T. Ngo

Tohoku University, Japan

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Publications (5)11.07 Total impact

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    ABSTRACT: Recently, the issue of energy consumption of the UEs in heterogeneous networks has rapidly become a research focus area of the entire telecommunications community. This issue is obviously critical because the energy consumption of UEs can severely degrade their already limited battery capacity. In this article, we consider a heterogeneous network environment comprising base stations (each of which is also referred to as an eNB) with embedded storage that can serve as an effective cache-based traffic offloading technology in scenarios where many UEs simultaneously want to access popular contents of sports matches, live music events, and so forth. However, if many UEs are connected to only a few eNBs, they suffer from degraded throughput and increased transmission time. This longer transmission time eventually leads to increased energy consumption of UEs. To deal with this challenge, we propose an algorithm to reassign UEs to eNBs to minimize the total energy consumption of UEs with the constraint that their throughput is guaranteed. The effectiveness of our proposed algorithm is evaluated through computer-based simulations.
    IEEE Wireless Communications 08/2014; 21(4):70-76. · 3.74 Impact Factor
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    ABSTRACT: The movable and deployable resource unit (MDRU)-based network provides communication services in disaster-struck areas where the lack of spectrum and energy resources is intensified due to the high demand from users and the power outages after a disaster. The MDRU-based network attempts to apply spectrum- and energy-efficient methods to provide communications services to users. However, existing works in this field only consider spectrum efficiency or energy efficiency separately, in spite of the tradeoff relationship between them. Thus, we propose a scheme to improve the utilization of both spectrum and energy resources for better system performance. The considered MDRU-based network is composed of gateways deployed in the disaster area, which can replenish their energy by using solar panels. Our proposed scheme constructs a topology based on the top $k$ spectrum-efficient paths from each sender and applies a max flow algorithm with vertex capacities, which are the number of transmissions that each gateway can send, which is referred to as transmission capability. The transmission capability of each gateway is determined by its energy resource and distances to its neighbors. Furthermore, we show that the proposal can be used for multisender–multireceiver topologies. A new metric named spectrum–energy efficiency to measure both spectrum efficiency and energy efficiency of the network is defined. Through analyses, we prove that a value of $k$ exists such that the spectrum–energy efficiency of a given topology is maximized. Furthermore, our simulation results show that, by dynamically selecting appropriate value of $k$, the proposed scheme can provide better spectrum–energy efficiency than existing approaches. Moreover, our exper- mental results verify the findings of our analysis.
    IEEE Transactions on Vehicular Technology 01/2014; 63(5):2027-2037. · 2.06 Impact Factor
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    ABSTRACT: During the great east Japan earthquake on March 11, 2011, a lot of ICT resources - telecom switching offices, optical fiber links, and so forth - were completely or partially damaged due to the tremor and the resultant tsunami. As a consequence, the demand for ICT services explosively increased, mainly because the people of the affected areas were trying desperately to communicate with the outside world that led to a phenomenal rise in the network traffic. In the Nippon Telegraph and Telephone (NTT) East Corporation alone, 385 telephone offices stopped operating immediately following the earthquake because of power outages and disruption of facilities. Approximately 1.5 million users were cut off from using fixed-line telephone service. The demand for fixed-line and mobile telephone services jumped up to 10-50 times the usual. This gave rise to serious traffic congestion, and the emergency ICT networks and services could not deal with this issue sufficiently. This article proposes a network architecture that is resilient even through devastating disasters by effectively exploiting specially designed movable and deployable resource units, which we refer to as MDRUs. An MDRU having the ability to accommodate communication and information processing functions can be rapidly transported or moved to the disaster zone, and can be deployed within a reasonably short time to establish the network at the disaster site and launch ICT services. The concept and configuration of the network architecture based on the MDRU and its features are described in this article. Some preliminary simulation results are also reported to evaluate the performance of our adopted MDRU-based disaster resilient network.
    IEEE Network 01/2013; 27(4):40-46. · 2.85 Impact Factor
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    ABSTRACT: After disasters such as earthquakes and tsunamis, the network infrastructures might be extremely damaged or destroyed while Internet connection becomes much more necessary. Therefore, deploying networks in disaster areas has received much attention especially after the great earthquake in Japan on March 11, 2011. Among many kinds of networks, Wireless Mesh Network (WMN) is considered as one of the most suitable solutions because it can easily configure a network without any wired infrastructure. In our national project on disaster recovery network, we attempt to build a WMN connecting remaining routers (i.e., the routers that remain functional after the disaster) by using a Movable and Deployable Resource Unit (MDRU) as a base station, which has processing servers, storage servers, and Internet connectivity. However, in order to have a good network design, many experiments such as simulations need to be done beforehand. In this paper, we provide an adequate throughput evaluation of the deployed network with many configurations, which are close to reality. The results demonstrate that the network can, at the same time, provide basic Internet access to a significantly large population of users.
    Computing, Networking and Communications (ICNC), 2013 International Conference on; 01/2013
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    ABSTRACT: Although topology control has received much attention in stationary sensor networks by effectively minimizing energy consumption, reducing interference, and shortening end-to-end delay, the transience of mobile nodes in Mobile Ad hoc Networks (MANETs) renders topology control a great challenge. To circumvent the transitory nature of mobile nodes, k-edge connected topology control algorithms have been proposed to construct robust topologies for mobile networks. However, uniformly using the value of k for localized topology control algorithms in any local graph is not effective because nodes move at different speeds. Moreover, the existing k-edge connected topology control algorithms need to determine the value of k a priori, but moving speeds of nodes are unpredictable, and therefore, these algorithms are not practical in MANETs. A dynamic method is proposed in this paper to effectively employ k-edge connected topology control algorithms in MANETs. The proposed method automatically determines the appropriate value of k for each local graph based on local information while ensuring the required connectivity ratio of the whole network. The results show that the dynamic method can enhance the practicality and scalability of existing k-edge connected topology control algorithms while guaranteeing the network connectivity.
    IEEE Transactions on Wireless Communications 03/2012; 11(3):1158-1166. · 2.42 Impact Factor