• Current Optics and Photonics
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• 제8권4호
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• pp.406-415
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• 2024

The major error factors that degrade the efficiency of coherent beam combining (CBC) are numerically studied in a comprehensive manner, paying particular attention to phase, tip-tilt, polarization angle, and beam quality. The power in the bucket (PIB), normalized to the zero-error PIB, is used as a figure of merit to quantify the effect of each error factor. To maintain a normalized PIB greater than or equal to 95% in a 3-channel CBC configuration, the errors in phase, tip-tilt, and polarization angle should be less than 1.06 radians, 1.25 ㎛, and 1.06 radians respectively, when each of the three parameters is calculated independently with the other two set to zero. In a worst-case scenario of the composite errors within the parameter range for the independent-95%-normalized-PIB condition, the aggregate effect would reduce the normalized PIB to 83.8%. It is noteworthy that the PIB performances of a CBC system, depending on phase and polarization-angle errors, share the same characteristic feature. A statistical approach for each error factor is also introduced, to assess a CBC system with an extended number of channels. The impact of the laser's beam-quality factor M² on the combining efficiency is also analyzed, based on a super-Gaussian beam. When M² increases from 1 to 1.3, the normalized PIB is reduced by 2.6%, 11.8%, 12.8%, and 13.2% for a single-channel configuration and 3-, 7-, and 19-channel CBC configurations respectively. This comprehensive numerical study is expected to pave the way for advances in the evaluation and design of multichannel CBC systems and other related applications.

• 한국철도학회:학술대회논문집
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• 한국철도학회 1999년도 춘계학술대회 논문집
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• pp.208-217
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• 1999

This paper presents spectrum analysis technique to identify abnormal noise produced in diesel railcars. Coherence and coherent power spectrum are very useful in determining source identifications. Experiments are applied to two different cars with similar phenomena about the case of stop and running condition. Metal contact in mounting bracket part which is excited by excessive transmission vibration is found to cause abnormal noise.

• 대한전자공학회논문지TC
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• 제49권5호
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• pp.72-76
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• 2012

이동성이 있는 센서 노드로 구성 된 센서 네트워크를 효율적으로 사용하기 위하여 고려해야 할 사항 중 중요한 것이 자원의 효율적인 사용과 신호를 빠르고 정확하게 보내는 것이다. 본 논문에서는 무선 센서 네트워크 내에서 SNR(Signal to Noise Ratio)과 릴레이 노드의 신호 증폭률 그리고 노드와 노드 간 직접적으로 통신하는 경우 경로의 이득의 변화에 따른 무선 센서 네트워크의 특성에 대하여 분석하고 다수의 협동 릴레이 노드가 존재할 때 선택기준을 제시하고 이에 따른 영향을 정량적으로 평가하였다.

• 설비공학논문집
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• 제3권2호
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• pp.103-113
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• 1991

This paper presents a method for estimating the noise source contribution of a pulse combustion burner in a multiple input system where the input sources may be coherent each other. By coherence function method, it is found that the biggest part of the noise source in the pulse combustion burner is generated by the part of the combustion chamber. This analysis is modeled as three input / single output system because the noise generating mechanism of the pulse combustion burner is very complicated. The coherence function method is proved to be useful tool for the identification of noise source. The overall levels of the radiated source pressure by coherence function method are compared with those measured and calculated by the frequency response function approach. The experimental results have shown a good agreement with the results calculated by the coherence function method when the input sources are coherent strongly each other. The estimation of shield effect by FRF method indicates that significant reduction can be achieved in sound radiation if only transmission path generated by the part of combustion chamber is acoustically shield.

• 대한전자공학회논문지TC
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• 제41권11호
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• pp.67-72
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• 2004

본 논문에서는 cdma2000-1x FDD 시스템의 순방향 링크 송신 성능 향상을 위해 공통빔(파이럿, 페이징, 싱크 채널)과 개별통화 채널에 대한 진폭과 위상을 정합 시키는 빔 형성 기법을 제안한다. 기존의 기법에서는 상이한 두 채널간의 위상이 정확히 일치함을 가정하여 시스템의 성능이 평가 되었다. 그러나 실제 CDMA 시스템 환경에서는 두 채널간의 위상이 상이할 경우 비트 오율이 심각히 열화 되므로 현실적으로 기존의 제안된 방법은 사용할 수 없다. 따라서 기존의 순방향 빔 형성 기법과 달리 제안된 순방향 빔 형성기법, 즉 공통채널과 통화채널의 크기와 위상을 맞추기 위한 빔합성 기법을 적용하여 기존 방법대비 낮은 비트오율로 양질의 통신이 가능함을 확인하였다.

• Asian review of World Histories
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• 제4권1호
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• pp.83-123
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• 2016

We describe a vision of historical analysis at the world scale, through the digital assembly of historical sources into a cloud-based database, where machine-learning techniques can be used to summarize the database into a time-integrated actor-to-actor complex network. Using this time-integrated network as a template, we then apply the method of automatic narratives to discover key actors ('who'), key events ('what'), key periods ('when'), key locations ('where'), key motives ('why'), and key actions ('how') that can be presented as hypotheses to world historians. We show two test cases on how this method works. To accelerate the pace of knowledge discovery and verification, we describe how historians would interact with these automatic narratives through an online, map-based knowledge aggregator that learns how scholars filter information, and eventually takes over this function to free historians from the more important tasks of verification, and stitching together coherent storylines. Ultimately, multiple coherent storylines that are not necessary compatible with each other can be discovered through human-computer interactions by the map-based knowledge aggregator.

• 한국전자파학회논문지
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• 제16권2호
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• pp.182-188
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• 2005

본 논문에서는 IEEE 802.11b WLAN시스템에서 간섭 억제에 관한 효과적인 기술을 제시하였다 선형 제약형 적응 어레이 처리기는 동일 채널 간섭 신호와 다중 경로 효과를 효과적으로 감소시켜 WLAN 장치의 수신 성능 향상을 위하여 제안되었다. 공간 유화 방법은 다중경로 환경에서 원하는 신호의 왜곡을 방지하는데 사용되며, integral null 지정 방법은 실내 환경에서 어레이 성능을 향상시키기 위해 제안하였다. 공간 유화 방법이 선형 제약형 방법보다 성능이 보다 우수하였으며, integral null지정 방법을 적용한 어레이 시스템이 가장 좋은 성능을 나타내었다. 본 논문에서 제안된 방법은 동일 채널 신호, 다중 경로 신호, 그리고 긴WPAN 간섭을 효과적으로 제거함으로써 WLAN의 성능이 향상될 것으로 기대된다.

• 전기전자학회논문지
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• 제22권3호
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• pp.599-604
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• 2018

계단 주파수 레이더는 송신 펄스의 주파수를 일정한 간격으로 증가시켜 넓은 합성 대역폭을 생성함으로써 고해상도 거리추정을 구현하는 방식이다. 그러나 이동표적의 경우에는 거리-도플러 결합 현상으로 인해 정확한 거리 추정이 어렵게 된다. 본 논문에서는 초기에 코히어런트 펄스열을 갖는 계단 주파수 레이더 파형을 이용하여 이동표적의 속도를 추정하고, 거리-도플러 결합 현상을 보상함으로써 얻어지는 고해상도 거리 추정 과정을 분석하고, 시뮬레이션을 통해 이를 검증하였다.

• Journal of Positioning, Navigation, and Timing
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• 제3권3호
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• pp.107-116
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• 2014

When an incoming Global Positioning System (GPS) signal is acquired, pull-in search performs a finer search of the Doppler frequency of the incoming signal so that phase lock loop can be quickly stabilized and the receiver can produce an accurate pseudo-range measurement. However, increasing the accuracy of the Doppler frequency estimation often involves a higher computational cost for weaker GPS signals, which delays the position fix. In this paper, we show that the Doppler frequency detectable by a long coherent auto-correlation can be accurately estimated using a complex-weighted sum of consecutive short coherent auto-correlation outputs with a different Doppler frequency hypothesis, and by exploiting this we propose a noise resistant, low-cost and highly accurate Doppler frequency and phase estimation technique based on a reverse directional application of the finite rate of innovation (FRI) technique. We provide a performance and computational complexity analysis to show the feasibility of the proposed technique and compare the performance to conventional techniques using numerous Monte Carlo simulations.

• Bulletin of the Korean Chemical Society
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• 제34권2호
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• pp.465-469
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• 2013

Proton transfer reaction is one of the most fundamental processes in chemistry and life science. Excited state intramolecular proton transfer (ESIPT) has been studied as a model system of the proton transfer, since it can be conveniently initiated by light. We report ESIPT reaction dynamic of 1-hydroxy-anthraquione (1-HAQ) in solution by highly time-resolved fluorescence. ESIPT time of 1-HAQ is determined to be $45{\pm}10$ fs directly from decay of the reactant fluorescence and rise of the product fluorescence. High time resolution allows observation of the coherent vibrational wave packet motion in the excited state of the reaction product tautomer. The coherently excited vibrational mode involves large displacement of the atoms, which shortens the distance between the proton donor and the acceptor. With the theoretical analysis, we propose that the ESIPT of 1-HAQ proceeds barrierlessly with assistance of the skeletal vibration, which in turn becomes excited coherently by the ESIPT reaction.