NTT Technical Review
July 2011 Vol. 9 No. 7
Front-line Researchers
Masaya Notomi, Senior Distinguished Scientist,
NTT Basic Research Laboratories

Feature Articles on Technical Solutions to
Real-world Problems
Simulation System for Estimating Hazardous Voltages
Induced in Telecommunication Cables by Power Faults
Fault Cases and Countermeasures for
Field Assembly Connectors in Optical Access Facilities
Troubleshooting Tool for FLET’S TV
Corrosion Diagnosis Equipment for
Inspecting Suspension Cables

Regular Articles:
New Paradigm toward Realizing Quantum Computers
Measurement-based Quantum Computation andthe Fault-tolerant System
Reducing the Resources in Measurement-only
Quantum Computation

Regular Articles
Speech Dereverberation Using Linear Prediction
Virtual Private Network Authentication System
Featuring High Extensibility and Availability: AAA

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External Awards/Papers Published inTechnical Journals and Conference Proceedings
External Awards/Papers Published inTechnical Journals and Conference Proceedings

Front-line Researchers

Toward the Ultimate in
Optical Integration Technology
Masaya Notomi
Senior Distinguished Scientist,
NTT Basic Research Laboratories
Photonic crystal, which can function as a photonic insulator(something not found in nature), is expected to provide major
breakthroughs in optical integration technology. Dr. Masaya
Notomi, Senior Distinguished Scientist at NTT Basic ResearchLaboratories, is a driving force behind research that aims toachieve on-chip optical networks by applying the special properties of photonic crystal. We asked him about his research experiences and what led him to take up the challenges of photonics.

Photonic crystal:achieving optical integrated circuits—Dr. Notomi, could we begin by hearing about the
research that you are now pursuing?
I am now researching the creation of structures
called photonic crystals using nanofabrication techniques with the aim of developing optical large-scale
integration technologies that have so far been difficult
to achieve. A major objective of this research is toresolve problems in existing electronics technologies
by introducing genuine photonic network technologies in information-processing chips.
In electronic circuits, the more you raise performance the more you increase energy consumption
and heat. In optics, however, increasing processing
speed causes almost no increase in energy consumption. In other words, the energy consumption and heatgeneration can be minimized by introducing opticsbased processing in circuits. With this in mind,
researchers are attempting to incorporate optical wiring and circuits into electronic circuits using a variety
of methods.
To begin with, achieving large-scale integrated cir

cuits by optical means inside a chip requires the fabrication of devices and wires (that is, optical waveguides) whose structures completely confine light. It
also requires that the size of individual devices andwires be exceedingly small to achieve large-scale
integration. In this regard, photonic crystal can function as a photonic insulator and confine light in a verysmall space, which makes it applicable to the fabrication of optical integrated circuits. By the way,although there are a number of naturally occurring
insulators that block the flow of electricity, no photonic insulators exist in nature. The artificial structureof photonic crystal makes it possible to achieve photonic insulators for the first time.
Furthermore, in the processing of signals for a logic
operation in information-processing circuits, somepacket signals must be made to wait until the others
arrive so that the computation can be performed. To
date, however, optical circuits have been incapable ofmaking optical signals wait, and this has been one of
their weak points. Photonic crystals, however, cansignificantly slow down the propagation speed oflight. In the laboratory, we have been successful inusing photonic crystal to produce extremely slow
light with a speed about 1/50,000 that of ordinary
NTT Technical Review

Front-line Researchers

Tanabe et al. Nature Photonics (2007)
Slow lightLight delay by using high-Q-nanoresonator
Input pulse

Output pulse

When light travels slowly
Interaction increases→overcomes the weak point of light

8.4 µmLight delay by strong dispersion waveguide (2001)
Group refractive index~100

Intensity

1.45 ns
Reference sample
Photonic crystal

100

Group Index
Light speed became
1/1000
Notomi et al. PRL (2001)

60

Group index

wd=0.65 W

80

10
8
6
4
2
0

40
20

−3 −2 −101

wd=1.0 W
1420 1460 1500
Wavelength (nm)

2

3

4

5

6

7

Time (ns)
Pulse transmission velocity = 5800 m/s

wd=1.0 W

0
1460

1480
1500
1520
Wavelength (nm)Light speed became 1/50,000

Fig. 1. Strange nature of photonic crystal: slow light.

light (Fig. 1).
Photonic crystal has various peculiar properties in
addition to slow light. One example is the phenomenon of negative refraction whereby the refractive
index is made negative by using a special type ofphotonic crystal (Fig. 2). In the natural world, therefractive index is positive for all substances, but anegative value can lead to unexpected and interesting
phenomena. For example, a simple flat surface canfunction as a lens if it has a negative refractive
index.
Ongoing innovation in optical technologies—What kinds of results is your research team currently aiming for?
One use of photonic crystal that we have been
thinking about is the development of ultralow-energy
and ultrasmall high-performance optical devices foruse in information-processing equipment. Up to now,the large amount of power consumed by microproVol. 9 No. 7 July 2011

cessor units (MPUs), for example, has prevented
them from being incorporated into compact information-processing equipment such as mobile phones.
The characteristics of electronic circuits inside MPUs
are such that the energy cost of information transfer
increases with the bit rate, which means that it is difficult to significantly reduce power consumption
while raising processing capacity. However, if we
could incorporate optical technologies into MPUs,
the energy cost of information transfer would notincrease for higher bit rates, which should make itpossible to drive circuits by using much less power
than in the case of electronic devices even as processing speed increases. In our research, we have alreadydemonstrated that photonic crystal can be used to
drastically reduce operating energy in a number of
optical devices (Fig. 3).
In addition to low power consumption, anotherrequirement here is the integration of individual
devices. Our aim is to develop optical integration
technologies so that we can achieve optical integrated
circuits while maintaining this energy-saving effect.
Front-line Researchers

Point source
Negative refractionnImaging effect

Negative
PhC

Negative PhC

−n

Air
PhC

PhC: photonic chrystal

Fig. 2. Strange nature of photonic crystal: negative refraction.

Switching time

1 ns
100 ps(Energy x time)
10

−25

10−24

10−22

10−23

10−21

PhC cavity

Ring cavity

10−26
10 ps

Our results
10−27

PhC-MZI

SOA-MZI
Nonlinear-fiber

1 ps
ISBT
100 fs

1 fJ

x (3) Waveguide

1 pJ

1 nJ

Switch energy
Overcame trade-off threshold of conventional devices.
MZI: Mach-Zehnder interferometer
SOA: semiconductor optical amplifier
ISBT: intersubband transition

Fig. 3. Operation speed versus energy consumption.

Research on photonic crystals is still at the fundamental level, but we can expect optical integration
technologies—if successfully developed—to be
applied to commercial products sometime in the
future.
At the same time, high-precision photonic crystals
will be needed to fabricate high-quality optical devices, and the key to this is the development of advancedfabrication techniques. Over the ten years that ourteam has been in existence, we have been making
steady improvements in the performance of photonic
crystal and have been working to improve our fabrication techniques. It is acknowledged that NTT has
been able to fabricate high-quality photonic-crystal
waveguides from an early stage because of these
efforts. Establishing high-precision fabrication
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Front-line Researchers

techniques is extremely important for later research.
Looking back at the history of electronic circuits,
we can see how vacuum tubes were followed by transistors and how technologies for integrating transistors on a chip were developed by Intel and othercompanies. In a similar way, innovations in opticaltechnologies are now appearing one after another. Up
to now, only individual optical devices could be
developed, but their evolution toward integrated optical circuits is beginning.
If fabrication techniques continue to progress, we
can expect optical technologies to somehow be incorporated into chips in about ten years time. I believethat photonic-crystal technology will play an important role in this evolution.
Adoption of a new theme and formation ofa cross-organization team in caravan form
—Could you tell us about the path that you took toyour current research endeavors?
During my university days, I researched the peculiar behavior of electrons confined to one-dimensional states by synthesizing crystals having a pseudo
one-dimensional structure and cooling them toextremely low temperatures. Then, after joining NTT
Laboratories in 1988, I undertook research on structures that could confine electrons to one-dimensionalor even zero-dimensional states with particular application to optical devices. At that time, my superiors inthis research group were being transferred, and in theend, I came to continue this research on my own. I
was still just a new, young researcher, and there was
not much of a budget, but upon looking back, I could
say that this was a research environment that gave memuch freedom.
After performing this research for about ten years,
I had written a number of papers and had completedenough research to write my doctoral thesis. It was atthat time that my superior asked me what I thought
about changing research theme. He was also very
generous in telling me that “Whatever you do is fineso please select a new theme of your own choice.” Ithen spent about one year in Sweden as a visiting
researcher during which time I completed my doctoral thesis and thought about my next research
theme.
Around that time, photonic crystal research was
passing from the concept phase to the testing of operating principles using microwaves. While I was in
Sweden, such testing in the optical wavelength region
Vol. 9 No. 7 July 2011

began. In Japan, photonic crystal research had begun
only in some universities, but the worldwide numberof researchers involved in this field was starting toclimb.
Moreover, the photonic crystals were relatively
large structures with feature dimensions greater than
100 nm, which was about one order of magnitudelarger than the structures that I had fabricated in past
research. Such a large structure was technically easy
to fabricate. Taking all of this into consideration, I
decided that photonic crystals would be my next
research theme.
At that time, I also hatched a plan to shift fromindividual research as I had been doing to researchthat I would perform with fellow researchers. We
called this our caravan plan in which we would
recruit researchers with an interest in photonic crystals not only from the organization to which I
belonged but also from other research laboratories.
This does not mean to say that many researchers
suddenly offered to participate, but people from several groups started to express interest and I was eventually able to form a cross-organization team. At NTT
Laboratories, there is actually a lot of interdisciplinary research, but most of this involves the development of practical products over a two- or three-year
period. Fundamental research themes like the onethat we were pursuing with photonic crystals wereactually quite rare. This cross-organization team wasable to conduct research on photonic crystals using
fabrication and measurement techniques that were
not previously available, which I believe had a great
effect on results.
Pushing yourself into a state
conducive to inspiration—Please tell us about your current research team.
At present, there are seven members from the same
research laboratory and several members from otherlaboratories. Our research on photonic crystals isbroken down into several themes with several members dealing with each. Of utmost concern here is thateach and every theme should be something that willgrow over time. For this reason, I leave everyday
research to team members, but I think about mediumand long-term research themes by forecasting thestate of development several years in the future andthen working backwards to the present.
Looking for a theme on one’s own from a blankslate is certainly not unpleasant, but there is pressureFront-line Researchers

just the same. And the fact is that, if you don’t push
yourself into such a state, you cannot expect inspiration or novel ideas to form. Pushing oneself mentally,
though, is not necessarily fun. When I find myself insuch a situation, I think back to my experiences as avisiting researcher in Sweden, when I would think
every day about what I should be researching. Thoseexperiences are proving to be quite useful even today.
I am also very grateful to my superior who gave me
the time to ponder my next move.
Research work, though, is often a matter of trial anderror and also of detours as researchers grope foranswers. Consequently, there must be some marginand flexibility in the time, people, and organizations
specified for a research project to prevent researchers
from coming to a standstill. In this regard, I think that
researchers at NTT Laboratories are fortunate to beworking in an environment conducive to basic
research.

Photo 1. Lake Namtso, Tibet.

Becoming a better researcher
by understanding your research theme

Traveling to Tibet alone—Do you have any hobbies or interests outside ofresearch?
Yes, I do. After entering the workforce and while I
was still single, I would often travel by myself to places
overseas. I first travelled to popular destinations inNorth America and Europe, for example, but in time,
I got bored with such trips, and I eventually started tovisit countries like Tibet, India, and Vietnam.
I thought that Tibet, in particular, was fascinating,and my trip to Lake Namtso (or Gnam-mtso) in thehighlands of Tibet left a strong impression on me.
This lake is located in a rural area several hundred
kilometers from the capital; there are no roads leading up to it, so it cannot be accessed by normal modesof transportation. But wanting to go there, I gatheredsome information after arriving in Tibet, and I foundthat I could charter a jeep. The driver and I set out inthe direction of the lake with no roads to go by. Aftermuch difficulty, we eventually arrived. I thought that
Lake Namtso was too beautiful for this world. Situated at an elevation of 4718 m, it is the highest saltlake in the world. This lake is so magnificent that it isproudly called the “sacred lake” or the “lake nearest
to heaven” in the Tibetan language (Photo 1).
After I got married and had children, my solo trips
came to an end, but I still love to walk around town.
On my days off, I often take my kids with me onwalks to temples, art galleries, and other interesting
places.
—Dr. Notomi, could you please leave us with someadvice for young researchers?
I would be happy to. First of all, it is helpful to havestaying power over the long term with respect to one’s
research theme. All researchers, myself included, canlose spirit if research results do not turn out as expected. But that’s when you must have the fortitude towithstand such painful periods and not give up. Thisis something that I tell myself all the time.
Secondly, you must discover for yourself what it isabout your research theme that is interesting or exciting. If you don’t develop on your own a deep understanding of the possibilities behind your research
theme and what that theme might mean to the field inquestion, you will never discover just how interesting
your research theme can be. And if you can’t discoverthat, you will not be able to continue your researchover the long term.
It is relatively easy to get a budget for research thatis currently fashionable, and in recent years, evenuniversity students have been switching over to suchresearch in great numbers. I guess some researchers
are worried that they will be left behind if they don’t
get on board immediately with such fashionable
research. However, I feel that this kind of impatience
in research can be very dangerous. This is because
most of research is a series of failures, but that does
not mean to say that there are no successes, few
though they may be. Researchers must continue toresearch while accumulating the results of those
NTT Technical Review

Front-line Researchers

successful experiments over time. Even if failures
should continue, researchers that can discover on
their own the fascinating aspects of their researchshould be able to use those experimental failures to
get on a path toward success. In this way, researchers
should be able to continue researching without getting discouraged even if successes are few and far
between.
In this sense, I believe that discovering what is
exciting in your research by yourself is an importantqualification for a researcher.

Masaya Notomi
Senior Distinguished Scientist, Group Leader,
Photonic Nanostructure Research Group, NTT
Basic Research Laboratories.
He received the B.E., M.E., and Dr.Eng. degrees
in applied physics from the University of Tokyoin 1986, 1988, and 1997, respectively. He joined
NTT Optoelectronics Laboratories in 1988. Since
then, his research interest has been to control the
optical properties of materials and devices byusing artificial nanostructures. He has been in
NTT Basic Research Laboratories since 1999.
During 1996–1997, he was a visiting researcher
at Linköping University in Sweden. He was aguest associate professor in the Department ofApplied Electronics at Tokyo Institute ofTechnology (2003–2009) and is currently a guest
professor in the Department of Physics at Tokyo
Institute of Technology. He received the 2006–
2008 IEEE/LEOS Distinguished Lecturer Award,
JSPS (Japan Society for the Promotion of
Science) Prize in 2009, the Japan Academy
Medal in 2009, and the Commendation for
Science and Technology from the Minister of
Education, Culture, Sports, Science andTechnology (Prize for Science and Technology,
Research Category) in 2010. He is a member of
the Japan Society of Applied Physics, theAmerican Physical Society, the Optical Societyof America, and IEEE.

Vol. 9 No. 7 July 2011

Feature Articles on Technical Solutions to Real-world Problems

Simulation System for
Estimating Hazardous Voltages
Induced in Telecommunication
Cables by Power Faults
Abstract

This article introduces a system for simulating hazardous voltages induced in telecommunication
cables in the event of power faults in high-voltage power transmission systems. When the estimatedinduction voltages exceed human safety limits, mitigation measures such as shielding the cables andusing insulation (including the replacement of metallic telecommunication cable by optical fiber) are
required. However, to date most of the work of calculating and estimating induced voltages has depended on manual operations. The new simulation system greatly reduces most of the manual operations byusing a personal computer and a telecommunication cable database.

1. Introduction
Power faults in high-voltage power transmission
lines can be caused by lightning, typhoons, andpower-line insulation breakdown, as shown in Fig. 1.
Figure 1(a) shows a power fault current flowing to theground after a power fault was caused by lightning.
Figure 1(b) shows the mechanism of power induction

Telecommunication
cable

Lightningφ

Aerial ground line
(lightning arrester)

Fault occurs.

ePower lineφlIe
Steel tower

e

(Vm)

e(Vm): induced voltage
Ie: power fault current
l: length
ϕ : magnetic flux

Earth(a) Power fault caused by lightning(b) Power induction in telecommunication cables

Fig. 1. Power fault in high voltage power transmission lines.
† NTT EAST
Ota-ku, 144-0053 JapanNTT Technical Review

Feature Articles

Telecommunication cable
Maintenance operators

Photo. 1. Electric shock risk to maintenance operators.

between a power line and a telecommunication cablerunning parallel to it. When a power fault occurs, ahigh electromagnetic field can be generated by thepower fault current. This field also generates electromagnetic induction ac (alternating current) voltages
in the telecommunication cable. When a maintenance
operator touches the telecommunication cable, as
shown in Photo 1, after a power fault has occurred,
the operator receives a shock from the ac voltage
induced in the telecommunication cable. The induced
voltage causes ac to flow through the operator’s body,typically via a hand. Such an electric shock can cause
secondary accidents such as the operator being
knocked over or falling to the ground and may evenlead to death if the ac flow through the operator’sheart exceeds the safety limits.
To ensure the safety of maintenance operators,
NTT and electric power companies have agreed toestablish countermeasures for hazardous voltages
caused by power induction. These agreements prescribe the calculation of induced voltage beforehand
and the implementation of countermeasures whenever the voltage exceeds the limits shown in Table 1[1]. The limits depend on the interruption time of thepower fault. In the case of a high-stability power line
having a short interruption time, the voltage inducedin the telecommunication cable should be no morethan 650 Vrms. In any case, the voltage induced in atelecommunication cable needs to be calculated
beforehand, and if it exceeds the limits, countermeasures must be taken such as increasing the insulation
on metallic cables, changing to a telecommunication
cable with a high shielding effect, changing the routesof the telecommunication cables, or upgrading frommetallic telecommunication cable to optical fiber.
2. Problems with existing calculation methods
To calculate voltages in telecommunication cables[2], we need the routes of power lines and telecommunication cables and also the power fault currents.
The power line routes and power fault currents are
given as paper-based maps and figures by the powercompanies. The telecommunication cable routes are
obtained from NTT itself. The power-line and telecommunication-cable routes are on different maps, sothese maps must be manually copied to a new map inorder to obtain the distances between the power lines
and telecommunication cables; this takes many hours
per cable. Moreover, it has become increasingly difficult to build up knowledge about induced voltage
calculation and induction countermeasures because
employees with induction estimation skills are retiring and the in-house workforce is shrinking. Thissituation has generated a need for a tool that can estimate induced voltages simply while maintaining highaccuracy.
3. Simulation system
Our new simulation system is compared with theexisting method in Fig. 2. There are five steps in theprocedure for obtaining induced voltage in telecommunication cables.
(1)	Gather power-line and telecommunication-cableroute information.

Table 1. Hazardous voltage limits in case of power fault.
Application conditions

Limiting value

High-stability power line having a continuous ground-fault
current lasting no longer than 0.06 s

650 V

High-stability power line having a continuous ground-fault
current lasting between 0.06 and 0.1 s

430 V

Other power lines

300 V* A power line having a transmission voltage of 100 kV or greater, a low frequency offaults, and a fault interval no longer than 0.1 s

Vol. 9 No. 7 July 2011

Feature Articles

Digitization of power-line routes

Induction calculation flow
Existing method

New simulation system

Paper-based

Digitize power-line routes(from paper-based
information).
Extract and digitize
telecommunication-cableroutes from plant records.

(1) Prepare maps andpower-line andtelecommunication-cableroute information
beforehand.

Add power-line routes toplant-record map.

(2) Prepare separationdistance map (power line
and telecommunication
cable).

ManualAutomated(3) Calculate mutual
inductance (power line
and telecommunication
cable).

Manual(partiallyautomated)

Automated(4) Calculate induced
current (grounding-current
curves).

ManualAutomated

Calculateinduction for eachtelecommunication-cable route.

Calculate all
telecommunication-cableroutes in a batch.

(5) Calculate induced
voltage.

Inductive area of
power-line induction(yellow interval: within
5 km of power lines)

Incorporate digital data
about induced current.

Draw perpendicular lines (in green)from power line to telecommunication
cable and automatically calculatedistance.
List calculation
results in a batch.

Fig. 2. Induction estimation flow and associated screen images.

(2)	Make a distance map of the separations between
power lines and telecommunication cables.
(3)	Calculate mutual inductance between powerlines and telecommunication cables from the
distance map obtained in (2).
(4) Calculate the power fault current.
(5) Calculate the induced voltage.
In the existing method, steps (2)–(4) involve timeconsuming manual operations. The maps of powerlines and telecommunication cables are copied byhand, so high-level skills are required to reduce
errors. When the number of telecommunication cableroutes is high, e.g., 100 or 1000, it is hard to check all
induced voltages for relevant telecommunication
cables. By contrast, the new simulation system,
which runs on a personal computer, is not based on
manual operations and has three good characteristics:
automatic preparation of the distances between apower line and telecommunication cables on the map,
automatic calculation of induced voltage, and batch
calculation of induction for all telecommunicationcable routes.
3.1 Automatic preparation of distances on map
The first step in the existing method is to acquire
information about power-line routes (from papermaps) from the power company and manually overlay that information onto the NTT paper-based plant
record. The next step is to project perpendicular lines
from the power line to sections of the telecommunication cable using rulers and squares and to calculate
the average distance for each section. By contrast, the
new system enables power-line information on apaper map to be incorporated into an electronic telecommunication cable route database via a scannerand lets a user digitize coordinate data (latitude &longitude) of a power-line route by simply clicking
on the route on a screen, thereby automating the creation of a map showing the distances between the
power line and telecommunication cables (or routes).
In this way, it is possible to automatically obtain amap of the separation distances along a power line
(distance map), which in turn makes it possible tocalculate mutual inductances between the power line
and telecommunication cables automatically.
3.2 Automatic calculation of induced voltage
Voltage induced in the telecommunication cable
can be calculated by multiplying induced current(grounding current) by mutual inductance as computed from the distance map. In the existing method,
induced-current values are also provided by the
NTT Technical Review

Feature Articles

power company in the form of paper-based graphs.
The new tool enables this graph data to be input into
the system via a scanner so that induced-current values can be digitized by clicking on those data curves
on the screen. This enables induced-current values
needed for calculating induced voltage to be extracted
automatically.
3.3 B
 atch calculation of induction for all telecommunication-cable routes
The new tool can also automatically determine
whether any telecommunication-cable route is within
the induction-impact range of a power line and simultaneously calculate induction for all telecommunication-cable routes in a batch. The operations needed to
calculate and estimate induction can therefore bedone much more efficiently. The precision of calculations is also improved through the use of map datathat has not been input manually and is thus moreaccurate, which reduces the errors in induction estimation.

voltage to be digitized, thereby reducing computation
time and improving computational accuracy. As aresult, it has become possible to respond promptly toplans for raising voltage in power transmission lines,
reduce dispersion in calculation results, and makemore accurate decisions about the need to implement
countermeasures to power induction.
In fiscal year 2010, we used the results of field trials
to improve the system’s workability and operability,
and in response to the wishes of NTT branch offices
involved in this trial, we implemented a function for
estimating induced voltage from multiple powertransmission lines. We also plan to implement a function for estimating voltage induced by other types ofpower-transmission systems such as in electric railways in addition to power transmission lines andinvestigate the networking of the induction-estimation tool to make calculations more efficient and tolink databases. Our objective is to implement theseenhancements in the field beginning in fiscal year2011.

4. Conclusion

References

NTT has developed a powerful new simulation system for estimating induction with a focus on hazardous voltages induced in telecommunication cables bypower faults in high-voltage power transmission
lines. This simulation system enables power-line
routing information needed for calculating induced

Vol. 9 No. 7 July 2011

[1]

[2]

“Calculation Method to Estimate Electrostatic and Electromagnetic
Induction in the Design of Telecommunication Facilities,” NTT Technical Review, Vol. 5, No. 8, 2007.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2007
08sf3.html“Induction (Vol. 1),” The Telecommunications Association, Tokai
Division, p. 127, 1981 (in Japanese).

Feature Articles on Technical Solutions to Real-world Problems

Fault Cases and Countermeasures
for Field Assembly Connectors
in Optical Access Facilities
Abstract

We have investigated faults with field assembly connectors that had been breaking down in opticalaccess facilities. These faults were caused by fiber scratching, fracture of the mechanical splice part,
incorrect fiber lengths, incorrect cleaving, incorrect joining, and so on. All these causes can be eliminated by countermeasures such as correct use of tools and proper procedures.

1. Introduction
NTT EAST and NTT WEST had more than 15.1
million customers using optical fiber broadband services (FLET’S HIKARI) at the end of March 2011.
This rapid expansion has been accompanied by unexpected faults in optical access facilities. NTT EAST
Technical Assistance and Support Center provides
consultation on a variety of fault cases and strives tomaintain a high-reliability network. This article introduces fault cases and countermeasures for field
assembly connectors, which are used to join opticalfibers in optical access facilities.
2. Field assembly connectors
A field assembly connector is composed of threemain parts: the ferrule, mechanical splice, and clamp.
There are several kinds of field assembly connectors
such as the field assembly small-sized (FAS) connector. The connector assembly procedure has six steps
as follows:(1)	Strip the 0.25-mm-diameter fiber coating fromthe optical fiber using a mechanical stripper.
(2)	Clean the stripped fiber (bare fiber) with alcohol.
(3)	Cut the bare optical fiber to the correct length
using a fiber cleaver.
(4)	Insert this bare optical fiber into the mechanical† NTT EAST
Ota-ku, 144-0053 Japan

splice part inside the field assembly connector.
(5)	Join it to the built-in optical fiber inside the ferrule.
(6)	Fix the position of the bare optical fiber by
releasing the wedge from the mechanical spliceinside the connector.
3. Fault cases with field assembly connectors
In fiscal year 2009, we investigated faults with field
assembly connectors that had broken down in a certain region of Japan. The direct causes of the faults
are summarized in Fig. 1. We also found that theseconnectors often have multiple causes in addition to
the direct causes. All of the direct causes of connector
faults are detailed below in order of occurrence frequency.
3.1 Fiber scratching
3.1.1 Fault cases
A scratch on an optical fiber can grow with timeand eventually lead to fiber breakage. Examples offailures of this kind are shown in Fig. 2. Such scratches can be made when the sheath is removed from theoptical fiber drop cable if the blades of the wire nippers damage the fiber. They can also be made when
the fiber coating is removed from the coated opticalfiber if coating debris from previous stripping wasleft on the blades of the mechanical stripper.
3.1.2 Countermeasures(1)	Remove the sheath carefully without the nipper
blades damaging the optical fiber, as shown in
NTT Technical Review

Feature Articles

Other: 9%
Incorrect joining: 7%
Fiber scratching: 35%
Incorrect cleaving: 12%

Incorrect fiber lengths: 12%

Fracture of mechanical splice part: 25%

Fig. 1. Summary of fault causes.

(a) Scratching caused by nipper bladesFig. 4. Fracture of the mechanical splice part.

(b) Scratching caused by coating debris

Fig. 2. Examples of scratched optical fibers.

Good
Good

Good

Bad

Grabbing the
top andbottom of aconnector
may push an
adjacent
connector.

(a) Connector insertion/removal(a) Correct sheath removal (b) Cleaning before coating removal

Fig. 3. Countermeasures to fiber scratching.

(b) Connector finger-grip

Fig. 5. C ountermeasures to fracture of the internalmechanical splice part.

Fig. 3(a), so as not to scratch the fiber.
(2)	Clean the mechanical stripper before removing
the fiber coating, as shown in Fig. 3(b).
3.2 Fracture of mechanical splice part3.2.1 Fault cases
When an optical connector is carelessly inserted
into or removed from equipment, lateral force can be
accidently applied to an adjacent connector. This
force can damage the mechanical splice part inside
the connector and lead to fracture of this part (Fig. 4).
The fracture may result in fiber breakage. In addition,
Vol. 9 No. 7 July 2011

when the connector is inserted into or removed fromequipment at an inclination to the optical connector
adapter, force can be applied to the connector, which
can cause the mechanical splice part to fracture.
3.2.2 Countermeasures(1)	Be careful not to push adjacent optical connectors (Fig. 5(a)). Hold the finger-grip of an optical connector correctly when inserting or
removing it (Fig. 5(b)).
(2)	Insert an optical connector into or remove one
Feature Articles

from equipment horizontally with respect to theoptical connector adapter.
3.3 Incorrect optical fiber length
3.3.1 Fault cases
Failure to set the optical fibers in the tools correctly
can lead to an incorrect length and may result in fiber
breakage or an increase in loss. Examples of incorrect
optical fiber lengths are shown in Fig. 6.
3.3.2 Countermeasures(1)	Set the bare and coated optical fiber in theproper tools correctly.
(2)	Use the scale marks on the fixture of the field
assembly connector to measure the bare andcoated optical fiber lengths before joining to thebuilt-in optical fiber.
3.4 Incorrectly cleaved fiber end
3.4.1 Fault cases
If there are no problems with the fiber cleaver, thefiber will be cleaved correctly and have a correct, flat,and smooth end perpendicular to the fiber axis. However, if there are problems, the fiber end will have an
incorrect and uneven end (Fig. 7). An incorrectly
cleaved fiber end can lead to abnormal joining to thebuilt-in optical fiber and may result in an increase inloss. A fiber end can be incorrectly cleaved for several reasons. For example, the fiber end can becleaved incorrectly with a dropped cleaver or one thathas struck something because a fiber cleaver is precisely fabricated and is a sensitive tool. Moreover, adefective or worn cutter blade may also lead to anincorrectly cleaved fiber end.
3.4.2 Countermeasures(1)	Inspect a cleaved fiber end regularly (about oncea week) to ensure good cleaving quality (using
the display screen of an arc fusion splicer toview the cleaved fiber ends).
(2)	Replace fiber cleaver blades (when incorrect
cleaving is noticed or about once a month).
(3)	Request the manufacturer to repair the fiber
cleaver if no improvements result from theabove countermeasures.



Gap caused by setting mistake

Bad

Bad

Gap caused by setting mistake

Fig. 6. Examples of incorrect optical fiber lengths.

NormalAbnormalFig. 7. Fiber ends (normal and abnormal).

3.5 Incorrect join to built-in optical fiber
3.5.1 Fault cases
Failures to correctly join an optical fiber to thebuilt-in optical fiber in the mechanical splice part canlead to a large gap, which may result in an increase inloss.
3.5.2 Countermeasures(1)	Ensure that the wedge is correctly installed
before inserting the bare optical fiber.
(2)	Join the bare optical fiber to the built-in opticalfiber and check that the fiber is sagging appropriately before releasing the wedge and fix thefiber.
4. Conclusion
This article introduced fault cases and countermeasures for field assembly connectors. All of the causesof faults can be eliminated by countermeasures suchas correct use of tools and proper procedures. Looking forward, NTT EAST Technical Assistance andSupport Center aims to accumulate and disseminate avariety of techniques and technologies for maintaining high-reliability network facilities.

NTT Technical Review

Feature Articles on Technical Solutions to Real-world Problems

Troubleshooting Tool for FLET’S TV
Abstract

NTT EAST Technical Assistance & Support Center has developed a tool called the RF Checker fortroubleshooting problems in the homes of FLET’S TV customers (RF: radio frequency). It can be used
to check video-signal levels and video quality using simple operations, which should make in-hometroubleshooting much more efficient.

1. Introduction

2. RF Checker

FLET’S TV is a video broadcasting service thatenables users to receive various types of broadcasts
such as terrestrial digital broadcasts and BS (broadcast satellite) digital broadcasts through broadcast
services offered by OptiCast Inc. via the FLET’S
HIKARI optical fiber broadband service available
from NTT EAST and NTT WEST. The FLET’S TV
service model is shown in Fig. 1. First, the reception/delivery base receives video signals from satellites
and terrestrial transmission towers via antennas andpasses them to an optical transmitter after subjecting
them to level adjustment, noise filtering, and otherprocesses at head-end equipment. Next, the opticaltransmitter converts these electrical video signals toan optical signal, and transmits it. Finally, the NTT
optical-fiber accommodation station amplifies andsplits this optical signal at the video optical line terminals (V-OLTs), optically multiplexes it with an
optical signal conveying FLET’S HIKARI communication signals from a Gigabit Ethernet OLT (GEOLT) using an internal wavelength division multiplexing (WDM) splitter, and transmits them to acustomer’s home over a single strand of optical fiber.
At the customer’s home, an optical network unit
(ONU) converts the WDM optical signals back toelectrical video and communication signals. ThisONU (GV-ONU) combines a V-ONU with a GEONU. The video signals arrive at television (TV) setsin the home via various types of wiring and devices
such as coaxial cables, booster amplifiers, distributors, and branching filters.

2.1 Development background
Since its launch in July 2008, FLET’S TV has
increased its subscriber base steadily and further
growth is expected in the future. Moreover, while
maintenance of the coaxial-cable section in the customer’s home was originally handled by the broadcasters, it was decided that NTT EAST and NTT
WEST would take over maintenance for some users
from March 2010 and April 2010, respectively.
Therefore, NTT needed a means of performing
prompt and accurate maintenance and troubleshooting in this coaxial-cable section. In response to thisneed, NTT EAST Technical Assistance & Support
Center developed the RF Checker as a measurement
tool that can be used by on-site technicians in the field
to judge video quality even without specialized skills(RF: radio frequency). The RF Checker has the minimum necessary functions for measuring the qualityof FLET’S TV in order to be an inexpensive and convenient handheld device that excels in operability andportability in the field.

† NTT EAST
Ota-ku, 144-0053 Japan

2.2 Features
The features of the RF Checker are summarized
below (Fig. 2 and Table 1).
(1)	Supports terrestrial digital, BS digital, and CS(communication satellite) digital (Sky PerfecTV! e2)(2) Measures the following items:· Level· BER (pre): bit error rate before correction· BER (post): bit error rate after correction· CNR: carrier-to-noise ratio(3)	Evaluates measured values (level, BER, andCNR) on the basis of a threshold value and
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Feature Articles

Satellites

Service contents• Terrestrial digital broadcasts• Analog terrestrial broadcasts• BS digital broadcasts• BS analog broadcasts• Sky PerfecTV! e2 broadcasts• FM broadcasts

Transmission tower

Customer’s home Distributor
Booster amplifier

Antennas

Coaxial cableTV
NTT

Coaxial cable

GE-OLT

Opticaltransmitter

V-OLT

HE HE

Optical cable

Reception/delivery base

Internal
WDM splitter

GV-ONU

splitterTV

Optical-fiber accommodation station
FM: frequency modulation
HE: head-end equipment

Fig. 1. FLET’S TV service model.

Screenshot

Operation buttons

OFDM: 27 ch (557 MHz)LEVEL: 70.5 dBµV
BER (PRE): 5.2e-7
BER (POST): 0.0e+0
CNR: 28.1 dB

Power

DT
(MENU)

+UP

−DOWN

BS
(NEXT)

110°CS(ENTER)

OK

Power: On/off switch for power and backlight+UP: Channel up−DOWN: Channel down

OFDM: orthogonal frequency division multiplexing

DT: Select terrestrial digital broadcast
Channels 13–62 can be tested (channel allocation in Japan).
BS: Select BS digital broadcast
Channels 1–15 can be tested.
110° CS: Select 110° CS digital broadcast
Channels ND2–ND24 can be tested.
Other: Up to 100 measurement results can be stored in memory
Data can be transferred to a personal computer via USB.

Fig. 2. RF Checker.

Table 1. Specifications of the RF Checker.
Display method

Liquid crystal display (LCD) with backlight

Dimensions

86 mm (width) × 43 mm (depth) × 163 mm (height)

Measurement items

Level
BER (pre)BER (post)
CNR

Measurement range (level) 40 to 100 dBµV
Connection
interface

RF terminal

75 Ω (F-type connector)USBmini-USB connector

Power supply

Four type-3 (AA) dry-cell batteries

Accessories

Protective case, dedicated USB cable, user manual* Dry-cell batteries and coaxial cables for measurements
are not included and need to be obtained separately.

Vol. 9 No. 7 July 2011

Feature Articles

Customer’s home

Distributor
Booster amplifier

GV-ONU

Coaxial cableTV

TV

Optical cable

RF Checker

Fig. 3. RF Checker usage points.

displays OK or NG (okay or no good). Threshold values may be set as desired.
(4)	Saves measurement results in internal memory(maximum 100 items)(5)	Transfers saved data to a personal computer via
a USB (universal serial bus) connection
2.3 UseThe RF Checker is operated as follows. It is turnedon by briefly holding down the power-supply button
and then connected to a coaxial cable (F-type connector) inside the customer’s home. Next, the button for
the type of broadcast (terrestrial digital, BS, or CS) tobe measured is pressed and a channel is selected bypressing the +UP or –DOWN button to begin automatic measurement. Results are displayed within a

few seconds all together on one screen. They consistof level, BER (pre), BER (post), and CNR as well as
OK or NG indicating whether those results are
acceptable according to the threshold values.
An on-site technician using the RF Checker during
maintenance work can easily perform troubleshooting by measuring video quality at the GV-ONU video
output terminal, at the coaxial terminal in each roomin the coaxial-cable section, at the input/output terminals of the booster amplifier and distributors, or at the
coaxial connector nearest to a TV.
Actual troubleshooting begins from the TV side
when video problems occur in only one TV set andfrom the ONU side when video problems occur inmore than one TV set. Using the RF Checker, thetechnician can observe the display of OK or NG atmultiple measurement points (or the display of “×under” instead of OK/NG in the case of a low level)
and can conclude that a problem exists at the point
where the display of OK/NG or “× under” changes.
RF Checker usage points are shown in Fig. 3.
3. Concluding remarks
The RF Checker was released in February 2010. Asof the end of March 2011, about 590 units are in use
by the maintenance departments of NTT EAST and
WEST and partner companies. Looking forward, we
expect the number of problem incidents to increase as
the use of FLET’S TV expands, but we also expect
the use of the RF Checker as a troubleshooting tool
inside customer homes to make repairs more efficientand raise the level of maintenance.

NTT Technical Review

Feature Articles on Technical Solutions to Real-world Problems

Corrosion Diagnosis Equipment for
Inspecting Suspension Cables
Abstract

NTT has developed equipment for diagnosing corrosion in aerial suspension cables as an inspection
tool. Although suspension cables use corrosion-resistant strands of steel wire having high anti-corrosion
properties, cables in coastal areas can still suffer from localized corrosion inside the nozzle section of aterminal box. This diagnostic equipment can analyze such hidden corrosion nondestructively.

1. Introduction
Aerial metallic cables and optical fiber cables aresupported by a steel suspension cable running
between utility poles. Modern suspension cables are
made of highly corrosion-resistant strands of steel
wire having both high anti-corrosion and anti-fatigue
properties. In coastal areas, however, localized corrosion can still occur under the sealing tape in the nozzle section of a terminal box, as shown in Fig. 1. Such
localized corrosion may often exist despite theabsence of corrosion on the outside of the nozzle section, so a conventional visual inspection will fail todetect any corrosion at all. For a full inspection, it istherefore necessary to dismantle the nozzle, so theinspection is long and costly. NTT therefore felt theneed for equipment that could diagnose suspensioncable corrosion inside the nozzle section nondestructively.
2. Inspection tool
The diagnostic equipment consists of a main unit,
ultrasonic sensor, fixture, and cable (Fig. 2). Ultrasonic pulses are transmitted inside the suspension
cable from the ultrasonic sensor, as shown in Fig. 3.
At local corrosion sites, the ultrasonic waves arereflected back to the sensor, where they are detected.

Localized corrosion
Localized corrosion occurs beneath sealing tape.

Suspension cable

Photograph of dismantled nozzle section
Cable

NozzleNo. 4 sealing tapeNo. 2 protective PVC tapeFig. 1. Example of localized corrosion in a suspension cable inside the nozzle section of a terminal box.
† NTT EAST
Ota-ku, 144-0053 JapanNTT Technical Review

Feature Articles

The waveforms of the reflected waves enable thepresence and extent of corrosion to be determined.
For better positioning accuracy and reproducibilityof ultrasonic sensor results in corrosion inspections,the sensor is attached to the suspension cable using a
fixture so that it emits ultrasonic waves into two wire
strands at a time. Since the suspension cable consistsof six outer wire strands surrounding a seventh central strand, the sensor must be put at three positions toinspect the six outer wire strands. In addition, onereference measurement is also performed to ensurethat the surface conditions of the suspension cable do
not affect the results, so a total of four measurements
must be taken.
A polymer sheet is inserted between the ultrasonicsensor and the suspension cable as a contact medium.
Although it is common to use a gel such as glycerinwith ultrasonic sensors, if the gel were to get among
the cable strands, it could itself act as a source of corrosion, so for this reason and for better operability inthe field, we decided to use a polymer sheet applicable to suspension cable diagnosis.
An example of the waveform of waves reflectedfrom a corroded section is shown in Fig. 4. This
waveform is determined by the degree of corrosion
and the distance of the corrosion site from the ultrasonic sensor. It is analyzed in the background on the
diagnostic equipment and only the result of the corrosion level is displayed on the screen in an easy-tounderstand manner.

Main unit

Cable

Fixture

16 cm

25 cm
Weight: 2.4 kg

Ultrasonic sensor

Fig. 2. Appearance of diagnostic equipment.

Diagnostic equipment
Ultrasonic sensor
Corrosion in nozzle section
Terminal box

Nozzle section

Corrosion-resistantsuspension cable

Fig. 3. Principle of diagnosis by ultrasonic waves.

(%)

100

The inspection method is shown in Fig. 5. A maintenance worker inspects a suspension cable from anaerial work platform positioned next to the terminalbox. The inspection procedure is presented to themaintenance worker via figures and photographs displayed on the screen of the main unit one step at atime. Referring to these instructions, the maintenanceworker attaches the ultrasonic sensor to the suspension cable using the fixture and proceeds to takemeasurements. If six wire strands are to be measured,for example, the instructions will use figures orimages to show how the ultrasonic sensor should be
slid along the suspension cable to measure threestrand pairs. The worker will also be prompted tovisually check for rust on the outside of the nozzle
section emerging from the nozzle’s interior because
corrosion progressing inside the nozzle section eventually appears as red rust outside the nozzle section.
In short, the visual detection of rust is a sure sign ofVol. 9 No. 7 July 2011

Intensity

3. Inspection method

Waveform of waves reflected fromcorroded/thinned section

50

0

0

50

100
150
Distance

200

250 (mm)Fig. 4. W aveform of waves reflected from corroded
section.

internal corrosion and evidence that the suspension
cable needs renovation as soon as possible. Finally,
the diagnostic equipment displays the diagnosis
Feature Articles

Field use of
the equipment

Simply hold the sensor
against the suspension
cable using the fixture.
Very simple!

Typical diagnosis display
Evaluation results

Level 4
Please renovate immediately.

Fig. 5. Example of suspension-cable corrosion diagnosis
in the field.



results. As shown in the example in Fig. 5, theseresults simply indicate the level of corrosion and theneed for renovation.
4. Conclusion
It has been more than twenty years since corrosionresistant suspension cables were first introduced, but
because they are becoming increasingly used incoastal areas as well, we can expect the need for
localized-corrosion inspections to grow. The suspension-cable corrosion diagnostic equipment can significantly reduce the inspection time and enable alarge number of inspections of facilities by eliminating the need to dismantle nozzles of terminal boxes.
As a result, effective maintenance becomes possible.

NTT Technical Review

Regular Articles: New Paradigm toward Realizing Quantum Computers

Measurement-based
Quantum Computation andthe Fault-tolerant System
Yuuki Tokunaga†
Abstract

Quantum computation provides us with a new form of information processing that surpasses current
computer technology. In a new model of quantum computation, measurement-based quantum
computation, the computation proceeds via simple measurements on previously prepared quantum
entangled states. This model has attracted attention for its good potential for realizing quantum
computers. This article reviews the basic concepts of measurement-based quantum computation and theelegant fault-tolerant system called topological one-way quantum computation.

1. Introduction
The principle of current information processing
performed by computers and networks is based on thephysics of classical electromagnetic dynamics. Information processing technology based on this principle
has blossomed in the 20th century as electronics.
Meanwhile, a new principle of physics, quantum
dynamics, was discovered in the early 20th century
for describing the world on the small scale: the scaleof the smallest particles and sub-particles. After awhile, in the late 20th century, a new principle ofinformation processing based on quantum physics
started to be considered. As a result, groundbreaking
information processing was found to be possible,
such as efficient computation of prime factorization,
unconditionally secure communication, and teleportation of quantum information [1]. However, quantum information processing is not easy to realize
because the quantum states are difficult to manipulate
precisely and are not stable for a long time; i.e., theyeasily decohere. A lot of study has been going on toachieve the breakthrough needed to realize quantum
information processing.
† NTT Information Sharing Platform Laboratories
Musashino-shi, 180-8585 Japan

In this article, I review a new computational model
for achieving quantum computers called measurement-based quantum computation. Section 2 describes
the basic concepts and advantages of measurementbased quantum computation and compares it with theconventional computational model: the circuit model.
Section 3 describes an important new fault-tolerantsystem for quantum computation, topological quantum computing, that can be performed on the basis of
the measurement-based quantum computation modelin an elegant way. Section 4 describes research on
measurement-based quantum computation at NTT
laboratories. Section 5 mentions the outlook for
future research.
2. Measurement-based quantum computation
From the beginning of the study of quantum computation in around 1990, the realization of quantum
computers has been considered on the basis of a standard computational model: the circuit model (Fig. 1).
It has a few basic gates such as single-qubit (quantumbit) rotation gates and an important two-qubit interaction gate: the controlled-NOT gate. The input state to
the combination of the basic gates achieves universal
quantum computation. However, one of the main difficulties in realizing quantum computers is how to
NTT Technical Review

Regular Articles

Cluster state
Input

OutputQuantum gatesFig. 1. Circuit model.

|α 〉

Input

Output

Input
Gates

Classical
informationBell
measurement

Computational flow

Fig. 3. One-way quantum computation.
CNOT |α , β 〉Entangled state

Input

|β 〉

Bell
measurement

OutputClassical
information

Fig. 2. Teleportation-based gate.

make the controlled-NOT gate. It is not easy to obtainan appropriate interaction between particles such asatoms or photons. To date, many experiments toward
the controlled-NOT gate have been performed.
A whole new paradigm for quantum computers
started in around 2000. Researchers began to reconsider the computational model in order to utilize theunique features of quantum physics. They thoughtthat there might be a completely different computational model for quantum computers than the one
based on the idea of the classical circuit model
because the physics is completely different from classical physics. The great feature of quantum states isthe existence of entanglement. A breakthrough idea
for utilizing entanglement for quantum computers
was described by Gottesman and Chuang [2]. They
proposed a new model of quantum gates that operateby means of quantum teleportation. Quantum teleportation is a quantum information transmission
process that consumes the quantum entanglement incompensation for sending quantum information. We
can obtain the output of a controlled-NOT gate byoperating quantum teleportation using a special
entangled state (Fig. 2). The entangled state can be
regarded as a computational resource for the quantumgate.
A more sophisticated computational model basedVol. 9 No. 7 July 2011

on the teleportation-based gate is one-way quantum
computation (Fig. 3) [3]. First, a special entangledstate, called a cluster state, is prepared beforehand.
After that, one only has to perform single-qubit measurement on the cluster state and one can then perform universal quantum computation, in which theentangled cluster state plays the roles of quantum
gates and the information flow. The cluster state is a
specific entangled state and it can be prepared before
computation is started. The entanglement preparation
is easier than computation because the target state is
known beforehand. Actually, we can even utilize a
non-deterministic gate having a low success probability in order to generate the entangled resource [4], [5].
Once the computational resource of the entanglement
has been prepared, one only needs to perform singlequbit measurements; this is a very simple task compared with two-qubit interaction gates, so the newcomputational model significantly decreases the difficulty of realizing quantum computers.
3. Fault-tolerant topologicalquantum computation
We also need to protect the quantum computing
from decoherence and from the effects of several
kinds of noise. Fortunately, error correction codes areavailable for quantum computing and we can perform
fault-tolerant quantum computing by applying anerror correction procedure appropriately during computation. A well-known fault-tolerant system basedon the circuit model used a quantum linear code andconcatenated codes (Fig. 4) [6], [7]. A standard quantum linear code encodes a single qubit into several
qubits. It can tolerate a single bit-flip and phase-flip
Regular Articles

Error correction

Error correction

Fig. 4. Concatenated code.

02

Identical

1

3

6

2’
5

4
7

Identical
6’

Toric code
0’

1’

Fig. 5. Topological code.

error, but if more than one error occurs in the encodedstate, then the error correction does not work well.
Concatenated coding solves this problem. It recursively uses the linear code as shown in Fig. 4. It provides greater error tolerance if the error rate per basic
computational unit is less than a certain threshold.
Reliable quantum computing can be executed byusing code concatenation of sufficient depth with theerror rate below the threshold. The threshold of faulttolerant systems based on the circuit model is known
to be around 1% [7]. However, it is assumed here thatany quantum gate can be achieved between widely
separated particles, but the interaction between particles normally becomes weaker with increasing distance between particles. Thus, this assumption is
unnatural from the physical viewpoint. Though we
can rewrite the quantum gate between spatially separate particles into combinations of quantum gates
between nearest-neighbor sites, the number of consumed gates becomes larger in that case and thethreshold of the fault-tolerant system becomes significantly small.
Considering the shortage of concatenated codes for
fault-tolerant systems, elegant quantum codes treating physical nearest-neighbor interactions were proposed [8]. They are called topological codes or surface codes; the name originates from the topologicalnature of the code. The important point is that a topological code uses only nearest-neighbor interactions.
This is a realistic scenario compared with the circuit
model. An example of a two-dimensional (2D) topological code, toric code, is shown in Fig. 5. The qubits
are on the edges of the 2D lattice and entangled as a
topological code state. In toric code, the qubits on theendpoints of the 2D lattice are identical to those at theother sides; that is, the 2D lattice is on the surface ofa torus, which gives us the degrees of freedom forencoding logical qubits. The entangled code state canbe prepared through the use of only nearest-neighborinteractions and the error-checking operations canalso be operated by nearest-neighbor interactions andsingle-site measurements. The encoding size can beenlarged by expanding the lattice size. Toric code canalso be rewritten on a square lattice with boundaries
by constructing the equivalent for the hole of thetorus and introducing the same topology. That makes
it easier to prepare multiple logical qubits. Topological fault-tolerant quantum computation is known to
be possible in a 2D square lattice with nearest-neighbor interactions during computation, where theobtained noise threshold is 0.75% [9]. Moreover, aspecial form of 3D cluster state becomes a resource
NTT Technical Review

Regular Articles

of fault-tolerant topological one-way quantum computation with noise threshold of 0.11% [10]. In thiscase, we can perform fault-tolerant quantum computing with only single-qubit measurements after preparing the 3D cluster states. The thresholds can be
improved by devising better encoding and decoding
methods [11], [12]. Recently, the important realistic
case of errors with a high loss rate [13] and of nondeterministic entangling gates [14], [15] have also
been investigated.
4. NTT research on measurement-based
quantum computation
Here, I briefly describe research on measurementbased quantum computation at NTT Laboratories. In
2008, my colleagues and I experimentally demonstrated a simple scheme for generating a four-photon
entangled cluster state and basic operations for oneway quantum computing using the produced state[16]. We showed that the output state fidelities surpass classical bounds, which indicates that the entanglement in the produced state essentially contributes
to the quantum operation. However, the obtained
fidelities have not been sufficiently high for fault-tolerant quantum computation. In order to perform
quantum computing fault-tolerantly, we must improve
the gate fidelities experimentally. At the same time, it
is important to improve the theory of fault-tolerant
quantum computing so that we can utilize realistic
imperfect devices. For example, in linear optics, twoqubit gates are intrinsically nondeterministic owing
to the linearity of the interaction [4], [17]. In othersystems, it is also often the case that a large amountof error such as photon loss and detector inefficiency
can be detected in heralded ways, and one can postselect successful events [16], [18]. In 2010, we proposed a scalable way to construct a 3D cluster state
for fault-tolerant topological one-way quantum computation using such nondeterministic two-qubit gateswith a low success probability [14]. We showed thatfault-tolerant topological one-way quantum computation can be performed with a two-qubit gate successprobability of less than 1/2 provided that the unheralded error probability of the two-qubit gate is sufficiently small. This work showed for the first time thatrealistic imperfect nondeterministic gates with a success probability of less than 1/2 can also be utilized
for fault-tolerant quantum computing. The other issue
in measurement-based quantum computation, reducing the computational resources, is discussed in thesecond Regular Article [19], which treats a different
Vol. 9 No. 7 July 2011

model of measurement-based quantum computation.
5. Future outlook
Having reviewed the concepts and favorable features of measurement-based quantum computationand the fault-tolerant system, I would like to mention
the outlook for future research. On the theoreticalside, an important task is to find a more realistic
physical model for realizing measurement-based
quantum computation. There have been studies forfinding Hamiltonians whose ground states are universal resources for measurement-based quantum computation [20], [21]. On the experimental side, somedemonstrations of measurement-based quantum
computation have been performed [16], [18], [22].
However, a lot of problems remain to be overcome.
The most important one is how to obtain scalability inquantum computers. For optical quantum computation, a new high-efficiency single-photon source isnecessary [23]. A promising candidate for preparinga large entangled resource for measurement-based
quantum computation is ultracold atomic gas in an
optical lattice [24]. The important issue for future
work on the optical lattice is single-site addressing[25], [26]. Another important candidate is the use ofsolid-state artificial atoms such as quantum dots or
dopants in solids for stationary qubits and the use of
atom-photon interactions with cavity quantum electrodynamics for the quantum gates [27]. In conclusion, measurement-based quantum computation provides us with great features toward realizing quantum
computers and has high potential for both theoreticaland experimental research in the future.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]

M. A. Nielsen and I. L. Chuang, “Quantum Computation and Quantum Information,” Cambridge University Press, 2000.
D. Gottesman and I. Chuang, “Demonstrating the Viability of Universal Quantum Computation Using Teleportation and Single-qubit
Operations,” Nature, Vol. 402, pp. 390–393, 1999.
R. Raussendorf and H. J. Briegel, “A One-way Quantum Computer,”Phys. Rev. Lett., Vol. 86, No. 22, pp. 5188–5191, 2001.
D. E. Browne and T. Rudolph, “Resource-efficient Linear Optical
Quantum Computation,” Phys. Rev. Lett., Vol. 95, No. 1, p. 010501,
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L.-M. Duan and R. Raussendorf, “Efficient Quantum Computation
with Probabilistic Quantum Gates,” Phys. Rev. Lett., Vol. 95, No. 8, p.
080503, 2005.
D. Aharonov and M. Ben-Or, “Fault-tolerant Quantum Computation
with Constant Error Rate,” SIAM Journal on Computing, Vol. 38, No.
4, pp. 1207–1282, 2008.
E. Knill, “Quantum Computing with Realistically Noisy Devices,”
Nature, Vol. 434, pp. 39–44, 2005.
A. Y. Kitaev, “Fault-tolerant Quantum Computation by Anyons,”
Annals of Physics, Vol. 303, No. 1, pp. 2–30, 1997.



Regular Articles[9]

R. Raussendorf and J. Harrington, “Fault Tolerant Quantum Computation with High Threshold in Two Dimensions,” Phys. Rev. Lett., Vol.
98, No. 19, p. 190504, 2007.
[10] R. Raussendorf, J. Harrington, and K. Goyal, “A Fault-tolerant Oneway Quantum Computer,” Annals of Physics, Vol. 321, No. 9, pp.
2242–2270, 2005.
[11] R. Raussendorf, J. Harrington, and K. Goyal, “Topological Fault-tolerance in Cluster State Quantum Computation,” New J. Phys., Vol. 9,199, 2007.
[12] D. Wang, A. Fowler, and L. Hollenberg, “Surface Code Quantum
Computing with Error Rates over 1%,” Phys. Rev. A, Vol. 83, No. 2,020302(R), 2011.
[13] S. Barrett and T. Stace, “Fault Tolerant Quantum Computation with
Very High Threshold for Loss Errors,” Phys. Rev. Lett., Vol. 105, No.
20, 200502, 2010.
[14] K. Fujii and Y. Tokunaga, “Fault-tolerant Topological One-way Quantum Computation with Probabilistic Two-qubit Gates,” Phys. Rev.
Lett., Vol. 105, No. 25, 250503, 2010.
[15] Y. Li, S. Barrett, T. Stace, and S. Benjamin, “Fault Tolerant Quantum
Computation with Nondeterministic Gates,” Phys. Rev. Lett., Vol.
105, No. 25, 250502, 2010.
[16] Y. Tokunaga, S. Kuwashiro, T. Yamamoto, M. Koashi, and N. Imoto,“Generation of High-fidelity Four-photon Cluster State and Quantumdomain Demonstration of One-way Quantum Computing,” Phys. Rev.
Lett., Vol. 100, No. 21, 210501, 2008.
[17] E. Knill, R. Laflamme, and G. Milburn, “A Scheme for Efficient
Quantum Computation with Linear Optics,” Nature, Vol. 409, pp.
46–52, 2001.
[18] P. Walther, K. J. Resch, T. Rudolph, E. Schenck, H. Weinfurter, V.
Vedral, M. Aspelmeyer, and A. Zeilinger, “Experimental One-way
Quantum Computing,” Nature, Vol. 434, , pp. 169–176, 2005.

[19] Y. Takahashi, “Reducing the Resources in Measurement-only Quantum Computation,” NTT Technical Review, Vol. 9, No. 7, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2011
07ra2.html[20] T.-C. Wei, I. Affleck, and R. Raussendorf, “Affleck-Kennedy-LiebTasaki State on a Honeycomb Lattice is a Universal Quantum Computational Resource,” Phys. Rev. Lett., Vol. 106, No. 7, 070501,
2011.
[21] M. Van den Nest, A. Miyake, W. Dür, and H. J. Briegel, “Universal
Resources for Measurement-based Quantum Computation,” Phys.
Rev. Lett., Vol. 97, No. 15, 150504, 2006.
[22] W.-B. Gao, A. M. Goebel, C.-Y. Lu, H.-N. Dai, C. Wagenknecht, Q.
Zhang, B. Zhao, C.-Z. Peng, Z.-B. Chen, Y.-A. Chen, and J.-W. Pan,
“Teleportation-based Realization of an Optical Quantum Two-qubit
Entangling Gate,” Proc. of the Natl. Acad. Sci. USA, Vol. 107, pp.
20869–20874, 2010.
[23] J. L. O’Brien, “Optical Quantum Computing,” Science, Vol. 318, No.
5856, pp. 1567–1570, 2008.
[24] I. Bloch, “Quantum Coherence and Entanglement with Ultracold
Atoms in Optical Lattices,” Nature, Vol. 453, pp. 1016–1022, 2008.
[25] A. Daley, M. Boyd, J. Ye, and P. Zoller, “Quantum Computing with
Alkaline-Earth-Metal Atoms,” Phys. Rev. Lett., Vol. 101, No. 17,170504, 2008.
[26] K. Shibata, S. Kato, A. Yamaguchi, S. Uetake, and Y. Takahashi, “A
Scalable Quantum Computer with Ultranarrow Optical Transition of
Ultracold Neutral Atoms in an Optical Lattice,” Appl. Phys. B, Vol.
97, No. 2, pp. 753–758, 2009.
[27] T. D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and J.
L. O’Brien, “Quantum Computers,” Nature, Vol. 464, pp. 45–53,
2010.

Yuuki Tokunaga

Research Scientist, Distinguished Researcher,Okamoto Research Laboratory, NTT Information Sharing Platform Laboratories.
He received the bachelor’s degree in integrated
human studies in mathematical science from
Kyoto University in 1999, the M.S. degree in
information science from the University ofTokyo in 2001, and the Ph.D. degree in materials
physics from Osaka University in 2007. He
joined NTT Information Sharing Platform Laboratories in 2001. Since then, he has been engaged
in research on quantum information science,
especially on optical quantum information processing. He received the 2008 Inoue ResearchAward for Young Scientists and the NTT Information Sharing Laboratory Group Director’sAward in 2008 and 2010. He is a member of the
Physical Society of Japan.



NTT Technical Review

Regular Articles: New Paradigm toward Realizing Quantum Computer

Reducing the Resources in
Measurement-only
Quantum Computation
Yasuhiro Takahashi†
Abstract

This article describes a recent result concerning the construction of a small set of projective
measurements required for implementing universal quantum computation, that is, for implementing an
arbitrary unitary operation. Projective measurements are used as the main resources for implementing
an arbitrary unitary operation in the measurement-only quantum computation model in contrast toconventional models. Resource minimization is important not only for realizing a quantum computer
based on the measurement-only quantum computation model, but also for understanding thecomputational power of projective measurements.

1. Introduction
Quantum computers are expected to enable highspeed computing and many researchers have madenumerous attempts to realize them. However, a practical quantum computer has not yet been made. Mostof the attempts are based on the standard model ofquantum computation, which is called the quantum
circuit model or gate-based quantum computation[1], [2]. The main resources for universal quantum
computation, that is, for implementing an arbitrary
unitary operation, are unitary gates that perform onequbit and two-qubit unitary operations. A computation proceeds by applying unitary gates to an input
state to transform it into an appropriate output one.
Projective measurements of the output state give us
the final output of the computation, which is classicalinformation.
In 2003, on the basis of an idea described by Gottesman and Chuang [3], Nielsen proposed a model ofquantum computation [4] strikingly different fromthe standard model. This model is called teleportation-based or measurement-only quantum computation. The main resources required for universality are† NTT Communication Science Laboratories
Atsugi-shi, 243-0198 Japan

only projective measurements. A computation proceeds by performing projective measurements on an
input state to transform it into an appropriate output
one. Projective measurements on the output state giveus the final output of the computation. This model has
recently attracted attention since it allows us to makecompletely new attempts to realize quantum computers. An important problem is to minimize the resources required for universality. From the practicalstandpoint, solutions to this problem will contributeto the performance of unitary operations on a quantum computer since it will be able to use only a limited amount of resources. On the theoretical side, they
will contribute to our understanding of the computational power of projective measurements.
Let us consider the problem of minimizing theresources required for universality in measurementonly quantum computation. There have been manystudies in this direction [5]–[8]. The best known
result is that a set consisting of one two-qubit projective measurement and infinitely many kinds of onequbit projective measurements with one ancillary
qubit is sufficient for universal quantum computation[8]. Since it is impossible to decrease the number of
two-qubit projective measurements and the numberof ancillary qubits, we shall focus on one-qubit projective measurements. Thus, our problem is restated
NTT Technical Review

Regular Articles

0

1
eiθ
0 −
1
2
2

Z
1
1
0 +
1
2
2

X

θ

1
eiθ
0 +
1
2
2

Y
1

1
1
0 −
1
2
2

(cosθ )X+(sinθ )YFig. 1. Unit three-dimensional sphere representing onequbit states.

as the problem of constructing a small set of onequbit projective measurements such that the set with
one two-qubit projective measurement and one ancillary qubit is sufficient for universal quantum computation.
The state of a qubit corresponds to a point on theunit three-dimensional sphere (Fig. 1) [1] and a onequbit projective measurement corresponds to an axis,
which is a line through the origin of the sphere. There
are two points of intersection between an axis and thesphere. A one-qubit projective measurement proba­bilistically projects a one-qubit state into one of thetwo states corresponding to the two intersection
points on the sphere. The best known result requires
the set consisting of one-qubit projective measurements corresponding to all the axes of the sphere’s
X-Y plane and the Z axis [8]. Until recently, it was notknown whether a smaller set of one-qubit projective
measurements could be constructed for universal
quantum computation.
In this article, I describe my recent result that thiscan be done [9]. Specifically, I show that the set consisting of one-qubit projective measurements corresponding to all the axes of the sphere’s X-Y plane(with one two-qubit projective measurement and one
ancillary qubit) is sufficient for universal quantum
computation. In other words, I show that the onequbit projective measurement corresponding to the Z
axis can be removed from the best known set. A key
finding is that the one-qubit projective measurement
corresponding to the Y axis can often be used in placeof the one-qubit projective measurements corresponding to the X and Z axes. In particular, a key
ingredient of my procedure for implementing an arbitrary one-qubit unitary operation is a simplified version of quantum teleportation (called state transfer)based on the one-qubit projective measurement corresponding to the Y axis.

Vol. 9 No. 7 July 2011

2. Measurement-only quantum computation
2.1 Q
 uantum states and projective measurements
As described above, the state of a qubit corresponds
to a point on the unit three-dimensional sphere(Fig. 1). The two points of intersection of the Z axis
and the sphere are represented by |0〉 and |1〉. The
other points on the sphere correspond to superposition states of |0〉 and |1〉. For example, the two points
of intersection of the X axis and the sphere are
1 |0〉 + 1 |1〉 and 1 |0〉 − 1 |1〉. Moreover, the two
−
−
−
−
√2
√2
√2
√2
points of intersection of the Y axis and the sphereare1 |0〉 + i |1〉 and 1 |0〉 − i |1〉. In general, a one−
−
−
−
√2
√2
√2
√2
qubit state is represented as a |0〉 + b |1〉, where a andb are complex numbers such that |a |2 + |b |2=1.
A one-qubit projective measurement corresponds
to an axis L, which is a line through the sphere’s origin. It probabilistically projects a one-qubit state into
one of the two states corresponding to the two points
of intersection of L and the sphere. The probability
depends on the state being measured. The measurement gives us the classical outcome (1 or -1) representing which of the two states the original state is
projected into by the measurement. We call such ameasurement an L-measurement. For example, theZ-measurement of a qubit in state a |0〉 + b |1〉 projects
the state into |0〉 with probability |a |2 and into |1〉 withprobability |b |2. In general, an axis of the sphere’s X-Yplane is represented by (cosq )X+(sinq )Y for somereal number q∈[0,2p). It corresponds to the one-qubit
projective measurement that probabilistically projects a one-qubit state into one of the two states
1 |0〉 + eiq |1〉 and 1 |0〉 - eiq |1〉.
−
−
−
−
√2
√2
√2
√2
2.2 Measurement-based quantum circuits
A computational procedure in measurement-only
quantum computation can be represented by a measurement-based quantum circuit (very similar to thestandard quantum circuit [2]). It consists of wires andmeasurement gates that correspond to qubits and projective measurements, respectively. In a circuit diagram, a wire is represented by a horizontal line and ameasurement gate is represented by the axis symbol
corresponding to the projective measurement on thewires on which it is performed. Information flows
through the circuit from left to right.
As an example, consider the measurement-basedRegular Articles

quantum circuit depicted in Fig. 2. It represents thefollowing procedure, where the initial state of qubit 1
is |j〉 and that of qubit 2 is an arbitrary one-qubit
state.
(1) Perform the X-measurement on qubit 2.
(2)	Perform the Z⊗Z-measurement on qubits 1 and
2.
(3) Perform the X-measurement on qubit 1.
I do not give details of the Z⊗Z-measurement here,
but in general it generates a two-qubit state that cannot be represented as a product form (called entangledstate).
The procedure outputs the state s |j 〉 on qubit 2 forsome unitary operation s, which is in a special classof unitary operations called Pauli operations. An
important point is that s is determined by the classical outcomes of the projective measurements in theprocedure, and the inverse of s can be performed by
the Y-measurements (and one two-qubit projective
measurement and one ancillary qubit). Thus, s can beeasily removed and thus ignored. That is, the procedure transfers an arbitrary one-qubit state from qubit
1 to qubit 2 (up to Pauli operations) and thus is calledstate transfer [6]. It can be regarded as a simplified
version of quantum teleportation.
As shown in [6], state transfer is a key ingredient ofa procedure for implementing an arbitrary one-qubit
unitary operation. More precisely, a procedure for
implementing such an operation (up to Pauli operations) can be obtained by replacing the projective
measurements in the state transfer with ones that
depend on the desired operation. For example, a procedure for implementing a Hadamard operation H
can be obtained by replacing the first measurementwith the Z-measurement and the second measurementwith the Z⊗X-measurement, where H is the one-qubit
unitary operation that maps |0〉 and |1〉 to
1 |0〉 + 1 |1〉 and 1 |0〉 − 1 |1〉, respectively.
−
−
−
−
√2
√2
√2
√2
3. Universal set of projective measurements
I will show that the set consisting of the one-qubit
projective measurement corresponding to the axis
(cosq )X+(sinq )Y for any q∈[0,2p) (with one twoqubit projective measurement and one ancillary
qubit) is sufficient for universal quantum computation. To do this, I will show that an arbitrary unitary
operation can be implemented using only the above
projective measurements. Since an arbitrary unitary
operation can be implemented by combining onequbit unitary operations with a two-qubit unitary


ϕ 1

Z

X

⊗
2

X

σϕ

Z

Fig. 2. M easurement-based quantum circuit proposed forstate transfer [6].

ϕ 1

Z

Y

⊗
2

Y

Z

σϕ

Fig. 3. State transfer based on Y-measurements.

operation [1], it suffices to show how to implement
these operations.
3.1 New state transfer
A key ingredient of my procedure for implementing
an arbitrary one-qubit unitary operation is a new statetransfer based on the Y-measurements. Consider thefollowing procedure (Fig. 3), where the initial state of
qubit 1 is |j 〉 and that of qubit 2 is an arbitrary onequbit state.
(1) Perform the Y-measurement on qubit 2.
(2)	Perform the Z⊗Z-measurement on qubits 1 and
2.
(3) Perform the Y-measurement on qubit 1.
The procedure can be shown to be a state transfer,which transfers the input state from qubit 1 to qubit 2.
It is obtained by replacing the X-measurements in theprevious state transfer with the Y-measurements. Thatis, this is an example of the case where the Y-measurements can be used in place of the X-measurements.
3.2 I mplementations of one-qubit and two-qubit
unitary operations
I will deal with one-qubit unitary operations first. I
can show that, by replacing the Z⊗Z-measurement in
the new state transfer with the Z⊗X-measurement,
the resulting procedure is the one for implementing
H. An important point is that it uses only Y-measurements, though the previous procedure for implementing H uses the X- and Z-measurements as describedabove. This can be considered an example of the casewhere the Y-measurements can be used in place of the
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Regular Articles

X- and Z-measurements. That is, the new state transfer provides an H implementation procedure that
requires only a small set of one-qubit projective measurements. By generalizing the method for transforming the new state transfer into an H implementation procedure, I can obtain a procedure for implementing an arbitrary one-qubit unitary operation (upto Pauli operations). Moreover, I can show that it
requires only the one-qubit projective measurement
corresponding to the axis (cosq )X+(sinq )Y for anyq∈[0,2p).
As a two-qubit unitary operation, it suffices to deal
with the controlled-Z operation LZ that maps |0〉|0〉,|0〉|1〉, |1〉|0〉, and |1〉|1〉 to |0〉|0〉, |0〉|1〉, |1〉|0〉, and−|1〉|1〉, respectively. The remaining problem is toobtain a procedure for implementing LZ (up to Pauli
operations) using only the one-qubit projective measurement corresponding to the axis (cosq )X+(sinq )Y
for any q∈[0,2p). Though it is difficult to implement
LZ directly, I can show that there is a two-qubit unitary operation similar to LZ such that combining itwith one-qubit unitary operations implements anarbitrary unitary operation and that it can be implemented by using only Y-measurements. Thus, the set
consisting of the one-qubit projective measurement
corresponding to the axis (cosq )X+(sinq )Y for anyq∈[0,2p) is sufficient for universal quantum computation.
4. Conclusions and future work
I examined the problem of minimizing the resources required for universality in measurement-only
quantum computation and described a small set ofprojective measurements sufficient for universal
quantum computation. A key ingredient of my procedures is state transfer based on the Y-measurements.
It would be interesting to consider approximate universality in measurement-only quantum computation[10] because a small approximately universal set ofprojective measurements is particularly important inpractice. Moreover, it would be interesting to investigate the resources required for other important problems, such as graph state preparation [9].
References[1]

M. A. Nielsen and I. L. Chuang, “Quantum Computation and Quantum Information,” Cambridge University Press, 2000.
[2] Y. Takahashi, “Quantum Arithmetic Circuits: a Survey,” IEICE Trans.
Fundamentals, Vol. E92-A, No. 5, pp. 1276–1283, 2009.
[3] D. Gottesman and I. L. Chuang, “Demonstrating the Viability of Universal Quantum Computation Using Teleportation and Single-qubit
Operations,” Nature, Vol. 402, pp. 390–393, 1999.
[4] M. A. Nielsen, “Quantum Computation by Measurement and Quantum Memory,” Phys. Lett. A, Vol. 308, No. 2-3, pp. 96–100, 2003.
[5] D. W. Leung, “Quantum Computation by Measurements,” International Journal of Quantum Information, Vol. 2, No. 1, pp. 33–43,
2004.
[6] S. Perdrix, “State Transfer Instead of Teleportation in Measurementbased Quantum Computation,” International Journal of Quantum
Information, Vol. 3, No. 1, pp. 219–223, 2005.
[7] A. M. Childs, D. W. Leung, and M. A. Nielsen, “Unified Derivationsof Measurement-based Schemes for Quantum Computation,” Phys.
Rev. A, Vol. 71, No. 3, 032318, 2005.
[8] P. Jorrand and S. Perdrix, “Unifying Quantum Computation with
Projective Measurements Only and One-way Quantum Computation,” Proc. of SPIE Quantum Informatics 2004, Vol. 5833, pp. 44–51,
2005.
[9] Y. Takahashi, “Simple Sets of Measurements for Universal Quantum
Computation and Graph State Preparation,” International Journal of
Quantum Information, Vol. 8, No. 6, pp. 1001–1012, 2010.
[10] S. Perdrix, “Towards Minimal Resources of Measurement-based
Quantum Computation,” New Journal of Physics, Vol. 9, No. 6, 206,
2007.

Yasuhiro Takahashi

Scientist, Computing Theory Research Group,
Innovative Communication Laboratory, NTT
Communication Science Laboratories.
He received the B.S. and M.S. degrees in mathematics from Tohoku University, Miyagi, and the
Ph.D. degree in engineering from the University
of Electro-Communications, Tokyo, in 1998,
2000, and 2008, respectively. He joined NTT
Communication Science Laboratories in 2000
and has been engaged in research on the designand optimization of quantum circuits including
measurement-based ones. His research interests
include quantum computing, computational
complexity theory, and cryptography. He is amember of the Information Processing Society ofJapan and the Institute of Electronics, Information and Communication Engineers.

Vol. 9 No. 7 July 2011

Regular Articles

Speech Dereverberation Using Linear
Prediction
Keisuke Kinoshita† and Tomohiro Nakatani
Abstract

This article describes a method of reducing reverberation in an observed signal to mitigate problems
experienced in speech communication systems, such as hands-free mobile telephones, videoconferencing
systems, hearing aids, and voice-controlled robots. It focuses on dealing with the effect of latereverberations, which are known to be a major cause of the degradation of automatic speech recognition(ASR) performance and loss of speech intelligibility. Experimental results show that this method canprovide substantial improvements in ASR performance and audible quality under severely reverberant
conditions.

1. Introduction
The last decade has seen the rapid development andpervasiveness of speech technologies, such as handsfree (mobile) telephones, videoconferencing, andhearing aids. In the near future, we can expect to see
a dramatic spread of human-machine communication
systems, for example, voice-operated electrical appliances and intelligent communication robots, whichhave already been partially launched and are attracting attention in the market. The main user benefit ofhands-free telephones is that they enable the user towalk around freely without wearing a headset or
microphone, so they provide a natural communication style. Users of hearing-aid applications obviously benefit from better hearing capability that helps
them to interact more fluently with other people. The
realization of communication robots will undoubtedly lead to numerous innovative services and technologies, and the benefits brought by these technologies will be literally beyond our imagination.
In all these examples, the position of the target
speaker can be at a considerable distance from themicrophone. As a result, the observed signal at the
microphones can be degraded by reverberation
caused by reflection from walls, floors, ceilings, and† NTT Communication Science Laboratories
Soraku-gun, 619-0237 Japanfurniture. The reverberant speech signal recorded at
the m-th microphone is generally modeled as:
L-1
xm(n) = ∑ hm(k)s(n−k), (1)=[s*hm](n),

(2)

k=0

where s(n) denotes clean speech and hm(n) the room
impulse response (RIR) between the source signal
and the m-th microphone, which is assumed to betime invariant in this article. [f*g](n) stands for the
convolution of f and g. The acoustic system treated in
Eq. (1) is shown in Fig. 1. A dereverberation method
is generally applied to the received microphone signal xm(n) to estimate the desired signal s(n). It should
be noted that most of the existing acoustic signal processing techniques, e.g., automatic speech recognition (ASR), source separation techniques, and noise
reduction techniques [1]–[6], completely fail or experience dramatically reduced performance when reverberation is present. In addition, even after a considerable number of investigations, dereverberation in realenvironments still remains one of the most challenging speech signal processing tasks to this day. Thus,
the investigation of dereverberation algorithms isevidently important.
Reverberant speech is generally assumed to consistof three parts: a direct-path response, early reflections, and late reverberation. In this article, the early
NTT Technical Review

Regular Articles

reflections are defined as the reflection componentsthat arrive after the direct-path response within a timeinterval of about 30 ms, and the late reverberation as
all the latter reflections. Since late reverberation is
known to be a major cause of ASR performance degradation and speech intelligibility loss, this article
focuses on dealing with the effect of late reverberation. The two most serious detrimental effects causedby late reverberation are summarized below.

phonemes are well separated in time. Now, if theanechoic signal in Fig. 2(a) is reverberated, forinstance, with the RIR measured in an office at a distance of 0.5 m from the source, the received signal
tends to show the characteristics shown in Fig. 2(b).
In Fig. 2, we simulated a situation with RT60 of about
0.6 s, where RT60 is the time required for reflectionsof a direct sound to decay by 60 dB below the level ofthe direct sound. The smearing of the phonemes intime is clearly noticeable in both the spectrogram andthe waveform. Owing to this smearing, the empty
spaces between words and syllables are filled up, andsubsequent phonemes overlap [7]–[9]. These distortions result in an audible difference between theanechoic speech and the reverberant speech and leadto degraded speech intelligibility and fidelity. Thesedetrimental perceptual effects are primarily causedby late reverberation, and they generally increase
with increasing distance between the source andmicrophone.

(1) Effects on waveform and spectrogram
The effects of late reverberation on speech areclearly visible in the spectrogram and waveform representation. The spectrogram and waveform of an
anechoic speech signal are depicted in Fig. 2(a). The

x1(n)

hM(n)
xM(n)
M: number of microphones(b)

Amplitude (index)(a)Amplitude (index)

Fig. 1. Acoustic system treated here.

0.8
0
−0.9

0

Time (s)

1.9

0.8
0
−0.9

0

Time (s)1.9Frequency (kHz)

x2(n)

h1(n)

Frequency (kHz)(2) Effect on ASR performance
The performance of ASR systems depends heavily
on the quality of the input speech. While reasonable
recognition performance is commonly achieved
when the source-microphone distance is small, theperformance tends to decrease drastically as the distance increases. To explain the reason, we show a
block diagram of a typical speech recognition system
in Fig. 3. In the system, first, acoustic features suchas Mel frequency cepstral coefficients (MFCCs) are
extracted from the speech signal using a short timeframe (e.g., 30 to 50 ms) of the speech signal. These

h1(n)

s(n)

4

0

0

Time (s)

1.9

4

0

0

Time (s)1.9

Fig. 2. Waveforms and spectrograms of (a) clean speech signal and (b) reverberant speech signal.

Vol. 9 No. 7 July 2011

Regular Articles

Acousticmodel

Speech signal

Feature extraction

Language
model

Decoding

Text

Fig. 3. Typical ASR system configuration.



70

Bad

60
Word error rate (%)

acoustic features are meant to characterize theessential information present in the speech signal.
Next, on the basis of these acoustic features, the most
likely text is found by the decoder using two types ofknowledge sources: an acoustic model and a language model. The acoustic model contains acoustic
knowledge required to decode the features into phonemes, and the language model contains linguisticknowledge required to decode these phonemes intowords or sentences. These models should be trainedusing a set of training data prior to the decoding step.
In most cases, the acoustic model is trained on a setof acoustic features extracted from a clean (i.e.,
undistorted) speech signal. Thus, if the input signal to
the ASR system is distorted, for example, by reverberation, the acoustic model mismatches the input
signal, which leads to degraded recognition performance.
The influence of reverberation on the performanceof a state-of-the-art speech recognition system [10],which has been developed at NTT Communication
Science Laboratories, is shown in Fig. 4. The word
error rate (WER) of continuous speech recognition isshown for various distances in an environment with areverberation time of 0.6 s. The reverberant signals
were generated by convolving anechoic speech signals taken from the Japanese newspaper article
speech (JNAS) corpus [11] with a synthetic RIR. The
solid line represents the WER for reverberant speech,
while the star (★) shows that for clean speech forreference. Note that in this figure, the WER increases
with increasing source-microphone distance. From
this simple example, it is clear that the effects ofreverberation on the ASR system are rather severe.
Similar results are obtained if the reverberation time
is varied, for example, from 0.1 s to 1.0 s with a fixedmicrophone-source distance of 1 m.

50
40
30
20
10

Good

0

0.5
1.0
1.5
2.0
Distance between microphone and speaker (m)Fig. 4. T endency of the WER in a reverberant environment.
The solid line represents the WER of the reverberant
speech, while the star represents that of clean
speech.

2. Difficulty of speech dereverberation
The problem of speech dereverberation has beenviewed as one of the most difficult tasks in the fieldof acoustic signal processing research. To explain the
difficulty of speech dereverberation, we show theprocess of reverberant speech generation in Fig. 5. As
you can see from the figure, first, the clean speech
signal s(n) is generated as a convolution of white
noise u(n) and the impulse response of the vocal tract
filter a(n), i.e., s(n)=[u*a](n), and then the reverberant speech xm(n) is generated according to Eq. (2).
That is, the observed signal xm(n) can be alternatively
formulated as

NTT Technical Review

Regular Articles

x1(n)
h1(n)
u(n)

α(n)

s(n)

h2(n)

x2(n)

hM(n)
M: number of microphonesxM(n)Fig. 5. Process of clean speech and reverberant speech
generation.

xm(n)=[u*a*hm](n).

(3)

To recover the clean speech s(n) with the observationof only xm(n), it is necessary to distinguish two
unknown impulse responses contained in the observed
signal, i.e., a(n) and hm(n), and remove only the effectdue to hm(n). Since it is not trivial to distinguish these
two unknown impulse responses on the basis of only
the observed signal, speech dereverberation problem
has remained as unsolved problem for many years.
Some researchers have proposed a subspace method for estimating the RIRs by distinguishing these
two unknown impulse responses [12]. This method
can work effectively in the case of a well-conditioned
problem, where the order of the RIR is small and its
inverse filter can be calculated in a numerically stablemanner. However, real reverberant environments are
generally regarded as ill-conditioned problems,
where the RIR order is more than several thousand
milliseconds, and the calculation of its inverse filter
often becomes numerically unstable. Therefore, thesubspace method could not work effectively with real
recordings. In this article, we propose a dereverberation algorithm that can appropriately distinguish theabovementioned two unknown impulse responses
even in the case of an ill-conditioned problem. Thus,
it is suitable for speech dereverberation.
3. Dereverberation based on
multichannel linear prediction
It is known that linear prediction algorithms [13]are very powerful for estimating the inverse filter ofthe unknown system. One advantage of using linear
prediction is that it is very robust in the case of illconditioned problems. However, the conventional
linear prediction algorithm does not have mechanism
for distinguishing two unknown impulse responses
included in the observation process, so it cannot be
Vol. 9 No. 7 July 2011used for the speech dereverberation as it is. To makethe linear prediction algorithm suitable for speech
signal dereverberation, in this section, we introduce adereverberation algorithm based on the generalized
linear prediction algorithm, namely multi-step linear
prediction (MSLP) [14].
MSLP is designed to estimate and suppress only
late reverberation, appropriately distinguishing itfrom the vocal tract filter a(n). Importantly, thelength of the vocal tract filter a(n) is, in general, relatively short compared with that of the RIR hm(n), suchas 30 to 100 ms, while the RIR length can be several
hundred milliseconds or sometimes more than a second. By taking advantage of this inherent speechproperty, i.e., the difference in the lengths of two
unknown impulse responses, we can correctly estimate the late reverberation which arrives after the
direct path-response with a delay of more than thelength of a(n).
First, let us modify Eq. (1) to clearly define the latereverberation component to be estimated with
MSLP:
D-1
L-1
xm(n) = ∑ hm(k)s(n−k) + ∑ hm(k)s(n−k), 
k= 0

k=D

=dm(n)+rm(n),

(4)

where D is the step-size parameter used in MSLP,
dm(n) denotes the mixture of the direct signal andearly reflections, and rm(n) denotes the late reverberation. Now, if the room transfer function does notshare common zeros, it is known that the above equation can be reformulated into the following autoregressive process using multichannel MSLP:
M K-1
xm(n) =∑ ∑ wm,i(k)xi(n−D−k) + dm(n), 
i=1k=0(5)where K is the length of the linear prediction filter, M
is the number of microphones, and wm,i(n) are theprediction coefficients used to predict the observed
signal at the m-th microphone at the present timeusing the past observed signals at the i-th microphone. As we can see from Eq. (5), the observed signal xm(n) can be expressed as the addition of the signal components that can be predicted from the pastobserved signal, and the direct signal plus the earlyreflections, dm(n), which cannot be predicted fromthe past observed signal. Note that, we can also see
that, by comparing Eqs. (4) and (5), the first term in
Eq. (5) can be regarded as the estimate of the latereverberation component. After estimating the prediction coefficients, we can suppress the lateRegular Articles

reverberation as in the following inverse filtering
form
M K-1
ŝ1(n) =x1(n)−∑ ∑ w1,i (k)xi(n−D−k)i=1k=0(6)

For simplicity, with Eq. (6), we show only the caseof suppressing the late reverberation contained in
x1(n).
Needless to say, it is essential to estimate wm,i(n) as
accurately as possible in order to efficiently suppress
the late reverberation. In [14], the minimum meansquare error criterion is presented for estimating
wm,i(n) as
w1=E{x(n−D)x(n−D)T}+ E{x(n−D)x1(n)T},
wherex(n)=[x1(n)T,x2(n)T,...,xM(n)T],
xm(n)=[xm(n),xm(n−1),...,xm(n−L+1)],
wm=[wm,1T,wm,2T,...,wm,MT]T,
wm,iT=[wm,i(0),wm,i(1),...,wm,i(L−1)]T.

(7)

4. Dereverberation experiments

With this estimation scheme, we can show that thelinear prediction coefficients for achieving accurate
dereverberation can be obtained if step-size parameter D is set as D > TS, where TS is defined as
E{s(n)s(n')}=0 if |n−n'|> TS .

(8)Here, Ts corresponds to the maximum period of time
during which the clean speech signal is assumed tomaintain a non-negligible autocorrelation value. It
should be noted that the clean speech signal is known
to have a larger autocorrelation value only within ashort-time region due to the characteristics of the
vocal tract. In other words, while the short-term correlation of reverberant speech can be affected by bothclean speech signal component and early reflection,
its long-term correlation is mostly dominated by only
the late reverberation effect. Since TS corresponds
roughly to the length of the vocal tract filter a(n) if

Input lInput M

Multichannel
Multistep LP
Copy LP coefficients Wm
Late rverberation
suppression
SignalOutputFilter parameter

Fig. 6. Schematic diagram of our method.



we set D sufficiently larger than the length of a(n),MSLP can efficiently utilize the long-term correlation as in Eq. (7) and estimate the late reverberation
precisely, distinguishing it from the vocal tract filtera(n). With our experiment, we found that the method
could estimate an accurate late reverberation component when we used D of 30 ms.
The processing diagram of our dereverberation
method based on multichannel MSLP is shown in
Fig. 6. First, using multichannel MSLP, we estimate
the prediction coefficients for estimating late reverberations at the i-th microphone. Then, on the basis of
the estimated coefficients, we perform inverse filtering as in Eq. (6) to achieve the dereverberation. A
more robust way of achieving this inverse filtering ispresented in [14], and its efficiency has been demonstrated.

We carried out dereverberation experiments inseverely reverberant environments and evaluated theperformance of our method in terms of spectrograms
and ASR performance.
Spectrograms of clean speech, reverberant speech
at a distance of 1.5 m, and speech dereverberated byour method using four microphones are shown in
Fig. 7. The effect of the method can be clearly seen.
The harmonic structure of the speech signal is wellrestored, and the separation of the phonemes in timeis well reconstructed. The improvement in audible
quality can be confirmed in [15]. The WER as a function of the distance between the microphone andspeaker is shown in Fig. 8. The dashed line shows the
WER of the reverberant speech, and the solid line
shows that of the signal processed by our method.
The recognition result for clean speech is also plottedby the star (★) as a reference value for the lowest
possible WER, i.e., 4.4%, that can be achieved withthis ASR system for this recognition task. As seenfrom the figure, if the reverberant speech is not subjected to any preprocessing, the WER increases
greatly with distance. Our method achieved a substantial reduction in the WER for all the tested reverberant conditions.
5. Concluding remarks
A speech signal captured by a distant microphone
is smeared by reverberation, which severely degrades
the ASR performance and the audible quality ofspeech signal. In this article, we introduced a novel
NTT Technical Review

Regular Articles(a)Frequency (kHz)

6

0

0

Time (s)2.40Time (s)2.40Time (s)2.4(b)

Frequency (kHz)

6

0

(c)

Frequency (kHz)60Fig. 7. Spectrograms of (a) clean speech, (b) reverberant speech, and (c) speech dereverberated by our method using four
microphones.

70
No processing
Word error rate (%)

60

Our method

50

Clean speech(beseline)

40
30
20
10
0

0.5
1.0
1.5
2.0
Distance between microphone and speaker (m)Fig. 8. WER as a function of distance between microphone and speaker.

dereverberation method based on the multichannel
linear prediction and showed its efficiency. It can beused, for example, as an efficient preprocessor forVol. 9 No. 7 July 2011

ASR system and as a useful speech enhancement tool
for audio postproduction engineers [15].

Regular Articles

References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]

J. Benesty, S. Makino, and J. Chen, “Speech Enhancement,” SpringerVerlag, New York, NY, USA, 2005.
T. F. Quatieri, “Discrete-time Speech Processing: Principles and Practice,” Prentice Hall, Upper Saddle River, NJ, USA, 1997.
M. Brandstein and D. Ward, “Microphone Array,” Springer-Verlag,
New York, NY, USA, 2001.
H. L. V. Trees, “Optimum Array Processing,” Wiley-Interscience,
New York, NY, USA, 2002.
S. Haykin, “Adaptive Filter Theory, 3rd ed.,” Upper Saddle River, NJ:
Prentice-Hall, 1996.
S. Haykin ed., “Unsupervised Adaptive Filtering: Blind Source Separation,” Wiley-Interscience, New York, NY, USA, 2000.
V. O. Knudsen, “The Hearing of Speech in Auditoriums,” J. Acoust.
Soc. Am., Vol. 1, No. 1, pp. 56–82, 1929.
R. H. Bolt and A. D. MacDonald, “Theory of Speech Masking byReverberation,” J. Acoust. Soc. Am., Vol. 21, No. 6, pp. 577–580,
1949.
A. K. Nábélek, R. Letowski, and F. M. Tucker, “Reverberant Overlapand Self-masking in Consonant Identification,” J. Acoust. Soc. Am.,

Keisuke Kinoshita

Researcher, NTT Communication Science
Laboratories.
He received the M.Eng. and Ph.D. degrees
from Sophia University, Tokyo, in 2003 and2010, respectively. Since joining NTT Communication Science Laboratories in 2003, he has
been engaged in research on speech and audio
signal processing. His research interests include
speech enhancement, robust automatic speech
recognition, and music signal processing. He
received the 2006 IEICE Paper Awards and the2009 ASJ Technical Development Awards. He isa member of IEEE, Acoustical Society of Japan(ASJ), and the Institute of Electronics, Information and Communication Engineers (IEICE).

Vol. 86, No. 4, pp. 1259–1265, 1989.
[10] T. Hori, C. Hori, Y. Minami, and A. Nakamura, “Efficient WFST-based
One-pass Decoding with On-the-fly Hypothesis Rescoring in
Extremely Large Vocabulary Continuous Speech Recognition,” IEEE
Trans. Speech, Audio and Language Processing, Vol. 15, No. 4, pp.
1352–1365, 2007.
[11] “JNAS: Japanese Newspaper Article Sentences,” (in Japanese).
http://www.mibel.cs.tsukuba.ac.jp/_090624/jnas/[12] S. Gannot and M. Moonen, “Subspace Methods for Multi Microphone
Speech Dereverberation,” EURASIP Journal of Applied Signal Process., Vol. 2003, No. 11, pp. 1074–1090, 2003.
[13] T. Kailath, A. H. Sayed, and B. Hassibi, “Linear Estimation,” Upper
Saddle River, NJ: Prentice Hall, 2000.
[14] K. Kinoshita, M. Delcroix, T. Nakatani, and M. Miyoshi, “Suppression of Late Reverberation Effect on Speech Signal Using Long-term
Multiple-step Linear Prediction,” IEEE Trans. Audio, Speech, andLanguage Processing, Vol. 17, No. 4, pp. 534–545, 2009.
[15] Sound demonstration of the speech dereverberation software developed on the basis of the principle presented in this article.
http://www.tacsystem.com/en/products/softwares/000563.php

Tomohiro Nakatani

Senior Research Scientist, Supervisor, NTT
Communication Science Laboratories.
He received the M.Eng. and Ph.D. degrees
from Kyoto University in 1991 and 2002, respectively. Since joining NTT as a researcher in 1991,
he has been investigating speech enhancement
technologies for developing intelligent human–machine interfaces. He received the 2006 IEICE
Paper Award and the 2009 ASJ Technical Development Award. He has been a member of theIEEE Signal Processing Audio and Acoustics
Technical Committee since 2009. He is a senior
member of IEEE and a member of IEICE and
ASJ.

NTT Technical Review

Regular Articles

Virtual Private Network
Authentication System Featuring
High Extensibility and Availability:
AAA
Kenichi Matsui†, Kenji Ota, Hiroyuki Kurita,
Hitoshi Nagao, Kenichi Mase, and Shinya Matsumoto
Abstract

NTT Network Service Systems Laboratories has developed an authentication system, called AAA,that authenticates users when they try to access a virtual private network (VPN). It features both highextensibility, which is the ability to flexibly combine multiple authentication methods depending on how
a given user uses a VPN service, and high availability, which is the ability to avoid service disruption
even in the event of a serious disaster.

1. Introduction
As the computerization of office operations
advances, it has become common to interconnect
local area networks in multiple office sites to carryout office operations in an integrated manner. This
interconnection is increasingly being done using virtual private network (VPN) services, which arecheaper than leased-line services. In Japan, the use of
VPNs now surpasses that of leased lines, especially inthe case of enterprise users: about half of all enterprise users now opt for VPNs [1].
Unlike leased lines, VPNs are configured on a public network, which can be accessed by the generalpublic. Therefore, an authentication system is needed
to ascertain that someone trying to access a VPN has
the authority to do so.
A VPN authentication system must provide threebasic functions: an authentication function to identify
the user correctly, an authorization function to check
whether the user has the authority to use the requested† NTT Network Service Systems Laboratories
Musashino-shi, 180-8585 Japan

service according to the contract with the user, and an
accounting function to record the user’s usage. These
three functions are collectively known as AAA. A
VPN authentication system equipped with thesefunctions can authenticate users, determine whether
or not the user should be permitted to access a given
service, and manage usage records for all users.
Although VPN authentication systems normallyuse only a user ID (identification) and password forauthentication and authorization, rising concerns
about security are leading to demands for the use ofmultiple authentication factors, such as identificationof the user’s access line, in addition to user ID andpassword [2]. VPN authentication systems are
required to be able to provide different combinationsof authentication and authorization flexibly depending on the needs of each service.
As VPN services become widely utilized, anincreasing number of enterprises are applying VPNs
to systems that require high availability, such as theircore business systems, but a failure in such systems
could cause considerable damage. Therefore, VPN
authentication systems need to provide high availability to avoid service disruption even in the event of
NTT Technical Review

Regular Articles

AAA system
Primary site
SOprocessing
interface

Customer
information
OpS
Operations forcustomermanagement

Log
collection
interface

Alarm
monitoring
OpS
Operations forequipment
operation andmaintenance

Secondary siteAAA
AAA
database Session management databasefunction
Log/SO function
AAA log/
AAA log/
SO server
SO server
Authentication andauthorization function
AAAAAA
authentication
authentication
serverserverRADIUSRADIUS
processor
processor

Alarmmonitoring
interface

RADIUS interface

NTU

NTU

NTU

Fig. 1. Overall configuration of AAA.

a serious disaster.
To meet these requirements, we have developed a
VPN authentication system, called AAA, that has
high extensibility and availability in processing
authentication and authorization. This article
describes the configuration, functions, and featuresof this system, which has already been imple­
mented.
2. System configurations and functions
2.1 Configuration
The overall configuration of our system is shown in
Fig. 1. The AAA consists of three parts. The AAA
authentication server performs authentication andauthorization. The AAA log/SO server collects andformats logs, processes service orders (SOs), which
are users’ service provisioning requests, in collaboration with the operations system (OpS), and manages
session information. The AAA database stores information about authentication derived from service
orders, information about equipment, and information about established sessions.
In preparation for serious disasters, the AAA is
duplicated at two sites. The number of AAA authentication servers can vary depending on the number ofconnected network termination units (NTUs), which
control user terminals.

Vol. 9 No. 7 July 2011

2.2 Authentication and authorization functions
The AAA exchanges authentication information
with NTUs using RADIUS (Remote Authentication
Dial-In User Service), which is a user authentication
security protocol. The basic operation of the system
up to the establishment of a VPN connection is as follows (Fig. 2):(1)	The router or home gateway in the user network
sends the user ID and password to the NTU torequest a VPN connection.
(2)	The NTU requests the AAA to authenticate theuser using the network information (e.g., line
identification) that it possesses, in addition to
the user ID and password.
(3)	The AAA determines whether the connection
should be permitted by referring to the authentication and authorization request from the network terminal unit and the user’s authorization
conditions stored in the database. It sends its
decision back to the NTU.
(4)	The NTU either starts or rejects the requested
VPN connection depending upon the reply fromthe AAA. If a connection is to be started, theNTU asks the AAA to start recording the session
for accounting. The AAA records the session
information, which shows the state of the user’sconnection (access line identification, connection start time, IP address given, etc. (IP: Internet protocol)).
Regular Articles(3) AAA determines whether to accept
authentication and authorization by referring
to request information and database.

(2) NTU requests, and responds
to, authentication and authorization
using RADIUS protocol.
(1) Router sends user ID and password
to request a VPN connection.

3 VPN authentication system: AAA
2

4

1

PC

2

LAN
HGW, router,
etc.

NTU
5

User networkLAN
HGW, router,etc.

(4) NTU requests start of accounting
function using RADIUS protocol. AAA
records start of user’s usage.
(5) NTU connects the HGW androuter and starts VPN communication.

1

PC

4

NTU

User network
IP network

HGW: home gateway
LAN: local area network
PC: personal computer

Fig. 2. Basic authentication operation up to VPN connection establishment.

(5)	The NTU connects the user terminal, and VPN
communication is started.
In this way, the AAA authenticates and authorizes
VPN users in a highly secure manner on the basis of
the user ID, password, and line identification (in thisexample).
2.3 VPN session management function
The AAA holds the session information about theuser who has been connected to the VPN after authentication has been completed. This session information is used to prevent double logins from the sameuser and is also referred to when maintenance staff
check the connection state to deal with a complaint
from the user.
As the user’s connection state changes (connection
establishment or release), the session information
held by the AAA is updated on the basis of information contained in RADIUS messages sent by the
NTU. If the RADIUS messages fail to reach the
AAA owing to packet loss or a fault in the NTU, thesession information held by the AAA may become
different from the user’s actual connection state.
To resolve such a discrepancy, the AAA collects
the user’s connection information from the NTU
and automatically corrects the session information

concerned. Specifically, this function operates
as follows (Fig. 3):(1)	The AAA log/SO server obtains the session
information list from the AAA database.
(2)	It obtains the session information list from theNTU.
(3)	It compares the two lists: if they do not match,
it replaces the session information in the AAA
database by that held by the NTU.
This session information correction can be executedwithout suspending the operation of the AAA andenhances the reliability of the session information.
Session information can also be sent to devices otherthan the AAA for use in authentication by upper-layer
applications, such as ASP (application service provider) services and SaaS (software as a service).
2.4 SO function
The SO function updates the SO information needed for VPN authentication in collaboration with the
OpS. It registers, updates, deletes, or refers to SO
information in the AAA database in response to an
SO request sent by the OpS. In addition, it asks the
NTU to release the user’s session.
The SO function and the OpS exchange messages
written in XML (extensible markup language) using
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Regular Articles

AAA log/SO server
2

User
network

2

NTU

1

3

AAA
database
User
network
NTU

AAA
authentication
serverFig. 3. Operation of session information correction.

the message exchange protocol SOAP (simple object
access protocol) to ensure versatility and extensibility
in authentication.
3. Features
3.1 Provision of usage history reports
The AAA can provide the history of each user’s
authentication and authorization in the form of areport.
The VPN authentication server normally outputs
the information included in RADIUS messages sentfrom NTUs, to the authentication and authorizationlog. However, in a VPN service, which can beaccessed from lines of various types, the NTUs used
may also vary. Attributes included in RADIUS messages can vary depending on the type of NTU. Therefore, it is not straightforward to generate a usage
history report from authentication and authorizationlogs.
To solve this problem, items that are output in usage
history reports (events, such as the start and end ofusage, date and time when each event occurred, user
ID, line number, etc.) are defined for each type of
NTU.
3.2 E
 xtensibility of authentication and authorization function
The number of items that the authentication andauthorization function must check is growing. To beable to cope with such an increase flexibly throughthe simple addition of necessary functions, the AAA
has extended interfaces (Fig. 4).
The extended processing part obtains or changes
Vol. 9 No. 7 July 2011

information needed in the processing of authentication and authorization, such as user ID, password, andnetwork information, through an extended interface.
When a new authentication and authorization module
is added, the module obtains the necessary information and returns processing results to the basic processing part through the extended interface so that theprocessing results will be reflected in the authentication and authorization. Since extended functions canbe added without modification of the AAA’s basic
processing part, new authentication and authorization
capabilities can be easily added to meet new userneeds.
An extended interface is available for each of thethree stages of processing: processing concerning allusers, processing concerning only legitimate users,
and processing concerning authorization. This
arrangement makes it possible to select an extended
interface appropriate for a specific processing operation or to select an appropriate method of adding
functions in order to achieve high-speed processing.
For example, processes executed for only legitimate
users, such as authentication and authorization processes, for which the occurrence number is proportional to the number of user sessions, can be applied
after the authentication of individual users. This will
limit the number of users handled by these processes
and thus eliminate wasteful processing.
3.3 C ompatibility of availability and high-speed
processing
In preparation for major disasters, the AAA has aredundancy configuration in which a system isinstalled at each of two sites: the primary site and theRegular Articles

Reception

Unsuccessful calls included
Authentication
requests

Basic processing

Only legitimate users’ calls

Processing
concerning all users

User
authentication
Extendedintereface

Authentication
requests

Only successful calls

Extended processing

Extendedinterface

Processing
concerning legitimate
users

Processingof IP address
Processing
concerning
authorization

Extendedintereface

Authentication
requests

Response

Fig. 4. Extensibility of the authentication/authorization process.

AAA
Primary site

Secondary site

Database

Database

Node #0

Node #1

Node #0

UpdateUpdateAuthentication
andauthorization

Node #1

Authentication
andauthorization
Switchover

Authentication requests

Authentication requests
NTUs

Fig. 5. Redundancy configuration of the AAA system.

secondary site (Fig. 5). Both systems are kept active.
If the system at the primary site is disrupted, all
NTUs switch over to the system at the secondary site,which continues to provide authentication and author­
ization.
To support the switchover to the secondary-site
system, the databases at the two sites must be syn

chronized in real time to ensure consistency in the
checking of double logins and other processing. To
meet this requirement, database updating is executedat the two sites simultaneously so that the contents ofthe two databases are always synchronized. To
enhance the authentication response performance,
access to the databases is minimized. For example,
NTT Technical Review

Regular Articles

the system that has received a request forauthentication refers only to its own database, and it
sends an authentication reply to the requesting NTU
only after it has successfully updated its database.
The other database will be updated after the authentication reply has been sent. This arrangement makes it
possible to keep duplicate data for authentication andauthorization at two sites while ensuring high-speed
authentication. For even higher availability, each site
has a cluster of two data servers.
4. Conclusion
The AAA authenticates users when they try toaccess a VPN. It features both high extensibility and

Kenichi Matsui

Senior Research Engineer, Third Promotion
Project, NTT Network Service Systems Laboratories.
He received the B.E. degree in informationengineering and the M.S. degree in information
sciences from Tohoku University, Miyagi, in
1995 and 1997, respectively. He joined NTT in
1997 and studied IP networking technology
including IP multicast management, quality-ofservice management, and traffic engineering.
During 2005–2008, he was engaged in commercial development of IPTV and video-on-demand
services. He is currently focusing on R&D of the
Next Generation Network. He is a member of theInstitute of Electronics, Information and Communication Engineers (IEICE), the Information
Processing Society of Japan (IPSJ), and IEEE.

Kenji Ota

Research Engineer, Third Promotion Project,
NTT Network Service Systems Laboratories.
He received the B.S. and M.S. degrees in mathematics from Keio University, Kanagawa, in
1989 and 1991, respectively. In 1991, he joined
NTT Software Laboratories, where he engaged
in R&D related to requirement engineering. He
has been engaged in R&D concerning software
engineering, IPv6 network operations, andsecure network access. During 2001–2004, he
was engaged in developing commercial services
for the Global IP-VPN. His work is currently
focused on R&D of the Next Generation Network. He is a member of IPSJ.

Hiroyuki Kurita

Engineer, Third Promotion Project, NTT Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in information and communication engineering from the
University of Tokyo in 2005 and 2007, respectively. He joined NTT Information Sharing Platform Laboratories in 2007 and studied network
security including authentication and network
attachment control. His work is currently focusedon R&D of the Next Generation Network. He isa member of IEICE.

Vol. 9 No. 7 July 2011high availability. It is currently used in the FLET’S
VPN Wide Service (offered by NTT EAST sinceAugust 2010 and by NTT WEST since November2009). Building upon the core authentication technology developed through these research and development activities, we will study technologies thatenable users to access a variety of network services
on VPNs, such as ASP and cloud services, safely,
securely, and conveniently.
References[1][2]

Nikkei BP, “Fact-finding Survey of Networks 2010,” NIKKEI NETWORK, No. 123, pp. 046–047, July 2010 (in Japanese).
B. Nagel, “Password Seeks Partner for Long-term, Secure Relationship,” CIO, Vol. 11, No. 2, pp. 28–31, 2010.

Hitoshi Nagao

Senior Research Engineer, Third Promotion
Project, NTT Network Service Systems Laboratories.
He received the B.E. degree in informationengineering from Tokushima University in 1993.
He joined NTT in 1993. He is currently focusing
on R&D of the Next Generation Network.

Kenichi Mase

Research Engineer, Third Promotion Project,
NTT Network Service Systems Laboratories.
He received the B.E. degree in electrical engineering from Toyo University, Tokyo, in 1991.
He joined NTT in 1991. He is currently focusing
on R&D of the Next Generation Network.

Shinya MatsumotoResearch Engineer, Third Promotion Project,
NTT Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in electronics and electrical engineering from Doshisha
University, Kyoto, in 1987 and 1989, respectively. He joined NTT Communication Switching
Laboratories in 1989 and studied intelligent networks. His work is currently focused on R&D of
the Next Generation Network.

NTT around the WorldNTT Com Asia
Total ICT Solutions Strengthen Hong Kongas the ICT Hub in Asia
Abstract

As the keystone of Mainland China, Hong Kong is poweredby NTT Com Asia’s comprehensive ICT (information andcommunications technology) solutions, which are strengthening its position as the regional ICT hub. It also supports thestaggering growth of business activities in the region. This
article introduces the office in Hong Kong and various ICT
solutions offered.

1. Introduction
NTT Com Asia Limited (NTT Com Asia) [1] wasestablished in Hong Kong in 1999. As the key arm of
NTT Communications’ Asia operations, it has devoted its best efforts to becoming the premiere ICT Solution Partner for enterprises in Greater China. In collaboration with its affiliate HKNet Company Limited(HKNet) [2], NTT Com Asia offers the best connectivity, datacentre, enterprise hosting, cloud, and managed services in the region. It has positioned itself as
the communication gateway for global enterprises todevelop and manage their operations in Hong Kongand mainland China. NTT Com Asia and HKNet currently employ around 300 industry professionals,
working in three offices in Hong Kong. Earlier inMay, the company expanded its operations andopened a new branch in Macao to meet the growing
demands in that city.
2. Total ICT solutions
2.1 Connectivity services
Utilising NTT Communications’ Tier I IP backbone
and the Arcstar™ private network, NTT Com Asia
and HKNet offer a full array of global and local connectivity services in Hong Kong from MPLS (multiprotocol label switching), IPVPN (Internet protocol
virtual private network), leased line, IPv4/IPv6 (IPversions 4 & 6) transit, and a content delivery network to teleconferencing services. Thanks to its
prime location in Asia, Hong Kong has developed asa regional ICT and financial hub with tremendous
demand for top-quality network services.
2.2 Datacentre and managed services
NTT Communications Hong Kong Data Centre [3](Fig. 1) and HKNet Kwai Chung Data Centre [4],which were designed to world-class standards, offer
the best-in-class datacentre service in the region.
Both facilities are carrier neutral, providing highly
diverse options, good coverage, and high-availability
failover solutions.
Officially launched in 2009, NTT Communications
Hong Kong Data Centre is a dedicated datacentre
located in a highly secure and robust environment in
Tai Po, Hong Kong. This state-of-the-art seven storey
datacentre provides a world-class datacentre service
with a total gross area of 212,100 sq. ft. (19,705 m2)
and optimal power and cooling systems. It was
designed with a Tier III+ fully redundant infrastructure. Every aspect of the facility is equipped with atleast N+1 redundancy, and a multiple-source andmultiple-path design was used to ensure business
continuity.
This facility is accredited with ISO 27001 and 9001
for its Information Security Management System andQuality Management. In addition to these ISO
NTT Technical Review

NTT around the World

Fig. 1. NTT Communications Hong Kong Data Centre.

certifications of its high standard of operation performance, NTT Com Asia has recently scooped theaccolade of Silver Award for Best Green ICT [5]bestowed on the NTT Communications Hong Kong
Data Centre at The Hong Kong ICT Awards 2011 forits green management approach and achievements.
Given its success in improving energy efficiency by
10% in the past two years, NTT Com Asia set forth
another ambitious target of a further 10% efficiency
enhancement within this year.
Last year, NTT Communications announced its
plan to develop a green efficient, premium Tier IVready datacentre [6] to meet the demands of the market. 30,000 m2 of land in Tseung Kwan O, Hong
Kong, was acquired for the purpose-built structure.
Upon completion, the new datacentre will have a total
gross floor area of 70,000 m2, composed of two fivestorey datacentre buildings and one six-storey officebuilding. Scheduled to begin operations in 2013, it isexpected to be the first Tier IV-ready datacentre in thecity and the largest one developed by a datacentre
service provider in the city, complementing Hong
Kong’s position as an ICT hub in Asia.
The company’s expansion plans have accelerated toaccommodate exceptional levels of demand. Clients
include multinationals from the financial services,
telecommunications, logistics, and technology sectors as well as information technology (IT) and web
companies originating from more than fifteen countries, including mainland China.
NTT Com Asia also provides an array of Managed
Services [7] and worldwide technical support worldwide 24 hours a day, 7 days a week.
2.3 Enterprise hosting
For over ten years, NTT Com Asia has striven toVol. 9 No. 7 July 2011

optimise solutions that help enterprises to maximise
their IT resource usage and enhance their business
operations. NTT Com Asia’s Enterprise Hosting Services are highly flexible and designed to support
rapid deployment and implementation in order tofacilitate enterprises’ IT initiatives on any scale fromlocal ad hoc projects to extensive global initiatives.
It offers a comprehensive portfolio of enterprisegrade hosting services from managed infrastructure
outsourcing to subscription-based cloud solutions.
These solutions support the extension of IT capabilities such as storage and servers in real time over the
network on an as-needed basis, which helps enterprises to operate efficiently and flexibly while at thesame time meeting the demand for increased mobility.
NTT Com Asia’s Cloud Hosting solutions utilise
VMware’s virtualisation technology and NTT Communications’ top-notch datacentre and Tier 1 Global
IP Network. The comprehensive portfolio of cloudservices, including Global Virtualization Service,
Virtual Infrastructure, Virtual Desktop, and VirtualServer Hosting, helps enterprises to build their cloud
platform and achieves the highest level of agility andefficiency for their businesses.
3. R&D incubation centre
NTT Com Asia takes pride in developing a researchand development (R&D) incubation centre in Asia tonurture ICT technologies and innovation in Asia. By
creating innovative ideas and driving R&D development, it hopes to bring about a breakthrough inadvanced technologies in the ICT industry and lay thefoundations for the future.
Last year, it organised the first ICT for the Future
R&D Forum in Hong Kong. This featured cuttingedge technologies from NTT’s R&D Centres thathave enabled industry leaders to remain resilient
while still being at the forefront of the latest technological trends (Figs. 2–5). The forum showcased
approximately 20 exhibits from NTT’s R&D Centres
covering a wide spectrum of areas from the pursuit ofTechnologies for Human Life and Society, Technologies for Business; Energy and Environment Technologies; and Science, Engineering, and Network Technologies.
There were also demonstrations and thought-provoking keynote speeches delivered by NTT’s Noritaka Uji, Senior Executive Vice President; Dr. Katsuhiko Kawazoe, Vice President and Chief Producer;
and Dr. Kenji Yokoyama, General Manager from theNTT around the World

Fig. 4. 4
 K Digital Cinema, the world’s first digital cinema
distribution and management system, drew wide
attention.

Fig. 2. Noritaka Uji opened the ICT for the Future R&D
Forum with a keynote address.

Fig. 5. Interactive Digital Signage, which comprises
SpotAd and NTT Docomo’s Recommender
System and presents customised ads according torealtime movement and customer preferences andprofiles, was also popular.

Fig. 3. NTT Com Asia unveiled cutting-edge technologies.

R&D Planning Department as well as guest speaker
Professor Matthew Ming-Fai Yuen, Acting VicePresident for Research and Development of The
Hong Kong University of Science and Technology.
The event provided a platform for the exchange ofviews and successfully attracted hundreds of leadersfrom top corporations and the research community.
Through the incubation of technology seeds, we lookforward to further strengthening the strategic positionof Hong Kong as the region’s ICT hub.

Communications. With the accelerating demand for
total ICT solutions for enterprises in Asia, we arebullish about its outlook and will strive to further
strengthen Hong Kong’s position as the region’s ICT
hub.
References
[1]
[2]
[3]
[4]
[5]

4. Looking forward[6]

NTT Com Asia will continue to provide high-quality innovative ICT solutions to the strategic market inHong Kong, Macao, and Greater China for NTT[7]

NTT Com Asia.
http://www.ntt.com.hk
HKNET.
http://www.hknet.com
NTT Communications Hong Kong Data Centre.
http://www.hk.ntt.com/en/products/data-centre/hong-kong-data-centre.html
http://www.hknet.com/en/ict-solution/hong-kong-data-centre/kwaichung-data-centre/server-colocation.html
http://www.hk.ntt.com/en/about-us/newsevents/press-releases/pressreleases/article/ntt-com-asia-achieves-10-improvement-in-energyefficiency-for-hong-kong-data-centre.html
http://www.hk.ntt.com/en/about-us/newsevents/press-releases/pressreleases/article/ntt-communications-closes-land-acquisition-the-firstservice-provider-to-build-a-tier-iv-ready-dat.html
http://www.hk.ntt.com/en/products/managed-services/managed-services.html

NTT Technical Review

NTT around the WorldNTT COM ASIA — short column
Dim sum is gaining in popularity in Japan. It isa range of delightful Chinese snacks that are
mostly served hot in bamboo baskets accompanied by Chinese tea. I am sure that most of you
have tried it and gained at least an indirect taste ofthis cultural aspect of Hong Kong. People say thatyou have not really been to Hong Kong if you
didn’t try dim sum during your trip. Here, thesedelightful, mouth-watering snacks are consumed
at breakfast and lunch. Some of my favourites are
steamed shrimp dumplings, pork dumplings, barbecued pork buns, and beef balls. Seeing thesemini arts pieces, which are handmade in bite
sizes, you will learn a bit of the local culture in
Hong Kong.
When you go into a Chinese restaurant at lunch

arrives at your table fairly fast. Sometimes thebamboo baskets are stacked up high and evenblock your view of your companions. This actually makes the great experience of dining in a dimsum restaurant. Joining the gang and being part ofthe bustling environment is really fun.
The experience is an accurate reflection ofHong Kong culture. People here like sharing,straightforwardness, and speed. Efficiency isalways the top priority. In the office, people
always try to get things done in the quickest possible way, which is sometimes a contrast to thecourtesy and sedate procedures of Japanese culture. Decision-making processes are amazingly
quick and I enjoy working in the heart of Asia.
Hong Kong is a fast moving city, just like Tokyoin Japan or New York in the USA. People walkextremely fast, especially during office hours.
The city also has a pleasant side. The worldfamous dazzling Victoria Harbour and relaxing
country parks are some of the areas that I am fondof for outings.

Dim sum is served inbamboo baskets.
Steamed beef dumplings:
one of my favourite dim
sumtime or at the weekend, you may find the noiseand crowds intimidating, especially if you are afirst timer. Dim sum restaurants in Hong Kong arebusy, loud, and fast-paced. That is what makes
them great, but the clamour and clatter can also be
off-putting. I like this kind of group experience
where a selection of dishes is ordered and sharedaround the table. Many of the tables in the restaurants, which are mostly round, can seat from fourto eight or even twelve people. Sitting alone israre.
Soon after your order has been placed, the foodVol. 9 No. 7 July 2011

View of Victoria Harbour
from The Peak.

Tsing Ma Bridge.

Hong Kong is truly a gourmet’s delight. I am
sure that NTT Com Asia staff members will bedelighted to recommend some of their favourite
dishes during your next visit to Hong Kong.
Lucy Leung, NTT Com Asia Ltd.



External Awards
Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology,
The Young Scientist’s Prize
Winner: Yoshitaka Taniyasu, NTT Basic Research Laboratories
Date: Apr. 23, 2011
Organization: The Ministry of Education, Culture, Sports, Science& Technology in Japan

Naoto Yoshimoto*2, and Kiyomi Kumozaki*3

For “Research on Crystal Growth of Aluminum Nitride and Its
Application to Deep Ultraviolet Light-emitting Device”.
Dr. Taniyasu has grown n-type and p-type aluminum nitride (AlN)
layers for the first time to apply them to semiconductor devices.
Using these layers, he has successfully demonstrated an AlN p-njunction light-emitting diode with a wavelength of 210 nm, the shortest ever reported for semiconductor solid-state light sources.

For “Adaptive Power Saving Mechanism for 10 Gigabit Class PON
Systems”.
This paper proposes a power saving mechanism with a variablesleep period to reduce the power consumed by optical network units(ONUs) in passive optical network (PON) systems. In the PON systems based on time division multiplexing (TDM), sleep and periodic
wake-up (SPW) control is an effective ONU power saving technique.
However, the effectiveness of SPW control is fully realized only if thesleep period changes in accordance with the traffic conditions. This
paper proposes an SPW control mechanism with a variable sleep
period. The proposed mechanism sets the sleep period according totraffic conditions, which greatly improves the power saving effect. In
addition, the protocols needed between an optical line terminal (OLT)and ONUs are described on the assumption that the proposed mechanism is applied to 10-Gbit/s (10G) class PON systems, i.e., IEEE
802.3av 10G-EPON and FSAN/ITU-T 10G-PON systems. The
validity of the proposed mechanism is confirmed by numerical simulations.
Published as: R. Kubo, J. Kani, Y. Fujimoto, N. Yoshimoto, and K.
Kumozaki, “Adaptive Power Saving Mechanism for 10 Gigabit ClassPON Systems,” IEICE Trans. Communications, Vol. E93-B, No. 2,
pp. 280–288, Feb. 2010.

The Institute of Physics (IOP) Fellow
Winner: Hiroshi Yamaguchi, NTT Basic Research Laboratories
Date: May 18, 2011
Organization: Institute of Physics
For personal contribution to the advancement of physics as a discipline and a profession.
The Council of the Institute is concerned to ensure that the Fellowship of the Institute includes all those who have made an important
contribution to physics, to the profession of physicist, or as physicists
in their chosen career.
Best Paper Award
Winners: Ryogo Kubo*1, Jun-ichi Kani*2, Yukihiro Fujimoto*2,*1	 Keio University*2 NTT Access Network Service Systems Laboratories*3 Mitsubishi Electric Corporation

Date: May 19, 2011
Organization: Institute of Physics

Papers Published in Technical Journals andConference Proceedings
Electroluminescence and Capacitance-voltage Characteristics of Single-crystal n-type AlN (0001)/p-type Diamond(111) Heterojunction Diodes
K. Hirama, Y. Taniyasu, and M. Kasu
Appl. Phys. Lett., Vol. 98, No. 011908, 2011.
n-type single-crystal AlN (0001) layers were grown on diamond(111) substrates by metalorganic vapor phase epitaxy. We observed
current-injected emission at a wavelength of 235 nm at room temperature in an n-type AlN/p-type diamond heterojunction diode. The
emission is attributed to free-exciton recombination in diamond.
From capacitance-voltage measurements of the n-type AlN/p-type
diamond heterojunction, we determined that the AlN/diamond heterojunction exhibits the staggered (type-II) band alignment with a

conduction band offset (DEC) of 3.5 eV and a valence band offset(DEV) of 4.0 eV.

Security of Cryptosystems Using Merkle-Damgård in the
Random Oracle Model
Y. Naito, K. Yoneyama, L. Wang, and K. Ohta
IEICE Trans. Fundamentals, Vol. E94-A, No. 1, 2011.
Since the Merkle-Damgård hash function (MDFH) that uses a
fixed input length random oracle as a compression function is not
indifferentiable from a random oracle (RO) because of the extension
attack, there is no guarantee of the security of cryptosystems that are

NTT Technical Review

Papers Published in Technical Journals and Conference Proceedings

secure in the RO model when RO is instantiated with MDHF. This
fact motivates us to establish a criteria methodology for confirming
cryptosystem security when RO is instantiated with MDHF. In thispaper, we confirm cryptosystem security by using the following
approach: 1) Find a weakened random oracle (WRO) that leaks values needed to realize the extension attack. 2) Prove that MDHF is
indifferentiable from WRO. 3) Prove cryptosystem security in the
WRO model. The indifferentiability framework of Maurer, Renner,and Holenstein guarantees that we can securely use the cryptosystem
when WRO is instantiated with MDHF. Thus, we concentrated onsuch finding a WRO. We propose the Traceable Random Oracle(TRO), which leaks values enough to permit the extension attack. By
using TRO, we can easily confirm the security of the OAEP encryption scheme and variants of it. However, there are several practical
cryptosystems whose security cannot be confirmed by TRO (e.g.,
RSA-KEM). This is because TRO leaks values that are irrelevant to
the extension attack. Therefore, we propose another WRO, Extension
Attack Simulatable Random Oracle (ERO), which leaks just thevalue needed for the extension attack. Fortunately, ERO is necessary
and sufficient to confirm the security of cryptosystems under MDHF.
This means that the security of any cryptosystem under MDHF isequivalent to that under the ERO model. We prove that RSA-KEM issecure in the ERO model.

Illuminant Color Estimation by Hue Categorization Basedon Gray World Assumption
H. Kawamura, S. Yonemura, J. Ohya, and N. Matsuura
Proc. of the SPIE, Vol. 7873, pp. 787312–787312-12, San Francisco, USA, 2011.
This paper proposes a gray-world-assumption-based method for
estimating an illuminant color from an image by hue categorization.
The gray world assumption hypothesizes that the average color of all
the objects in a scene is gray. However, it is difficult to estimate an
illuminant color correctly if the colors of the objects in a scene aredominated by certain colors. To solve this problem, our method uses
the opponent color properties that the average of a pair of opponent
colors is gray. Thus, our method roughly categorizes the colors
derived from the image based on hue and selects them one by one
from the hue categories until selected colors satisfy the gray world
assumption. In our experiments, we used three kinds of illuminants(i.e., CIE standard illuminants A and D65 and a fluorescent light) andtwo kinds of data sets. One data set satisfies the gray world assumption and the other does not. Experimental results show that estimatedilluminants are closer to the correct ones than those obtained with theconventional method and the estimation errors for using CIE standard
illuminants A and D65 in our method are within the barely noticeable
difference in human color perception.

Optical Spectrum Control Circuit Using an Arrayed-waveguide Grating and Tunable Phase Shifters
K. Kato, Y. Ikuma, H. Takahashi, T. Mizuno, and H. Tsuda
IEICE Electronics Express, Vol. 8, No. 6, pp. 391–396, 2011.
We proposed and fabricated an optical spectrum control circuit
using an arrayed-waveguide grating (AWG) and an array of channel
waveguides with tunable phase shifters. We found that the spectralphase and amplitude of a modulated optical signal could be arbitrarily controlled if the number of channel waveguides was set to be
more than twice the number of waveguides in the AWG. As a first
demonstration, we successfully obtained a flat band-pass filter function with the fabricated device by controlling the tunable phase shifters to control the interference between the light propagating throughVol. 9 No. 7 July 2011

them.

Efficient Combination of Likelihood Recycling and Batch
Calculation for Fast Acoustic Likelihood Calculation
A. Ogawa, S. Takahashi, and A. Nakamura
IEICE Trans. Information and Systems, Vol. E94-D, No. 3, pp.
648–658, 2011.
This paper proposes an efficient combination of state likelihood
recycling and batch state likelihood calculation for accelerating
acoustic likelihood calculation in an HMM-based speech recognizer.
Recycling and batch calculation are based on different technicalapproaches, i.e., the former is a purely algorithmic technique while
the latter fully exploits the computer architecture. To accelerate therecognition process further by combining them efficiently, we introduce conditional fast processing and acoustic backing-off. Conditional fast processing is based on two criteria. The first criterion,
potential activity, is used to control not only the recycling of state
likelihoods at the current frame but also the precalculation of state
likelihoods for several succeeding frames. The second criterion, reliability, and acoustic backing-off are used to control the choice ofrecycled or batch-calculated state likelihoods when they are contradictory in the combination and to prevent word accuracies fromdegrading. Large vocabulary spontaneous speech recognition experiments using four machines with different CPUs under two environmental conditions showed that, compared with the baseline recognizer, recycling and batch calculation (our combined acceleration
technique) further reduced both of the acoustic likelihood calculation
time and the total recognition time. We also performed detailed
analyses to reveal each technique’s acceleration and environmental
dependency mechanisms by classifying types of state likelihoods andcounting each of them. The analysis results confirmed the effectiveness of the combined acceleration technique.

Efficient Optical Flow Estimation Method Using Optimal
Weighting Parameter of Sinusoidal Pattern
H. Sakaino
The Institute of Image Information and Television Engineers, Vol.
65, No. 3, pp. 382–394, 2011 (in Japanese).
In the Horn and Schunck optical flow method, a weighting parameter a of a motion smoothness constraint plays an important role indetermining the estimation accuracy of optical flow. However, conventional methods of optimizing a have been based on an empiricalselection or cross-validation, where a global optimization is done ata high computational cost. Thus, a more efficient optimization
method is needed. We first assume that real images can be approximated by a two-dimensional sinusoidal wave function on the basis ofan example of a previously used texture analysis. Two image features—the amplitude (standard deviation of image brightness) andwave number using the sinusoidal wave function—are used to analyze and model the relationship between the optimal a and two image
features. From the analyzed model, the optimal a can be used to
locally minimize the estimation error of optical flow. Because these
two simple image features of given real images are used, the optimal
a can be estimated efficiently. Experimental results for optical flow
estimation accuracy show that our proposed method outperforms
conventional optical flow ones.

Papers Published in Technical Journals and Conference Proceedings

A Rat Model for Measuring the Effectiveness of Transcranial Direct Current Stimulation Using fMRI
Y. Takano, T. Yokawa, A. Masuda, J. Niimi, S. Tanaka, and N.
Hironaka
Neurosci Lett., Elsevier, Vol. 491, No. 1, pp. 40–43, 2011.
Transcranial direct current stimulation (tDCS) is one of the noteworthy noninvasive brain stimulation techniques, but the mechanismof its action has remained unclear. With the aim of clarifying themechanism, we developed a rat model and measured its effectivenessusing fMRI. Carbon fiber electrodes were placed on the top of thehead over the frontal cortex as the anode and on the neck as the cathode. The stimulus was 400- or 40-μA current applied for 10 min after
a baseline recording in an anesthetized condition. The 400-μA stimulation significantly increased signal intensities in the frontal cortex
and nucleus accumbens. This suggests that anodal tDCS over thefrontal cortex induces neuronal activation in the frontal cortex and inits connected brain region.

Word Alignment with Synonym Information
H. Shindo, A. Fujino, and M. Nagata
Information Processing Society of Japan, Vol. 4, No. 2, pp. 13–22,
2011.
We present a novel framework for word alignment that incorporates monolingual synonym knowledge to improve word alignment
performance. We think that synonym information is helpful to overcome the data sparseness problem of word alignment since there arevarious lexical forms representing the same meaning in a bilingual
corpus. However, synonym relations depend heavily on context or
domain since a word in natural language is ambiguous. We designed
a synonym probabilistic model with a topic model, which uses synonym information according to the context. Moreover, we propose aword alignment framework that jointly trains our synonym modeland conventional bilingual model. The experimental results show thatour proposed method obtained better results compared with cases
where synonym or context information is not used.

ting problem by utilizing unlabeled data in the target domain with thelabeled data for both training of discriminative and generative models
composing a classifier. Our experimental results for text classification using three test collections confirmed that the classification
performance obtained with the proposed method was better than thatwith JESS-CM in most cases of the task setting.

Non-data-aided Wide-range Frequency Offset Estimatorfor QAM Optical Coherent Receivers
T. Nakagawa, M. Matsui, T. Kobayashi, K. Ishihara, R. Kudo, M.
Mizoguchi, and Y. Miyamoto
Proc. of OFC 2011, Vol. 2011, No. OMJ1, pp. 1–3, Los Angeles,USA.
We propose and experimentally demonstrate a novel blind frequency offset estimator for coherent quadrature amplitude modulation (QAM) receivers. Its frequency offset estimation range is morethan three times the conventional estimation range.

Wide-range BER Measurement Scheme by Estimating
BER of Discarded Frames for 10 G-EPON Systems
N. Ikeda, K. Terada, H. Uzawa, A. Miyazaki, S. Shigematsu, M.
Urano, and T. Shibata
Proc. of OFC 2011, Vol. 2011, No. OTh, pp. OThT6, Los Angeles,
USA.
This paper describes a new BER measurement method for obtaining the BER by estimating the number of error bits in discarded
frames by using the rate of discarded frames. The BER is obtained
precisely by the method.

Multi-sized Sphere Packing: Computational Modeling and
Formulation for the Packing Density in Containers
S. Yamada, J. Kanno, and M. Miyauchi
Information Processing Society of Japan, Vol. 4, No. 2, pp. 23–30,
2011 (in Japanese).
This article provides a mathematical formula for determining the
optimal sizes of two different sized spheres to maximize the packing
density when randomized loose packing is used in containers withvarious shapes. The formula was evaluated with numerous computer
simulations involving over a million of spheres.

Origin of Exciton Emissions from an AlN p-n Junction
Light-emitting DiodeY. Taniyasu and M. Kasu
Appl. Phys. Lett., Vol. 98, No. 131910, 2011.
Exciton emissions from an AlN light-emitting diode with an
improved emission efficiency of 1×10−4% were observed at 5.94 eV(208.7 nm) and 6.10 eV (203.2 nm) for current injection. The emission at 5.94 eV is attributed to an exciton emission originating fromthe crystal-field split-off valence band (CH-exciton emission).
Owing to the large carrier-phonon interaction, the CH-exciton emission is accompanied by its phonon replicas. The emission at 6.10 eV
is attributed to another exciton emission originating from heavy/lighthole valence bands (HH/LH-exciton emission). From the emission
energies, considering residual strain, the crystal-field splitting energy
was determined to be −165 meV.

Robust Semi-supervised Learning for Labeled Data Selection Bias
A. Fujino, N. Ueda, and M. Nagata
Information Processing Society of Japan, Vol. 4, No. 2, pp. 31–42,
2011.
We propose a robust semi-supervised learning method for designing good classifiers with a high generalization ability from labeleddata whose distribution differs largely from that of test data in a target
domain. Although JESS-CM is one of the most successful semisupervised learning methods that achieved the best published resultsin natural language processing tasks, it has an overfitting problem in
our task setting. We expect the proposed method to solve the overfit-

Measuring Sweeping Echoes in Rectangular Cross-section Reverberant Fields
K. Kiyohara, K. Furuya, Y. Haneda, and Y. Kaneda
Acta Acustica united with Acustica, Vol. 97, No. 2, pp. 278–283,
2011.
We investigated a new acoustical phenomenon, which we callsweeping echoes, in a two-dimensional (2D) space. Sweeping echoes
in a three-dimensional (3D) space have recently been reported. We
first investigated the regularity of reflected sound in a 2D regularly
shaped space on the basis of number theory. The reflected pulse
sound train has almost equal intervals between pulses on the squaredtime axis as in a 3D space. This regularity of the arrival time ofNTT Technical Review

Papers Published in Technical Journals and Conference Proceedings

reflected pulse sounds generates sweeping echoes whose frequencies
increase linearly with time. Computer simulation of room acoustics
shows good agreement with the theoretical results. We first describe
our number-theory-based investigation of a square cross-section.
Next, we describe rectangular cross-sections with various aspectratios investigated using the same theory as used for the square. We
also discuss our measurements of sweeping echoes in a long hallway.
We propose a method for extracting the sweep rates of sweeping
echoes by calculating their correlation with a time stretched pulse.
We analyzed the sweeping echoes for a source and receiver at thecenter of a rectangular cross-section. These sweeping echoes wereperceived not only at the exact center position but also around thecenter.

Underdetermined Convolutive Blind Source Separation
via Frequency Bin-wise Clustering and Permutation Alignment
H. Sawada, S. Araki, and S. Makino
IEEE Trans. Audio, Speech, and Language Processing, Vol. 19,
No. 3, pp. 516–527, 2011.
This paper presents a blind source separation method for convolutive mixtures of speech/audio sources. The method can even be
applied to an underdetermined case where there are fewer microphones than sources. The separation operation is performed in thefrequency domain and consists of two stages. In the first stage, frequency-domain mixture samples are clustered into each source by an
expectation-maximization (EM) algorithm. Since the clustering is
performed in a frequency bin-wise manner, the permutation ambiguities of the bin-wise clustered samples should be aligned. This issolved in the second stage by using the probability of each sample
belonging to the assigned class. This two-stage structure makes it
possible to attain good separation even under reverberant conditions.
Experimental results for separating four speech signals with three
microphones under reverberant conditions show the superiority of the
new method over existing methods. We also report separation results
for a benchmark data set and live recordings of speech mixtures.

Chapter 1. Integration of Statistical-model-based Voice
Activity Detection and Noise Suppression for Noise
Robust Speech Recognition
M. Fujimoto
Recent Advances in Robust Speech Recognition Technology, Bentham Science Publishers, 2011.
This chapter addresses robust front-end processing for automatic
speech recognition in noisy environments. To recognize corrupted
speech accurately, it is necessary to use methods that are robust
against various types of interference. Usually, noise suppression isused for the front-end processing of speech recognition in the presence of noise. Voice activity detection (VAD) is also used for frontend processing to eliminate the redundant non-speech period. VAD
and noise suppression are typically combined as series processing.
VAD and noise suppression should not be assumed to be separate
techniques because the output information of these methods is mutually beneficial. Thus, this chapter introduces the integrated front-end
processing of VAD and noise suppression, which can utilize eachother’s input-output information.

Vol. 9 No. 7 July 2011

Proposal of the Switching-control Algorithm for 3D-MEMS
Optical Switch Module and Its Demonstration
M. Minakami, J. Yamaguchi, and S. Nemoto
The Japan Society for Precision Engineering, Vol. 77, No. 4, p.
383, 2011 (in Japanese).
A switching-control algorithm is necessary in order to connect the
optical paths between input and output ports by using MEMS (microelectromechanical systems) mirrors in a free-space optical system.
We propose a search algorithm that finds the maximum optical powerby using the motion control technique of MEMS mirrors. We apply
the algorithm to peak search control for optical path connection withmaximum optical power and to optical power stabilization control in
the case of environment changes, such as temperature change. The
results confirm that the algorithm is suitable for practical use.

100Gb/s Ethernet Inverse Multiplexing Based on Aggregation at the Physical Layer
K. Hisadome, M. Teshima, Y. Yamada, and O. Ishida
IEICE Trans. Communications, Vol. E94-B, No. 4, pp. 904–909,
2011.
We propose a packet-based inverse multiplexing method to allow
scalable network access with a bigger-pipe physical interface. The
method is based on aggregation at the physical layer (APL) that fragments an original packet-flow and distributes the fragments among
an adequate number of physical links or networks. It allows us toshare wavelengths and/or bandwidth resources in optical networks.
Its technical feasibility at the speed of newly standardized 100Gb/s
Ethernet (100GbE) was successfully evaluated by implementing theinverse multiplexing logic functions on a prototype board. We demonstrated super-high-definition video streaming and huge file transfer by transmitting 100GbE MAC (media access control) frames overmultiple 10GbE physical links via inverse multiplexing.

Multi-layer Hypercube Photonic Network Architecture for
Intra-datacenter Network
T. Sakano, A. Kadohata, Y. Sone, A. Watanabe, and M. Jinno
IEICE Trans. Communications, Vol. E94-B, No. 4, pp. 910–917,
2011.
The popularity of cloud computing services is driving the boom inbuilding mega-datacenters. This trend is forcing significant increases
in the required scale of the intra-datacenter network. To meet thisrequirement, this paper proposes a photonic network architecture
based on a multi-layer hypercube topology. The proposed architecture uses the cyclic-frequency arrayed waveguide grating (CF-AWG)
device to realize a multi-layer hypercube and properly combinesseveral multiplexing systems that include time division multiplexing(TDM), wavelength division multiplexing (WDM), wave-band division multiplexing (WBDM), and space division multiplexing (SDM).
An estimation of the achievable network scale reveals that the proposed architecture can achieve a petabit-to-exabit-per-second-class,large-scale hypercube network with existing technologies.

Debating Diversity
M. Seyama
The Society of Polymer Science, Japan, Vol. 60, No. 5, p. 323,
2011 (in Japanese).
This paper provides comments about the debate on diversity andmakes a suggestion.

