an adaptive receiver for the time and frequency selective fading channel an adaptive receiver is presented in this paper for the reception of linearly modulated signals transmitted over a time and frequency selective fading channel the channel is modeled as a truncated power series which represents the dispersive fading channel as a sum of three elementary flat fading channels the proposed receiver consists of a sequence estimator with a parallel channel estimator the channel estimator recovers the instantaneous fading processes associated with each elementary channel and filters them to generate one step predictions of each fading process some implementation difficulties and solutions are also discussed computer simulations using quadrature phase shift keying qpsk and channels with moderate delay spreads and fade rates have been used to evaluate the performance of the receiver the results show that our technique has potential in channels with delay spread of about 20 signal to noise ratio snr greater than 15 db and applications requiring bit error rates ber s less than 10 sup 2

the wssus pulse design problem in multicarrier transmission optimal link adaption to the scattering function of wide sense stationary uncorrelated scattering wssus mobile communication channels is still an unsolved problem despite its importance for next generation system design in multicarrier transmission such link adaption is performed by pulse shaping i e by properly adjusting the transmit and receive filters for example pulse shaped offset quadrature amplitude modulation oqam systems have recently been shown to have superior performance over standard cyclic prefix orthogonal frequency division multiplexing ofdm while operating at higher spectral efficiency in this paper we establish a general mathematical framework for joint transmitter and receiver pulse shape optimization for so called weyl heisenberg or gabor signaling with respect to the scattering function of the wssus channel in our framework the pulse shape optimization problem is translated to an optimization problem over trace class operators which in turn is related to fidelity optimization in quantum information processing by convexity relaxation the problem is shown to be equivalent to a convex constraint quasi convex maximization problem thereby revealing the nonconvex nature of the overall wssus pulse design problem we present several iterative algorithms for optimization providing applicable results even for large scale problem constellations we show that with transmitter side knowledge of the channel statistics a gain of 3 6 db in signal to interference and noise ratio sinr can be expected

non linear dynamic analysis of anisotropic cylindrical shells containing a flowing fluid abstract an analytical model is presented to predict the influence of non linearities associated with fluid flow on the dynamic behaviour of a structure consisting of shells and a surrounding fluid medium the model requires the use of two linear operators to control the equilibrium of the shell and the velocity potential a linear boundary condition of impermeability and a non linear dynamic boundary condition the method is based on thin shell theory and the non rotational flow of non viscous fluids in combination with finite element analysis it is applicable to non uniform thin anisotropic cylinders subjected to different boundary conditions the displacement functions for the wall and liquid column are derived from sanders equations and from the velocity field associated with the column respectively the set of matrices describing their relative contributions to equilibrium is determined by exact analytical integration the coupled equations are solved for the non flow problem for cases of fluid flow certain analytical modifications are proposed to restore the situation to conventional modal analysis the non linear equations of motion are solved by the fourth order runge kutta numerical method the frequency variations are then determined with respect to the amplitude of the motion the trends of the non linearities are of a softening type

nonlinear stability of cylindrical shells subjected to axial flow theory and experiments abstract this paper is concerned with the nonlinear dynamics and stability of thin circular cylindrical shells clamped at both ends and subjected to axial fluid flow in particular it describes the development of a nonlinear theoretical model and presents theoretical results displaying the nonlinear behaviour of the clamped shell subjected to flowing fluid the theoretical model employs the donnell nonlinear shallow shell equations to describe the geometrically nonlinear structure the clamped beam eigenfunctions are used to describe the axial variations of the shell deformation automatically satisfying the boundary conditions and the circumferential continuity condition exactly the fluid is assumed to be incompressible and inviscid and the fluid structure interaction is described by linear potential flow theory the partial differential equation of motion is discretized using the galerkin method and the final set of ordinary differential equations are integrated numerically using a pseudo arclength continuation and collocation techniques and the gear backward differentiation formula a theoretical model for shells with simply supported ends is presented as well experiments are also described for i elastomer shells subjected to annular external air flow and ii aluminium and plastic shells with internal water flow the experimental results along with the theoretical ones indicate loss of stability by divergence with a subcritical nonlinear behaviour finally theory and experiments are compared showing good qualitative and reasonable quantitative agreement

continuous phase modulation part i full response signaling the continuous phase modulation cpm signaling scheme has gained interest in recent years because of its attractive spectral properties data symbol pulse shaping has previously been studied with regard to spectra for binary data and modulation index 0 5 in this paper these results have been extended to the m ary case where the pulse shaping is over a one symbol interval the so called full response systems results are given for modulation indexes of practical interest concerning both performance and spectrum comparisons are made with minimum shift keying msk and systems have been found which are significantly better in e b n 0 for a large signal to noise ratio snr without expanded bandwidth schemes with the same bit error probability as msk but with considerably smaller bandwidth have also been found significant improvement in both power and bandwidth are obtained by increasing the number of levels m from 2 to 4

lois de repartition des diviseurs des entiers friables according to a general probabilistic principle the natural divisors of friable integers i e integers free of large prime factors should normally present a gaussian distribution we show that this indeed is the case with conditional density tending to 1 as soon as the standard necessary conditions are met furthermore we provide explicit essentially optimal estimates for the decay of the involved error terms the size of the exceptional set is sufficiently small to enable recovery of the average behaviour in the same optimal range our argument combines the saddle point method with new large deviations estimates for the distribution of certain additive functions

symbol by symbol map demodulation of cpm and psk signals on rayleigh flat fading channels demodulation using the symbol by symbol maximum a posteriori probability map algorithm is presented the algorithm is derived for the case of continuous phase modulation cpm signals transmitted over rayleigh flat fading channels and a corresponding receiver structure is specified it is shown that the map algorithm requires computing for each trellis branch the sum of the products of the weights of all paths through the trellis which pass through that branch and that this generic computational problem can be solved efficiently by an approach that uses a forward and backward recursion through the trellis simulation results are presented which show both the hard and soft decision performance of the map receiver to be robust even in the presence of fade rates of up to 30 of the symbol rate the application of the receiver concept to phase shift keying psk signals is also discussed and then evaluated via simulation the concept of joint demodulation and decoding using iterative processing techniques is introduced it is shown that the map receiver is well suited for iterative processing applications due to its use of a priori symbol probabilities and its production of optimal soft decisions simulation results for the reception of quaternary psk qpsk show that the bit error rate ber performance of the iterative map receiver can approach that of a receiver operating with perfect knowledge of the fading process

nonlinear vibrations of circular cylindrical shells with different boundary conditions large amplitude geometrically nonlinear vibrations of circular cylindrical shells with different boundary conditions and subjected to radial harmonic excitation in the spectral neighborhood of the lowest resonances are investigated in particular simply supported shells with allowed and constrained axial displacements at the edges are studied in both cases the radial and circumferential displacements at the shell edges are constrained elastic rotational constraints are assumed they allow simulating any condition from simply supported to perfectly clamped by varying the stiffness of this elastic constraint two different nonlinear thin shell theories namely donnell s and novozhilov s theories are used to calculate the elastic strain energy the formulation is also valid for orthotropic and symmetric cross ply laminated composite shells geometric imperfections are taken into account comparison of calculations to experimental and numerical results available in the literature is performed both empty and fluid filled shells are investigated by using a potential fluid model the nonlinear equations of motion are studied by using a code based on arclength continuation method that allows bifurcation analysis

gallager bounds for noncoherent decoders in fading channels recently gallager s bounding techniques have been used to derive tight performance bounds for coded systems in fading channels most works in this field have thus far dealt with coherent decoding this paper develops gallager bounds for noncoherent systems in fading channels unlike coherent decoding the exact error probability of a noncoherent decoder detector conditioned on the fading coefficients does not admit a closed form expression this difficulty is overcome in this paper by employing the chernoff technique although it weakens the bounds to some extent the chernoff technique enables the derivations of the limit before average lba bound and gallager bounds in closed form for noncoherent fading channels numerical examples show that the proposed bounds are convergent and are tighter than the conventional union bound

error performance of multiple symbol differential detection of psk signals transmitted over correlated rayleigh fading channels the error performance of multiple symbol differential detection of uncoded qpsk signals transmitted over correlated rayleigh fading channels is studied the optimal detector is presented along with an exact expression for the corresponding pairwise error event probability it is shown that multiple symbol differential detection is a very effective strategy for eliminating the irreducible error floor associated with a conventional differential detector in all of the cases investigated a detector with an observation interval as small as two symbols is sufficient for this purpose it is also found that the error performance of a multiple symbol differential detector is not sensitive to the mismatch between the decoding metric and the channel fading statistics

aspects on single symbol signaling on the frequency flat rayleigh fading channel the optimal single symbol detector on the rayleigh fading channel computes a functional quadratic form in the time continuous received signal a drawback is that closed form solutions of integral equations based on the channel statistics are required this makes simplified discrete receivers attractive a class of suboptimal receivers that transforms the received random process to a set of discrete observables is derived the set of observables constitutes a random vector in a finite dimensional receiver signal space given this vector the maximum likelihood detector computes a quadratic form in the received vector the discretization implies a loss of information therefore such a detector is not in general optimal given the received time continuous signal the purpose is however to achieve close to optimal performance when the number of observables becomes large this class of detectors is analyzed using exact error probability calculations which reveal several interesting properties the length of the observation interval and the number of discrete observables have significant influences on the error probability when the time variations of the fading process are rapid compared with the symbol duration by increasing the number of observables the error floor is lowered and the implicit diversity order is increased this implicit diversity arises as soon as more than one observable per symbol interval is used and is a consequence of the information bearing signal being a random process matched filter receivers use few discrete observables per symbol interval and thus suffer from high error floors and low implicit diversity orders on fast fading channels the error probability is highly dependent on the shapes and durations of the modulator waveforms for instance pulses of long duration give lower error probabilities than shorter pulses and for a certain type of orthogonal waveforms there is no error floor

entiers friables inegalite de turan kubilius et applications we generalize the classical turan kubilus inequality to friable integers and obtain sharp estimates for the involved constant in large subdomains several applications are derived in probabilitic number theory

natural vibrations and stability of shells of revolution interacting with an internal fluid flow abstract a finite element algorithm is proposed to investigate the dynamic behavior of elastic shells of revolution containing a quiescent or a flowing inviscid fluid in the framework of linear theory the fluid behavior is described using the perturbed velocity potential the shell behavior is treated in the framework of the classical shell theory and variational principle of virtual displacements incorporating a linearized bernoulli equation for calculation of hydrodynamic pressure acting on the shell the problem reduces to evaluation and analysis of the eigenvalues in the connected system of equations obtained by coupling the equations for velocity perturbations with the equations for shell displacements for cylindrical shells the results of numerical simulations are compared with recently published experimental analytical and numerical data the paper also reports the results of studying the dynamic behavior of shells under various boundary conditions for the perturbed velocity potential the investigation made for conical shells has shown that under certain conditions an increase in the cone angle can change a divergent type of instability to a flutter type

optimal joint detection estimation in fading channels with polynomial complexity the problem of sequence detection in frequency nonselective time selective fading channels when channel state information csi is not available at the transmitter and receiver is considered in this paper the traditional belief is that exact maximum likelihood sequence detection mlsd of an uncoded sequence over this channel has exponential complexity in the channel coherence time thus for slowly varying channels i e channels having coherence time on the order of the sequence length the complexity appears to be exponential in the sequence length in the first part of this work it is shown that exact mlsd can be computed with only polynomial worst case complexity in the sequence length regardless of the operating signal to noise ratio snr for equal energy signal constellations by establishing a relationship between the aforementioned complexity and the rank of the correlation matrix of the fading process an understanding of how complexity of the optimal mlsd receiver varies as the channel dynamics change is provided in the second part of this paper the problem of decoding turbo like codes in frequency nonselective time selective fading channels without receiver csi is examined using arguments similar to the ones used for the mlsd case it is shown that the exact symbol by symbol soft decision metrics sbssdms implied by the min sum algorithm can be evaluated with polynomial worst case complexity in the sequence length regardless of snr for equal energy signal constellations finally by simplifying some key steps in the polynomial complexity algorithm a family of fast approximate algorithms is derived which yield near optimal performance

nonlinear dynamics of harmonically excited circular cylindrical shells containing fluid flow abstract in the present study the geometrically nonlinear vibrations of circular cylindrical shells subjected to internal fluid flow and to a radial harmonic excitation in the spectral neighbourhood of one of the lowest frequency modes are investigated for different flow velocities the shell is modelled by donnell s nonlinear shell theory retaining in plane inertia and geometric imperfections the fluid is modelled as a potential flow with the addition of unsteady viscous terms obtained by using the time averaged navier stokes equations a harmonic concentrated force is applied at mid length of the shell acting in the radial direction the shell is considered to be immersed in an external confined quiescent liquid and to contain a fluid flow in order to reproduce conditions in previous water tunnel experiments for the same reason complex boundary conditions are applied at the shell ends simulating conditions intermediate between clamped and simply supported ends numerical results obtained by using pseudo arclength continuation methods and bifurcation analysis show the nonlinear response at different flow velocities for i a fixed excitation amplitude and variable excitation frequency and ii fixed excitation frequency by varying the excitation amplitude bifurcation diagrams of poincare maps obtained from direct time integration are presented as well as the maximum lyapunov exponent in order to classify the system dynamics in particular periodic quasi periodic sub harmonic and chaotic responses have been detected the full spectrum of the lyapunov exponents and the lyapunov dimension have been calculated for the chaotic response they reveal the occurrence of large dimension hyperchaos

further results on differential space time modulations some novel results on the space time differential modulation schemes described by b m hochwald and w sweldens see ibid vol 48 p 2041 52 2000 and v tarokh and h jafarkhani ieee j select areas commun vol 18 p 1169 74 2000 are derived under the assumption of a quasi static multiple antenna channel first the optimality in the maximum likelihood sense of the one shot detection algorithms of hochwald and sweldens and of tarokh and jafarkhani is proved then the expression of the pairwise error probability for these detectors is derived and as a particular case the bit error rate is computed for the binary phase shift keyed scheme of tarokh and jafarkhani finally a per survivor processing psp detection algorithm for this class of modulations is illustrated numerical results evidence both the superiority of the psp strategy over the one shot space time differential detectors and the accuracy of the above mentioned bit error rate formula

decision feedback differential detection based on linear prediction for 16dapsk signals transmitted over flat ricean fading channels in this article prediction based decision feedback differential detection df dd for 16 level differentially encoded amplitude phase shift keying is proposed unlike previously reported df dd schemes this scheme provides a performance gain over conventional differential detection under general ricean fading conditions a further important advantage of the novel scheme is that it is able to compensate a small carrier frequency offset the linear predictor coefficients may be updated using the recursive least squares algorithm which can start blind i e without a priori knowledge about the channel statistics and without a training sequence this makes the scheme attractive for application in mobile communications since the statistics of a nonstationary mobile channel can be tracked

nonlinear dynamics and stability of cantilevered circular cylindrical shells conveying fluid abstract in this paper the nonlinear dynamics of thin circular cylindrical shells with clamped free boundary conditions subjected to axial internal flow is theoretically analyzed for the first time the nonlinearity is geometric and is related to the large deformation of the structure the nonlinear model of the shell is based on the flugge shell theory in this model in plane inertia terms and all the nonlinear terms due to the mid surface stretching are retained the fluid is considered to be inviscid and incompressible and its modelling is based on linearized potential flow theory the fluid behaviour beyond the free end of the shell is described by an outflow model which characterizes the fluid boundary condition at the free end of the shell at the clamped end however it is assumed that the fluid remains unperturbed the fourier transform method is used to solve the governing equations for the fluid and to obtain the hydrodynamic forces the extended hamilton principle is utilized to formulate the coupled fluid structure system and a direct approach is employed to discretize the space domain of the problem the resulting coupled nonlinear odes are integrated numerically and bifurcation analyses are performed using the auto software results indicate that the shell loses stability through a supercritical hopf bifurcation giving rise to a stable periodic motion limit cycle the amplitude of this oscillation grows with flow velocity until it loses stability to nonperiodic oscillatory motion namely quasiperiodic and chaotic oscillation the values of the critical flow velocities for various length to radius ratios obtained by nonlinear theory agree well with available experimental data

bit interleaved coded differential space time modulation bit interleaved coded differential space time modulation for transmission over spatially correlated ricean flat fading channels is discussed for improved noncoherent detection without channel state information at the receiver iterative decoding employing hard decision feedback and prediction based metric computation is applied the performance is assessed based on the associated cutoff rate analytical expressions for the bit error rate and the outage probability respectively and simulations it is shown that the proposed scheme offers high power efficiency exploiting both space and time diversity while the computational complexity is kept at a relatively low level

non linear dynamics and stability of circular cylindrical shells containing flowing fluid part i stability abstract the study presented is an investigation of the non linear dynamics and stability of simply supported circular cylindrical shells containing inviscid incompressible fluid flow non linearities due to large amplitude shell motion are considered by using the non linear donnell s shallow shell theory with account taken of the effect of viscous structural damping linear potential flow theory is applied to describe the fluid structure interaction the system is discretiszd by galerkin s method and is investigated by using a model involving seven degrees of freedom allowing for travelling wave response of the shell and shell axisymmetric contraction two different boundary conditions are applied to the fluid flow beyond the shell corresponding to i infinite baffles rigid extensions of the shell and ii connection with a flexible wall of infinite extent in the longitudinal direction permitting solution by separation of variables they give two different kinds of dynamical behaviour of the system as a consequence of the fact that axisymmetric contraction responsible for the softening non linear dynamical behaviour of shells is not allowed if the fluid flow beyond the shell is constrained by rigid baffles results show that the system loses stability by divergence

maximum likelihood synchronization equalization and sequence estimation for unknown time varying frequency selective rician channels this paper develops a receiver structure to perform jointly maximum likelihood ml synchronization equalization and detection of a linearly modulated signal transmitted over a time varying frequency selective rician faded channel corrupted by additive white gaussian noise awgn the receiver is particularly suited to a fast fading channel where other receivers that rely on estimating the channel cannot track it quickly enough the signal mean and autocovariance are needed and a scheme is proposed for estimating these quantities adaptively the receiver processes the specular and diffuse components corresponding to the signal mean and autocovariance separately processing the known specular component is the classical detection problem the unknown diffuse component is processed by predictors we show that the predictors can achieve synchronization in a novel manner if synchronization is required a union bound on the receiver s bit error rate ber is derived and it tightly bounds simulated bers in fast fading at high signal to noise ratios snrs

convergence of a ml parameter estimation algorithm for ds ss systems in time varying channels with strong interference an unbiased maximum likelihood ml channel parameter estimation algorithm for direct sequence spread spectrum systems with strong interference is discussed in this paper the algorithm includes correcting terms to the extended kalman filter ekf based on the gradient of the negative log likelihood function of the output of a conventional matched filter by an asymptotic analysis the algorithm is shown to determine the actual parameters a complete implementation of the algorithm is given and its transient behavior is examined by computer simulations results show the ml algorithm albeit optimal in the sense of unbiased parameter estimation is less robust than the modified ekf described in the first reference

nonlinear supersonic flutter of circular cylindrical shells the aeroelastic stability of simply supported circular cylindrical shells in supersonic flow is investigated nonlinearities caused by large amplitude shell motion are considered by using the donnell nonlinear shallow shell theory and the effect of viscous structural damping is taken into account two different in plane constraints are applied to the shell edges zero axial force and zero axial displacement the other boundary conditions are those for simply supported shells linear piston theory is applied to describe the fluid structure interaction by using two different formulations taking into account or neglecting the curvature correction term the system is discretized by galerkin projections and is investigated by using a model involving seven degrees of freedom allowing for traveling wave flutter of the shell and shell axisymmetric contraction results show that the system loses stability by standing wave flutter through supercritical bifurcation however traveling wave flutter appears with a very small increment of the freestream static pressure that is used as the bifurcation parameter a very good agreement between theoretical and existing experimental data has been found for flutter amplitudes the influence of internal static pressure has also been studied

maximum likelihood sequence estimation of cpm signals transmitted over rayleigh flat fading channels a method for the sequential updating of log likelihood functions for maximum likelihood sequence estimation is presented it is shown that in a general case this method can be implemented using kalman filtering techniques for the special case of rayleigh flat fading and continuous phase modulation cpm signaling this approach is shown to lead to an attractive receiver structure this type of receiver called the linear predictive receiver can be implemented in the form of the viterbi algorithm with the trellis updates being computed using a bank of finite pulse response fir filter and square operations simulation results are presented that demonstrate the superiority of the linear predictive receiver over receivers employing differential detection in the presence of fast fading it is shown that the optimal linear predictive receiver does not possess an irreducible error rate for a class of rayleigh fading channels used to model terrestrial mobile channels

mlse for correlated diversity sources and unknown time varying frequency selective rayleigh fading channels yu and pasupathy see ibid vol 43 p 1534 44 1995 derived a maximum likelihood sequence estimation mlse receiver structure for unknown time varying frequency selective rayleigh fading channels and uncorrelated diversity sources this receiver design is extended to the case of correlated diversity sources correlated diversity sources typically arise with space diversity where constraints on antenna volume require that diversity antennae be placed too closely together analytic and simulated bit error rate ber curves are presented for receivers which exploit and ignore the correlation in the former case we find a small ber improvement that reduces with decreasing correlation however for a fixed receiver complexity superior performance is achieved when the correlation is ignored

decision feedback differential detection of mdpsk for flat rayleigh fading channels in this paper decision feedback differential detection df dd of m ary differential phase shift keying mdpsk signals which has been introduced previously for the additive white gaussian noise awgn channel by leib et al 1988 and edbauer 1992 is extended to flat rayleigh fading channels the corresponding df dd metric is derived from the multiple symbol detection msd metric and for genie aided df dd an exact expression for the bit error rate ber of qdpsk m 4 is calculated furthermore the dependence of ber on the power spectrum of the fading process is investigated for feedback filters of infinite order it is shown that in this case for ideally bandlimited fading processes the error floor of conventional differential detection dd can be removed entirely simulation results confirm that both msd and df dd with feedback filters of finite order can reduce the error floor of conventional dd significantly df dd thereby causes considerably less computational load

multiple symbol differential sphere decoding in multiple symbol differential detection msdd for power efficient transmission over rayleigh fading channels without channel state information blocks of n received symbols are jointly processed to decide on n 1 data symbols the search space for the maximum likelihood ml estimate is therefore complex n 1 dimensional and maximum likelihood msdd ml msdd quickly becomes computationally intractable as n grows mackenthun s low complexity msdd algorithm finds the ml estimate only for rayleigh fading channels that are time invariant over an n symbol period for the general time varying fading case however low complexity ml msdd is an unsolved problem in this letter we solve this problem by applying sphere decoding sd to ml msdd for time varying rayleigh fading channels the resulting technique is referred to as multiple symbol differential sphere decoding msdsd

adaptive mlse for dpsk in time and frequency selective channels an adaptive equalizer structure based on a state space formulation and maximum likelihood sequence estimation mlse is developed for a time varying frequency selective channel differential phase shift keying dpsk is assumed at the transmitter and effective differential decoding is performed at the receiver the standard models of a time varying linear channel and the karhunen loeve kl expansion underpin the receiver structure analytical and simulation results for the receiver are shown the resulting receiver is a per survivor structure

theory and experiments for large amplitude vibrations of empty and fluid filled circular cylindrical shells with imperfections abstract the large amplitude response of perfect and imperfect simply supported circular cylindrical shells to harmonic excitation in the spectral neighbourhood of some of the lowest natural frequencies is investigated donnell s non linear shallow shell theory is used and the solution is obtained by the galerkin method several expansions involving 16 or more natural modes of the shell are used the boundary conditions on the radial displacement and the continuity of circumferential displacement are exactly satisfied the effect of internal quiescent incompressible and inviscid fluid is investigated the non linear equations of motion are studied by using a code based on the arclength continuation method a series of accurate experiments on forced vibrations of an empty and water filled stainless steel shell have been performed several modes have been intensively investigated for different vibration amplitudes a closed loop control of the force excitation has been used the actual geometry of the test shell has been measured and the geometric imperfections have been introduced in the theoretical model several interesting non linear phenomena have been experimentally observed and numerically reproduced such as softening type non linearity different types of travelling wave response in the proximity of resonances interaction among modes with different numbers of circumferential waves and amplitude modulated response for all the modes investigated the theoretical and experimental results are in strong agreement

a robust receiver structure for time varying frequency flat rayleigh fading channels a receiver structure for unknown time varying frequency flat rayleigh fading channels is proposed unlike the maximum likelihood receiver structure this receiver does not need any knowledge of the channel autocovariance or the received signal to noise ratio and yet analytic and simulation results show that its bit error rate performance is close to the optimal structure the receiver is applicable to signals with constant or approximately constant envelopes or to linearly modulated signals using nyquist pulses

iterative diversity detection for correlated continuous time rayleigh fading channels an antenna array is proposed as a means of achieving a space diversity effect that partly overcomes the severity of continuous time rayleigh fading channels the investigated channel is assumed to be frequency nonselective with correlated diversity links where the correlation is related to the array geometry and the spatial and doppler dispersions further the error performance is improved by bit interleaving and channel coding where the encoders channel is viewed as a serially concatenated system a convolutional code constitutes the outer code whereas a differential encoder and the fading channel having truncated memory form a joint inner code in order to obtain a practical detector structure it is desirable to perform iterative decoding by applying some a posteriori probability app algorithms for this purpose we propose a novel generalization of the well known bahl cocke jelinek raviv 1974 algorithm that calculates the apps over channels having memory numerical results indicate that iterative decoding becomes more powerful when the exploited channel memory depth is extended also the error performance is significantly improved by introducing multiple antennas the interleaver gain is however seen to be quite moderate in contrast to additive white gaussian noise channels

a cfd based model for the study of the stability of cantilevered coaxial cylindrical shells conveying viscous fluid abstract a new numerical cfd based model is presented for the study of the effects of unsteady viscous forces on the stability of a cantilevered flexible cylindrical shell concentrically located inside a rigid cylinder with incompressible viscous flow in the annulus and with stagnant fluid within the shell flugge s modified equations are used to describe shell motions taking into account the steady viscous forces due to flow pressurization and traction effects on the shell these equations are subsequently solved by the finite difference method the unsteady viscous forces exerted on the shell are determined from flow perturbations governed by the linearized unsteady navier stokes equations which are solved using a recently developed finite difference based time marching technique with artificial compressibility it was found that the analytical results obtained for a particular set of system parameters are in excellent quantitative agreement with experiment although the predicted type of instability is not always the same as that observed the unsteady viscous effects tend to become reduced with diminishing annular gap width provided that the gap is moderately small

generalized polynomial based receiver for the flat fading channel this letter presents a simple polynomial predictor based sequence detector for the rayleigh nonselective fading channel unlike the polynomial predictor based sequence detector proposed by borah and hart the new receiver is not restricted to constant envelope modulation schemes analytical and simulated results are presented in some instances the proposed receiver performs within 6 db of the equivalent maximum likelihood sequence estimation receiver

autocovariance preserving estimator ape interpretation of the mlsd metric for rayleigh fading channels this paper describes a new interpretation of the maximum likelihood sequence detector metric for a linearly modulated signal and an unknown time varying dispersive rayleigh fading channel the metric which involves the channel and noise autocovariances is transformed into the intuitive mahalanobis distance squared euclidean distance in white noise between the observations and a reference estimated from them a novel estimator called the autocovariance preserving estimator is obtained that trades off increased estimation bias for reduced sequence error probability

non linear vibration analysis of fluid filled cylindrical shells abstract a theoretical analysis is presented for determining the elastic non linear vibrations of a prestressed thin walled cylindrical shell filled with an ideal fluid for the vibrations of the shell itself the dynamic version of the sanders non linear equations for the case of moderately small rotations is employed modal expansions are used for the displacements of the shell middle surface that are required to satisfy the classical simply supported boundary conditions and the circumferential periodicity condition the fluid is taken as non viscous and incompressible and the coupling between the deformable shell and this medium is taken into account the velocity potential is expanded in terms of harmonic functions which satisfy the laplace equation term by term the galerkin method is used to reduce the problem to a system of coupled algebraic non linear equations for the modal amplitudes solutions are presented to show the effects of fluid and shell parameters on the non linear vibrations of the shell

nyquist 3 pulse shaping in continuous phase modulation continuous phase modulation cpm schemes using nyquist 3 pulse shaping and correlative codes are popular candidates for mobile radio applications this paper presents a detailed study of the effects of nyquist 3 pulse shaping on the spectral characteristics of cpm schemes with a modulation index of 1 2 the influence of pulse parameters such as its shape bandwidth coding polynomial and truncation length on spectral characteristics such as main lobe fractional power bandwidths and out of band power levels is thoroughly investigated error performance is also briefly analyzed among uncoded schemes a new scheme using a pulse satisfying both nyquist 3 and nyquist 2 criteria is pointed out as a promising candidate and as a good compromise between spectral and power efficiencies

on integers free of large prime factors the number l x y of integers y has been given satisfactory estimates in the regions y exp log log x 5 3 6 in the intermediate range only very crude estimates have been obtained so far we close this gap and give an expression which approximates l x y uniformly for x y 2 within a factor 1 o log y log x log y y as an application we derive a simple formula for ti cx y fl x y where 1 c y we also prove a short interval estimate for l x y

sur les lois locales de la repartition du k ieme diviseur d un entier let d j n j 1 tau n denote the increasing sequence of divisors of an integer n and write lambda k d for the natural density of the set of those integers n with d k n d answering a question raised by erd h os we show that lambda k d attains its maximum for d k 1 2 o 1 where k k log log k log 2 which by a theorem of ramanujan is roughly the size of the smallest integer having k divisors a key step of the proof consists in establishing a sublinearity property of the counting function psi 1 x y of y friable square free integers not exceeding x this is achieved via a complete study of psi 1 x y using the saddle point method which in particular also enables a precise description of the gaussian behaviour of this function in certain ranges 2000 mathematical subject classification 11n25 11n37

predictable processes and wold s decomposition a review the concepts of predictability and band limitedness are reexamined and a simple proof of wold s decomposition is presented in the context of mean square estimation

non linear dynamics and stability of circular cylindrical shells containing flowing fluid part ii large amplitude vibrations without flow abstract the non linear response of empty and fluid filled circular cylindrical shells to harmonic excitations is investigated both modal and point excitations have been considered the model is suitable to study simply supported shells with and without axial constraints donnell s non linear shallow shell theory is used the boundary conditions on radial displacement and the continuity of circumferential displacement are exactly satisfied the radial deflection of the shell is expanded by using a basis of seven linear modes the effect of internal quiescent incompressible and inviscid fluid is investigated the equations of motion obtained in part i of this study are studied by using a code based on the collocation method the validation of the present model is obtained by comparison with other authoritative results the effect of the number of axisymmetric modes used in the expansion on the response of the shell is investigated clarifying questions open for a long time the results show the occurrence of travelling wave response in the proximity of the resonance frequency the fundamental role of the first and third axisymmetric modes in the expansion of the radial deflection with one longitudinal half wave and limit cycle responses modes with two longitudinal half waves are also investigated

non linear dynamic analysis of orthotropic open cylindrical shells subjected to a flowing fluid abstract a theory is presented to predict the influence of non linearities associated with the wall of the shell and with the fluid flow on the dynamics of elastic thin orthotropic and non uniform open cylindrical shells submerged and subjected simultaneously to an internal and external fluid the open shells are assumed to be freely simply supported along their curved edges and to have arbitrary straight edge boundary conditions the method developed is a hybrid of thin shell theory fluid theory and the finite element method the solution is divided into four parts in part one the displacement functions are obtained from sander s linear shell theory and the mass and linear stiffness matrices for the empty shell are obtained by the finite element procedure in part two the modal coefficients derived from the sanders koiter non linear theory of thin shells are obtained for these displacement functions expressions for the second and third order non linear stiffness matrices of the empty shell are then determined through the finite element method in part three a fluid finite element is developed the model requires the use of a linear operator for the velocity potential and a linear boundary condition of impermeability with the non linear dynamic pressure we develop in the fourth part three non linear matrices for the fluid the non linear equation of motion is then solved by the fourth order runge kutta numerical method the linear and non linear natural frequency variations are determined as a function of shell amplitudes for different cases

noncoherent sequence detection of differential space time modulation approximate maximum likelihood noncoherent sequence detection nsd for differential space time modulation dstm in time selective fading channels is proposed the starting point is the optimum multiple symbol differential detection for dstm that is characterized by exponential complexity by truncating the memory of the incremental metric a finite state trellis is obtained so that a viterbi algorithm can be implemented to perform sequence detection compared to existing linear predictive receivers a distinguished feature of nsd is that it can accommodate nondiagonal constellations in continuous fading error analysis demonstrates that significant improvement in performance is achievable over linear prediction receivers by incorporating the reduced state sequence detection techniques performance and complexity tradeoffs can be controlled by the branch memory and trellis size numerical results show that most of the performance gain can be achieved by using an l state trellis where l is the size of the dstm constellation

an adaptive maximum likelihood receiver for correlated rayleigh fading channels the paper develops a receiver structure for random gaussian signals in additive noise based on the classic maximum likelihood m l estimator correlator derivation of kailath 1960 and applies it to differential phase shift keying dpsk on the correlated rayleigh fading channel it is shown to lower the error floor found in the performance of conventional dpsk receivers by orders of magnitude in addition the maximum likelihood procedure is shown to make uncorrelated symbol decisions the performance of both conventional and optimal receivers which require knowledge of the channel statistics is examined analytically a recursive channel adaptive version of the optimal receiver utilizing decision feedback to estimate the channel statistics is developed its simulated performance shows no penalty compared to theoretical calculations which require explicit knowledge of the channel statistics

sur l inegalite de turan kubilius friable the exact value of the friable turan kubilius constant is determined when u log x log y tends to a finite limit for the set of y friable integers not exceeding x

multisampling decision feedback linear prediction receivers for differential space time modulation over rayleigh fast fading channels novel decision feedback df linear prediction lp receivers which process multiple samples per symbol interval in conjunction with optimal sample combining are proposed for differential space time modulation dstm over rayleigh fast fading channels performance analysis demonstrates that multisampling df lp receivers outperform their symbol rate sampling counterpart in fast fading substantially in addition an asymptotically tight upper bound on the pairwise error probability is derived in view of this bound the design criterion of dstm for fast fading is the same as that for block wise static fading to avoid the estimation of the second order statistics of the channel a polynomial model based df lp receiver is proposed it can approach the performance of the optimum df lp receiver at high signal to noise ratios provided fading is moderate

detection by multiple trellises in this paper we present a novel pragmatic approach referred to as detection by multiple trellises to perform trellis based detection over realistic channels more precisely we consider channels with unknown parameters and apply the concept of detection by multiple trellises to forward backward fb algorithms the key idea of our approach consists first of properly quantizing the channel parameters and then considering replication of coherent fb algorithms operating on parallel trellises one per hypothetical quantized value in order to make the receiver robust against a possibly time varying channel parameters the proposed soft output algorithms perform a proper manipulation of the forward and backward metrics computed by the parallel fb algorithms at regularly spaced trellis steps we consider two significant examples of application detection over i phase uncertain channels and ii fading channels the performance of the proposed algorithms is investigated considering differentially encoded de quaternary phase shift keying qpsk and iterative detection schemes based on low density parity check ldpc codes besides having a low complexity the proposed soft output algorithms turn out to be robust flexible blind in the sense that no knowledge of the channel parameter statistics is required and highly parallelizable as it is desirable in high throughput future wireless communication systems

optimal parametric feedforward estimation of frequency selective fading radio channels estimation of time varying dispersive radio channels is one of the most important tasks of receiver synchronisation state of the art adaptive estimators employing decision feedback suffer from error propagation and limited robustness against faster fading in this paper the optimal feedforward channel estimator using only known training symbols is systematically derived statistical channel information is assumed to be available for moderately rapid fading channels snapshot assumption the optimal estimator is shown to be divided into the two subtasks of maximum likelihood acquisition and subsequent wiener filtering of the acquired quantities if perfect training sequences resulting in optimal performance are used the wiener filtering operation collapses into independent filtering of the individual acquired tap estimates and the resulting channel estimator becomes efficient and flexible the performance of the optimal estimator is evaluated for important cases a design example of a near optimal hf channel estimator receiver is discussed linear interpolation is used to reduce the computational complexity of receiver coefficient adjustment simulation results confirm the superiority and robustness of feedforward synchronization having near optimal performance at reduced complexity

adaptive mlsde using the em algorithm the theory of adaptive sequence detection incorporating estimation of channel and related parameters is studied in the context of maximum likelihood ml principles in a general framework based on the expectation and maximization em algorithm a generalized ml sequence detection and estimation gmlsde criterion is derived based on the em approach and it is shown how the per survivor processing and per branch processing methods emerge naturally from gmlsde gmlsde is developed into a real time detection estimation algorithm using the online em algorithm with coupling between estimation and detection by utilizing titterington s 1984 stochastic approximation approach different adaptive ml sequence detection and estimation mlsde algorithms are formulated in a unified manner for different channel models and for different amounts of channel knowledge available at the receiver computer simulation results are presented for differentially encoded quadrature phase shift keying in frequency flat and selective fading channels and comparisons are made among the performances of the various adaptive mlsde algorithms derived earlier

a blind particle filtering detector of signals transmitted over flat fading channels a new particle filtering detector pfd is proposed for blind signal detection over flat rayleigh fading channels whose model coefficients are unknown the detector employs a hybrid importance function and a mixture kalman filter it also incorporates an auxiliary particle filtering strategy with a smoothing kernel in the resampling step further by considering practical information of communication systems and the physical interpretation of the adopted second order autoregressive ar channel model a fully blind particle filtering implementation is developed the structure of the proposed pfd can be easily adapted to other system requirements simulations are provided that demonstrate the performance of the new pfd

particle filtering for demodulation in fading channels with non gaussian additive noise an efficient particle filtering algorithm is developed to solve the problem of demodulation of m ary modulated signals under conditions of fading channels in the presence of non gaussian additive noise simulations for mdpsk signals are presented the results show that the algorithm outperforms the current methods

diversity combining for ds ss systems with time varying correlated fading branches diversity combining of multiple time varying and correlated fading branches is investigated for direct sequence spread spectrum systems the correlated branches are modeled and estimated jointly as a vector autoregressive var process the joint estimation is shown to provide performance gain over separate estimation of the fading branches the parameter matrices of the var model are estimated via the method of expectation maximization em with two algorithms the first algorithm using results from kalman smoothing provides a closed form solution to the maximization problem in the iterative em procedure however the iterative em kalman algorithm operates repeatedly on a batch of training data and results in large storage requirements and long processing delays to overcome these disadvantages a new algorithm with only forward time recursions is proposed that approximates the iterative em solution and efficiently adapts to slowly changing doppler spreads as a result the new algorithm significantly reduces memory and training sequence requirements through computer simulations a near ideal bit error rate performance is found for both algorithms and the efficacy of the new adaptive algorithm for channels with changing doppler spreads is demonstrated