SLAW: Self-Similar Least-Action Human Walk

Kyunghan Lee,Seongik Hong,Seong Joon Kim,Injong Rhee,Song Chong IEEE 2012 IEEE/ACM transactions on networking Vol.20 No.2

<P>Many empirical studies of human walks have reported that there exist fundamental statistical features commonly appearing in mobility traces taken in various mobility settings. These include: 1) heavy-tail flight and pause-time distributions; 2) heterogeneously bounded mobility areas of individuals; and 3) truncated power-law intercontact times. This paper reports two additional such features: a) The destinations of people (or we say waypoints) are dispersed in a self-similar manner; and b) people are more likely to choose a destination closer to its current waypoint. These features are known to be influential to the performance of human-assisted mobility networks. The main contribution of this paper is to present a mobility model called Self-similar Least-Action Walk (SLAW) that can produce synthetic mobility traces containing all the five statistical features in various mobility settings including user-created virtual ones for which no empirical information is available. Creating synthetic traces for virtual environments is important for the performance evaluation of mobile networks as network designers test their networks in many diverse network settings. A performance study of mobile routing protocols on top of synthetic traces created by SLAW shows that SLAW brings out the unique performance features of various routing protocols.</P>

Enhancing p-Type Thermoelectric Performances of Polycrystalline SnSe via Tuning Phase Transition Temperature

Lee, Yong Kyu,Ahn, Kyunghan,Cha, Joonil,Zhou, Chongjian,Kim, Hyo Seok,Choi, Garam,Chae, Sue In,Park, Jae-Hyuk,Cho, Sung-Pyo,Park, Sang Hyun,Sung, Yung-Eun,Lee, Won Bo,Hyeon, Taeghwan,Chung, In American Chemical Society 2017 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.139 No.31

<P>SnSe emerges as a new class of thermoelectric materials since the recent discovery of an ultrahigh thermoelectric figure of merit in its single crystals. Achieving such performance in the polycrystalline counterpart is still challenging and requires fundamental understandings of its electrical and thermal transport properties as well as structural chemistry. Here we demonstrate a new strategy of improving conversion efficiency of bulk polycrystalline SnSe thermoelethics. We show that PbSe alloying decreases the transition temperature between Pnma and Cmcm phases and thereby can serve as a means of controlling its onset temperature. Along with 1% Na doping, delicate control of the alloying fraction markedly enhances electrical conductivity by earlier initiation of bipolar conduction while reducing lattice thermal conductivity by alloy and point defect scattering simultaneously. As a result, a remarkably high peak ZT of similar to 1.2 at 773 K as well as average ZT of similar to 0.5 from RT to 773 K is achieved for Na-0.01(Sn1-xPbx)(0.99)Se. Surprisingly, spherical-aberration corrected scanning transmission electron microscopic studies reveal that NaySn1-xPbxSe (0 < x <= 0.2; y = 0, 0.01) alloys spontaneously form nanoscale particles with a typical size of similar to 5-10 nm embedded inside the bulk matrix, rather than solid solutions as previously believed. This unexpected feature results in further reduction in their lattice thermal conductivity.</P>

Mobile Data Offloading: How Much Can WiFi Deliver?

Kyunghan Lee,Joohyun Lee,Yung Yi,Injong Rhee,Song Chong IEEE 2013 IEEE/ACM transactions on networking Vol.21 No.2

<P>This paper presents a quantitative study on the performance of 3G mobile data offloading through WiFi networks. We recruited 97 iPhone users from metropolitan areas and collected statistics on their WiFi connectivity during a two-and-a-half-week period in February 2010. Our trace-driven simulation using the acquired whole-day traces indicates that WiFi already offloads about 65% of the total mobile data traffic and saves 55% of battery power without using any delayed transmission. If data transfers can be delayed with some deadline until users enter a WiFi zone, substantial gains can be achieved only when the deadline is fairly larger than tens of minutes. With 100-s delays, the achievable gain is less than only 2%-3%, whereas with 1 h or longer deadlines, traffic and energy saving gains increase beyond 29% and 20%, respectively. These results are in contrast to the substantial gain (20%-33%) reported by the existing work even for 100-s delayed transmission using traces taken from transit buses or war-driving. In addition, a distribution model-based simulator and a theoretical framework that enable analytical studies of the average performance of offloading are proposed. These tools are useful for network providers to obtain a rough estimate on the average performance of offloading for a given WiFi deployment condition.</P>

Max Contribution: An Online Approximation of Optimal Resource Allocation in Delay Tolerant Networks

Kyunghan Lee,Jaeseong Jeong,Yung Yi,Hyungsuk Won,Injong Rhee,Song Chong IEEE 2015 IEEE transactions on mobile computing Vol.14 No.3

<P>In this paper, a joint optimization of link scheduling, routing and replication for delay-tolerant networks (DTNs) has been studied. The optimization problems for resource allocation in DTNs are typically solved using dynamic programming which requires knowledge of future events such as meeting schedules and durations. This paper defines a new notion of approximation to the optimality for DTNs, called snapshot approximation where nodes are not clairvoyant, i.e., not looking ahead into future events, and thus decisions are made using only contemporarily available knowledges. Unfortunately, the snapshot approximation still requires solving an NP-hard problem of maximum weighted independent set (MWIS) and a global knowledge of who currently owns a copy and what their delivery probabilities are. This paper proposes an algorithm, Max-Contribution (MC) that approximates MWIS problem with a greedy method and its distributed online approximation algorithm, Distributed Max-Contribution (DMC) that performs scheduling, routing and replication based only on locally and contemporarily available information. Through extensive simulations based on real GPS traces tracking over 4,000 taxies and 500 taxies for about 30 days and 25 days in two different large cities, DMC is verified to perform closely to MC and outperform existing heuristically engineered resource allocation algorithms for DTNs.</P>

New Bioreactor technology using double horizontal pipe system

( Kyunghan Lee ),( Keun Jhang ),( Nakjoo Kim ) 한국폐기물자원순환학회 2011 ISWA Vol.2011 No.0

CarrierMix: How Much Can User-side Carrier Mixing Help?

Lee, Joohyun,Lee, Kyunghan,Kim, Yeongjin,Chong, Song IEEE Computer Society 2017 IEEE TRANSACTIONS ON MOBILE COMPUTING Vol.16 No.1

<P>Energy consumption for cellular communication is increasingly gaining importance in smartphone battery lifetime as the bandwidth of wireless communication and the demand for mobile traffic increase. For energy-efficient cellular communication, we tackle two energy characteristics of cellular networks: (1) transmission energy highly varies upon channel condition, and (2) transmission of a packet accompanies unnecessary tail energy waste. Under the objective of transmitting packets when the best channel is provided as well as a number of packets are accumulated, we propose a new mobile collaboration framework 'CarrierMix' that aggregates smart devices across multiple heterogeneous cellular carriers. Compared to the standalone operation, even without a buffering delay, CarrierMix allows better channel and reduces more tail energy in a statistical point of view. To maximize the energy benefit while maintaining the fairness among the nodes in collaboration, we further develop a dynamic programming framework providing the optimal algorithm of CarrierMix and its approximated heuristic. Trace-driven simulations on our experimental HSPA/EVDO/LTE network traces show that CarrierMix of five devices achieves up to 42 percent of energy reduction.</P>

Resource-Efficient Mobile Multimedia Streaming With Adaptive Network Selection

Joohyun Lee,Kyunghan Lee,Choongwoo Han,Taehoon Kim,Song Chong IEEE 2016 IEEE transactions on multimedia Vol.18 No.12

<P>From the advancements of mobile display and network infrastructure, mobile users can enjoy high quality-mobile video streaming anywhere, anytime. However, most mobile users are still reluctant to use high quality video streaming when they are mobile due to costly cellular data and high energy consumption. In this work, we develop scheduling algorithms for resource-efficient mobile video streaming, which minimize the weighted sum objective of cellular cost and energy consumption. We first model the scheduling problem as a Markov decision process and propose an optimal scheduling algorithm based on dynamic programming. Then, we derive a heuristic algorithm that approximates the optimal algorithm. To evaluate the performance of proposed algorithms, we run simulation over YouTube video traces with audience retention graphs and mobility/connectivity traces in public transportation (e.g., commuting). Through extensive simulations, we show that our proposed scheduling algorithm has negligible performance loss compared to the optimal scheduling algorithm, where it saves 59% of cellular cost and 41% of energy compared to the YouTube default scheduler. We also implement our scheduling algorithm on an Android platform, and experimentally evaluate the performance compared to existing streaming policies.</P>

TCP 소켓 버퍼로 인한 지연시간 증폭 현상에 관한 실험적 연구

이경한(Kyunghan Lee),박신익(Shinik Park) 한국통신학회 2016 한국통신학회 학술대회논문집 Vol.2016 No.1

Commoncode: a code-reuse platform for wireless network experimentation

Junhee Lee,Jinsung Lee,Kyunghan Lee,Song Chong Institute of Electrical and Electronics Engineers 2012 IEEE communications magazine Vol.50 No.3

<P>Experimentation of a wireless network protocol over the air is of significant interest. However, it is more rarely performed than simulation because of the difficulties in coding and debugging as well as lack of scalability and repeatability. In this article, the concept of a code-reuse platform making use of a simulation code directly for real experiments is revisited as an efficient and easy means of experimentation. Furthermore, an architecture and key components of an idealistic code-reuse platform are suggested, and then CommonCode, the most advanced codereuse platform, is proposed. Through extensive simulations and experiments using Common-Code for the same codes, we demonstrate that CommonCode is valid and accurate in terms of protocol performance, and simultaneously fast and easy in terms of protocol development.</P>

골육세포 성장 촉진을 위한 스마트 써피스 텍스처링 임플란트 스템 제작 기술

김경한(Kyunghan Kim),이제훈(Jaehoon Lee),박종권(Jongkweon Park),진석원(Sukwon Jin),최완해(Wanhae Choi),이홍진(Hongjin Lee) Korean Society for Precision Engineering 2014 한국정밀공학회지 Vol.31 No.5