continuation of acoustic near fields this paper deals with the analytic continuation of a coherent pressure field specified on a finite sheet located close to and conformal to the surface of a vibrator this analytic continuation is an extension or extrapolation of the given measured field into a region outside and tangential to the original finite sheet and is based on the green s function the transfer function relating acoustic quantities on the two conformal surfaces the continuation of the measured pressure field is an inverse problem that requires the use or regularization theory especially when noise is present in the data an iteration algorithm is presented that is successful in continuing the pressure field into the tangential sheet the results are accurate close to the original boundary and taper decay toward zero with distance away from it the algorithm is tested on numerical and experimental data from a point driven rectangular plate results show the successful extrapolation continuation of this data into an area nearly double that of the original pressure field this algorithm is not limited to planar surfaces and can be applied to arbitrarily shaped surfaces

unilateral global bifurcation for a class of quasilinear elliptic systems and applications abstract in this paper we establish a unilateral bifurcation result for a class of quasilinear elliptic system strongly coupled extending the bifurcation theorem of 1 to this aim we use several results such that fredholm operator of index 0 theory eigenvalues of elliptic operators and the krein rutman theorem lastly we apply this result to some particular systems arising from population dynamics and determine a region of existence of coexistence states

an analysis of nonlinear direct sequence correlators an analysis of the performance of nonlinear correlation reception of direct sequence signals in single and multiuser channels is presented the communications channel is modeled as containing non gaussian background noise and in some cases multiple access interference as well the error probability behavior is studied asymptotically as the lengths of the spreading codes increase without bound and conditions on the spreading sequences are obtained that assure asymptotic achievement of single user performance in a multiuser system a long spreading sequence approximation to the average error probability is also derived and this result is applied to the analysis of smooth limiting correlation receivers in impulsive channels simulation results are also provided to verify the analysis average bit error probabilities are computed by monte carlo simulations for linear hard limiting and smooth limiting correlation receivers in both single and two user impulsive channels the simulation results are compared to the error rates by asymptotic approximations for the smooth limiter and also to those from previous studies on linear and hard limiting correlators

nonparametric multiuser detection in non gaussian channels existing multiuser detection techniques in wireless systems are based on the assumption that some information on the parameters of the probability density function pdf of ambient noise is available such information may not be available in all cases particularly for non gaussian and impulsive noises or may change depending on circumstances in this paper we present a technique for multiuser detection that does not require any a priori knowledge about the noise parameters this method is based on using pseudo norms for linear nonparametric regression analytical and simulation results show that the proposed method offers an improved or at least comparable performance over existing robust techniques in the absence of any information on the nature of noise in the environment the increased computational complexity is marginal compared to existing parametric detectors in addition the proposed nonparametric detector is portable in the sense that it does not need to be tuned for different noise models without any considerable degradation of performance we also show that in non gaussian noise the performance of blind adaptive nonparametric multiuser detectors is better than that of robust multiuser detectors

symbol to symbol performance evaluation of densely populated asynchronous ds cdma in gaussian and impulsive noise environments a fourier bessel series approach this letter considers the well established poisson impulse noise model with a random amplitude area distributed according to the spl epsiv mixture gaussian probability density function this model is used to develop fast converging truncated fourier bessel fb series expressions required to evaluate accurately the performance of a binary phase shift keying bpsk modulation scheme as part of a densely populated asynchronous direct sequence code division multiple access a ds cdma configuration the average error probability of the correlation receiver is easily and quickly obtained here even for the case of a large number of users k 100 unlike earlier attempts in this area in which the use of other analysis approaches e g closed form approach or taylor series expansion approach could not exceed the limit of k 2 and k 24 in an spl epsiv mixture impulsive and or gaussian noise environment respectively

quasilinear elliptic problems interacting with its asymptotic spectrum under suitable assumptions on the coefficients of the matrix a x u and on the nonlinear term f x u we study the quasilinear problem in bounded domains rn div a x u u f x u x u 0 x we extend the semilinear results of landesman lazer j math mech 19 1970 609 and of ambrosetti prodi in a primer on nonlinear analysis cambridge university press cambridge 1993 for resonant problems the existence of positive solution is also considered extending to the quasilinear case the classical result by ambrosetti rabinowitz j funct anal 14 1973 349 in this case the result is obtained as a corollary of the previous multiplicity result in the ambrosetti prodi framework

multiuser detection in flat fading non gaussian channels this paper deals with the problem of multiuser detection in direct sequence code division multiple access cdma fading channels with impulsive noise this issue arises in practical situations because in many realistic wireless channels the ambient channel noise is impulsive resulting from various natural and man made impulsive sources an m estimator based structure for noncoherent demodulation of differentially phase shift keyed dpsk signals transmitted simultaneously via a cdma flat fading channel and embedded in impulsive noise is proposed and analyzed analytical and numerical results show that in highly impulsive noise the performance gain afforded by the proposed multiuser detector can be substantial when compared to the linear decorrelating detector for dpsk with little attendant increase in algorithmic complexity

a detailed analysis on local bifurcation from infinity for nonlinear elliptic problems in this paper we analyze the local side of the bifurcation from infinity at the first eigenvalue of several elliptic operators we underline that the key to decide the local behavior of the bifurcation lies in the sign of certain integral involving all the values of the nonlinearity

on the structure of unbounded positive solutions to the quasilinear logistic equation abstract for the quasilinear logistic equation p u m x u u in u 0 on with p 1 m x l q we obtain the existence of the maximal unbounded positive solutions their asymptotic behavior and global bifurcation from the trivial solution

combined helmholtz equation least squares method for reconstructing acoustic radiation from arbitrarily shaped objects a combined helmholtz equation least squares chels method is developed for reconstructing acoustic radiation from an arbitrary object this method combines the advantages of both the hels method and the helmholtz integral theory based near field acoustic holography nah as such it allows for reconstruction of the acoustic field radiated from an arbitrary object with relatively few measurements thus significantly enhancing the reconstruction efficiency the first step in the chels method is to establish the hels formulations based on a finite number of acoustic pressure measurements taken on or beyond a hypothetical spherical surface that encloses the object under consideration next enough field acoustic pressures are generated using the hels formulations and taken as the input to the helmholtz integral formulations implemented through the boundary element method bem the acoustic pressure and normal component of the velocity at the discretized nodes on the surface are then determined by solving two matrix equations using singular value decomposition svd and regularization techniques also presented are in depth analyses of the advantages and limitations of the chels method examples of reconstructing acoustic radiation from separable and nonseparable surfaces are demonstrated

measurement of structural intensity using boundary element method based nearfield acoustical holography the regularization method for measurement of structural intensity si using boundary element method bem based nearfield acoustical holography nah is proposed spatial derivatives of normal displacement are necessary to obtain the structural intensity the derivative operations amplify high wavenumber component of measurement noise and contaminate the measurement result of si to overcome this difficulty regularization method for measurement of si using fast fourier transform based nah has been introduced in this paper this regularization method is modified for the bem based nah the bem based nah avoids the aperture replication problem therefore measurement aperture for bem based nah can be set smaller than that for fft based nah the effectiveness of the proposed method is demonstrated by experiments

the use of partially measured source data in near field acoustical holography based on the bem in applying the conformal near field acoustical holography nah to actual source identification problems it is often possible to determine the velocity at certain points of the source surface in advance this partially known velocity data would reduce the problem size and permit better reconstruction accuracy in this paper the effectiveness of using partially measured source data in the conformal nah is investigated which uses the boundary element method a vibro acoustic transfer matrix and measured field pressure data which is involved with the boundary integral equation are reorganized in order to deal with the partially measured surface velocities for a baffled vibrating panel simulations were performed by varying the number of velocity known nodes in addition the effect of measurement error is investigated for two extreme positioning methods of velocity known nodes without regularization the reconstructed error can be reduced considerably by employing some of the source data and this err

use of nonsingular boundary integral formulation for reducing errors due to near field measurements in the boundary element method based near field acoustic holography in the conformal near field acoustic holography nah using the boundary element method bem the transfer matrix relating the vibro acoustic properties of source and field depends solely on the geometrical condition of the problem this kind of nah is known to be very powerful in dealing with the sources having irregular shaped boundaries when the vibro acoustic source field is reconstructed by using this conformal nah one tends to position the sensors as close as possible to the source surface in order to get rich information on the nonpropagating wave components the conventional acoustic bem based on the kirchhoff helmholtz integral equation has the singularity problem in the close near field of the source surface this problem stems from the singular kernel of the green function of the boundary integral equation bie and the singularity can influence the reconstruction accuracy greatly in this paper the nonsingular bie is introduced to the nah calculation and the holographic bie is reformulated the effectiveness of nonsingular bem has been investigated for the reduction of reconstruction error through interior and exterior examples it is shown that the resolution of predicted field pressure could be improved in the close near field by employing the nonsingular bie because the bem based nah inevitably requires the field pressure measured in the close proximity to the source surface the present approach is recommended for improving the resolution of the reconstructed source field

fast fourier transform and singular value decomposition formulations for patch nearfield acoustical holography nearfield acoustical holography nah requires the measurement of the pressure field over a complete surface in order to recover the normal velocity on a nearby concentric surface the latter generally coincident with a vibrator patch nah provides a major simplification by eliminating the need for complete surface pressure scans only a small area needs to be scanned to determine the normal velocity on the corresponding small area concentric patch on the vibrator the theory of patch nah is based on 1 an analytic continuation of the patch pressure which provides a spatially tapered aperture extension of the field and 2 a decomposition of the transfer function pressure to velocity and or pressure to pressure between the two surfaces using the singular value decomposition svd for general shapes and the fast fourier transform fft for planar surfaces inversion of the transfer function is stabilized using tikhonov regularization and the morozov discrepancy principle experimental results show that root mean square errors of the normal velocity reconstruction for a point driven vibrator over 200 2700 hz average less than 20 for two small concentric patch surfaces 0 4 cm apart reconstruction of the active normal acoustic intensity was also successful with less than 30 error over the frequency band

signed rank nonparametric multiuser detection in non gaussian channels we present a novel nonparametric multiuser detector for non gaussian channels that is based on the signed rank norm for linear regression analytical and simulation results show that the proposed detector offers similar or better performance as compared to the minimax robust detector but without requiring any a priori information on the noise the complexity of this detector is lower than that of the pseudo norm nonparametric detector stated previously by the authors this is due to the fact that in contrast to the latter it is not necessary to compute the intercept parameter for the signed rank detector proposed in this correspondence we analyze the behavior of the blind version of this detector and show that it outperforms the blind minimax detector we also show that this detector has a bounded influence function and hence it is robust

data extrapolation method for boundary element method based near field acoustical holography a new data extrapolation method for boundary element method bem based near field acoustical holography nah is proposed to reduce an error of the reconstructed result obtained from the pressure measured on an aperture small compared with the structure the finiteness of the measurement aperture is a serious impediment to actual large scale implementation of nah because nah requires the measurement of the pressure field over a complete surface of structure to eliminate the requirement the wave number space data extrapolation method for fast fourier transform fft based nah has been proposed in this paper the extension of this data extrapolation method to bem based nah is presented the effectiveness of this method is demonstrated by experiments the experiment results confirm that the reconstruction error is sufficiently suppressed by the proposed method

the relationship of singular value decomposition to wave vector filtering in sound radiation problems singular value decomposition svd has recently been suggested as a filtering tool to analyze acoustic radiation the relationship between this filtering technique and the more familiar wave vector filtering is examined for a geometry with a preferred axis the source modes are found to consist approximately of a single wave number and the accompanying radiation modes are thus beams some simple examples are worked out to illustrate the general behavior of the filtering and some of the properties of the beams are given

an overview of the inverse problem for sound reconstruction in interior spaces calculation of the sound field between noise sources and a closed two dimensional measurement surface is an inverse propagation problem a powerful and popular method for the reconstruction of sound fields in this space is based on boundary element methods applied to the helmholtz integral equation unfortunately due to the numerical nature of these solutions the physics associated with backtracking backpropagating the field from the measurement surface to the source surfaces is hidden analytic models are used to view this backpropagation as a deconvolution operation on the measured field the nature of this deconvolving function called the inverse propagator is examined it is a singular function it is spatially local and it unsoothes the measured pressure field an understanding of the nature of the inverse propagator is critical to successful solution of the inverse problem by bem methods especially the regularization process as well as the sucessful development of three dimensional sound projection methods for sound synthesis furthermore this understanding leads to new insights into the solution of the inverse problem work supported by onr and nasa

effect of sensor proximity over the non conformal hologram plane in the near field acoustical holography based on the inverse boundary element method near field acoustical holography nah is a useful tool for the identification and visualization of vibro acoustic sources in particular nah can be applied to many practical sources having irregular shape if the inverse boundary element method bem is employed once the relation between the source and the radiated field is defined in the transfer matrix modeled by the bem the reconstruction of acoustic parameters on the source surface can be conducted by the multiplication of the inverse transfer matrix and the field data measured over the hologram surface the usual practical way to measure the field data radiated from an irregular shaped source is to adopt a regular hologram surface which can be a flat cylindrical or spherical shape for the measurement ease then the hologram surface is not conformal to the source surface and the resulting transfer matrix becomes further ill posed than the conformal case to investigate the effect of sensor proximity and distance variability on the reconstruction error simulation and measurement were conducted for an interior problem comprised of a parallelepiped rigid box with a vibrating end plate flat tilted and randomized hologram surfaces were adopted in the test it was shown that the reconstruction error is greatly affected by the conditioning of transfer matrix which is related to the positioning of sensor to improve the reconstruction accuracy an investigation was conducted to find proper field points among a large number of overdetermined field points the number of field data was reduced gradually under various reduction schemes using condition number effective independence value and sensor distance it was demonstrated that the quality of reconstruction result given by the non conformal measurement can be improved by removing some field points that contribute to the ill conditioning of the inverse problem a small improvement of the reconstruction accuracy was observed by reducing the field points in the overdetermined situation however further reduction of the field points becoming an underdetermined situation yielded a drastic improvement of the reconstruction accuracy

reconstruction of vibro acoustic fields using hybrid nearfield acoustic holography hybrid nearfield acoustical holography is utilized to reconstruct the vibro acoustic fields generated by an arbitrary structure vibrating in an unbounded fluid medium this hybrid nah is based on the modified helmholtz equation least squares method that expresses the field acoustic pressure in terms of expansions of outgoing and incoming waves the expansion coefficients are determined by solving an over determined linear system of equations obtained by matching the assumed form solution to a finite number of acoustic pressures measured in the field via least squares the measurements are taken over a conformal surface around the source at close range so that the evanescent waves can be captured once the expansion coefficients are specified enough field acoustic pressures are regenerated on a conformal surface around the source and are taken as input to the helmholtz integral formulation implemented numerically by the boundary element method the acoustic pressure and normal velocity on the source surface are reconstructed using singular value decomposition modified tikhonov regularization and generalized cross validation method this hybrid nah enables one to obtain a satisfactory reconstruction cost effectively because the majority of the input data are calculated but not measured numerical examples of reconstructing the vibro acoustic quantities on the surfaces of an engine block and a highly elongated cylinder are demonstrated

application of bem boundary element method based acoustic holography to radiation analysis of sound sources with arbitrarily shaped geometries a method employing holograms that conform with arbitrarily shaped sources has been developed for enhancing conventional near field acoustic holography which has been limited to sources with simple geometries e g planar or cylindrical surfaces four holography transformation algorithms have been developed based on acoustic holography theory and the boundary element method bem singular value decomposition svd has been incorporated into the algorithms in order to alleviate the ill posed nature frequently encountered in backward reconstruction of a field a pulsating sphere a cylinder with spherical endcaps and a vibrating piston set in a rigid sphere have been adopted in a numerical simulation for verifying the algorithms satisfactory agreement has been achieved between the holographically transformed results and the analytical solutions

a non singular boundary integral equation for acoustic problems abstract a weakly singular boundar integral equation and a non singular one are derived for acoustic problems by using the one dimensional wave propagation mode the degree of the singularities appearing in the conventional boundary integral equation can be reduced since the strong singularity is removed in the weakly singular equation the jump term of the field pressure expression in the very vicinity of the boundary disappears it is shown that the weak singularity can be also removed for a smooth boundary in the non singular representation and thus special treatments for singular integral are no longer needed in order to test the proposed method several simple problems have been solved by employing the triangular element with standard gaussian quadrature integration points the results obtained show the accuracy of the method in the near field as well as on the boundary

error probability for direct sequence spread spectrum multiple access communications part ii approximations approximations are obtained for the average probability of error in an asynchronous direct sequence spread spectrum multiple access communications system both binary and quaternary systems are considered and the chip waveforms are allowed to be arbitrary time limited waveforms with time duration equal to the inverse of the chip rate the approximation is based on the integration of the characteristic function of the multiple access interference the amount of computation required to evaluate this approximation grows only linearly with the product of the number of simultaneous transmitters and the number of chips per bit the accuracy of the approximation is extremely good in most cases but it can be improved if necessary by an application of a series expansion numerical results are presented for specific chip waveforms and signature sequences

a statistical and physical mechanisms based interference and noise model for array observations a statistical noise model is developed from mathematical modeling of the physical mechanisms that generate noise in communication receivers employing antenna arrays such models have been lacking for cases where the antenna observations may be statistically dependent from antenna to antenna the model is developed by generalizing an approach for single antenna cases suggested by middleton 1967 1974 1976 1977 the model derived here is applicable to a wide variety of physical situations the focus is primarily on problems defined by middleton to be class a interference the number of noise sources in a small region of space is assumed to be poisson distributed and the emission times are assumed to be uniformly distributed over a long time interval finally an additive gaussian background component is included to represent the thermal noise that is always present in real receivers

performance of ds ssma communications in impulsive channels part i linear correlation receivers the performance of digital linear correlation receivers is studied in a multiuser environment there are assumed to be two types of sources interfering with data transmission multiple access interference and additive channel noise which is attributed to impulsive noise sources in the environment the contribution of multiple access interference is examined by considering k asynchronous users transmitting simultaneously over a linear channel using the binary psk direct sequence spreadspectrum multiple access ds ssma technique alternatively the effects of the non gaussian impulsive channel in such a system are studied by modeling the samples of noise after front end filtering errorprobability performance under these conditions is compared to that for additive white gaussian noise awgn channels due to computational complexity exact analysis is limited here to systems utilizing short spreading sequences computationally simple methods are proposed for approximating the average error probability when the length of the signature sequences is large furthermore some asymptotic results are obtained for the case of infinitely long sequences in all cases performance variation is examined as the shape of the noise density varies with snr held constant the results of this analysis indicate that the presence of impulsive noise can cause significant performance degradation over that predicted from an awgn model even when the total noise power does not increase

sound source reconstruction using inverse boundary element calculations whereas standard boundary element calculations focus on the forward problem of computing the radiated acoustic field from a vibrating structure the aim in this work is to reverse the process i e to determine vibration from acoustic field data this inverse problem is brought on a form suited for solution by means of an inverse boundary element method since the numerical treatment of the inverse source reconstruction results in a discrete ill posed problem regularization is imposed to avoid unstable solutions dominated by errors in the present work the emphasis is on tikhonov regularization and parameter choice methods not requiring an error norm estimate for choosing the right amount of regularization several parameter choice strategies have been presented lately but it still remains to be seen how well these can handle industrial applications with real measurement data in the present work it is demonstrated that the l curve criterion is robust with respect to the errors in a real measurement situation in particular it is shown that the l curve criterion is superior to the more conventional generalized cross validation gcv approach for the present tire noise studies

on the reconstruction of the vibro acoustic field over the surface enclosing an interior space using the boundary element method the vibrational velocity sound pressure and acoustic power on the vibrating boundary comprising an enclosed space are reconstructed by the boundary element method based on the measured field pressures the singular value decomposition is used to obtain the inverse solution in the least square sense and to express the acoustic modal expansion between the measurement and source fields in general such an inverse operation has been considered an ill posed problem having a divergence phenomenon involved with extremely small measurement errors the ill conditioned nature of the acoustic inverse problem is caused by the singularity of the transfer matrix which produces nonradiating wave components in order to minimize the singularity and to also reduce the number of measurement points optimal measurement positions are determined by the effective independence method regularization methods are used to stabilize the reconstructed field by suppressing nonradiating components resulting in the singular transfer

nonnegative solutions for the degenerate logistic indefinite sublinear equation the goal of this paper is to study the nonnegative steady states solutions of the degenerate logistic indefinite sublinear problem we combine bifurcation method and linking local subsuper solution technique to show the existence and multiplicity of nonnegative solutions we employ a change of variable already used in a different context and the spectral singular theory to prove uniqueness results

on optimum and nearly optimum data quantization for signal detection the application of companding approximation theory to the quantization of data for detection of coherent signals in a noisy environment is considered this application is twofold allowing for greater simplicity in both analysis and design of quantizers for detection systems most computational methods for designing optimum most efficient quantizers for signal detection systems are iterative and are extremely sensitive to initial conditions companding approximation theory is used here to obtain suitable initial conditions for this problem furthermore the companding approximation idea is applied to design suboptimum quantizers which are nearly as efficient as optimum quantizers when the number of levels is large in this design iteration is not needed to derive the parameters of the quantizer and the design procedure is very simple in this paper we explore this approach numerically and demonstrate its effectiveness for designing and analyzing quantizers in detection systems

direct sequence spread spectrum multiple access communications with random signature sequences a large deviations analysis a direct sequence spread spectrum multiple access bit error probability analysis is developed using large deviations theory let m denote the number of interfering spread spectrum signals and let n denote the signature sequence length then the large deviations limit is as n to infinity with m fixed a tight asymptotic expression for the bit error probability is proven and in addition recent large deviations results with the importance sampling monte carlo estimation technique are applied to obtain accurate and computationally efficient estimates of the bit error probability for finite values of m and n the large deviations point of view is compared also to the conventional asymptotics of central limit theory and the associated gaussian approximation the gaussian approximation is accurate and the ratio m n is moderately large and all signals have roughly equal power in the near far situation however the gaussian approximation is quite poor in contrast large deviations techniques are more accurate in the near far situation and it is here that these methods provide some important practical insight

optimum diversity detection over fading dispersive channels with non gaussian noise we consider the problem of m ary signal detection over a single input multiple output simo channel affected by frequency dispersive rayleigh distributed fading and corrupted by additive non gaussian noise modeled as a spherically invariant random process we derive both the optimum detection structure and a suboptimal reduced complexity receiver based on the low energy coherence approach interestingly both detection structures are canonical i e they are independent of the actual noise statistics we also carry out a performance analysis of both receivers with reference to the case that the channel is affected by a frequency selective fading and for a binary frequency shift keying signaling format the results obtained through both a chernoff bounding technique and monte carlo simulations reveal that the adoption of diversity also represents a suitable means to restore performance in the presence of dispersive fading and impulsive non gaussian noise interestingly it is also shown that the suboptimal receiver incurs a limited loss with respect to the optimum unrealizable receiving structure

threshold detection in narrow band non gaussian noise the middleton class a narrow band non gaussian noise model 9 12 is examined it is shown that this noise model which is known to fit closely a variety of non gaussian noises can itself be closely approximated by a computationally much simpler noise model it is then shown by numerical examples that for the problem of locally optimum detection the simplest form of this approximation yields nearly optimal asymptotic performance the performance of other simple suboptimal threshold detectors in class a noise is also examined finally a useful relationship between the class a model and the e mixture model is developed

an adaptive spatial diversity receiver for non gaussian interference and noise standard linear diversity combining techniques are not effective in combating fading in the presence of non gaussian noise an adaptive spatial diversity receiver is developed for wireless communication channels with slow flat fading and additive non gaussian noise the noise is modeled as a mixture of gaussian distributions and the expectation maximization em algorithm is used to derive estimates for the model parameters the transmitted signals are detected using a likelihood ratio test based on the parameter estimates the new adaptive receiver converges rapidly its bit error rate performance is very close to optimum when relatively short training sequences are used and it appears to be relatively insensitive to mismatch between the noise model and the actual noise distribution simulation results are included that illustrate various aspects of the adaptive receiver performance

some superlinear problems with nonlocal diffusion coefficient abstract we study a superlinear elliptic problem with a non local diffusion coefficient we show that there exists a drastic change on the structure of the set of positive solutions when the non local coefficient grows fast enough to infinity we combine mainly sub super and bifurcation methods to obtain our results

design of an optimal wave vector filter for enhancing the resolution of reconstructed source field by near field acoustical holography nah in near field acoustical holography using the boundary element method the reconstructed field often diverges due to the presence of small measurement errors in order to handle this instability in the inverse problem the reconstruction process should include some form of regularization for enhancing the resolution of source images the usual method of regularization has been the truncation of wave vectors associated with small singular values although the determination of an optimal truncation order is difficult in this article an iterative inverse solution technique is suggested in which the mean square error prediction is used a statistical estimation of the minimum mean square error between measured pressures and the model solution is required for yielding the optimal number of iterations the continuous curve of an optimal wave vector filter is designed for suppressing the high order modes that can produce large reconstruction errors experimental results from a baffled radiator reveal that the reconstruction errors can be reduced by this form of regularization by at least 48 compared to those without any regularization in comparison to results using the optimal truncation method of regularization the new scheme is shown to give further reductions of truncation error of between 7 and 39 for the example in this article

implicit methods of solution to integral formulations in boundary element method based nearfield acoustic holography two integral equation methods are considered which can be inverted to provide the surface velocity and or pressure given a measurement of the pressure on an imaginary surface in the nearfield of a vibrating or scattering body this problem is central to nearfield acoustical holography nah the integral equations are discretized using boundary element methods bems the integral equation methods considered are 1 an indirect formulation method based on the single layer integral equation and 2 a direct formulation method based on a system of equations derived from the helmholtz kirchhoff integral equation the formation of integral equations from the mentioned methods will not involve the explicit inversion of matrices but instead will require this inversion to be done implicitly since these methods back track the sound field from the measurement surface to the surface of the source vibrator they are ill posed in nature and tikhonov regularization are used to stabilize the reconstruction problems a

adaptive bayesian multiuser detection for synchronous cdma with gaussian and impulsive noise we consider the problem of simultaneous parameter estimation and data restoration in a synchronous cdma system in the presence of either additive gaussian or additive impulsive white noise with unknown parameters the impulsive noise is modeled by a two term gaussian mixture distribution bayesian inference of all unknown quantities is made from the superimposed and noisy received signals the gibbs sampler a markov chain monte carlo procedure is employed to calculate the bayesian estimates the basic idea is to generate ergodic random samples from the joint posterior distribution of all unknown and then to average the appropriate samples to obtain the estimates of the unknown quantities adaptive bayesian multiuser detectors based on the gibbs sampler are derived for both the gaussian noise synchronous cdma channel and the impulsive noise synchronous cdma channel a salient feature of the proposed adaptive bayesian multiuser detectors is that they can incorporate the a priori symbol probabilities and they produce as output the a posteriori symbol probabilities that is they are soft input soft output algorithms hence these methods are well suited for iterative processing in a coded system which allows the adaptive bayesian multiuser detector to refine its processing based on the information from the decoding stage and vice versa a receiver structure termed the adaptive turbo multiuser detector

orthogonal codes for robust low cost communication orthogonal coding schemes known to asymptotically achieve the capacity per unit cost cpuc for single user ergodic memoryless channels with a zero cost input symbol are investigated for single user compound memoryless channels which exhibit uncertainties in their input output statistical relationships a minimax formulation is adopted to attain robustness first a class of achievable rates per unit cost arpuc is derived and its utility is demonstrated through several representative case studies second when the uncertainty set of channel transition statistics satisfies a convexity property optimization is performed over the class of arpuc through utilizing results of minimax robustness the resulting cpuc lower bound indicates the ultimate performance of the orthogonal coding scheme and coincides with the cpuc under certain restrictive conditions finally still under the convexity property it is shown that the cpuc can generally be achieved through utilizing a so called mixed strategy in which an orthogonal code contains an appropriate composition of different nonzero cost input symbols

performance of ds ssma communications in impulsive channels ii hard limiting correlation receivers for pt i see ibid vol com 35 no 11 p 1179 88 1987 in part i it was demonstrated that impulsive channel noise can be a serious detriment to the performance of direct sequence spread spectrum multiple access ds ssma communications when conventional linear correlation reception is used here a hard limiting correlator as an alternative for reception of multiple access transmission in impulsive channels is considered for k asynchronous binary psk ds ssma users sharing a linear channel corrupted by impulsive noise that is modeled at the output of the front end filter of the receiver techniques are developed for analyzing bit error probabilities of this hard limiting receiver by exact computation for short spreading sequences by approximation for longer spreading sequences and by asymptotic limits for infinitely long spreading sequences performance is compared to that of the linear correlator under a variety of conditions showing that hard limiting correlation reception can offer substantial improvement over conventional systems in impulsive channels however the linear receiver is more effective against multiple access noise only and so a tradeoff emerges between rejection of impulsive noise and rejection of multiple access interference

on nonlinear direct sequence detectors in arbitrary power limited interference communication over a waveform channel corrupted by additive white gaussian noise and by an unknown and arbitrary interference of bounded power is considered for this channel an upper bound is presented for the worst case error probability of a communication system comprising a direct sequence spread spectrum modulator and a nonlinear correlation receiver it is shown that this bound is exponentially tight as the number of chips used in the modulator becomes large this bound is evaluated for several detector nonlinearities numerical examples and comparisons to the performance of a pure gaussian noise channel are also given