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Use MU-MIMO at your own risk — Why we don’t get Gb/s Wi-Fi
Choi, Hyunwoo,Gong, Taesik,Kim, Jaehun,Shin, Jaemin,Lee, Sung-Ju Elsevier 2019 Ad hoc networks Vol.83 No.-
<P><B>Abstract</B></P> <P>With the ever growing popularity of mobile devices, the demand for wireless bandwidth has also increased, with the mobile users now expecting wireless network quality similar to what they experience with wired networks. Wireless LANs have evolved over the last twenty years, with major breakthrough technologies such as OFDM (Orthogonal Frequency Division Multiplexing), MIMO (Multiple Input Multiple Output), and MU (Multi-User)-MIMO. The latest IEEE 802.11ac standard supports up to 6.9 Gb/s theoretical capacity, but it could only be achieved with 8-streams in a “perfect” environment. Commercial 802.11ac <I>wave 2</I> APs that include MU-MIMO capability, have only recently been made available in the market. We deployed a few APs from different vendors (that uses chipsets from different vendors) in various office environments and measured user throughput on smartphone mobile devices. We observe an enormous gap between theory and practice, with MU-MIMO often providing less throughput than SU (Single User)-MIMO in various network environments. We analyze the root cause of performance issues and suggest future research directions to achieve Gb/s Wi-Fi in practical deployments.</P>
Kim, Taesik,Woo, Sang Inn,Choi, Eunsoo,Jung, Young-Hoon Elsevier 2018 Computers and geotechnics Vol.94 No.-
<P><B>Abstract</B></P> <P>This paper introduces a new method for quantifying the morphological evolution of the strain response envelope (SRE). The Fourier series approximation was used to trace the morphological characteristics of SREs. Two SREs computed via numerical simulations using the Modified Cam Clay (MCC) model and three SREs obtained from stress-probing experiments on Chicago clay were used to validate the proposed method of Fourier series approximation. The results show that using a harmonic number of 1 or 2, the target shape of the SRE can be approximated within a 10% error.</P>
Seamless Transfer를 위한 계통연계형 인버터의 간접전류 제어기법
유태식(Taesik Yu),최세완(Sewan Choi),김효성(Hyosung Kim) 전력전자학회 2005 전력전자학술대회 논문집 Vol.- No.-
Distributed generation (DG) systems fall in islanding operation when they still in operation even when the main grid is out of electric power. Islanding operation is further classified into intentional islanding and unintentional islanding operations. In intentional islanding operation, the DG backs up critical loads while it separates from the main grid on islanding detection. Intentional islanding operation increases utilization of the DG system during the islanding operation. This paper proposes reasonal inverter topology and its control algorithm for seamless transfer of DG systems in intentional islanding operation.
Jung Taesik,Choi Bo Yoon,Jang Minho,Kim Taehee,Seo EunHye,Kim Jin Kook 대한의학회 2023 Journal of Korean medical science Vol.38 No.43
Background: Coronavirus disease 2019 (COVID-19) is known to have a high incidence of loss of smell and taste. However, studies in the early stages of the COVID-19 pandemic have evaluated these symptoms using subjective surveys and simple olfactory tests only. Hence, we compared the olfactory and gustatory characteristics of patient groups with COVID-19 olfactory dysfunction (C19OD) and non-COVID-19 postinfectious olfactory dysfunction (PIOD) using an objective olfactory test and evaluated the significance of olfactory training in both patient groups. Methods: We retrospectively analyzed the medical records of 14 patients with a decreased sense of smell after having positive COVID-19 polymerase chain reaction results, and 56 patients with PIOD with no history of confirmed COVID-19. Participants were evaluated using the Korean version of the Sniffin’ stick (KVSS) II, and chemical gustometry and olfactory training was assessed during their first visit. Olfactory training was then re-evaluated after an average of 8 (± 6) weeks. Results: The average age of participants in the C19OD group was lower than in those in the non-COVID-19 PIOD group. The proportion of men in the C19OD group was higher than in the non-COVID-19 PIOD group. At baseline assessment, the C19OD group had better olfactory and gustatory functions. After olfactory training, the non-COVID-19 PIOD patient group showed a significant increase in all KVSS II Total, T, D, and I scores, but there was a non-significant increase in all scores in the C19OD group. Conclusion: The C19OD group had better olfactory and gustatory function than the nonCOVID-19 PIOD group at the initial assessment. After olfactory training, there was an increase in olfactory function test scores in both groups. Olfactory training may be helpful in C19OD, as in non-COVID-19 PIOD.
최종호(Jongho Choi),구본휘(Bonwhee Koo),김태식(Taesik Kim) 한국지반환경공학회 2015 한국지반환경공학회논문집 Vol.16 No.2
도심지 구간에 위치한 지반공학 관련 공사 현장에서는 충분한 안전율 확보와 더불어 사용성도 확보해야 한다. 이를 위해서는 공사 중 인접지반에 발생하는 변형을 제어해야 하므로 지반구조물과 관련된 기술 요구사항도 더욱 높아지고 있다. 사용성 확보를 위해서는 지반의 변형을 예측해야 하므로 지반의 강성을 분석해야 한다. 특히 강성의 비선형성과 이방성은 주요인자라 할 수 있다. 본 연구에서는 깊은 굴착 중 지반이 경험하는 응력경로에 따른 강성의 비선형성과 이방성에 대해 실내시험 결과와 전산 해석 결과를 비교하였다. 강성은 변형률이 작은 미소변형률 구간에서 비선형성이 두드러졌으며, 응력경로에도 큰 영향을 받는 것으로 나타났다. 실내시험 결과 압축은 작은 강성을 인장은 큰 강성을 나타내었으며, 깊은 굴착 시 지지벽체 뒤는 압축이 앞은 인장이 지배적인 응력경로를 나타내었다. 굴착 중 발생하는 지반의 변형을 정확히 예측하기 위해서는 지반이 경험하는 응력경로에 따라 변하는 강성의 변화 특성을 고려해야 한다. In urban area, many design projects related to geotechnical projects are controlled by serviceability rather than stability requirements. Accordingly, control of ground deformation has become more crucial and many researchers have studied soil stiffness. Recent experimental studies on the stress-strain response of Chicago glacial clays showed that the nonlinearity and anisotropy are the two key factors in evaluating the soil stiffness. In this study, experimental results are applied to analyze the deep excavation site locating in downtown Chicago. The stress paths observed from the observation points located behind and front of the supporting wall yield typical stress paths. Changes in soil stiffness nonlinearity and anisotropy were discussed by comparing experimental and computed stress paths. The stiffness anisotropy were significant even at the first few excavations. The stiffness degradation characteristics are significantly different according to relative location to the support wall even at the same elevation.