• KSII Transactions on Internet and Information Systems (TIIS)
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• 제8권7호
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• pp.2231-2245
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• 2014

Cognitive Radio (CR) has very promising potential to improve spectrum utilization by allowing unlicensed Secondary Users (SUs) to access the spectrum dynamically without disturbing licensed Primary Users (PUs). Mitigating interference is a fundamental problem in CR scenarios. This is particularly problematic for deploying CR in cellular networks, when users are located at the cell edge, as the inter-cell interference mitigation and frequency reuse are critical requirements for both PUs and SUs. Further cellular networks require higher cell edge performance, then SUs will meet more challenges than PUs. To solve the performance decrease for SUs at the cell edge, a novel Dynamic Spectrum Allocation (DSA) scheme based on Game Theory is proposed in this paper. Full frequency reuse can be realized as well as inter-cell interference mitigated according to SUs' sensing, measurement and interaction in this scheme. A joint power/channel allocation algorithm is proposed to improve both cell-edge user experience and network performance through distributed pricing calculation and exchange based on game theory. Analytical proof is presented and simulation results show that the proposed scheme achieves high efficiency of spectrum usage and improvement of cell edge SUs' performance.

• 대한토목학회논문집
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• 제11권3호
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• pp.1-10
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• 1991

본 논문에서는 개선된 감절점(degenerated) 쉘 유한요소의 복합적충을 갖는 쉘구조에의 적용성을 고찰하였다. 본 논문의 개선된 쉘 요소는 shear locking 해결에 우수한 결과를 보인 가정된 전단변형도를 대치사용하고, membrane locking 현상을 제거하기 위해 평면내 변형도의 구성시 감차적분을 행하며, 쉘요소 자체의 거동을 보완하기 위해 비적합변위형을 선택적으로 추가하였다. 본 요소는 shear/membrane locking이 발생하지 않으며, 전달가능한 거짓 영에너지모드도 나타나지 않는다. 유한변형을 고려한 기하학적 비선형 방정식을 total Lagrangian 수식화를 시용하여 정형화 하였고, 비선형 수치해석은 Newton-Raphson 반복법으로 반복 계산한다. 여러 예제해석을 통하여 본 개선된 쉘 유한요소의 유용성과 정확도를 고찰하였다.

• Steel and Composite Structures
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• 제28권3호
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• pp.331-347
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• 2018

In this paper history of stresses, strains, radial and circumferential displacements of a functionally graded thick-walled hollow cylinder due to creep phenomenon is investigated. The cylinder is subjected to an arbitrary non-axisymmetric two dimensional thermo-mechanical loading and uniform magnetic field along axial direction. Using equilibrium, strain-displacements and stress-strain relations, the governing differential equations of the problem containing creep strains are derived in terms of radial and circumferential displacements. Since the displacements are varying with time due to creep phenomenon, an analytical solution is not available for these equations. Thus, a semi-analytical procedure based on separation of variables and Fourier series together with a numerical procedure is employed. The numerical results indicate that the non-axisymmetric loading and the material grading index have significant effect on stress redistributions. Moreover, by proper selection of material for any combination of non-axisymmetric loading, one can arrive suitable response for the cylinder to achieve optimal design. With some simplifications, the results are validated with the existing literature.

• 한국정밀공학회지
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• 제34권2호
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• pp.115-123
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• 2017

This paper presents the design and the control performance of a novel dynamic compliant-arm support with parallel elastic actuators that was developed to assist with the daily living activities of those whose arms are compromised by muscular disease or the aging process. The parallel elastic-arm support consists of a compliant mechanism with combined passive and active components for human interaction and to reach the user's desired positions. The achievement of these tasks requires impedance control, which can change the virtual stiffness, damping coefficients, and equilibrium points of the system; however, the desired-position tracking by the impedance control is limited when the end-effector weight varies according to the equipping of diverse objects. A prompt algorithm regarding weight calibration and friction compensation is adopted to overcome this problem. A result comparison shows that, by accurately assessing the desired workspace, the proposed algorithm is more effective for the accomplishment of the desired activities.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제9권11호
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• pp.4291-4310
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• 2015

This paper investigates the traffic offloading over unlicensed bands for two-tier multi-mode small cell networks. We formulate this problem as a Stackelberg game and apply a hierarchical learning framework to jointly maximize the utilities of both macro base station (MBS) and small base stations (SBSs). During the learning process, the MBS behaves as a leader and the SBSs are followers. A pricing mechanism is adopt by MBS and the price information is broadcasted to all SBSs by MBS firstly, then each SBS competes with other SBSs and takes its best response strategies to appropriately allocate the traffic load in licensed and unlicensed band in the sequel, taking the traffic flow payment charged by MBS into consideration. Then, we present a hierarchical Q-learning algorithm (HQL) to discover the Stackelberg equilibrium. Additionally, if some extra information can be obtained via feedback, we propose an improved hierarchical Q-learning algorithm (IHQL) to speed up the SBSs' learning process. Last but not the least, the convergence performance of the proposed two algorithms is analyzed. Numerical experiments are presented to validate the proposed schemes and show the effectiveness.

• 자원ㆍ환경경제연구
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• 제17권1호
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• pp.147-164
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• 2008

주택에 포함된 여러 가지 환경적 요인에 대한 가치추정을 시도할 경우 전형적인 헤도닉 가격기법에서는 시장균형을 가정한다. 이때 개별 요인들의 추정가치는 시장가격에 근거한 암묵적 가치가 된다. 따라서 이 접근법의 논리는 기본적으로 현시선호이론을 배경으로 한다. 그런데 주택시장의 참여자들이 주택과 관련된 요인들에 대해 완전한 정보를 갖지 못하거나 판단이 애매할 경우에는 전형적인 헤도닉 접근법이 제공하는 추정치의 적절성에 대해 의문이 제기될 수 있다. 이 문제의 해결방안으로 퍼지 헤도닉 기법이 제안된 바 있다. 본 논문은 우리나라 주요 공항주변의 비행기 소음이 주택가격에 미치는 영향을 추정하기 위해 퍼지 헤도닉 기법을 응용함으로써 추정치에 포함된 불확실성 또는 모호성 부분을 파악하고자 하였다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제8권4호
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• pp.1324-1343
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• 2014

In recent years, the problem of spectrum mobility in Cognitive Radio (CR) Networks has been widely investigated. In order to utilize spectrum resources completely, many spectrum handoff techniques based on game theory have been proposed, but most studies only concern that users how to achieve better payoffs, without much attention to the diverse needs of users. In this paper, we propose a new channel-switching model based on game theory, using a prioritized approach to meet the diverse needs of users in two different modes (CQ and PS). At the same time, this paper proposes some acceleration techniques to reach the Nash equilibrium more efficiently. We evaluate the performance of the proposed schemes depending on priority using real channel availability measurements, and conclude that the channel quality function (CQ) mode provide better service for priority user but the plan-sorting (PS) mode can be more suitable in multiple priority users exist scene.

• Steel and Composite Structures
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• 제16권3호
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• pp.325-337
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• 2014

The strength and buckling problem of a five layer sandwich beam under axial compression or bending is presented. Two faces of the beam are thin aluminium sheets and the core is made of aluminium foam. Between the faces and the core there are two thin binding glue layers. In the paper a mathematical model of the field of displacements, which includes a share effect and a bending moment, is presented. The system of partial differential equations of equilibrium for the five layer sandwich beam is derived on the basis of the principle of stationary total potential energy. The equations are analytically solved and the critical load is obtained. For comparison reasons a finite element model of the beam is formulated. For the case of bended beam the static analysis has been performed to obtain the stress distribution across the height of the beam. For the axially compressed beam the buckling analysis was carried out to determine the buckling load and buckling shape. Moreover, experimental investigations are carried out for two beams. The comparison of the results obtained in the analytical and numerical (FEM) analysis is shown in graphs and figures. The main aim of the paper is to present an analytical model of the five layer beam and to compare the results of the theoretical, numerical and experimental analyses.

• 한국태양에너지학회 논문집
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• 제36권3호
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• pp.9-16
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• 2016

This paper discusses a refined model for investigating the micro-mechanical behavior of beam-like structures, which are composed of various elastic moduli and complex geometries varying through the cross-section directions and are also periodically-repeated and heterogeneous along the axial direction. Following the previous work (Lee and Yu, 2011), the original three-dimensional static problem is first formulated in a unified and compact form using the concept of decomposition of the rotation tensor. Taking advantage of the smallness of the cross-sectional dimension-to-length parameter and the micro-to-macro heterogeneity, while also performing homogenization along the dimensional reduction simultaneously, the variational asymptotic method is rigorously used to construct a total energy function, which is asymptotically correct up to the second order. Furthermore, through the transformation procedure based on the pure kinematic relations and the linearized equilibrium equations, a generalized Timoshenko model is systematically established. For the purpose of dealing with realistic and complex geometries and constituent materials at the microscopic level, this present approach is incorporated into a commercial analysis package. A few examples available in literature are used to demonstrate the consistency and efficiency of this proposed model, especially for the structures, in which the effects of transverse shear deformations are significant.

• 복합신소재구조학회 논문집
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• 제2권3호
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• pp.25-29
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• 2011

몇 가지 섬유 배향각을 가진 적층판은 층수가 증가하면 $D_{16}$, $B_{16}$, $D_{26}$ 및 $B_{26}$ 강성이 감소하게 되어 특별직교이방성 판처럼 거동함을 밝히고, 간단한 공식들을 개발하여 발표한 바 있다. 대부분의 교량이나 건물의 상판은 형상비가 큰 경우가 많은데, 이런 구조물의 평형방정식에 대한 종방항 모멘트항(Mx)의 영향은 매우 작아서, 더욱 간단한 해석이 가능하다. 본 논문에는 이러한 문제들에 대한 연구결과를 제시하였다.