• Steel and Composite Structures
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• v.19 no.5
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• pp.1115-1144
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• 2015

Current climate conditions along with advances in technology make further design and verification methods for structural strength and reliability of wind turbine towers imperative. Along with the growing interest for "green" energy, the wind energy sector has been developed tremendously the past decades. To this end, the improvement of wind turbine towers in terms of structural detailing and performance result in more efficient, durable and robust structures that facilitate their wider application, thus leading to energy harvesting increase. The wind tower industry is set to expand to greater heights than before and tapered steel towers with a circular cross-section are widely used as more capable of carrying heavier loads. The present study focuses on the improvement of the structural response of steel wind turbine towers, by means of internal stiffening. A thorough investigation of the contribution of stiffening rings to the overall structural behavior of the tower is being carried out. These stiffening rings are placed along the tower height to reduce local buckling phenomena, thus increasing the buckling strength of steel wind energy towers and leading the structure to a behavior closer to the one provided by the beam theory. Additionally to ring stiffeners, vertical stiffening schemes are studied to eliminate the presence of short wavelength buckles due to bending. For the purposes of this research, finite element analysis is applied in order to describe and predict in an accurate way the structural response of a model tower stiffened by internal stiffeners. Moreover, a parametric study is being performed in order to investigate the effect of the stiffeners' number to the functionality of the aforementioned stiffening systems and the improved structural behavior of the overall wind converter.

• Journal of the Korea Society for Simulation
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• v.17 no.4
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• pp.41-50
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• 2008

In this study, an improved scheme for distance-based registration (DBR) is proposed and its performance is analyzed. In the DBR, when a mobile station (MS) enters a new cell, it calculates the distance between last registered cell and current cell and registers its location if the distance reaches reference distance D. In this study, two-location DBR (TDBR) is proposed to improve the performance of the DBR. In the TDBR, an MS stores not only last registered location area (LA) but also previously registered LA, and then no registration is needed when the MS crosses two LAs stored already. However, since the TDBR may increase paging cost, trade-off is necessary between decreased registration cost and increased paging cost. In this study, the performances of two schemes are analyzed and compared using 2-dimensional random walk mobility model in hexagonal cell configuration. We show that our mathematical analysis is accurate by comparing with simulation. From the numerical results for various circumstances, it is shown that our proposed TDBR outperforms current DBR in most cases.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2007.04a
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• pp.577-582
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• 2007

A variational formulation for plane elasticity problems is derived based on an isogeometric approach. The isogeometric analysis is an emerging methodology such that the basis functions in analysis domain arc generated directly from NURBS (Non-Uniform Rational B-Splines) geometry. Thus. the solution space can be represented in terms of the same functions to represent the geometry. The coefficients of basis functions or the control variables play the role of degrees-of-freedom. Furthermore, due to h-. p-, and k-refinement schemes, the high order geometric features can be described exactly and easily without tedious re-meshing process. The isogeometric sensitivity analysis method enables us to analyze arbitrarily shaped structures without re-meshing. Also, it provides a precise construction method of finite element model to exactly represent geometry using B-spline base functions in CAD geometric modeling. To obtain precise shape sensitivity, the normal and curvature of boundary should be taken into account in the shape sensitivity expressions. However, in conventional finite element methods, the normal information is inaccurate and the curvature is generally missing due to the use of linear interpolation functions. A continuum-based adjoint sensitivity analysis method using the isogeometric approach is derived for the plane elasticity problems. The conventional shape optimization using the finite element method has some difficulties in the parameterization of boundary. In isogeometric analysis, however, the geometric properties arc already embedded in the B-spline shape functions and control points. The perturbation of control points in isogeometric analysis automatically results in shape changes. Using the conventional finite clement method, the inter-element continuity of the design space is not guaranteed so that the normal vector and curvature arc not accurate enough. On tile other hand, in isogeometric analysis, these values arc continuous over the whole design space so that accurate shape sensitivity can be obtained. Through numerical examples, the developed isogeometric sensitivity analysis method is verified to show excellent agreement with finite difference sensitivity.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.21 no.3
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• pp.281-285
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• 2008

Eigenvalue reduction schemes approximate the lower eigenmodes that represent the global behavior of the structures. In the previous study, we proposed a two-level condensation scheme (TLCS) for the construction of a reduced system. And we have improved previous TLCS with combination of the iterated improved reduced system method (IIRS) to increase accuracy of the higher modes intermediate range. In this study, we apply previous improved TLCS to multi-level sub-structuring scheme. In the first step, the global system is recursively partitioned into a hierarchy of sub-domain. In second step, each uncoupled sub-domain is condensed by the improved TLCS. After assembly process of each reduced sub-eigenvalue problem, eigen-solution is calculated by Lanczos method (ARPACK). Finally, Numerical examples demonstrate performance of proposed method.

• Journal of Semiconductor Engineering
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• v.1 no.1
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• pp.1-12
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• 2020

Handling precise timing in high-speed transceivers has always been a primary design target to achieve better performance. Many different approaches have been tried, and one of those is utilizing the beneficial nature of injection locking. Though the phenomenon was not intended for building integrated circuits at first, its coupling effect between neighboring oscillators has been utilized deliberately. Consequently, the dynamics of the injection-locked oscillator (ILO) have been explored, starting from R. Adler. As many aspects of the ILO were revealed, further studies followed to utilize the technique in practice, suggesting alternatives to the conventional frequency syntheses, which tend to be complicated and expensive. In this review, the historical analysis techniques from R. Adler are studied for better comprehension with proper notation of the variables, resulting in numerical results. In addition, how the timing jitter or phase noise in the ILO is attenuated from noise sources is presented in contrast to the clock generators based on the phase-locked loop (PLL). Although the ILO is very promising with higher cost effectiveness and better noise immunity than other schemes, unless correctly controlled or tuned, the promises above might not be realized. In order to present the favorable conditions, several strategies have been explored in diverse applications like frequency multiplication, data recovery, frequency division, clock distribution, etc. This paper reviews those research results for clock multiplication and data recovery in detail with their advantages and disadvantages they are referring to. Through this review, the readers will hopefully grasp the overall insight of the ILO, as well as its practical issues, in order to incorporate it on silicon successfully.

• Journal of the Korea Concrete Institute
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• v.17 no.4 s.88
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• pp.521-526
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• 2005

Prestressing tendons in a nuclear containment building dome are non-axisymmetrically arranged in most cases. However, simple axisymmetric modeling of the containment has been often employed in practice to estimate structural behavior for the axisymmetric loadings such as an internal pressure. In this case, the axisymmetric approximation is required for the actual tendon arrangements in the dome. Some procedures are proposed that can implement the actual 3-dimensional tendon stiffness and prestressing effect into the axisymmetric model. Prestressing tendons, which are arranged in 3 or 2-ways depending on a containment type, are converted into an equivalent layer to consider the stiffness contribution in meridional and hoop directions. In order to reflect the prestressing effect, equivalent load method and initial stress method are devised and the corresponding loads or stresses are derived in terms of the axisymmetric model. In a companion paper, the proposed schemes are applied into CANDU and KSNP(Korean Standard Nuclear Power Plant) type containments and are verified through some numerical examples comparing the analysis results with those of the actual 3-dimensional model.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.17 no.8
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• pp.1811-1819
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• 2013

In this paper, we carried out a study on how to find an appropriate helper node for cooperative communications, the role of which is very import to enhance system throughput of wireless communication system. The busy tone cooperative MAC (BT-COMAC) protocol proposed in this paper is a new cooperative MAC protocol with a reactive helper node scheme and maximizes the benefits of existing schemes while making up for their shortcomings. We conducted performance evaluation of this new protocol using computer simulation experiment. System throughput in bps and channel access delay are utilized as performance measures. We used a random way point mobility model where every communication node moves independently one another, and slow fading channel where every communication node decided its transmission rate with received power basis. Numerical results show that the new MAC protocol enhances system throughput as much as 15% of the existing scheme.

• Journal of KIISE:Information Networking
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• v.30 no.1
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• pp.48-64
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• 2003

Recently, as the number of mobile users keeps increasing explosively, the location management to track the mobile users in mobile communication systems is being more important. However, the previous location management schemes have been used static location information without the consideration of a user's moving direction. This results in unnecessary pagings, consequently it increases location management costs. In this paper, we propose a mew location management costs. In this paper, we propose a new location the cell occurred a location update. It also determines whether the location update will be executed or not, by the offset operation of direction vector. Thus, a user's paging areas are generated dynamically along the user's moving direction. The wide of paging areas is also determined dynamically. Besides, we present analytic model for our scheme. To compare with our scheme, the distance-based scheme is analyzed. The numerical result shows that our scheme is more efficient than the distance-based scheme in the most cases except a low CMR. Particularly if a user move into a specific direction, our scheme has the lower location management cost than that of the distance-based scheme.

• KIPS Transactions on Computer and Communication Systems
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• v.1 no.3
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• pp.205-218
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• 2012

Mobility management of the mobile nodes and provision of seamless handover is crucial to an efficient support for the global roaming of the mobile nodes in next-generation wireless networks. Mobile IPv6 and mobility management in extended IP layer, which highly depend on traffic characteristics and user mobility models, were proposed by the IETF. Therefore, to evaluate the in-depth performance about these factors is important. Generally, the performance of IPv6-based mobility management protocol is evaluated through simulation. This paper shows the correlation between network parameters and performance metrics through numerical results, which is investigated how influence handoff latency and packet loss. And this paper uses mathematical analysis of the system parameters, such as the subnet residence time, the packet arrival rate and delay in wireless connection through the analytical framework which evaluate the performance of IPv6-based mobility management protocol.

• Journal of Satellite, Information and Communications
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• v.6 no.2
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• pp.20-25
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• 2011

The authors have proposed novel TDD-CDMA systems with Pre-Rake transmit diversity schemes (system 1 and system 2) using multiple transmit antennas in [2] and have also evaluated the system performance through the theoretical analysis and computer simulation. However, the performance of system 2 which transmit a signal using all antennas has not been evaluated for multi-user environment. Therefore in this paper, we analyze the performance of system 2 for multi-user environment and compare the performance with that of the already proposed system 1 which chooses only one antenna. From the numerical results, it is found that system 2 outperforms system 1 as the number of users increases while system 1 outperforms system 2 at a small number of users. Therefore in order to achieve the best system performance, the Pre-Rake transmit diversity type should be selected at the base station according to the number of users.