local stability of dynamical processes in random media abstract a particular type of random dynamical process is considered in which the stochasticity is introduced through randomly fluctuating parameters a method of local multipliers is developed for treating the local stability of such dynamical processes corresponding to infinite dimensional dynamical systems the method is illustrated by several examples by the random diffusion equation random wave equation and random schrodinger equation the evolution equation for the density matrix of a quasiopen statistical system subject to the action of random surrounding is considered the stationary solutions to this equation are found to be unstable against arbitrary small finite random perturbations the notion of random structural stability is introduced

symbol synchronization for digital signals with multisegment trajectory digital data transition tracking loop the symbol synchronizer presented is an all digital data transition tracking loop which permits simple implementation and voltage control of the loop bandwidth the equivalent loop integration constant is changed by changing the threshold level of a comparator at the output of the loop phase error detector phase error probability distribution function its variance and the average probability of cycle slipping are analyzed analytically for the general case the theory of finite markov chains is used to analyze statistical properties of the tracking error primarily with regard to the phase error distribution and its variance the average probability of the cycle slipping the synchronizer time response and bandwidth as an example numerical results for generalized tamed frequency modulation gtfm with b 0 62 and r 0 36 are presented and discussed

interpolation in digital modems ii implementation and performance for pt i see ibid vol 41 no 3 p 502 208 1993 properties of a specific class of interpolators that are based upon polynomials are discussed several implementations are described one of which is particularly convenient in practical hardware simulations demonstrate that simple interpolators give excellent performance in many cases two point linear interpolation is adequate if better performance is needed classical four point third order polynomials could be used better yet a novel four point interpolating filter with piecewise parabolic impulse response can have performance superior to that of the standard cubic interpolator and still be implemented much more simply the nco based control method presented in part i is shown to be equivalent to a conventional phase locked loop and its operation is verified by simulation

the modified cramer rao bound and its application to synchronization problems we introduce the modified cramer rao bound crb which like the true crb is a lower bound to the error variance of any parameter estimator the modified crb proves useful when in addition to the parameter to be estimated the observed data also depend on other unwanted parameters the relationship between the modified and true crb is established and applications are discussed regarding the estimation of carrier frequency offset carrier phase and timing epoch in linearly modulated signals modified crbs for phase and timing estimation have been already discussed in previous works where it is shown that several practical carrier phase and clock recovery circuits do attain such bounds frequency discrimination instead is not so well represented in the literature and a significant contribution of this paper is the calculation of the modified crb for frequency estimation this bound is compared with the performance of some frequency detectors and it is concluded that further work is needed in search of more efficient frequency discrimination methods

sequence estimation and synchronization from nonsynchronized samples the authors study the problem of maximum likelihood sequence estimation and synchronization from samples of the output of a matched filter taken at integer multiples of the symbol rate which is assumed perfectly known by the receiver a general analysis is presented of sampled receivers that handle arbitrary baseband pulse shapes and arbitrary sampling rates it is observed that the optimal processing of the matched filter samples consists of digital interpolation followed by symbol by symbol decoding when sampling is at or above the nyquist rate or viterbi decoding when sampling is below the nyquist rate performance is studied through the cramer rao bound on mean square estimation error and a lower bound on error probability

smooth dynamics and new theoretical ideas in nonequilibrium statistical mechanics this paper reviews various applications of the theory of smooth dynamical systems to conceptual problems of nonequilibrium statistical mecanics we adopt a new point of view which has emerged progressively in recent years and which takes seriously into account the chaotic character of the microscopic time evolution the emphasis is on nonequilibrium steady states rather than the traditional approach to equilibrium point of view of boltzmann the nonequilibrium steady states in presence of a gaussian thermostat are described by srb measures in terms of these one can prove the gallavotti cohen fluctuation theorem one can also prove a general linear response formula and study its consequences which are not restricted to near equilibrium situations at equilibrium one recovers in particular the onsager reciprocity relations under suitable conditions the nonequilibrium steady states satisfy the pairing theorem of dettmann and morriss the results just mentioned hold so far only for classical systems they do not involve large size i e they hold without a thermodynamic limit

derivation of gardner s timing error detector from the maximum likelihood principle in a recent paper 1 gardner described a new timing error detector for sampled receivers the detector derives timing information without decision direction from a bpsk or qpsk signal sampled at rate 2 t symbol interval t we will show how this detector can be derived from the maximum likelihood principle and thus give insight into the nature of the algorithm

robustness of centrality measures against network manipulation abstract node centrality is an important quantity to consider in studying complex networks as it is related to many applications ranging from the prediction of network structure to the control of dynamics on networks in the literature much effort has been devoted to design new centrality measurements however the reliability of these centrality measurements has not been fully assessed particularly with respect to the fact that many real networks are facing different kinds of manipulations such as addition removal or rewiring of links in this paper we focus on the robustness of classic centrality measures against network manipulation our analysis is based on both artificial and real networks we find that the centrality measurements are generally more robust in heterogeneous networks biased link manipulation could more seriously distort the centrality measures than random link manipulation moreover the top part of the centrality ranking is more resistant to manipulation

efficient maximum likelihood based clock and phase estimators for oqpsk signals in this paper we propose an algorithm for joint carrier phase and timing estimation with oqpsk modulations the derivation is based on the maximum likelihood criterion and exploits a very efficient algorithm for the detection of differentially encoded m psk symbols already described in literature though we are mainly interested in measuring the phase and clock parameters estimates of the transmitted symbols are also obtained as by products the resulting scheme has a feedforward structure and provides phase and timing information in a fixed time differently from closed loop architectures it can be implemented in digital form and is particularly suitable for burst mode transmissions its performance is investigated by simulation and the results are compared with cramer rao bounds it turns out that the estimation accuracy is very close to the theoretical limits even with short observation intervals and small values of the excess bandwidth in such conditions the proposed estimators largely outperform other schemes already known in literature their superiority becomes less significant as the signal bandwidth increases

a class of data aided timing recovery schemes this paper develops a class of data aided timing recovery schemes whose structure and operating principle are reminiscent of zero forcing equalizer adaptation the schemes exhibit no pattern dependent jitter and are applicable to a variety of modulation codes and reception techniques they come in both oversampled and baud rate forms and permit maximum likelihood tracking performance whenever the oversampling factor satisfies the sampling theorem subject to this condition even simple schemes can be close to optimum it is shown that various existing schemes belong to the class under study or are closely related to it

symbol synchronization using signal samples and interpolation a method for symbol synchronization using two unsynchronized signal samples per symbol is presented signal values between the sample points are calculated by interpolation and then processed digitally to find a suitable strobe point the interpolated value at the strobe point is used for data detection or input to an equalizer simulations show that low variance timing estimates can be made for pulses with raised cosine spectra degraded by quadratic delay distortion

design of optimal interpolation filter for symbol timing recovery we propose an optimal interpolation filter for symbol timing recovery in a digital receiver where the input analog to digital a d conversion sampling clock is not synchronized to the transmitter symbol clock the optimal filter is designed by minimizing the mean square error mse at the output of the receiver assuming that the correct timing difference between the nonsynchronized input samples and the correct strobe synchronized to the symbol is given the mse minimization procedure results in a system of linear equations which can easily be solved to yield the optimal filter coefficients we also analyze the symbol tracking performance of a symbol synchronizer that employs the proposed optimal interpolation filter although the proposed optimal filter is designed assuming the availability of the correct timing difference we show that it minimizes the timing estimation error variance even when the computed timing difference values deviate from the ideal we also show that the timing estimation is virtually unbiased if the length of the interpolation filter is greater than or equal to 4 simulation results are included to show the performance improvement realizable by employing the optimal interpolation filter

performance study of a near maximum likelihood code aided timing recovery technique in this paper we propose a new code aided ca timing recovery algorithm for various linear constant modulus constellations based on the maximum likelihood ml estimator the first contribution is the derivation of a soft estimator expression of the transmitted symbol instead of its true or hard estimated value which is fed into the timing error detector ted equation the proposed expression includes the log likelihood ratios llrs obtained from a turbo decoder our results show that the proposed ca approach achieves almost as good results as the data aided da approach over a large interval of snr values while achieving a higher spectral efficiency we also derive the corresponding ca cramer rao bounds crb for various modulation orders contrarily to former work we develop here the crb analytical expression for different m psk modulation orders and validate them through comparison to empirical crb obtained by monte carlo iterations the proposed ca estimator realizes an important gain over the nondata aided approach nda and achieves a smaller gap when compared to its relative ca crb especially at moderate snr values where modern systems are constrained to work

network robustness and random processes abstract we introduce two new measures of network robustness and apply them to four different strategies the measures are independent from the number of nodes in the network and have the strong potential to cover a large portfolio of applications using the monte carlo methods we demonstrate how to approximate the measures the methods are based on random interruption of links with suitable constraints which represent the above mentioned strategies we introduce two networks with obvious varying robustness to demonstrate the measures we also demonstrate how to employ the measures in order to improve the robustness of the networks by adding one new link we further indicate that the measures are able to identify the infrequently connected parts of the network and suggest the most appropriate improvement we also discuss the consequences of the obtained results and the possible applications of the measures

probabilistic approach to pattern selection abstract the problem of pattern selection arises when the evolution equations have many solutions whereas observed patterns constitute a much more restricted set an approach is advanced for treating the problem of pattern selection by defining the probability distribution of patterns then the most probable pattern naturally corresponds to the largest probability weight this approach provides the ordering principle for the multiplicity of solutions explaining why some of them are more preferable than others the approach is applied to solving the problem of turbulent photon filamentation in resonant media

a bpsk qpsk timing error detector for sampled receivers a simple algorithm for detection of timing error of a synchronous band limited bpsk or qpsk data stream is proposed the algorithm requires only two samples per symbol for its operation one of the two samples is also used for the symbol decision derivation of the s curve reveals a sinusoidal shape

one dimensional gibbs states and axiom a diffeomorphisms on etudie la mecanique statistique en equilibre de systemes reticules classiques a une dimension a interactions a decroissance exponentielle pour de tels systemes les transformees de fourier des fonctions de correlation de paires sont meromorphes dans une bande et les residus des poles peuvent s exprimer en termes de distributions de gibbs on applique ces resultats aux diffeomorphismes de l axiome a de smale

robustness of network measures to link errors in various applications involving complex networks network measures are employed to assess the relative importance of network nodes however the robustness of such measures in the presence of link inaccuracies has not been well characterized here we present two simple stochastic models of false and missing links and study the effect of link errors on three commonly used node centrality measures degree centrality betweenness centrality and dynamical importance we perform numerical simulations to assess robustness of these three centrality measures we also develop an analytical theory which we compare with our simulations obtaining very good agreement

extended gardner detector for improved symbol timing recovery of m psk signals in a recent paper the modified gardner detector has been proposed for advanced symbol timing recovery of m ary phase shift keying signals compared with the original algorithm the self noise jitter can be considerably reduced for highly bandlimited systems and smaller values of m in this letter a completely different approach is investigated which improves the self noise performance for larger values of m as well

microscopic origins of irreversible macroscopic behavior time asymmetric behavior as embodied in the second law of thermodynamics is observed in individual macroscopic systems it can be understood as arising naturally from time symmetric microscopic laws in accord with the ideas of thompson maxwell and boltzmann alternative explanations based on equating irreversible macroscopic behavior with the mixing type of behavior already present in the time evolution of ensembles probability distributions of certain systems having only a few degrees of freedom are in my opinion unnecessary misguided and misleading

expansion exponents for nonequilibrium systems abstract local expansion exponents for nonequilibrium dynamical systems described by partial differential equations are introduced these exponents show whether the system phase volume expands contracts or is conserved in time the ways of calculating the exponents are discussed the principle of minimal expansion provides the basis for treating the problem of pattern selection the exponents are also defined for stochastic dynamical systems the analysis of the expansion exponent behavior for quasi isolated systems results in the formulation of two other principles the principle of asymptotic expansion tells that the phase volumes of quasi isolated systems expand at asymptotically large times the principle of time irreversibility follows from the asymptotic phase expansion since the direction of time arrow can be defined by the asymptotic expansion of phase volume