• Journal of Communications and Networks
• /
• v.7 no.4
• /
• pp.462-470
• /
• 2005

We propose a multi-rate and multi-BEP transmission scheme using adaptive overlapping pulse-position modulator (OPPM) and optical power controller in optical code division multiple access (CDMA) networks. The proposed system achieves the multi-rate and multi-BEP transmission by accommodating users with different values of OPPM parameter and transmitted power in the same network. The proposed scheme has advantages that the system is not required to change the code length and number of weight depending on the required bit rate of a user and the difference of bit rates does not have so much effect on the bit error probabilities (BEPs). Moreover, the difference of transmitted powers does not cause the change of bit rate. We analyze the BEPs of the four multimedia service classes corresponding to the com­binations of high/low-rates and low/high-BEPs and show that the proposed scheme can easily achieve distinct differentiation of the service classes with the simple system configuration.

• Journal of Korea Port Economic Association
• /
• v.34 no.4
• /
• pp.125-140
• /
• 2018

This study uses daily price data on three major types of crude oil (Brent, Dubai, and WTI) and BDI from January 2, 2009 to June 29, 2018, to compare the relationship between crude oil prices and BDI for rate of change and volatility. Unlike previous studies, the correlation between BDI and crude oil prices was analyzed both the rate of change and variability, VARs, Granger Causality Test, and the GARCH and DCC models were employed. The correlation analysis, indicated that the crude oil price change rate and volatility affect the BDI change rate and that BDI volatility affects the crude oil price change rate and volatility. The relationship between oil prices and BDI is identified, but their correlation is low, which is likely a result of lower dependence on crude oil as demand for natural gas increases worldwide and demand for renewable energy decreases. These trends could result in lower correlations over time. Therefore, focusing on the changing demand for raw materials in future investments in international shipping(real economy) and oil markets and macroeconomic analysis is necessary.

• Proceedings of the Korean Reliability Society Conference
• /
• 2005.06a
• /
• pp.115-120
• /
• 2005

This paper introduces models for preventive maintenance policies and considers periodic preventive maintenance policy with minimal repair when the failure of system occurs. It is assumed that minimal repairs do not change the failure rate of the system. The failure rate under prevention maintenance received an effect by a previously prevention maintenance and the slope of failure rate increases the model where it considered. Also the start point of failure rate under prevention maintenance considers the degradation of system and that it increases quotient, it assumed. Per unit time it bought an expectation cost from under this prevention maintenance policy. We obtain the optimal period time and the number for the periodic preventive maintenance by using Nakagawa's Algorithm, which minimizes the expected cost rate per unit time. Finally, it suppose that the failure time of a system has a Weibull distribution as an example and we obtain an expected cost rate per unit time the optimal period time and the number when cost of replacement and cost of minimal repair change.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.4 no.3
• /
• pp.187-198
• /
• 1996

In this study, the performance of an automobile heat storage system using PCM is numerically simulated. For the analysis of system performance. The phase-change of the PCM and the transient forced convective heat transfer for the HTF are considered simultaneously as a conjugate problem. The phase-change behavior is effectively analyzed using a concept of thermal resistance. From the correlations of phase change rate and heat transfer due to the variations of flow rate of HTF around PCM, the automobile heat storage system performance is predicted. The present results amy be used as the fundamental information for the design of automobile heat storage system.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.7 no.3
• /
• pp.353-358
• /
• 1995

The emission characteristics of a semi-Bunsen type burner for gas boilers were studied experimentally. The experimental results reveal that nitric oxide emission increases with fuel flow rate. It is linearly proportional to total fue flow rate at a small amount of fuel up to 0.4 liters per minute. It does not change significantly within the range of fuel flow rate from 0.4 to 1.2 liters per minute per nozzle and increases at large fuel flow rate. The carbon monoxide emission reveals to be dependent upon the fuel flow rate per each nozzle and the number of fuel injection nozzles. Diameter of an injection nozzle could have an effect on the emission characteristics of this type of burners. However, there is no marked change in the nitric oxide emission if the total fuel flow rate is same with different nozzle sizes.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.22 no.4
• /
• pp.675-684
• /
• 1997

The bit rate control algorithm which is capable of handing scene change is proposed. In MPEG-2 TM5, block variance is used to measure block activity. But block variance is not consistent with human visual system and does not differenciate the distribution of pixel values within the block. In target bit allocation process of TM5, global complexity, obtained by results of previous coded pictures, is used. Since I pictures are spaced relatively far apart, their complexity estimate is not very accurate. In the proposed algorithm local variance is used to measure block activity and detect scene change. Local variance, using deviation from the mean of neighboring pixels, well represents the distribution of pixel values within the block. If scene change is detected, the local variance information is used for target bit allocation process. Allocating target bits for I picture, the average local variance difference between previous and current I picture is considered. The experimental results show that the proposed algorithm can detect scene change very precisely and gives better picture quality and higher PSNR values than MPEG-2 TM5.

• Journal of Hydrogen and New Energy
• /
• v.1 no.1
• /
• pp.1-8
• /
• 1989

The hydriding kinetic mechanism and the change of the hydriding reaction rate of $MmNi_{4.5}Al_{0.5}$ during the thermally induced hydrogen absorption-desorption cycling are investigated. Comparison of the reaction rate data which are obtained by the pressure sweep method with the theoretical rate equations suggests that the hydriding rate controlling step has changed from the dissociative chemisorption of hydrogen molecules at the surface to the hydrogen diffusion through the hydride phase with the increase of the hydriding fraction. These hydriding kinetic mechanism is not changed during the cycling. However, the intrinsic hydriding reaction rate of $MmNi_{4.5}Al_{0.5}$ after 5500 cycles increases significantly comparing with the activated one. It is suggested that the change of the hydriding kinetic behavior due to intrinsic degradation of $MmNi_{4.5}Al_{0.5}$ can be interpreted as follows ; the formation of nickel cluster at the surface of the sample and the host metal atom exchange in bulk by thermal cycling.

• Proceedings of the Korean Society for Agricultural Machinery Conference
• /
• 1996.06c
• /
• pp.947-952
• /
• 1996

The effect of fluctuating temperature on the respiration of vegetables has been investigated. Spinach was selected as the experimental material because of its high respiratory activity and kept under the condition that temperature changed alternately at low and high levels every 4 hours. The low-high level temperature combination was set in $1-10^{\circ}C,{\;}1-20^{\circ}C{\;}and{\;}1-30^{\circ}C$. Respiration was evaluated in terms of quantity of $CO_2$ evolved from spinach. The evolution rate of $CO_2$ was determined by a change in its concentration. The evaluation rate of $CO_2$ followed closely the temperature change. In the temperature combinations at $1-10^{\circ}C{\;}and{\;}1-20^{\circ}C$, the relationship between $CO_2$ evolution rate and temperature was found to be able to express by Arrhenius law, while at $1-30^{\circ}C$, it did not obey the law.

• Journal of Korean Society for Quality Management
• /
• v.22 no.3
• /
• pp.119-123
• /
• 1994

A frequently recurring question posed by researchers concerns a test of a constant failure rate against the alternative of a failure rate involving a single chang-point. Park (1988) has presented a new test procedure for testing constant failure rate against a bathtub shaped (upside-down bathtub shaped) failure rate. assuming that the proportion of population that fails at or before the change point of failure rate is known. Jeong (1992) has extended Park's test to randomly censored data under the same assumption. In this paper, we have investigated efficiency loss to the presence of censoring.

• Tribology and Lubricants
• /
• v.33 no.5
• /
• pp.228-233
• /
• 2017

Chemical Mechanical Polishing of chemically stable sapphire substrates is dominantly affected by the mechanical processing of abrasives, in terms of the material removal rate. In this study, we investigated the effect of electrostatic force between the abrasives and substrate, on the polishing. If potassium chloride (KCl) is added to slurry, water molecules are decomposed into $H^+$ and $OH^-$ ions, and the amount of ions in the slurry changes. The zeta potential of the abrasives decreases with an increase in the amount of $H^+$ ions in the stern layer; consequently, the electrostatic force between the abrasives and substrate decreases. The change in zeta potential of abrasives in the slurry is affected by the slurry pH. In acidic zones, the amount of ions bound to the abrasives increases if the amount of $H^+$ ions is increased by adding KCl. However, in basic zones, there is no change in the corresponding amount. In acidic zones, zeta potential decreases as molar concentration of potassium increases; however, it does not change significantly in basic zones. The removal rate tends to decrease with increase in molar amount of potassium in acidic zones, where zeta potential changes significantly. However, in basic zones, the removal rate does not change with zeta potential. The tendencies of zeta potential and that of the frictional force generated during polishing show strong correlation. Through experiments, it is confirmed that the contact probability of abrasives changes according to the electrostatic force generated between the abrasives and substrate, and variation in removal rate.