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Short-term fading model for signals reflected by ocean surfaces in underwater acoustic communication
Joonsuk Kim,Il-Suek Koh,Yongshik Lee IET 2015 IET COMMUNICATIONS Vol.9 No.9
<P>A short-term fading model based on rough surface theory is proposed to estimate stochastic characteristics of signals reflected by time-varying ocean surfaces in underwater acoustic channels. With this method, probability density functions of the fluctuations in the propagation delay and magnitude, and the Doppler spectral densities are provided with respect to wind speeds. After conducting several experiments and verifying them by comparing with the measurements, it is shown that the model accurately estimates the stochastic characteristics. The proposed model is valid when the transmitter and receiver locate sufficiently far away from the ocean surface.</P>
Kim, Joonsuk,Chun, Kwang Min,Song, Soonho,Baek, Hong-Kil,Lee, Seung Woo Elsevier 2017 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.42 No.39
<P><B>Abstract</B></P> <P>The effects of hydrogen on the combustion characteristics, thermal efficiency, and emissions of a turbo gasoline direct-injection engine with exhaust gas recirculation (EGR) were investigated experimentally at brake mean effective pressures of 4, 6, and 8 bar at 2000 rpm. Four cases of hydrogen energy fraction (0%, 1%, 3% and 5%) of total fuel energy were studied. Hydrogen energy fraction of total fuel energy was hydrogen energy in the sum of energy of consumed gasoline and added hydrogen. The test results demonstrated that hydrogen addition improved the combustion speed and reduced cycle-to-cycle variation. In particular, cylinder-to-cylinder variation dramatically decreased with hydrogen addition at high EGR rates. This suggests that the operable EGR rate can be widened for a turbo gasoline direct-injection engine. The improved combustion and wider operable EGR rate resulted in enhanced thermal efficiency. However, the turbocharging effect acted in opposition to the thermal efficiency with respect to the EGR rate. Therefore, a different strategy to improve the thermal efficiency with EGR was required for the turbo gasoline direct-injection engine. HC and CO<SUB>2</SUB> emissions were reduced but NO<SUB>X</SUB> emissions increased with hydrogen addition. The CO emissions as a function of engine load followed different trends that depended on the level of hydrogen addition.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Hydrogen effects on the performance of T-GDI engine with EGR are investigated. </LI> <LI> Variations of cycle to cycle and cylinder to cylinder decreased with hydrogen. </LI> <LI> EGR limit was extended by hydrogen. </LI> <LI> ITE increased with hydrogen but trends for ITE were different with engine load. </LI> <LI> HC, CO<SUB>2</SUB> emissions decreased, NO<SUB>X</SUB> emissions increased with hydrogen. </LI> </UL> </P>
Kim, Joonsuk,Chun, Kwang Min,Song, Soonho,Baek, Hong-Kil,Lee, Seung Woo Elsevier 2018 APPLIED ENERGY Vol.228 No.-
<P><B>Abstract</B></P> <P>Lean combustion has been identified as a way of improving the thermal efficiency of gasoline engines. However, it is difficult to operate gasoline engines in lean conditions, because the stability of the combustion deteriorates. Hydrogen has properties that can be exploited to improve the combustion stability, and hence solve the combustion instability problem. In this paper, we describe an experimental study into the effects of hydrogen on the combustion stability, performance, and emissions at various air/fuel ratios using a four-cylinder 2.0-L turbo gasoline direct-ignition engine. The engine speed was fixed at 2000 RPM with a brake mean effective pressure of 2–18 bar. The results of these experiments indicate that combustion is improved, thermal efficiency enhanced, and emissions reduced except for NO<SUB>X</SUB>. Especially, in terms of combustion stability, the cycle-to-cycle variation and cylinder-to-cylinder variation decreased when hydrogen was added. This was caused by two effects. First, when the brake mean effective pressure was 10 bar or below, the burn duration was reduced by the fast flame speed, high adiabatic temperature, and high diffusion coefficient of hydrogen. This led to an increase in the combustion stability and thermal efficiency. Second, when the brake mean effective pressure was above 10 bar, knock resistance was enhanced by hydrogen: spark timing could be advanced upon hydrogen addition. Therefore, the combustion stability and performance improved. Additionally, the improvements to the combustion, especially in terms of combustion stability, became more pronounced as the in-cylinder air–fuel mixture became leaner: the lean limit was thus extended.</P> <P><B>Highlights</B></P> <P> <UL> <LI> H<SUB>2</SUB> effects on performance of T-GDI engine with various air/fuel ratios are studied. </LI> <LI> Spark timing could be retarded on MBT zone and could be advanced on knock zone. </LI> <LI> With H<SUB>2</SUB>, CA0-50 was improved on all, but CA50-90 worsened on BMEP 8 to 16 bar. </LI> <LI> Variations of cylinder-to-cylinder and cycle-to-cycle are reduced by H<SUB>2</SUB> addition. </LI> <LI> Thermal efficiency is improved by H<SUB>2</SUB> addition but optimum conditions differed. </LI> </UL> </P>
Kim, Kyung-su,Cho, Hye Jin,Lee, Jookyeong,Ha, Seonggyun,Song, Sun Gu,Kim, Seunghun,Yun, Wan Soo,Kim, Seong Kyu,Huh, Joonsuk,Song, Changsik American Chemical Society 2018 Macromolecules Vol.51 No.20
<P>Stimuli-responsive polymeric systems are of considerable interest due to their potential applications in environment-adaptive technologies such as smart surfaces. Traditionally, such systems can be constructed either by dynamic noncovalent (supramolecular) or dynamic covalent chemistry, but the use of both chemistries in one system may offer unique opportunities for structural diversity and various controllability. Herein, we report that hydrazone-pyridinum conjugates, which can be dynamically exchanged by transimination, assemble to form one-dimensional nanowires due to <I>direct intermolecular interactions</I> (without metal-ion coordination). The self-assembly process can be controlled not only by dynamic covalent chemistry but also by pH adjustment. The hydrazone-pyridinum conjugates are transformed to merocyanine-type dyes of distinctive negative solvatochromism via deprotonation, which also affects their self-assembly. Such a dual control of the dynamic molecular assembly will provide unique way to develop diverse smart nanomaterials with multistimuli-responsiveness.</P> [FIG OMISSION]</BR>
802.11 WLAN: history and new enabling MIMO techniques for next generation standards
Kim, Joonsuk,Lee, Inkyu Institute of Electrical and Electronics Engineers 2015 IEEE communications magazine Vol.53 No.3
<P>IEEE 802.11, which designs wireless local area networks (WLAN), is one of the most successful standards in wireless communication systems. In this article, we review the history of WLAN standards, and provide technical overviews of the recent development of WLAN systems. Especially, as the original inventor and the proposer, we focus on beamforming and compressed feedback schemes, which have been adopted in 802.11 WLAN standards, to improve the throughput for a multiple-input multiple-output (MIMO) system. These techniques are essential to maximize the downlink system throughput for multiple user transmission as well as for single user transmission. Also, we present discussions on new technologies to further enhance user throughput, which are currently considered for future WLAN systems.</P>
Kim, Joonsuk,Lee, Yongshik Professional Technical Group on Microwace Theory a 2017 IEEE Transactions on Microwave Theory and Techniqu Vol. No.
<P>A method based on the Z-transform is demonstrated to cascade transmission line sections of unequal lengths and achieve multiband and/or wideband properties. Unlike previous methods based on the Z-transform, the proposed method associates the z(-1) term with the infinitesimal length of the unit segment. Consequently, any electrical length can be expressed in the z(-1) form. Therefore, while the previous methods are restricted to cascading lines of a commensurate length only, the proposed method can be applied to those of incommensurate lengths. Furthermore, the z-polynomials for triple or more bands can be constructed by multiplying a combination of single-and dual-band polynomials, which remarkably simplifies the entire procedure to develop multiband and/or wideband circuits with virtually no limitation in terms of the number and/or combination of frequencies. The validity and practicality of the proposed method is verified with experimental results for various impedance transformers for real loads and Wilkinson power dividers; the results show the independence in the number and combination of operating frequencies as well as a dual-band operation with a center frequency ratio as large as 7.75.</P>