Analyzing the Performance of Hybrid Diversity-Beamforming Systems in Spatially Correlated Channels

Hojoong Kwon,Byeong Gi Lee 한국통신학회 2006 한국통신학회 학술대회논문집 Vol.2006 No.7

Inter-Cell Interference Management for Next-Generation Wireless Communication Systems

Hojoong Kwon,Soomin Ko,Hanbyul Seo,이병기 한국통신학회 2008 Journal of communications and networks Vol.10 No.3

In this paper, we examine what changes the nextgeneration wireless communication systems will experience in terms of the technologies, services, and networks and, based on that, we investigate how the inter-cell interference management should evolve in various aspects.We identify that the main driving forces of the future changes involve the data-centric services, new dynamic service scenarios, all-IP core access networks, new physical-layer technologies, and heavy upload traffic. We establish that in order to cope with the changes, the next-generation intercell interference management should evolve to 1) set the objective of providing a maximal data rate, 2) take the form of joint management of power allocation and user scheduling, 3) operate in a fully distributed manner, 4) handle the time-varying channel conditions in mobile environment, 5) deal with the changes in interference mechanism triggered by the new physical-layer technologies, and 6) increase the spectral efficiency while avoiding centralized coordination of resource allocation of the users in the uplink channel.

Frictional Characteristics of Nanoporous SiO2/TiO2 Film with Treated Surface by Chemical Mechanical Polishing (CMP)

Hojoong Kim,Daekwang Woo,Ji Hun Kim,Min Suk Kang,Jeong Kuk Shon,Ji Man Kim,Young Ze Lee,Taesung Kim 한국표면공학회 2008 한국표면공학회 학술발표회 초록집 Vol.2008 No.-

Opportunistic multi-channel CSMA protocol for OFDMA systems

Hojoong Kwon,Seonwook Kim,Byeong Lee IEEE 2010 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS Vol.9 No.5

<P>In this letter, we present an opportunistic multichannel CSMA protocol for OFDMA systems in frequency-selective fading channel. In order to opportunistically exploit the frequency selectivity, an OFDM-based single-channel CSMA protocol can employ water-filling power allocation as it allows a single station to utilize the whole frequency bandwidth at each time instant. In contrast, the proposed OFDMA-based multichannel CSMA protocol achieves the multi-user diversity gain by arranging multiple stations to transmit on their favorable sub-channels simultaneously. The proposed protocol is designed as an opportunistic version of the original multi-channel CSMA protocol, which was proposed in our previous work, and adopts a channel-adaptive backoff (or transmission probability control) algorithm, enabling each station to attempt data transmission on its high-SNR sub-channel with a high probability. The opportunistic multi-channel CSMA turns out to improve the total data rate significantly over the opportunistic single-channel CSMA, and to achieve almost full multi-user diversity gain despite the distributed operation.</P>

Generalized CSMA/CA for OFDMA systems: protocol design, throughput analysis, and implementation issues

Hojoong Kwon,Hanbyul Seo,Seonwook Kim,Byeong Gi Lee IEEE 2009 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS Vol.8 No.8

<P>In this paper, we present a multi-channel carrier sense multiple access with collision avoidance (CSMA/CA) protocol for orthogonal frequency division multiple access (OFDMA) systems. The CSMA/CA system in conventional single-channel operation has the advantage of not requiring the signaling for bandwidth request and allocation over the scheduled access system but it sacrifices system efficiency significantly. We overcome the system efficiency limitation by exploiting the OFDMA system features that multiple stations can transmit simultaneously on different sub-channels, and the stations can obtain the channel information of all the sub-channels at each time instant. The OFDMA-based multi-channel CSMA/CA protocol is designed to enable the stations to contend with each other for channel access both in time and frequency domains through a two-dimensional backoff scheme. The protocol takes a generalized form of the conventional single-channel CSMA/CA protocol: It allows to segment the channel bandwidth into multiple narrow-band random access channels and adjust the transmission probability according to the multi-channel activity in a flexible manner. For throughput analysis, we devise a multi-channel p-persistent CSMA protocol that can emulate the multi-channel CSMA/CA protocol and then analyze its saturated throughput. The simulation and analysis results reveal that the proposed system performs far better than the single-channel CSMA/CA system while using the same total bandwidth and even close to the scheduled access system. Further, we examine how stringent time synchronization is required among the constituent stations in implementing of the OFDMA-based multi-channel CSMA/CA system.</P>

Analysis of Key Factors for High Yield AMOLED Display

Hojoong Kim,Chanhyuk Jung,Jang-Yeon Kwon,Songkuk Kim IEEE 2015 Journal of display technology Vol.11 No.9

<P>To figure out the key factors of the reliability of AMOLED displays, we analyzed the luminance degradation based on a large-scale simulation. The initial non-uniformity and aging parameters were incorporated into the luminance decay model. For each set of parameters, we simulated the luminance degradation of 10,000 panels for 100,000 operating hours and analyzed by two criteria; contrast deviation and luminance variation of panels to determine failures. Stability of TFTs is the most significant parameter for realizing highly reliable AMOLED display. Stability of OLEDs is also potential factor when which of TFTs is sufficient.</P>

A Minimum Data-Rate Guaranteed Resource Allocation With Low Signaling Overhead in Multi-Cell OFDMA Systems

Hojoong Kwon,Won-Ick Lee,이병기 한국통신학회 2009 Journal of communications and networks Vol.11 No.1

In this paper, we investigate how to do resource allocation to guarantee a minimum user data rate at low signaling overhead in multi-cell orthogonal frequency division multiple access (OFDMA) wireless systems. We devise dynamic resource allocation (DRA) algorithms that can minimize the QoS violation ratio (i.e., the ratio of the number of users who fail to get the requested data rate to the total number of users in the overall network). We assume an OFDMA system that allows dynamic control of frequency reuse factor (FRF) of each sub-carrier. The proposed DRA algorithms determine the FRFs of the sub-carriers and allocate them to the users adaptively based on inter-cell interference and load distribution. In order to reduce the signaling overhead, we adopt a hierarchical resource allocation architecture which divides the resource allocation decision into the inter-cell coordinator (ICC) and the base station (BS) levels. We limit the information available at the ICC only to the load of each cell, that is, the total number of sub-carriers required for supporting the data rate requirement of all the users. We then present the DRA with limited coordination (DRA-LC) algorithm where the ICC performs load-adaptive inter-cell resource allocation with the limited information while the BS performs intra-cell resource allocation with full information about its own cell. For performance comparison, we design a centralized algorithm called DRA with full coordination (DRA-FC). Simulation results reveal that the DRA-LC algorithm can perform close to the DRA-FC algorithm at very low signaling overhead. In addition, it turns out to improve the QoS performance of the cell-boundary users, and achieve a better fairness among neighboring cells under non-uniform load distribution.

Evolution of Inter-Cell Interference Management in Next-Generation Wireless Communication Systems

Hojoong Kwon,Soomin Ko,Hanbyul Seo,Byeong Gi Lee 대한전자공학회 2008 ICEIC:International Conference on Electronics, Inf Vol.1 No.1

In this paper, we investigate how the inter-cell interference management should evolve in the next-generation systems. A new paradigm has been created in inter-cell interference management due to the advent of the next-generation wireless communication systems. The main driving force involves datacentric services, new dynamic service scenarios, all-IP core access networks, and new physical-layer technologies. We establish that the next-generation inter-cell interference management should 1) set the objective of providing a maximal data rate, 2) take the form of joint management of power allocation and user scheduling, 3) operate in a fully distributed manner, and 4) deal with the change in interference mechanism triggered by the new physical-layer technologies.

Bending and Strain Sensitivities in a Helicoidal Long-Period Fiber Gratings

Hojoong Jung,Woojin Shin,Jun Ki Kim,Seung-Han Park,Do-Kyeong Ko,Jongmin Lee,Kyunghwan Oh IEEE 2009 IEEE photonics technology letters Vol.21 No.17

<P>In this letter, we report bending and strain sensitivities of helicoidal long-period fiber gratings fabricated by twisting single-mode fibers during CO<SUB>2</SUB> laser irradiation. Linear spectral shifts of the resonant wavelengths under the bending and tensile strain were observed with the sensitivities of -11.7 nm/m<SUP>-1</SUP> and -1.1 pm/muepsiv, respectively. The corresponding transmission power variations at the resonance wavelength were 4.1 dB/m<SUP>-1</SUP> and 2.2 times 10<SUP>-4</SUP> dB/muepsiv, respectively. Detailed measurement techniques and sensor applications are discussed.</P>

Novel Gateways and Sensor Nodes Applying an Object Identifier to Monitor Gas Facilities

Hojoong Kim,Hyunjune Shin,Junho Shin 보안공학연구지원센터 2015 International Journal of Smart Home Vol.9 No.11

With the technological development of the Internet of Things (IoT), a wide variety of IoT-based monitoring systems have been developed in various domains. An IoT-based monitoring system provides an administrator with efficient management and information collection. Due to such advantages, the demand for monitoring systems in the gas industry domain has dramatically increased. In this paper, we propose two kinds of novel gateways and sensor nodes that apply an object identifier (OID) to monitor gas facilities. The proposed gateways and sensor nodes collect gas and environment data with various sensors and communicate with a monitoring server. If an exceptional event happens at the inspection site where our devices are installed, sensor nodes can take action by issuing control commands from the monitoring server. Moreover, we have also designed an OID that provides resources and devices in a gas monitoring system with a unique identification. By applying an OID to each message for transmission among sensor nodes, gateways, and monitoring servers, gas facilities and related devices can be accurately and safely managed. To evaluate the proposed devices, we properly installed our gateways and sensor nodes near operating gas facilities and verified their operation.