a novel decomposition analysis of nonlinear distortion in ofdm transmitter systems in orthogonal frequency division multiplexing ofdm transmitter systems nonlinear distortion is a major impairment caused by power amplifier pa the distortion is constituted by numerous noise like inter modulation products imps although the overall behavior of nonlinear imp distortion has been well studied in large multi carrier contexts such as in ofdm the knowledge of individual imps and their quantitative interrelationships together with the derivation of behavioural knowledge and the predictability of consequences arising from such interrelationships is a fairly unexplored topic this is the subject of this paper the powers of the very numerous individual imps generated are analytically derived based on a bessel fourier pa behavioural model and it is proven that these powers have only a small number of distinct powers proven also is that the ratios of these power values are constants and that these constants are invariant with the particular pa s nonlinearity characteristics or operating point further it is proven that specific types of imps dominate nonlinear distortion effects finally it is shown that imp power spectra can be derived from a simple product of two polynomials which are quadrature functions of frequency and pa operating point respectively application possibilities for findings presented in this paper such as their potential to contribute efficiency improvements in ofdm system modulation fidelity calculations over 1100 times faster than conventional methods are also outlined in the conclusions

theoretical analysis and performance of ofdm signals in nonlinear awgn channels orthogonal frequency division multiplexing ofdm baseband signals may be modeled by complex gaussian processes with rayleigh envelope distribution and uniform phase distribution if the number of carriers is sufficiently large the output correlation function of instantaneous nonlinear amplifiers and the signal to distortion ratio can be derived and expressed in an easy way as a consequence the output spectrum and the bit error rate ber performance of ofdm systems in nonlinear additive white gaussian noise channels are predictable both for uncompensated amplitude modulation amplitude modulation am am and amplitude modulation pulse modulation am pm distortions and for ideal predistortion the aim of this work is to obtain the analytical expressions for the output correlation function of a nonlinear device and for the ber performance the results in closed form solutions are derived for am am and am pm curves approximated by bessel series expansion and for the ideal predistortion case

nonlinear equalization of hammerstein ofdm systems a practical orthogonal frequency division multiplexing ofdm system can generally be modelled by the hammerstein system that includes the nonlinear distortion effects of the high power amplifier hpa at transmitter in this contribution we advocate a novel nonlinear equalization scheme for ofdm hammerstein systems we model the nonlinear hpa which represents the static nonlinearity of the ofdm hammerstein channel by a b spline neural network and we develop a highly effective alternating least squares algorithm for estimating the parameters of the ofdm hammerstein channel including channel impulse response coefficients and the parameters of the b spline model moreover we also use another b spline neural network to model the inversion of the hpa s nonlinearity and the parameters of this inverting b spline model can easily be estimated using the standard least squares algorithm based on the pseudo training data obtained as a byproduct of the hammerstein channel identification equalization of the ofdm hammerstein channel can then be accomplished by the usual one tap linear equalization as well as the inverse b spline neural network model obtained the effectiveness of our nonlinear equalization scheme for ofdm hammerstein channels is demonstrated by simulation results

one block method for computing the generalized stress intensity factors for laplace s equation on a square with a slit and on an l shaped domain a highly accurate approximation for the coefficients of the series expansion of the solution of laplace s equation around the singular vertex which are called the generalized stress intensity factors gsifs is obtained by one block method the method is demonstrated for the slit problem which has a strong singularity and for the popular problem on an l shaped domain furthermore the extremely accurate series segment solution for each of the problems is obtained by taking an appropriate number of calculated gsifs numerical results are presented through tables and figures

thermal generalized stress intensity factors in 2 d domains lately methods that are capable of predicting failure initiation and propagation in structural components subjected to thermal loads have been sought it is postulated as in the theory of linear elastic fracture mechanics that the methods should correlate experimental observed failures to parameters characterizing the thermoelastic stress field in the vicinity of failure initiation points failures due to thermal loading occur for example in integrated circuits which are assemblages of dissimilar materials with different thermal and mechanical properties addressed in chapter 9 the mismatch of elastic constants and thermal expansion coefficients causes stress intensification at corners of interfaces and may lead to mechanical failure

modeling of singular stress fields using finite element method the paper discusses methods of modeling of singular stress fields in problems with angular corners a novel method of analytical constraints has been proposed in this method the relations between the displacements of the finite element nodes are assumed to conform to the analytical solution the method of analytical constraints has been used for calculations of the stress intensity factors and of the coefficients of the two consecutive terms of the asymptotic solution in the case of elements with cracks and v notches under uniaxial and biaxial loading singular finite elements have been applied and various mesh discretisations have been used

an analysis of band limited communication systems from amplifier efficiency and distortion perspective considering growing concerns toward green communications not only their bandwidth efficiency but also their power consumption is expected to gain increasing importance for future communication systems a power amplifier pa is one of the most power consuming analog devices at the transmitter in communication systems and a precise estimation of the power consumption at the transmitter circuit requires the knowledge of its efficiency that depends on the distribution of input signal envelope and the amount of input back off in this paper we develop a simple analytical model that can be used for estimating achievable efficiency of a linear pa as well as the amount of resulting nonlinear distortion for a given modulated signal exclusively from its envelope distribution based on this model the trade off relationships between the efficiency and signal to distortion power ratio for several spectrally efficient linear modulations are examined it is also shown that in some special cases of gaussian input and memoryless nonlinearity pa models the results can be represented in simple closed form expressions

digital predistortion for power amplifiers using separable functions this paper is concerned with digital predistortion for linearization of rf high power amplifiers hpas it has two objectives first we establish a theoretical framework for a generic predistorter system and show that if a postdistorter exists then it is also a predistorter and therefore the predistorter and postdistorter are equivalent this justifies the indirect learning methods for a large class of hpas second we establish a systematic and general structure for a predistorter that is capable of compensating nonlinearity for a large variety of hpas this systematic structure is derived using approximation by separable functions and avoids selection of predistorters based on the assumption of hpa models traditionally done in the literature

numerical treatment of vertex singularities and intensity factors for mixed boundary value problems for the laplace equation in r 3 a numerical method for the computation of the singular behavior of the solution of the laplace equation is proposed it is shown that the accuracy of the computed stress intensity factor by the h p and h text p version of the finite element method has the same order as the square of the error of the solution measured in the energy norm numerical examples are given

on the accuracy of the gaussian approximation for the evaluation of nonlinear effects in ofdm signals the almost gaussian nature of ofdm orthogonal frequency division multiplexing signals with high number of subcarriers n is widely employed to characterize nonlinearly distorted ofdm signals and to evaluate the corresponding performance in this paper we study the accuracy of the gaussian approach when evaluating nonlinear effects in ofdm signals with finite number of subcarriers showing the strengths and limitations of this approach it is shown that the decomposition in useful and self interference components is valid even for a reduced number of subcarriers the gaussian approximation of the nonlinear self interference at the subcarrier level is very accurate provided that n2 is high however the nonlinear distortion levels slightly lower than the ones obtained with the gaussian approximation with relative errors dropping with 1 n leading to somewhat pessimistic sir levels signal to interference ratio

maximum likelihood detection of nonlinearly distorted multicarrier symbols by iterative decoding this paper proposes a new method for decoding multicarrier symbols with severe nonlinear distortion the first part evaluates mutual information expressions for practical nonlinear models and shows the performance bounds for commonly used receiver structures then we derive the maximum likelihood ml sequence estimator which unfortunately has an exponential complexity due to the nonlinear distortion this extremely large complexity can be reduced with a simple algorithm that iteratively estimates the nonlinear distortion thereby reducing the exponential ml to the standard ml without nonlinear distortion the proposed method can be used to reduce the peak to average power ratio of multicarrier signals by clipping the transmit sequence it can also be used to correct any nonlinear distortion present in transmitter receiver amplifiers that are operating close to saturation

frequency independent and frequency dependent nonlinear models of twt amplifiers simple two parameter formulas are presented for the functions involved in the amplitude phase and the quadrature nonlinear models of a twt amplifier and are shown to fit measured data very well also a closed form expression is derived for the output signal of a twt amplifier excited by two phase modulated carriers and an expression containing a single integral is given when more than two such earriers are involved finally a frequencydependent quadrature model is proposed whose parameters are obtainable from single tone measurements

modeling of complex valued wiener systems using b spline neural network in this brief a new complex valued b spline neural network is introduced in order to model the complex valued wiener system using observational input output data the complex valued nonlinear static function in the wiener system is represented using the tensor product from two univariate b spline neural networks using the real and imaginary parts of the system input following the use of a simple least squares parameter initialization scheme the gauss newton algorithm is applied for the parameter estimation which incorporates the de boor algorithm including both the b spline curve and the first order derivatives recursion numerical examples including a nonlinear high power amplifier model in communication systems are used to demonstrate the efficacy of the proposed approaches

commutators and companion matrices over rings of stable rank 1 abstract we consider the group gl n a of all invertible n by n matrices over a ring a satisfying the first bass stable range condition we prove that every matrix is similar to the product of a lower and upper triangular matrix and that it is also the product of two matrices each similar to a companion matrix we use this to show that when n 3 and a is commutative every matrix in sl n a is the product of two commutators

a partially decoupled rls algorithm for volterra filters in this correspondence we consider a partially decoupled variation of the rls algorithm it is based on a constrained optimization of the cumulative filter error using the higher order sets of filter weights to improve on the performance of the lower order weight sets whose values are already established from this constrained optimization a recursive algorithm is developed whose form closely resembles the standard volterra rls algorithm but with structural differences that arise from eliminating the dependence of the lower order weight sets on the higher order weight sets while retaining the dependence of the higher order weights on the lower order weights the resulting algorithm while suboptimal requires less computational effort than the fully coupled version converges to steady state in the same amount of time and is shown by example not to exhibit a substantial degradation in performance

spectral precoding for constant envelope ofdm spectrally precoded orthogonal frequency division multiplexing ofdm and constant envelope ofdm ce ofdm are two modified ofdm formats that enhance the signaling features of ofdm while allowing for the efficient implementation by discrete fourier transform and the insertion of guard intervals to counteract channel dispersion the spectrally precoded ofdm signal provides very small power spectral sidelobes decaying asymptotically as f 2j 2 with j integer valued design parameter thereby suppressing sidelobe powers effectively and achieving higher spectral efficiency than nonprecoded rectangularly pulsed ofdm which has power spectral sidelobes decaying asymptotically as f 2 the ce ofdm signal exhibits a unity peak to average power ratio and thus enables the efficient use of transmitter s power amplifier in this paper a specific ce ofdm block signaling format is proposed to incorporate spectral precoding with an aim to suppressing sidelobe powers more effectively than conventional nonprecoded ce ofdm the general constraint on spectral precoding is developed to ensure that the power spectral sidelobes of the spectrally precoded ce ofdm block signal with arbitrary data statistics decay asymptotically as f 2j 2 some spectral codes previously found for spectrally precoded ofdm are shown to meet the constraint and thus suited for spectrally precoded ce ofdm specifically spectral performance is analyzed for the spectrally precoded ce ofdm block signal with independent and identically distributed data to illustrate the prevailing spectral advantage the corresponding receiver is also developed to demodulate the spectrally precoded ce ofdm block signal over dispersive channels

constant envelope ofdm this paper describes a transformation technique aimed at solving the peak to average power ratio papr problem associated with ofdm orthogonal frequency division multiplexing constant envelope ofdm ce ofdm transforms the ofdm signal by way of phase modulation to a signal designed for efficient power amplification at the receiver the inverse transformation phase demodulation is applied prior to the conventional ofdm demodulator the performance of ce ofdm is analyzed in additive white gaussian noise awgn and fading channels ce ofdm is shown to achieve good performance in dense multipath with the use of cyclic prefix transmission in conjunction with a frequency domain equalizer fde by way of computer simulation and hardware realization ce ofdm is shown to compare favorably to conventional ofdm

on the power amplifier nonlinearity in mimo transmit beamforming systems in this paper single carrier multiple input multiple output mimo transmit beamforming tb systems in the presence of high power amplifier hpa nonlinearity are investigated specifically due to the suboptimality of the conventional maximal ratio transmission maximal ratio combining mrt mrc under hpa nonlinearity we propose the optimal tb scheme with the optimal beamforming weight vector and combining vector for mimo systems with nonlinear hpas moreover an alternative suboptimal but much simpler tb scheme namely quantized equal gain transmission qegt is proposed the latter profits from the property that the elements of the beamforming weight vector have the same constant modulus the performance of the proposed optimal tb scheme and qegt mrc technique in the presence of the hpa nonlinearity is evaluated in terms of the average symbol error probability and mutual information with the gaussian input considering the transmission over uncorrelated quasi static frequency flat rayleigh fading channels numerical results are provided and show the effects on the performance of several system parameters namely the hpa parameters numbers of antennas quadrature amplitude modulation modulation order number of pilot symbols and cardinality of the beamforming weight vector codebook for qegt

rf power amplifier linearization through amplitude and phase predistortion this paper presents a novel technique for power amplifier linearization in digital microwave radio systems the proposed technique is based on the use of a predistortion circuit whose am am and am pm responses are separately implemented as polynomial approximations of the respective responses of the ideal linearizer the proposed scheme is shown to attain superior performance in comparison with other well known predistortion structures such as those based on the cancellation of third or fifth order distortion with no substantial aggravation in implementation complexity

solving laplacian problems with boundary singularities a comparison of a singular function boundary integral method with the p hp version of the finite element method we solve a laplacian problem over an l shaped domain using a singular function boundary integral method as well as the p hp finite element method in the former method the solution is approximated by the leading terms of the local asymptotic solution expansion and the unknown singular coefficients are calculated directly in the latter method these coefficients are computed by post processing the finite element solution the predictions of the two methods are discussed and compared with recent numerical results in the literature

the effect of transfer function estimation errors on the filtered x lms algorithm the filtered x lms algorithm which is commonly implemented in active noise and vibration control systems requires an estimate of the cancellation path transfer function to maintain algorithm stability this correspondence considers the effect of errors in this estimate on the stability of the algorithm implemented in the time domain concluding that while a maximum phase error of spl plusmn 90 spl deg is a bound for stability there is no simple relationship between error and stability within this region

complex valued b spline neural networks for modeling and inverting hammerstein systems many communication signal processing applications involve modeling and inverting complex valued cv hammerstein systems we develop a new cv b spline neural network approach for efficient identification of the cv hammerstein system and effective inversion of the estimated cv hammerstein model in particular the cv nonlinear static function in the hammerstein system is represented using the tensor product from two univariate b spline neural networks an efficient alternating least squares estimation method is adopted for identifying the cv linear dynamic model s coefficients and the cv b spline neural network s weights which yields the closed form solutions for both the linear dynamic model s coefficients and the b spline neural network s weights and this estimation process is guaranteed to converge very fast to a unique minimum solution furthermore an accurate inversion of the cv hammerstein system can readily be obtained using the estimated model in particular the inversion of the cv nonlinear static function in the hammerstein system can be calculated effectively using a gaussian newton algorithm which naturally incorporates the efficient de boor algorithm with both the b spline curve and first order derivative recursions the effectiveness of our approach is demonstrated using the application to equalization of hammerstein channels

an adaptive nonlinear prefilter for compensation of distortion in nonlinear systems this article concerns the linearization of a nonlinear system by connecting a nonlinear volterra prefilter tandemly with the nonlinear system and by adaptively adjusting the coefficients of the prefilter a nonlinear filtered x lms algorithm for updating the coefficients of a nonlinear prefilter as proposed is shown to be viewed as an extension of the well known filtered x lms algorithm

novel adaptive nonlinear predistorters based on the direct learning algorithm the adaptive nonlinear predistorter is an effective technique to compensate for the nonlinear distortion existing in digital communication and control systems however available adaptive nonlinear predistorters using indirect learning are sensitive to measurement noise and do not perform optimally other available types are either slow to converge complicated in structure and computationally expensive or do not consider the memory effects in nonlinear systems such as a high power amplifier hpa in this paper we first propose several novel adaptive nonlinear predistorters based on direct learning algorithms the nonlinear filtered x rls nfxrls algorithm the nonlinear adjoint lms nalms algorithm and the nonlinear adjoint rls narls algorithm using these new learning algorithms we design adaptive nonlinear predistorters for an hpa with memory effects or for an hpa following a linear system because of the direct learning algorithm these novel adaptive predistorters outperform nonlinear predistorters that are based on the indirect learning method in the sense of normalized mean square error nmse bit error rate ber and spectral regrowth moreover our developed adaptive nonlinear predistorters are computationally efficient and or converge rapidly when compared to other adaptive nonlinear predistorters that use direct learning and furthermore can be easily implemented we further simplify our proposed algorithms by exploring the robustness of our proposed algorithm as well as by examining the statistical properties of what we call the instantaneous equivalent linear iel filter simulation results confirm the effectiveness of our proposed algorithms

analysis of bonded and fastened repairs by the p version of the finite element method abstract the development of efficient and reliable methods for the design and analysis of fastened structural connections and bonded joints is among the most important and most challenging problems in aerospace applications because these connections are common sites of failure initiation and there is a complicated nonlinear interaction between the fasteners and the connected plates realistic representations of fastened connections must account for the diameter of fastener holes the stiffness of the fasteners the effects of interference fitting or looseness of fit contact between the fasteners and the connected plates and yielding of the plates similarly the treatment of bonded joints is complicated by the fact that bonded patches are typically composed of several thin layers of orthotropic material and the adhesive material may yield under load requiring a nonlinear analysis consequently the complexity of the models is such that it is difficult and time consuming to treat them using conventional finite element or boundary element procedures fortunately these problems lend themselves to standardization in fea based handbook libraries structural connections can be described in a parametric form therefore topologically similar families of connections need to be meshed only once the fea based handbook library provides for the archival and recall of entries for analysis users need only enter the parameters the finite element mesh being associative is adjusted automatically therefore users need not be concerned with meshing or other details of the analysis process the finite element solution and the computation of all data of interest are performed automatically and the results are displayed in tabular and graphical forms automatic quality control tests are performed and reported for purposes of documentation there are provisions for the computation of margins of safety the advanced fea based handbook library produces results of sufficient reliability to permit certification of repairs and joints based on computed information this will be demonstrated by examples

reduction of the envelope fluctuations of multi carrier modulations using adaptive neural fuzzy inference systems in this paper a novel scheme for reducing the envelope fluctuations in multi carrier signals applying adaptive neural fuzzy inference systems anfis is proposed and analyzed once trained with signals with very low envelope fluctuations such as those obtained by the active constellation expansion approximate gradient project ace agp algorithm anfis approximately reaches a similar reduction as with ace agp for multi carrier signals without the complexity and the large convergence time of conventional ace agp we show that our approach is less complex than other previous schemes and with better performance

sequence detection in nonlinear channels a convenient alternative to analog predistortion a new maximum likelihood sequence detection receiver for spectrally efficient linear modulations on bandlimited bandpass nonlinear channels is proposed the receiver is based on oversampling the received signal corrupted by noise and nonlinear distortion contrary to other solutions in the literature in the proposed technique there is no need for a bank of matched filters and the receiver front end reduces to a single lowpass filter for a given peak power level a performance gain can be achieved over more traditional approaches to transmission on nonlinear channels such as those based on predistortion if a moderate spectral expansion is allowed to analyze the receiver performance the concept of distance spectrum is employed since the minimum distance alone cannot account for a reliable performance evaluation both analysis and simulation are carried out for realistic narrowband nonlinear channels possibly employing reduced state sequence detection appreciable gain margins are confirmed to be possible in these realistic cases

factorization of transformations over a local ring abstract let v be a finitely generated free module over a local ring r and an invertible linear transformation for v then is a product of simple mappings in addition the determinant of each simple factor but one can be chosen to be any given unit in r the smallest number of factors needed is not greater than r 1 where r is the codimension of the vector space that is associated with the module of vectors fixed under

performance of combined trellis coded modulation and nonlinear cancellation the performance of nonlinear cancellation schemes is limited by the accuracy of tentative decisions this correspondence analyzes the performance of combined trellis coded modulation and nonlinear cancellation a scheme is proposed to make use of the redundancy of the code it is shown that performance close to optimum can be achieved with this scheme

stability analysis of closed loop algorithms for cancelling nonlinear memory effects in a nonlinear memory system using a nonlinear adaptive filter as a predistorter is an effective compensation technique predistorters based on closed loop algorithms typically exhibit better linearity than those based on indirect learning algorithms however the steady state of a closed loop algorithm may be affected by external noise or error signals in this paper the long term stability of a closed loop algorithm is analyzed using perturbation and iteration analyses it is shown that because of accumulation of historical errors it is difficult for typical closed loop algorithms to converge when the number of iterations becomes large to address this problem a modified closed loop algorithm is proposed by reducing the effect of accumulated historical errors the long term stability of the algorithm is largely improved as verified by numerical simulations

optimal volterra kernel estimation algorithms for a nonlinear communication system for psk and qam inputs a fifth order volterra kernel estimation algorithm which is optimal in the least mean square error sense for a bandpass nonlinear system is derived the algorithm is based on some characteristics of i i d circularly symmetric zero mean complex valued gaussian random variables the proposed algorithm can be used to identify a nonlinear system under uniformly i i d rectangular m qam input and under uniformly i i d m psk input m spl ges 4 with modest modification the same approach has been used to derive an optimal volterra kernel estimation algorithm up to the third order however in some cases a third order model is not of high enough order to capture the nonlinear system characteristics a simulation example is given to show the necessity of deriving a fifth order volterra kernel estimation algorithm and to test for the correctness of the algorithm

numerical analysis of singularities in two dimensions part 2 computation of generalized flux stress intensity factors summary a numerical method for the computation of the generalized flux stress intensity factors gfifs gsifs for the asymptotic solution of linear second order elliptic partial differential equations in two dimensions in the vicinity of singular points is described special attention is given to heat transfer and elasticity problems the singularities may be caused by re entrant corners and abrupt changes in material properties such singularities are of great interest from the point of view of failure initiation the eigenpairs computed in a companion paper characterize the straining modes and their amplitudes the gfifs gsifs quantify the amount of energy residing in particular straining modes for this reason failure theories directly or indirectly involve the gfifs gsifs this paper addresses a general method based on the complementary weak formulation for determining the gfifs gsifs numerically as a post solution operation on the finite element solution vector importantly the method is applicable to anisotropic materials multi material interfaces and cases where the singularities are characterized by complex eigenpairs an error analysis is sketched and numerical examples are presented to illustrate the effectiveness of the technique

identification of certain time varying nonlinear wiener and hammerstein systems the problem of identification and tracking of time varying nonlinear systems is addressed in particular the wiener system that consists of a dynamic time varying linear part followed by a fixed nonlinearity and the hammerstein system in which the order of these two blocks is reversed are studied the extended kalman filter ekf algorithm is applied it is also shown that this algorithm can be reformulated in terms of a nonlinear minimization problem with a quadratic inequality constraint in order to ensure exponential stability resulting in the algorithm cekf as indicated by means of numerical examples this latter algorithm is less sensitive to the chosen initialization than the ekf the proposed algorithms depend on certain second order statistics that may be unknown in a typical scenario a method for estimation of these quantities is proposed it is demonstrated that the suggested algorithms can be successfully applied to the problem of acoustic echo cancelation

multiscale computations with a combination of the h and p versions of the finite element method the numerical treatment of structural problems is often difficult if the global system behavior is affected by phenomena on different length scales when simulating multiscale problems it is essential to find a discretization that equally reflects all aspects of the problem with sufficient accuracy the present paper is concerned with the formulation of a finite element procedure which combines the p method to resolve global features of a problem with an h method to resolve local features thus the advantageous properties of both standard procedures can be optimally exploited the implementation provides the further advantage that the numerical treatment of the problems on the different length scales can be done with independent finite element discretizations to solve the overall problem an iteration scheme with an adaptive solution strategy is developed benchmark problems and an example with relevance to soil mechanics are presented

finite element stress extraction by the complementary energy principle this paper presents a new method for accurate pointwise stress extraction from finite element solutions applied to two dimensional linear elastostatic problem shaving bounded value stresses the method denoted by sec stress extraction by complementary principle is based on the complementary energy principle applied overalocal domain in thepost processing phase detailed formulation of the sec method is provided and numerical experiments with the h and p versions of the finite element method are presented for a family of exact solutions characterized by varying degree of smoothness it is shown that on the boundaries of the domain as well as in the interior accurate pointwise stresses are obtained and the relative error in the pointwise stresses converges with a rate which is as fast as the relative error measured in energy norm or faster importantly the sec method in conjunction with the p version of the finite element method is virtually independent of the poisson s ratio and is equally applicable to nearly in compressible materials 1997 by john wiley sons ltd

on products of three triangular matrices over associative rings abstract in this paper we characterize an associative ring over which any n n n 2 square matrix a can be written as lum where l m are lower triangular u is upper triangular and l and u have all their diagonal entries 1

optimal convergence analysis for the extended finite element method we establish some optimal a priori error estimate on some variants of the extended finite element method xfem namely the xfem with a cut off function and the standard xfem with a fixed enrichment area the results are established for the lame system homogeneous isotropic elasticity and the laplace problem the convergence of the numerical stress intensity factors is also investigated we show some numerical experiments which corroborate the theoretical results

a new boundary element approach of modeling singular stress fields of plane v notch problems in this paper a new boundary element be approach is proposed to determine the singular stress field in plane v notch structures the method is based on an asymptotic expansion of the stresses in a small region around a notch tip and application of the conventional be in the remaining region of the structure the evaluation of stress singularities at a notch tip is transformed into an eigenvalue problem of ordinary differential equations that is solved by the interpolating matrix method in order to obtain singularity orders degrees and associated eigen functions of the v notch the combination of the eigen analysis for the small region and the conventional be analysis for the remaining part of the structure results in both the singular stress field near the notch tip and the notch stress intensity factors sifs examples are given for v notch plates made of isotropic materials comparisons and parametric studies on stresses and notch sifs are carried out for various v notch plates the studies show that the new approach is accurate and effective in simulating singular stress fields in v notch crack structures

numerical analysis of singularities in two dimensions part 1 computation of eigenpairs a numerical method is described for the computation of eigenpairs which characterize the exact solution of linear second order elliptic partial differential equations in two dimensions in the vicinity of singular points the singularities may be caused by re entrant corners and abrupt changes in boundary conditions or material properties such singularities are of great interest from the point of view of failure initiation the eigenpairs characterize the straining modes and their amplitudes quantify the amount of energy residing in particular straining modes for this reason failure theories directly or indirectly involve the eigenpairs and their amplitudes this paper addresses the problem of determining the eigenpairs numerically on the basis of the steklov formulation numerical results are presented for several cases importantly the method is applicable to three dimensional cases

a method of harmonic extension for computing the generalized stress intensity factors for laplace s equation with singularities abstract the solution of the dirichlet problem for laplace s equation on a special polygon is harmonically extended to a sector with the center at the singular vertex this is followed by an integral representation of the extended function in this sector which is approximated by the mid point rule by using the extension properties for the approximate values at the quadrature nodes a well conditioned and exponentially convergent with respect to the number of nodes algebraic system of equations are obtained these values determine the coefficients of the series representation of the solution around the singular vertex of the polygonal domain which are called the generalized stress intensity factors gsifs the comparison of the results with those existing in the literature in the case of motz s problem show that the obtained gsifs are more accurate moreover the extremely accurate series segment solution is obtained by taking an appropriate number of calculated gsifs

a theoretical characterization of nonlinear distortion effects in ofdm systems this paper presents a theoretical characterization of nonlinear distortion effects in orthogonal frequency division multiplexing ofdm transmission systems in the theoretical framework developed it is shown that the effects on the decision variables of the in band distortion introduced by a bandpass memoryless nonlinearity can be described by means of a complex gain and an additive gaussian term with zero mean and suitable variance analytical expressions for gain and variance are given the conditions which allow this description are emphasized and discussed as a consequence a completely analytical procedure to evaluate error probability is also obtained and illustrated using ofdm discrete multitone modulation dmt systems with rectangular pulse shaping for the soft envelope limiter nonlinearity a closed form is derived a comparison with simulation results is carried out to verify the accuracy of this method

performance of volterra and mlsd receivers for nonlinear band limited satellite systems this paper evaluates the performance of volterra equalizers and maximum likelihood sequence detection mlsd receivers for compensation of signal distortion in nonlinear band limited satellite systems in addition the performance of a receiver with a fractionally spaced equalizer followed by a volterra equalizer is studied fse volterra equalizer for the equalizers adaptation of the equalizer weights is considered including a multiple step lms algorithm which improves the convergence characteristics two mlsd receiver structures are considered the optimum receiver consisting of a matched filter bank followed by a viterbi 1967 detector and a suboptimum receiver consisting of a single receiver filter followed by a viterbi detector the performance of the mlsd receivers is then compared to that of the equalizers

cancellation of nonlinearities in bandpass qam systems an orthogonalized input output characterization of nonlinear bandpass quadrature amplitude modulation systems with memory is developed the orthogonal expansion is used with a nonlinear canceller structure to counteract the effect of nonlinear distortion introduced by high power amplifiers at the transmitter it is shown through simulations that when the transmitted symbols are known at the canceller relatively simple structures can eliminate the effect of the nonlinearity even when the transmitted symbols are not known at the canceller the improvement in performance with the nonlinear canceller with respect to a linear receiver is significant

nonlinear channel equalization with gaussian processes for regression we propose gaussian processes for regression gpr as a novel nonlinear equalizer for digital communications receivers gpr s main advantage compared to previous nonlinear estimation approaches lies on their capability to optimize the kernel hyperparameters by maximum likelihood which improves its performance significantly for short training sequences besides gpr can be understood as a nonlinear minimum mean square error estimator a standard criterion for training equalizers that trades off the inversion of the channel and the amplification of the noise in the experiment section we show that the gpr based equalizer clearly outperforms support vector machine and kernel adaline approaches exhibiting outstanding results for short training sequences

adaptive volterra filters for active control of nonlinear noise processes this paper presents a volterra filtered x least mean square lms algorithm for feedforward active noise control the research has demonstrated that linear active noise control anc systems can be successfully applied to reduce the broadband noise and narrowband noise specifically such linear anc systems are very efficient in reduction of low frequency noise however in some situations the noise that comes from a dynamic system may he a nonlinear and deterministic noise process rather than a stochastic white or tonal noise process and the primary noise at the canceling point may exhibit nonlinear distortion furthermore the secondary path estimate in the anc system which denotes the transfer function between the secondary source secondary speaker and the error microphone may have nonminimum phase and hence the causality constraint is violated if such situations exist the linear anc system will suffer performance degradation an implementation of a volterra filtered x lms vfxlms algorithm based on a multichannel structure is described for feedforward active noise control numerical simulation results show that the developed algorithm achieves performance improvement over the standard filtered x lms algorithm for the following two situations 1 the reference noise is a nonlinear noise process and at the same time the secondary path estimate is of nonminimum phase 2 the primary path exhibits the nonlinear behavior in addition the developed vfxlms algorithm can also be employed as an alternative in the case where the standard filtered x lms algorithm does not perform well

the singular function boundary integral method for 3 d laplacian problems with a boundary straight edge singularity abstract three dimensional laplace problems with a boundary straight edge singularity caused by two intersecting flat planes are considered the solution in the neighbourhood of the straight edge can be expressed as an asymptotic expansion involving the eigenpairs of the analogous two dimensional problem in polar coordinates which have as coefficients the so called edge flux intensity functions efifs the efifs are functions of the axial coordinate the higher derivatives of which appear in an inner infinite series in the expansion the objective of this work is to extend the singular function boundary integral method sfbim developed for two dimensional elliptic problems with point boundary singularities g c georgiou l olson g smyrlis a singular function boundary integral method for the laplace equation commun numer methods eng 12 1996 127 134 for solving the above problem and directly extracting the efifs approximating the latter by either piecewise constant or linear polynomials eliminates the inner infinite series in the local expansion and allows the straightforward extension of the sfbim as in the case of two dimensional problems the solution is approximated by the leading terms of the local asymptotic solution expansion these terms are also used to weight the governing harmonic equation in the galerkin sense the resulting discretized equations are reduced to boundary integrals by means of the divergence theorem the dirichlet boundary conditions are then weakly enforced by means of lagrange multipliers the values of the latter are calculated together with the coefficients of the efifs the sfbim is applied to a test problem exhibiting fast convergence of order k 1 k being the order of the approximation of the efifs in the l 2 norm and leading to accurate estimates for the efifs

on the optimum multicarrier performance with memoryless nonlinearities it is widely recognized that orthogonal frequency division multiplexing ofdm signals are very prone to nonlinear distortion effects which can lead to significant performance degradation however recent results have showed that nonlinear distortion effects do not necessarily mean performance degradation and can actually lead to performance improvements relative to conventional linear ofdm schemes in this paper we consider the effects of bandpass memoryless nonlinear devices on ofdm signals and study the optimum asymptotic performance for both nondispersive channels and severely time dispersive channels we present analytical methods for obtaining the euclidean distance between two ofdm signals that are subjected to different nonlinear characteristics these results are then employed for obtaining the average asymptotic gain relative to conventional linear ofdm schemes in nondispersive channels and the gain distribution for severely time dispersive channels with rayleigh distributed fading on the different multipath components our analytical results which are shown to be very accurate indicate that the optimum detection of ofdm schemes with strong nonlinear distortion effects allows significant gains when compared with conventional linear ofdm schemes

the post processing approach in the finite element method part 2 the calculation of stress intensity factors abstract in the context of a model problem the authors describe post processing techniques for the calculation of generalized stress intensity factors they discuss two broad classes of methods one involving an influence function and the other related to the well known energy release principle of fracture mechanics an error analysis is sketched and two numerical examples are given to illustrate the effectivity of the techniques this is the second in a series of three papers author

numerical methods for extracting edge stress intensity functions in anisotropic three dimensional domains the solution to elasticity problems in three dimensional 3 d polyhedral domains in the vicinity of an edge is represented by a family of eigen functions similar to 2 d domains complemented by shadow functions and their associated edge stress intensity functions esifs which are functions along the edge these are of major engineering importance because failure theories directly or indirectly involve them in isotropic materials one may compute analytically the eigen functions and their shadows z yosibash n omer m costabel m dauge edge stress intensity functions in polyhedral domains and their extraction by a quasi dual function method int j fract 136 2005 37 73 used in conjunction with the quasi dual function method m costabel m dauge z yosibash a quasi dual function method for extracting edge stress intensity functions siam j math anal 35 5 2004 1177 1202 for extracting esifs from finite element solutions however in anisotropic materials and multi material interfaces the analytical derivation becomes intractable and numerical methods are mandatory herein we use p finite element methods p fem for the computation of the eigen pairs and shadow functions together with their duals having computed these the p fem is used again to obtain a fe solution from which we extract approximations of the esifs based on a family of adaptive hierarchical jacobi polynomials of increasing order numerical examples for 3 d isotropic and anisotropic materials are provided for which the eigen pairs and shadow functions are numerically computed and esifs extracted these examples show the efficiency and high accuracy of the numerical approximations 2007 elsevier b v all rights reserved

a singular function boundary integral method for laplacian problems with boundary singularities a singular function boundary integral method for laplacian problems with boundary singularities is analyzed in this method the solution is approximated by the truncated asymptotic expansion for the solution near the singular point and the dirichlet boundary conditions are weakly enforced by means of lagrange multiplier functions the resulting discrete problem is posed and solved on the boundary of the domain away from the point of singularity the main result of this paper is the proof of convergence of the method in particular we show that the method approximates the generalized stress intensity factors i e the coefficients in the asymptotic expansion at an exponential rate a numerical example illustrating the convergence of the method is also presented

factorization of stably invertible matrices we prove in this article that every stably invertible matrix is similar to the product of two cyclic matrices furthermore we represent a stably invertible matrix as the product of a diagonal matrix and some commutators

partially decoupled volterra filters formulation and lms adaptation the adaptation of volterra filters by one particular method the method of least mean squares lms while easily implemented is complicated by the fact that upper hounds for the values of step sizes employed by a parallel update lms scheme are difficult to obtain in this paper we propose a modification of the volterra filter in which the filter weights of a given order are optimized independently of those weights of higher order using this approach we then solve the minimum mean square error mmse filtering problem as a series of constrained optimization problems which produce a partially decoupled normal equation for the volterra filter from this normal equation we are able to develop an adaptation routine that uses the principles of partial decoupling that is similar in form to the volterra lms algorithm but with important structural differences that allow a straightforward derivation of bounds on the algorithm s step sizes these bounds can be shown to depend on the respective diagonal blocks of the volterra autocorrelation matrix this produces a reliable set of design guidelines that allow more rapid convergence of the lower order weight sets

a class of signal processing schemes for reducing the envelope fluctuations of cdma signals in this paper we present a class of nonlinear signal processing schemes to reduce the envelope fluctuations in the downlink transmission within code division multiple access cdma systems these schemes are compatible with conventional cdma receivers since they just require some modifications at the transmitter side the proposed transmitter structure includes a nonlinear operation in the time domain followed by a linear frequency domain filtering operation we also include a semi analytical statistical characterization of the transmitted signals which takes advantage of the gaussian like nature of cdma signals with a high number of users this characterization is used for an analytical performance evaluation of the proposed techniques a set of performance results is presented showing that we can reduce significantly the envelope fluctuations of the transmitted signals while maintaining the spectral occupation of the corresponding conventional cdma signals moreover the performance degradation due to the nonlinear distortion effects on the transmitted signals can be kept relatively low

intersymbol interference due to linear and nonlinear distortion the paper provides a recursion model for the calculation of the probability density function pdf of intersymbol interference isi which is caused by the combined effect of linear channel dispersion and of nonlinear distortion the nonlinearity introduces statistical interdependencies between the interfering symbols and these dependencies are implicitly taken into account in a trellis structured recursion rule the results were verified by time consuming monte carlo simulation and show e g that the nonlinear characteristic of a high power amplifier hpa reduces in some cases the error probability caused by the linear dispersion surprisingly the isi due to the nonlinear amplifier is increased in the case of offset modulation

mean weight behavior of the filtered x lms algorithm a new stochastic analysis is presented for the filtered x lms fxlms algorithm the analysis does not use independence theory an analytical model is derived for the mean behavior of the adaptive weights the model is valid for white or colored reference inputs and accurately predicts the mean weight behavior even for large step sizes the constrained wiener solution is determined as a function of the input statistics and the impulse responses of the adaptation loop filters effects of secondary path estimation error are studied monte carlo simulations demonstrate the accuracy of the theoretical model

performance analysis of coded multicarrier spread spectrum systems in the presence of multipath fading and nonlinearities in this paper we analyze the effects of a nonlinear amplifier on the performance of convolutionally coded multicarrier spread spectrum systems in the presence of multipath fading two performance measures bit error rate ber and adjacent channel power ratio acpr are considered to assess the effects of in band interference and out of band interference spectral regrowth from the nonlinear amplifier respectively a memoryless polynomial model is used to represent the bandpass amplifier amplitude nonlinearities am am and a slow frequency nonselective rayleigh fading channel is assumed for each modulated carrier analytical results for the ber for uncoded systems and upper bounds on the ber for coded systems are presented the optimum amplifier output power backoffs for both systems which yield the smallest system power consumption are also determined the obtained acpr shows its slow inverse linear falloff with the amplifier output backoff

superconvergent extraction of flux intensity factors and first derivatives from finite element solutions a superconvergent method for the computation of the derivatives of the solution and the coefficients of the asymptotic expansion at singular points is presented for the laplace problem in two dimensions the algorithm utilizes the complementary weak form on a localized small domain mathematical analysis demonstrates the superconvergent behavior and numerical experiments support our analysis this method is well suited for anisotropic multi material singular interface problems

semi analytical performance evaluation in satellite digital links in the presence of interference a model for the semi analytic performance evaluation of digital satellite radio links in the presence of interference on both the uplink and the downlink is presented error probability on the linear portion of the link is estimated using simulation to determine the moments of the interfering signal samples and analyzing the effect of an undetermined phase difference among carriers the nonlinear portion of the link is modeled using a series expansion of the nonlinearity the output terms are then separated allowing construction of the conditional probability densities required in the error probability computation results are carried out for m qam m ary quadrature amplitude modulation and m psk m ary phase shift keying modulation systems