Iterative cancellation of clipping noise in multilevel quadrature amplitude modulation multi-carrier CDMA system

An iterative reconstruction method that was proposed for clipping noise cancellation in orthogonal frequency domain multiplexing (OFDM) systems is applied to multilevel quadrature amplitude modulation-based multi-carrier (MC) code division multiple access signals in downlink. The iterative method uses a known distortion function, maybe a nonlinear one such as a successive clipping and filtering process, in the iterations to give an approximation of inverse of the distortion process and then the iterative method removes distortion under a convergence condition. The authors show that MC signal with properly chosen clipping threshold satisfies convergence conditions of the iterative method. In contrast to some of the other reconstruction-based techniques, this method requires no extra bandwidth and side information and it can be implemented with reasonable complexity. Furthermore, the authors show that the proposed iterative scheme can be enhanced by using an extra frequency bandwidth. Exploiting extra bandwidth improves the performance of the reconstruction-based methods in case of using successive clipping and filtering. Simulation results will be used to demonstrate achievable bit-error-rate improvement by the proposed enhanced iterative scheme.     

Mixed explicit-implicit iterative finite element scheme for diffusion-type problems: I. Theory

A Galerkin finite element formulation of diffusion processes based on a diagonal capacity matrix is analysed from the standpoint of local stability and convergence. The theoretical analysis assumes that the conductance matrix is locally diagonally dominant, and it is shown that one can always construct a finite element network of linear triangles satisfying this condition. Time derivatives are replaced by finite differences, leading to a mixed explicit-implicit system of algebraic equations which can be efficiently solved by a point iterative technique. In this work the accelerated point iterative method is adopted and is shown to converge when the conductance matrix is locally diagonally dominant. Several examples are included in Part II of this paper to demonstrate the efficiency of the new approach.     

Design of disassembly sequences using search strategies. Application of IDA* in state diagrams

The main aim of a disassembly process is to reuse components and reduce the negative impact it has on the environment. Reaching an efficient and effective sequence of disassembly has become one of the major concerns in this field. This article refers to the use of iterative deepening A* search (artificial intelligence), in the design of the disassembly sequence. This method is applied to the state diagrams, a simple representation of the different states in a disassembly sequence. This way, the design process gains efficiency by finding the final sequence in the first iteration; therefore removing the need to analyse all possible sequences, since the best possible sequence is known in the first iteration. The method can also be applied to mechanical, electromechanical and electronic appliances with the aim of obtaining components for reuse or final disposal. Experimental results demonstrate the applicability and effectiveness of the methodology.     

加权微粒群算法在模型配准中的应用

微粒群算法目前已经在很多领域得到了广泛的应用。根据微粒群算法收敛较快的权值范围,建立加权函数,将其运用到速度进化过程中,并在进化过程中分群优化,使得改进的微粒群算法在迭代初期具有较好的全局收敛能力,在迭代后期具有较好的局部收敛能力,从而可以实现维护全局和局部搜索能力的平衡。将该算法运用于散乱点云与三维CAD模型的配准问题中,并与基本微粒群算法进行对比,具有更好的配准结果,迭代收敛更快。     

改进迭代过程的车轨耦合振动数值解法及应用

针对列车-轨道耦合振动迭代求解过程,结合Newmark-β 积分格式,提出一种基于有限元法与非线性接触理论的改进迭代过程数值解法。考虑分别建立车辆系统和轨道系统振动方程,在耦合和解耦迭代过程中,构造松弛因子函数和收敛准则函数,简化轮轨界面协调适应条件,利用轮轨相互作用力在两子系统之间的快速迭代实现动态耦合关系的高效求解。此算法增强了对迭代收敛精度、迭代过程稳定性的控制,同时也减小了程序设计的难度。应用此算法分别对竖错和路基沉降两类典型线路缺陷引起的车轨振动响应进行了算例对比和分析,计算结果表明,改进解法在迭代速度和迭代稳定性上具有优势,可广泛应用于高速铁路车辆运行和轨道结构动力学问题的分析中。     

Theoretical verification of the displacement adjustment and springforward algorithms for springback compensation

Now that Finite Element springback prediction has become possible, springback compensation can also be carried out in the context of a forming simulation, before actual production tools are made. The Displacement Adjustment (DA) and Springforward (SF) methods were applied to an analytical bar stretch-bending model, in order to gain insight about the influence of material, process and geometrical parameters on springback and compensation. The DA method was investigated in both a one-step and iterative variant. In one-step DA, a compensation factor is required. This factor can be directly calculated for the analytical model. The results can be used as a guideline for industrial processes, where such a calculation is not possible. Finally, it was shown that iterative DA leads to better tool shapes than SF, and that practical and computational problems make the use of SF impossible in an industrial context.     

Application of the general Jacobi diagonalization method to the optical properties of a medium perturbed by an external field

An analytical approach is presented for studying the convergence of the general Jacobi method applied to diagonalizing the second-rank tensors that describe the optical properties of a medium subjected to an external field. This approach utilizes the fact that the components of such tensors are usually given in field-free principal axes as power series in the field strength, neglecting terms beyond a chosen power of the field. It is shown that for a biaxial or uniaxial medium, the finite number of iterations, which guarantees exact reduction of all the initial terms up to the required power in the series expansions of all off-diagonal elements, can always be found. Moreover, a fixed sequence of rotations in the Jacobi algorithm can be predicted. These findings allow one to derive analytical formulas in noniterative form for a given highest order of the effects being considered and also to optimize numerical iterative diagonalization procedures. Formulas for eigenvalues and eigenvectors applicable to biaxial and uniaxial mediums perturbed by the linear and quadratic effects are presented. Illustrations are given of the electro-optic and piezo-optic effects for the point group 3m. Conditions for biaxial and uniaxial perturbation of a uniaxial crystal are discussed.     

A unidimensional convergence test for matrix iterative processes applied to Strapdown Navigation

The investigation of the convergence properties of matrix iterative processes usually involves test matrices of high order. This fact may prohibit an analytic approach to the problem. In this paper a method is presented which converts the multidimensional test procedure into a scalar one. The method is presented in conjunction with the problem of matrix orthogonalization which exists in Strapdown Inertial Navigation. Three examples are presented in which the convergence of matrix orthogonalization techniques is investigated. The examples demonstrate the use of the unidimensional convergence test in determining the order of the processes and in finding sufficient conditions for convergence. Numerical results are presented.     

An arc-length method including line searches and accelerations

This paper describes a method for introducing line searches into the arc-length solution procedure. Such line searches may be used at each iteration to calculate an optimum scalar step-length which scales the normal iterative vector. In practice, a loose tolerance is provided so that on many iterations the line searches are avoided. However on ‘difficult iterations’, the line searches are shown to lead to a substantial improvement in the convergence characteristics. A simple single-parameter acceleration is also developed using line search concepts. The new arc-length method is applied to both the geometrically nonlinear analysis of shallow shells and the materially nonlinear analysis of reinforced concrete beams and slabs. Significant improvements are demonstrated in relation to the standard arc-length method.     

ITERATIVE SOLUTION OF LARGE THREE-DIMENSIONAL BEM ELASTOSTATIC ANALYSES USING THE GMRES TECHNIQUE

The GMRES (Generalized Minimal RESidual) iterative method is receiving increased attention as a solver for the large dense and unstructured matrices generated by boundary element elastostatic analyses. Existing published results are predominantly for two-dimensional problems, of only medium size. When these methods are applied to large three-dimensional problems, which actually do require efficient iterative methods for practical solution, they fail. This failure is exacerbated by the use of the pre-conditioning otherwise desirable in such problems. The cause of the failure is identified as being in the orthogonalization process, and is demonstrated by the divergence of the ‘true’ residual, and the residual calculated during the GMRES algorithm. It is shown that double precision arithmetic is required for only the small fraction of the work comprising the orthogonalization process, and exploitation of this largely removes the penalties associated with the use of double precision. Additionally, it is shown that full re-orthogonalization can be employed to overcome the lack of convergence, extending the applicability of the GMRES to significantly larger problems. The approach is demonstrated by solving three-dimensional problems comprising ∼4000 and ∼5000 equations. © 1997 John Wiley & Sons, Ltd.     

Three-phase power-flow by direct ZBR method for unbalanced radial distribution systems

This paper introduces a novel three-phase power flow approach for unbalanced radial distribution systems. The proposed approach is developed based on the branch frame of reference, rather than the traditional bus frame of reference. On the basis of the branch frame of reference, a simple direct iterative method can be applied. Hence, the proposed approach may be called the `direct Z _BR method'. Basic graph theory and injection current technique are also applied in the proposed approach. The clear theoretical foundation and the simple topology of the radial distribution network make the proposed method efficient and reliable. To demonstrate the better convergence performance and the efficiency of the proposed approach, four three-phase IEEE test feeders are used for comparisons. The test results show that the proposed method has robust convergence characteristics and high performance, especially for large-scale radial distribution systems.     

增生算子方程的具误差的Ishikawa迭代序列的收敛率估计

设X是一实的Banach空间,T：X→Y是一Lipschitz的增生算子。本文证明了具误差的Ishikawa迭代序列强收敛到方程x Tx=f的唯一解：并得一个一般的收敛估计式。若T：X→X是一Lipschitz的强增生算子,则具误差的Ishikawa迭代序列强收敛到方程Tx=f的唯一解。本文结果推广和发展了现有的相应结果。     

Spatio-temporal coding in complex media for optimum beamforming: the iterative time-reversal approach

Spatio-temporal encoding in transmit and receive modes is of major importance in the development of ultrasound imaging devices. Classically, the assumption of constant sound speed in the medium allows one to restrict the beamforming process to the application of a cylindrical time-delay law on the elements of a multiple-transducer array. Here is proposed an iterative time-reversal method capable of taking into account all the heterogeneities of the medium, concerning density, speed of sound, and absorption variations. It will be shown that this iterative focusing process converges toward a spatio-temporal inverse filter focusing, the first step of the process being a time-reversal focusing on the targeted point. This method can be seen as a calibration process and has been successfully applied to transskull focusing and intraplate echoes suppression. It is leading the way to promising applications such as high-resolution ultrasonic brain imaging and high-resolution focusing through complex reverberating media, in nondestructive testing and telecommunications. This work highlights the advantages of using spatio-temporal coding to focus through complex media. Such codes require the use of fully programmable, multichannel electronics to implement this technique in real time.     

Application of a Modified Algorithm of the Method of Conjugate Gradients in Finite-Element Analyses

For the solution of systems of linear algebraic equations by the finite-element method, we consider a generalized method of conjugate gradients with preconditioning matrix constructed by using the transition matrix for the method of symmetric upper relaxation. It is shown that the rate of the iterative process can be doubled. We present the results of the numerical analysis of the rate of convergence of the iterative process in the solution of two-dimensional model problems of the theory of elasticity and linear fracture mechanics with the help of the classical and modified algorithms of the method of conjugate gradients with preconditioning matrix of the method of symmetric upper relaxation. __________ Translated from Problemy Prochnosti, No. 6, pp. 89 – 102, November – December, 2005.     

Iterative solution techniques in boundary element analysis

Iterative techniques for the solution of the algebraic equations associated with the direct boundary element analysis (BEA) method are discussed. Continuum structural response analysis problems are considered, employing single- and multi-zone boundary element models with and without zone condensation. The impact on convergence rate and computer resource requirements associated with the sparse and blocked matrices, resulting in multi-zone BEA, is studied. Both conjugate gradient and generalized minimum residual preconditioned iterative solvers are applied for these problems and the performance of these algorithms is reported. Included is a quantification of the impact of the preconditioning utilized to render the boundary element matrices solvable by the respective iterative methods in a time competitive with direct methods. To characterize the potential of these iterative techniques, we discuss accuracy, storage and timing statistics in comparison with analogous information from direct, sparse blocked matrix factorization procedures. Matrix populations that experience block fill-in during the direct decomposition process are included. With different degrees of preconditioning, iterative equation solving is shown to be competitive with direct methods for the problems considered.     

Issues in convergence improvement for non-linear finite element programs

Systems of non-linear equations as they arise when analysing various physical phenomena and technological processes by the implicit Finite Element Method (FEM) are commonly solved by the Newton–Raphson method. The modelling of sheet metal forming processes is one example of highly non-linear problems where the iterative solution procedure can become very slow or diverge. This paper focuses on techniques to overcome these numerical difficulties. Several methods to generate initial guesses within the radius of convergence are proposed. Appropriate stopping criteria for the iterative procedure are discussed. A combination of various line search methods with the continuation method is proposed. The efficiency and robustness of these numerical procedures are compared based on a set of test examples. A particular form of line search was identified which allows the stable and efficient solution of highly non-linear sheet metal forming problems. Even though the present investigations were motivated by the application of the implicit FEM to the simulation of sheet metal forming processes, the findings are general enough to be applicable to a wide spectrum of non-linear FEM applications. © 1997 John Wiley & Sons, Ltd.     

On the preconditioned conjugate gradient method for solving (A – λB)X =0

Classical iterative methods when applied to the partial solution of the generalized eigenvalue problem Ax =λ Bx , may yield very poor convergence rates particularly when ill-conditioned problems are considered. In this paper the preconditioned conjugate gradient (CG) method via the minimization of the Rayleigh quotient and the reverse power method is employed for the partial eigenproblem. The triangular splitting preconditioners employed are obtained from an incomplete Choleski factorization and a partial Evans preconditioner. This approach can dramatically improve the convergence rate of the basic CG method and is applicable to any symmetric eigenproblem in which one of the matrices A , B is positive definite. Because of the renewed interest in CG techniques for FE work on microprocessors and parallel computers, it is believed that this improved approach to the generalized eigenvalue problem is likely to be very promising.     

DYNAMIC PROGRAMMING,REDUCTION OF DIMENSIONALITY AND MATRIX EIGENVALUE COMPUTATIONS

This paper is a continuation of earlier papers by Ng & Sancho for solving certain dynamic programming problems, An iterative algorithm, based on a priori deduction from Bellman's principle of optimality, is developed. The technique is applied to evaluate the smallest eigenvalue of positive definite symmetric matrices. It is shown to use only minimal storage requirements as compared to the traditional dynamic programming approach. Solutions on test matrices compare favorably in accuracy and convergence speed with other numerical solutions. The significance of the method is that it provides a means of reducing Bellman's “curse of dimensionality” and broadens the scope of problems that can effectively be solved with the dynamic programming approach     

有限元──迭代法

本文提出基于有限元分析结构变更后的迭代法，文中导出了迭代公式，讨论了收敛速度并附有算例，用本文方法计算变更后的新结构，不论结构刚度变化区域大小均可节约计算时间，从而克服了广义结构变更定理应用中的局限性。     

Browder-Petryshyn型严格半伪压缩映射的强收敛性

通过引入比严格伪压缩映射更一般的严格半伪压缩映射的概念,得到了任意实Banach空间中带误差的Ishikawa迭代法生成的序列强收敛于严格半伪压缩映射的不动点的一些充要条件;作为应用,给出了序列强收敛于不动点的一个充分条件,从而将相关结果从q-一致光滑实Banach空间延拓至任意实。Banach空间;并给出了Ishikawa迭代序列的误差估计,所用的证明方法与已有的不同且更为简单。