• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.18 no.10
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• pp.1196-1202
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• 2007

In this paper, the effects of performance improvement far the OFDM based satellite communications are analyzed with applying the CI(Carrier Interferometry)-OFDM and channel coding scheme considering the group delay and gain ripple characteristic as well as the nonlinear characteristic of the transponder. Comparing the BER between traditional OFDM and CI-OFDM, the degree of performance improvement is presented in AWGN channel environments for specified backoff condition of HPA. The simulations are performed with the 36 MHz bandwidth of transponder channel, 120 Mbps transmission rate, and 16 QAM modulation scheme between ideal and worst case condition. It is shown that the improvement measure by the CI-OFDM and channel coding for the group delay and nonlinear characteristic outperforms that for the gain ripple in terms of performance degradation presented by the individual characteristics. And the simulation results show that the effects of improvement by the CI-OFDM outperforms the effect by the applied channel coding, particularly in worst case condition.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.11 no.1
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• pp.64-72
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• 1986

The error rate equation fo DS-BPSK(Direct Sequence Binary Phase Shift Keying) signal transmitted through the nonlinear satellite transponder has been derived in the cochannel interference and downlink Gaussian noise environment. The input to the satellite transponder is the superposition of DS-BPSK signal with one interfere which is a cochannel wide-band PN signal. The error rate performance of DS-BPSK system has been evaluated and shown in figures in terms of carrier to interference power ratio(CIR), downlink signal to noise power ratio(downlink SNR) and process gain. In the analysis, it has been shown that the use of a hard limiter in DS-BPSK satellite system leads to the generation of narrow-band intermodulation products which is independent of the process gain. Also it is known that the error rate performance can be improved in the low levels (below 10dB) of CIR as the CIR increase. As the process gain varies from 10 to 100 the curve gives the about 10 dB gain in downlink SNR to maintain a fixed error rate.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2001.10a
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• pp.688-692
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• 2001

본 논문에서는 무선 통신로 환경에서 발생하는 페이딩 현상과 전자기기에서 방출되는 임펄스성 전자간섭파에 따른 MC DS-CDMA와 DS-CDMA 시스템의 성능을 비교 분석하였다. 결과에 의하면, 페이딩과 임펄스 전자간섭파에 의해 시스템에 심각한 영향을 미치게 되는데 특히, 직접파 성분이 존재하는 페이딩 보다는 임펄스 전자간섭파의 발생빈도와 크기에 따라 두 시스템 성능이 크게 좌우됨을 알 수 있었다. 이에 대한 성능개선 기법으로 다이버시티 기법과 BCH(15, 7) 부호화 기법을 적용하여 직접파가 존재하는 페이딩과 강한 임펄스 간섭 환경에서도 만족할 만한 성능을 얻을 수 있었다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.1A
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• pp.84-90
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• 2005

In Korea, transition to digital TV broadcasting started at the metropolitan Seoul area and has expanded to the metropolitan area in 2004. However, until 2010 analog NTSC broadcasting will co-exist with digital TV. In this situation, digital TV may have the same channel as an analog NTSC broadcasting. To remove the effect of the analog NTSC intereference, DTV receivers adopt an NTSC rejection filter. In this paper, analysis on properties of the filter, such as a rejection property and noise performance degradation, is presented.

• Proceedings of the Optical Society of Korea Conference
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• 2000.08a
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• pp.240-241
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• 2000

최근에 개발된 강력한 레이저는 원자가 tunneling 이온화 될 수 있는 충분한 세기를 가지고 있다$^{(1)}$ . Tunneling 이온화 된 전자는 원자의 Coulomb 퍼텐셜 보다 레이저 장에 의한 영향이 훨씬 크므로 기존의 perturbation 이론으로는 이들간의 상호작용을 설명하기에 부족하였다. 뿐만 아니라 이온화된 전자가 레이저 장의 방향이 바뀜에 따라 재결합함으로써 발생되는 고차조화파는 기존의 perturbation 이론으로는 설명될 수 없는 발생효율이 차수에 상관없이 거의 일정한 평탄영역을 가지고 있음이 실험적으로 관측되었다$^{(2)}$ . 강력한 레이저 장과 원자들과의 상호작용에 의해서 생성된 고차조화파는 기존의 비선형 현상으로 설명되어지는 이차 조화파나 삼차 조화파와 같이 레이저의 간섭성을 그대로 물려 받으므로 극자외선 영역의 간섭광원으로써 이의 활용가치가 높을 것으로 보고 있다. 현재에는 기존의 엑스선 레이저로 가능했던 고밀도 플라즈마 밀도 측정등과 같이 극자외선 간섭광이 필요로 하는 연구등에 활용 되어지고 있다. 뿐만 아니라 고차조화파는 효율이 거의 일정한 넓은 파장 영역인 평탄 영역을 가지므로 이의 넓은 파장 영역을 이용하면 아토초 수준의 짧은 극자외선 광원을 만들 수 있을 것으로 이론적으로 예견되어 지고 있다$^{(3)}$ . (중략)

• 한국가시화정보학회:학술대회논문집
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• 2002.11a
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• pp.117-120
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• 2002

An experimental research has been carried out for flow control of the shock wave/turbulent boundary-layer interaction utilizing aeroelastic mesoflaps. Various shapes and thicknesses of the mesoflap are tested to achieve different deflections of the flap, and ail the results are compared to the solid-wall reference case without flow-control mechanism. Quantitative variation of skin friction has been measured downstream of the interactions using the laser interferometer skin friction meter, and qualitative skin friction distribution has been obtained by observing the interference fringe pattern on the oil-film surface. A strong spanwise variation in the fringe patterns with a narrow region of separation near the centerline is noticed to form behind the shock structure, which phenomenon is presumed partially related to three-dimensional flow structures associated with both the sidewalls and the bottom test surface. The effect of the shape of the cavity is also observed and it is noticed that the shape of the cavity is not negligible.

• Journal of the Society of Naval Architects of Korea
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• v.34 no.2
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• pp.90-103
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• 1997

A numerical procedure is described for predicting the motion and structural responses of the very large floating offshore structures supported by multiple 3-D floating bodies of arbitrary shape in multi-directional irregular waves. The developed numerical approach taking into account of the hydrodynamic interactions among the multiple floating bodies is based on a combination of the 3-D source distribution method, the wave interaction theory, the finite element method and the spectral analysis method to get the significant values of the dynamic responses in the multi-directional irregular waves. The effects of wave interactions and directionality on the dynamic responses of a very large offshore structure, which is semisubmersible ring type, are numerically examined.

• Journal of the Korean Society of Propulsion Engineers
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• v.7 no.3
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• pp.45-52
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• 2003

Detailed flowfield resulting from the secondary sonic gas injection into a divergent section of supersonic conical nozzle has been numerically investigated. The three-dimensional flowfield associated with the bow-shock/boundary-layer interaction inside the nozzle has been solved by Reynolds-averaged Navier-Stokes equations with an algebraic and $\kappa$-$\varepsilon$ turbulence model. The numerical results have been compared with the experimental results for the identical flow conditions, and it is shown that the comparison is satisfactory Effects of different injection pressures of the secondary jet on the shock/boundary-layer interactions and the overall flow structure inside the nozzle have been investigated. The vortex structures behind the shock interaction and wall pressure variations have also been studied.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.20 no.12
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• pp.2226-2234
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• 2016

Orthogonal Frequency Division Multiplexing (OFDM) technique uses multiple sub-carriers for the data transmission. Therefore, Inter Carrier Interference(ICI) is generated because of nonlinear high power amplifier and carrier frequency offset. Wireless OFDM transmission over Doppler fading channels also causes ICI. In OFDMA(Orthogonal Frequency Division Multiplexing Access), multiple sub-carriers are allocated to each user. Therefore, inter carrier interference causes interference to other users. I evaluate the BER performance of OFDMA systems in frequency selective fading channel, considering Multi-User Interference (MUI) owing to the carrier frequency offset, the nonlinear high power amplifier, and the Doppler fading. In the uplink OFDMA, multi-user interference introduces larger BER degradation than in the downlink. I explain the reason and obtain the required characteristics of the nonlinear amplifier and the value of frequency offset for good BER performance. And I also analyze the BER degradation upon Doppler fading channel.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.7
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• pp.481-487
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• 2017

The interaction between a shock wave and a boundary layer causes boundary layer separation, shock train, and in some cases, strong unsteadiness in the flow field. Such a situation is also observed in a shock tube, where the reflected shock wave interacts with the unsteady boundary layer. However, only a few studies have been conducted to investigate the shock train phenomenon in a shock tube. In the present study, numerical studies were conducted using the two-dimensional axisymmetric domain of a shock tube, and compressible Navier-Stokes equations were solved to clarify the flow characteristics of shock train phenomenon inside a shock tube. A detailed wave diagram was developed based on the present computational results, which were validated with existing experimental data.