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Activity Probability-Based Performance Analysis and Contention Control for IEEE 802.11 WLANs
Choi, Junsu,Byeon, Seongho,Choi, Sunghyun,Lee, Kwang Bok IEEE Computer Society 2017 IEEE TRANSACTIONS ON MOBILE COMPUTING Vol.16 No.7
<P>In this paper, we develop a contention window (CW) control scheme for practical IEEE 802.11 wireless local area networks (WLANs) that have node heterogeneity in terms of the traffic load, transmission rate, and packet size. We introduce activity probability, i.e., the probability that a node contends for medium access opportunities at a given time. We then newly develop a performance analysis model that enables analytic estimation on the contention status including the collision probability, collision time, back-off time, and throughput with comprehensive consideration of node heterogeneity. Based on the newly developed model, we derive the theoretically ideal contention status, and develop a CW control scheme that achieves the ideal contention status in an average sense. We perform extensive NS-3 simulations and real testbed experiments for evaluation of both the proposed performance analysis model and CW control scheme. The results show that the proposed model provides accurate prediction on the contention status, and the proposed CW control scheme achieves considerable throughput improvement compared to the existing schemes which do not comprehensively consider node heterogeneity.</P>
Adaptive remote radio head control for cloud radio access networks
Choi, Junsu,Sohn, Illsoo,Lee, Kwang Bok Springer (Biomed Central Ltd.) 2016 Eurasip Journal on Wireless Communications and Net Vol.2016 No.-
<P>In this paper, we develop an adaptive remote radio head (RRH) control scheme to maximize the network capacity of frequency division duplexing (FDD)-based cloud radio access networks. We focus on a realistic performance metric that considers reference signal (RS) transmission overhead. Finding the optimal subset of RRHs that maximize the network capacity is formulated as an integer programming problem. We develop two efficient algorithms based on greedy search and linear programming relaxation combined with gradient ascent search, respectively. Our simulation results reveal that a larger number of antennas do not always guarantee capacity increase in real communication environments due to RS transmission overhead. The proposed scheme adaptively determines the subset of RRHs considering RS transmission overhead and provides significant capacity gain over previous approaches.</P>
Sounding Node Set and Sounding Interval Determination for IEEE 802.11ac MU-MIMO
Choi, Junsu,Choi, Sunghyun,Lee, Kwang Bok IEEE 2016 IEEE Transactions on Vehicular Technology VT Vol.65 No.12
<P>In this paper, we propose a sounding control scheme for IEEE 802.11ac multiuser multiple-input multiple-output (MU-MIMO). The proposed scheme comprehensively considers the long-term characteristics of a network environment, including the downlink traffic loads and channel coherence times of wireless links, and jointly determines the sounding node set and sounding interval to maximize the long-term expected MU-MIMO throughput gain in consideration of sounding overhead. To this end, we analytically formulate an MU-MIMO throughput gain maximization problem, considering the network environment and sounding overhead. We conduct MIMO channel measurement in practical wireless local area network (WLAN) environments and evaluate the performance of the proposed scheme by employing the real channel data traces. Simulation results verify that the proposed scheme adaptively determines the sounding node set and sounding interval, according to the network environment, and outperforms the existing scheme, which considers the channel coherence times only.</P>
Choi, Joonhyeong,Kim, Wansun,Kim, Donguk,Kim, Seonha,Chae, Junsu,Choi, Siyoung Q.,Kim, Felix Sunjoo,Kim, Taek-Soo,Kim, Bumjoon J. American Chemical Society 2019 Chemistry of materials Vol.31 No.9
<P>Mechanical properties of conducting polymers are an essential consideration in the design of flexible and stretchable electronics, but the guidelines for the material design having both high mechanical and electrical properties remain limited. Here we provide an important guideline for the design of mechanically robust, electroactive polymer thin films in terms of the molecular weight of the polymers. These studies based on a highly efficient, representative n-type conjugated polymer (P(NDI2OD-T2)) revealed a marked enhancement in mechanical properties across a narrow molecular weight range, highlighting the existence of a critical molecular weight that can be exploited to engineer films that balance processability and mechanical and electronic properties. We found the thin films formed from high molecular weight polymers (i.e., number-average molecular weight (<I>M</I><SUB>n</SUB>) ∼ 163 kg mol<SUP>-1</SUP>) to exhibit superior mechanical compliance and robustness, with a 114-fold enhanced strain at fracture and a 2820-fold enhanced toughness, as compared to those of low molecular weight polymer films (<I>M</I><SUB>n</SUB> = 15 kg mol<SUP>-1</SUP>). In particular, we observed a jump in the mechanical properties between the <I>M</I><SUB>n</SUB> = 48 and 103 kg mol<SUP><B>-</B>1</SUP>, yielding a 26-fold enhanced strain at fracture and a 160-fold enhanced toughness. The significant improvement of tensile properties indicates the presence of a critical molecular weight at which entangled polymer networks start to form, as supported by the analysis of the thermal and crystalline properties, specific viscosity, and microstructure. Our work provides useful guidelines for the design of conjugated polymers with recommendations for the best combinations of mechanical robustness and electrical performance for flexible and stretchable electronics.</P> [FIG OMISSION]</BR>
Choi, Woongjin,Lee, Junsu,Lee, Yunho,Ahn, Kyunghan,You, Tae-Soo Royal Society of Chemistry 2017 Dalton Transactions Vol. No.
<▼1><P>Eu10.74(2)K0.26Bi9.14(2)Sn0.86 is the first example of having both cationic and anionic p-dopants in a single compound.</P></▼1><▼2><P>Two Zintl phase thermoelectric compounds of Eu11−xKxBi10−ySny (<I>x</I> = 0, 0.26(1); <I>y</I> = 0.86(2), 1.93(2)) have been synthesized by a high-temperature solid-state reaction and arc-melting methods. The two isotypic crystal structures are characterized by both single-crystal and powder X-ray diffractions, and adopt a tetragonal Ho11Ge10-type structure (space group <I>I</I>4/<I>mmm</I>, <I>Z</I> = 2, Pearson code <I>tI</I>84) containing nine crystallographically independent asymmetric atomic sites in a unit cell. The chemical compositions are confirmed by EDS analysis. The complex crystal structure of the two title compounds can be described as an assembly of three different types of co-facial polyhedra formed by cations and 3-dimensional anionic frameworks surrounding these polyhedra. A quaternary title compound, Eu10.74(2)K0.26Bi9.14(2)Sn10.86, which simultaneously contains both cationic and anionic p-dopants in a single compound, was successfully crystallized for the first time in the A11M10 (A = alkaline-earth metals, rare-earth metals; M = triels, tetrels, pnictogens) series. In particular, two different types of p-dopants K and Sn show particular site-preferences, respectively, where K and Sn prefer to occupy the cationic Wyckoff 4<I>e</I> site and the anionic Wyckoff 8<I>h</I> site. These noticeable site preferences can be elucidated by either a size-factor criterion for the K-doping case or by an electronic-factor criterion for the Sn-doping case. The tight-binding linear muffin–tin orbital calculations show that as the double p-doping is applied to the Eu11−xKxBi10−ySny system, some extra holes are generated on the electronic structures according to the density of states curves. However, a series of thermoelectric property measurements prove that this extra hole-carrier doping is hardly effective enough to completely suppress a bipolar conduction of holes and electrons due to the rigid metallic band structure of the title system.</P></▼2>