• Journal of KIISE:Databases
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• v.37 no.5
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• pp.228-237
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• 2010

Although conventional index structures provide various nearest-neighbor search algorithms for high-dimensional data, there are additional requirements to increase search performances as well as to support index scalability for large scale data. To support these requirements, we propose a distributed high-dimensional indexing structure based on cluster systems, called a Distributed Vector Approximation-tree (DVA-tree), which is a two-level structure consisting of a hybrid spill-tree and VA-files. We also describe the algorithms used for constructing the DVA-tree over multiple machines and performing distributed k-nearest neighbors (NN) searches. To evaluate the performance of the DVA-tree, we conduct an experimental study using both real and synthetic datasets. The results show that our proposed method contributes to significant performance advantages over existing index structures on difference kinds of datasets.

• Proceedings of the Korean Information Science Society Conference
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• 2003.10b
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• pp.223-225
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• 2003

분산 데이터베이스 시스템 환경에서는 특정 노드로 집중되는 부하의 분산이나 가용성 및 안정성 제공을 위해 데이터 분할기법 (fragmentation)과 복제기법(replication)을 사용한다. 이때 전송된 데이터에 대한 기존의 색인 재활용 기법과 벌크 로딩(bulk loading) 기법은 효율적인 색인 구성을 위해 논리적인 페이지 포인터를 물리적 주소로 변환하는 물리적 사상구조를 필요로 하거나, 색인 구성시간과 검색성능 모두를 향상시키지 못하는 문제점을 지닌다. 본 논문에서는 이와 같은 문제점을 해결하기 위해 색인 전송기법을 제안한다. 본 기법은 색인 재활용을 위해 물리적 사상구조를 추가로 유지하거나, 검색 성능을 향상시키기 위해 전체 데이터 집합을 정렬하는 것이 아니라, 데이터가 전송될 사이트에 색인구조물 저장하기 위한 물리적 공간은 예약하고 예약된 공간에 색인구조를 전송, 기록함으로써 색인 구성비용을 줄이게 된다. 또한 예약된 공간을 연속적인 페이지구조로 구성함으로써 색인 구성 시 자식노드에 대한 위치정보를 예상하여 부모노드가 지니는 자식노드에 대한 위치정보 기록 비용을 줄일 수 있다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.11B
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• pp.1607-1611
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• 2001

The dependence of the performance of 40-Gb/s optical transmission using short pulses on the fiber chromatic dispersion is numerically studied. When very short pulses are used, the wide spectrum of the optical signal and the chromatic dispersion of the fiber interact in such a way that results in the reduction of nonlinear impairments of the transmission performance. The degree of this reduction is determined by the combined effects of chromatic dispersion of the fiber and the strength of the optical signal and the transmission distance. When 3ps-long pulses were used for the transmission, the eye-closure penalty was highest with the dispersion D=4ps/nm/km.

• 한국정보통신설비학회:학술대회논문집
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• 2003.08a
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• pp.40-41
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• 2003

대용량 정보 통신 선로로 이용되고 있는 단일모드 광섬유에서, 초고속. 장거리 광신호 전송에서 발생하는 색분산(Chromatic dispersion)을 보상하기 위하여 비주기 광섬유 격자(Chirped fiber Bragg grating ; CFG)를 적용하여 그 특성을 조사, 분석 하였다. 10Gbps의 전송 시스템에서 80km 단일모드 광섬유 전송후의 색분산에 의한 광신호 품질 열화 및 CFG에 의한 보상 특성을 측정하기 위하여 BER 과 eye pattern 테스트를 하였다. 또한 기존에 색분산 보상용으로 사용되고 있는 분산 보상 광섬유(Dispersion compensation fiber ; DCF)와 특성 비교을 하였다.

• Journal of the Korea Society of Computer and Information
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• v.14 no.10
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• pp.73-82
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• 2009

This paper proposes a variance recovery method for character strokes that can be missed in applying the previously proposed color variance approach in text detection of natural scene images. The previous method has a shortcoming of missing the color variance due to the fixed length of horizontal and vertical windows of variance detection when the character strokes are thick or long. Thus, this paper proposes a variance recovery method by using geometric information of bounding boxes of connected components and heuristic knowledge. We have tested the proposed method using various kinds of document-style and natural scene images such as billboards, signboards, etc captured by digital cameras and mobile-phone cameras. And we showed the improved text detection accuracy even in the images of containing large characters.

• The Journal of the Korea institute of electronic communication sciences
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• v.8 no.8
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• pp.1235-1240
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• 2013

We investigate an approximate analytic expression of signal distortion caused by the interaction between self-phase modulation (SPM) and dispersion in the dispersion-managed optical transmission where the dispersion is optimally compensated. From the analytic study, we obtain the analytic expression of the signal distortion in dispersion-managed optical transmission system. To confirm the validity of the analytic expression, we show that the eye-opening penalties calculated by the analytic expression correspond with the simulation results. Using the analytic result, we can easily estimate the signal distortion without complex nonlinear simulations.

• Proceedings of the Optical Society of Korea Conference
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• 2000.02a
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• pp.82-83
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• 2000

초고속 광통신 시스템 구현에 있어서 핵심기술의 하나인 광섬유의 색분산 제어와 함께 그 정확한 측정을 위한 여러 방법들이 연구되어 왔으나, 기존의 방법들은 실험 장치가 복잡하고 광대역의 광원을 필요하므로 저가의 시스템 구현 및 실제적 응용이 어렵다.$^{(1)}$ 실제 시스템에 장착되어 있는 필드에서 응용 될 수 있기 위해서는 소형이며 온도와 같은 외부 환경에 민감하지 않은 시스템을 필요로 한다. 본 연구에서는 실제필드에서의 광섬유에 대한 정확한 색분산 측정을 위한 시스템 개발을 목적으로 이득스위칭된 다모드레이저 및 고분산광섬유(Highly Dispersive Fiber)를 이용한 색분산 측정시스템을 제안한다. (중략)

• Korean Journal of Optics and Photonics
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• v.10 no.2
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• pp.169-173
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• 1999

We have experimentally investigated the eye margin characteristics of intensity modulated 10Gb/s NRZ signals in optically amplified systems depending on optical signal-to-noise ratio and chromatic dispersion. For the practical system application, the minimum optical signal-to-noise ratio was 25 dB. We utilized the negative chirped transmitter and a dispersion compensation fiber to compensate the chromatic dispersion in single mode fiber. We found the optimum eye opening and receiver sensitivity in case of the residual dispersion of 0~+935 ps/nm.

• The Journal of the Korea institute of electronic communication sciences
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• v.8 no.9
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• pp.1343-1349
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• 2013

In optical fiber communications, several newly-developed signal formats are used to obtain the best performance within limited spectral bandwidth. CSRZ (carrier-suppressed return-to-zero) format is one of the new signal formats, which has better spectral efficiency and better robustness to dispersion than RZ (return-to-zero) format. Thus it is widely used for demonstrating high-speed optical communication systems. In an earlier research, we proposed a clock-extraction method of CSRZ signal to monitor chromatic dispersion. However, the clock-frequency component extracted by the clock-extraction method can be affected by the bandwidth of a transmitter or a receiver. Therefore, in this paper, we investigate the effect of bandwidth on the chromatic dispersion monitoring of CSRZ signal based on clock-frequency component. As a result, we propose a couple of robust clock-extraction methods to monitor chromatic dispersion in CSRZ signal.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.13 no.6
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• pp.462-469
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• 1988

Choromatic dispersion of single mode optical fibers limits the maximum bit rate in the single mode optical transmission systems. In this paper we calculate chromatic dispersion and dispersion parametes(zero dispersion wavelength, dispersion slope) for concatenated single mode optical fibers form the chromatic dispersion and dispersion parameters of individual single mode optical fibers, and compare theoretical valuse with those measrued by double demodulation method. Calculated values are coincident with measured ones reasonably. The standard deviation is 0.77-1.28nm for zero dispersion wavelength and 0.56-0.75PS/km.nm2 for dispersion slope. The standard deviation of zero dispersion wavelength is inverse proportional to the optical fiber lenght.