Low Number Of Iterations Research Articles

A modified gravitational search algorithm for slope stability analysis

This paper first proposes an effective modification for the gravitational search algorithm. The new strategy used an adaptive maximum velocity constraint, which aims to control the global exploration ability of the original algorithm, increase its convergence rate and thereby to obtain an acceptable solution with a lower number of iterations. We testify the performance of the modified gravitational search algorithm (MGSA) on a suite of five well-known benchmark functions and provide comparisons with standard gravitational search algorithm (SGSA). The simulated results illustrate that the modified GSA has the potential to converge faster, while improving the quality of solution. Thereafter, the proposed MGSA is employed to search for the minimum factor of safety and minimum reliability index in both deterministic and probabilistic slope stability analysis. The factor of safety is formulated using a concise approach of the Morgenstern and Price method and the advanced first-order second-moment (AFOSM) method is adopted as the reliability assessment model. The numerical experiments demonstrate that the modified algorithm significantly outperforms the original algorithm and some other methods in the literature.

고속 처리와 성능 향상을 위한 LDPC 코드 기반 결정 궤환 등화기

본 논문에서는 OFDM 시스템에서 고속 처리와 성능 향상을 위한 LDPC 코드 기반 결정 궤환 등화기(Decision Feedback Equalizer: DFE)를 제안한다. LDPC 코드는 우수한 오류 정정 능력과 Shannon의 채널 용량에 근접하는 성능을 갖는다. 그러나, 많은 parity 검사 행렬과 반복 횟수를 가진다는 단점이 있다. 제안된 시스템에서는 판정된 신호와 복호기 사이의 신호의 MSE(Mean Square Error)를 등화기로 피드백한다. 이러한 방법을 사용하면 추정된 채널 응답을 보정해 주기 때문에 성능을 향상시킬 수 있다. 또한, 동일한 성능에서 피드백이 포함되지 않은 시스템보다 낮은 반복 횟수를 갖기 때문에 시스템의 복잡도를 줄일 수 있다. 시뮬레이션을 통해 다중 경로 채널에서 CFO(Carrier Frequency Offset)와 위상 잡음이 고려된 OFDM 시스템의 성능을 평가하여 제안 시스템의 우수성을 보인다. In this paper, we propose a decision feedback equalizer based on LDPC(Low Density Parity Check) code for the fast processing and performance improvement in OFDM system. LDPC code has good error correcting capability and its performance approaches the Shannon capacity limit. However, it has longer parity check matrix and needs more iteration numbers. In our proposed system, MSE(Mean Square Error) of signal between decision device and decoder is fed back to equalizer. This proposed system can improve BER performance because it corrects estimated channel response more accurately. In addition, the proposed system can reduce complexity because it has a lower number of iterations than system without feedback at the same performance. Simulation results evaluate and show the performance of OFDM system with the CFO and phase noise in multipath channel.

Improved Iterative Coordinated Beamforming Based on Singular Value Decomposition for Multiuser Mimo Systems With Limited Feedforward

Coordinated beamforming based on singular value decomposition is an iterative method to jointly optimize the transmit beamformers and receive combiners, to achieve high levels of sum rates in the downlink of multiuser systems, by exploiting the multi-dimensional wireless channel created by multiple transmit and receive antennas. The optimization is done at the base station and the quantized beamformers are sent to the users through a low rate link.In this work, we propose to optimize this algorithm by reducing the number of iterations and improving its uncoded bit error rate performance. Simulation results show that our proposal achieves a better bit error rate with a lower number of iterations than the original algorithm.

Evaluation of two approaches to motion-corrected PET image reconstruction

Motion correction is a major task in PET. This study aims to understand the behaviour of different approaches to motion-corrected image reconstruction in terms of convergence rates and the properties of the images obtained. We have studied two approaches that have been suggested to motion-correct PET gated data. The first method, RRA, is based on independent reconstructions of each gate which are then registered to a reference gate and averaged. On the other hand, in the second method, MCIR, all data are used with the motion information within a single reconstruction. As a first step, this investigation compares the methods with no motion present. Multiple simulations have been produced with counts approximating a 5 min PET thorax acquisition. The image bias and variance are the main figures of merit. The results show different convergence and statistics performance between the two methods. RRA converges faster than MCIR but not at the correct value. Considering the trade-off between bias and variance, it is seen that MCIR outperforms RRA. It has been also demonstrated that, although at higher iterations RRA has lower variance and higher bias than MCIR, at a low number of iterations the performance of the methods becomes closer. This investigation has revealed the statistical and computational discrepancies between RRA and MCIR when motion is not present and MCIR is shown to be clearly superior.

The effects of power control on the optical CDMA random access protocol

In this work, the performance of the hybrid system that combines the distributed power control algorithm (DPCA) with the random access protocol as a novel and simple scheme of achieving a high performance in decentralized optical code division multiple access (OCDMA) networks has been investigated. The multiple access interference (MAI) and the near–far problem effects have been considered. The DPCA’s advantage lies in its characteristics being effectively implemented to each node, since only local parameters are necessary. The principal results have shown that the network throughput and delay are strongly affected by the near–far problem and the DPCA works to solve this problem. Hence, the introduction of a certain level of the power control to the random access temporally coded (1D) or the time-wavelength coded (2D) OCDMA networks has demonstrated profitability of the throughput increase and the delay reduction. As a consequence, the proposed system configuration with the DPCA using a very low number of iterations has resulted in a better throughput and simultaneously in a delay decrease when compared to the system without power control mechanisms.

Determination of bone density distribution in proximal femur by using the 3D orthotropic bone adaptation model

Most of the models developed and presented in the literature about bone remodelling assume bone as isotropic material. During this study, a structural model which has orthotropic bone properties is developed to simulate bone behaviour under mechanical loads. For a three-dimensional finite element modelling of proximal femoral bone, a computer code was written to investigate bone property changes owing to mechanical loads. The bone density distribution was determined after stress analysis was carried out in an iterative method. At every time step the bone properties were calculated depending on density change. Additionally, the effects of the bone remodelling parameters and sensors on numerical results were investigated. The most realistic bone density distribution in the proximal femur is obtained with strain energy density sensor. Generally initial bone density values do not have any effect on density distribution but at the low iteration numbers they may have some effects. Lastly, the density distribution of proximal femoral bone is more realistic for the dead zone size less than 0.35 and the stimulus value less than 2500.

AB-OSEM reconstruction for improved Patlak kinetic parameter estimation: a simulation study

The non-negativity constraint inherently present in OSEM reconstruction successfully reduces the standard deviation in cold regions but at the cost of introducing a positive bias, especially at low iteration numbers. For low-count data, as often encountered in short-duration frames in dynamic imaging protocols, it has been shown that it can be advantageous (in terms of bias in the reconstructed image) to remove the non-negativity constraint. In this work two competing algorithms that do not impose non-negativity in the reconstructed image are investigated: NEG-ML and AB-OSEM. It was found that the AB-OSEM reconstruction outperformed the NEG-ML reconstruction. The AB-OSEM algorithm was then further developed to allow a forward model that includes randoms and scatter background terms. In addition to static reconstruction the current analysis was extended to consider the important case of kinetic parameter estimation from dynamic PET data. Simulation studies (comparing OSEM, FBP and AB-OSEM) showed that the positive bias obtained with OSEM reconstruction can be avoided in both static and parametric imaging through use of a negative lower bound in AB-OSEM reconstruction (i.e. by lifting the implicit non-negativity constraint of OSEM). When quantification tasks are considered, the overall error in the estimates (composed of both bias and standard deviation) is often of primary concern. An important finding of this work is that in most cases the activity concentration and the kinetic parameters obtained from images reconstructed using AB-OSEM showed a lower overall root mean squared error compared to the popular choices of either OSEM or FBP reconstruction for both cold and warm regions. As such, AB-OSEM should be preferred instead of the standard OSEM and FBP reconstructions when kinetic parameter estimation is considered. Finally, this work shows example parametric images from the high-resolution research tomograph obtained using the different reconstruction methods.

ON INTRINSIC MODE FUNCTION

Empirical Mode Decomposition (EMD) has been widely used to analyze non-stationary and nonlinear signal by decomposing data into a series of intrinsic mode functions (IMFs) and a trend function through sifting processes. For lack of a firm mathematical foundation, the implementation of EMD is still empirical and ad hoc. In this paper, we prove mathematically that EMD, as practiced now, only gives an approximation to the true envelope. As a result, there is a potential conflict between the strict definition of IMF and its empirical implementation through natural cubic spline. It is found that the amplitude of IMF is closely connected with the interpolation function defining the upper and lower envelopes: adopting the cubic spline function, the upper (lower) envelope of the resulting IMF is proved to be a unitary cubic spline line as long as the extrema are sparsely distributed compared with the sampling data. Furthermore, when natural spline boundary condition is adopted, the unitary cubic spline line degenerates into a straight line. Unless the amplitude of the IMF is a strictly monotonic function, the slope of the straight line will be zero. It explains why the amplitude of IMF tends to be a constant with the number of sifting increasing ad infinitum. Therefore, to get physically meaningful IMFs the sifting times for each IMF should be kept low as in the practice of EMD. Strictly speaking, the resolution of these difficulties should be either to change the EMD implementation method and eschew the spline, or to define the stoppage criterion more objectively and leniently. Short of the full resolution of the conflict, we should realize that the EMD as implemented now yields an approximation with respect to cubic spline basis. We further concluded that a fixed low number of iterations would be the best option at this time, for it delivers the best approximation.

A channel detection technique based on Max-Log-MAP algorithm for wireless indoor system

This study introduces a new low complexity Max-Log-MAP based channel estimation technique operating in a multi-path band limited channel. The proposed estimation technique is based on an iterative procedure derived through the maximum a posteriori probability approach. Wireless communication channels often undergo frequency selective fading that leads to inter-symbol interference, which limits the performance of wireless indoor systems. Performance of the algorithms is assessed by combined analysis and simulation. Results are presented and compared against the performance of conventional Log-MAP algorithm. The obtained results show that the required bit-error-rate performance can be achieved with a lower number of iterations using the proposed low complexity algorithm compared to using the conventional Log-MAP algorithm.

Iterated rippled noise discrimination at long durations

Iterated rippled noise (IRN) was used to study discrimination of IRN stimuli with a lower number of iterations from IRN stimuli with a higher number of iterations as a function of stimulus duration (100-2000 ms). Such IRN stimuli differ in the strength of the repetition pitch. In some cases, the gain used to generate IRN stimuli was adjusted so that both IRN stimuli in the discrimination task had the same height of the first peak in the autocorrelation function or autocorrelogram. In previous work involving short-duration IRN stimuli (<500 ms), listeners were not able to discriminate between IRN stimuli that had different numbers of iterations but the same height of the first peak in the autocorrelation function. In the current study, IRN discrimination performance improved with increases in duration, even in cases when the height of the first peak in the autocorrelation was the same for the two IRN stimuli. Thus, future studies involving discrimination of IRN stimuli may need to use longer durations (1 s or greater) than those that have been used in the past.

SU-FF-J-115: Quantification of Uptake Volumes in PET Images for Treatment Response Monitoring: Challenges and Solutions

Purpose: Monitoring treatment response in PETimages by quantifying changes in the spatial distribution of radioactivity concentration over time requires accurate determination of volumes. Thresholding methods are usually based on finding volume‐recovering thresholds for known objects, such as spheres, in a discrete voxel grid. Measuring these volumes introduces discretization errors into the threshold calibration process. In addition, thresholds depend strongly on the object‐to‐background activity ratio which is usually unknown a‐priori. In this work, the efficacy of a simple pre‐processing step is investigated that makes the thresholds largely independent of the object‐to‐background activity ratio. Methods:PET scans of a standard imaging phantom containing 6 spheres were acquired in 3D mode on a GE Discovery ST scanner for different object‐to‐background activity ratios including no background. Images were reconstructed using FORE‐OSEM with a variable number of iterations. Thresholds in images where the background activity was subtracted were compared to the thresholds in the no‐background images. Discretization errors were quantified by placing mathematically accurate contours of the spheres into the voxel grid of the PET dataset. Volumes were computed by the treatment planningsoftware.Results: Using a low number of iterations, volumes of all spheres were accurate to within 10% using thresholds derived from the background‐corrected images. A higher number of iterations degraded the accuracy significantly. In coarse voxel grids common in PETimaging (2.3 × 2.3 × 3.3 mm), small volumes were consistently overestimated by as much as 30% depending on the position of the spheres with respect to the grid. Conclusions: Threshold calibration is sensitive to voxel grid resolution. For images acquired in 3D mode and reconstructed with a low number of iterations, a simple background correction method allows for accurate volume quantification independently of initial background activity and allows for a significant simplification of the threshold calibration process.

An approximate method for evaporation problem with mushy zone

In the present paper, a simple mushy zone model is used to track the moving boundaries in an evaporation problem in which the vapor is removed upon formation. Two main parameters for the mushy zone model are analyzed as well as their effect on the movement of the moving boundaries and the thickness of the mushy zone. A new approximate method is developed for analysis and tracking the moving boundaries appears throughout the process. The proposed method mainly based on applying the boundary integral equation corresponding to each phase in such a way that the associated boundary and initial conditions as well as energy equations at the moving boundaries achieved with minimum error and low number of iterations. The results of the present paper seem to be good because there are neither analytical or numerical solutions available.

On the behavior of phm distributed schedulers for input buffered packet switches

iSLIP and parallel hierarchical matching (PHM) are distributed maximal size matching schedulers for input-buffered switches. Previous research has analyzed the hardware cost of those schedulers and their performance after a small number of iterations. In this paper, we formulate an upper bound for the number of iterations required by PHM to converge. Then, we compare the number of iterations required by iSLIP and PHM to achieve a maximal throughput under uniform Bernoulli traffic, by means of simulation. Finally, we obtain the corresponding delay performances, which are similar. The results suggest that PHM has both the advantages of previous hierarchical matching algorithms (low hardware complexity) and iSLIP (low number of iterations).

Interrelation between the kinetic constant and the reaction order in pyrolysis

A numerical method for calculating the kinetic constants of the thermal decomposition of heterogeneous solids is proposed, considering the interdependence between the pre-exponential factor and the reaction order. This method is applied to the pyrolysis of poly-(ethylene terephtalate) with satisfactory results. The kinetic constants calculated with this method during the optimization goes preferably to parameters with correct physical meaning, and therefore, a lower number of iterations in the optimization calculation system is needed. The interdependence of the pre-exponential factor and the reaction order restricts the possible solutions of the calculations, and produces more valid solutions in the calculation procedure.

An evaluation of the effect of filtering in 3-D OSEM reconstruction by using data from a high-resolution PET scanner

We evaluated the effect of filtering in the three-dimensional (3-D) ordered subset expectation maximization (OSEM) algorithm for reconstruction of projection data obtained with a high-resolution 3-D positron emission tomography (PET) scanner. For this study, we used the inter-update Metz filtered OSEM (IMF-OSEM) algorithm, which was developed by the PARAPET project. IMF-OSEM is an implementation of the OSEM algorithm with some additional capabilities such as inter-update filtering. The projection data were acquired using the high-resolution PET camera developed at the University of Texas M. D. Anderson Cancer Center (MDAPET). This prototype camera, which is a multiring scanner without any septa, has a transaxial resolution of 2.8 mm, which allows better evaluation of the algorithm. We scanned three phantoms: a cylindrical uniform phantom, a cylindrical phantom containing four small lesion phantoms, and the Hoffman brain phantom. The effect of inter-filtering in OSEM reconstruction was evaluated by computing the noise level of the reconstructed images of the uniform phantom, studying the contrast recovery for the hot lesions in a warm background, and visually inspecting images especially those of the Hoffman brain phantom. In addition, the effect of post-filtering on the reconstructed images was evaluated. For the high statistics data, a good compromise between contrast recovery and noise level was achieved using 20-50 iterations for the plain OSEM algorithm. By visually inspecting the images of the Hoffman brain phantom and hot lesions, we observed that the plain OSEM algorithm, especially when followed by post-filtering, and the inter-update filtering with Metz power of 1 could reasonably reproduce the phantom's structure. We also found that inter-update filtering has the potential to produce a noise level and contrast recovery comparable with that using the plain OSEM algorithm at a lower iteration number; however, it also has a greater tendency to develop noise artifacts.

Optimization of Land Use in Afforestation Areas Using Evolutionary Self-Organization

Abstract This article presents a cellular automaton approach to the optimization of land use planning in afforestation areas. The case study deals with 110 ha of recently afforested land in Denmark. A modified two-dimensional automaton containing 2,345 active cells is used for optimizing the land use setup in this area. Four land use alternatives are considered: pasture, beech (Fagus sylvatica L.), Norway spruce (Picea abies [L.] Karst.), and oak (Quercus robur L.). The objective is to maximize a weighted sum of soil expectation value, scale-dependent costs, recreational value, and the value of structural variation. It is demonstrated that for this sort of problem, appropriately modified cellular automata may yield acceptable solutions within a comparatively low number of iterations. Furthermore, for a simplified hypothetical problem, it is shown that the cellular automaton approach was able to identify optimal solutions within reasonable time. FOR. SCI. 48(3):543–555.

VORTEX SIZE CONTROL FOR ORIFICES CASCADE IN A RECTANGULAR DUCT

Vortex formation and shedding downstream obstructions may be assumed to be one of the main sources of flow induced vibration and noise in pipes, ducts, and control valves. The level of the produced noise depends on the size of the formed vortex and its shedding frequency hence, controlling the size of the formed vortex may be one of the main factors that help in attenuation of the generated noise. The vortex size may be estimated from the reattachment length of the separated flow downstream these obstructions. In the present work, vortex formation downstream a single orifice and two orifices in series in a rectangular duct have been investigated numerically and experimentally. A numerical solution has been coded to solve the flow governing equations in the primitive variables using the finite difference technique. The solution has been carried out for laminar, 2-D and incompressible flow field at Reynolds number ranges from 50 to 400 for orifice height to duct height ratio of 0.5 and for the inter distance between the two orifices from 0.2H to 7H. The velocity field, the streamlines, and the vorticity field have been determined. The code is written in visual C-language to achieve low computational time and lower number of iterations. An experimental investigation has been carried out on 2-D laminar flow visualization table. The streamlines of water flow throughout the single orifice and two orifices in the rectangular duct have been visualized for various Reynolds number and orifice height to duct height ratio, D/H, values. The experimental and the theoretical results showed a considerable agreement. The results showed a minimum vortex size as well as lowest circulation in case of using an inter distance of about (1-D/H). Finally, the results showed that the control of the inter distance between the two orifices or similarly between any two obstructions in a rectangular duct may be of special interest to reduce the size of the generated vortex and consequently to attenuate the vibration and noise.

Application of the fast multipole method to the generalized forward–backward iterative algorithm

In a previous work, the generalized forward–backward (GFB) method was proposed to compute the scattering from targets on rough ocean-like surfaces. In this paper, we develop an acceleration of the GFB method based on the fast multipole method (FMM). The FMM is adapted to reduce the operational cost associated with the iterative computations in the target regions. The proposed method is shown to converge in a low number of iterations, and allows a significant reduction in the computational and storage costs with respect to the conventional GFB formulation. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26: 78–83, 2000.

Assessment of the reliability of 2D inversion of apparent resistivity data

The reliability of inversion of apparent resistivity pseudosection data to determine accurately the true resistivity distribution over 2D structures has been investigated, using a common inversion scheme based on a smoothness‐constrained non‐linear least‐squares optimization, for the Wenner array. This involved calculation of synthetic apparent resistivity pseudosection data, which were then inverted and the model estimated from the inversion was compared with the original 2D model. The models examined include (i) horizontal layering, (ii) a vertical fault, (iii) a low‐resistivity fill within a high‐resistivity basement, and (iv) an upfaulted basement block beneath a conductive overburden. Over vertical structures, the resistivity models obtained from inversion are usually much sharper than the measured data. However, the inverted resistivities can be smaller than the lowest, or greater than the highest, true model resistivity. The substantial reduction generally recorded in the data misfit during the least‐squares inversion of 2D apparent resistivity data is not always accompanied by any noticeable reduction in the model misfit. Conversely, the model misfit may, for all practical purposes, remain invariant for successive iterations. It can also increase with the iteration number, especially where the resistivity contrast at the bedrock interface exceeds a factor of about 10; in such instances, the optimum model estimated from inversion is attained at a very low iteration number. The largest model misfit is encountered in the zone adjacent to a contact where there is a large change in the resistivity contrast. It is concluded that smooth inversion can provide only an approximate guide to the true geometry and true formation resistivity.

Application of the fast multipole method to the generalized forward-backward iterative algorithm

In a preious work, the generalized forward)backward () GFB method was proposed to compute the scattering from targets on rough ocean-like surfaces. In this paper, we deelop an acceleration of () the GFB method based on the fast multipole method FMM . The FMM is adapted to reduce the operational cost associated with the iteratie computations in the target regions. The proposed method is shown to conerge in a low number of iterations, and allows a significant reduction in the computational and storage costs with respect to the conentional GFB formulation. Q 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26: 78)83, 2000. method. In both GFB methods GFB 6 and SA)GFB 7 , the solution is obtained through an iterative process based on the same general concepts as the FB method, combined with the direct MoM solution of the region containing the obsta- cle. So, the computational costs of the GFB methods can be 2 . . considered as the above-mentioned O N or O N costs, respectively, for the sea-surface integrations plus the addi- tional cost associated with the conventional MoM solution of the obstacle region. The latter cost may be predominant when the targets become electrically large. The MoM solution of a single obstacle region results in a dense n = n linear system, where n is the number of MoM basis functions in the obstacle region. The memory required 2 .