Kwanghoon Kim

Analysis of stresses generated during hydrogen extraction from and injection into Ni(OH)2/NiOOH film electrode

Abstract Stresses generated during the hydrogen extraction from and injection into a Ni(OH) 2 /NiOOH film electrode in 0.1 M KOH solution are analysed by means of a laser beam deflection technique combined with potentiostatic current transient and electrochemical quartz crystal microbalance (EQCM) techniques. From the measured values of the film thickness, the elapsed time for phase boundary movement (PBM) and the potential step given between the hydrogen extraction/injection potential and the plateau potential, the velocity and the mobility of PBM are determined to be 2.3×10 −6 –5.4×10 −6 cm s −1 and 3.5×10 −10 –2.6×10 −9 cm 2 s −1 V −1 , respectively. From the mass change measured by means of EQCM, it is suggested that, during hydrogen extraction, the PBM continues until the insertion of K + ions begins to occur simultaneously with the desertion of such neutral species as H 2 O molecules or KOH molecules. During hydrogen injection, the PBM is accompanied by the extraction of K + ions, followed by the insertion of neutral species. From the mass change transient and the deflection transient, simultaneously measured with the current transient, it is suggested that the sharp rise of tensile and compressive deflections in the initial stage is traced back to the PBM. The following relaxation of tensile and compressive deflections is attributed to the extraction/insertion of neutral species. Most of the stresses developed during the hydrogen extraction and injection originate mainly from the PBM, not the extraction/insertion of K + ions and neutral species.

Optimum Clustered Pilot Sequence for OFDM Systems under Rapidly Time-Varying Channel

As channel time-variation increases, orthogonality among subcarriers in an orthogonal frequency division multiplexing (OFDM) symbol can be destroyed because of the relatively long symbol period, whereupon intercarrier interference (ICI) appears, and hence an irreducible error floor occurs. It was shown recently that grouping pilot tones into a number of equally spaced clusters can yield better channel estimation against the doubly selective channel than placing each pilot tone in an equally spaced manner. However, a random pilot sequence was used in the literature, and an optimal sequence has not yet been studied. This paper presents how to optimize the clustered pilot sequence. First, this paper (a) proves the existence of an optimum pilot sequence and (b) suggests a guideline for finding an optimum sequence. Second, this paper shows that an optimum sequence is independent of the signal-to-noise ratio and Doppler rate, and the sequence is generally optimum for any channels. Third, this paper verifies through analysis and simulation that the coded-OFDM system with an optimized cluster pilot sequence can yield a smaller mean square channel estimation error and lower bit error rate than the system with an equidistance pilot or a random cluster pilot under a rapidly time-varying channel.

An Affine Projection Algorithm With Periodically Evolved Update Interval

We propose a novel affine projection algorithm (APA) that updates the weights intermittently. While the conventional APA updates the weights at each time instant, the proposed APA performs an intermittent update of the weights through dynamically adjusting the update interval. The adjustment of the update interval is accomplished by comparing the squared output error with a threshold derived from the steady-state mean-squared error. Experimental results show that the proposed algorithm has improved performance in terms of its convergence rate and steady-state error while reducing the number of updates.

Rate-Compatible SFBC-OFDM under Rapidly Time-Varying Channels

An inter-carrier interference (ICI) occurs in an orthogonal frequency division multiplexing (OFDM) system when a channel is rapidly time-varying. The proposed scheme employs a polynomial cancellation code (PCC) to suppress this ICI, and uses a linear complex field (LCF) code as a transmit-diversity technique to make the transmission rate compatible up to {nT} / {(nT+1)}, where nT is the number of transmit antennas used. The conventional orthogonal code (OC), such as the Alamouti code, with accompanying PCC cannot achieve this rate. Both analysis and simulation results verify that the proposed scheme can achieve higher channel capacity as well as lower bit error rate (BER) than the other schemes in rapidly time-varying channels, even with a linear receiver and inaccurate channel estimation.

New Subcarrier Allocation for Uplink-OFDMA under Time-Varying Channels

Inter-carrier interference (ICI) and multiuser interference (MUI) occur in an uplink orthogonal frequency division multiple access system if a channel between a mobile user and a base station is rapidly time-varying. To reduce the ICI and MUI, this letter proposes a subcarrier allocation method. The key idea is to allocate a high-speed users subcarrier near a low-speed users one. Then, a wide Doppler spread of a high-speed user can be spilled over a low-speed users subcarrier band with insignificant influence, and the signal-to-interference-plus-noise ratio (SINR) at each high-speed users subcarrier can be maximized. This can provide a significant performance improvement of multiuser detection in an uplink-OFDMA.

Effects of Subcarrier Interleaving on LDPC Coded MC-CDMA Systems

This paper describes the effect of subcarrier interleaving on low density parity check (LDPC) coded multi-carrier code division multiple access (MC-CDMA) system. Subcarrier interleaving in MC-CDMA system can achieve the randomization effect of burst error in frequency do- main, but it generates the distortion among the spreading codes due to the destruction of orthogonality. In channel-coded system ,h o wever, the achievable frequency diversity effect associated with the subcarrier interleaving in frequency domain also depends on the channel coding gain. So, there is a trade off between frequency diversity gain via sub- carrier interleaving or channel coding, and destruction of orthogonality. From the simulation results, frequency diversity gain by channel coding is more effective than subcarrier interleaving, especially for system with high order modulations and with low code rates.

Hybridization of XRF/XPS and scatterometry for Cu CMP process control

This paper demonstrates the synergy between X-rays techniques and scatterometry, and the benefits to combine the data to improve the accuracy and precision for in-line metrology. Particular example is given to show that the hybridization addresses the challenges of aggressive patterning. In 10nm node back-end-of-line (BEOL) integration, we show that the hybridized data between scatterometry and simultaneous X-Ray Fluorescence (XRF) and X-ray Photoelectron Spectroscopy (XPS) provided the closest dimensional correlation to TEM results compared to the individual technique and CDSEM.

Enhanced phase tracking for unique word based SC-FDE on frequency selective channels

This paper proposes a new residual carrier frequency offset (CFO) estimation technique for the unique word (UW) based single-carrier transmission with frequency domain equalizer (SC-FDE) on frequency selective channels. In extremely frequency selective channels, UW symbols for CFO estimation can also be corrupted by the inter-symbol interference (ISI). Since the UW can be included in the equalizer, we employ the FDE before the CFO estimation. Moreover, we utilize the repeated pattern of UW symbols to improve the estimation accuracy.

Analysis of multimedia streaming service over server-based many-to-many overlay multicast

Among many overlay multicast algorithms, there is one that uses powerful and dedicated servers as multicast routers in the application level The multicast tree is constructed on the servers, which is referred to as server-based overlay multicast (SOM) Clients who wish to join just access one of the servers The SOM is adequate for real time multimedia applications, such as video conferencing In this paper, we want to find the answer to the following question: “What is the best form of network topology that has minimum cost and maintains a minimum quality of service (QoS) of multimedia application?” To answer this question, we made analytical models of SOM with three different topologies–linear, star and hybrid types that combine the linear and star topologies–and compared their performances We also tried to answer the maximum number of clients that can be served by a group of servers for a given QoS such as tolerable delay We validated the analysis with an extensive simulation.

An efficient channel tracking method for OFDM based high mobility wireless multimedia system

Preamble based channel estimation is widely used in OFDM based wireless multimedia system. However, the channel response varies significantly during one transmission packet due to high mobility. We propose new channel estimation method that tracks the channel variation along the time. We take advantage of the pilots occupied for the phase tracking to catch the variation of the channel responses with slightly modified pilot structure. For the accurate channel tracking, the variation trend of the channel is examined by correlation analysis. In here, we see the linear channel variation and so we use the linear interpolation for channel tracking. Without loss of data rate, the method reduces the error floor of bit error rate (BER) dramatically. Simulation results demonstrate the capability of the proposed channel tracking scheme even though the Doppler shift is very large. Moreover, the proposed method shows more competitive performance in convolutional coded systems used in many wireless multimedia application like IEEE 802.11a.