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Block Type 파일럿 배치를 적용한 OFDM 시스템의 등화 기법 개선
김환우,김지헌,Kim Whan-Woo,Kim Ji-Heon 한국음향학회 2006 韓國音響學會誌 Vol.25 No.3
This paper is concerned with a equalization technique for Orthogonal Frequency Division Multiplexing (OFDM) systems based on a block type pilot arrangement over slow fading channels. The bit rates obtained in underwater channels are relatively modest compared to some other communication channels such as cellular phones or indoor wireless systems. Consequently. the Doppler effect is the important parameter in tracking a channel. In case of a coherent demodulation scheme, the residual mean phase errors due to Doppler frequency may be fatal for the performance of the system. The equalizer could not solely handle mean Doppler shift. To account for the common Doppler effect a phase error tracking loop is used with the frequency equalizer. so that the rotation errors are avoided. Furthermore. simulations show that we can reduce the computational load of the tracking loop with negligible effect on performance. 본 논문은 slow 페이딩 채널에서 block type 파일럿 배치에 기반한 OFDM (Orthogonal Frequency Division Multiplexing) 시스템의 등화 기법을 다루고 있다. 수중 채널에서 얻을 수 있는 비트 속도는 셀룰러 폰이나 실내 무선 시스템과 같은 여타 통신 채널에 비해 상대적으로 낮은 편이며, 따라서 채널 추적시 도플러 효과가 중요한 파라미터가 된다. Coherent 복조 방식의 경우 도플러 주파수에 의한 잔여 평균위상에러는 시스템 성능에 치명적으로 작용할 수 있으며, 등화기만으로는 평균 도플러 쉬프트 효과에 대처하지 못할 수 있다. 공통 도플러 효과에 대처하기 위해 주파수 등화기와 더불어 위상에러 추적회로를 사용하여 회전 에러를 배제할 수 있다. 아울러 성능 저하를 최소화하는 수준에서 추적회로의 연산 부담을 줄일 수 있음을 시뮬레이션을 통해 증명하였다.
김환우 충남대학교 1991 論文集 Vol.14 No.1
The performance of the second-order Born approximation method for the ultrasound tomography algorithm is studied by comparing the scattered fields computed using the first-order Born equation, the first-order Rytov equation and the second-order Born equation. The second-order Born equation turns out to provide more desirable results than the other two equations. Phantom images with resolutions of up to 1 pixel x 1 pixel are satisfactorily reconstructed using the second-order Born equation. When the view angle is limited, good reconstruction results are also obtained using multi-frequency incident fields.
소형 물체의 검사를 위한 고해상도 미세 초점 X선 단층 촬영 시스템
구자용,김영주,이승석,김환우 韓國非破壞檢査學會 1998 한국비파괴검사학회지 Vol.18 No.3
광원의 직경이 5㎛ 수준인 X선을 이용하여 미세 초점 X-선 단층촬영장치(computerized tomography system)를 제작하고 제어 및 계산 프로그램을 개발하였다. 본 장치는 비파괴검사용으로서 직경20mm 이내의 시험체를 검사할 수 있다. 단층영상을 얻기 위한 계산 알고리즘으로서는 convolution back projection 알고리즘을 사용하였으며 광원의 형상을 평행 빔과 부채꼴 빔(fan beam)등 두 종류를 사용하여 결과를 비교하였다. 장치의 제어 및 계산은 개인용 컴퓨터에서 동작하도록 하였다. 제작된 CT 장치와 프로그램을 이용하여 제작된 시편에 대하여 단층 영상을 복원하고 시편의 내부 구조 및 결함을 관찰하였다. 단층영상을 얻은 결과 실제 모습과 일치하는 재료내부의 결함에 대한 정보를 얻을 수 있었으며 해상도는 20~30㎛정도인 것으로 판단되었다. A computerized tomography system was developed using the X-ray source that has diameter of 5 micrometer. The system is used for the nondestructive testing of specimens with diameter below 20mm. The convolution back projection algorithm was adopted for the reconstruction of cross sectional image, and the shape of the X-ray beam was let parallel beam or fan beam to compare each resultant image. Our CT system was constructed to operate based on the personal computer. The sectional images of the fabricated specimens were reconstructed and analyzed. The reconstructed images well coincided with real images taken with optical microscope and gave us enough reports on the defects in the ceramic specimen. The resolution of the system regarded as about 20~30 micrometers.
金永銓,洪元杓,任洪彬,鄭海駿,李廷玟,權五憲,成世鎭,金煥宇,李文洙,康泰賢 충남대학교 공업교육연구소 1980 論文集 Vol.3 No.1
This paper is intended to provide a guideline to improve technical high school teacher education in the College of Industrial Education of Chungnam National University. The thesis consists of three parts, namely, pedagogic education, equalizing difference of basic knowledge due to dual criteria of student entry requirements and employment potentials for graduates. The main conclusions and recommendations are summarized as follows; 1. The portion of pedagogic subjects of the College is rated at 12.5% or 20 credits and was concluded insufficient in comparison of those of foreign colleges. These subjects should be intensified with a balanced education in major subjects taking into account that the entry of the college does not require the pre-experiences in the major fields. 2. In order to produce desirable teachers, it is recommendable to reinforce the general education, especially in the field of psychology. 3. The dual curriculum system was verified not to be so satisfactory for the students from general high schools and technical high schools in equalization of basic knowledge competence. Further and consecutive analysis should be undertaken to secure better education. 4. The number of student enrollment was analyzed to be much larger than the demand scale of technical high school teachers. Therefore counter-measures for surplus supply should be searched for example reduction in enrollment number, supplemental education to transfer surplus graduates into general middle and high school teachers of technical subjects, employment control of graduates from general engineering colleges, etc.
Transitional Butterworth-Chebyshev 近似式을 구하는 프로그램 開發에 關한 연구
金煥宇 충남대학교 공업교육연구소 1982 論文集 Vol.5 No.1
In this thesis, a computer program is developed to find the Transitional Butterworth-Chebyshev approximations. By using it, we can find any transitional approximation, including Butterworth and Chebyshev approximations, whose dimension ranges from 1 to 10 and which has the cut-off frequency of 1 rad/sec, ripple facter ε, and transition constant k which ranges from 0 to the dimension of the approximation, and we can get its magnitude and phase characteristics. As an example, five 8-th order approximations whose transition constants k's are 0, 2, 4, 6, and 8 respectively are presented, and their magnitude and phase characteristics are calculated.
능동 저역 여파기 설계를 위한 프로그램 개발에 관한 연구
金煥宇 충남대학교 공업교육연구소 1982 論文集 Vol.5 No.2
A computer program was developed to find the values of circuit components in an active lowpass filter network. A fourth-order low-pass filter is realized by the use of the program, from which |G(if)| is obtained and compared with the theoretical values.