• Korean Journal of Optics and Photonics
• /
• v.27 no.3
• /
• pp.106-113
• /
• 2016

In this paper, we deal with the wavefront compensation capability of a silicon carbide (SiC) deformable mirror (DM) with 37 actuators for adaptive optics. The wavefront compensation capability of the SiC DM is predicted by computer simulation and examined by actual experiments with a closed-loop adaptive optics system consistsing of a light source, a phase plate, a SiC DM, a high speed Shack-Hartmann sensor, and a control computer. Distortion of wavefront is caused by the phase plate in the closed-loop adaptive optics system. The distorted wavefront has a peak-to-valley (PV) wavefront error of $0.3{\mu}m{\sim}0.9{\mu}m$ and root-mean-square (RMS) error of $0.06{\mu}m{\sim}0.25{\mu}m$. The high-speed Shack-Hartmann sensor measures the wavefront error of the distortion caused by the phase plate, and the SiC DM compensates for the distorted wavefront. The compensated wavefront has residual errors lower than $0.1{\mu}m$ PV and $0.03{\mu}m$ RMS. Consequently, we conclude that we can compensate for the distorted wavefront using the SiC DM in the closed-loop adaptive optics system with an operating frequency speed of 500 Hz.

• Proceedings of the Acoustical Society of Korea Conference
• /
• autumn
• /
• pp.295-298
• /
• 2004

본 논문에서는 잡음에 오염된 신호의 지각관계를 해석하여 지각 필터 응답 제어를 통한 음성 신호 개선 알고리즘을 제안한다. 음성 신호 개선 기법은 단일 채널환경에서 사람의 청각시스템에서의 주파수 변별력을 나타내는 각각의 임계대역에 대한 전체 에너지를 나타내는 임계대역 에너지의 지각적인 확산의 영향 즉, 마스킹 확산의 영향을 나타내는 자극에너지를 이용하여 신호와 잡음 에너지에 의해 변화하는 잡음에 의한 신호의 마스킹 구간을 검출하여 묵음 구간 추출 잡음 필터응답과 추정 잡음 오차를 보상시킨 필터응답을 통한 지각 필터 응답을 보상하여 신호를 개선하는 방법이다. 실험 결과 제안한 방법을 통해 SNR에 개선과 음질 개선 효과를 얻을 수 있음을 테스트를 통해 확인하였다.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
• /
• 2010.10a
• /
• pp.784-786
• /
• 2010

Implementation possibility of inline dispersion management (DM) using bi-end schemes, which consist of one single mode fiber (SMF) and two dispersion compensating fiber (DCF) placed at front and rear of SMF, respectively, is investigated for compensating total dispersion accumulated in a span of WDM transmission links. It is confirmed that if net residual dispersion (NRD) is decided to be ${\pm}10\;ps/nm$ then bi-end scheme is effective to compensate for WDM channels with wide launching power range.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.16 no.4
• /
• pp.821-828
• /
• 2012

In this paper, we report that the differences between RF power levels can be improved in wavelength division multiplexing - radio over fiber (WDM-RoF) system by using a photonic crystal fiber. In a WDM-RoF system, each WDM channel experiences different received RF power level fluctuation in remote node (RN) because of wavelength-dependent dispersion. Since each WDM channel experiences different power fluctuation, the RF power fluctuation acts as a design constraint in viewpoint of network design. We designed a photonic crystal fiber to improve the effect of wavelength- dependent dispersion on RF power fluctuation. Also, we analyzed the wavelength-dependent difference of inter-channel RF power fluctuations.

• The Proceeding of the Korean Institute of Electromagnetic Engineering and Science
• /
• v.6 no.2
• /
• pp.45-53
• /
• 1995

In mobile communication, the performance of QAM technique is poor under deep fading even though Rayleigh channel characteristics are estimated and compensated. In this paper, we analyze the performance of 3/4 Trellis coded 16QAM in Rayleigh channel where burst errors are randomized by bit reversal block interleaver, and TCM coder is designed by computer search to be adequate for Rayleigh channel. The simulation results show that the interleaver has 2dB gain, the modified Viterbi decoder has 2.5dB, and diversity has 9dB gain at $P_{b}b/=10^{-3}$. This indicates that the 3/4 TC 16QAM is applicable to digital mobile communication with good performance.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.8 no.6
• /
• pp.581-588
• /
• 1997

Time-variant mobile radio fading channels distort amplitude, frequency and phase of a transmitted signal. On channels that have time-variant spectral nulls, the conventional equalizers which have low convergence speed and high sensitiveness to phase distortion yields very poor error rate performance. In this paper, the performance of a combined structure with the data-recycling algorithm and fractionally spaced equalizer (FSE) has been investigated on time-variant mobile radio fading channels through computer simulations and compared to other kinds of equalizers. The results show that the data-recycling FSE has excellent capabilities for tracking rapidly time-variant mobile channels and effective compensation for phase distortion.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.36 no.10B
• /
• pp.1210-1215
• /
• 2011

In this paper, a supervised adaptive equalization algorithm based on probability density function (PDF) matching method is introduced and its decision-feedback version is proposed for underwater communication channels with strong impulsive noise and severe multipath characteristics. The conventional least mean square (LMS) algorithm based on mean squared error (MSE) criterion has shown to be incapable of coping with impulsive noise and multipath effects commonly shown in underwater communications. The linear PDF matching algorithm, which shows immunity to impulsive noise, however, has revealed to yield unsatisfying performance under severe multipath environments with impulsive noise. On the other hand, the proposed nonlinear PDF matching algorithm with decision feedback proves in the simulation to possess superior robustness against impulsive noise and multipath characteristics of underwater communication channels.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.30 no.1A
• /
• pp.55-61
• /
• 2005

Mobile communication systems are required not only to support high-data-rate transmissions in favorable channel conditions but also to be able to tolerate hostile environments possibly encountered by cellular communications. This paper compares CDMA(Code Division Multiple Access), OFDM(Orthogonal Frequency Division Multiplexing), and MC-CDMA(Multi-Carrier CDMA)with inaccurate channel estimates and low SNR environments. The equations and simulation results show that the performance losses in CDMA systems due to imprecise channel estimates are not considerable while frequency-hopping of OFDM systems can result in more than l0dB SNR losses. Also, this paper show that frequency-spreading of MC-CDMA can be very helpful for channel compensation performances than frequency-hopping or time-spreading of OFDM.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.25 no.6B
• /
• pp.1127-1135
• /
• 2000

The beamforming procedure in Adaptive Array Antenna System is affected by signal degradation and data rate due to DS-CDMA characteristics. Until this time, a lot of techniques are suggested to overcome this problems. This paper shows the simulation result about the beamforming performance of symbol level system that process slow data rate, compensated signal by despreading procedure in front of beamformer, and that of chip level system that process chip level signal without it. we analysis the performance using MSE, beam pattern, scattering points of beamformer output. chip level system is superior to symbol level system in time varying channel, while the performance of them didn't have difference in static channel.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.15 no.2
• /
• pp.299-306
• /
• 2011

In this paper, a blind adaptive equalization algorithm based on PDF-distance minimization and a set of Delta functions is introduced and its superior robustness against impulsive noise and multipath characteristics of underwater communication channels is proved. The conventional CMA based on MSE has shown to be incapable of coping with impulsive noise, and correntropy blind algorithm has also revealed to yield not satisfying performance for the mission. On the other hand, the blind adaptive equalization algorithm based on PDF-distance minimization and a set of Delta functions has been proved to solve effectively the problem of impulsive noise and multipath characteristics of underwater communication channels through theoretical and simulation analysis.