Jae Hyuk Choi

The IEEE 802.11 standard for wireless local area networks (WLANs) employs a medium access control (MAC), called distributed coordination function (DCF), which is based on carrier sense multiple access with collision avoidance (CSMA/CA). The collision avoidance mechanism utilizes the random backoff prior to each frame transmission attempt. The random nature of the backoff reduces the collision probability, but cannot completely eliminate collisions. It is known that the throughput performance of

Advances in information technology have enabled ubiquitous health monitoring at home, which is particularly useful for patients, who have to live alone. We have focused on the automatic and unobtrusive measurement of biomedical signals and activities of patients. We have constructed wireless communication networks in order to transfer data. The networks consist of Bluetooth and Wireless Local Area Network (WLAN). In this paper, we present the concept of a ubiquitous-Bedroom (u-Bedroom) which is

One of the key challenges in designing a rate adaptation scheme for IEEE 802.11 wireless LANs (WLANs) is to differentiate bit errors from link-layer collisions. Many recent rate adaptation schemes adopt the RTS/CTS mechanism to prevent collision losses from triggering unnecessary rate decrease. However, the RTS/CTS handshake incurs significant overhead and is rarely activated in today's infrastructure WLANs. In this paper we propose a new rate adaptation scheme that mitigates the collision effec

Rate adaptation is one of the basic functionalities in today's 802.11 wireless LANs (WLANs). Although it is primarily designed to cope with the variability of wireless channels and achieve higher system spectral efficiency, its design needs careful consideration of cross-layer dependencies, in particular, link-layer collisions. Most practical rate adaptations focus on the time-varying characteristics of wireless channels, ignoring the impact of link-layer collisions. As a result, they may lose t

We fabricated gated field emitter arrays with a novel focusing structure of electron beams, where the focusing electrode concentrically surrounded each gate hole. Carbon nanotube emitters were screen printed inside an amorphous-Si concave well far below the gate. It was theoretically and experimentally verified that the concave well structure effectively focused the emitted electron beams to their designated phosphor pixels by modulating focusing gate voltages. For the vacuum packaged field emis

한국 역사 속 과학탐방은 1998년부터 개발, 시행되고 있다. 한국 역사 속 과학 탐방의 효과에 대해서는 일부 논의가 있었으나, 실질적인 연구는 부족하였다. 이 연구에서는 한국 역사 속 과학탐방의 목적을 문화재에 대한 과학적 안목 형성 관점에서 화성 과학탐방의 사례와 함께 논의하였다. 문화재의 과학적 우수성 평가 준거를 고려하여 연구자의 안내된 질문과 수렴적 및 발산적 과학 탐구로 구성된 화성(華城) 과학 탐방 자료가 개발되고 지도 방략이 구안되었다. 본 연구에서는 문화재의 과학적 우수성 평가 준거를 고려한 화성 과학 탐방을 지도하여 중학생들이 문화재의 과학적 우수성을 올바로 평가할 수 있는 분별 능력, 즉 문화재에 대한 과학적 안목을 형성하는지 여부를 조사 하고자 하였다. 서울 지역 중학교 과학 특별활동반 10명을 연구 대상으로 하였다. 탐방 전과 후에 실시된 '문화재의 과학적 우수성 평가하기' 설문과 탐방 후 면담을 통해 탐방을 통한 관점 변화를 분석하여 과학 탐방 활동으로 문화

Gait-based gender classification is a challenging task since people may walk in different directions with varying speed, gait style, and occluded joints. The majority of research studies in the literature focused on gender-specific joints, while there is less attention on the comparison of all of a body’s joints. To consider all of the joints, it is essential to determine a person’s gender based on their gait using a Kinect sensor. This paper proposes a logistic-regression-based machine learning

Multi-Target Multi-Camera Tracking (MTMCT), which aims to track multiple targets within a multi-camera network, has recently attracted considerable attention due to its wide range of applications. The main challenge of MTMCT is to match local tracklets (i.e., sub-trajectories) obtained by different cameras and to combine them into global trajectories across the multi-camera network. This paper addresses the cross-camera tracklet matching problem in scenarios with partially overlapping fields of

Detecting misbehaving users in wireless networks is an important problem that has been drawing considerable attention. Even though there is a plethora of work on 802.11 wireless local area networks (WLANs), most existing schemes employ behavior-based anomaly detection, assuming that the backoff-time information of each transmitting node is available to the monitoring node. Unfortunately, it is practically infeasible to obtain the accurate backoff value chosen by other transmitting nodes because

During the crystallization process of amorphous SrBi2Ta2O9 (SBT) thin films at 800 °C in a dry O2 ambient, we have found elliptical nuclei in the initial nucleation state. These elliptical grains are preferentially oriented to [110] direction in the [11̄0] direction of projection. Elliptical grain growth keeps [110] as increasing the annealing time at 800 °C. Transmission electron microscopy and selected-area electron diffraction pattern indicate that the origin of 〈110〉-oriented crystallization

The Pohang Accelerator Laboratory is going to build an X-ray free electron laser (FEL) based on self-amplified spontaneous emission (SASE). This new machine will utilize the existing 2.5-GeV injection linac to the storage ring by upgrading its energy up to 3.7 GeV or more. The target wavelength will be 3 - 4.5 angstrom, and its third harmonic 1 - 1.5 angstrom will also be used. The project will proceed in two stages: In the first stage, a vacuum-ultra-violet (VUV) SASE machine with 385 MeV will

Mobile WiFi hotspots have become increasingly popular as a new innovative wireless Internet access technology. Although it has received little attention, adaptive, dynamic (re)assignment of channels in a mobile hotspot router - typically, a smartphone or a laptop or a tablet equipped with heterogeneous network interfaces like 3G/4G, WiFi, Bluetooth or ZigBee - is key to mobile hotspot performance. To fill this important gap, we present a novel scheme, called Ex2R, that finds the best WiFi servic