• 한국자동차공학회논문집
• /
• 제1권3호
• /
• pp.34-45
• /
• 1993

This study aims to analyze the mixing characteristics of hydrogen considered as a new fuel for internal combustion engines. As the physical property of helium gas is similar to that of hydrogen, helium gas was used in this study. To analyze the steady and unsteady behavior of jet, helium gas was injected into the stationary atmosphere at the normal temperature and pressure. Concentration of helium gas in the center of jet flow is in inverse proportion with axial distance from the nozzle tip. This agrees with the free jet theory of Schlichting. The relative equation for dimensionless concentration to radial/axial distance the axial distance of potential core region, the cone angle a of the jet flow and the relative equation for arriving distance of the front of jet flow to the lapse of time are obtained. But free jet theory of Schlichting in the dimensionless concentration is not in agreement with the present experimental results of the distance of the radial direction. It needs more study. When the arrival frequency of jet flow is used as a parameter, the transition area changing from unsteady flow area into steady flow area becomes gradually wider downstream, but its ratio for the whole unsteady flow area gradually decreases.

• Journal of Communications and Networks
• /
• 제7권4호
• /
• pp.450-461
• /
• 2005

In this paper, we derive analytical expressions for the exact pairwise error probability (PEP) of a space-time coded system operating over spatially correlated fast (constant over the duration of a symbol) and slow (constant over the length of a code word) fad­ing channels using a moment-generating function-based approach. We discuss two analytical techniques that can be used to evaluate the exact-PEPs (and therefore, approximate the average bit error probability (BEP)) in closed form. These analytical expressions are more realistic than previously published PEP expressions as they fully account for antenna spacing, antenna geometries (uniform linear array, uniform grid array, uniform circular array, etc.) and scattering models (uniform, Gaussian, Laplacian, Von-mises, etc.). Inclusion of spatial information in these expressions provides valuable insights into the physical factors determining the performance of a space-time code. Using these new PEP expressions, we investigate the effect of antenna spacing, antenna geometries and azimuth power distribution parameters (angle of arrival/departure and angular spread) on the performance of a four-state QPSK space-time trellis code proposed by Tarokh et al. for two transmit antennas.

• 한국음향학회지
• /
• 제16권1호
• /
• pp.60-66
• /
• 1997

신호도착방향 추정에 관한 대부분의 연구는 신호원을 점신호원이라 두고 이루어져 왔으나, 실제 환경에서는 신호원을 퍼진 신호모형으로 두면 더 알맞게 추정할 수 있을 때가 많다. 신호원이 어떤 영역에 걸쳐 퍼져 있을 때 우리는 잘 알려진 신호도착방향 추정 방법들을 직접 사용할 수 없다. 이 논문에서는 2차원 퍼진 신호원을 중심각과 퍼짐의 정도로 나타내고, 3차원 공간에서 두단계 기법을 써서 모수형 퍼진 신호원의 수직, 수평각을 추정하였다.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• 제7권10호
• /
• pp.2338-2356
• /
• 2013

In cognitive radio networks, acquiring the position information of the primary user is critical to the communication of the secondary user. Localization of primary users can help improve the efficiency with which the spectrum is reused, because the information can be used to avoid harmful interference to the network while simultaneity is exploited to improve the spectrum utilization. Despite its inherent inaccuracy, received signal strength based on range has been used as the standard tool for distance measurements in the location detection process. Most previous works have employed the path-loss propagation model with a fixed value of the path loss exponent. However, in actual environments, the path loss exponent for each channel is different. Moreover, due to the complexity of the radio channel, when the number of channel increases, a larger number of RSS measurements are needed, and this results in additional energy consumption. In this paper, to overcome this problem, we propose using the Bayesian compressive sensing method with a calibrated path loss exponent to improve the performance of the PU localization method.

• 제어로봇시스템학회:학술대회논문집
• /
• 제어로봇시스템학회 2004년도 ICCAS
• /
• pp.1568-1572
• /
• 2004

This paper proposes a new online calibration algorithm for the array antenna system. As you know, the several previous calibration methods for the mutual coupling did not estimate but measure mutual coupling effect at the real or test-bed system directly. Therefore we suggest some idea to compensate the calibration errors due to mutual coupling effect and mismatch in cables and electronic modules without the off-line calibration. In this work, we can calibrate the array antenna system under the operation of the system using Independent Component Analysis(ICA). This is what is called an online calibration. As you know, the ICA method has permutation and scaling problems. However, we solve problems of the ICA method and apply it to the calibration of an array antenna. The method simultaneously estimates the DOA(Direction of Arrival) of the signals, and calibrates the array for that specific angle. The proposed algorithm is evaluated by computer simulation and its behavior is illustrated by a numerical example.

• 한국항해항만학회:학술대회논문집
• /
• 한국항해항만학회 2004년도 춘계학술대회 논문집
• /
• pp.305-309
• /
• 2004

레이더, 소나 및 육해상 통신시스템에 있어서, 어레이 센서에 입사하는 수신신호가 가지는 도래방향 및 신호전력과 같은 파라미터 추정문제는 수십년 동안 많은 연구관심이 되어 왔으며, 현재에도 다양한 응용영역에서 중요한 문제 중의 하나이다. 수신신호의 파라미터 추정방법으로서 종래의 MUSIC 또는 ESPRIT와 같은 방법은 역행렬 연산과 고유치 계산이 필요하므로 계산적 복잡성이 부가된다. 본 논문에서는 어레이 안테나의 기본 신호모델로부터 파라미터 추정의 문제를 비선형 최소화 문제로 정식화 하여 역행렬과 고유치 연산을 요구하지 않는 수신신호의 파라미터 동시추정 알고리즘을 제안하고 평가한다.

• 한국인터넷방송통신학회논문지
• /
• 제17권5호
• /
• pp.79-84
• /
• 2017

본 논문은 다중 경로 다중입력 (Multiple-input multiple-output : MIMO) 채널에서 space alternating generalized expectation-maximization(SAGE) 알고리즘을 이용하여 채널 파라미터 추정 성능을 확인한다. 성능을 비교하기 위해 781 대역의 제주도 측정지역에서의 채널 파라미터로부터 시변 채널 환경 채널 파라미터 추정을 SAGE 알고리즘을 통해 추정하고 원본 데이터와 비교한다. 이를 통해 SAGE 알고리즘의 성능을 확인할 수 있으며 일반적인 파라미터 계산보다 SAGE 알고리즘을 통해 추정하는 것이 지연 확산(Delay Spread), 도래각 확산(Arrive of Angular Spread)이 적어 정확 측면에서 성능이 뛰어나고 안정적이다.

• Journal of Astronomy and Space Sciences
• /
• 제40권4호
• /
• pp.237-245
• /
• 2023

Laser communication has been considered as a novel method for earth observation satellites with generation of high data volume. It offers faster data transmission speeds compared to conventional radio frequency (RF) communication due to the short wavelength and narrow beam divergence. However, laser beams are refracted due to atmospheric turbulence between the ground and the satellite. Refracted laser beams, upon reaching the receiver, result in angle-of-arrival (AoA) fluctuation, inducing image dancing and wavefront distortion. These phenomena hinder signal acquisition and lead to signal loss in the course of laser communication. So, precise alignment between the transmitter and receiver is essential to guarantee effective and reliable laser communication, which is achieved by pointing, acquisition, and tracking (PAT) system. In this study, we simulate the effectiveness of tip/tilt compensation for more efficient laser communication in the satellite-ground downlink. By compensating for low-order terms using tip/tilt mirror, we verify the alleviation of AoA fluctuations under both weak and strong atmospheric turbulence conditions. And the performance of tip/tilt correction is analyzed in terms of the AoA fluctuation and collected power on the detector.

• 한국전자통신학회논문지
• /
• 제12권5호
• /
• pp.777-782
• /
• 2017

신호의 도래각(: Angle-of-Arrival, AOA) 추정기 및 간섭 제거기 등으로 구성된 적응 빔형성기 기반의 신호정보 수집(: Signal Intelligence, SIGINT) 시스템은 레이더나 위성 등과 같은 각종 장비를 활용하여 다양한 신호정보를 수집하기 위한 최첨단 기술이다. 본 논문에서는 도래각 추정기와 적응 빔형성기로 구성된 효율적인 신호정보 수집 시스템의 구조를 제안한다. 다양한 신호의 도래각 정보를 추정하기 위해 MUSIC(: Multiple Signal Classification) 알고리즘을 사용하고, 불필요한 간섭 신호를 제거하기 위해 MVDR(: Minimum Variance Distortionless Response) 기법을 사용한다. 또한, 컴퓨터 시뮬레이션을 통해 제안된 적응 빔형성기의 성능을 평가한다.

• 한국음향학회지
• /
• 제20권4호
• /
• pp.81-86
• /
• 2001

126-kHz양상태 해수면 음파산란 측정을 강원도 삼척시 연안에서 수행하였다. 수평입사각을38％, 52％로 고정시킨 상태에서 트랜스듀서와 청음기의 거리를 변화시키면서 산란각의 변화에 따른 전방산란 신호의 변화를 관찰하였다. 해저면 산란신호와는 달리 해수면 산란신호는 해수면 유동성에 의해서 도달시간 변이와 진폭의 변이를 나타내었으며 신호간의 상관관계도 불규칙적인 경향을 나타내었다. 전방산란강도를 계산하여 Kirchhoff approximation (KA)과 small slope approximation (SSA)을 비교한 결과 두 모델 모두 실측치와 잘 일치하였다.