• KSII Transactions on Internet and Information Systems (TIIS)
• /
• v.13 no.5
• /
• pp.2357-2380
• /
• 2019

In this paper, we investigate the impact of hardware impairments (HWIs) on the performance of a downlink massive MIMO system. We consider a single-cell system with maximum ratio transmission (MRT) as precoding scheme, and with all the HWIs characteristics such as phase noise, distortion noise, and amplified thermal noise. Based on the system model, we derive closed-form expressions for a typical user data rate under two scenarios: when a common local oscillator (CLO) is used at the base station and when separated oscillators (SLOs) are used. We also derive closed-form expressions for the downlink transmit power required for some desired per-user data rate under each scenario. Compared to the conventional system with ideal transceiver hardware, our results show that impairments of hardware make a finite upper limit on the user's downlink channel capacity; and as the number of base station antennas grows large, it is only the hardware impairments at the users that mainly limit the capacity. Our results also show that SLOs configuration provides higher data rate than CLO at the price of higher power consumption. An approach to minimize the effect of the hardware impairments on the system performance is also proposed in the paper. In our approach, we show that by reducing the cell size, the effect of accumulated phase noise during channel estimation time is minimized and hence the user capacity is increased, and the downlink transmit power is decreased.

• Journal of the Microelectronics and Packaging Society
• /
• v.25 no.4
• /
• pp.89-93
• /
• 2018

Ceramic substrates applied to power modules of electric vehicles are required to have properties of high thermal conductivity, high electrical insulation, low thermal expansion coefficient and resistance to abrupt temperature change due to high power applied by driving power. Aluminum nitride and silicon nitride, which are applied to heat dissipation, are considered as materials meeting their needs. Therefore, in this paper, the properties of aluminum nitride and silicon nitride as radiator plate materials were compared through a commercial analysis program. As a result, when the process of applying heat of the same condition to aluminum nitride was implemented by simulation, the silicon nitride exhibited superior impact resistance and stress resistance due to less stress and warping. In terms of thermal conductivity, aluminum nitride has superior properties as a heat dissipation material, but silicon nitride is more dominant in terms of reliability.

• Journal of The Korean Society of Integrative Medicine
• /
• v.12 no.1
• /
• pp.97-108
• /
• 2024

Purpose : In this study, we aimed to determine the impact of the COVID-19 pandemic on nursing students' adjustment to college life by focusing on their empathic ability, perceived stress, and resilience. Methods : We applied a descriptive survey research design, which included a self-report questionnaire. The participants comprised 307 nursing students in B city. The data were analyzed by calculating the percentages, means, standard deviations, t-tests, ANOVA, Scheffé test, Pearson's correlation coefficients, and hierarchical regression using SPSS 23.0. Results : The participants' empathic ability score was 3.30±.42, perceived stress score 1.85±.49, resilience score 3.44±.64, and adjustment to college life score 3.25±.52. Adjustment to college life was positively correlated with resilience (r=.43, p<.001) but negatively correlated with perceived stress (r=.27, p<.001). Factors affecting adjustment to college life include, among general characteristics in Model 1, in descending order, major satisfaction-satisfied (β=.54, p<.001), interpersonal conflict: never (β=.26, p=.018), health status: healthy (β=.25, p=.002), character: positive (β=.21, p=.006), character: optimistic (β=.19, p=.015), parents' economic power: high (β=.15, p=.047), and gender: male (β=.11, p=.016). Model 1 was statistically significant (F=11.67, p<.001), and the explanatory power was 41 %. In Model 2, empathic ability, perceived stress, and resilience were added as independent variables. When including the dependent variables, the factors that most influenced adjustment to college life were perceived stress (β=-.37, p<.001), major satisfaction-satisfied (β=.36, p<.001), health status-healthy (β=.25, p<.001), gender-male (β=.10, p=.015), and resilience (β=.10, p=.029). Model 2 was statistically significant (F=17.65, p<.001), and the explanatory power was 56 %. Conclusion : We found that gender, major satisfaction, health status, perceived stress, and resilience affected adjustment to college life among nursing students who had experienced the COVID-19 pandemic. To increase their ability to adjust to college life, a gender-specific intervention program should be developed that can improve the students' health status, major satisfaction and resilience, and reduce their perceived stress.

• Journal of the Institute of Electronics Engineers of Korea SD
• /
• v.44 no.12
• /
• pp.1-6
• /
• 2007

In this study, we have performed the channel modification of the conventional MHEMT (metamorphic high electron mobility transistor) to improve the breakdown characteristics. The Modified channel consists of the InxGal-xAs channel and the InP sub channel instead of the InxGa1-xAs channel. Since InP has the lower impact ionization coefficient in comparison with In0.53Ga0.47As, we have adopted the InP-composite channel in the modified MHEMT. We have investigated the breakdown mechanism and the RF characteristics for the conventional and the InP- composite channel MHEMTs. From the measurement results, we have obtained the enhanced on and off-state breakdown voltages of 2.4 and 5.7 V, respectively. Also, the increased RF characteristics have brought about the decreased output conductance for the InP-composite channel MHEMT. The cut-off frequency (fT) and the maximum oscillation frequency (fmax) for the InP-composite Channel MHEMT were 160 GHz and 230 GHz, respectively. It has been shown that the InP-composite channel MHEMT has the potential applications for the millimeter wave power device.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.19 no.1
• /
• pp.69-74
• /
• 2018

With the increasing development of industrial society and the availability of high quality electrical energy, the simplification of operation and maintenance procedures is required, in order to ensure the reliability and safety of electrical systems. In this paper, the dielectric breakdown characteristics of $N_2-O_2$ mixed gas solid insulation, which is used as an alternative to SF6 in various electric power facilities, are verified. When the gas mixture has a composition ratio similar to that of the atmosphere, the dielectric breakdown characteristics are relatively stabilized. It was confirmed that the breakdown voltage of the gas in the electrode near an equal electric field increased with increasing pressure according to Paschen's rule. The breakdown voltage of the surface increased linearly with increasing pressure, and the difference was caused by the mixing ratio of $O_2$ gas. This change in the surface insulation breakdown voltage was caused by the influence of the electrically negative $O_2$ gas and the intermolecular collision distance. In this study, the influence of the intermolecular impact distance was larger (than that in the absence of the electrically negative $O_2$ gas). The breakdown voltage relation applicable to Teflon according to the surface insulation characteristics was calculated. The characteristics of the surface insulation properties of Teflon, which is used as a solid insulation material, were derived as a function of pressure. It is thought that these results can be used as the basic data for the insulation design of electric power facilities.

• Clean Technology
• /
• v.23 no.1
• /
• pp.54-63
• /
• 2017

Volatile organic compounds (VOCs) are widely used in both industrial and domestic activities. VOCs are one of the most unpleasant, frequently complaint-rousing factors of pollution around the world. It is now necessary to research and develop an alternative technology that could overcome the problems of the existing odor-control and VOC-eliminating techniques. In this study, essential oil and photocatalytic process was applied in the removal of benzene and toluene, typical VOCs in petrochemistry plant. therefore, this study conducted experiments on the selection of appropriate essential oil, photodegradation, hydroxyl radical generation capacity. The removal efficiency and reaction rate were performed to selecte the type and concentration of essential oil. As a result, removal efficiency of Hinoki Cypress oil was approximately 70% and reaction rate of Hinoki Cypress was high. The results of photolysis experiment, photocatalytic oxidation process showed that the decomposition efficiency of VOCs increased considerably with increasing UV lamp power. In addition, the conversion of VOCs was increased up to $0.1gL^{-1}$ photocatalysts. The hydroxyl radicals measure was performed to determine the ability to generate hydroxyl radicals. The analytical result showed that high $TiO_2$ concentration and lamp power was produced many hydroxyl radical. Experiments of the removal efficiency and reaction rate were performed using essential oil and photooxidation. As a result, the removal efficiency showed that the removal efficiency was increased high temperature and reaction time. The activation energy was calculated from the reaction rate equation at various temperature condition. Activation energy was approximately $18kJmol^{-1}$.

• Journal of Biomedical Engineering Research
• /
• v.28 no.2
• /
• pp.187-194
• /
• 2007

The purpose of this study is to determine the characteristics of ground reaction force(GRF) in golf swing for various slopes of flat lie and uphill lies of 5 and 10 degrees. Five right-handed professional golfers were selected for the experiment and the 7 iron club was used. We used four forceplates to measure GRF and synchronized with the three-dimensional motion analysis system. Results showed that slope did not affect the total time for golf swing, but the time until the impact had a tendency to slightly increase for the uphill lie(p<0.05). The medial-lateral GRF of the right foot increased toward the medial direction during back swing, but less increases were found with the angle of uphill lie(p<0.05). The GRF of the left foot increased rapidly toward the medial direction at the uncocking and the impact during down swing, but decreased with the increase in the angle of uphill lie(p<0.05). The anterior-posterior GRF of both feet showed almost the same for different slopes. With the slopes, the vertical GRF of the right foot increased, but the vertical GRF of left foot decreased(p<0.05). Uphill lies would have negative effect to provide the angular momentum during back swing, restricting pelvic and trunk rotations, and to provide the precise timing and strong power during down swing, limiting movements of body's center of mass. The present study could provide valuable information to quantitatively analyze the dynamics of golf swing. Further study would be required to understand detailed mechanism in golf swing under different conditions.

• International Commerce and Information Review
• /
• v.14 no.3
• /
• pp.351-379
• /
• 2012

Understanding the cultures of other countries is not as fast as the speed of internationalization. As globalization advances, interest and research on the characteristics of national culture differences have been increasing. One of the important challenges for business leaders is the ability to understand and deal with cross-cultural management. But there is a lack of research on cross-cultural management and suitable type of leadership. In order to achieve this objective, this study, adopts Hofstede cultural dimensions, and finds the impact of these kinds of leadership on organizational performance. Feminism, collectivism, power distance, and uncertainty avoidance were classified into four factors of cultural orientation that affect the selection of initiating structure and consideration leadership. In this research, among virtue factors of leadership, structure initiation and leadership consideration are focused on how to affect organizational effectiveness of the result factor by national culture. This research has concentrated on how leadership-styles among national-cultures have influence on organizational-effectiveness.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• v.7 no.8
• /
• pp.1933-1954
• /
• 2013

Multiple wireless devices are being widely deployed in Intelligent Transportation System (ITS) services on the road to establish end-to-end connection between vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) networks. Vehicular ad hoc networks (VANETs) play an important role in supporting V2V and V2I communications (also called V2X communications) in a variety of urban environments with distinct topological characteristics. In fact, obstacles such as big buildings, moving vehicles, trees, advertisement boards, traffic lights, etc. may block the radio signals in V2X communications. Their impact has been neglected in VANET research. In this paper, we present a realistic and efficient radio propagation model to handle different sizes of static and moving obstacles for V2X communications. In the proposed model, buildings and large moving vehicles are modeled as static and moving obstacles, and taken into account their impact on the packet reception rate, Line-of-sight (LOS) obstruction, and received signal power. We use unsymmetrical city map which has many dead-end roads and open faces. Each dead-end road and open faces are joined to the nearest edge making a polygon to model realistic obstacles. The simulation results of proposed model demonstrates better performance compared to some existing models, that shows proposed model can reflect more realistic simulation environments.

• Korean Journal of Materials Research
• /
• v.22 no.1
• /
• pp.46-53
• /
• 2012

In this study, mechanical tests and microstructural analyses including TEM analyses with EDX of precipitates in modified 9Cr-1Mo steel were carried out to determine the cause of embrittlement observed after heat-treatment, which limits the usage of the alloy for power plants. Mod. 9Cr-1Mo steel specimens at austenite temperature were quenched to the molten salt baths at $760^{\circ}C$ and $700^{\circ}C$, in which the specimens were kept for 10 min ~ 10 hr with subsequent air-cooling. Impact tests showed that the impact value dropped abruptly when the specimens were kept longer than 30 min at $\sim760^{\circ}C$ reaching to minima in about 1 hr, and then increasing at further retention. The tensile strength of the specimens reached the minimum value without much change afterward, whereas the values of elongation showed the same trend as that of the impact value. The isothermally heat-treated steel at $700^{\circ}C$ also showed a minimum impact value in about 1 hr. These results suggest that the isothermal heattreatment at 760 and $700^{\circ}C$ for about 1 hr induces temporal embrittlement in Mod. 9Cr-1Mo steel. The microstructural examination of all the specimens with extraction replica of the carbides revealed that the specimens with temporal embrittlement had $Cr_2C$, indicating that the cause of the embrittlement was the precipitation of the $Cr_2C$. In addition, TEM/EDX results showed that the Fe/Cr ratio was 0.033 to 0.055 for $Cr_2C$, whereas it was 0.48 to 0.75 for $Cr_{23}C_6$, making the distinction of the $Cr_2C$ and $Cr_{23}C_6$ possible even without direct electron diffraction analyses.