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Takeo Fujii

Bio: Takeo Fujii is an academic researcher from University of Electro-Communications. The author has contributed to research in topics: Cognitive radio & Wireless. The author has an hindex of 18, co-authored 363 publications receiving 1678 citations. Previous affiliations of Takeo Fujii include Tokyo University of Agriculture and Technology & Keio University.


Papers
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Journal ArticleDOI
TL;DR: It is shown that the proposed probabilistic interference constraint method with a radio environment map (REM) for spatial spectrum sharing has a higher spectrum sharing opportunity than the path loss-based method, even if only a small amount of measurement data is available.
Abstract: This paper proposes a probabilistic interference constraint method with a radio environment map (REM) for spatial spectrum sharing. The REM stores the spatial distribution of the average received signal power. We can optimize the accuracy of the measurement-based REM using the Kriging interpolation. Although several researchers have maintained a continuous interest in improving the accuracy of the REM, sufficient study has not been done to actually explore the interference constraint considering the estimation error. The proposed method uses ordinary Kriging interpolation for the spectrum cartography. According to the predicted distribution of the estimation error, the allowable interference power to the primary user is approximately formulated. Numerical results show that the proposed method can achieve the probabilistic interference constraint asymptotically. Additionally, we compare the performance of the proposed technique with three methods: the perfect estimation, the path loss-based method, and the Kriging-based method without the error prediction. The comparison results show that the proposed method has a higher spectrum sharing opportunity than the path loss-based method, even if only a small amount of measurement data is available. It is also shown that the proposed method dramatically improves the outage probability of the interference power compared to the conventional Kriging-based method.

68 citations

Proceedings ArticleDOI
05 Dec 2005
TL;DR: A multi-hop system that autonomously avoids the transmission in the interference area by recognizing the radio environment in each relay node is presented and the frequency sharing system with adaptive route selection according to the surrounding radio environment can be realized.
Abstract: In this paper, we focus on the cognitive radio, which reconfigures the radio transceiver according to the surrounding radio environment. We introduce a development to an ad-hoc cognitive radio that can realize a frequency sharing system under existing communication systems using a multi-hop small power communication. Here, we discuss the surrounding radio environment recognition methods in the distributed terminal cooperated with the media access protocols. In this paper, we present a multi-hop system that autonomously avoids the transmission in the interference area by recognizing the radio environment in each relay node. By using the proposed system, the frequency sharing system with adaptive route selection according to the surrounding radio environment can be realized

63 citations

Journal ArticleDOI
TL;DR: A soft decision based cooperative sensing method for cognitive radio (CR) networks for opportunistic frequency usage is proposed and it is presented that detection performance of soft decisionbased CS outperform detectionperformance of hard decision based CS.
Abstract: SUMMARY In this paper, we propose a soft decision based cooperative sensing method for cognitive radio (CR) networks for opportunistic frequency usage To identify unused frequency, CR should exploit sensing technique to detect presence or absence of primary user and use this information to opportunistically provide communication among secondary users while performance of primary user should not be deteriorated by the secondary users Because of multipath fading or shadowing, the detection of primary users may be significantly difficult For this problem, cooperative sensing (CS), where gathered observations obtained by multiple secondary users is utilized to achieve higher performance of detection, has been investigated We design a soft decision based CS analytically and analyze the detector in several situations, ie, signal model where single-carrier case and multi-carrier case are assumed and two scenarios; in the first scenario, SNR values of secondary users are totally equal and in the second scenario, a certain SNR difference between secondary users is assumed We present numerical results as follows The first scenario shows that there is little difference between the signal models in terms of detection performance The second scenario shows that CS is superior to non-cooperative sensing In addition, we presents that detection performance of soft decision based CS outperform detection performance of hard decision based CS

39 citations

Journal ArticleDOI
TL;DR: This paper addresses the research question: can feedforward neural network (FFNN)-based path loss modeling improve the accuracy of Kriging and shows that the FFNN is capable of improving Kriged in such a distributed network case.
Abstract: This paper addresses the research question: can feedforward neural network (FFNN)-based path loss modeling improve the accuracy of Kriging? Radio propagation factors, which consist of path loss and shadowing, can accurately be obtained via crowdsourcing with Kriging In most works on Kriging-aided radio environment mapping, measurement datasets are first regressed via linear path loss modeling to ensure spatial stationarity of the shadowing However, in practical situations, the path loss often contains an anisotropy owing to terrain and obstacle effects Thus, Kriging may not perform an optimal interpolation because of the errors in path loss modeling In this paper, an FFNN is used for path loss modeling Then, ordinary Kriging is applied to interpolate the shadowing We first evaluate the performance of this method in a case where the transmitter is fixed It is shown that this method does not improve Kriging in a large-scale and fixed transmitter system; although the FFNN outperforms OLS in path loss modeling Then, this method is extended to distributed wireless networks where transmitters are arbitrarily located, such as in mobile ad hoc networks (MANETs) and vehicular ad hoc networks (VANETs) The results of a measurement-based experiment show that the FFNN is capable of improving Kriging in such a distributed network case

36 citations

Proceedings ArticleDOI
01 Dec 2011
TL;DR: This demonstration will first present animated results of a TV spectrum measurement campaign along the entire portion of Interstate I-90 located in the US state of Massachusetts, and demonstrate a cyber-physical proof-of-concept lab implementation of the previously developed control and data channel assignment schemes for vehicle-to-vehicle communications over (TV) white space.
Abstract: Future vehicular communications systems are expected to utilize the vacant channels (white spaces) of the spectrum, otherwise allocated for specific designated use. One such candidate of white space comes from the TV broadcast band. In this demonstration, we will first present animated results of a TV spectrum measurement campaign along the entire portion of Interstate I-90 located in the US state of Massachusetts. Next, we will demonstrate a cyber-physical proof-of-concept lab implementation of our previously developed control and data channel assignment schemes for vehicle-to-vehicle communications over (TV) white space. Finally we will show a video of actual vehicle to vehicle communications field tests conducted in Japan using TV white space.

35 citations


Cited by
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Journal ArticleDOI

[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

Journal ArticleDOI

6,278 citations

Journal ArticleDOI
TL;DR: Recent advances in research related to cognitive radios are surveyed, including the fundamentals of cognitive radio technology, architecture of a cognitive radio network and its applications, and important issues in dynamic spectrum allocation and sharing are investigated in detail.
Abstract: With the rapid deployment of new wireless devices and applications, the last decade has witnessed a growing demand for wireless radio spectrum. However, the fixed spectrum assignment policy becomes a bottleneck for more efficient spectrum utilization, under which a great portion of the licensed spectrum is severely under-utilized. The inefficient usage of the limited spectrum resources urges the spectrum regulatory bodies to review their policy and start to seek for innovative communication technology that can exploit the wireless spectrum in a more intelligent and flexible way. The concept of cognitive radio is proposed to address the issue of spectrum efficiency and has been receiving an increasing attention in recent years, since it equips wireless users the capability to optimally adapt their operating parameters according to the interactions with the surrounding radio environment. There have been many significant developments in the past few years on cognitive radios. This paper surveys recent advances in research related to cognitive radios. The fundamentals of cognitive radio technology, architecture of a cognitive radio network and its applications are first introduced. The existing works in spectrum sensing are reviewed, and important issues in dynamic spectrum allocation and sharing are investigated in detail.

1,329 citations

Journal ArticleDOI
28 Apr 2009
TL;DR: The use of cooperative spectrum sensing in cognitive radio systems to enhance the reliability of detecting primary users and a cognitive space-time-frequency coding technique that can opportunistically adjust its coding structure by adapting itself to the dynamic spectrum environment are considered.
Abstract: Cognitive radio is an exciting emerging technology that has the potential of dealing with the stringent requirement and scarcity of the radio spectrum. Such revolutionary and transforming technology represents a paradigm shift in the design of wireless systems, as it will allow the agile and efficient utilization of the radio spectrum by offering distributed terminals or radio cells the ability of radio sensing, self-adaptation, and dynamic spectrum sharing. Cooperative communications and networking is another new communication technology paradigm that allows distributed terminals in a wireless network to collaborate through some distributed transmission or signal processing so as to realize a new form of space diversity to combat the detrimental effects of fading channels. In this paper, we consider the application of these technologies to spectrum sensing and spectrum sharing. One of the most important challenges for cognitive radio systems is to identify the presence of primary (licensed) users over a wide range of spectrum at a particular time and specific geographic location. We consider the use of cooperative spectrum sensing in cognitive radio systems to enhance the reliability of detecting primary users. We shall describe spectrum sensing for cognitive radios and propose robust cooperative spectrum sensing techniques for a practical framework employing cognitive radios. We also investigate cooperative communications for spectrum sharing in a cognitive wireless relay network. To exploit the maximum spectrum opportunities, we present a cognitive space-time-frequency coding technique that can opportunistically adjust its coding structure by adapting itself to the dynamic spectrum environment.

1,176 citations

Journal ArticleDOI
Ning Lu1, Nan Cheng1, Ning Zhang1, Xuemin Shen1, Jon W. Mark1 
TL;DR: The challenges and potential challenges to provide vehicle-to-x connectivity are discussed and the state-of-the-art wireless solutions for vehicle-To-sensor, vehicle- to-vehicle, motorway infrastructure connectivities are reviewed.
Abstract: Providing various wireless connectivities for vehicles enables the communication between vehicles and their internal and external environments. Such a connected vehicle solution is expected to be the next frontier for automotive revolution and the key to the evolution to next generation intelligent transportation systems (ITSs). Moreover, connected vehicles are also the building blocks of emerging Internet of Vehicles (IoV). Extensive research activities and numerous industrial initiatives have paved the way for the coming era of connected vehicles. In this paper, we focus on wireless technologies and potential challenges to provide vehicle-to-x connectivity. In particular, we discuss the challenges and review the state-of-the-art wireless solutions for vehicle-to-sensor, vehicle-to-vehicle, vehicle-to-Internet, and vehicle-to-road infrastructure connectivities. We also identify future research issues for building connected vehicles.

936 citations