Browsing by Author "Chang, Li-Jen"

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An Error-Floor Reduction Technique for Short-Length LT Codes
(2014-01-01)
Chang, Li-Jen
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Wang, Chung-Hsuan
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Zao, John K.
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資訊工程學系
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電機資訊學士班
Luby transform (LT) codes are often employed during best-effort packet transfers to offer rateless erasure protection. Efficient as they are, these randomized codes with a small number of input symbols often post an inevitable performance trade-off between the decoding failure rates in their waterfall and error-floor regions. In order to surmount this trade-off, we propose a new encoding strategy that requires a portion of the high-degree output symbols of an LT code to abandon the conventional equally-probable input selection strategy; instead, connect themselves to some specially selected low-degree input symbols. Our simulation results show that shortlength LT codes with merely 10(3) input symbols yield significantly lower symboVhlock failure rates at a small overhead of reception as they employ the proposed strategy. These performanceenhanced rateless codes can have potential applications in realtime multimedia broadcasting.
An integrated approach with new strategies for QSAR models and lead optimization
(2017-01-01)
Hsu, Hui-Hui
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Hsu, Yen-Chao
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Chang, Li-Jen
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Yang, Jinn-Moon
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生物科技學系
Background: Computational drug design approaches are important for shortening the time and reducing the cost for drug discovery and development. Among these methods, molecular docking and quantitative structure activity relationship (QSAR) play key roles for lead discovery and optimization. Here, we propose an integrated approach with core strategies to identify the protein-ligand hot spots for QSAR models and lead optimization. These core strategies are: 1) to generate both residue-based and atom-based interactions as the features; 2) to identify compound common and specific skeletons; and 3) to infer consensus features for QSAR models. Results: We evaluated our methods and new strategies on building QSAR models of human acetylcholinesterase (huAChE). The leave-one-out cross validation values q(2) and r(2) of our huAChE QSAR model are 0.82 and 0.78, respectively. The experimental results show that the selected features (resides/atoms) are important for enzymatic functions and stabling the protein structure by forming key interactions (e.g., stack forces and hydrogen bonds) between huAChE and its inhibitors. Finally, we applied our methods to arthrobacter globiformis histamine oxidase (AGHO) which is correlated to heart failure and diabetic. Conclusions: Based on our AGHO QSAR model, we identified a new substrate verified by bioassay experiments for AGHO. These results show that our methods and new strategies can yield stable and high accuracy QSAR models. We believe that our methods and strategies are useful for discovering new leads and guiding lead optimization in drug discovery.
OPTIMIZED RATELESS UEP CODES FOR SCALABLE VIDEO STREAMING
(2012)
Zao, John K.
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Hornansky, Martin
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Diao, Pei-Lun
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Wang, Bo-Wei
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Wang, Chung-Hsuan
Wireless video streaming often suffers from heavy and varying amount of packet loss. Short-length Luby Transform (SLLT) codes are often employed to offer necessary rateless or universal erasure protection. Nevertheless, a proficient method for finding the optimal degree distributions of SLLT codes is still missing although the asymptotic behaviors of infinite-length LT codes have long been deduced. In this paper, we propose a practical approach to find the optimal degree distributions of LT codes with arbitrary input block lengths. Our approach starts with a new fitness model for the LT codes based on three performance measurements: their coding overhead e, decoding failure ratio r and failure probability p. We then employed the exponential natural evolution strategy (xNES) along with a suitable choice of initial degree distribution and a proper transformation between the genotypes and the phenotypes to search for the optimized degree distributions. The optimized SLLT codes designed using this approach far outperforms the ones obtained through asymptotic approximation. As an example application, we constructed a family of rateless convolutional UEP codes using these optimized SLLT codes as the rateless postcodes. These codes offer superb universal unequal erasure protection to wireless H.264/SVC broadcasting with only 16%-20% coding overhead. This design exercise, however, made clear the necessity of matching the rateless erasure correction capability of the SLLT postcode with the error-rate sensitive UEP capability of the convolutional precode. The effect of code matching was demonstrated clearly in the improvement of rate-distortion performance of video playback.
Performance Analysis of Improved LT Codes With the Removal of Low Degree Input Symbols
(2020-02-01)
Wang, Chung-Hsuan
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Li, Yun-Han
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Chang, Li-Jen
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電信工程研究所
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Institute of Communications Engineering
Luby transform (LT) codes suffer from undesired error floors since a certain amount of input symbols may remain uncovered by output symbols due to the uniformly random encoding process. Several variants of LT codes presented previously aiming at removing low-degree input symbols assure that all input symbols can be covered by output symbols after encoding in high probability and have been verified to effectively reduce error floors for short and moderate block lengths. However, in this work, we show that those variants might not be asymptotically good even with the removal of low-degree input symbols in the encoding process.
Revealing the Atomic Defects of WS2 Governing Its Distinct Optical Emissions
(2018-01-24)
Lin, Yung-Chang
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Li, Shisheng
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Komsa, Hannu-Pekka
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Chang, Li-Jen
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Krasheninnikov, Arkady V.
Defects and their spatial distribution are crucial factors in controlling the electronic and optical properties of semiconductors. By using scanning transmission electron microscopy and electron energy loss spectroscopy, the type of impurities/defects in WS2 subdomains with different optical properties is successfully assigned. A higher population of Cr impurities is found in the W-terminated edge domain, while the S-terminated domain contains more Fe impurities, in accordance with the luminescence characteristics of chemical-vapor-grown WS2 of a hexagonal shape. In agreement with the first-principles calculations, the domains with Cr substitutional dopants exhibit strong trion emission. Fe atoms tend to gather into trimer configuration and introduce deep acceptor levels which compensate the n-type doping and suppress trion emission. It is also discovered that the domain with higher luminescence but smaller defect concentration tends to get oxidized more rapidly and degrade the 2D structure with many triangular holes. Excitons tend to accumulate at the edges of the oxidized triangular holes and results in enhanced PL emission. The findings indicate that choosing stable elements as dopant and controlling the number of specific edge structures within a crystal domain of 2D transitional metal dichalcogenides can be a new route to improve the optical properties of these materials.
Scanning Moire Fringe Method: A Superior Approach to Perceive Defects, Interfaces, and Distortion in 2D Materials
(2020-05-26)
Lin, Yung-Chang
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Ji, Hyun Goo
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Chang, Li-Jen
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Chang, Yao-Pang
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Liu, Zheng
Scanning moire fringe (SMF) is a widely utilized technique for the precise measurement of the strain field in semiconductor transistors and heterointerfaces. With the growing challenges of traditional chip scaling, two-dimensional (2D) materials turn out to be ideal candidates for incorporation into semiconductor devices. Therefore, a method to efficiently locate defects and grain boundaries in 2D materials is highly essential. Here, we present a demonstration of using the SMF method to locate the domain boundaries at the nearly coherent interfaces with sub-angstrom spatial resolution under submicron fields of views. The strain field of small angle grain boundary and lateral heterojunction are instantaneously found and precisely determined by a quick SMF method without any atomic resolution images.