. M. Fejer, R. L. Byer, andW. R. Bosenberg, “Quasi-phase-matched 1.064-μm-pumped OpticalParametric Oscillator in Bulk Periodically Poled LiNbO3” Opt. Lett.,Vol. 20, No. 1, pp. 52–54, 1995.
[15] M. Asobe, O. Tadanaga, T. Yanagawa, H. Itoh, and H. Suzuki,“Reducing Photorefractive Effect in Periodically Poled ZnO- andMgO-doped LiNbO3 Wavelength Converters,” Appl. Phys. Lett., Vol.
78, No. 21, 3163, 2001.

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Converter Using Direct-bonded PPZnLN Ridge Waveguide,” IEEE J.
Quantum Electron., Vol. 46, No. 8, pp. 1206–1213, 2010.
[17] N. Takanashi, T. Kashiwazaki, T. Kazama, K. Enbutsu, R. Kasahara,
T. Umeki, and A. Furusawa, “4-dB Quadrature Squeezing with Fibercoupled PPLN Ridge Waveguide Module,” J. Quantum Electron., Vol.
56, No. 3, 6000100, 2020.
[18] T. Kashiwazaki, K. Enbutsu, T. Kazama, O. Tadanaga, T. Umeki, and
R. Kasahara, “Over-30-dB Phase-sensitive Amplification Using a
Fiber-pigtailed PPLN Waveguide Module,” Proc. of Nonlinear Optics
2019, OSA Technical Digest (Optical Society of America, 2019),
paper NW3A.2, Hawaii, USA, July 2019.

Takahiro Kashiwazaki[19] H. P. Yuen, “Two-photon Coherent States of the Radiation Field,”
Phys. Rev. A, Vol. 13, No. 6, 2226, 1976.
[20] W. Asavanant, Y. Shiozawa, S. Yokoyama, B. Charoensombutamon,
H. Emura, R. N. Alexande, S. Takeda, J. Yoshikawa, N. C. Menicucci,
H. Yonezawa, and A. Furusawa, “Generation of Time-domainmultiplexed Two-dimensional Cluster State,” Science, Vol. 366, pp.
373–376, 2019.
[21] S. Takeda and A. Furusawa, “Toward Large-scale Fault-tolerant
Universal Photonic Quantum Computing,” APL Photonics, Vol. 4, No.
6, 60902, 2019.
[22] H. P. Yuen and V. W. S. Chan, “Noise in Homodyne and Heterodyne
Detection,” Opt. Lett., Vol. 8, No. 3, pp. 177–179, 1983.

Koji Enbutsu

Researcher, Heterogeneous Materials andDevices Research Group, NTT Device Technology Laboratories.
He received a B.E. and M.E. in materials science from Keio University, Kanagawa, in 2013
and 2015. In 2015, he joined NTT Device Technology Laboratories, where he has been engaged
in research on waveguide devices for opticalcommunication and quantum computation. He isa member of the Institute of Electronics, Information, and Communication Engineers (IEICE)
and the Japan Society of Applied Physics(JSAP).

Research Engineer, Heterogeneous Materials
and Devices Research Group, NTT Device Technology Laboratories.
He received a B.E. and M.E. in electronics
engineering from Hokkaido University in 1994
and 1996. In 1996, he joined NTT Opto-electronics Laboratories, where he engaged in researchon organic optical waveguides for optical communications and electro-optic crystals and theirdevices. In 2007, he joined NTT Access Network
Service Systems Laboratories, where he engaged
in research on optical fiber testing and monitoring. He is a member of IEICE and JSAP.

Takeshi Umeki

Osamu Tadanaga

Distinguished Researcher, HeterogeneousMaterials and Devices Research Group, NTT
Device Technology Laboratories.
He received a B.S. in physics from Gakushuin
University, Tokyo, in 2002 and M.S. in physics
and Ph.D. in nonlinear optics from the Universityof Tokyo, in 2004 and 2014. He joined NTT
Photonics Laboratories in 2004, since then he has
been involved in research on nonlinear opticaldevices based on periodically poled LiNbO3
waveguides. He was a visiting researcher at the
National Institute of Standards and Technology(NIST), Boulder, CO in 2020. He is a member of
JSAP, IEICE, and the Institute of Electrical andElectronics Engineers (IEEE) Photonics Society.

Senior Research Engineer, HeterogeneousMaterials and Devices Research Group, NTT
Device Technology Laboratories.
He received a B.E. and M.E. in materials science and engineering from Kyoto University in
1993 and 1995 and a Ph.D. in nonlinear optics
from the University of Tokyo in 2009. He joined
NTT Opto-electronics Laboratories in 1995 andhas been involved in research on surface-normal
modulators, vertical cavity surface emitting
lasers, and quasi-phase-matched LiNbO3 wavelength converters. He is a member of IEICE andJSAP.

Takushi Kazama

Senior Research Engineer, HeterogeneousMaterials and Devices Research Group, NTT
Device Technology Laboratories.
He received a B.E. in electrical and electronics
engineering and M.E. and Dr.Eng. in mathematical and material science from Kobe University,
Hyogo, in 1998, 2000, and 2003. In 2003, hejoined NTT Photonics Laboratories, where heengaged in research on silica-based optical waveguides. From 2008 to 2009, he was a visiting
researcher at Ghent University, Belgium, where
he engaged in research on silicon photonics.
After returning to NTT, he has been engaged inresearch on indium phosphide high-speed modulators and nonlinear optical devices based on
PPLN waveguides. He is a member of JSAP,
IEICE, and the IEEE Photonics Society.

Research Engineer, Heterogeneous Materials
and Devices Research Group, NTT Device Technology Laboratories.
He received a B.S. and M.S. in electrical engineering from The University of Tokyo in 2009
and 2011. In 2011, he joined the NTT Photonics
Laboratories, where he has been engaged inresearch on nonlinear optical devices based on
PPLN waveguides. He is a member of IEICE and
JSAP.

NTT Technical Review

Vol. 19 No. 11 Nov. 2021

Kei Watanabe

58

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Development of Beyond 100G Optical
Cross Connect (B100G-OXC) System
Takeshi Kawasaki, Takeshi Seki, Ken Ito,
Yasutaka Sugano, Hiroki Date, Hiroki Kawahara,
Daisaku Shimazaki, and Hideki Maeda
Abstract

The rapid increase in Internet traffic is expected due to the spread of network services such as 5G(fifth-generation mobile communication), Internet of Things, and cloud. To economically expand thecapacity in the core network of the NTT Group, NTT Network Innovation Center developed the Beyond
100G Optical Cross Connect (B100G-OXC) system, which enables beyond 100-Gbit/s per wavelength
optical transmission. This article gives an overview of this system.
Keywords: optical transport, DWDM, multi-level modulation

1. Capacity enhancement in the core network
NTT has expanded the capacity of optical transportsystems to build both an efficient and economical
optical transport network to address the constantincrease in communication traffic in the core network, as shown in Fig. 1. In the current optical transport system, which has been deployed, we achieved amaximum of 8-Tbit/s capacity by multiplexing 80
channels of an optical signal with 100-Gbit/s per
wavelength through the use of digital coherent technology*1. To further enhance system capacity, wedeveloped the Beyond 100G Optical Cross Connect(B100G-OXC) system as a higher-capacity opticaltransport system by increasing the signal speed.
2. Technical features of B100G-OXC system
The system configuration of the B100G-OXC system is shown in Fig. 2. It is an optical transport system based on dense wavelength-division multiplexing (DWDM) and digital coherent technology(DWDM is the technology to transmit multiple wavelengths in a single fiber).
The B100G-OXC system is also called ROADM(reconfigurable optical add/drop multiplexer) since it
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can configure optical nodes as “add,” “drop,” or“through” for any arbitrary optical path in a maximum of eight directions. The main technical featuresof the B100G-OXC system are as follows:•	400-Gbit/s multi-carrier optical transmission•	Long-distance transmission of 100-Gbit/s optical signals•	Adaptation to low-loss optical fiber for longerdistance transmission•	Accommodation of the 400 Gigabit Ethernet(GbE) client interface
2.1 400-Gbit/s multi-carrier optical transmission
There are several ways to enhance capacity. One is
to increase the number of wavelengths for multiplexing, and the other is to increase the signal speed of the
optical channel per wavelength. In addition, there aremainly three ways to increase the signal speed of an
optical channel [1]:1. Increase the rate of modulation*1 Digital coherent technology: The technology that improves bothsensitivity and spectral efficiency by using a coherent detection
scheme with which an optical signal is treated as a wave. It also
provides high-speed signal processing that can compensate forthe distortion of waveforms when transmitting through opticalfiber.

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1P<3rd Gen>
Digital coherent technology

System capacity (bit/s/fiber)

100T
10T
1T
100G

B100G-OXC(16 Tbit/s)<2nd Gen>
DWDM technology
Optical amplification
technology

8 Tbit/s(100G x 80ch)<1st Gen>
Electrical multiplexer
TDM technology

10G
1G
100M
1990

1980

2000

2010

2020

Year
TDM: time division multiplexing

Fig. 1. Development history of optical transport systems at NTT.

Terminal PC for
surveillance and control

400-Gbit/s
multi-carrier opticaltransmission

Optical cross connect

NE-OpS

Adaptation to low-loss optical fiber
for longer-distance transmission
Optical amplifier

Max. of 8-directions

Transponder

Client signal
GbE/
10GbE/
100GbE/
400GbE

B100G-OXC
NE-OpS server

Optical fiber

Transponder

Transponder

Transponder

Transponder

Transponder

Distributed Raman amplifier

Transponder

100 Gbit/s/
200 Gbit/s/Long-distancetransmission of 100-Gbit/s
optical signals

Transponder

Accommodation of
400GbE client interface
Wavelength

NE-OpS: network operation system
PC: personal computer

Fig. 2. Overview of B100G-OXC system.

2. Increase the degree of the multiple modulation/
demodulation scheme
3. Enable multi-carrier optical transmission
With the B100G-OXC system, we increased thetransmission rate per wavelength from 100 to 200
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Gbit/s by changing the modulation scheme from dual
polarization-quadrature phase shift keying (DPQPSK), which is used in the conventional 100-Gbit/sbased optical system, to DP-16 quadrature amplitude
modulation (QAM), maintaining the baud rate at 32
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Table 1. Modulation scheme for the B100G-OXC system.
Data rate

100G

200G

400G

DP-QPSK

DP-16QAM

DP-16QAM(Multi-carrier opticaltransmission)

Q

Q

Q

Modulation format

Constellation map

I

I

I

Baud rate at single
wavelength

32 Gbaud

32 Gbaud

32 Gbaud

Number of symbols(bit/symbol)

2

4

4

Number of opticalcarriers

1

1

2

Image of optical
spectrum
Frequency
Transmission distanceApprox. 1000 km

Gbaud, which is that of the conventional system(Table 1). We also achieved maximum 500-km transmission at a 400-Gbit/s data rate with no regenerationby using 2 x 200-Gbit/s optical signals.
2.2 L ong-distance transmission of 100-Gbit/s
optical signals
In the core network, long-distance transmission is
as critical as capacity enhancement in terms of building a network more economically. There is generally
a trade-off between increasing the transmission rate
(higher-capacity transmission) and transmission distance. Although we can achieve a higher rate of transmission by increasing the degree of the multi-level
modulation scheme (for example, from DP-QPSK to
DP-16QAM), transmission distance can be decreased.
In the B100G-OXC system, we adopted DP-QPSK,
which is the same modulation scheme as in the conventional 100-Gbit/s-based optical transport system
to achieve 1000-km transmission without regeneration, which is almost double that of the conventional
system. The B100G-OXC system also enables us tochange the mode of the modulation scheme as well astransmission rate through software, so it is possible
for us to select the optimum type of modulation more
flexibly on the basis of the target distance of theroute.

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Frequency
Approx. 500 km

Frequency
Approx. 500 km

2.3 A daptation to low-loss optical fiber for
longer-distance transmission
We attribute 400-Gbit/s multi-carrier 500-km transmission to the fact that we adapted the B100G-OXC
system to the new cutoff shifted fiber (CSF, compliedwith ITU-T*2 G.654.E) as well as single mode fiber(SMF, compliant with ITU-T G.652) or dispersion
shifted fiber (DSF, compliant with ITU-T G.653).
CSF offers a lower loss profile than that of the othertypes of fibers for longer transmission. It also securesa larger effective area*3 by expanding the diameter ofthe optical fiber core, which allows us to suppress thedegradation of optical signals due to nonlinear effects
that can occur during the transmission through thefiber. As a result, we can achieve longer transmission
distance. The general optical characteristics of SMF,
DSF, and CSF are listed in Table 2 [2].
We also achieved high-power distributed Raman
amplification (DRA) in the B100G-OXC system as a
key technology for longer-distance transmission, as
shown in Fig. 3. DRA amplifies optical signals byusing optical fiber as an amplified medium, so its gain
*2 ITU-T: The Telecommunication Standardization Sector of International Telecommunication Union*3 Effective area: Indicator of how the optical power is distributed
around the core in the fiber. The larger its indicator, the weaker
the nonlinear effect that occurs in the fiber during transmission
since the optical-power density becomes lower.

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Table 2. Types of optical fibers and their characteristics.
Optical fiber type

DSF

CSF

G.652

G.653

G.654.E< 1260< 1270< 1530< 0.35< 0.35< 0.23Chromatic dispersion (ps/nm/km)*2, 3

16−23

2−8

17−23

Effective area (µm2)*2, 3

−80

−50

110−125

ITU-T standard
Cutoff wavelength*1 (Max) (nm)*2
Loss coefficient (Max) (dB/km)*2, 3

SMF*1 Cutoff wavelength is the minimum wavelength that the light in the fiber can transmit as a single mode.
If the wavelength of the light is shorter than the cutoff wavelength, it can transmit as multi-mode.
*2 Above figures are extracted from ITU-T Recommendation, document [2], and manufactures’ catalog.
*3 Value at 1550-nm wavelength.

Optical amplifier

CSF

Optical amplifier

DRA: Amplification by using optical fiber as anamplified medium

High-power lightsource for DRA

Optimum design of optical power
level to comply with IEC safety
standard IEC 60825-2

Optical power
level

Higher gain by high-power
DRA
Distance (longitudinal)

Fig. 3. High-power DRA.

characteristic depends on the fiber characteristics.
Initially lower gain of DRA was assumed for CSF
since it has a relatively larger core than those of otherfibers. We then achieved high-power DRA to obtainsufficient gain even under CSF. Safety design is also
important for long-term operation because we need tohandle a high-power excitation light source, which
has around 100 mW of output power. With the
B100G-OXC system, we determined the optimum
condition for the excitation light source for DRA toachieve maximum transmission performance while
maintaining safety by complying with IEC*4 608252, which is the international standard on the safety of
optical-fiber communication systems.

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2.4 Accommodation of 400GbE client interface
In the B100G-OXC system, we achieved theaccommodation of the 400GbE*5 client interface in
combination with 400-Gbit/s multi-carrier opticaltransmission. This allows us to connect with newlarge-scale routers or Layer 2 switches. The 400GbE
interface has been standardized and is now widelyused mainly in datacenters. Figure 4 shows a schematic of how 400GbE is transmitted. The incoming*4 IEC: International Electrotechnical Commission*5 400GbE: The new specification for the high-speed Ethernet clientinterface standardized as international standard (Institute of Electrical and Electronics Engineers (IEEE) 802.3bs). The data-transport speed is 400 Gbit/s, which is quadruple that of 100GbE(compliant with IEEE 802.3ba).

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200 Gbit/s

Line side
400GbE frame

Optoelectronic
convertor

Client side
OpticalmoduleFramer & signal
processing unit

Optoelectronic
convertor

Carrier 1
Frequency
Carrier 1 Carrier 2
Carrier 2

Mapping of 400GbE signal to 2 optical carriers
400GbE

OTUC4

OTUC
OTUC

OTUC
OTUC
OTUC
OTUC

Carrier 1

OTUC x 2
OTUC
OTUC

OTUC x 4

Carrier 2

OTUC x 2

Fig. 4. Multi-carrier transmission of 400GbE signals.

400GbE signal is mapped onto 4 x OTUC*6 frames in
the framer and signal processing unit inside the transponder (transceiver). Each OTUC frame has a
100-Gbit/s bit rate. Two OTUC frames are modulated
in a single optical carrier, which has a 200-Gbit/s rate,then 400GbE data are transmitted using 2 x 200-Gbit/s
optical carriers [3].

expanding transmission capacity.
References
[1]

[2]

3. Conclusion and future perspectives
We achieved both high-capacity and longer-distance transmission to build a core transport network
more economically. NTT is now researching anddeveloping the All-Photonics Network [4], which isone of the innovative optical transport platforms to
implement IOWN (Innovative Optical and Wireless
Network). We are aiming to further increase the speedof optical signals per wavelength such as 400 Gbit/s
or 1 Tbit/s. We will also target furthering high-capacity transmission by using multi-band transmission [5]to expand the transmission-wavelength range orusing space-division multiplexing technology such as
multi-core fibers [6]. We will continue to work on
further improving optical transport systems and

[3]

[4]

[5]

[6]

H. Maeda, K. Saito, T. Kotanigawa, S. Yamamoto, F. Hamaoka, M.
Yoshida, M. Suzuki, and T. Seki, “Field Trial of 400 Gbps
Transmission Using Advanced Digital Coherent Technologies,” J.
Lightw. Technol., Vol. 35, No. 12, pp. 2494–2499, June 2017.
K. Kikuchi, “Optical Information Network,” Ohmsha, 2002 (inJapanese).
K. Shintaku, S. Kuwahara, and S. Kobayashi, “Trends inStandardization of Mapping and Multiplexing Technologies for
Optical Transport Networks,” NTT Technical Review, Vol. 18, No. 12,
pp. 103–108, 2020.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202012gls.html
A. Itoh, “Initiatives Concerning All-Photonics-Network-related
Technologies Based on IOWN,” NTT Technical Review, Vol. 18, No.
5, pp. 11–13, 2020.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202005fa1.html
T. Seki, H. Kawahara, T. Miyamura, H. Maeda, K. Hara, S. Kaneko,
and J. Kani, “Evolution of Transmission Technology toward theRealization of All-photonics Network,” J. IEICE, Vol. 104, No. 5, pp.
463–469, 2021 (in Japanese).
T. Sakamoto, T. Mori, T. Matsui, T. Yamamoto, and K. Nakajima,“Research and Development of Next Generation Optical Fiber Using
Multiple Spatial Channel,” NTT Technical Review, Vol. 15, No. 6,2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201706fa5.html*6 OTUC: A 100-Gbit/s element (slice) of an optical channel transport unit-Cn (OTUCn). The index Cn denotes n × 100G, where n
is a positive integer and C means 100G.

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Takeshi Kawasaki

Senior Research Engineer, Transport System
Project, NTT Network Innovation Center.
He received a B.S. in physics from Keio University, Tokyo, in 2001 and M.A. in arts and sciences from the University of Tokyo in 2003. He
joined NTT Network Service Systems Laboratories in 2003 and studied optical transmission
systems. From 2011 to 2014, he was involved ininternational submarine cable projects at NTT
Communications. He also worked on the development project for optical transport systems andthe development project for the Beyond 100G
Optical Cross Connect system at NTT Network
Innovation Center.

Takeshi Seki

Senior Research Engineer, Transport Network
Innovation Project, NTT Network Service Systems Laboratories.
He received a B.S. and M.S. in electronics andapplied physics from Tokyo Institute of Technology in 2002 and 2004. In 2004, he joined NTT
Network Service Systems Laboratories, where
he was engaged in research on optical crossconnect systems. He is currently developing
wavelength division multiplexing (WDM) transmission systems. He is a member of the Instituteof Electronics, Information and Communication
Engineers (IEICE).

Ken ItoHiroki Date

Senior Research Engineer, Transport System
Project, NTT Network Innovation Center.
He received a B.E. and M.E. from Tokyo Institute of Technology in 2006 and 2008. He joined
NTT Network Service Systems Laboratories in
2008, where he was engaged in the R&D oftransport network systems and centralized control of IP and optical transport networks. From2016 to 2020, he was in charge of deploying new
optical transport network systems at NTT Communications. He is currently engaged in the R&D
of optical cross-connect systems with advanced
maintenance and operation functions.

Hiroki Kawahara

Engineer, Division of Infrastructure Design,
Department of Platform Services, NTT Communications Corporation.
He received a B.E. and M.E. in electrical, electronics, and information engineering from Osaka
University in 2009 and 2011. He joined NTT
Network Innovation Laboratories and NTT Network Service Systems Laboratories in 2011 andagain in 2017, where he was involved in the R&D
of large-capacity optical cross-connect and transmission systems. He is currently with NTT Communications. He is a member of IEICE.

Daisaku Shimazaki

Senior Research Engineer, Transport System
Project, NTT Network Innovation Center.
He received a B.E. and M.E. from Waseda
University, Tokyo, in 2005 and 2007. He joined
NTT Network Service Systems Laboratories in
2007, where he was engaged in the research anddevelopment (R&D) of transport network systems. He is currently developing 400-Gbit/s
packet transport systems. He is a member of
IEICE.

Senior Research Engineer, Supervisor, Transport System Project, NTT Network Innovation
Center.
He received a B.S. in applied chemistry and
M.S. in materials science from Keio University,
Kanagawa, in 1999 and 2001. He joined NTT
Network Service Systems Laboratories in 2001,
where he researched and developed multi-layer
network virtualization as a research engineer. He
has been in his current position since January
2014 and is engaged in the development project
for the Beyond 100G Optical Cross Connect
system at NTT Network Innovation Center. He isa member of IEICE.

Yasutaka Sugano

Hideki Maeda

Senior Research Engineer, Transport System
Project, NTT Network Innovation Center.
He received a B.E. from Tokyo Denki University in 1995. He joined NTT Communications in
2003, where he was engaged in the developmentof the wide area Ethernet system. In 2019, hemoved to NTT Network Service Systems Laboratories, where he has been engaged in the development project for beyond 100-Gbit/s packet
transport systems.

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Group Leader, Transport Network Systems
Project, NTT Network Innovation Center.
He received a B.S. and M.S. in electrical engineering from Tokyo University of Science in1992 and 1994, and received a Ph.D. from Hokkaido University in 2020, where he researchedand developed long-haul large-capacity transmission systems. He was the recipient of the
TELECOM System Technology Award from the
Telecommunication Advancement Foundation in
2019. Dr. Maeda is a senior member of IEICE.

64

Global Standardization Activities

Standardization Activities in the AsiaPacific Region—New Management
Structure Approved at the 33rd AsiaPacific Telecommunity
Standardization Program (ASTAP)
Meeting
Noriyuki Araki
Abstract

The 33rd Asia-Pacific Telecommunity Standardization Program (ASTAP) meeting was held in June
2021 in an online format. ASTAP aims at strengthening standardization activities in the information andcommunication technology field in the Asia-Pacific Telecommunity and contributing to the establishment of international standards as a region. This article describes the main results of ASTAP-33 and theindustry workshop held during the meeting.
Keywords: APT, ASTAP, standardization

1. Introduction
The Asia-Pacific Telecommunity (APT) is an international organization promoting information andcommunication technology (ICT) sector development in the Asia-Pacific region. It was established in1979 and has 38 member countries [1].
The APT Standardization Program (ASTAP) wasestablished in 1998 to promote standardization activities in APT [2]. The main objectives are to (1) contribute to international standardization by building acooperative framework for standardization within the
APT region, (2) foster standardization activists within the APT region and support skill development in
the ICT field by members within the region, especially members of developing countries, and (3) makeAPT common proposals to international standardization organizations, such as the International Telecommunication Union (ITU), as regional standardization
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organizations. It is important to use ASTAP as aplatform for building partnerships in the APT region
and contributing to developing countries. It is also
considered to be an important issue as a strategy forinternational collaboration on standardization in
Japan. The 33rd ASTAP meeting (ASTAP-33) was
held from June 7 to 11, 2021, and about 190 people
from 17 member countries participated. At the meeting, an industry workshop on the fifth-generation
mobile communication system (5G) and emergency
communications was held, and 142 people participated.
2. New management structure of ASTAPThe terms of office of the chair and vice-chair of
ASTAP are set to allow a maximum of two terms foreach term of three years. ASTAP-32 was held in
November 2020 at the timing of the expiration of the65Global Standardization Activities

Table 1. Organization and management structure of ASTAP.
Group

Chair

Vice-chair

ASTAP

Dr. Hyoung Jun Kim (Korea)

Dr. Hideyuki Iwata (TTC, Japan)
Mr. Xiaoyu You (China)

WG Policy and Strategic Co-ordination (WG PSC)

Mrs. Nguyen Thi Khanh Thuan (Vietnam)

Mr. Kaoru Kenyoshi (NICT, Japan)
Mr. Wu Tong (China)

Expert Group Bridging the Standardization Gap(EG BSG)Mrs. Nguyen Thi Khanh Thuan (Vietnam)

Mr. Ki-Hun Kim (Korea)
Dr. Hideyuki Iwata (TTC, Japan)

Expert Group Green ICT and EMF Exposure(EG GICT&EMF)

Dr. Sam Young Chung (Korea)

Mr. Min Prasad Aryal (Nepal)
Mr. Nur Akbar Said (Indonesia)
Mr. Uttachai Mannontri (Thailand)Expert Group ITU-T Issues (EG ITU-T)

Mr. Kaoru Kenyoshi (NICT, Japan)

Mr. Nguyen Van Khoa (Vietnam)

Expert Group Policies, Regulatory and Strategies(EG PRS)

Ms. Nadia Hazwani Yaakob (Malaysia)–

WG Network and System (WG NS)

Dr. Joon-Won Leen (Korea)

Dr. Hiroyo Ogawa (NICT, Japan)

Expert Group Future Network and Next Generation
Networks (EG FN&NGN)

Dr. Joon-Won Lee (Korea)

Mr. Kazunori Tanikawa (NICT, Japan)

Expert Group Seamless Access Communication
Systems (EG SACS)

Dr. Hiroyo Ogawa (NICT, Japan)–

Expert Group Disaster Risk Management and Relief
System (EG DRMRS)

Mr. Noriyuki Araki (NTT, Japan)–

WG Service and Application (WG SA)

Ms. Miho Naganuma (NEC, Japan)

Dr. Jee-In Kim (Korea)

Expert Group Internet of Things Application/Services(EG IOT)

Dr. Toru Yamada (NEC, Japan)

Dr. Seung-yun Lee (Korea)
Ms. Haihua Li (China)

Expert Group Security (EG IS)

Ms. Miho Naganuma (NEC, Japan)

Dr. Heuisu Ryu (Korea)

Expert Group Multimedia Application (EG MA)

Dr. Hideki Yamamoto (OKI, Japan)

Dr. Dong il Seo (Korea)

Expert Group Accessibility and Usability (EG AU)

Dr. Jee-In Kim (Korea)

Ms. Wantanee Phantachat (Thailand)terms of office of the chair and vice-chairs of ASTAP.
A new chair and vice-chairs were elected, and thework plans were confirmed at the meeting of theWorking Group (WG), including the review of working methods in response to the worldwide spread of
COVID-19 and the confirmation of the continuationof the organizational structure. Due to the COVID-19
pandemic, ASTAP-32 was held online for only 2days, so the Expert Group (EG) meeting by technology field was not held, and substantive discussion on
the new management structure started with ASTAP33.
Table 1 shows the organization structure of ASTAP
and the new management structure. At ASTAP-32,
Dr. Hyoung Jun Kim (Electronics and Telecommunications Research Institute (ETRI), Korea) was
appointed as the new chair, who served as vice-chair
until the previous meeting, and Dr. Hideyuki Iwata(The Telecommunication Technology Committee(TTC), Japan) and Dr. Xiaoyu You (The China Academy of Information and Communications Technology (CAICT), China) were appointed as vice-chairs.
The continuation of the chairs and vice-chairs of the
WG and EG was also confirmed and approved,
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respectively. At ASTAP-33, Mr. Kaoru Kenyoshi(National Institute of Information and Communications Technology (NICT), Japan) was newly appointed as vice-chair of WG PSC (Policy and Strategic
Co-ordination), and Mr. Noriyuki Araki (NTT,Japan), author of this article, was appointed as chairof EG DRMRS (Disaster Risk Management andRelief System).
3. Industry workshop
At ASTAP-33, the ASTAP vice-chair organized anindustry workshop on topics of interest to APT member countries. Based on the results of the questionnaire to APT member countries, ten presentations
were made in each session on two topics of 5G and
emergency communications from Japan, China,
Korea, Thailand, etc. The commercial introduction of
5G has started in APT countries including Japan,
China, and Korea, and it is a topic of interest for
countries that are about to start commercialization.
Natural disasters, such as typhoons and torrential
rains, have occurred frequently not only in Japan but
also throughout the APT region, and the social impact
66

Global Standardization Activities

Table 2. Industry workshop program.
Industry workshop on “5G & Emergency communications”
Introductory Remarks
by Hideyuki Iwata, TTC, Japan
Industry Workshop Corresponding Member

Session 1:
5G

Part I: 5G
Chair: Dr. Seungyun Lee, ETRI, Republic of Korea
1. 5G Technology, Standard and Industry Development by Yongming Liang, Huawei, P.R. China
2. Application of 5G in the Industrial Internet by Zongxiang Li, CAICT, P.R. China
3. Pioneering 5G Broadcast: Building on Multiple Generations of Cellular Broadcast Technology Leadership by Michael
Seongill Park, Qualcomm, Rep. of Korea
4. 5G Network Deployment and Service Development by Xiaoyin Zhao, China Telecom, P.R. China
5. Moving Towards Autonomous Networks Together by Wong Leon, Rakuten Mobile, Inc, Japan
Session 2:
Emergency
communications

Conclusion

Part II: Emergency communications
Chair: Mr. Noriyuki Araki, NTT, Japan
1. The Role of Emergency Messages in Covid-19 and the Future Direction in 5G by Seung-hee Oh, ETRI, Rep. of Korea
2. An intelligent Big Data Analysis System for Fire Management Using NB-IoT by Lun Li, CAICT, P.R. China
3. Enhancing Emergency Medical Service (EMS) Operation with Digital Technology: Case Study in Thailand by
Teerawat Issariyakul, National Telecom Public Company Limited, Thailand
4. Himawari Real-Time: A Web Application and Sharing System of Large-scale Data and High-resolution Images of
Himawari Meteorological Satellite across Asia-Pacific Region for Disaster Mitigations by Ken Takeshi Murata, NICT,
Japan
5. Resilient Information and Communication Technologies by Toshiaki Kuri, NICT, Japan
Conclusion of Industry Workshop
Mr. Xiaoyu You, CAICT, P.R. China

NB-IoT: narrow band Internet of Thingsof COVID-19 continues to grow. Table 2 shows the
program of the industry workshop.
Some of the presentations at the industry workshop
were discussed in technically relevant EGs such as
EG FN&NGN (Future Network and Next Generation
Networks) and EG DRMRS, and new technical documents (APT Report) on related technologies were
proposed as future work items.
4. Key output documents
At ASTAP-33, input documents were discussed at
WG and EG meetings, and ten APT Reports and two
Liaison statements to other standards bodies wereapproved as output documents at the plenary meeting. Table 3 lists the main output documents (APT
Reports and Liaisons) that have been approved.
Five APT Reports have been proposed by Japan andhave led the development of documents, accounting
for more than half the documents approved at thismeeting. The following two documents were proposed by TTC where NTT serves as an expert committee member:•	“Handbook to Introduce ICT Solutions for theCommunity in Rural Areas” (OUT-10)•	“APT Report on Traffic Accidental Record and
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its Analysis Method’s Guidelines in Asia” (OUT18)
The former is a compilation of examples of ICT
solution demonstration experiments in rural areas
conducted in cooperation with partners in Asian
countries and using the APT Project, etc. In this case,we added and revised a medical solution for tuberculosis in Myanmar. The latter indicates the importanceof devising countermeasures by recording and analyzing accidents for the purpose of reducing traffic
accidents. In this report, we investigated the situationof accident-record collection and analysis in Asian
countries and summarized the record items andanalysis methods as guidelines.
5. Future plans
Due to the fact that ITU events, such as World Telecommunication Standardization Assembly and World
Telecommunication Development Conference, havebeen postponed until 2022, and that there is a possibility that the schedule will be changed again depending on the COVID-19 situation, we determined that it
would be difficult to decide the date and venue of
ASTAP-34, and the schedule of ASTAP-34 has not
yet been decided. It is hoped that the ASTAP meetings
67

Global Standardization Activities

Table 3. Approved main output documents (APT Reports and Liaisons).
Title

Document number

Handbook to Introduce ICT Solutions for the Community in Rural Areas

ASTAP-33/OUT-10

Guideline on referencing int’l standards in developing national standards

ASTAP-33/OUT-11

Report of the Compliance Label of Communication Devices Implemented by APT Member Countries

ASTAP-33/OUT-12

APT Report on VoLTE Interoperability

ASTAP-33/OUT-13

Liaison Statement to ITU-D SG2 Q5/2

ASTAP-33/OUT-14

APT Report on Requirement of Transmitter in Coherent Radio over Fiber System

ASTAP-33/OUT-15

Revised APT Report on Radio-over-Fiber Relay Link for Indoor Communication Systems

ASTAP-33/OUT-16

APT Report on Traffic Accident Record and its Analysis Method’s Guidelines in Asia

ASTAP-33/OUT-18

Security Guidelines for Open Source Software

ASTAP-33/OUT-19

APT Report on interactive multimedia service on IPTV/CATV in the Asia-Pacific Region

ASTAP-33/OUT-20

Liaison Statement to ITU-T SG16, SG9 and ITU-D SG1

ASTAP-33/OUT-21

Report of Surveying Mobile Accessibility in the AP region
CATV: cable television
IPTV: Internet protocol television
ITU-D: ITU Telecommunication Development Sector

will be held in person to promote standardization
activities in the Asia-Pacific region and use ASTAP
more effectively as a forum for deepening cooperation with APT member countries.

ASTAP-33/OUT-24
ITU-T: ITU Telecommunication Standardization Sector
SG: Study GroupVoLTE: voice over Long-Term Evolution

References[1][2]

APT, https://www.apt.int/
ASTAP, https://www.apt.int/APTASTAP

Noriyuki Araki

Manager, Standard Strategy, Research andDevelopment Planning Department, NTT.
He received a B.E. and M.E. in electrical andelectronic engineering from Sophia University,
Tokyo, in 1993 and 1995. He joined NTT AccessNetwork Service Systems Laboratories in 1995,
where he researched and developed operationand maintenance systems for optical fiber cable
networks. He has been contributing to standardization efforts in ITU-T Study Group (SG)6 since2006. He was the rapporteur of Question 6 of
ITU-T SG6 from 2006 to 2008 and the rapporteur of Question 17 of ITU-T SG15 from 2008 to
2012. He also served as the chairman of the
ITU-T Focus Group on Disaster Relief Systems
and Network Resilience and Recovery. He has
been the vice-chairman of ITU-T SG15 since2013. He also contributes to the activities ofInternational Electrotechnical Commission(IEC) Technical Committee 86 (fiber optic systems). He received the ITU-AJ award from the
ITU Association of Japan in 2017. He is a member of the Institute of Electronics, Information
and Communication Engineers (IEICE).

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Event Report: NTT Communication
Science Laboratories Open House
2021
Kenichi Arai, Go Kato, Kenta Niwa, Hsin-I Liao,
Daiki Nasu, and Tomoki Ookuni
Abstract

NTT Communication Science Laboratories Open House 2021 was held online, the content of which
was published on the Open House 2021 web page at noon on June 3rd, 2021. The videos of 5 talks and29 exhibits presented our latest research efforts in information and human sciences and were played
more than 6000 times within the first month after the event.
Keywords: information science, human science, artificial intelligence

1. Overview
NTT Communication Science Laboratories (NTTCS Labs) celebrated its 30th anniversary this year.
NTT CS Labs has aimed to establish cutting-edge
technologies that enable heart-to-heart communication between people and people and between people
and computers. We are thus working on a fundamental theory that approaches the essence of humans andinformation science, as well as on innovative technologies that will transform society. NTT CS Labs’
Open House is held annually to introduce the resultsof our basic research and innovative leading-edge
research with many hands-on intuitive exhibits to
those who are engaged in research, development,
business, and education.
Open House 2021 was held virtually due to measures to prevent the further spread of COVID-19. The
latest research results were published with recorded
lecture videos on the Open House 2021 web page atnoon on June 3rd [1]. The content attracted many
views within a month not only from NTT Group
employees but also from businesses, universities, andresearch institutes. The event content is still available.
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This article summarizes the event’s research talks
and exhibits.
2. Keynote speech
Dr. Takeshi Yamada, vice president and head of
NTT CS Labs, presented a speech entitled “No matter
how far apart, my heart will always be with you –Exploring the essence of communication that will
create a spiritually rich society –,” in which he lookedback upon the history of NTT and establishment of
NTT CS Labs then introduced present and future
cutting-edge basic research and technologies(Photo 1).
For the past 30 years since its founding on July 4,
1991, NTT CS Labs has been developing innovativetechnologies such as media processing and machinelearning that approach and even surpass human capabilities and seeking out fundamental principles infields such as cognitive neuroscience and brain science that afford a deeper understanding of humans.
After introducing the latest research results of NTT
CS Labs from the three perspectives of “how to communicate accurately and efficiently” (information),69

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Photo 1. K eynote speech (Dr. Takeshi Yamada). “No matter how far apart, my heart will always be with you – Exploring theessence of communication that will create a spiritually rich society –.”“how to communicate by what means” (media), and“what is being indicated” (meaning), Dr. Yamada
declared that NTT CS Labs will boldly and persistently tackle new challenges for the next 30 years toenable communication that can be conveyed to theheart through research that only NTT CS Labs canconduct.
3. Research talks
The following three research talks highlighted
recent significant research results and high-profile
research themes. Each talk introduced some of thelatest research results and provided background andan overview of the research. After each talk, we
received many questions from participants in realtime for the question-and-answer (Q&A) session,confirming that the viewers had a high interest in our
research.
(1)	“The coming ages when AI becomes our conversation partners – Cutting edge of conversational systems with large-scale neural networkmodels –,” Dr. Hiroaki Sugiyama, Innovative
Communication Laboratory
Dr. Hiroaki Sugiyama explained chatting dialoguesystems for satisfying people’s desire for dialogue
through natural chatting with people. NTT CS Labs
has been researching various dialogue systems for along time, including research on giving systems personalities and improving the smoothness of dialogue
by organically linking multiple dialogue robots. With
the recent rapid development in deep learning, English social dialogue systems using large-scale deep
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learning have been proposed. He introduced details
of NTT CS Labs’ latest deep learning-based Japanesesocial dialogue system and its achievements andremaining issues (Photo 2).
(2)	“Looking more, acting better – New concept ofeye-hand coordination for skilled action –,” Dr.
Naotoshi Abekawa, Human and Information
Science Laboratory
Dr. Naotoshi Abekawa explained skillful motor
control using the mechanism of coordinated eye andarm motion as an example. One key issue in developing user-friendly information and communication
technology is to understand “behavior” or “action,”that is, to understand the essence of “motion.”
Although we seem to control motions easily, motor
controls are achieved by sophisticated brain mechanisms, including the control of eye movements toobtain target information and the generation of limb
movements. In his talk, he focused on the question,“Why is the eye important for skilled motor actions?”,
introduced explanations from the literature, and proposed an interpretation based on his findings(Photo 3).
(3)	“Developing AI that pays attention to who you
want to listen to – Deep learning based selective
hearing with SpeakerBeam –,” Dr. Marc Delcroix, Media Information Laboratory
Dr. Marc Delcroix discussed approaches to achieve
computational selective hearing. Selective hearingrefers to the ability to focus on listening to a desired
speaker, even in a noisy environment such as a cocktail party. He first introduced SpeakerBeam, which is70

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Photo 2. R esearch talk (Dr. Hiroaki Sugiyama). “The coming ages when AI becomes our conversation partners – Cuttingedge of conversational systems with large-scale neural network models –.”Photo 3. R esearch talk (Dr. Naotoshi Abekawa). “Looking more, acting better – New concept of eye-hand coordination forskilled action –.”a deep-learning-based method he and his coresearchers proposed to extract speech of a desired
target speaker in a mixture of several speakers byusing a few seconds of pre-recorded audio data of thetarget speaker. He then presented recent research on(1) extension to multi-modal processing, where heand his co-researchers use video of the lip movementof the target speaker in addition to audio pre-recording, (2) integration with automatic speech recognition, and (3) the generalization to the extraction ofarbitrary sounds (Photo 4).
4. Research exhibition
The Open House featured 29 exhibits displaying
NTT CS Labs’ latest research results. We categorized
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them into four areas: Science of Machine Learning,
Science of Communication and Computation, Science of Media Information, and Science of Humans.
Each exhibit prepared videos explaining the latest
results and were published on the event web page
(Photo 5). Several provided online demonstrations ordemo videos to make them closer to direct demonstrations. We also introduced a Q&A system in which
page visitors can freely post questions and comments
and CS Labs’ researcher can answer them. There
were more than 130 public questions, and some
expressed the enthusiasm of the visitors, such as longcomments and professional questions.
The following list, taken from the Open House
website, summarizes the research exhibits in eachcategory.
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Photo 4. R
 esearch talk (Dr. Marc Delcroix). “Developing AI that pays attention to who you want to listen to – Deep learning
based selective hearing with SpeakerBeam –.”Photo 5. Exhibition web page.

4.1 Science of Machine Learning• Learning how to learn from various datasets–	Meta-learning from tasks with heterogeneous
attribute spaces –• Ask me how to make a fair decision for everyone–	Learning individually fair classifier based on
causality –NTT Technical Review

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• Ask me anything about network structure–	Indexing graph structure with decision diagrams –• No labels? Count on me!
–	S elf-supervised adaptation for unknown
domains/classes –• Finding groups through connection relationships–	Relational data model with infinite flexibility –72

Information• Finding features in data fast and accurately–	Acceleration of feature selection with group
regularization –• Privacy-aware machine learning–	D istributed learning algorithm and medical
application –• Estimating risk of infection in a city–	People flow reconstruction based on anonymous
sensor data –4.2 Science of Communication and Computation•	Being greedy makes quantum computers work
well–	E conomic rationality makes cloud quantum
computing reliable –• Revealing hidden structures behind sentences–	Neural rhetorical structure parsing with pseudolabeled data –• Two experts, one result–	Fusing two experts for enhancing their specialties –• Can a chatbot mediate trust between humans?
–	Bridging doctor-patient rapport through a chatbot –• Recipes for enjoy-talking conversational systems–	Development of transformer-based conversational systems –4.3 Science of Media Information• Detecting faint sound by light–	Non-contact sound measurement by precisioninterferometry –• Extracting voices out of noise & reverberation–	Joint signal separation, dereverberation, andnoise reduction –• AI that acquires knowledge just by watching TV–	Crossmodal learning for concept acquisition ofhuman movements –• Real-time speech emotion controller using face–	Emotional voice conversion via facial expression
recognition –• Visualizing touched places for Corona prevention–	Touched places detection using heat traces bythermography –• Telestethoscope: Looking into body by listening–	Biomedical sound analysis utilizing physicalcharacteristics –4.4 Science of Humans• Is falling birthrate related to population density?
–	Prediction and demonstration via life history
theory –NTT Technical Review

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• Move of magnetic fields, move by magnetic fields–	Magnetact: A magnetic force-based tactile technology –•	Make hard objects soft, make rough objects
smooth–	Simple method for modulating tactile texture –•	Sense of touch connects our hearts beyond distance–	Empathetic telecommunication by vibrotactile
transmission –• Why do mothers approach to infants’ “crying”?
–	O xytocin as a neural regulator of maternal
implicit approach –• Auditory attention that appears in the eye–	Relation between microsaccades and auditory
spatial attention –• Blink pattern of elite car race drivers–	Professional drivers always blink at the sametime over laps –• Essence of “keeping your eye on the ball”– Eye-hand coordination in motor learning –• How do you define your dominant hand?
–	Quantifying motor-skill performance using asmartphone –• Brain functions to recognize and hit a fastball–	Brain mechanisms for quick judgment and motor
control –5. Special lecture
We asked Professor Asa Ito, professor of Tokyo
Institute of Technology, to give a special lecture
entitled “Bodies in the remote era,” have a talk withNTT Fellow Makio Kashino, and review the research
exhibitions. In the lecture, based on her research on
haptic communication with the visually impaired, shetalked about the problems of remote communicationand the potential of the body (haptics). As an exampleof how trust was created through haptic communication, she shared an episode in which she simulated theexperience of a visually impaired person being
accompanied by a runner, and the rope connected to
the runner quickly transmitted subtle informationsuch as emotions and sensations, resulting in a surprising level of trust. She mentioned, however, that
remote communication tends to be a one-way transmission of information, making it difficult to build a
relationship of trust. She also introduced her researchon watching sports with the visually impaired,remotely operated alter-ego robot communication,and memory of sensation and haptic perception. She
mentioned that in haptic remote communication, we
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can sometimes feel the existence of a person because
we cannot touch their actual body, and that she has
sensed the possibility for a new form of communication.
6. Concluding remarks
Open House 2021 was held as an event to present
our latest results on a website. The lecture videos
were viewed more than 6000 times in June by various
segments of users. Participants provided many valu-

Kenichi Arai
Senior Research Scientist, Signal Processing
Research Group, Media Information Laboratory,
NTT Communication Science Laboratories.
He received a B.S., M.S., and Ph.D. fromWaseda University, Tokyo, in 1991, 1993, and2003. He joined NTT Communication Science
Laboratories in 1993 and has been engaged inresearch on the application of nonlinear dynamics and stochastic systems to signal processing.
His current research interests include physicalrandom number generators based on a chaotic
laser and the prediction of intelligibility using an
automatic speech recognition system. He is amember of the Institute of Electronics, Information and Communication Engineers, the PhysicalSociety of Japan (JPS), and the Acoustical Society of Japan (ASJ).
Go KatoSenior Research Scientist, Computing Theory
Research Group, Media Information Laboratory,
NTT Communication Science Laboratories.
He received a doctor of science in physics fromThe University of Tokyo in 2004. He joined NTT
Communication Science Laboratories the same
year and has been engaged in the theoreticalinvestigation of quantum information. He is
especially interested in mathematical structures
emerging in the field of quantum information. He
is a member of JPS.

Kenta Niwa
Distinguished Researcher, Learning and Intelligent System Research Group, Innovative Communication Laboratory, NTT Communication
Science Laboratories.
He received a B.E., M.E., and Ph.D. in information science from Nagoya University in 2006,
2008, and 2014. Since joining NTT in 2008, hehas been engaged in research on microphonearray signal processing. From 2017 to 2018, he
was a visiting researcher at Victoria University of
Wellington, New Zealand and involved withresearch on distributed machine learning andmathematical optimization. He is especiallyinterested in distributed system optimization,such as model training on edge computing. He
also belongs to NTT Computer and Data Science
Laboratories. He was awarded the Awaya Prize
by ASJ in 2010. He is a member of the Instituteof Electrical and Electronics Engineers (IEEE).

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Vol. 19 No. 11 Nov. 2021

able opinions that have encouraged us to pursue further research activities through interactive means ofcommunication with CS Labs researchers such as the
Q&A system and Q&A session during the lectures. In
closing, we would like to offer our sincere thanks to
all the participants of this online event.
Reference[1]

Website of NTT Communication Science Laboratories Open House
2021, http://www.kecl.ntt.co.jp/openhouse/2021/index_en.html

Hsin-I Liao
Senior Research Scientist, Human Information
Science Laboratory, NTT Communication Science Laboratories.
She received a Ph.D. in psychology from the
Department of Psychology, National Taiwan
University, in 2009. She joined NTT Communication Science Laboratories in 2012 and has been
studying auditory salience, music preference,and preference of visual images. She has also
explored the use of pupillary response recording
to correlate human cognitive functions such as
auditory salience and preference decision. From2007 to 2008, she was a visiting student at California Institute of Technology, USA, where she
studied visual preference using recorded eye
movements and visual awareness using transcranial magnetic stimulation. She received a Best
Student Poster Prize of the Asia-Pacific Conference on Vision in 2008, a Travel Award of the
Association for the Scientific Study of Consciousness in 2011, and a Registration Fee
Exemption Award of the International Multisensory Research Forum in 2011. She is a memberof the Vision Sciences Society, Association for
Research in Otolaryngology, and Japan Neuroscience Society.
Daiki Nasu
Research Scientist, Kashino Diverse Brain
Research Laboratory, NTT Communication Science Laboratories.
He received a Ph.D. in medicine from the
Graduate School of Medicine, Osaka University,
in 2014. He joined NTT Communication Science
Laboratories in 2016 and has been engaged inresearch on human motor control and biomechanics in sports. He has recently been focusing
on the relationship between sports performanceskill and brain function in athletes. He is a member of the Society for Neuroscience, the Japan
Society of Physical Education, Health and Sport
Sciences, the Japanese Society of Biomechanics.
Tomoki Ookuni
Senior Research Scientist, Research and Planning Section, NTT Communication Science
Laboratories.
He received a B.A. in economics from Kyoto
University in 1990 and M.A. in marketing science from Osaka Prefecture University in 2004.
He joined NTT in 1990 and has been engaged inmanaging research and development at both NTT
WEST and NTT since 1998. He joined NTT
Communication Science Laboratories in 2019.

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External Awards
Best Paper Award
Winners: Ryosuke Matsuo, Jun Shiomi, Tohru Ishihara, Hidetoshi
Onodera, Kyoto University; Akihiko Shinya, Masaya Notomi, NTT
Nanophotonics Center/NTT Basic Research Laboratories
Date: June 3, 2021
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE)
For “Methods for Reducing Power and Area and BDD-based Optical Light Circuits.”
Published as: R. Matsuo, J. Shiomi, T. Ishihara, H. Onodera, A.
Shinya, and M. Notomi, “Methods for Reducing Power and Area and
BDD-based Optical Light Circuits,” IEICE Trans. Fundamentals,
Vol. E102-A, No. 12, pp. 1751–1759, Dec. 2019.
Young Researcher Award
Winner: Yoko Sogabe, NTT Computer and Data Science Laboratories
Date: August 6, 2021
Organization: The Institute of Image Electronics Engineers of Japan
For “Deep Learning-based Image Reconstruction for Compressive
Spectral Imaging with Spectrally Varying PSF.”
Published as: Y. Sogabe, M. Miyata, F. Kobayashi, S. Sugimoto, T.
Kurozumi, T. Hashimoto, and Y. Hiwasaki, “Deep Learning-based
Image Reconstruction for Compressive Spectral Imaging with Spectrally Varying PSF,” Proc. of Media Computing Conference 2021,
June 2021.
Infographics VQA runners-up
Winners: Ryota Tanaka and Kyosuke Nishida, NTT Human Informatics Laboratories
Date: September 6, 2021
Organization: 16th International Conference on Document Analysis
and Recognition (ICDAR 2021)
For “IG-BERT: Learning Text-Icon-Layout Representations andArithmetic Operations for Infographic Understanding.”
Published as: R. Tanaka and K. Nishida, “IG-BERT: Learning TextIcon-Layout Representations and Arithmetic Operations for Infographic Understanding,” ICDAR 2021, Apr. 2021.
Best Presentation Award / Best Industry Presentation
Award
Winners: Kenji Tanaka, Yuki Arikawa, Tsuyoshi Ito, NTT Device
Technology Laboratories; Kazutaka Morita, NTT Software Innovation Center; Naru Nemoto, NTT Device Technology Laboratories;
Fumiaki Miura, NTT Software Innovation Center; Kazuhiko Terada,
NTT Device Technology Laboratories; Junji Teramoto, NTT Software Innovation Center; Takeshi Sakamoto, NTT Device Technology
Laboratories
Date: September 10, 2021
Organization: The IEICE Technical Committee on Reconfigurable
Systems (RECONF)
For “With GPU-FPGA Heterogeneous Computing, Highly Effective Communication for Distributed Deep Learning.”
Published as: K. Tanaka, Y. Arikawa, T. Ito, K. Morita, N. Nemoto,F. Miura, K. Terada, J. Teramoto, and T. Sakamoto, “With GPUFPGA Heterogeneous Computing, Highly Effective Communication

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for Distributed Deep Learning,” IEICE-RECONF2020-19, Sept.
2020.
Distinguished Contributions Award
Winner: Yasushi Takatori, NTT Access Network Service Systems
Laboratories
Date: September 14, 2021
Organization: IEICE Communications Society
For his contributions to the IEICE Communications Society as a
sub-editor of EB journal.
Distinguished Contributions Award
Winner: Yusuke Asai, NTT Access Network Service Systems Laboratories
Date: September 14, 2021
Organization: IEICE Communications Society
For his contributions to the IEICE Communications Society as a
sub-editor of EB journal.
Distinguished Contributions Award
Winner: Wataru Yamada, NTT Access Network Service Systems
Laboratories
Date: September 14, 2021
Organization: IEICE Communications Society
For his contributions to the IEICE Communications Society as ajournal reviewer.
Distinguished Contributions Award
Winner: Tomoki Murakami, NTT Access Network Service Systems
Laboratories
Date: September 14, 2021
Organization: IEICE Communications Society
For his contributions to the IEICE Communications Society as ajournal reviewer.
Distinguished Contributions Award
Winner: Motoharu Sasaki, NTT Access Network Service Systems
Laboratories
Date: September 14, 2021
Organization: IEICE Communications Society
For his contributions to the IEICE Communications Society as ajournal reviewer.
Distinguished Contributions Award
Winner: Takeshi Onizawa, NTT Access Network Service Systems
Laboratories
Date: September 14, 2021
Organization: IEICE Communications Society
For his contributions to the IEICE Communications Society as a
sub-editor of JB journal.
Distinguished Contributions Award
Winner: Hayato Fukuzono, NTT Access Network Service Systems
Laboratories

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Papers Published in Technical Journals and Conference Proceedings

Date: September 14, 2021
Organization: IEICE Communications Society
For his contributions to the IEICE Communications Society as ajournal reviewer.

IEICE ComSoc Distinguished Contributions Award
Winner: Tatsuya Shimada, NTT Access Network Service Systems
Laboratories
Date: September 15, 2021
Organization: IEICE Communications Society
For his contribution to the IEICE Communications Society as amember of the Council of Technical Committee Representatives.

Papers Published in Technical Journals andConference Proceedings
Task Clustering Method Using User Interaction Logs toPlan RPA Introduction
Y. Urabe, S. Yagi, K. Tsuchikawa, and H. Oishi
Proc. of the 19th International Conference on Business Process
Management (BPM 2021), Lecture Notes in Computer Science, Vol.
12875, pp. 273–288, Springer, Cham., August 2021.
Robotic process automation (RPA) software is a powerful tool that
can automate business operations to reduce manual labor while
improving operational quality by eliminating input errors. In order toefficiently and effectively improve business operations with RPA, it
is necessary to clarify the types and volumes of actual business
operations being performed by the employees and improve operations that have a large volume and are performed repeatedly. Userinteraction (UI) logs consist of users’ activities performed on thecomputer and can be collected regardless of the business system orapplication to understand how employees work. However, it is difficult to understand the types and volumes of the executed tasks fromsuch data because the task types are not recorded explicitly. In thiswork, we propose a method that clusters UI logs into task types tohelp analyzers identify high-volume and repetitive tasks for RPA
introduction. As the operation types differ by task type, we utilize thischaracteristic to analyze the co-occurrence of operations and segment
UI logs into a sequence of the same task types. Then, we perform
clustering based on the operation types contained in the segments. We
evaluated our approach using UI logs generated from actual scenarios
in a workplace, and report the results and limitations.

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Vol. 19 No. 11 Nov. 2021

Gaze Control during Reaching Is Flexibly Modulated to
Optimize Task Outcome
N. Abekawa, H. Gomi, and J. Diedrichsen
Journal of Neurophysiology, Vol. 126, No. 3, pp. 816–826, Sept.
2021.
When reaching for an object with the hand, the gaze is usually
directed at the target. In a laboratory setting, fixation is strongly
maintained at the reach target until the reaching is completed, a phenomenon known as “gaze anchoring.” While conventional accountsof such tight eye-hand coordination have often emphasized the internal synergetic linkage between both motor systems, more recent
optimal control theories regard motor coordination as the adaptive
solution to task requirements. We here investigated to what degree
gaze control during reaching is modulated by task demands. We
adopted a gaze-anchoring paradigm in which participants had toreach for a target location. During the reach, they additionally had tomake a saccadic eye movement to a salient visual cue presented at
locations other than the target. We manipulated the task demands byindependently changing reward contingencies for saccade reaction
time (RT) and reaching accuracy. On average, both saccade RTs andreach error varied systematically with reward condition, with reachaccuracy improving when the saccade was delayed. The distributionof the saccade RTs showed two types of eye movements: fast saccades with short RTs, and voluntary saccade with longer RTs.
Increased reward for high reach accuracy reduced the probability offast saccades but left their latency unchanged. The results suggest thatgaze anchoring acts through a suppression of fast saccades, a mechanism that can be adaptively adjusted to the current task demands.

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