Browsing by Author "Shieh, WY"

Now showing 1 - 7 of 7

A novel architecture for multilane-free-flow electronic-toll-collection systems in the millimeter-wave range
(2005-09-01)
Shieh, WY
;
Lee, WH
;
Tung, SL
;
Ho, CD
;
資訊工程學系
An architecture for simultaneously performing multitarget tracking and multidata communication suitable for millimeter-wave multilane-free-flow electronic-toll-collection (ETC) systems is presented. This architecture combines the idea of frequency multiplexing in communication systems and the technique of target tracking in the pulse-Doppler radar. For target tracking, we make use of pulse ranging by the aid of pulse compression and fine target-direction determination by amplitude comparison to obtain high resolution in the radial direction (downrange), as well as both lateral directions (cross ranges). This architecture can be utilized for both active and passive onboard-unit (OBU) systems. Another important advantage of this architecture is that it can easily identify the passing vehicles not equipped with an OBU. This will activate some subsequent enforcement activities against the violation vehicles.
A statistics-based approach to incrementally update inverted files
(2003)
Shieh, WY
;
Chung, CP
;
資訊工程學系
;
Department of Computer Science
Many information retrieval systems use the inverted file as indexing structure. The inverted file, however, is not suited to supporting incremental updates when new documents are to be added to an existing collection. Most studies suggest dealing with this problem by sparing free space in an inverted file for future updates. In this paper, we propose a run-time statistics-based approach to allocate the spare space. This approach estimates the space requirements in an inverted file using only a little most recent statistical data on space usage and document update request rate. For best indexing speed and space efficiency, the amount of the spare space to be allocated is determined by adaptively balancing the trade-offs between reorganization count and space utilization. Simulation results show that the proposed space-sparing approach significantly avoids reorganization in updating an inverted-file, and in the meantime, unused free space can be well controlled such that the file access speed is not affected.
A statistics-based approach to incrementally update inverted files
(2005-03-01)
Shieh, WY
;
Chung, CP
;
資訊工程學系
;
Department of Computer Science
Many information retrieval systems use the inverted file as indexing structure. The inverted file, however, requires inefficient reorganization when new documents are to be added to an existing collection. Most studies suggest dealing with this problem by sparing free space in an inverted file for incremental updates. In this paper, we propose a run-time statistics-based approach to allocate the spare space. This approach estimates the space requirements in an inverted file using only a little most recent statistical data on space usage and document update request rate. For best indexing speed and space efficiency, the amount of the spare space to be allocated is determined by adaptively balancing the trade-offs between reorganization reduction and space utilization. Experiment results show that the proposed space-sparing approach significantly avoids reorganization in updating an inverted file, and in the meantime, unused free space can be well controlled such that the file access speed is not affected. (C) 2003 Elsevier Ltd. All rights reserved.
A tree-based inverted file for fast ranked-document retrieval
(2003)
Shieh, WY
;
Chen, TF
;
Chung, CP
;
資訊工程學系
;
Department of Computer Science
Inverted files are widely used to index documents in large-scale information retrieval systems. An inverted file consists of posting lists, which can be stored in either a document-identifier ascending order or a document-weight descending order. For an identfier-ascending-order posting list, retrieving ranked documents necessitates traversal of all postings, whereas for the weight-descending-order posting list, performing Boolean queries involves very complex processing. In this paper, we transform a posting list to a tree-based structure, called the n-key-heap posting tree, to speedup ranked-document retrieval for Boolean queries. In this structure, the orders of document identfiers and document weights are preserved simultaneously. To preserve the identifier order, the edge pointers are designed to maintain numerical order in the posting tree. To preserve the weight order, greater-weight postings are stored in higher tree nodes by the heap property. We model these criteria to a tree-construction problem and propose an efficient algorithm to construct an optimal posting tree having the minimal access time.
An inverted file cache for fast information retrieval
(2003-07-01)
Shieh, WY
;
Shann, JJJ
;
Chung, CP
;
資訊工程學系
;
Department of Computer Science
The inverted file is the most popular indexing mechanism used for document search in an information retrieval system (IRS). However, the disk I/O for accessing the inverted file becomes a bottleneck in an IRS. To avoid using the disk I/O, we propose a caching mechanism for accessing the inverted file, called the inverted file cache (IF cache). In this cache, a proposed hashing scheme using a linked list structure to handle collisions in the hash table speeds up entry indexing. Furthermore, the replacement and storage mechanisms of this cache are designed specifically for the inverted file structure. We experimentally verify our design, based on documents collected from the TREC (Text REtrieval Conference) and search requests generated by the Zipf-like distribution. Simulation results show that the IF cache can improve the performance of a test IRS by about 60% in terms of the average searching response time.
Inverted file compression through document identifier reassignment
(2003-01-01)
Shieh, WY
;
Chen, TF
;
Shann, JJJ
;
Chung, CP
;
資訊工程學系
The inverted file is the most popular indexing mechanism for document search in an information retrieval system. Compressing an inverted file can greatly improve document search rate. Traditionally, the d-gap technique is used in the inverted file compression by replacing document identifiers with usually much smaller gap values. However, fluctuating gap values cannot be efficiently compressed by some well-known prefix-free codes. To smoothen and reduce the gap values, we propose a document-identifier reassignment algorithm. This reassignment is based on a similarity factor between documents. We generate a reassignment order for all documents according to the similarity to reassign closer identifiers to the documents having closer relationships. Simulation results show that the average gap values of sample inverted files can be reduced by 30%, and the compression rate of d-gapped inverted file with prefix-free codes can be improved by 15%. (C) 2002 Elsevier Science Ltd. All rights reserved.
Leakage current reduction of chemical-vapor-deposited Ta2O5 films on rugged polycrystalline silicon electrode for dynamic random access memory application
(1999-04-01)
Lin, M
;
Chang, CY
;
Huang, TY
;
Shieh, WY
;
電子工程學系及電子研究所
In this paper we describe the electrical and physical characterizations of Ta2O5 films on rapid thermal nitrided (RTN) rugged polycrystalline silicon electrodes for 256 M dynamic random access memory (DRAM) application. To overcome the higher leakage on Ta2O5 films with rugged poly-Si bottom electrodes, we have successfully employed a light oxidation on rugged poly-Si grains for improving the acute angle of surface morphology, and a post-treatment with rapid thermal nitridation of N2O on Ta2O5 films to reduce the leakage. In addition, a RTN pre-treatment is also applied to prevent rugged poly-Si oxidation during a Ta2O5 film deposition that compromises the effective Ta2O5 thickness. Our results show that Ta2O5 film with an effective thickness of 1.4 nm (physical thickness=10 nm) and a low leakage current density of less than 1 X 10(-8) A/cm(2), suitable for 256 M DRAM application, can be achieved. Our results also show that while a light oxidation on rugged poly-Si grains could improve the acute angle between the rugged poly-Si and its congruent amorphous silicon sub-layer, over oxidation on rugged poly-Si grains could degrade the leakage current and the time dependent dielectric breakdown (TDDB) characteristics of the stacked capacitor.