• IEIE Transactions on Smart Processing and Computing
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• 제3권5호
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• pp.275-284
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• 2014

This paper considers two-tier networks consisting of macrocells and femtocells operating in the same spectrum. This paper proposes a femtocell spectrum reuse scheme that determines the shared spectrum and transmit power for the femtocells to mitigate the effects of cross-tier interference between the macrocells and femtocells. The proposed scheme provides macrocell throughput that is unaffected by the increasing number of femtocells per cell site and improves the femtocell signal quality at the same time by limiting the cross-tier interference. This study analyzed the per-tier signal-to-interference ratio (SIR) and outage probability of the proposed scheme to investigate the macrocell and femtocell performance. The total throughput of the proposed scheme was analyzed based on the outage probabilities. The analysis and numerical results proved that high femtocell throughput can be achieved using only a small fraction of the spectrum while protecting the macrocell throughput. As a result, an improved total throughput was achieved enforcing higher spatial reuse.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2008년도 춘계 학술발표회 초청강연 및 논문집
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• pp.599-604
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• 2008

The dispersive phase velocity of a wave propagating through a system is an important parameter and carries valuable information in non-destructive tests related to multilayered systems such as a soil site. The dispersive phase velocity of a wave can be determined using the phase spectrum, which is easily evaluated through the cross power spectrum. However, the phase spectrum as determined using the cross power spectrum is sensitive to background noise which always exists in the field. This causes difficulties in the determination of the dispersive phase velocities. In this paper, a new method to evaluate the phase spectrum using the harmonic wavelet transform is proposed. The proposed method can successfully remove background noise effects. To evaluate the validity of the proposed method, numerical simulations of multi-layered systems were performed. Phase spectrums by the proposed method were found to be in good agreement with the actual phase spectrums under conditions characterized by heavy background noise. This shows the potential of the proposed method.

• Earthquakes and Structures
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• 제11권6호
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• pp.1143-1164
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• 2016

Time Delay of Arrival (TDOA) estimation methods based on correlation function analysis play an important role in the micro-seismic event monitoring. It makes full use of the similarity in the recorded signals that are from the same source. However, those methods are subjected to the noise effect, particularly when the global similarity of the signals is low. This paper proposes a new approach for micro-seismic monitoring based on cross wavelet transform. The cross wavelet transform is utilized to analyse the measured signals under micro-seismic events, and the cross wavelet power spectrum is used to measure the similarity of two signals in a multi-scale dimension and subsequently identify TDOA. The offset time instant associated with the maximum cross wavelet transform spectrum power is identified as TDOA, and then the location of micro-seismic event can be identified. Individual and statistical identification tests are performed with measurement data from an in-field mine. Experimental studies demonstrate that the proposed approach significantly improves the robustness and accuracy of micro-seismic source locating in mines compared to several existing methods, such as the cross-correlation, multi-correlation, STA/LTA and Kurtosis methods.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제8권5호
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• pp.1514-1531
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• 2014

Femtocell provides better coverage and higher spectrum efficiency in areas rarely covered by macrocells. However, serious two-tier interference emerging from randomly deploying femtocells may create dead zones where the service is unavailable for macro-users. In this paper, we present adopting cognitive radio spectrum overlay to avoid intra-tier interference and incorporating spectrum underlay and overlay to coordinate cross-tier interference. It is a novel centralized control strategy appropriate for both uplink and downlink transmission. We introduce the application of proper spectrum sharing strategy plus optimal power allocation to address the issue of OFDM-based femtocells interference-limited downlink transmission, along with, a low-complexity suboptimal solution proposed. Simulation results illustrate the proposed optimal scheme achieves the highest transmission rate on successfully avoiding two-tier interference, and outperforms the traditional spectrum underlay or spectrum overlay, via maximizing the opportunity to transmit. Moreover, the strength of our proposed schemes is further demonstrated by comparison with previous classic power allocation methods, in terms of transmission rate, computational complexity and signal peak-to-average power ratio.

• 대한기계학회논문집B
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• 제22권6호
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• pp.725-733
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• 1998

The pressure fluctuation on the surface of a submerged body has been recognized as a dominant noise source. There have been many studies concerning the flow induced noise on a flat plate. However, the noise over an axisymmetric body has not been well reported. This paper addresses the way in which we have investigated the mechanism of noise generation due to an axisymmetric body. The associated experiments and signal processing methods are introduced. A 3-dimensional axisymmetric body whose length and diameter were 2 m and 10.4 cm, was prepared as a test specimen. The wall pressure on the surface of the body was measured in a large scale low noise wind tunnel at KIMM(Korea Institute of Machinery and Metals). To measure the wall pressure, we used two microphone arrays which were tangential and normal to the flow. Based on the measured signal, frequency-wavenumber spectrum which explains the structure of turbulence noise, was estimated. Tangential to the flow, there exists convective ridge at a relatively higher wavenumber region; this can cause spatial aliasing. To circumvent this problem, the cross spectrum was interpolated. The interpolation has been performed by unwrapping the phase and smoothing the cross spectrum. The phase unwrapping was done based on the Corcos model; the phase of cross spectrum decreases linearly with the distance between microphones. Aforementioned signal processings are possible by employing the experimental results that the estimated wavenumber spectrum quite resembles the Corcos model. We try to modify the Corcos model which is applicable to the flat plate, by altering the magnitude of cross spectrum to fit the experimental data more accurately. We proposed that this wavenumber spectrum model is suitable for the 3-dimensional axisymmetric body. Normal to the flow, there exists a little correlation between signals of different microphones. The circumferential wavenumber spectrum contains uniform power along the wavenumbers.

• 대한조선학회논문집
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• 제51권2호
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• pp.122-129
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• 2014

Identification of the whipping response out of the combined wave-vibration response of a flexible sea going vessel is one of the most interesting research topic from ship designer's point of view. In order to achieve this goal, a novel methodology based on the wavelet cross-correlation technique was proposed in this paper. The cross-correlation of the wavelet power spectrum averaged across the frequency axis was introduced to check the similarity between the combined wave-vibration response and impulse response. The calculated cross-correlation of the wavelet power spectrum was normalized by the auto-correlation of the each spectrum with zero time lag, eventually providing the cross-correlation coefficient that stays between 0 and 1, precisely indicating the existence of the impulse response buried in the combined wave-vibration response. Additionally, the weight function was introduced while calculating the cross-correlation of the two spectrums in order to filter out the signal of lower frequency so that the accuracy of the similarity check becomes as high as possible. The validity of the proposed methodology was checked through the application to the artificially generated ideal combined wave-vibration signal, together with the more realistic signal obtained by running 3D hydroelasticity program WISH-Flex. The correspondence of the identified whipping instances between the results, one from the proposed method and the other from the calculated slamming modal force, was excellent.

• 한국음향학회지
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• 제31권7호
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• pp.449-460
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• 2012

발전소 건설 시에 설치된 지하매설 배관이 대부분 20년 이상 경과됨에 따라서 지하매설 배관의 누설탐지의 중요성이 점차 부각되고 있다. 본 논문의 목적은 발전소와 같이 매설배관 주변에 회전체 기계잡음이 존재하는 경우에 배관의 누설위치 추정성능을 개선하기 위한 방법을 연구하는 것이며, 이러한 목적을 달성하기 위하여 주파수영역에서 배관 주변의 회전체 기계잡음을 효과적으로 제거할 수 있는 방법을 제안하였다. 또한 상호스펙트럼의 위상정보를 이용하여 배관의 누설위치를 추정하는 방법을 제안하였다. 제안된 방법의 타당성 평가를 위하여 실험을 수행하였으며, 실험결과 회전체에서 발생하는 주기적인 잡음성분을 제거함으로써, 배관 누설을 탐지하는 상호상관함수기법의 성능을 개선할 수 있음을 확인하였다.

• 한국음향학회지
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• 제22권8호
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• pp.660-669
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• 2003

본 논문은 음원 근처의 홀로그램 평면에서 측정된 음압에 대한 상호 파워 스펙트럼으로부터 홀로그램 평면에서의 음압 분포를 구하고 획득된 음장을 공간 변환하여 음원의 음장을 구하기 위한 이론을 설명하였으며, 홀로그램 평면에서의 상호 파워 스펙트럼으로부터 모든 지점에서의 음압을 구하기 위해 비선형 방정식에 대한 Taylor 급수를 전개하고 Newton-Raphson 법을 이용하여 계산하는 방법과 음원 영역으로의 역방향 전파시 발생되는 오차를 줄이기 위한 파수 필터를 제시하였다. 무한 배플 내의 원판형 진동체 수중 음원에 대한 모의실험을 통해 결과를 고찰하고 제시된 이론을 검증하였다.

• 한국지반공학회논문집
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• 제24권4호
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• pp.115-123
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• 2008

지반과 같은 다층으로 구성된 시스템을 통과하여 전파되는 파의 분산특성은 표면파 지반조사에서 매우 중요한 요소이다. 이러한 표면파의 분산곡선(주파수위상속도 곡선)은 크로스 파워 스펙트럼에 의해 결정되는 위상 스펙트럼을 사용하여 쉽게 결정할 수 있다. 그러나 크로스 파워 스펙트럼에 의해 결정되는 분산곡선은 현장에 항상 존재하는 잡음에 의해 쉽게 손상되며, 손상된 분산곡선을 사용하는 경우 잘못된 조사 결과를 도출하기도 한다. 본 논문에서는 이러한 기존 방법의 문제점을 해결하고자 하모닉 웨이브릿 변환을 이용한 새로운 위상 스펙트럼 결정방법을 제안하고 이를 분산곡선을 결정에 적용하였다. 제안된 방법은 효과적으로 현장에 존재하는 배경잡음의 효과를 제거할 수 있다. 제안된 방법의 타당성을 검증하기 위하여, 지반을 모사한 다층구조에 대해 수치모의실험을 수행하였다. 수치모의실험을 통하여 제안된 방법이 심한 잡음조건하에서도 효과적으로 신뢰할 수 있는 위상 스펙트럼과 분산곡선을 결정할 수 있음을 볼 수 있었으며, 이를 통하여 제안된 방법의 타당성을 확인하였다.

• 대한의용생체공학회:학술대회논문집
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• 대한의용생체공학회 1998년도 추계학술대회
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• pp.185-186
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• 1998

The discrete Wigner-distribution(DWD) was implemented for the time-frequency analysis of heart rate variability signals. The smoothed cross-DWD was used to estimate time-varying power spectrum. Spurious cross-terms were suppressed using a smoothing data window and a Gauss frequency window. The DWD is very easy to implement using the FFT algorithm. Experiment show that the DWD follows well the instantaneous changes of spectral content of heart rate variability signals, which characterize the dynamics of autonomic nervous system response.