NTT Technical Review
March 2013 Vol. 11 No. 3
View from the Top
Hiromichi Shinohara
Director and Executive Vice President, Director of Researchand Development Planning Department, NTT

Feature Articles: Wireless Access Technologies that
Enhance Convenience by Network Linkage
Current Status, Issues, and Future Perspective ofWireless Home Networks
Public Wi-Fi Initiatives at NTT DOCOMO
High-speed Wireless LAN for
Broadband Wireless Home Networks
Flexible Wireless System for
User-centric Wireless Home Network
Scalable M2M Wireless Access System

Regular Articles
Virtual Sensor Construction Technique forParticipatory Sensing Environments

Global Standardization Activities
Trends in the International Standardization of
HVDC Power Feeds

External Awards
External Awards

View from the Top

Listen Carefully to the Voice of
Stakeholders and Produce Resultsthat Exceed their Requests—Sustainable, Open, Agile, and Global
R&D as Keywords

Hiromichi Shinohara
Director and Executive Vice President,
Director of Research andDevelopment Planning Department, NTT
Overview

NTT’s pioneering research and development (R&D) division continues to pursue innovation in theface of great change in the industry. We asked Hiromichi Shinohara, NTT Executive Vice President andDirector of the Research and Development Planning Department, to tell us how he plans to manage thiseffort with a strategy based on four motivating keywords.

Facing change with the right attitude
based on four keywords––Mr. Shinohara, you have been involved in researchand development (R&D) management and planningfor quite some time. Looking back at the path taken todate, how do you think the NTT Group should posture
itself going forward?
In the past, most R&D at NTT was undertaken bycreating five- and ten-year visions as in a planned
economy and working accordingly to meet the objectives set therein. Today, however, it goes without saying that world trends are changing at an increasingly
rapid pace.


In contrast to the previous era in which new technologies brought great change to society, today it iscustomer needs that are becoming a major factor driving the world we live in. At the same time, intensecompetition and other factors in the external environment are having a big impact on the market.
Under these conditions, I don’t think we will beable to survive unless we combine three different
approaches. The first is pursuing R&D with a longterm vision, the second is pursuing R&D that anticipates customer needs and creates new trends, and thethird is cultivating a development mindset that allows
us to quickly identify and respond to new social
trends over the short term. This calls for a new management style.
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I can describe my management policy as “sustainable, open, agile, and global.” I would like to explain
the meaning of these four keywords.
Technology development that blossoms
over many years of effort—adopting asustainable approach to research
As the word implies, “sustainable” means the ability to continue on. This is a keyword directed toward
our researchers. It’s easy to terminate a certain line ofresearch but nearly impossible to resume it. When
research is being done on a certain topic, it may happen that a totally unexpected discovery comes alongthat puts that research on a path different from theoriginal objective. In any case, research must be continued in order to bear fruit.
For example, R&D of optical fiber technology
began about two years before I entered NTT and has
continued ever since to produce the amazing resultsthat we see today. The concept of “scrap and build”
may sound appealing, but when it comes to researchand development, being resolute in purpose is a must.
So to all our fine researchers, I say, “Let’s pursue
research and development in a sustainable manner.”
Adopting an open attitude:“raising our hand” to reveal ourselves
At present, the areas covered by information communications are extensive—much more so than in thetelephone era—and the supporting technologies arebroad as well. It should be obvious that it is becoming
impossible for us to cover all of these areas by ourselves especially if we are to face the changes aroundus with a sense of urgency. Researchers, however, are
apt to want to research a certain topic all the way fromstart to finish; they crave that feeling of individual
accomplishment. This is why I am firmly committed
to a theme of open innovation and collaboration andam accelerating its adoption. And here, I’m not justtalking about tie-ups with outside companies. I
believe it’s important that we work with parties
within the NTT Group and the NTT R&D Laboratories
Group, too. Furthermore, I don’t mean collaboration
just in the complementary sense in which we ask others to provide the technological expertise we don’t
possess. It is also important that we search out cooperative methods in which we can make “1+1 = 3” bysynergistically joining forces.
Let me give an example of how collaboration
within NTT R&D produced an effective result.
Vol. 11 No. 3 Mar. 2013

There was a need at one time to install optical fiber
into older condominium buildings, but the existing
conduits had little extra space to accept new wiring.
The group researching and developing optical fiber
and the group researching cable materials consequently joined forces in a joint project to tackle thisproblem. In the end, they succeeded in developing
thin and low-friction indoor optical cables that could
be smoothly slipped into tight places, thereby resolving the problem. This was the first cable of this typein the world, and try as they might, other companies
around the world could not duplicate it. I think it’sfair to say that such a result could be achieved precisely because research and development at NTT
R&D is so broad-based.
I can also give an example of how collaboration
between NTT R&D and a party outside NTT is producing good results. I’m referring specifically toJubatus, which is a distributed processing frameworkfor deep analysis of big data in real time, which we
jointly developed with some young developers of aJapanese venture company. Up until now, our development style had been for the most part to placeorders with manufacturers for products that we decided to develop based on our technology. Now, however, we are changing this style and adopting anapproach in which we try to produce something in ajoint manner.
In addition, our R&D efforts are producing resultsthat can be used and applied for purposes other thancommunications. One example is Revtrina, a reverberation control system that received a Japan Audio
Society prize in December 2012. It was originallydeveloped as a technology for improving the soundquality of phone calls, but we encountered opportunities to provide this technology to companies in fields
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outside of communications for adoption in audio
equipment and other devices. Incidentally, as a technology that can control reverberation without having
to reprocess the original sound, it has been highly
praised by music distribution companies that own the
publication rights to the music of rock bands such as
Queen and the Rolling Stones, and it has even been
adopted by movie theaters for use in screening movies. In short, Revtrina has gained a good reputation
among professionals in various fields.
Moreover, as announced in a recent press release,
we have successfully fabricated a conductive composite material that is mild on the skin by coatingfiber with a conductive polymer. This product demonstrates the potential to continuously monitor aperson’s heartbeat and to perform electrocardiogram
tests via a piece of clothing the person is wearing, andit holds the promise of tie-ups with companies in the
medical field and other industries. Looking forward,
I would like to see more of our R&D results be put tosuch positive use.
Although the NTT name is and has always been
known for communications, a diverse array of
byproducts has been coming out of the R&D process
for communications. For example, the laser wasdeveloped for use in communications, but it has alsocome to be particularly useful in identifying the district where rice was produced. This is done by lasinga grain of rice to determine its water content. While I
don’t think such applications will make a name for
NTT as strongly as communications has, I think it isimportant that we have many technologies that canmake a viable contribution to society by simply looking at them in a slightly different way. I also believethat we should use our technologies to develop busi-

ness in new fields to provide the firm with newsources of revenue in addition to those from voicerelated operations. In short, we should apply anddevelop the technological assets that we as a company
possess.
In this way, I think it will become possible for us tocontribute to new fields and create value by collaborating with other industries with which we have had
no points of contact in the past. To this end, I would
like to see us take on a mindset such that when determining whether the technologies that we are researching and developing can be put to good use, we consider not just our own ideas but also the ideas of othercompanies and groups that wish to collaborate.
Likewise, I would like to see us become accustomed
to creating opportunities for others to get to know ourtechnologies.
We also need to reflect on a few things in relation toNTT’s customary approach of taking the initiative insearching out a partner for collaboration purposes. To
begin with, ideas in this regard have so far onlyreflected the viewpoint of NTT. This creates a narrowperspective with the result that a hunt for a partnertakes more time. However, making this a bidirectional process can shorten the time involved as well as
broaden our perspective. In addition, each prospective partner has its own practices, dynamics, andneeds. Analyzing such characteristics takes time initself. If, on the other hand, we make an effort toreveal ourselves and “raise our hand” to attract attention to the technologies that we possess, it should bepossible to promptly get the partner we need to perform the selection process for us and become a valued
partner.
Responding to customer needs with a sense ofurgency: agile, on-sight development
Our past approach to research and development
was to provide NTT’s business companies withsound, complete products, or, to put it another way,
products “scoring a full 100 points.” However, considering that even “80-point” products should be
acceptable to customers, I think we should make achange to a “beta version” approach in which we
provide such products as promptly as possible tomeet customer needs. Then, once these products arein the customers’ hands, we can make on-site visits tothe business companies and refine these products asneeded while listening to what customers have tosay.
NTT DOCOMO’s voice-activated concierge



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service called “Shabette Concier” is one example ofthis approach. The original technology used here foranswering user questions on a mobile phone wasdeveloped by NTT DOCOMO itself. The service
evolved, however, when NTT researchers visited
NTT DOCOMO and worked with their staff to further research and develop the technology. This collaborative process resulted in the introduction ofJapanese-language processing and a more enjoyable
conversational experience.
We have begun to apply our technologies to otherfields as well. For example, with the aim of improving customer satisfaction and creating even better
services, we have brought voice-recognition, emotion-estimation, and other technologies from NTT
R&D to NTT contact centers where comments andclaims are received so that customer input can beanalyzed in a more accurate manner.
––Has this 80-point concept been well received?
First of all, establishing specific criteria for achieving “80 points” in a product is difficult. Although we
may know when a certain product has a “passing”
grade, we may be totally in the dark as to what customer needs must be satisfied to achieve a 100-point,
complete product. I think that the portion of productdevelopment that must be performed to reach a perfect score requires a great deal of effort since it
requires that we first determine what it is that customers really need and then accurately reflect theirdemands in the product. It’s exactly because we live
in an era of diverse customer needs that deciding on
what makes a 100-point product is so difficult. This is
why it is necessary to complete a product at sites
close to the market.
Becoming a global leader in newworld markets and fields
NTT R&D has expanded its activities to include
standardization and tie-ups with overseas universities
as well as the global provision of R&D results.
Last year, we succeeded in selling more than 10
billion yen worth of optical devices and encryption
technology for delivering video of the Olympic
Games, the World Cup, and other events. Furthermore,in addition to tie-ups with venture companies andcooperative activities with the open source community, we are also expanding our proposals for new
open source software on a global basis.
At the same time, the international expansion ofVol. 11 No. 3 Mar. 2013

NTT business is accelerating rapidly. In this regard, I
would like to actively provide technologies that wehave researched and developed to overseas groupcompanies that have been increasing in number
through merger and acquisition activities. I also thinkthat we need to create an environment conducive tocreating new products as a global business strategy. In
the United States, open innovation thrives, markets
are active, regulations are relatively easy to satisfy,
and venture companies are abundant. I want R&D to
support NTT’s global strategy announced in lastyear’s Medium-Term Management Strategy by setting up a new R&D base in North America that is
blessed with this open environment, and using thisbase as a means of providing the services that are
researched and developed in North America throughout the world.
––Please tell us more about this R&D base in North
America and other activities tied to NTT’s MediumTerm Management Strategy.
Preparations are now in place to open this North
America R&D base in April 2013. The plan is tolocate the base on the west coast and to begin with the
R&D of security- and cloud-related technologies andservices. I would like to start off with 30–40 employees and expand the staff gradually over time. We plan
to build a vigorous team composed of both locally
hired personnel and staff members drawn from groupcompanies, and with this team, we intend to promote
open innovation without delay from North America
while listening closely to market demands. The current name planned for this R&D base is NTT
Innovation Institute, Inc. (provisional), or NTT I3 forshort. As opposed to creating a perfect organization
for this base from the very start, we intend to makeadjustments as needed as the institute grows.
Furthermore, with an eye to the next stage of R&D,
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we will need to achieve a simple, friendly, andpersonalized UI/UX (user interface/user experience).
Let’s take the smartphone as an example; it has beencalled a very convenient device, but is it really thatconvenient for all users? Some people think smartphones have too many functions that actually makethem less convenient than they thought. I would liketo combine smartphone technology with fundamental
technologies like voice recognition and image processing from NTT R&D so that we can provide userfriendly products for all types of people that can becustomized with customer input instead of products
having a uniform set of specifications.
Becoming sustainable and open by working
with conviction and listening to stakeholders––Mr. Shinohara, please leave us with a few words
for NTT researchers.
First of all, I would like to see our researchers
approach their work in a multifaceted way: not just on
a theme-driven basis but also in a sustainable andopen manner. Additionally, instead of simply taking
the development requests received from NTT’s business companies in a literal fashion, I would ask
researchers to reflect on and search out the real, substantial meaning of those requirements. That is to say,
I would like our researchers to listen carefully to what
the business companies—NTT stakeholders—have

to say, and to then combine their requests with theirown knowledge of technology trends with the aim ofproviding technologies and services that even exceed
those requests.
Finally, I would like our researchers to believe inthemselves and work with conviction. To each andevery researcher I say: “Please carry out your duties
with purpose while absorbing all the stimuli you needfrom the outside.” This is what I look forward to seeing NTT researchers do.

Interviewee profile
Career highlights
Hiromichi Shinohara joined Nippon Telegraphand Telephone Public Corporation (now NTT) in
1978. He took up the post of Project Manager in
NTT Access Network Systems Laboratories in
1998, General Manager of the Access Network
Service Systems Laboratories of the Information
Sharing Laboratory Group in 2003, and Directorof the Information Sharing Laboratory Group in
2007. He became Senior Vice President andDirector of the Research and Development
Planning Department of NTT in 2009 and took up
his present position in June 2012.

NTT Technical Review

Feature Articles: Wireless Access Technologies that Enhance
Convenience by Network Linkage

Current Status, Issues, and Future
Perspective of Wireless Home
Networks
Yasushi Takatori, Masato Mizoguchi, Kazuhiro Uehara,
Shuichi Yoshino, Mitsuru Harada, and Kazuyasu Okada
Abstract

Wireless access is now widely available in home networks. This allows network access from anywherein the home. When wireless home networks become more secure and reliable and can be deployed moreeasily, more people will be able to use home network services. This article explains the current status
and issues of home networks. It also introduces the concept of a user-centric wireless home network thatis under development at NTT Network Innovation Laboratories.

1. IntroductionMore and more people have started using wirelessaccess at home in recent years. The changes in theway home networking is used are the backdrop of thistrend. As shown in Fig. 1, at the beginning of theInternet era in the early 1990s, the only networked
device was the desktop personal computer (PC), andfew people were using the Internet at home. In contrast, optical broadband network access has now
penetrated the entire country, and various network
devices, e.g., smart phones, tablets, gaming machines,
and PC peripheral devices, are now used at home. In
addition, people are using home networks in various
ways, e.g., data communication using the Internet,
sound communication using a wireless headset andwireless microphone, and communication with sensing devices. This change has been possible because of
the introduction of various wireless systems as shownin Fig. 2. There are three major wireless access systems: (1) wireless local area networks (LANs), used
for Internet access [1], (2) Bluetooth, used for wireless headsets and healthcare appliances, and (3) ZigBee, used for communication with sensing devices
for network services such as visualizing the power
consumption of home appliances.
Most smartphones and tablets now use wirelessLANs. In home networks, they are mainly used fordata communications. The current wireless LAN
standards are IEEE 802.11b/g for the 2.4-GHz band,IEEE 802.11a for the 5-GHz band, and IEEE 802.11n
for the 2.4/5-GHz band. The next standards, i.e.,IEEE 802.11ac and IEEE 802.11ad, are currently
under development and will apply to gigabit throughput. Draft documents have already been issued, andprocedures are being developed to complete the standard. Wireless LANs based on the IEEE 802.11acdraft are expected to be available by April or May of
2013. Wireless broadband access will undoubtedly
become more common after the launch of the IEEE
802.11ac standard.
Bluetooth has also become a popular wireless system. It uses the 2.4-GHz band and is based on theIEEE 802.15.1 standard, which targets wireless personal area networks (PANs) whose service area issmaller than that of a wireless LAN. In 2009, Bluetooth 4.0 was developed with reduced power consumption, and various mobile devices started deploying Bluetooth. It is mainly used for voice communications or music appliances such as wireless headsets
and wireless mikes. It is expected to be applied tovarious appliances in the near future, and larger numbers of Bluetooth-enabled products are predicted to
be shipped.
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Core network(NGN)

Core network
ISDN/ADSL

Optical
Wired
LAN

Modem
USB

ONU Wired/wirelessbroadband router

Ethernet

Home

Home

ADSL: asymmetric digital subscriber line
ISDN: integrated services digital network
LAN: local area network

NGN: Next Generation Network
ONU: optical network unit

Fig. 1. Changes in home network usage.

100 G
Next generation
wireless LAN

10 G
Wireless LANs

.11ad

.11ac

1G
Data rate (bit/s)

.11n
100 M

LTE-Advanced
LTE

.11a
10 M

Cellular

.11g

.11b

HSDPA

802.11

1M

WCDMA
100 k

GSM
PDC

1995

2000

HSPDA: high-speed downlink packet access
GSM: global system for mobile communications

2005

2010

2015

Year

LTE: Long Term Evolution
PDC: personal digital cellularFig. 2. Acceleration in use of wireless LAN.

A machine-to-machine (M2M) network is also considered to be an affordable wireless system that canconnect not only high-end smart devices but alsoVol. 11 No. 3 Mar. 2013

anything in the home to the network. ZigBee, which
is a sensor network system based on the IEEE
802.15.4 standard, is available for use in homeFeature Articles

network services. IEEE 802.11ah is also under development to support M2M communication in the 920MHz band.
However, some issues have arisen in the aforementioned evolution of wireless networks that must be
addressed to further evolve wireless home networks.
These issues are described in detail in the next section.
2. Issues in wireless home networks
There are three key issues regarding home networks: (1) providing support for a wider variety ofwireless systems, (2) preventing interference andimproving actual throughput, and (3) simplifying thesetting and certification procedure. These issues are
explained below.
(1)	Providing support for a wider variety of wirelesssystems:
Each wireless standard has been optimized for a
specific purpose. These standards have been amended
in order to enhance performance by incorporating thelatest technologies. Although this has expanded theapplication area of wireless systems, there are still
too many wireless standards, and it is difficult forusers to select the most appropriate wireless system.
It is necessary to develop a wireless home network
platform that supports various standards. This willallow users to buy what they want without worrying
about connectivity to their own home network.
(2)	Preventing interference and improving actual
throughput:
Wireless systems in home networks use unlicensed
frequency bands where multiple wireless systems
may coexist. The possibility of the coexistence ofother wireless systems was low just a few years ago
because of the low deployment rate. However, therecent proliferation of wireless systems has increased
the possibility of coexistence, which will increase therate of interference within the same system andbetween different systems. Interference degrades theactual system throughput, especially in high population zones such as train stations or urban areas. The
use of the 5-GHz band is an effective solution to
mitigate the influence of interference because mostcurrent wireless systems use the 2.4-GHz band. However, it is insufficient to handle the predicted increasein mobile traffic. Improving the spectral efficiency
will be necessary to maintain high system throughput
in high-density wireless system areas.
(3)	Simplifying the setting and certification procedure:When people start using a home network to communicate with things such as sensing devices, thenumber of wireless devices will drastically increase.
The setting procedures may be very complex forusers because they differ according to the wirelesssystem. It is necessary to create a mechanism that
does not require the user to implement any setting
procedures. Thus, both the settings and the certification procedure will need to be simplified in future
wireless access systems.
When the above issues are resolved, users candeploy and extend their wireless home network without concerns for the security and reliability of thewireless network. The next section reviews theresearch and development (R&D) of the wirelesshome network underway at NTT Network Innovation
Laboratories.
3. R&D of user-centric wireless home network
To address the issues described in the previous section, NTT Network Innovation Laboratories is developing a user-centric wireless home network. Its key
concepts are detailed below.
The user-centric wireless home network allows
people to use any applications and devices without
concern for wireless connectivity. It enables users to:(1) select wireless devices without worrying about
connectivity to their home network, (2) operate
devices without having to set them first to avoid interference, and (3) carry out certification procedureswith reduced complexity. These benefits are describedin more detail as follows:(1)	Wireless device selection without worrying
about connectivity to their own home network:
The user-centric wireless home network provides aplatform that enables users to deploy arbitrary wireless systems for various purposes, e.g., the Internet
for data communication, a wireless headset and mikefor sound communication, and sensing devices formonitoring home equipment, without worrying about
connectivity to their own home network. These features are enabled by the use of software technology
for processing radio signals and by a flexible accesspoint (AP), which supports a wide variety of wirelesssystems. Since the flexible AP can be used for new
standards by updating the radio signal processing
software at the network server, users do not need tobuy new access points for new wireless devices.
Thus, users can select their wireless devices without
ever worrying about the connectivity.
(2)	No need to input settings or operations in
NTT Technical Review

Feature Articles

Quantum improvement in the system
capacity per area by spatial andfrequency resource management
Coordinated wireless LAN
HGW

HGW

HGW

HGW

HGW

HGW

HGW

HGW

Adaptation to the new generation
wireless access standards bysoftware replacement

Flexible accesspoint

Network control
engines
Wireless access, authentication,interference management,
compressed transmission of radiowave data, monitoring andmaintenance

Coordination

Radio-wave
Home ICT platform
data(OSAP)(compressedtransmission) Software distribution
Access point information
Network
Controller
Control information

HGW: home gateway
ICT: information and communications technology
OSAP: OSGi Service Aggregation Platform

Fig. 3. User-centric wireless home networks.

devices to avoid interference
Interference within the same system as well as
between different systems can be suppressed by
employing the wireless access platform where beam
patterns and channels at multiple APs are coordinatedand controlled over the network. Moreover, by adaptively changing beam patterns and channels according to the environment changes, spectrum resources
can be effectively shared by multiple wireless systems. This increases the capacity per area even when
multiple wireless systems coexist in the same area.
Therefore, users need not worry about interference
from other systems.
(3)	Simplified certification
The certification procedure in the user-centric wireless home network is triggered by the network,whereas it is triggered by the network terminals incurrent general wireless systems. This new procedure
reduces the complexity of certification, especially in
M2M networks, where users do not directly operate
the devices, and in short-range wireless access systems with very high throughput using millimeterwave bands.
The key technology to realize the user-centric wireless home network is the control of APs over the
network with secure and reliable access. R&D is
being conducted to implement the architecture of AP
network control as shown in Fig. 3, where remotely
controlled functions such as the certification andinterference avoidance are handled by network conVol. 11 No. 3 Mar. 2013

trol. The functions at the network control engine utilize the OSGi (Open Services Gateway initiative)
technology of the next-generation home network
platform [2].
The Feature Article in this issue entitled “Public
Wi-Fi Services at NTT DOCOMO,” [3] introduces
the current services and development activities
underway at NTT DOCOMO to improve the user
experience for mobile users. The next three Articles
introduce technologies that support the user-centric
wireless home network. “High-speed Wireless LAN
for Broadband Wireless Home Networks” [4]describes technology to manage interference while
coordinating multiple APs. “Flexible Wireless System for User-centric Wireless Home Network” [5]describes technology that makes it possible to avoid
having to use individual APs for various wirelesssystems. “Scalable M2M Wireless Access System”[6] describes technology that enables access to/fromanything.
References
[1]

[2]

K. Nagata, Y. Kojima, T. Hiraguri, and Y. Takatori, “Wireless Local
Area Network Standardization of IEEE 802.11 and the Wi-Fi Alliance,” NTT Technical Review, Vol. 8, No. 4, 2010.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2010
04gls.html
T. Yamazaki, A. Kawanobe, T. Nunobiki, K. Obana, T. Yahara, K.
Mizuno, K. Nakamura, Y. Koike, Y. Mihara, and T. Orisaka, “HomeICT Framework Technology that Provides Easy, Safe, Reliable, andConvenient Home ICT Services,” NTT Technical Journal, Vol. 22, No.

Feature Articles

[3]

[4]

5, pp. 23–27, 2010 (in Japanese).
T. Ninagawa and K. Hirama, “Public Wi-Fi Initiatives at NTT DOCOMO,” NTT Technical Review, Vol. 11, No. 3, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
03fa2.html
T. Ichikawa, K. Ishihara, T. Murakami, B. A. Hirantha Sithira Abeysekera, Y.Asai, Y.Takatori, and M.Mizoguchi, “High-speed Wireless
LAN for Broadband Wireless Home Networks,” NTT Technical
Review, Vol. 11, No. 3, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
03fa3.html

Yasushi Takatori

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. degree in electrical andcommunication engineering and the M.E. degree
in system information engineering from Tohoku
University, Miyagi, in 1993 and 1995, respectively, and the Ph.D. degree in wireless communication engineering from Aalborg University,
Denmark, in 2005. He joined NTT in 1995. Heserved as a co-chair of COEX Adhoc in IEEE
TGac from 2009 to 2010. He was a visiting
researcher at the Center for TeleInFrastruktur,
Aalborg University, from 2004 to 2005. Hereceived the Young Engineer’s Award from theInstitute of Electronics, Information and Communication Engineers (IEICE) in 2000, and theBest Paper Award of IEICE in 2011. He is asenior member of IEICE and a member of
IEEE.

[5]

[6]

S. Yoshino, T.Kaho, H.Shiba, H.Toshinaga, T. Yamada, M. Kobayashi,
and Y.Shirato, “Flexible Wireless System for User-centric WirelessHome Network,”NTT Technical Review, Vol. 11, No. 3, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
03fa4.html
M. Harada, T.Kumagai, Y. Shimizu, K. Suzuki, N. Mochizuki,
Y.Fujino, and A.Yamagishi, “Scalable M2M Wireless Access
System,”NTT Technical Review, Vol. 11, No. 3, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
03fa5.html

Communication Society. He served as a General Co-Chair of the 6th International
Conference on Cognitive Radio Oriented Wireless Networks and Communications,
CrownCom, in May 2011. He was a Guest Editor-in-Chief of the Special Section on
Wireless Distributed Networks, IEICE Transactions on Communications, December
2010. He received the Young Engineer’s Award and Excellent Paper Award fromIEICE in 1995 and 1997, respectively, the 1st YRP (Yokosuka Research Park) Awardin 2002, and the 18th Telecom System Technology Award from the Telecommunications Advancement Foundation in 2003, respectively. He is a senior member ofIEEE and IEICE.

Shuichi Yoshino

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees inmechanical engineering from Kanazawa University, Ishikawa, in 1990 and 1992, respectively. He
joined NTT in 1992 and worked on satellite
Internet system and wireless networking technology development. He is currently engaged in
R&D on wireless technology for ubiquitous services.

Masato Mizoguchi

Senior Manager, R&D Produce Group, NTT
Department III (R&D Strategy Department).
He received the B.E. and M.E. degrees in electrical engineering from Tokyo University of Science in 1989 and 1991, respectively. Since joining NTT in 1991, he has mainly been engaged in
R&D of the personal handy-phone system (PHS)and high data rate wireless LANs such as those
covered by IEEE 802.11a. Since 2004, he has
been working in the R&D Planning Team forcomprehensive commercialization functions. He
received the Young Researcher’s Award and theBest Paper Award from IEICE in 1998 and 2000,
respectively. He is a member of IEICE andIEEE.

Kazuhiro Uehara

Senior Research Engineer, Supervisor, Group
Leader, Wireless Systems Innovation Laboratory,
NTT Network Innovation Laboratories.
He received the B.E., M.E., and Ph.D. degrees
from Tohoku University, Miyagi, in 1987, 1989,
and 1992, respectively. In 1992, he joined NTT
and engaged in research on array antennas, active
antennas, and indoor propagation in the millimeter-wave and microwave frequency bands. From1997 to 1998, he was a Visiting Associate at the
Department of Electrical Engineering, California
Institute of Technology, USA. From 2003 to
2010, he was a part-time lecturer at the Department of Electrical Engineering, Tohoku University. His current interests include R&D of software defined radio and cognitive radio systems
and millimeter-wave multi-gigabit wireless systems. He is currently serving as Chair of theTechnical Committee on Software Radio, IEICE

Mitsuru Harada

Senior Research Engineer, Supervisor, Group
Leader of Wireless Communication Circuits
Research Group, Smart Devices Laboratory,
NTT Microsystem Integration Laboratories.
He received the B.S. and M.S. degrees in physics from Tsukuba University, Ibaraki, in 1985
and 1987, and the Ph.D. degree in electronicengineering from Tohoku University, Miyagi, in
2003. In 1990, he joined NTT Atsugi ElectricalCommunication Laboratories, where he engaged
in research on thin-film SOI devices. Since 1997,
he has been researching high-speed/low-power
CMOS circuits for wireless terminals.

Kazuyasu Okada

Director, Vice President, Wireless System
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received the B.E. degree from Hokkaido
University in 1983. Since joining Nippon Telegraph and Telephone Public Corporation (nowNTT) in 1983, he has been engaged in R&D ofsatellite transponder and satellite communication
systems. He moved to NTT Communications in2001 and served as a project manager on a highspeed satellite access service for ships. He moved
to NTT Network Innovation Laboratories andtook up his current position in April 2008. Hereceived the Young Engineer’s Award fromIEICE in 1991.

NTT Technical Review

Feature Articles: Wireless Access Technologies that Enhance
Convenience by Network Linkage

Public Wi-Fi Initiatives at
NTT DOCOMO
Tetsuya Ninagawa and Kosuke Hirama
Abstract

Smartphone use and data traffic have increased dramatically, and to cope with this, NTT DOCOMO
is placing increasing importance on Wi-Fi services with the aim of raising customer satisfaction andaccelerating data offloading to reduce network load. This article outlines NTT DOCOMO’s public Wi-Fi
service and introduces expanded coverage, free-access campaign, and enhanced convenience as specificexamples of how NTT DOCOMO is promoting Wi-Fi use among its customers.

1. Overview of Wi-Fi environment
The recent spread of smartphones in society has led
to an increasing number of customers using devices
equipped with Wi-Fi functions. In response to thistrend, NTT DOCOMO is paying increasing attention
to Wi-Fi with the aim of providing a comfortablecommunications environment and raising customer
satisfaction.
At the same time, mobile data traffic has been
increasing year on year in parallel with this proliferation of smartphones. At NTT DOCOMO, traffic isslated to increase by about 12 times by fiscal year
2015 compared to 2011 levels [1]. To respond effectively to this dramatic increase in traffic, NTT
DOCOMO’s plan is to provide high-quality communications by expanding capacity through a migration
to Xi (crossy) LTE (Long Term Evolution) services,adopting smaller zones, controlling the transmission
speeds of heavy users, and reducing network load
through use of Wi-Fi (data offloading).
2. Features of docomo Wi-Fi
NTT DOCOMO launched its Mzone public Wi-Fi
service in 2002. It was initially assumed that this service would be used for data communications via
personal computers. Positioned as a complement toFOMA 3G (third generation) circuits, Mzone marked
the start of the provision of a high-speed communication service, although the locations providing Mzone
were initially limited. Then, with the aim of enhancing convenience and increasing usage, NTT DOCOMO launched domestic roaming for Mzone in 2003,
international roaming and the establishment of Wi-Fi
locations within subway stations in 2004, and use
within railway cars in 2006. In addition, the service
name was changed to docomo Wi-Fi in 2012.
To provide service in more locations, docomo WiFi makes frequent use of shared access points
installed by NTT Broadband Platform Incorporated(NTT BP) for more efficient operations. Given thatNTT DOCOMO and other Wi-Fi providers are
expanding their Wi-Fi coverage, the renting and sharing of NTT BP’s access points in this way prevents
the inefficient construction of facilities by multiple
providers at the same location while also avoiding theradio interference that would result from the operation of multiple access points.
3. Promotion of docomo Wi-Fi
As described above, NTT DOCOMO is placing
increasing importance on Wi-Fi with the aim of raising customer satisfaction and accelerating data
offloading. Specifically, the company is undertaking
a variety of initiatives to get even more customers to
use and enjoy Wi-Fi under even more scenarios.
3.1 Expanded coverage
As of the end of March 2012, docomo Wi-Fi use
was available at around 8400 access points
NTT Technical Review

Feature Articles

No. of accesspoints

Airports, subway
stations

Railway/bullet-train
stations

Sept. 2012: 70,000 access points
Mar. 2013: 120,000–150,000 access points(Wi-Fi available here!)

Fast food restaurants

Convenience
storesCafes

02 003 004 005 006 007 008 009 010 011 012
2
2
2
2
2
2
2
2
2
2
Fiscal year

20

Fig. 1. docomo Wi-Fi locations and number of access points.

throughout Japan mainly in train stations, airports,
cafes, fast food restaurants, and convenience stores.
Since then, NTT DOCOMO has been working to
expand Wi-Fi locations to commercial facilities andsmall eating and drinking establishments and has
rapidly increased the number of access points to
70,400 as of the end of September 2012. The plan is
to continue with this expansion of Wi-Fi coverage(Fig. 1). In this expansion, NTT DOCOMO selects
the optimum method of installing an access point at a
certain location in collaboration with NTT BP fromthe viewpoints of quality and prompt deployment.
For example, either an optical circuit or Xi circuit canbe used as a backhaul line depending on the volume
of Wi-Fi traffic estimated for the location in question.
In short, optical circuits can ensure sufficient bandwidth for high traffic locations, while using Xi circuits for small eating and drinking establishments canfacilitate the prompt expansion of Wi-Fi locations.
3.2 Free-access campaign
For NTT DOCOMO customers, the docomo Wi-Fi
service is mainly provided as an optional ISP service
such as sp-mode and mopera at a monthly fee of 315
yen (tax included). Customers can apply for docomo
Wi-Fi at docomo shops or by telephone, personal
computer, sp-mode, etc. Individuals not having an
NTT DOCOMO circuit or ISP contract can use
docomo Wi-Fi for a monthly fee of 1575 yen (taxincluded).
To further promote the use of docomo Wi-Fi, NTT
DOCOMO is conducting a free-access forever campaign from September 1, 2012 to March 31, 2014 inwhich the monthly fee of 315 yen will be waived forcustomers subscribing to a designated packet flat-rate
Vol. 11 No. 3 Mar. 2013service, an ISP service, and an ISP optional service.
These customers will continue to receive free accesseven after the campaign period ends provided thatthey continue to satisfy applicable conditions.
3.3 Enhanced convenience
Since most docomo Wi-Fi users access the service
not from personal computers but from smartphones
and tablets, we started providing an app called docomo Wi-Fi simple connect for Android smartphones
and tablets in January 2012. With this app, there is noneed for customers to make service set identification(SSID) or security settings on their own at the startup
of service, and ID/password settings for connecting
to docomo Wi-Fi are greatly simplified. The app
enables a customer at a docomo Wi-Fi location toconnect to the service by simply tapping a docomo
Wi-Fi widget on the screen. This app has significantly improved the ease of using docomo Wi-Fi(Fig. 2).
4. Future activities
Wi-Fi is becoming all the more important for making the communications environment even more
comfortable and enjoyable for users. At the time of
the Great East Japan Earthquake in March 2011, theNTT Group made Wi-Fi access points available freeof charge in the stricken area. In the same way, NTT
DOCOMO is working to make docomo Wi-Fi available at the time of a disaster as a network that complements FOMA and Xi. In addition, docomo Wi-Fi
already provides a filtering function that restricts
access to potentially harmful sites, and in the future,
NTT DOCOMO plans to expand its efforts inFeature Articles

Features of the “docomo Wi-Fi easy connection” app- Greatly simplifies initial settings- Displays signal intensity on widget- Turns connection ON/OFF by widget

Outside area
(Wi-Fi not connected)

Inside area
(Wi-Fi not connected)

Connection complete

Tap toswitchFig. 2. Features of the docomo Wi-Fi simple connect app.

attracting even more customers to docomo Wi-Fi bysetting up an environment in which Wi-Fi can be used
in a safe and secure manner and improving its affinity
with FOMA and Xi services.

Tetsuya Ninagawa

Director, Ubiquitous Services Department,
NTT DOCOMO.
He joined NTT Mobile Communications Network (now NTT DOCOMO) in 1993 and worked
on network planning and corporate strategies. He
is currently engaged in planning of public andin-house Wi-Fi services.



Reference[1]

NTT DOCOMO Medium Term Vision 2015 Report.
http://www.nttdocomo.com/about/core_foundation/core_foundation.
pdf

Kosuke Hirama

Manager, Ubiquitous Services Department,
NTT DOCOMO.
He joined NTT DOCOMO in 2000 and worked
on development of 2G and 3G packet communication core networks. He is currently engaged inplanning of the public Wi-Fi service.

NTT Technical Review

Feature Articles: Wireless Access Technologies that Enhance
Convenience by Network Linkage

High-speed Wireless LAN for
Broadband Wireless Home Networks
Takeo Ichikawa, Koichi Ishihara, Tomoki Murakami,
B. A. Hirantha Sithira Abeysekera, Yusuke Asai,
Yasushi Takatori, and Masato Mizoguchi
Abstract

This article introduces cooperative transmission techniques for wireless local area networks (WLANs)
to mitigate interference between neighboring wireless home networks by establishing the cooperation ofmultiple WLAN access points (APs) on the next-generation home ICT (information and communications technology) platform. The cooperative transmission techniques achieve stable transmission ofvideo data (such as for high-definition televisions (TVs)) and data traffic offloading of mobile devices
such as smartphones from cellular networks to fixed broadband networks.

1. Introduction
The number of digital network-capable devices
used in homes has rapidly been increasing in recent
years due to the digitization of consumer electronics
appliances such as televisions (TVs), video recorders,cameras, and music players. Furthermore, new digital
devices such as digital photo frames, tablet personal
computers (PCs), and smartphones are becoming
more and more popular. This trend is expected toincrease further in the near future with the introduction of various kinds of sensor terminals. Wireless
local area networks (WLANs) can be used to connect
these various devices to broadband networks without
the need for cables. This convenience has led to therapid increase in the use of WLANs in homes as wellas in offices.
However, the rapid growth in the use of smartphones is causing congestion in cellular networks.
One solution is traffic offloading to WLANs. Cellular
operators are expanding the areas where public
WLANs are available by introducing WLAN accesspoints (APs) that are capable of not only 2.4-GHzband but also 5-GHz-band transmission, and are distributing APs to their users.
This article first describes the evolution and challenges of WLAN and then introduces the cooperativetransmission techniques that NTT Network Innovation Laboratories is developing for the next-generation WLAN.
2. Evolution and challenges of wireless LAN
The first international WLAN standard was introduced in 1997 in Institute of Electrical and Electronics Engineers (IEEE) 802.11, which covered the useof the 2.4-GHz band. This band is called the ISM(industrial, scientific, and medical) band and is used
for medical and industrial equipment such as microwave ovens (even those used in the home). In thisband, a radio license is not required worldwide if theradio transmission power is less than the regulation
value. The 5-GHz band is addressed in new wireless
LAN standards IEEE 802.11a and IEEE 802.11n. In
the 5-GHz band, no interference occurs from microwave ovens in the home.
In the latest WLAN standard, IEEE 802.11n, thechannel width for wireless data transmission is 20 or
40 MHz; this means that there are 3 channels or 1channel in the 2.4-GHz band and 19 or 9 channels in
the 5-GHz band. Because the number of installed APs
has increased, the number of channels has become
insufficient in the 2.4-GHz band. Therefore, the useof the 5-GHz band is increasing. Mobile devices such
NTT Technical Review

Feature Articles(a) Conventional WLAN(b) Interference control between home networks

Interference between home networks
Interference from neighboring APs in dense
residential area degrades throughput.

Suppression of interference
APs mutually transmit null beams to stations
connected to the neighboring AP.
Null beamforming toneighboring cell

InterferenceHome A

Home B

Cooperation

Home A

Home BHome
ICT platform

Fig. 1. Interference control between home networks.

as smartphones are now being developed that havemulti-band capability (5-GHz band and 2.4-GHzband).
However, the next generation WLAN standard,IEEE 802.11ac, which targets system throughputs
above 1 Gbit/s and uses the 5-GHz band, is currently
being drafted by the IEEE 802.11 working group. In
IEEE 802.11ac, the channel width will be 80 or
160 MHz in order to achieve higher throughput. For
this reason, the number of channels in the 5-GHzband is being reduced to 2 or 4, and consequently, thenumber of channels in this band is expected to be
insufficient in the near future, just as in the 2.4-GHzband.
3. Cooperative transmission techniques for
wireless LAN
To accommodate the ever-increasing traffic, it isnecessary to further improve the frequency utilization
efficiency. We are developing cooperative transmission techniques for WLAN to improve frequency
utilization efficiency and achieve stable wireless datatransmission through the cooperation of multiple APs
using the next-generation home ICT (information andcommunications technology) platform. In this section, we introduce cooperative transmission techniques for WLAN that consist of interference control
between home networks, multi-user multi-channel
control, and network control.
3.1 Interference control between home networks
Problems caused by interference between homeVol. 11 No. 3 Mar. 2013

networks due to the spread of WLAN is becoming
apparent in public areas such as train stations and indense residential areas such as areas with many apartments, where neighboring APs use the same channel
because of the paucity of channels. In the IEEE
802.11ac specifications, as mentioned above, thenumber of channels will be reduced to 2 or 4 because
the channel width is 80 MHz (mandatory) or 160
MHz (optional), which means that interference
between home networks is likely to occur in the
5-GHz band in the near future. Given that two APs
using the same channel are adjoining, each home
network will have only half the average throughput of
conventional WLAN because the channel is sharedby time division multiple access based on the carrier
sense multiple access with collision avoidance(CSMA/CA) protocol (Fig. 1).
We are developing an interference control technique as one solution to the interference problem.
This technique links multiple APs by network transmission signals to achieve the cooperation of the APs
via the beamforming technology of multi-user multiple-input multiple-output (MU-MIMO) transmission.
MU-MIMO, which is a new technology for wireless broadband communication, is expected to be
adopted in IEEE 802.11ac as an option (Fig. 2) [1]. It
achieves high frequency-utilization efficiency bysending data to multiple stations (STAs) over a single
channel at the same time by controlling the directional characteristics of the radio signals by using
beamforming control technology so the signals do
not interfere with each other. We developed a wirelessFeature Articles(a) Single user MIMO(b) Multi-user MIMOA WLAN AP and stations share the same radio
frequency by TDMA.

WLAN AP beamforming control achieves SDMA
communication with diverse stations.

WLAN APWLAN AP

SDMA(space division multiple access)

TDMA(time division multiple access)- Cell throughput depends on the number ofantennas on the station.
- Adopted in IEEE 802.11n, WiMAX, LTE, etc.

- Holds promise as core technology for cellthroughput of 1 Gbit/s or more.
- Adoption in IEEE 802.11ac is expected.

Fig. 2. Multi-user MIMO technique.

WLAN access point

Simultaneousreceiving of highvolume content

Terminals (six in one case)Multi-user MIMO
transmission(six terminals
multiplexed)

Server(content
storage)

Video streaming
Channel state
information

Beamforming control using recursive
weight calculation smaller circuit scale

Up to 6

Transmission streams

Up to 6

Bandwidth

Up to 80 MHz

Per-stream data rate(physical layer)

Up to 270 Mbit/s

Total data rate(physical layer)

Up to 1.62 Gbit/s

Data transmission rate

Up to 1.37 Gbit/s

Data2
Total data rate (Gbit/s)

Stations

Compressed CSI feedback
shorter overhead time

Up to1.62 Gbit/s
With multi-user MIMO
1

UP
Without multi-user MIMO

0

1

2

3
4
Number of stations

5

6

Fig. 3. Multi-user MIMO transmission prototype.



NTT Technical Review

Feature Articles

AP collaboration

AP 1

AP 2

Mutual nullsteering
suppression ofinterference

STA 2

Bandwidth

STA 3

STA 4

80 MHzNumber of APs

2

Number of AP antennas

6

Number of STAs associated with AP

3

Number of STA antennas

1

Normalized system throughput

STA 1

STA 5

1.2

STA 6

STAs
(six in one case)With interference control

1
0.8

×2

0.6
0.4

Without interference control

0.2
0

0

1

2
3
4
Distance between STAs (m)5Fig. 4. Prototype of interference control system between home networks.

transmission prototype that employs MU-MIMO
transmission, and succeeded in transmitting data inreal time to six STAs at more than 1 Gbit/s for the firsttime in the world (Fig. 3) [2]–[4].
The interference control between home networks is
an extension of the beamforming technology of MUMIMO. An AP sends data to multiple STAs in its
home network without letting signals interfere with
the neighboring home network by controlling thedirectional characteristics of the radio signals, so that
two APs in each home network can send data to their
STAs over the same channel at the same time without
interference. As a result, the throughput is approximately double that of the conventional CSMA/CA
method (Fig. 4) [5].
3.2 Multi-user multi-channel control
APs specified in IEEE 802.11ac can perform highspeed communication using the 160-MHz-bandwidthchannel as an option. In addition, to ensure compatibility with the conventional IEEE 802.11a and802.11n protocols, the APs communicate with IEEE
802.11a-specified STAs using 20-MHz-bandwidth
Vol. 11 No. 3 Mar. 2013

channels and with IEEE 802.11n STAs using channels up to the 40-MHz-bandwidth channel. Therefore, when an IEEE 802.11ac-specified AP uses a
20-MHz-bandwidth channel to transmit data to an
IEEE 802.11a STA, the frequency utilization efficiency degrades because the 140-MHz band isunused.
As one solution, we are developing a multi-user
multi-channel control technique in which the AP allocates a communication channel to each STA using
20-MHz-bandwidth units and transmits data to each
STA simultaneously by frequency division multiplexing (Fig. 5). This new technique achieves efficient useof frequency resources by transmitting data to each
STA using 20-MHz- and 120-MHz-bandwidth channels at the same time. If there is only one active STA
in a home network, the AP may use just the 20-MHzbandwidth channel to communicate with the STA. In
this case, by synchronizing the timing of data transmission with the neighboring AP, the AP and theneighboring AP can transmit data via the 20-MHzand 120-MHz-bandwidth channels at the same time.

Feature Articles

Frequency (channel bandwidth)

Frequency (channel bandwidth)

160
MHz

160
MHz

Data to802.11ac
STA

Data to802.11ac
STA

UnusedData to multi-user
multi-channel
capable STA

Guard band
Data to 802.11a STA

0

Time

Data to 802.11a STA

0

Time

Fig. 5. Multi-user multi-channel control.

2. Interference management
engine calculates the optimal
allocation of channels.

Home networks
APAP

1. APs send radio environment
information to interference
management engine.

AP

Interference
management engine

AP information
Controlfor

AP inController

n
matio

rol

Cont

Network

3. Interference management
engine indicates the appropriate
channel to each AP.

Fig. 6. Network control for wireless LAN.

3.3 Network control for wireless LAN
As mentioned above, the WLAN uses the CSMA/
CA method as the medium access control scheme.
Because CSMA/CA is based on autonomous distributed control, WLAN users can build a wireless home
network simply by using the AP; the channels are
selected freely by the users. However, in dense residential areas such as neighborhoods with many apartment buildings, a problem may occur where optimal
allocation of channels used by each AP is not performed, and frequency resources are not used effectively.
One solution that we are working on is a network
control technique for WLAN in which an interference
management engine is set on the broadband network.
This engine collects information on the radio environment such as channel utilization rates from eachAP, calculates the optimal allocation of channels, andinforms each AP of the appropriate channel (Fig. 6).
This technique enforces optimal channel usage automatically. In addition, techniques to provide a synchronization signal from the Ethernet-based network,
called synchronous ethernet (SyncE), and precisiontime protocol (PTP), have been standardized. These
techniques can achieve frequency, phase, and timesynchronization between APs, which is expected tomake interference control between home networks
more accurate by synchronizing the frequency andphase of radio signals transmitted by the APs.
4. Conclusion
WLAN is widely used today and has advanced considerably since it first came into use. These networks
NTT Technical Review

Feature Articles

are continuing to evolve and to achieve higher speed,larger capacity, and greater functionality that will
enable them to accommodate various data services inhomes, offices, and public areas.

[5]

T. Murakami, R. Kudo, Y. Asai, T. Kumagai, and M. Mizoguchi, “Performance evaluation of distributed multi-cell beamforming for MUMIMO systems,” Proc. of the 8th IEEE International Symposium on
Wireless Communication Systems (ISWCS 2011), pp. 547–551,
Aachen, Germany.

References
[1]
[2]
[3]
[4]

Q. H. Spencer, C. B. Peel, A. L. Swindlehurst, and M. Haardt, “Anintroduction to the multi-user MIMO downlink,” IEEE Communication Magazine, Vol. 42, No. 10, pp. 60–67, 2004.
NTT news release: World’s First Successful Multiuser-MIMO (MUMIMO) Transmission Above 1 Gbit/s, May 2010.
http://www.ntt.co.jp/news2010/1005e/100507a.html
T. Ichikawa, Y. Asai, and M. Mizoguchi, “World’s First Successful
1 Gbit/s Multi-user MIMO Wireless Transmission,” ITU-AJ New
Breeze, Vol. 23, No. 1, Winter 2011.
K. Ishihara, Y. Asai, R. Kudo, T. Ichikawa, and M. Mizoguchi,“Indoor experiments on real-time multiuser MIMO transmission in
wireless LAN systems,” Proc. of the 2012 IEEE Wireless Communications and Networking Conference (WCNC 2012), pp. 147–151,
Paris, France.

B. A. Hirantha Sithira Abeysekera

Researcher, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in communications engineering from Osaka
University in 2005, 2007, and 2010, respectively.
He joined NTT in 2010. His research interests
include design and performance evaluation ofnext generation wireless networks. He received
the IEEE VTS (Vehicular Technology Society)Japan Student Paper Award in 2009. He is amember of IEICE and IEEE.

Yusuke Asai

Takeo Ichikawa

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Waseda University,
Tokyo, in 1991 and 1993, respectively. Sincejoining NTT in 1993, he has participated in R&D
of PHS-based packet systems and high-speed
wireless LAN systems. He received the Young
Researcher’s Award from the Institute of Electronics, Information and Communication Engineers (IEICE) in 1999. He is a member of IEICE
and IEEE.

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received the B.E. and M.E. degrees in information electronics from Nagoya University,
Aichi, in 1997 and 1999, respectively. Since joining NTT in 1999, he has been engaged in R&D
of broadband MIMO-OFDM techniques for
wireless LAN systems. He has been participatingin the IEEE 802.11 working group and is one of
the co-chairpersons of the coexistence ad-hoc
group in task group ac. He received the Young
Researcher’s Award from IEICE in 2004. He is amember of IEEE.

Yasushi Takatori

Researcher, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in communications engineering from Tohoku
University, Miyagi, in 2004, 2006, and 2011,
respectively. Since April 2006, he has been withNTT Network Innovation Laboratories. His current research interests include digital signaltransmission techniques for broadband wirelessand fiber-optic communication systems. He
received the Young Engineer’s Award fromIEICE in 2009 and the 2011 IEICE RCS (Radio
Communication Systems) Active ResearchAward in 2011. He is a member of IEICE andIEEE.

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. degree in electrical andcommunication engineering and the M.E. degree
in system information engineering from Tohoku
University, Miyagi, in 1993 and 1995, and the
Ph.D. degree in wireless communication engineering from Aalborg University, Denmark, in2005. He joined NTT in 1995. He served as a
co-chair of COEX Adhoc in IEEE TGac from2009 to 2010. He was a visiting researcher at theCenter for TeleInFrastruktur, Aalborg University,from 2004 to 2005. He received the Young
Engineer’s Award from IEICE in 2000, and theBest Paper Award of IEICE in 2011. He is asenior member of IEICE and a member ofIEEE.

Tomoki Murakami

Masato Mizoguchi

Koichi Ishihara

Researcher, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees fromWaseda University, Tokyo, in 2006 and 2008,
respectively. He joined NTT Network Innovation
Laboratories in 2008. His current research interests include multiuser MIMO systems andresource allocation. He received the Young
Researcher’s Award from IEICE in 2010. He is amember of IEICE and IEEE.

Vol. 11 No. 3 Mar. 2013

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Tokyo University of Science in 1989 and 1991, respectively. He joined
NTT in 1991 where he has mainly been engaged
in R&D of personal communication systems andhigh data rate wireless LANs including the IEEE
802.11a systems. He received the Young
Researcher’s Award in 1998, the Best Paper
Award in 2000, and the Achievement Award in2006 from IEICE. He is a senior member of
IEICE and a member of IEEE.

Feature Articles: Wireless Access Technologies that Enhance
Convenience by Network Linkage

Flexible Wireless System for
User-centric Wireless Home Network
Shuichi Yoshino, Takana Kaho, Hiroyuki Shiba,
Hideki Toshinaga, Takayuki Yamada,
Mamoru Kobayashi, and Yushi Shirato
Abstract

We are developing a flexible wireless system with the aim of achieving a wireless home network thatusers can use without being aware of wireless systems and that will be easy for network operators tooperate and maintain. This article introduces an overview of the flexible wireless system, use cases forit, and key technologies needed to achieve it.

1. Introduction
1.1 Current wireless networks
Recently, considerable progress has been made in
developing broadband wireless local area networks(LANs) for the 2.4-GHz and 5-GHz bands, reviewing
frequency usage in the 920-MHz band to expand
machine-to-machine (M2M) communication applications, and improving home automation and home
security technology for the 300-MHz and 400-MHz
bands. Various wireless systems that utilize the characteristics of each frequency band have been developed. However, in order to use these systems in home
networks, users need a certain degree of knowledgeof wireless systems, and they need to carry out thefollowing tasks:
1)	Selecting a wireless system that suits their purposes2)	Configuring the system’s access point (AP) andbuilding the wireless network3)	Fixing hidden problems such as interference
caused by APs or wireless terminals in theneighborhood
1.2 Future home networks
In future home networks, the use of informationand communications technology (ICT) will make itpossible to further advance the development of smart
homes or smart communities that provide more var

ied and efficient home and social activities. At thattime, it will be necessary for everyone, even those
without wireless skills, to be able to use their wirelessnetwork easily. We are now researching and developing a flexible wireless system on a wireless home
network that will make the above three tasks unnecessary.
An image of the wireless home network of the
future, as well as that of a conventional system, isshown in Fig. 1. The future network features a single
flexible AP that supports all wireless systems fromlow-speed ubiquitous data transmission systems tohigh-speed wireless LANs. This makes it unnecessary for users to select a wireless system and build
their own wireless home network. Furthermore,through remote analysis of radio spectra and remotesupport of wireless operations, we can provide a usercentric wireless home network that will enable users
to use devices and applications without being awareof the wireless systems the network accommodates.
2. Technical overview of theflexible wireless system
To enable users to effortlessly access and use wireless home network services without even being awareof the wireless systems their network accommodates,
much less having to manage or monitor them, it isnecessary to build a common signal processing
NTT Technical Review

Feature Articles

Respond individually toeach wireless system.

Equipment installation andmanagement require special wirelessskills.

Spreading wireless usage(a) Conventional home network

Supporting
all wirelesssystems

Handling all radio
wave data at once

Stable management without users
being aware of wireless systems

Spreading wireless usage(b) Future home network

Fig. 1. Comparison of conventional and future home networks.

Digital signal processing- Implemented as software
Wireless signal
processing serverIEEE 802.11
a/b/g/nExample of
Flexible
generation change
access pointof wirelesssystems
IEEE 802.11ac

RF frontend- Wideband and multiband

Home
gateway

Network engine
servers
Signalprocessing
Compressedtransmission

OSGi technology
Compression rate

Common platform

Wireless data
analysis
Radio data in
the home
network…

Advanced wirelesssystems or
M2M systems

Software radio
distribution andresource
management

Analog-to-digital conversion
Digital-to–analog conversion- Containing a data compressionfunction for digitized radio waves

Fig. 2. System configuration.

platform that can accommodate a wide variety ofwireless systems. The configuration of our flexible
wireless system, which features such a platform, isshown in Fig. 2. The system comprises a flexible AP
Vol. 11 No. 3 Mar. 2013

in the home network, a wireless signal processing
server connected to the AP over the network, andengine servers to control them. In conventional wireless systems, an analog radio frequency (RF)Feature Articles

Table 1. Desired RF component performance.
Item
Frequency band

From 0.3 to 6 GHz

Receiving power

From –120 to +10 dBm

Power transmission

From –41 to +13 dBm

Independent gain control in each band

90 dB

Spurious emission–80 dBc

frontend, an analog-digital converter (ADC)/digitalanalog converter (DAC), and digital signal processing
hardware are compressed into a single equipment
unit. In our flexible wireless system, the analog RF
frontend and ADC/DAC are incorporated into theflexible AP. The digital signal processing is performed through cooperation between the AP and the
network server (Table 1). The system’s wideband andmultiband analog RF frontend, which allows multiple
frequency bands to be used simultaneously, makes it
possible for the single flexible AP to accommodate
various wireless terminals regardless of what frequency bands are used.
Cooperative software in the flexible AP and the
network server performs the digital signal processingthat needs to be processed differently for each accommodated wireless system. In addition, correspondence between the diverse wireless systems andenhancements made to them later can be achieved
merely by replacing the software. OSGi technology*1
makes it possible to replace the software in cooperation with the functions and services of the home
gateway. Furthermore, the transmission to the network server of radio wave data received in the flexible
AP makes it possible to balance the load of the signal
processing resources under high load conditions andto analyze the home radio environment in the network. Since there is a huge amount of digitized radio
wave data, the ADC’s data compression function is
deployed to transmit the data to the network server
through fiber access networks. By aggregating theseveral nearby radio environments at the network, thesystem can analyze in detail hidden problems such as
interference between different or identical systems.
These distributed functions are controlled by the network engine servers, and this is what achieves the*1	 OSGi technology: OSGi (Open Service Gateway initiative) is astandard framework for developing a Java-based module. It
makes it possible to deploy a software component called a bundle, to run in conjunction with it. It has been standardized by the
OSGi Alliance.

Desired valueuser-centric wireless home network.
3. Key technologies
In this section, we introduce two key technologies
that have enabled the development of the flexible AP.
One is wideband and multiband radio frequency circuits that can operate at frequencies from several
hundred megahertz up to 6 GHz. The other is datacompression of digitized radio waves.
3.1 W
 ideband and multiband radio frequency
circuits
Recent wireless communication systems such as
wireless LANs or cellular systems require unified
terminals and APs that operate in several frequency
bands. They also require wideband and multiband RF
circuits. Reports have been published on systems
using tunable band pass filters and multiband amplifiers [1]–[3]. Conventionally, there are two generalsystem compositions: 1) those using a number ofsingle-band circuits in parallel [4], and 2) those using
wideband circuits and digitized wideband radio wave
data. The former is advantageous in that it enables anumber of commercial circuits to be used; however,the problem is that the number of circuits increases
along with the number of frequency bands. The latteris advantageous in that it comprises only a small
number of circuits; however, it requires a high-speed
and expensive ADC that can process a huge amountof digitized radio wave data.
To overcome these problems, we developed a wideband low noise amplifier and a multiband simultaneous receiving mixer [5]. In contrast to conventional
mixers that use a single local oscillator (LO) signal
and receive a single RF signal, the mixer we developed uses multiple LO signals and receives multiple
RF signals simultaneously, as shown in Fig. 3. Generally, multiband wireless systems require a wide
dynamic range to handle multiple power levels with arelative difference of about 100 dB, which is caused
NTT Technical Review

Feature Articles

Widebandlow noiseamplifier

ATT

ATT

ATT

ADC

LPF

Network
Multiband simultaneousreceiving mixer

Nyquist sampling for decoding known signals
Random sampling for detecting unknown
signals (compressed sensing)Power difference
Frequency
difference

ADC and network

300 MHz–several GHz

DC 100 MHz- Minimize frequency difference.
- Minimize power difference.
- Change frequency allocation.

- Minimize number of ADC bits.
- Minimize amount of data.
ATT: attenuator
LPF: low pass filter

Fig. 3. Proposed multiband simultaneous receiver.

by the differences in terminal locations or transmission power levels of each wireless system. Our mixer
can control the conversion gain for each frequency
band independently by using variable attenuators tocontrol the relative power of the multiple LO signals.
This enables down-converted intermediate frequency(IF) signals to be controlled so that they all have thesame power levels. This in turn enables the ADC to
operate on fewer bits than for the case of a wide
dynamic range receiver, which reduces the amount ofradio wave data. The mixer can also change the frequency band allocation of the received signals; e.g., asignal to be demodulated is allocated to a lower frequency in the IF frequency band while others, such as
power-sensing signals, are set in higher frequencies.
This system configuration makes it possible to reduce
the number of radio circuits and to produce a radio
circuit that can be implemented in the batch digital
signal processing of any wireless system.

(industrial, scientific, and medical) band*2 is digitized, the amount of data per second becomes
2 Gbits*3. Therefore, it is necessary to apply a datacompression technique that can reduce the amount ofdata to between 1/100 and 1/1000 of the originalamount in order to enable the digitized radio wave
data to be transferred to the digital signal processing
server via the access networks. To minimize therequired bandwidth for transferring the radio wave
data, we focus on a certain feature, i.e., the sparsity ofradio wave data, and investigate data compressionthat makes use of compressed sensing technology[6].
An overview of the compressed sensing technology
is shown in Fig. 4. Compressed sensing is a new
method for solving ill-posed inverse problems byutilizing the sparsity of data [7]. It allows us to reconstruct data by using sub-Nyquist*4 rate information.
We have already achieved 1/400 data compression for

3.2 Data compression of digitized radio waves
Broadband radio waves received in the flexible AP
are digitized by an ADC so that they can be transferred to the digital signal processing server via fiber
access networks. However, the amount of data indigitized radio waves becomes very large; for example, if an entire radio wave of the 2.4-GHz ISM*2 ISM band: The ISM bands are radio bands reserved for the use of
RF energy for industrial, scientific, and medical purposes. They
are used by wireless LANs, Bluetooth, etc.
*3 The 100-MHz bandwidth of the ISM band is sampled at the Nyquist rate and quantized with the 10 bits used in wireless LAN
systems (100 MHz × 2 × 10 = 2 Gbit/s).
*4 Nyquist sampling: Nyquist sampling is sufficient to reconstruct a
signal completely from samples.

Vol. 11 No. 3 Mar. 2013



Feature Articles

Random sampling of radio wave data(compressed sensing)

Tim
e

Power

Radio wave data compression transfer

Frequency
Broadband radio waves
including multiple signals

Time
Sampling of radio
waves

Time
Data compression based on
the sparsity of radio waves

Reconstruction of radio wave data
(L1-minimization)

ReconstructionTime
Compressed radio wave data

Time
Reconstruction

A method for solving illposed inverse problem
utilizing a sparsity of
data

Fig. 4. Compressed sensing technology.

radio signal detection by using the frequency andtime domain sparsity of radio wave data and 1-bit
quantization. We will attempt to improve this to
1/1000 data compression by using the space domain
sparsity of radio wave data and a weighting method
based on known information on the presence of asignal [8]–[10].
4. Future efforts
In the future we can expect to see expanded use of
ICT and wireless communications along with thecontinuous review of frequency band usage, thedevelopment of new standards, and changes in new
generations of wireless systems. To meet the challenges of such changes, we plan to continue ourresearch and development work with the aim ofachieving a wireless home network that users can use
without being aware of wireless systems and that will
provide easy maintenance and operation for network
operators.
References[1]

K. Kawai, H. Okazaki, and S. Narahashi, “Center frequency, bandwidth, and transfer function tunable bandpass filter using ring resonator and J-inverter,” Proc. of the 39th European Microwave Conference(EuMC2009), pp. 1207–1210, Rome, Italy, 2009.
T. Furuta, A. Fukuda, K. Kawai, H. Okazaki, and S. Narahashi, “Compact 1.5 GHz to 2.5 GHz multi-band multi-mode power amplifier,”
IEICE Electronics Express, Vol. 8, No. 11, pp. 854–858, 2011.
[3] M. Kitsunezuka, K. Kunihiro, and M. Fukaishi, “Efficient Use of theSpectrum,” IEEE Microwave Mag., Vol. 13, No. 1, pp. 55–63, 2012.
[4] “UMTS/HSPA/GSM/GPRS/EDGE RF Transceiver LSI for Multimode/Multiband Communication,” FIND (Fujitsu semiconductor
magazine), Vol. 28, No. 1, 2010.
http://www.fujitsu.com/downloads/EDG/binary/pdf/find/28-1e/2.pdf[5] T. Kaho, Y. Yamaguchi, H. Shiba, K. Akabane, K. Uehara, and K.
Araki, “A Simultaneous Receiving Multi-band Mixer with Independent Gain Control,” Proc. of the Asia-Pacific Microwave Conference
2011 (APMC2011), pp. 383–386, Melbourne, Australia.
[6] D. Lee, T. Yamada, H. Shiba, Y. Yamaguchi, and K. Uehara, “Combined Nyquist and compressed sampling method for radio wave datacompression of a heterogeneous network system,” IEICE Trans. Commun., Vol. E93-B, No. 12, pp. 3238–3247, 2010.
[7] T. Tanaka, “Mathematics of Compressed Sensing,” IEICE Fundamentals Review, Vol. 4, No. 1, pp. 39–47, 2010 (in Japanese).
[8] T. Yamada, D. Lee, H. Shiba, Y. Yamaguchi, K. Akabane, and K.
Uehara, “Experimental results of compressed sensing reconstruction
for FSK signal demodulation in flexible wireless system,” Proc. of theSociety Conference of IEICE 2011, B-17-5, Hokkaido, Japan (inJapanese).
[9] D. Lee, T. Sasaki, T. Yamada, K. Akabane, Y. Yamaguchi, and K.
Uehara, “Spectrum Sensing for Networked System Using 1-bit Compressed Sensing with Partial Random Circulant Measurement Matrices,” Proc. of the IEEE 75th Vehicular Technology Conference(VTC2012-Spring), Yokohama, Japan.
[10] T. Yamada, D. Lee, H. Toshinaga, K. Akabane, Y. Yamaguchi, and K.
Uehara, “1-bit Compressed Sensing with Edge Detection for Compressed Radio Wave Data Transfer,” Proc. of the 18th Asia-PacificConference on Communications (APCC2012), Jeju, Korea.
[2]

NTT Technical Review

Feature Articles

Shuichi Yoshino

Takayuki Yamada

Takana Kaho

Mamoru Kobayashi

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees inmechanical engineering from Kanazawa University in 1990 and 1992, respectively. He joined
NTT Laboratories in 1992 and worked on satellite Internet system and wireless networking
technology development. He is currently engaged
in R&D of wireless technology for ubiquitous
services.

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
She received the B.S. and M.S. degrees inphysics from Tokyo Metropolitan University in1994 and 1996 and the Dr.Eng. degree in communication engineering from Tokyo Institute ofTechnology in 2007. Since joining NTT Radio
Communication Systems Laboratories in 1996,
she has been engaged in R&D of MMICs. From2010 to 2012, she was a Visiting Associate Professor at Research Institute of Electrical Communication, Tohoku University, Miyagi. She
received the Japan Microwave Prize at the 1998
Asia Pacific Microwave Conference and theYoung Researcher’s Award in 2004 from theInstitute of Electronics, Information and Communication Engineers (IEICE). She is a senior
member of IEICE.

Hiroyuki Shiba

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received the B.E. and M.E. degrees from
Gunma University in 1995 and 1997, respectively. Since joining NTT Wireless Systems
Laboratories (now NTT Network Innovation
Laboratories) in 1997, he has been engaged inresearch on software defined radio and cognitive
radio technologies. He received the Young Engineer’s Award from IEICE in 2001 and 2012, andthe 18th Telecom System Technology Award
from the Telecommunications Advancement
Foundation.

Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. degree in electronics from
Doshisha University, Kyoto, in 2005 and the
M.E. degree in communications and computerengineering from Kyoto University in 2007.
Since joining NTT Network Innovation Laboratories in 2007, he has been engaged in researchon flexible wireless systems. He received theBest Paper Award from IEICE Communications
Society in 2012. He is a member of IEICE andIEEE.

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees in electronic engineering from Shizuoka University in
1987 and 1989, respectively. Since joining NTT
Wireless Systems Laboratories in 1989, he has
been engaged in research on channel control forsatellite communications systems. He is currently engaged in R&D of flexible wireless systems
and their services.

Yushi Shirato

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Tokyo University of Science in 1990 and 1992, respectively. Since joining NTT Wireless Systems Laboratories in 1992,
he has been engaged in R&D of adaptive equalizers, modems for fixed wireless access systems,
and software defined radio systems. He is currently engaged in R&D of flexible wireless systems. He received the Young Engineer’s Award
from IEICE in 2000 and the 18th Telecom System Technology Award from the Telecommunications Advancement Foundation. He is a member of IEICE.

Hideki Toshinaga

Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees fromthe Department of Electronics and Communications of Waseda University, Tokyo, in 1994 and1996, respectively. He joined NTT Wireless
Systems Laboratories in 1996. He is currently
engaged in R&D of flexible wireless systems. He
is a member of IEICE.

Vol. 11 No. 3 Mar. 2013



Feature Articles: Wireless Access Technologies that Enhance
Convenience by Network Linkage

Scalable M2M
Wireless Access System
Mitsuru Harada, Tomoaki Kumagai, Yoshitaka Shimizu,
Kenji Suzuki, Nobuaki Mochizuki, Yosuke Fujino, andAkihiro Yamagishi
Abstract

NTT Network Innovation Laboratories is collaborating in developing a machine-to-machine (M2M)
wireless access method in order to expand the ways of communicating with machines. This method
utilizes a standard system of simplified management that improves communication quality by controlling the access points and wireless terminals from a network. This article outlines the system being
developed and introduces the functions and requirements of the wireless access M2M method. In addition, we describe how the method realizes the rapid construction of a wireless access network using
existing Wi-Fi home devices in a disaster area.

1. Background of M2M wireless accessIn recent years, considerable attention has been
focused on machine-to-machine (M2M) communication, which refers to communication between various
devices and is not necessarily limited to home devices. This form of communication utilizes various
types of information about the devices that network
servers collect, store, and analyze. M2M communications is expected to create new value in areas ranging
from health care, the environment, and ecology tosecurity and safety. Wireless access systems for M2M
communication play an important role these days asa means of accessing the devices. The 920-MHz frequency band (ARIB-STD 108) [1] became available
in Japan at the start of 2012, and wireless access systems for this band have been attracting a lot of attention with an eye towards increasing their use in the
future.
ZigBee has been established as an attractive standard for M2M wireless systems for the 920-MHz
band because it can use multi-hop network topology
and has relatively low power consumption characteristics. Zigbee private wireless systems allow users to
set up facilities that they operate themselves. To utilize wireless local area networks (WLANs), which

are also private wireless systems, users buy an accesspoint (AP) and a wireless terminal (WT) and input anauthentication key into them to establish authentication between them. This must be done for private
wireless systems that request operational management on the part of each user. However, for M2M
communication that must accommodate a huge number of WTs, this imposes a heavy burden on users
who have to set the authentication manually for eachindividual WT. Another problem that arises is unstable communication quality due to interference
between APs set up independently by many users. Yetanother is that communication protocol requirements,
e.g., IP (Internet protocol) or non-IP, differ according
to users’ purposes.
We aim to solve these problems without losing thebenefits of the private wireless system by means of ascalable M2M wireless access system. This system isbased on the standard method used in private wirelesssystems but has an added function that enables it tocontrol the APs and WTs from a network. This makes
it possible to avoid interference between APs and toconcentrate on operational management.
The salient feature of the M2M wireless system weare developing is that it is scalable. This enables thesizes of cells in the cellular communication system to
NTT Technical Review

Feature Articles

Protocol integration technology- XML transfer to IP/non-IP terminals- Supports multiple-protocols capabilities

Low-rate, wide area

High-rate, small area

Scalable wireless access- Standard method used forcommunication includes
variable speed capability.
Automatic

M2M wireless systemaccess point

Transmission ti
ming co
Chann
ntrol
e
l
Synchro assignmentnization contr
ol
t
n
nme
ol
el assig
Chann ization contr
n
ro
h
c
n
Sy

Interference management engine- Allocates and manages timing andfrequency of AP use- Allocates and manages terminal
communication timing- Determines a centralized terminal
accommodated in each AP

Network control
engine groupauthenti

cationon

enticati

tic auth

AutomaAuthentication control engine- Determines a centralized terminal
accommodated in each AP

Fig. 1. Features of M2M wireless access.

(a) Cell configuration of Zigbee
Base station(b) Effect of diversity technique
Antenna Base station
consolidation

Range expansion

Fig. 2. Concept of diversity technique.

be flexibly designed from large to small by varying
the cell size and communication speed according to
demand. We also aim to establish multiple-protocol
capabilities as the system’s communication protocol,
which will allow it to accommodate WTs using different protocols. Moreover, to achieve centralized
control of APs and WTs from a network, we designed
a WT that can be used without changing the connection settings to the APs when the WT moves amongcells. This cooperative operation stabilizes the connection quality (Fig. 1). We aim to build an economical wireless system for M2M communication byusing an AP that can use various commercial WTs
made by the standard method.

Vol. 11 No. 3 Mar. 2013

2. Features of scalable M2M wireless access
2.1 Scalable M2M wireless access
Scalable M2M wireless access enables the communication range to be flexibly controlled by adjusting
the transmit power and/or data rate. In outdoor environments, for instance, the communication range isabout 1 km with 250 mW of transmit power and a
10-kbit/s communication rate. Moreover, the communication range can be approximately doubled by
reducing the data rate to 1/10. In addition, as illustrated in Fig. 2, the communication range can be
expanded without reducing the data rate by applying
a diversity technique that consolidates antennas in thesame location and combines their reception power
levels.
In a non-line-of-sight environment between thetransmitter and receiver, buildings often reflect signals, and this produces areas of weak received signal
strength. In M2M communication where a fixed terminal position is assumed, a continuous communication failure known as fading frequently occurs as aresult of this local degradation. It is therefore essential to develop a solution to combat the problem offading.
The most common solution is to input a margin tocompensate for the local degradation. However, in
this solution, the margin limits the communication
range. To solve this unique problem in M2M communication and to expand the communication range
without reducing the data rate, we apply a diversity
technique.
Feature Articles

APL-A
HTTP/XMLIP

SEP2.0HTTP/XML
Zigbee IP

Non-IP

WiFi

IEEE802.15.4

IEEE802.15.4

APL-B

Unifying
The application interface
is unified.
The inter-conversion of
HTTP/CoAP is supported.
AP stores two or more
wireless systems.

Access point
APL-A/SEP2.0/APL-B
XML
CoAP

HTTP
IP
WiFi

Zigbee IP
IEEE
802.15.4

Non-IP
BluetoothAccess speed covers IP
and non-IP.

Fig. 3. Transparent XML transmission with HTTP/CoAP conversion function.

The application of diversity is generally limited in
M2M communication because of its low signal-processing capability. Therefore, we developed a new
method called frequency offset transmitter diversitythat transmits only signals from two or more antennas
at slightly different frequencies. This diversity method enables the communication range to be expanded
by about 1.8 times using six transmitter antennas.

These features make XML transmission possible at
low-performance terminals for M2M communication. Supporting the HTTP/CoAP inter-conversion inan AP (Fig. 3) also enables the protocols to be unified
into the application interface of an XML base to the
WT of a non-IP connection. This enables users todevelop applications without being base on the protocol of a WT.

2.2 M
 ulti-protocol correspondence of communication protocols
With respect to the communication protocol, an
HTTP (hypertext transfer protocol)/CoAP (constrained application protocol) conversion function is
mounted in an AP, and transparent XML (extensible
markup language) transmission is achieved without
being based on the protocol of a WT. Conventionally,
the communication protocol is implemented using
HTTP or an original system according to each application of M2M communication. This multi-protocol
approach is intended to unify various protocols into
the application interface of an XML base (Fig. 3).
Although HTTP is usually used as a transfer protocol
in IP connections, it is very difficult to achieve an IP
connection and to mount HTTP into the low-performance terminals in M2M communication. Therefore,
the IETF (Internet Engineering Task Force) has been
advancing the standardization of CoAP in order to
use it as a transfer protocol suitable for M2M communication. The main features of CoAP are that it iscompatible with HTTP and that it reduces therequired number of packet headers and sequences.

2.3 Network-initiated authentication
M2M communication accommodates a huge number of WTs, and the connection setup needs to beestablished easily and securely between APs and
WTs for authentication. Having to manually set individual WTs places an additional burden on users andshould therefore be phased out or reduced as much as
possible. WTs for M2M typically do not have anymeans of input (e.g., a keyboard or mouse), so theconnection setup and authentication procedure forthese terminals should be automated. In the system
we are contemplating, a private-certification-authority server issues a digital certificate to a WT that
contains a secure chip. After the power is activated,each WT automatically uses the certification serveron the network to set up the connection with the AP
for authentication. The tamper-resistant secure chip
the WT contains provides a safe and secure wirelessconnection environment.



2.4 C
 entralized control with AP and WT from a
network
Narrowband and limited frequency channels are
NTT Technical Review

Feature Articles

Internet

NTT building
WLAN AP with awireless terminal forM2M wireless access

Use available
optical fiber cables
to access theInternet.

Internet

NTT building
Construct amesh network of
WLAN APs.

Remote control of
APs through M2M
wireless accessInternet access via
home network
Smartphone

Access theInternet via meshnetwork of
WLAN APs.

Access point of
M2M wirelessaccess(a) Daily use(b) Disaster use

Fig. 4. Disaster recovery scenario using M2M wireless access.

used for M2M wireless access. An efficient approach
to WT accommodation is for network-initiated management to coordinate the AP operation. For example,
a cooperation scheme is needed where the network
carries out synchronization between APs and allocates the frequency channel and communication timing to APs and WTs in order to prevent interference
between adjacent APs and to enable the wide distribution of APs with fewer frequency channels. Another necessary scheme is one that ensures that data aretransferred without fail, even though the currency ofdata may be limited to some extent. This is done by afunction that distributes the WT’s access timing insuch a way as to prevent collision congestion when
packet concentration is high in the network. This feature is suitable for M2M communication and could be
effective for applications that do not require as high a
degree of instancy as in human communications.
3. Disaster recovery solution using
M2M wireless access
Smartphones with Wi-Fi capability have rapidly
come into widespread use. They enable users toaccess the Internet outdoors via a cellular network
and indoors via a home network, i.e., a WLAN. Consequently, the number of users who access the Internet with smartphones has recently been increasing.
However, these access networks may not function in
the event of a natural disaster such as an earthquake,so communication tools such as telephone and e-mailVol. 11 No. 3 Mar. 2013

may not be available. In that event, users would havedifficulty contacting relatives or gathering information about the disaster with these tools. To address
this problem, in addition to restoring telephone services, it is important to construct new networks using
existing Wi-Fi home devices, i.e., WLAN APs, toenable Internet access as quickly as possible. With
this objective in mind, we are developing a prototype
system that enables the rapid construction of a wireless access network by using M2M wireless access asa control link. This will make it possible for users toaccess the Internet with the Wi-Fi function on asmartphone, as shown in Fig. 4.
This system controls the Wi-Fi home (WLAN) APs
attached to the M2M wireless terminal from theM2M AP installed by a carrier in a disaster area via
an M2M wireless access link. Constructing relay
links among the APs provides wide area coverage forcommunication around the area where the M2M AP
is installed. Therefore, the system has two noteworthy
features. The first is easy deployment. New APs andcomplicated settings done by engineers are unnecessary since APs for household use are used to construct
the wireless access network for Internet access. The
system also provides flexibility in establishing connections. For connection to the wireless access network, the system makes available access operations
that any smartphone users can use, as well as thosethat only preregistered users can use. It should be
noted that in either case, considerable discussion on
the access operations is needed from the viewpoint ofFeature Articles

security. It is possible to access the Internet via
optical fiber cables for household use if they have notbeen damaged in the disaster.
The second feature the system offers is flexible
network management. When a disaster occurs, thenumber of available APs and the user distribution
tend to vary from hour to hour. To deal with thesevariations, i.e., to accommodate a large number ofusers and handle a huge volume of traffic effectively,
this system can control the network topology andassign the channel frequencies dynamically in accordance with the changes by using the M2M wirelessaccess as a control link.
Finally, we mention the system’s technical aspects.
We are mainly developing two technologies relevantto this system under the national project; one is ameans of effectively controlling data transmission of
M2M wireless access in order to reduce overhead,since this access offers very low transmission rates
ranging from several kilobits per second to several
hundred kilobits per second. The other is technology



to enable different WLAN AP devices to be controlled in a common manner.
Acknowledgment
This work is being partially conducted under thenational project, “The R&D on the reconfigurable
communication resource unit for disaster recovery,”
supported by the Ministry of Internal Affairs andCommunications (MIC), Japan.
References
[1]

[2]

The Association of Radio Industries and Businesses (ARIB), “Standard Specifications for 920-MHz Band Equipment,” 2012 (in Japanese).
http://www.arib.or.jp/english/html/overview/doc/1-STD-T108v1_
0.pdf
D. Uchida, H. Matsumura, S. Kuwano, F. Nuno, N. Mochizuki, S.
Kotabe, K. Suzuki, T. Fujita, and K. Mitani, “Technical Development
Activities for Wide-area Ubiquitous Network,” NTT Technical
Review, Vol. 8, No. 6, 2010.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2010
06sf2.html

NTT Technical Review

Feature Articles

Mitsuru Harada

Nobuaki Mochizuki

Senior Research Engineer, Supervisor, Group
Leader of Wireless Communication Circuits
Research Group, Smart Devices Laboratory,
NTT Microsystem Integration Laboratories.
He received the B.S. and M.S. degrees in physics from Tsukuba University, Ibaraki, in 1985
and 1987, and the Ph.D. degree in electronicengineering from Tohoku University, Miyagi, in
2003. In 1990, he joined NTT Atsugi ElectricalCommunication Laboratories, where he engaged
in research on thin-film SOI devices. Since 1997,
he has been researching high-speed/low-power
CMOS circuits for wireless terminals.

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received the B.E. and M.E. degrees fromTohoku University, Miyagi, in 1992 and 1994,
respectively. He joined NTT in 1994 and engaged
in R&D of a modulation/demodulation scheme
for a broadband 5-GHz wireless access systemand synchronization scheme for coded OFDM.
He is currently engaged in developing WAUN
wireless access systems. He is a member of
IEICE.

Tomoaki Kumagai

Yosuke Fujino

Yoshitaka Shimizu

Akihiro Yamagishi

Senior Research Engineer, Supervisor, NTT
Network Innovation Laboratories.
He received the B.E. and M.E. degrees in electrical and communication engineering, and the
Ph.D. degree in information science fromTohoku University, Miyagi, in 1990, 1992, and2008, respectively. Since joining NTT in 1992,
he has been engaged in R&D of personal communication systems and high-speed wireless
LAN systems. He received the Young Engineer’sAward from the Institute of Electronics, Information and Communication Engineers (IEICE) in1999. He is a member of IEICE and IEEE.

Wireless Systems Innovation Laboratories,
NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Tokyo Institute of Technology in 1995 and 1997, respectively. He joined
NTT Wireless Systems Laboratories in 1997. He
studied wide area sensor network (WASN) systems and contributed to WASN standardization
efforts in ITU-R. He is currently engaged inresearch on wireless access systems. He is amember of IEICE.

Wireless Systems Innovation Laboratory, NTT
Network Innovation Laboratories.
He received the B.E. and M.E. degrees in electrical and electronic engineering from Shizuoka
University in 2002 and 2004, respectively. He
joined NTT Network Innovation Laboratories in2004 and engaged in research on multiuser detection for a multiantenna, multiuser wireless system. He is currently working on the design of
WAUN physical layer protocols and algorithms.
He is a member of IEICE.

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Toyama University and
the Ph.D. degree in electrical engineering fromTohoku University, Miyagi, in 1986, 1988, and2005, respectively. In 1988, he joined NTT LSILaboratories, where he engaged in R&D of frequency synthesizer ICs. From 2000 to 2004, hestudied 1-V-operation CMOS RF circuits for
wireless systems. He is currently studying awireless terminal for the WAUN. He is a memberof the Institute of Image Information and Television Engineers of Japan.

Kenji Suzuki

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received the B.E. and M.E. degrees in electrical and electronic engineering from Tokyo
Institute of Technology in 1999 and 2001,
respectively. In 2001, he joined NTT Telecommunications Energy Laboratories, where heworked on a wireless transceiver LSI architecture
for low power dissipation. His interests include
analog and RF IC design for wireless communications. He is currently developing WAUN systems. He is a member of IEICE and IEEE.

Vol. 11 No. 3 Mar. 2013

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Virtual Sensor Construction
Technique for Participatory Sensing
Environments
Hiroshi Sato, Atsushi Yamamoto, Hisashi Kurasawa,
Hitoshi Kawasaki, Motonori Nakamura, andHajime Matsumura
Abstract

Much attention is being focused on participatory sensing, in which real-world data are collected using
personal mobile devices as sensor nodes to sense various conditions of the world we live in. In thisapproach, sensor nodes are not controlled by a sensing system; they move and sense independently.
Therefore, the sensing data may include noise or missing values and are therefore difficult to use as-is
for applications. We are investigating a technique to construct virtual sensors in order to provide reliable
and flexible sensing data. In this article, we introduce a construction technique that consists of three core
methods to enhance data quality: participation promotion using our Top of Worlds technique, noise
reduction, and missing value estimation. We also describe a simple demonstration we conducted thatshows how these factors enhance data quality.

1. Introduction
Worldwide smartphone use is currently estimatedto be 12% and is predicted to increase to 50% in five
years [1]. These mobile devices are usually equippedwith several sensors that capture visual, acoustic,
tactile, location, and other data. This makes themsuitable devices for participatory sensing, which has
been attracting global attention [2]. Participatory
sensing, also called human/people-centric sensing, isan approach for collecting and analyzing data inwhich individuals, acting alone or in groups, use theirpersonal mobile devices and cloud services to sense
and share various conditions of their surroundings.
We call such data real-world data. The concept ofparticipatory sensing is shown in Fig. 1. Thisapproach is an alternative to conventional sensing
environments where stationary sensor nodes that are
dominated, i.e., controlled, by a sensor system areinstalled in a target area. The conventional approachis not suitable for wide-area sensing because itrequires a large number of sensor nodes that are
expensive to install and maintain. Such costs are notincurred with the participatory sensing approach,which can be used with existing devices. The dataobtained in participatory sensing will be useful inapplications that have a significant socio-economic
impact. Such applications include, but are not limited
to, environmental monitoring, pedestrian navigation,
urban planning, pandemic prevention, disaster recovery, and energy management.
Recent advances achieved in sensor devices andwireless networking technologies mean that participatory sensing is becoming more and more possible;
however, technical challenges remain. One of the key
challenges is determining how to enhance defective
data. In participatory sensing, sensor nodes are notunder the control of a sensing system; they move andsense independently. The system cannot sense targets
systematically as in conventional controlled sensing
and cannot always obtain the intended data in theintended form when needed. That is to say, raw
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GPS

ACCEL

GYRO

LIGHT

ORIENT

Wi-Fi

ACCEL
GYRO
ORIENT

GPS

PRESSURE

CAMERA

ACCEL

TEMP

GRAVITY

ACCEL

HUMIDITY

GPS

LIGHT

MAGNET

Wi-Fi

GRAVITY

MAGNETCAMERA

Fig. 1. Concept of participatory sensing; users collect various kinds of data with their own smartphones.

participatory sensing data are defective and difficult
to use as-is for applications. Therefore, we believethat sensor virtualization, which provides reliablevirtual sensor data produced from related (actual)
data, is essential for participatory sensing.
We introduce here three techniques to enhance data
quality: participation promotion, noise reduction, andmissing-value estimation. These techniques enhance
the quality of participatory sensing data and makevirtual sensors reliable and flexible. In section 2, wediscuss various issues concerning participatory sensing data. Then we briefly review related work in section 3. In section 4, we explain the three data quality
enhancement techniques used in virtual sensor construction. In section 5, we discuss a demonstration ofvirtual sensors constructed using our techniques
based on a sample scenario. Finally, we conclude thearticle in section 6.
2. Issues concerning participatory sensing
This section addresses participatory sensing issues,
specifically defective data. The largest difference
between conventional and participatory sensing isthat sensor nodes of the former are dominated; i.e.,they are controlled by a system, whereas sensor nodesof the latter are not. In participatory sensing, sensing
conditions such as time, location, and angle dependon each individual’s behavior, and sensor module
specifications such as type and accuracy depend on
the individual devices; we cannot typically control
them. Moreover, whether or not to sense or to transmit data also depends on the will of individual participants. This means that most of the collected data
Vol. 11 No. 3 Mar. 2013is defective—sparse, noisy, and distorted. We discussthese three issues below.
2.1 Sparseness (insufficient quantity of data)
The first issue is data sparseness, namely, the insufficient supply of data. Participatory sensing does notwork without people’s participation. However, participants must use their own battery power and pay
the data transmission charge, so it may be difficult tocollect sufficient data solely by depending on volunteers; therefore, the collected sensing data may besparse. One way to collect data is to provide a financial incentive to people. This improves their extrinsic
motivation to participate; however, it may increase
sensing system operating costs. Furthermore, a psychological study [3] found that extrinsic motivation isunsustainable. Therefore, a method to promote participation by improving intrinsic motivation is necessary.
2.2 Noise (inaccurate data)
The second issue is noise, namely, inaccuracy. Participatory sensing involves sensor modules installedin personal mobile devices. In general, such sensor
modules are less accurate than specially tuned stationary sensors used in conventional sensing. Additionally, the accuracy of such personal use sensor
modules is probably not uniform because there aremany types of products. Therefore, the sensing data
may be noisy. We must reduce the noise and enhance
the accuracy of the data in order to use such data inapplications.

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2.3 Distortion (incomplete data)
The last issue is distortion of data, namely, incompleteness. In participatory sensing, the sensing timeand location are basically left up to the participants.
Participants can freely move, sense objects, andtransmit the sensing data. Consequently, it is easy tocollect sensor data in spaces where many people
gather, whereas it is difficult to collect data where
fewer people gather. In addition, the type of sensor tocarry depends on the participant’s device. For example, one participant’s device may be equipped withsensor modules A and B, while another participant’s
device may only have sensor module A. Therefore, itis easy to collect data from commonly used sensors
but difficult to do so from sensors that are used lessfrequently. As a result, participatory sensing data are
unevenly distributed in time and space and are therefore incomplete; in other words, they include many
missing values. We must compensate for any missing
values in applications.

small for most users.
We believe that a comprehensive approach toenhancing the quality of sensing data is important toprovide reliable virtual sensors for applications.
4. Virtual sensor

3. Related work

We are researching virtual sensor construction
techniques that enhance the quality of participatory
sensing data against the issues mentioned in section 2
and that produce virtual sensor data from enhanced
sensor data sets for various applications. The flow of
our construction technique is shown in Fig. 2. First,
sufficient sensor data are collected by using a technique to promote user participation. Second, the reliability of sensor data sets is enhanced using a noise
reduction technique. Finally, incomplete data arecompensated for using a multiple regression technique. Additionally, a new sensor data set that fuses
the data collected from all sensors is produced if necessary. In this section, we explain each of our data
quality enhancement techniques.

Many studies have been conducted on sensor virtualization for participatory sensing, and some havefocused on enhancing the quality of sensing data.
Weinschrott et al. [4] proposed virtual sensor abstraction algorithms, collectively called StreamShaper, formobile urban sensing. They focused on enhancing thecoverage of virtual sensors; however, they did nottake the reliability of virtual sensor values into
account. Ganti et al. [5] proposed a navigation service
called Green GPS that suggests the most fuel-efficient routes to drivers. They focused on compensating for missing values. Green GPS builds a generalized hierarchical model for various vehicle types andmerges the various sensor data by using the model;
this increases the service coverage, but it requires anelaborately tuned model.
The issue of participant motivation has also been
investigated in several studies. Reddy et al. [6] investigated a method to motivate users to participate insensing; however, they presuppose the incentive payment model. To motivate users to participate in aparagliding community service, Kaenel et al. [7]developed a method of presenting the rank of eachuser’s paragliding distance calculated from GPS data
as information for users to compare with other users.
However, because this method only ranks each useramong all other users, most users have little chance ofbeing ranked in the top group. Therefore, we believethat the motivational effect of this method would be

4.1 Participation promotion
Because participatory sensing requires input of sensor data from users, the users must feel motivated tocontinue participating. We focused on improving
users’ intrinsic motivation by promoting a sense of
superior achievement.
We proposed Top of Worlds [8], a method forstrengthening the motivation of users to participate insensing by presenting rankings in multidimensional
hierarchical sets. A previously proposed method [7]only ranks each user among all other users, so mostusers have little chance of being ranked in the topgroup and may have little motivation to continue. Topof Worlds creates many sets with varying granularity
to increase the chance of many users being ranked in
the top group and presents these rankings in thosesets. An example of values used to compare rankings
is the amount of sensor data transmitted by users. Topof Worlds is based on two hypotheses:- H1. If a user is ranked in the top group, her/his
motivation will increase compared to when theuser is not in the top group.
- H2. If a user is ranked in the top group in multiple sets, the coarser the granularity of the set is,
the more her/his motivation will increase.
On the basis of H1, our proposed method is composed of the following steps: 1. many sets are createdwith varying granularity to increase the chance ofmany users being ranked in the top group; 2. sets areNTT Technical Review

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Noise reduction in every sensor(reliable sensor data)

Complementation among sensors(complete sensor data)

Data fusion(fused sensor data)

Participation promotion(sufficient sensor data)

Fig. 2. Flow of virtual sensor construction.

PeriodEntireperiodIn pastweek

In past day

Attribute (address)
Tokyo

USA

Osaka
…

Japan
…

Worldwide

Attribute (age)30 years old

30s

31 years old
…

20s

…

All ages

Fig. 3. Hierarchical sets.

selected in which a user ranks in the top group. For
example, step 1 involves creating multidimensional
hierarchical sets by time period and by attribution(address, age, or sex). In step 2, sets are selectedamong multidimensional hierarchical sets in which auser ranks in the top group. We define hierarchicalsets based on time period (entire period, in the past
Vol. 11 No. 3 Mar. 2013

week (one-week period up to now), in the past day(one-day period)), address (worldwide, nationwide,statewide (citywide)), and age (all ages, a certain age
range, e.g., 20s and 30s, or a specific age), as shownin Fig. 3. We define a multidimensional hierarchicalset as a common subset of each hierarchical set.
On the basis of H2, if a user ranks in the top groupin multiple sets, Top of Worlds is composed of thefollowing additional steps: 3. a set with the coarsest
granularity (the longest period, largest address segment, or broadest age range) is selected; 4. information on the set and the rank is presented to the user
(we skip step 3 if the user ranks in the top group in
only one set).
An example of Top of Worlds is shown in Fig. 4. We
assume the user lives in Tokyo and is 31 years old. If
the user is given information such as You are 50,000thin Japan among all ages out of 100,000 people, theuser may not be motivated to continue. Therefore,
Top of Worlds selects a hierarchical set focused on
the user and presents information such as You areranked 1st in Tokyo among fifty 31-year-olds to theuser. As a result, the user will be motivated to continue participating. Thus, Top of Worlds effectively
increases the motivation of many users.
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Set: Japan, all ages

User
Set: Tokyo, 31-year-olds
Select hierarchical set focused on user.

Fig. 4. Example of Top of Worlds.

4.2 True value estimation
4.2.1 Basic idea
Participatory sensing produces noisy sensor data
due to the use of sensors with low accuracy or a mixof sensors with diverse performance capabilities. It ismeaningless to analyze each value in the noisy sensor
data in order to use it in applications. We have to estimate true values from the noisy sensor data to provide
reliable data for applications.
We recently proposed a method for statisticallyestimating the true values of population means byusing noisy sensor data as random samples [9]. Thisestimation method is aimed at accurately estimating
the population means while preserving their spatiotemporal patterns.
This method can be used to accurately estimate apopulation mean by expanding a spatio-temporal
region where a sample is obtained in order to increase
the sample size. This is because of the well-known
fact that an estimate for a population mean becomes
more accurate as the sample size increases.

However, expanding the region makes the spatiotemporal granularity of the estimate coarse, whichleads to a loss of spatio-temporal patterns of the
population means. This estimation method preserves
those spatio-temporal patterns by the use of a constraint condition in which the region must consist ofelements with the same population mean when theregion is expanded.
In other words, this estimation method partitions anentire spatio-temporal region into the largest possible
regions, each of which consists of elements with thesame population mean, and then estimates each population mean by using samples obtained in theregions.
4.2.2 Detailed steps of estimation method
The above-mentioned estimation method consistsof three steps: initialization, region expansion, andinterval estimation. The region expansion and interval estimation steps are repeated until the entire spatio-temporal region is covered. An overview of thisestimation method is shown in Fig. 5, where a spatiotemporal region is drawn as a simplified two-dimensional region.
(1) Initialization
In this step, the entire spatio-temporal region isdivided into elements of the same size, which areunited into regions in the region expansion step.
(2) Region expansion
In this step, one of the elements is randomly selected and added to a region, which is then expanded byuniting the neighboring elements with the same population mean.

Initialization

Region expansion

Interval estimation

Entire spatio-temporal region
is divided into same-sized
elements.

Elements with the same
population mean are united
into the same region.

A confidence interval for the
population mean of eachregion is estimated.

Region 2

Element 1 Element 2 Element 3

Element 4 Element 5 Element 6

Element 7 Element 8 Element 9

Region 1

Interval estimate 2Interval estimate 1

Region 3

Interval estimate 3

Fig. 5. Overview of estimating true values of population means.



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This step statistically tests the null hypothesis,
which states that the population mean of the region
equals that of each of the neighboring elements,
against the alternative hypothesis. If the null hypothesis is accepted or not rejected, this expansion step
basically adds each of the neighboring elements to
the region. However, a statistical hypothesis test may
produce a type II error, which is the incorrect acceptance of a false null hypothesis. Thus, this step adds
each of the neighboring elements to the region withthe statistical power, which is the probability of not
making the type II error. In this way, this step adheresto the constraint condition that the region consists ofelements with the same population mean when
expanding the region.
In this step, the region expansion with statistical
hypothesis testing is repeated until no neighboring
elements are newly added to the region, which maximizes the region under the constraint condition inorder to increase the sample size for estimation.
(3) Interval estimation
In this step, a confidence interval is statisticallyestimated for the population mean by using a sample
obtained in the expanded region. This ensures thatapplications can not only obtain a more accurate estimate but can also assess the accuracy of the estimate
by the width of the interval.
The above steps, particularly the region expansion
step, allow us to achieve the intended aim of accurately estimating the population means while preserving their spatio-temporal patterns.
4.3 Missing value estimation
4.3.1 Basic idea
In participatory sensing, the sensor data are often
affected by missing values, which makes the data
incomplete. The sensors and sensing area respectively depend on the mobile devices of the participants
and the participants’ trajectories. It is not feasible toforce a participant to stay in a target area as a stationary sensor.
Our goal was to accurately estimate the missing
values of a sensor from incomplete sensor data byusing multiple regression. A simple way of analyzing
incomplete sensor data would be to delete all themissing records and then use only the complete
records. However, excluding all records having morethan one missing value would result in a decrease in
the quantity of training sensor data available for estimation. Thus, we should select the minimum numberof sensors needed to estimate the missing values andexclude all records that have missing values of theVol. 11 No. 3 Mar. 2013selected sensors.
We previously proposed a method to estimate a
missing sensor value by using incomplete sensor data[10]. This estimation method was designed to
improve the regression model and increase the quantity of training sensor data. This method roughly
prunes inessential sensors by using small training
sensor data and improves estimation accuracy while
repeating sensor selection and updating the training
sensor data. An overview of this estimation method isshown in Fig. 6.
4.3.2 Detailed procedure
Let Rinit be incomplete sensor data that consist ofninit records rinit,1, rinit,2, … , rinit,ni. Each record
rinit,j ∈ Rinit has values of multiple sensors V, which
may include some missing values. Here, rinit,j (v) isrinit,j’s value of the sensor v ∈ V. Given the incomplete
sensor data Rinit as the initial training sensor data anda missing value Q(q) of a sensor q in one record Q asa query, we set the estimation task as estimatingQ(q).
Our estimation method consists of the following six
steps: inputting incomplete sensor data and queries,
maintaining training sensor data, conducting locallyweighted measurements, selecting sensors, evaluating accuracy, and doing the final estimation. The
steps from training sensor data maintenance to accuracy evaluation are repeated until the accuracy satisfies quality conditions.
(1) Incomplete sensor data and query inputThis estimation method receives Rinit, which isdetected through participatory sensing. The sensors
initially selected for regression are set to be theobserved sensors of the query record.
(2) Training sensor data maintenance
When arranging the training sensor data, our estimation method completely removes the records that
do not include the selected sensors from the incomplete sensor data, then puts the remaining records inorder.
(3) Locally weighted measurement
This estimation method weighs the nearest neighbor records for regression. This is for sensors that are
not generally correlated but locally correlated. For
example, illuminance and ultraviolet light are often
correlated if the spatio-temporal range is limited. The
weight is measured based on the selected sensors.
This means that the weight of a record updates every
correlated sensor selection.
(4) Sensor selection
This estimation method selects sensors correlated
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Delete all missing cases andarrange initial training data.

Arrange incomplete sensor data.

RecordsA

SensorsQuerysensor

1st-training dataQueryrecordSelect sensors locallycorrelated with query sensor.
Improve modelC…

RepeatSensors

Participatory sensing

Queryrecord Records

B

Query
record Records

Sensors with missing values

Need missing sensor value

Queryrecord Records

Sensors

Sensors

Selected
sensor

1st-training data

Increase training data

2nd-training data

Estimate

Update training data
including selected sensors.

Fig. 6. Overview of missing-value estimation.

with the sensor that includes the missing value by
using Lasso, which is an L1-norm regularization
technique. Lasso does both continuous shrinkage andautomatic variable selection at the same time. It is
widely used in high-dimensional regression for variable selection.
(5) Accuracy evaluation
The estimation method determines whether itshould continue the repeated steps. It stops repeating
the steps when one of the following two conditions is
satisfied. One is that the amount of training sensor
data does not increase after sensor selection. This isbecause the accuracy would not be expected to
improve. The other is that the accuracy measured by
K-fold cross-validation satisfies the predefined
threshold, which represents the mean squared error. If
these conditions are not satisfied, it executes the steps
from training sensor data maintenance to accuracy
evaluation again.
(6) Final estimation
The final estimation value is then calculated.



5. Demonstration
We developed a demonstration system to simulate
participatory sensing and indicate the effects of ourtrue-value and missing-value estimation methods
described in subsections 4.2 and 4.3. In this section,
we explain how the quality of participatory sensing
data is improved by using our estimation methods
with the demonstration system. Our demonstration
scenario is to visualize the fused values of five typesof participatory sensing values as a heat map. For thedata collection, we assume that 100 people participate in sensing.
5.1 Simulation of participatory sensing
We prepared five synthetic datasets of sensors, A,
B, C, D, and E, in a square area for the demonstration.
Their heat maps are shown in Fig. 7. Red indicates ahigher value and blue a lower value. The visualization
target X is the sum of the five sensor values. We
divide the 100 participants into five groups. Eachgroup consists of 20 people and is assigned one of the
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A

B

C

D

E

X

Fig. 7. Heat maps of original sensor data and sum of their values.

A

B

C

D

E

Fig. 8. Trajectories of participants.

A

B

C

D

E

Fig. 9. Heat maps of raw sensing data.

five types of sensors. We assume that each sensor
module has its own error rate, and participants walkaround the area while capturing sensor data.
To simulate participatory sensing, each participant
freely draws a curved line on the area instead of walking. The trajectories are shown in Fig. 8. The temporal axis is omitted from the simulation to simplify theheat maps, so all sensing actions of all participants
are regarded as concurrent. The sensing results areshown in Fig. 9. Noise and missing values can beseen compared to the original data in Fig. 7.
5.2 Effects of our two estimation methods
We estimated the population means for each sensor.
The results are shown in Fig. 10. The noise was
reduced compared to the raw data in Fig. 9; however,there were still many missing values. Moreover,points where all kinds of data exist are very rareVol. 11 No. 3 Mar. 2013

(Fig. 11). This means that it is impossible to calculate
and visualize X on the map.
In this demonstration, we directly compensated for
missing values of X instead of the values of each of
the five sensors A to E. We estimated the missing
values of X using other sensors’ values. These results
are shown in Fig. 12. Although some errors remained,almost all of the data were compensated for.
6. Conclusion
We discussed participatory sensing and pointed out
three defects that can occur in sensed data: sparseness, noise, and distortion. We also introduced threecore techniques to enhance the quality of participatory sensing data. A demonstration showed that ourenhancement techniques worked well. We believethat using our enhancement techniques in sensor
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A

B

C

D

E

Fig. 10. Results of estimating population means.

X

X

Fig. 11. Heat map of X (intersection only).

Fig. 12. Results of estimating missing values of X.

virtualization environments will make it possible tosupport many useful applications that use real-worlddata.
We are currently investigating ways to evaluate data
quality and value and developing sample applicationsof participatory sensing.

“Green GPS: A Participatory Sensing Fuel-efficient Maps Application,” Proc. of the 8th International Conference on Mobile Systems,
Applications, and Services (MobiSys 2010), pp. 151–164, San Francisco, CA, USA.
[6] S. Reddy, D. Estrin, M. Hansen, and M. Srivastava, “Examining
Micro-payments for Participatory Sensing Data Collections,” Proc. of
the 12th ACM International Conference on Ubiquitous Computing(Ubicomp 2010), pp. 33–36, Copenhagen, Denmark.
[7] M. V. Kaenel, P. Sommer, and R. Wattenhofer, “Ikarus: Large-scale
Participatory Sensing at High Altitudes,” Proc. of the 12th Workshop
on Mobile Computing Systems and Applications (HotMobile 2011),
Phoenix, AZ, USA.
[8] H. Kawasaki, A. Yamamoto, H. Kurasawa, H. Sato, M. Nakamura,and H. Matsumura, “Top of worlds: method for improving motivation
to participate in sensing services,” Proc. of the 14th ACM International Conference on Ubiquitous Computing (Ubicomp 2012), pp.
594–595, Pittsburgh, PA, USA.
[9] A. Yamamoto, H. Kawasaki, H. Kurasawa, H. Sato, M. Nakamura,and H. Matsumura, “Proposal of an Accuracy-aware Virtual Sensor
for Participatory Sensing,” Forum on Information Technology 2012
(FIT2012), Vol. 4, No. RM-001, pp. 17–20, Tokyo, Japan.
[10] H. Kurasawa, H. Sato, A. Yamamoto, H. Kawasaki, M. Nakamura,and H. Matsumura, “Variable Selection Method in Multiple Regression with Incomplete Sensor Data,” IEICE Technical Report,
USN2012-54, Vol. 112, No. 242, pp. 149–154, 2012.

References
[1]
[2]

[3]
[4]

[5]



Press release by Seed Planning, July 26, 2012 (in Japanese).
http://www.seedplanning.co.jp/press/2012/2012072601.html
J. Goldman, K. Shilton, J. Burke, D. Estrin, M. Hansen, N. Ramanathan, S. Reddy, V. Samanta, M. Srivastava, and R. West, “Participatory Sensing: A citizen-powered approach to illuminating the patterns
that shape our world,” Woodrow Wilson International Center for
Scholars, Washington, D.C., May 2009.
E. L. Deci and R. M. Ryan, “Intrinsic Motivation and Self-Determination in Human Behavior,” Plenum Press, New York, 1985.
H. Weinschrott, F. Dürr, and K. Rothermel, “Streamshaper: Coordination algorithms for participatory mobile urban sensing,” Proc. of the
7th IEEE International Conference on Mobile Ad-hoc and Sensor
Systems (IEEE MASS 2010), pp. 195–204, San Francisco, CA,
USA.
R. K. Ganti, N. Pham, H. Ahmadi, S. Nangia, and T. F. Abdelzaher,

NTT Technical Review

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Hiroshi Sato

Senior Research Engineer, Ubiquitous Service
Systems Laboratory, NTT Network Innovation
Laboratories.
He received the B.S. and M.S. degrees in mathematics from Tohoku University, Miyagi, in 1994
and 1996, respectively. In 1996, he joined NTT
Communication Science Laboratories, where heengaged in research on natural language processing. Since moving to NTT Network Innovation
Laboratories in 2007, he has been researchingubiquitous data management. His research interests include uncertain information processing.
He received the Best Paper Award of the Information Processing Society of Japan (IPSJ)National Convention in 1999. He is a member of
the IPSJ and the Japan Society for Fuzzy Theory
and Intelligent Informatics.

Atsushi Yamamoto

Research Engineer, Ubiquitous Service Systems Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees from
Kyushu University, Fukuoka, in 2001 and 2003,
respectively. He joined NTT Network Innovation
Laboratories in 2003. His research interests
include large-scale data processing and statisticalinference. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE) and IPSJ.

Hitoshi Kawasaki

Researcher, Ubiquitous Service Systems Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees in computer science from Keio University, Kanagawa,in 2007 and 2009, respectively. He joined NTT
Cyber Solutions Laboratories in 2009. He is currently studying data mining and persuasive technology. He is a member of IPSJ and IEICE. He
received the Best Paper Award for Young
Researchers at the 73rd Annual IPSJ Conference.

Motonori Nakamura

Senior Research Engineer, Supervisor, Ubiquitous Service Systems Laboratory, NTT Network
Innovation Laboratories.
He received the B.E. and M.E. degrees in information engineering from Nagoya University,
Aichi, in 1990 and 1992, respectively. Since joining NTT Switching System Laboratories in 1992,
he has been researching distributed telecommunication service software architectures, ad-hoc
routing protocols, and software platforms for
ubiquitous services. He is a member of IEICE
and IPSJ.

Hajime Matsumura
Hisashi Kurasawa

Researcher, Ubiquitous Service Systems Laboratory, NTT Network Innovation Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in information science and technology from the
University of Tokyo in 2006, 2008, and 2011,
respectively. He joined NTT Network Innovation
Laboratories in 2011 and has been studying data
mining. His research interests include similarity
search, distributed systems, and context-aware
computing. He is a member of IEICE, IPSJ, andthe Database Society of Japan.

Vol. 11 No. 3 Mar. 2013

Senior Research Engineer, Supervisor, Ubiquitous Service Systems Laboratory, NTT Network
Innovation Laboratories.
He received the B.E., M.E., and D.E. degrees inengineering from the University of Tokushima in
1987, 1989, and 2000, respectively. Since joining
NTT in 1989, he has been engaged in R&D of
STM switching systems, common software platforms for switching systems, distributed networkarchitecture and distributed network performance simulation, network-linked broadcasting
systems based on home servers, and business
operation support systems for FLET’S and NGN
services. He is currently researching ubiquitous
networking systems. He is a member of IEEEand IEICE.

10Global Standardization Activities

Trends in the International
Standardization of
HVDC Power Feeds
Toshimitsu Tanaka, Kaoru Asakura, andTadayoshi Babasaki
Abstract

Higher-voltage direct current (HVDC) power feeds are attracting attention as an efficient means ofsupplying 380-V DC electricity to datacenters and telecommunications facilities, and it is expected that
this technology will eventually be put to practical use in many places worldwide. In this article, we
describe the characteristics of HVDC power feeds and the international standardization efforts related tothis technology, including an overview of ITU-T Recommendation L.1200 of May 2012 (“Direct current
power feeding interface up to 400 V at the input to telecommunication and ICT (information and communications technology) equipment”).

1. Introduction
In recent years, with the arrival of cloud computing
and other network-enabled technologies, demand forcommunications and datacenters has continued to
grow. However, as more equipment is installed indatacenters, their power consumption is steadily
increasing. This trend is not exclusive to Japan, but isbeing seen all over the world. To realize a sustainable
low-carbon society, the need for greener, more energy-efficient information and communications technology (ICT) equipment and facilities has therefore
been recognized as a pressing issue.
Worldwide attention has recently been focused on
the idea of distributing direct current (DC) electric
power inside datacenters and supplying the servers
directly with higher-voltage DC (HVDC) power
feeds as a key technology for reducing losses [1], [2].
In response to this trend, international standards organizations such as the ITU-T (International Telecommunication Union, Telecommunication Standardization Sector) and IEC (International ElectrotechnicalCommission) are now working on the standardizationof device specifications, system configurations, safety measures, and component/device specifications
that are needed for the practical implementation ofthis technology.
2. Benefits of HVDC power feeds
A comparison of alternating current (AC) and DC
power feed systems in a datacenter is given in Fig. 1.
The internal components (CPU (central processingunit), memory, etc.) of the ICT equipment (serversetc.) operate with DC supply voltages such as 5 V and
3.3 V, and the backup storage battery for use during
power outages also runs on DC. With an AC power
feed, power conversion between AC and DC is typically performed four times in order to connect thisequipment, which results in conversion losses. By
contrast, with a DC power feed, the AC power can beconverted into DC and connected directly to the storage battery. Therefore, power conversion is typically
performed just twice (including DC/DC conversion
inside the equipment), so a DC feed can in principle
achieve better efficiency due to the need for fewer
power conversion stages than with an AC power
feed.
ITU-T Recommendation L.1200 assumes that a DC
voltage of 380 V will be used. This is because thisNTT Technical Review

Global Standardization Activities

AC power feed
No. of power conversion stages = 4

UPS
1
Commercial power supply

AC

DC

DC

2
DC

AC

100–240
VAC

3
AC

SwitchElectric generator

4
DC

DC

DC

CPU

ICT equipment

Storage battery

Large losses incurred due to AC/DC conversion
HVDC power feed
No. of power conversion stages = 2

Rectifier1
Commercial power supply

AC

380 VDCDC

SwitchElectric generator

UPS: uninterruptible power supply

Storage battery

2
DC

DC

CPU

ICT equipment

Since there are fewer conversion stages, the efficiency and reliability are improved.
Cables can also be made thinner, resulting in lower wiring and construction costs.

Fig. 1. Comparison of AC and HVDC power feed systems in a datacenter or communications building.

voltage has traditionally been used as the intermediate bus voltage of server power supply units worldwide, making it highly compatible with existing
equipment. Also, since the higher voltage means thatthe power current can be suppressed to approximately
one eighth that of a conventional DC 48 V power feedsystem used for communication equipment, it can
reduce the power distribution losses and heat dissipation. These characteristics make the power supply
equipment more efficient and compact and allow theuse of thinner power distribution cables, thereby
reducing the wiring and construction costs whileallowing the equipment to be laid out with greater
flexibility.
Another reason why this power feed system has
attracted international attention is the high reliabilityof DC power feed systems. Although there is always
a risk of breakdowns, DC power feeds are known to
be over ten times as reliable as AC power feeds
because the use of AC necessitates complex mechanisms such as frequency synchronization during
power switching. By contrast, with DC power feeds
the storage battery can be connected directly to thepower supply line, and the likelihood of breakdowns
is therefore smaller [3].
When applied to higher-voltage DC power feeds,
Vol. 11 No. 3 Mar. 2013

the designation HVDC corresponds to a voltage
approximately eight times higher than the DC 48 V
traditionally used to power communication equipment, and this is the definition that started to takehold in the field of communications facilities. However, HVDC is also used in the field of DC power
supply technology, where it refers to electrical powersystems operating at 100 kV or more. To avoid confusion, the ITU-T uses the designation “up to 400
VDC” instead of HVDC when referring to power
feed systems.
3. Activities of standards organizations
Discussions on the standardization of HVDC power
feeds operating at up to 400 V are mostly being heldby the ITU-T, the IEC, and the ETSI (European Telecommunications Standards Institute). These threeorganizations have coordinated their efforts throughliaison relationships enabling them to cooperate andexchange information.
The groundwork for standardization of HVDC
power feeds aimed at datacenters and communicationbuildings was started by the ITU-T in 2009. In May
2012, the ITU-T gave its approval to Recommendation L.1200 (“Direct current power feeding interfaceGlobal Standardization Activities

up to 400 V at the input to telecommunication and
ICT equipment”)* by study group 5 (SG5). In thesame year, the ETSI published a similarly specified
European standard EN300-132-3-1 (“Power supply
interface at the input to telecommunications and datacom (ICT) equipment”). NTT also participated in thediscussion on this ETSI standard as an associate
member.
The IEC has also continued to work proactively.
The SMB (Standardization Management Board)/SG4(low voltage DC distribution systems) is discussing
the issues of DC power distribution at voltages up to1500 V, as well as the items that require standardization, for application not only to communicationbuildings and datacenters but also to ordinary buildings, households, solar power generation facilities,
and to storage batteries (including electric vehicles).
It is also studying the technical issues related to DC
power delivery and a systematic approach to standardization, and is currently preparing a roadmap forthis field. TC (Technical Committee) 23/WG (Working Group) 8 is conducting draft deliberations onconnectors for DC 400-V power feeds (rated at 2.6 kW
and 5.2 kW), TC64 is studying the construction,
maintenance, and safety protection of indoor electrical facilities, and SC (Subcommittee) 23 is conducting fresh discussions relating to DC circuit breakers.
Another standardization organization is the EMerge
Alliance, which is also engaged in discussions relating to the preparation of standards [4]. This is a North
American industry organization that promotes the useof DC power distribution within buildings (for lighting etc.). It plans to publish a standards document
within the 2012 fiscal year. Another North Americanindustry organization called The Green Grid [5] aims
to improve the efficiency of resources used by data
centers and in 2010 published a white paper on
HVDC power feeds (Japanese translation publishedin 2011).
4. ITU-T SG5 standardization activities
Within ITU-T SG5, a working party focusing on
environment and climate change issues (WP3/5:
Working Party 3/Study Group 5), has been investigating how the use of ICT is affecting the environmentand has been working on related recommendations(Fig. 2). As of December 2012, WP3/5 has been
* When the recommendation was approved in June 2012, it was
titled “Specification of DC power feeding system interface”;
it was subsequently changed after a proposal was made by thecommittee.

addressing six questions (Qs). One of these questions(Q19/5) is being discussed in relation to three points
connected with HVDC power feeds: (1) specifications of ICT equipment power supplies compatible
with HVDC power feeds, (2) power feed system configurations, and (3) power feed performance evaluation methods.
5. Overview of L.1200
ITU-T Recommendation L.1200 is a basic specification relating to ICT equipment power supplies thatare compatible with HVDC power feeds. NTT andother Japanese members have contributed greatly tothe drafting work of this recommendation. This recommendation applies primarily to communication
carrier buildings and datacenters and describes theminimum requirements and specifications for achieving reliable communication services. An overview ofa power feed system to which L.1200 applies isshown in Fig. 3. The basic configuration is based ona DC 48-V system that is used worldwide. To show
the interface conditions (power supply specifications)at the input terminals of power supplies for ICT
equipment, the points of contact between the power
feed system and ICT equipment are defined as interface P, for which various types of electrical specifications are provided.
5.1 Operating voltage range of ICT equipment
A crucial item in determining the power source
specifications is the equipment’s operating voltage
range. Here, the voltage range is set by assuming a
configuration where electricity is supplied directlyfrom a storage battery, which is the main feature of a
DC power feed, and the standard voltage range waschosen to be 260–400 V. ICT equipment compatible
with newly installed HVDC power feeds is required
to continue operating stably within this voltage range
without any stoppages, failures, or malfunctions.
With regard to the voltage range setting when considering the operation of this storage battery during
power outages, there has been considerable support
from communications carriers in Europe.
In the initial discussions on the standardization ofvoltage ranges, no agreements could be reached forseveral years. Meanwhile, NTT was also participating in the discussions on the ETSI standard (EN300132-3-1), which was approved in 2011. Furthermore,
research in this area stimulated a lot of product development and verification trials, mainly in Europe andNorth America, and this subsequently led to the
NTT Technical Review

Global Standardization Activities

WP1 Damage prevention and safety
WP2 Electromagnetic fields: emission, immunity, and human exposure
WP3 ICT and climate change
Q13/5

Environmental impact reduction including e-waste

Q14/5

Setting up a low-cost sustainable telecommunication infrastructure forrural communications in developing countries

Q15/5

ICT and adaptation to the effects of climate change

Q16/5

Leveraging and enhancing ICT environmental sustainability

Q17/5

Energy efficiency for the ICT sector and harmonization of environmental
standards

Q18/5

Methodologies for the assessment of environmental impact of ICT

Q19/5

Power feeding systems

Mainly investigating the electrical
specifications of DC power feedsystems for ICT equipment (servers,routers, etc.)

Source: http://www.itu.int/en/ITU-T/studygroups/2013-2016/05/Pages/structure.aspx

Fig. 2. Questions being discussed by ITU-T SG5 WP3.

Power feed system

Commercial AC power

ICT equipment

RectifierAC current

DC current
AC/DC
converter

Storage battery

Interface P

Fig. 3. Overview of power feed system that L.1200 applies to (commercial power supply).

adoption of the voltage range proposed by Japan andEurope in ITU-T Recommendation L.1200. An
agreement was reached on the voltage range in 2011,
and standardization was achieved in 2012.

Vol. 11 No. 3 Mar. 2013

5.2 Dealing with transient voltage fluctuations
It is essential to consider transient voltage fluctuations in power supply devices for communications
equipment. As shown in Fig. 4, even when transient
voltage fluctuations occur due to a problem such as ablown fuse or tripped breaker in the power feed system, the ICT equipment must continue with normalGlobal Standardization Activities(2) Blown fuse / trippedbreaker(1) Short-circuit failure

Rectifier200 VACICT equipment compatible
with HVDC power feeds
380 VDCPSU(3) Propagation ofvoltage oscillation

Service continuity
PSU: power supply unit

Fig. 4. Propagation of voltage fluctuations.

Grounding riser

RectifierDistribution board

R+

Fuse

Point of contact+190 VDC–190 VDC

ICT
equipment

R–

Earth terminal
Worker (human body resistanceapprox. 1 kΩ, min 500 Ω)
High resistance(R+ = R−, e.g., 40 kΩ)

Protective earth

Fig. 5. Grounding configuration and earth fault currents in an HVDC power feed.

operations (service). The test methods reference theIEC61000-4 series, and the test equipment is compatible with existing commercial products. Tests are
performed with standard voltage fluctuations andvoltage drops relative to the test voltage chosen
according to system requirements.
5.3 Grounding and isolation
In this specification, grounding is implemented bya high-resistance scheme considering the safety not
only of the engineers using the ICT equipment but
also of the facility managers. An overview of thisgrounding scheme is shown in Fig. 5. When there isa ground fault or current leakage, the large interven

ing resistance suppresses the resulting current so thateven if someone touches the equipment, the flow ofcurrent is so weak as to cause no injury or discomfort.
Power sources for ICT equipment are also required touse isolating converters as shown in Fig. 6, and are
configured so that the primary side (DC feed) andsecondary side (e.g., motherboard) are isolated fromeach other in order to provide secure protection
against electric shocks.
6. Future prospects
HVDC power feed technology shows great promise
as a means of reducing the power consumption of
NTT Technical Review

Global Standardization Activities

ICT equipment
Isolation
DC power(400 V or less)Primary side(+)

Secondary side
PSU

Motherboard
etc.

(–)

Protective ground

Isolation

Interface P

Fig. 6. Isolation conditions of power supply unit in ICT equipment.

ICT equipment and facilities because the losses
incurred during power conversion are small in principle. It is also characterized by a strong affinity withDC power sources such as those used in solar electricity generation, electric vehicles, and fuel cells,
and is expected to be used in a wide range of applications. At the NTT Group, we are pressing ahead withinternational standardization efforts in order to promote the widespread use and technical developmentof DC power feed technology.
References
[1]

[2]

[3]
[4]
[5]

K. Asakura, T. Tanaka, and T. Babasaki, “Higher-voltage Direct Current Power-feeding System,” NTT Technical Review, Vol. 9, No. 2,
2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2011
02fa2.html
Y. Nozaki, “Development of Higher-voltage Direct Current Power
Feeding System for ICT Equipment,” NTT Technical Review, Vol. 7,
No. 10, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2009
10sf2.html
H. Ikebe, “Power Systems for Telecommunications in the IT Age,”
Proc. of the 25th International Telecommunications Energy Conference (INTELEC’03), pp. 1–8, Yokohama, Japan.
Emerge Alliance. http://www.emergealliance.org/
The Green Grid. http://www.thegreengrid.org

Vol. 11 No. 3 Mar. 2013

Toshimitsu Tanaka

Research Engineer, Energy Supply Technology
Group, Energy Systems Project, NTT Energy
and Environment Systems Laboratories.
He received the M.S. and Ph.D. degrees inelectronic engineering from Chiba University in2001 and 2007, respectively. He joined NTT in
2007. He has mainly been engaged in analyzing
the stability of DC power feeding systems fortelecommunications. He received the Young
Researcher’s Award from the Institute of Electronics, Information and Communication Engineers (IEICE) in 2006.

Kaoru Asakura

Senior Research Engineer, Supervisor, Energy
Systems Projects, NTT Energy and Environment
Systems Laboratories.
She received the B.E. and M.E. degrees inapplied chemistry from Keio University, Kanagawa, in 1993 and 1995, respectively. She joined
NTT Interdisciplinary Research Laboratories in
1995. Since then, she has been engaged in R&D
of battery management systems and DC power
feeding systems for telecommunications. She
received the ITU-AJ Award Encouragement
Award ICT Accomplishment Field, Japan in
2012.

Tadatoshi Babasaki

Group Leader of Energy Supply Technology
Group, Energy Systems Project, NTT Energy
and Environment Systems Laboratories.
He received the M.S. and Ph.D. degrees inelectronic engineering from Nagasaki University
in 1990 and 2011, respectively. He joined NTT
Applied Electronics Research Laboratories in1990. He has mainly been engaged in R&D of
DC power feeding systems and batteries fortelecommunication. He is a member of IEEE,
IEICE, and the Institute of Energy Economics,
Japan.



External Awards
CEATEC JAPAN 2012 Innovation Awards As Selected by
U.S. Journalists GrandPrix
Winners: NTT DOCOMO, Inc.
Date: October 4, 2012
Organization: CEATEC JAPANFor “translator Appli”.
http://www.ceatec.com/2012/en/event/event05_02_02.html
The 18th Asia-Pacific Conference on Communications(APCC 2012)
Best Paper Awards
Winners: Hiromasa Fujii, Shunji Miura, and Hidetoshi Kayama,
NTT DOCOMO, Inc. Research Laboratories
Date: October 16, 2012
Organization: APCC
For “Novel cognitive radio technique for using white space inpublic cellular networks”.
In this study, we investigate the potential of cognitive radio systems (CRSs) with the aim of using the white space (WS) in public
cellular networks (PCNs). We first clarify the difficulties specific to
PCN WS usage and propose a CRS technique with a Cognitive-radio
Supportive Accommodation System (CSAS) as a scheme to overcome the difficulties. The main concept of the scheme is that CSAS,
which is the primary system that owns the frequency band, agrees tosupport CRS functions required for spectrum sharing. The spectrum
sharing scheme is designed on the PCN signal formats so as to suppress inter-system control signal overheads. We outline the prototype
devices equipped with the proposed scheme and present some initial
evaluation results obtained on an indoor testbed composed of the
prototype devices.
Published as: H. Fujii, S. Miura, and H. Kayama, “Novel cognitive
radio technique for using white space in public cellular networks,”
Proc. of the 18th Asia-Pacific Conference on Communications

(APCC 2012), pp. 266–271, Jeju, Korea.
Best Paper Award
Winners: Yuki Kurauchi, Toshio Uchiyama, and Tadasu Uchiyama,
NTT Service Evolution Laboratories
Date: November 21, 2012
Organization: WebDB Forum
For “Inferring user profiles in social networks using Markov random field” (in Japanese).
If user profiles in social networks are estimated, we can analyze
users’ postings in each profile and utilize the profiles for marketing.
Studies have been done on inferring user profiles from their content,
but there is still room for improving accuracy. Thus, we advance thestudies by combining this idea with the knowledge that social neighbors tend to share traits with each other. We propose a method ofinferring true user profiles by modeling user profiles in social graphs
using a Markov random field. In our evaluations, the proposed
method removed 54% of artificial noise in the inferred psychographic profiles and improved 9.1% points of accuracy in the inferreddemographic profiles.
http://db-event.jpn.org/webdbf2012/award.html (in Japanese).
FY2012 Industrial Standardization Awards, Industrial Science and Technology Policy and Environment Bureau
Director-General’s Awards
Winner: Yoshiaki Tarusawa, NTT DOCOMO, Inc. Research Laboratories
Date: December 15, 2012
Organization: Ministry of Economy, Trade and Industry
For contributions to supporting international standardization
activities in various ways.
http://www.meti.go.jp/english/press/2012/1012_01.html

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