• Journal of Communications and Networks
• /
• 제6권2호
• /
• pp.182-191
• /
• 2004

In this paper, performances of different ultra-wideband (UWB) modulation schemes in the presence of timing jitter are evaluated and compared. Static and Rayleigh fading channels are considered. For fading channels, Oat and dispersive channels are assumed. First, bit error rate (BER) performances for each case are derived for a fixed value of timing jitter. Later, a uniform distribution of jitter is assumed to evaluate the performance of the system, and the theoretical results are verified by computer simulations. Finger estimation error is treated as timing jitter and an appropriate model is generated. Furthermore, a worst case distribution that provides an upper bound on the system performance is presented and compared with other distributions. Effects of timing jitter on systems employing different pulse shapes are analyzed to show the dependency of UWB performance on pulse shape. Although our analysis assumes uniform timing jitter, our framework can be used to evaluate the BER performance for any given probability distribution function of the jitter.

• Structural Engineering and Mechanics
• /
• 제10권2호
• /
• pp.181-195
• /
• 2000

A two-step procedure for the application of non linear constrained programming to the limit analysis of rigid brick-block systems with no-tension and frictional interface is implemented and applied to various masonry structures. In the first step, a linear problem of programming, obtained by applying the upper bound theorem of limit analysis to systems of blocks interacting through no-tension and dilatant interfaces, is solved. The solution of this linear program is then employed as initial guess for a non linear and non convex problem of programming, obtained applying both the 'mechanism' and the 'equilibrium' approaches to the same block system with no-tension and frictional interfaces. The optimiser used is based on the sequential quadratic programming. The gradients of the constraints required are provided directly in symbolic form. In this way the program easily converges to the optimal solution even for systems with many degrees of freedom. Various numerical analyses showed that the procedure allows a reliable investigation of the ultimate behaviour of jointed structures, such as stone masonry structures, under statical load conditions.

• Geomechanics and Engineering
• /
• 제29권3호
• /
• pp.331-338
• /
• 2022

Face stability analyses provides the most probable failure mechanisms and the understanding about parameters that need to be considered for the evaluation of ground movements caused by tunneling. After the Upper Bound Method (UBM) solution which can consider the influence of seepage forces and depth-dependent effective cohesion is verified with the numerical experiments, the probabilistic model is proposed to calculate the unbiased limiting tunnel collapse pressure. A reliability analysis of a shallow circular tunnel driven by a pressurized shield in a frictional and cohesive soil is presented to consider the inherent uncertainty in the input parameters and the proposed model. The probability of failure that exceeding a specified applied pressure at the tunnel face is estimated. Sensitivity and importance measures are computed to identify the key parameters and random variables in the model.

• ETRI Journal
• /
• 제44권1호
• /
• pp.117-124
• /
• 2022

In this paper, the performance of precoding-aided differential spatial modulation (PDSM) systems with optimal transmit antenna subset (TAS) selection is examined analytically. The average bit error rate (ABER) performance of the optimal TAS selection-based PDSM systems using a zero-forcing (ZF) precoder is evaluated using theoretical upper bound and Monte Carlo simulations. Simulation results validate the analysis and demonstrate a performance penalty < 2.6 dB compared with precoding-aided spatial modulation (PSM) with optimal TAS selection. The performance analysis reveals a transmit diversity gain of (N_T-N_R+1) for the ZF-based PDSM (ZF-PDSM) systems that employ TAS selection with N_T transmit antennas, N_S selected transmit antennas, and N_R receive antennas. It is also shown that reducing the number of activated transmit antennas via optimal TAS selection in the ZF-PDSM systems degrades ABER performance. In addition, the impacts of channel estimation errors on the performance of the ZF-PDSM system with TAS selection are evaluated, and the performance of this system is compared with that of ZF-based PSM with TAS selection.

• Structural Engineering and Mechanics
• /
• 제44권6호
• /
• pp.775-799
• /
• 2012

This work approaches the structural performance of masonry arches that have a small ratio between number of vossoirs and span length. The aim of this research is to compare and validate three different methods of analysis (funicular limit analysis F.L.A., kinematic limit analysis K.L.A. and plane stress Finite Element Analysis F.E.A.) with an experimental campaign. 18 failure tests with arches of different shapes and boundary conditions have been performed. The basic failure mechanism was the formation of enough hinges in the geometry. Nevertheless, in few cases, sliding between vossoirs also played a relevant influence. Moreover, few arches didn't reach the collapse. The FLA and KLA didn't find a solution close to the experimental values for some of the tests. The low number of vossoirs and joints become a drawback for an agreement between kinematic mechanism, equilibrium of forces and geometry constraints. FLA finds a lower bound whereas KLA finds an upper bound of the ultimate load of the arch. FEA is the most reliable and robust method and it can reproduce most of the mechanism and ultimate loads. However, special care is required in the definition of boundary conditions for FEA analysis. Scientific justification of the more suitability of numerical methods in front of classic methods at calculating arches with a few vossoirs is the main original contribution of the paper.

• Structural Engineering and Mechanics
• /
• 제46권1호
• /
• pp.19-37
• /
• 2013

The optimum design of base isolation system considering model parameter uncertainty is usually performed by using the unconditional response of structure obtained by the total probability theory, as the performance index. Though, the probabilistic approach is powerful, it cannot be applied when the maximum possible ranges of variations are known and can be only modelled as uncertain but bounded type. In such cases, the interval analysis method is a viable alternative. The present study focuses on the bounded optimization of base isolation system to mitigate the seismic vibration effect of structures characterized by bounded type system parameters. With this intention in view, the conditional stochastic response quantities are obtained in random vibration framework using the state space formulation. Subsequently, with the aid of matrix perturbation theory using first order Taylor series expansion of dynamic response function and its interval extension, the vibration control problem is transformed to appropriate deterministic optimization problems correspond to a lower bound and upper bound optimum solutions. A lead rubber bearing isolating a multi-storeyed building frame is considered for numerical study to elucidate the proposed bounded optimization procedure and the optimum performance of the isolation system.

• Structural Engineering and Mechanics
• /
• 제26권2호
• /
• pp.211-229
• /
• 2007

An experimental investigation to determine the transfer length of a seven-wire prestressing strand in different concretes is presented in this paper. A testing technique based on the analysis of bond behaviour by means of measuring the force supported by the prestressing strand on a series of specimens with different embedment lengths has been used. An analytical bond model to calculate the transfer length from an inelastic bond stress distribution along the transfer length has been obtained. A relationship between the plastic bond stress for transfer length and the concrete compressive strength at the time of prestress transfer has been found. An equation to predict the average and both the lower bound and the upper bound values of transfer length is proposed. The experimental results have not only been compared with the theoretical prediction from proposed equations in the literature, but also with experimental results obtained by several researchers.

• 농업생명과학연구
• /
• 제46권6호
• /
• pp.197-205
• /
• 2012

이 논문은 한육우 사육두수 주기를 분석하고 이 주기를 기초로 한육우 사육두수를 예측하였다. 이를 위해 일정한 주기를 가지고 정점과 저점을 반복하는 싸인함수를 활용하였다. 과거 한육우 사육두수의 주기는 짧게는 5년부터 길게는 12년까지의 패턴을 보여주었고, 이러한 주기는 시간이 흐름에 따라 점차 길어지는 양상을 보여주고 있다. 싸인함수를 활용하여 한육우 사육두수를 추정한 결과, 모형의 적합도가 0.95로 매우 높게 나타났다. 이 모형을 이용하여 한육우 사육두수를 예측한 결과, 사육주기에 따라 약간의 차이가 나지만 저점의 사육두수는 160~180만두 정도이고 정점의 사육두수는 320만두~370만두 수준으로 예측되었다. 이러한 예측결과는 사육두수 불안정성에 따른 농가 소득 불안정성을 내포하고 있어 그에 따르는 적절한 정책이 필요함을 시사한다.

• 대한기계학회논문집
• /
• 제19권5호
• /
• pp.1211-1219
• /
• 1995

An upper bound elemental technique (UBET) program has been developed to analyze forging load, die-cavity filling and effective strain distribution for flash and flashless forgings. The simulation for flash and flashless forgings are applied axisy mmetric and plane-strain closed-die forging with rib-web type cavity. Inverse triangular and inverse trapezoidal elements are used to analyze flashless forging. The analysis is described for merit of flashless precision forging. Experiments have been carried out with pure plasticine billets at room temperature. Theoretical predictions of the forging load and the flow pattern are in good agreement with experimental results.

• 대한기계학회논문집
• /
• 제15권5호
• /
• pp.1407-1416
• /
• 1991

본 연구에서는 Liu의 수식화를 바탕으로 가공 경화성을 고려하여 수식화를 재 구성하고, 유한요소 프로그램을 개발하여 평면 변형 인발문제를 극한 해석함으로써, 성형에 필요한 한계 하중 및 최적 속도장을 직접적으로 구하였다.수렴되어진 최적 속도장으로 각 요송에서의 변형률 속도, 변형률 및 격자 변형등을 수치적으로 계산함 으로써 가공에 따른 변형 특성도 파악하였다. 한계 하중은 항공기 구조용 소재인 알 루미늄 6061 재료를 이용하여 판재 인발 실험을 행함으로써 얻은 결과치와 비교 검토 하였으며, 유동 특성을 관찰하기 위하여 격자 왜곡(grid distortion) 실험을 하여 얻 은 변형 패턴과 수치 계산에서 구한 격자변형 패턴을 상호 비교하였다.