• Geophysics and Geophysical Exploration
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• v.15 no.4
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• pp.235-244
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• 2012

Nuclear magnetic resonance (NMR) logging has been considered one of the most complicated nevertheless, one of the most powerful logging methods for the characterization on of both rocks and natural fluids in formation. NMR measures magnetized signals (polarization and relaxation) between the properties of hydrogen nucleus called magnetic moment and applied magnetic fields. The measured data set contains two important petrophysical properties such as density of hydrogen in the fluids inside the pore space and the distinct decay rate for fluid type. Therefore, after the proper data processing, key petrophysical information, not only the quantities and properties of fluids but also supplies of rock characterization in a porous medium, could be archived. Thus, based on this information, several ongoing researches are being developed in estimating aspects of reservoir productivity information, permeability and wettability since it is the key to having correct interpretation. This study goes through the basic theory of NMR at first, and then reviews NMR logging tools as well as their technical characteristics. This paper also briefly discusses the basic knowledge of NMR simulation algorithm by using Random walk.

• 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.

• Geophysics and Geophysical Exploration
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• v.16 no.2
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• pp.97-105
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• 2013

Nuclear magnetic resonance (NMR) logging has become an important technique for formation evaluation, detecting interaction signals between H protons and applied magnetic fields. Measured decay signals called relaxation, contain important information about density of H protons and different decay rate due to its fluid type in the sensitive area. Thus, petrophysical information such as porosity, permeability and wettability can be estimated through the interpretation of the decay signals. Many researches on random walk simulation have been published, since a simulation method based on random walk for solving exponential decays was adapted in the early of 1950. This study first makes a review on NMR simulation researches, explains two most important methods: simulation with or without considering magnetic field gradient. Lastly, the study makes a comparison between NMR simulation responses with and without magnetic field gradient to show the importance to consider magnetic gradient to analyze the effects of magnetic gradients on NMR responses.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.10 no.3
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• pp.91-95
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• 2010

Using artificial neural network to use a search preference based on the user's information, the ranking of search results that will enable flexible searches can be improved. After trained in several different queries by other users in the past, the actual search results in order to better reflect the use of artificial neural networks to neural network learning. In order to change the weights constantly moving backward in the network to change weights of backpropagation algorithm. In this study, however, the initial training, performance data, look for increasing the number of lessons that can be overfitted. In this paper, we have optimized a lot of objects that have a strong advantage to apply genetic algorithms to the relevant page of the search rankings flexible as an object to the URL list on a random selection method is proposed for the study.

• The Journal of the Korea institute of electronic communication sciences
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• v.4 no.4
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• pp.265-269
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• 2009

A conference key agreement system is a scheme to generate a session key in a contributory manner in order to communicate with each other securely among participants. In this paper an efficient conference key agreement system is proposed by employing symmetric balanced incomplete block design(SBIBD), one class of block designs. The protocol presented not only minimizes the message overhead and message exchanging rounds but also makes every participant contribute evenly for generating a conference key. Our protocol constructs a conference key which takes modified Diffe-Helman form of ${\prod}_{i=0}^{v-1}R_i$, where v is the number of participants and $R_i$ is a random number generated from member i. In a special class of SBIBD, it takes only 3 rounds message exchange and message overhead is $O(v{\sqrt{v}})$. Our protocol can be proved as computationally difficult to calculate as discrete logarithms.

• Journal of the Korean Data and Information Science Society
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• v.21 no.6
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• pp.1155-1169
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• 2010

This study considers movement-based registration (MBR). In MBR, a mobile station (MS) performs location registration whenever the number of entering cells reaches the specified movement threshold M. MBR is simple and its implementation is quite straightforward. However, it may result in more registrations than other similar schemes. An improved MBR scheme called MBIR (MBR with implicit registration) was proposed to reduce registration cost of MBR and its approximated performance was analyzed. In this study, we point out some problems of the previous analysis method and propose an exact analysis method of MBIR. Using the exact analysis method, we show that the previous analysis includes considerable errors and improvement of MBIR in our study seems to be larger than in the previous study.

• Convergence Security Journal
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• v.8 no.4
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• pp.173-181
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• 2008

Recently, Worm epidemic models have been developed in response to the cyber threats posed by worms in order to analyze their propagation and predict their spread. Some of the most important ones involve mathematical model techniques such as Epidemic(SI), KM (Kermack-MeKendrick), Two-Factor and AAWP(Analytical Active Worm Propagation). However, most models have several inherent limitations. For instance, they target worms that employ random scanning in the network such as CodeRed worm and it was able to be applied to the specified threats. Therefore, we propose the probabilistic of worm propagation based on the Markov Chain, which can be applied to cyber threats such as Mass SQL Injection worm. Using the proposed method in this paper, we can predict the occurrence probability and occurrence frequency for each threats in the entire system.

• KDI Journal of Economic Policy
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• v.21 no.2
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• pp.55-103
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• 1999

본 연구의 목적은 외환위기의 주요 파급경로 중 하나인 금융부문의 환율위험 노출현상을 90년대 우리나라의 실제 데이터를 통하여 실증분석함에 있다. 동 연구를 수행함에 있어 장부상 나타나는 회계적 환율위험 대신 간접적인 경제적 위험을 포함하는 포괄적 환율위험을 분석의 대상으로 정의하고, 시장에서 평가되는 금융기관의 기업가치가 환율변동에 얼마나 민감하게 노출되어 있는가를 동 위험의 측정수단으로 채택하였다. 또한 랜덤워크모형과 더불어 일종의 자본자산가격결정모형(CAPM)에 환율위험요인을 추가하여 모형을 구성함으로써 실증분석의 이론적 적합성을 제고하였다. 시장평균환율제도가 채택된 90년 3월부터 최근까지를 표본기간으로 한 실증분석 결과는 다음과 같이 요약가능하다. 첫째, 산업별로는 은행 및 보험산업이 비교적 환율위험에 노출되지 않았던 반면, 종금 및 증권산업은 환율위험에 상대적으로 크게 노출되어 있었으며, 동 노출의 방향은 원화가치의 절하가 금융기관의 시장가치에 부정적인 영향을 미치는 방향으로 노출되어 있었다. 둘째, 종금 및 증권산업의 경우 95년 이전보다는 이후의 시기에 환율위험에 대한 노출도가 유의하게 나타나 90년대 후반기에 가속화된 자본거래 자유화가 이들 금융기관의 환율위험 노출정도를 증가시켰을 가능성을 시사하고 있다. 셋째, 은행 및 종금부문을 대상으로 개별 금융기관 주가의 패널자료를 이용하여 실증분석한 결과, 은행산업 또한 90년대에 걸쳐 환율위험에 유의하게 노출되어 있었던 것으로 분석되었다. 그러나 은행부문보다는 종금부문이 노출도의 크기나 통계적 유의도면에서 환율위험에 더욱 노출된 것으로 분석되었다. 넷째, 개별 금융기관의 환율위험 노출도를 추정한 결과 은행부문은 약 19%(상장은행 26사 중 5개), 종금부문은 약 52%(상장종금사 29사 중 15개)가 환율위험에 유의하게 노출되고 있었으며 이들 은행의 절반 이상 그리고 종금사의 대부분이 원화절하가 금융기관 시장가치에 부정적인 영향을 미치는 방향으로 노출되어 있었던 것으로 분석되었다. 동 실증분석 결과는 97년말 급격한 원화가치의 하락이 이들 금융기관, 특히 종금부문의 재무건전성 악화에 치명적인 영향을 끼침으로써 금융위기를 더욱 촉발시키는 한 메커니즘으로 작용하였음을 시사하고 있다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.1A
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• pp.34-40
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• 2009

In this paper, we derive the average received beampattern at each receive node which is randomly distributed within a disk when considering a collaborative beanforming between transmission nodes which are randomly distributed within another disk for ad hoc sensor network. Numerical results show that the radius of disk at the transmitter determines the beamwidth of mainbeam, and the sidebeam is affected by the number of transmitter nodes. We also consider K receive groups and divide the transmit sensor modes into K groups to support each receive group using beamforming, and derive the SINR at each receive node. Finally, we investigate the effects of beampattern and interference on total sum capacity among all receive node, and numerically determine the number of transmit nodes for each group which can achieve the maximal sum capacity.

• Journal of Korean Society of Transportation
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• v.21 no.3
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• pp.97-106
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• 2003

When a route choice is done under uncertainty, a driver has some expectation of traffic conditions that will occur according to the route chosen. This study tries to build a framework in which we can observe the learning behavior of the drivers' expectations of the travel time under nonstationary environment. In order to investigate how drivers have their subjective expectations on traffic conditions in response to public information, a numerical experiment is carried out. We found that rational expectations(RE) formation about the route travel time can be expressed by the adaptive expectation model when the travel time changes in accordance with the nonstationary process which consists of permanent shock and transient shock. Also, we found that the adaptive parameter of the model converges to the fixed value corresponding to the route conditions.