Chun-Hyun Paik

Performance analysis for closed-loop production systems with unreliable machines and random processing times

In this paper we propose new approximate methods for the performance analysis of closed-loop production systems with unreliable machines and random processing times. Each approximate method decomposes the production system consisting of K machines into a set of K subsystems, each subsystem consisting of two machines separated by a finite buffer. Then, each subsystem is analyzed by three different analyzing methods in isolation. The population constraint of the closed-loop production system is taken into account by prescribing that the sum of average buffer levels in the subsystems is equal to the number of customers in the closed-loop production system. We establish a set of equations that characterize unknown parameters of the servers in the subsystems. An iterative procedure is then used to determine the unknown parameters. Experimental results show that these methods provide a good estimation of the throughput.

A robust design of simulated annealing approach for mixed-model sequencing

The effectiveness of the solution method based on simulated annealing (SA) mainly depends on how to determine the SA-related parameters. A scheme as well as parameter values for defining an annealing schedule should be appropriately determined, since various schemes and their corresponding parameter values have a significant impact on the performance of SA algorithms. In this paper, based on robust design we propose a new annealing parameter design method for the mixed-model sequencing problem which is known to be NP-hard. To show the effectiveness of the proposed method, extensive computation experiments are conducted. It was found that the robust designed method outperforms the SA algorithm by McMullen and Frazier [McMullen, P.R., & Frazier, G.V. (2000). A simulated annealing approach to mixed-model sequencing with multiple objectives on a just-in-time line. IIE Transactions, 32, 679-686].

Integrated call control in a CDMA cellular system

Soft handoff is an essential component of code-division multiple-access (CDMA) digital mobile communication systems. While soft handoff has some advantages of system capacity and communication quality, it also has some disadvantages arising from the excessive use of channel resources. In this study, we propose a new control policy to alleviate effectively this excessive use of channel resources. For CDMA cellular mobile systems consisting of homogeneous cells, a variety of traffic parameters are estimated by the detailed traffic analysis, based on which a Markovian queueing model is developed to analyze the performance of the proposed call control policy. With a well-known performance objective, a mathematical problem of finding optimal call control parameters is proposed, along with its solution method.

Subgradient Approach for Resource Management in Multiuser OFDM Systems

This paper studies the problem of finding an optimal subcarrier and power allocation for downlink communication in the orthogonal frequency division multiplexing (OFDM) wireless systems. With the constraints on the power consumption and transmission rates for users, the problem of maximizing total throughput is mathematically formulated. The developed mathematical model is so general as to include almost of existing ones in which the original problems have been usually divided into smaller and tractable ones to make a practical application of some simple greedy algorithms. In this study, developed is the subgradient dual optimization method for the problem. Extensive experiments with realistic data show that in a reasonable time the subgradient method converges to the stable point, almost feasible to the original problem. We proved that the subgradient method is applicable to the dynamic resource allocation, by showing that environmental changes at a stable state need much lesser additional iterations to converge.

Comparison of emission estimates for non-CO2 greenhouse gases from livestock and poultry in Korea from 1990 to 2010

It has often been claimed that non-carbon dioxide greenhouse gases (NCGGs), such as methane, nitrous oxide and fluorinated greenhouse gases, are significant contributors to climate change. Here we nvestigate emission estimates of methane and nitrous oxide from livestock and poultry production, which is recognized as a major source of those NCGGs, in Korea over the period of 1990 through 2010. Based on the data on livestock and poultry populations, emission estimates of methane and nitrous oxide are first derived based on the Tier 1 approach. Then, the Tier 2 approach is adopted to obtain emission estimates of methane and nitrous oxide from cattle, which are known to be the largest sources of these NCGGs and account for about 70% of emissions from livestock and poultry in Korea. The result indicates that the Tier 2 estimates of methane and nitrous oxide emissions from enteric fermentation and manure management are significantly different from the Tier 1 estimates over the analysis period.

Handoff management without intercell hard handoffs in a multifrequency CDMA system

The paper addresses handoff management in a multifrequency code-division multiple-access system with nonuniform traffic loads over cells. We propose a new handoff scheme, adaptive handoff management (AHM), in which the intercell hard handoffs, which not only incur an equipment cost burden but also degrade call quality, are totally removed. Also strengthened in the AHM is the call-control capability of adaptively reflecting a changing traffic environment. Traffic and mobility analysis is first conducted on a single isolated cell with two concentric circle boundaries, thereby rendering two key performance measures. We then formulate a mathematical problem of finding the optimal parameters of the proposed AHM in which intracell hard handoffs are exploited most effectively. After reporting a simple solution process, computational experiments are conducted to illustrate the superiority of AHM in two key performance criteria.

Throughput bounding and simple approximation methods for exponential fork/join queueing networks with blocking

Exponential fork/join queueing networks (FJQNs) with finite buffers have been used as a major tool for evaluating the performances of manufacturing systems. In this study, we first suggest the throughput upper and lower bounds. Our upper-bounding method is elaborated on with general network configuration (acyclic configuration), while our lower bounds can be obtained only for networks with more specialized configuration. Next, developed is a simple approximation method for throughputs, which are based on decomposition/aggregation principles and structurally equivalent relations between different configurations.

Spectrum-efficient call control in a dual-mode TDMA cellular system

This study deals with a call control strategy in a dual-mode time-division multiple access (TDMA) cellular system, which provides services both to analog and digital calls. Since analog calls consume the frequency resource several times as much as digital calls, we consider a call control strategy of the threshold type that the number of active analog calls is restricted within a prespecified level. Given the arrival rates and the grade of service (GOS) for both types of calls in the cells, two nonlinear integer optimization problems are considered for a multicell system as well as for a single cell system. One problem is to find the threshold parameters for optimizing the relevant objective measures. The other is to obtain the minimum numbers of required channels in the cells satisfying the GOS of both types of calls. The solution methods for the two kinds of optimization problems are devised based on the properties of the objective function and the blocking probabilities of both call types. The efficiency of the proposed algorithms is verified by extensive computational experiments with realistic input data.

Greenhouse Gas Emission Inventory Calculation of Korean Glass Industry through the Bottom-up Production Process Analysis

The glass production is classified into an energy intensive industry. This study develops a systematic procedure to derive Greenhouse Gas (GHG) emission inventory for the Korean glass industry. Based on the bottom-up approach in which the energy intensity in each production process is characterized, the EBs (energy balances) of glass production processes are derived. And the GHG emission is calculated for each of four types of glasses-flat glass, container glass, fiber glass, and LCD glass.

Marginal Abatement Cost Analysis for the Korean Residential Sector Using Bottom-Up Modeling

A marginal abatement cost analysis has been conducte d to analyze the effects of abatement measures on greenhouse gas (GHG) emissions for the Korean residential sector. A bottom-up model using MESSAGE has been developed by defining the energy demand and constructing the reference energy system for the residential sector. A great amount of activity data has also been analyzed. Abatement potentials and related costs of individual abatement measures are investigated. The result from the marginal abatement cost analysis may provide general guidelines and proc edures for the establishment of GHG abatement polices.