• Structural Engineering and Mechanics
• /
• 제77권3호
• /
• pp.355-368
• /
• 2021

The time-varying mean (TVM) component of non-stationary wind speeds is commonly extracted utilizing empirical mode decomposition (EMD) in practice, whereas the accuracy of the extracted TVM is difficult to be quantified. To deal with this problem, this paper proposes an approach to identify and extract the optimal TVM from several TVM results obtained by the EMD. It is suggested that the optimal TVM of a 10-min time history of wind speeds should meet both the following conditions: (1) the probability density function (PDF) of fluctuating wind component agrees well with the modified Gaussian function (MGF). At this stage, a coefficient p is newly defined as an evaluation index to quantify the correlation between PDF and MGF. The smaller the p is, the better the derived TVM is; (2) the number of local maxima of obtained optimal TVM within a 10-min time interval is less than 6. The proposed approach is validated by a numerical example, and it is also adopted to extract the optimal TVM from the field measurement records of wind speeds collected during a sandstorm event.

• ETRI Journal
• /
• 제37권1호
• /
• pp.11-20
• /
• 2015

This paper deals with optimal power allocation for channel estimation of orthogonal frequency-division multiplexing uplinks in time-varying channels. In the existing literature, the estimation of time-varying channel response in an uplink environment can be accomplished by estimating the corresponding channel parameters. Accordingly, the optimal power allocation studied in the literature has been in terms of minimizing the mean square error of the channel estimation. However, the final goal for channel estimation is to enable the application of coherent detection, which usually means high spectral efficiency. Therefore, it is more meaningful to optimize the power allocation in terms of capacity. In this paper, we investigate capacity with imperfect channel estimation. By exploiting the derived capacity expression, an optimal power allocation strategy is developed. With this developed power allocation strategy, improved performance can be observed, as demonstrated by the numerical results.

• ETRI Journal
• /
• 제28권6호
• /
• pp.822-825
• /
• 2006

Based on superimposed training methods, a novel time-varying multipath channel estimation scheme is proposed for orthogonal frequency division multiplexing systems. We first develop a linear least square channel estimator, and meanwhile find the optimal superimposed sequences with respect to the channel estimates' mean square error. Next, a low-rank approximated channel estimator is obtained by using the singular value decomposition. As demonstrated in simulations, the proposed scheme achieves not only better performance but also higher bandwidth efficiency than the conventional pilot-aided approach.

• Journal of Communications and Networks
• /
• 제11권1호
• /
• pp.42-46
• /
• 2009

Superimposed training (SIT) design for estimating of time-varying multipath channels is investigated for mobile orthogonal frequency division multiplexing (OFDM) systems. The design of optimum SIT consists of two parts: The optimal SIT sequence is derived by minimizing the channel estimates' mean square error (MSE); the optimal power allocation between training and information data is developed by maximizing the averaged signal to interference plus noise ratio (SINR) under the condition of equal powered paths. The theoretical analysis is verified by simulations. For the metric of the averaged SINR against signal to noise ratio (SNR), the theoretical result matches the simulation result perfectly. In contrast to an interpolated frequency-multiplexing training (FMT) scheme or an SIT scheme with random pilot sequence, the SIT scheme with proposed optimal sequence achieves higher SINR. The analytical solution of the optimal power allocation is demonstrated by the simulation as well.

• 품질경영학회지
• /
• 제31권3호
• /
• pp.160-171
• /
• 2003

Criteria for the comparison of quality of asset portfolio management are risk and return. In this paper a method to use structural time-series model to determine an optimal portfolio for the improvement of quality of asset portfolio management is suggested. In traditional mean variance analysis expected return is assumed to be time-invariant. However, it is more realistic to assume that expected return is temporally dynamic and structural time-series model can be used to reflect time-varying nature of return. A data set from an insurance company was used to show validity of suggested method.

• 한국통신학회논문지
• /
• 제32권3C호
• /
• pp.338-347
• /
• 2007

본 논문에서는 순환 보호 구긴(cyclic-prefix)을 사용하는 시공간 블록 부호 (STBC: Space-Time Block-Coding) 단일 반송파 시스템에서 향상된 채널 성능을 위한 가중된 블록 적응형 주파수 영역 채널 추정기를 제안한다. 제안된 채널 추정기 구조는 필터 입력 신호에 대해 STBC로 구성된 블록을 형성하며, 이후 형성된 입력 블록에 대해 사후 오차 (a posteriori error)를 이용하는 가중된 LS (least-square) 규준을 적용하여 알고리즘을 유도한다. 또한 정적 채널에서 steady-state EMSE (excess mean-square error) 분석을 통해 블록 길이가 늘어남에 따라 EMSE를 분석한다. 전산 모의실험에서는 시변 TU (typical urban) 채널에서 블록 길이를 증가시킬수록 제안한 채널 추정기는 기존 NLMS와 RLS 채널 추정기들 보다 우수한 성능을 나타냄을 확인 할 수 있다.

• 한국통신학회논문지
• /
• 제33권7A호
• /
• pp.743-750
• /
• 2008

OFDM 방식은 주파수 선택적(frequency-selective) 채널 환경에서 고속 데이터 전송을 위한 효과적인 변조 기술이지만 시간 선택적(time-selective) 채널 환경에선 인접 채널간의 간섭(intercarrier interference) 현상으로 인해 시스템 성능이 저하되게 된다. 본 논문에선 먼저 MMSE(minimum mean squared error) 등화기와 직렬 간섭 제거방식(successive interference cancellation)이 결합된 MMSE-SIC 등화기법을 위한 새로운 recursive 알고리즘을 제안한다. 제안된 알고리즘은 최적의 검출 순서가 알려져 있을 경우 기존 MMSE-SIC 등화 방식의 복잡도를 크게 낮추면서도 동일한 성능을 나타낸다. 또한 ICI의 특성을 이용하여 어느 정도 성능을 유지하면서도 제안된 알고리즘의 복잡도를 보다 개선할 수 있다.

• Journal of the Optical Society of Korea
• /
• 제13권3호
• /
• pp.367-372
• /
• 2009

In this paper we propose an electronic domain timing phase selection scheme for the optical communication systems suffering from inter-symbol-interference (ISI) distortion due to chromatic dispersion (CD) or polarization mode dispersion (PMD). In the presence of CD/PMD a proper timing phase selection is important for discrete time domain equalizers, since different timing phases produce different nonlinear ISI channels of different severity. The proposed timing phase recovery scheme based on dispersion minimization (DM) practically approximates the optimal minimum mean squared error (MMSE) timing phase without training signals which reduces overall throughput substantially, especially in time-varying channels such as PMD. The simulation results show that the proposed DM timing agrees with MMSE timing phase, under proper normalization of the received signals, for various dispersion and OSNR.

• 대한기계학회논문집B
• /
• 제24권4호
• /
• pp.589-598
• /
• 2000

A pilot-scale Spray Drying Sorber (SDS) system was set up to evaluate the effect of spray characteristics on the desulfurization yield. The size distribution and the Sauter Mean Diameters of slurry droplets were measured in advance using the optical size measurement system, Malvern 2600. The desulfurization yield of the drying chamber by size was measured for the conditions of inlet gas and spray injection. As a reagent, 10% limestone slurry of $Ca(OH)_2$ was treated with flue gas containing $SO_2$, and the combustion gas analyzer and gas detectors were attached to measure the $SO_2$ concentration. With a flow rate of 144 Nm3/h and a temperature range of $200{\sim}300^{\circ}C$, the experiments were performed for the Stoichiometric Ratio (SR) of 1.0 to 3.0 and droplet mean diameter of 6.5 to $34.3{\mu}m$. In case of smaller spray droplets, the desulfurization efficiency improved due to the increase of total droplet surface area, while the reduction in evaporation time reduced the contact time between the droplets and $SO_2$ gas. In some typical region of droplet diameter, this negative effect, reduction of contact time, became dominant and the desulfurization yield decreases the desulfurization yield in spite of the expansion in absorption area. These results revealed that there exists the optimal size of spray droplets for a given state, which is determined by the compromise between the total surface area of slurry droplets and the evaporation time of droplets. The measurements also indicated that the inlet temperature of flue gas changes the optimal injection condition by varying the driving force for evaporation. The results confirm that the effect of the evaporation time of slurry droplets should be considered in analyzing the desulfurization yield as well as the total surface area, for it is a significant aspect of the correlation with the capabilities of $SO_2$ absorption in wet droplets. In conclusion, the optimal condition of spray can be determined based on these results, which might be applied to design or scale-up of SDS system.

• 대한전자공학회논문지TC
• /
• 제43권3호
• /
• pp.49-58
• /
• 2006

본 논문에서는 시변 주파수 선택적 페이딩 채널에서 OFDM (orthogonal frequency division multiplexing) 시스템을 위한 curve-fitting 채널추정 방법을 제안한다. 제안된 방법은 시간영역 및 주파수영역에서 1차원 curve-fitting을 통하여 smoothing과 interpolation을 순차적으로 수행함으로써 채널추정 정확도를 크게 개선할 수 있다. 먼저, 파일럿 심벌들을 사용하여 LS(least-Square) 추정치를 구하고, 이를 바탕으로 파일럿 밀도가 상대적으로 높은 영역을 선택하여 최소자승오차 기준에 따라 적절한 차수의 다항식으로 1차원 curve-fitting을 수행한다. 다음으로, 이 다항식을 이용하여 주어진 범위 내에 존재하는 LS 추정치들을 smoothing하고, interpolation 또는 prediction을 통하여 데이터 전송을 위하여 필요한 채널추정치들을 구한다. 이어서, 선택된 영역에서 얻어진 채널추정치들을 나머지 영역에서 또 다른 다항식을 사용하여 동일한 과정으로 1차원 curve-fitting을 통하여 smoothing과 interpolation을 수행함으로써 시간영역 및 주파수영역에서의 채널추정을 완료한다. 모의실험을 통하여 다양한 채널환경에서 MSE (mean square error) 및 BER (bit error rate) 성능을 분석한 결과, 제안된 방법이 기존의 채널추정 방법들에 비하여 월등히 우수하며, 최적의 Wiener 필터링 방법보다도 우수함을 보였다.