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  • 1995-1999  (2,974)
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  • 1995  (2,974)
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  • Electrical Engineering, Measurement and Control Technology  (3,008)
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  • Articles  (3,008)
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  • 1995-1999  (2,974)
  • 1945-1949  (34)
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  • 1
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    Circuits, systems and signal processing 14 (1995), S. 57-67 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The paper presents a simple method for solving the optimal (LQG) control problem on an infinite time horizon for linear plants described by proper rational transfer function matrices. Since the class of proper plants discussed in the paper is more general than the one commonly used, additional properties of solutions are presented. Two numerical examples illustrate the theoretical considerations. The examples have been performed on an IBM PC using the proposed algorithm. This algorithm is a part of a public ASMCS package developed by the author for analysis, synthesis, and simulations of multi-input, multi-output (MIMO) control systems.
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  • 2
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    Circuits, systems and signal processing 14 (1995), S. 135-143 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract We study the behavior of a Hilbert network (i.e., a finite or countably infinite network whose variables are in a Hilbert space and in which the associated total energy is finite) whose elements are affected by perturbations. More specifically, we will give estimates for a change of the current distribution caused by (a) perturbations of the elements of the nominal network when the voltage sources are fixed, and (b) a change of voltage sources in a network whose elements are perturbed. The conditions given in our theorems imply insensitivity and robust stability of the nominal network. The applications of the results are illustrated by an example of an infinite network.
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  • 3
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    Circuits, systems and signal processing 14 (1995), S. 473-494 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract A new lattice filter structure to model two-dimensional (2-D) autoregressive (AR) fields is proposed. The proposed structure utilizes and extracts the information contained in the backward prediction error fields and their delayed versions. The main idea is to use two sets of reflection coefficients corresponding to two quadrant filters and to increase the number of reflection coefficients with the order of the lattice filter. Increasing the number of reflection coefficients at each stage produces a sufficient number of independent parameters to model AR fields up to order three, which is an improvement over the existing 2-D lattice filter structures. The improvement is confirmed by computer simulations. In addition, a relationship between the reflection coefficients and the AR coefficients is derived. It is also shown that the entropy contained in the backward prediction error field vector of the proposed structure is closer to the input entropy when compared to those contained in existing 2-D lattice filters.
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  • 4
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    Circuits, systems and signal processing 14 (1995), S. 555-561 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract This paper addresses the problem of global asymptotic stability in recursive first hyperquadrant causalm-D digital filters. A set of simple-to-check 1-D conditions necessary for stability of them-D system is given. Generalizations tov-dimensional (v〈m) subsystems are also provided. These derived conditions are useful in determining asymptotic convergence ofm-D digital filters implemented in fixed or floating point arithmetic. The set of 1-D conditions can be considered the analogous result to practical bounded input bounded output (BIBO) stability for linearm-D systems.
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  • 5
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    Circuits, systems and signal processing 14 (1995), S. 633-637 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract A new efficient algorithm for calculating the weighting coefficients of maximally linear, FIR digital differentiators is presented. Simple closed-form explicit and recursive formulas are derived in a very straightforward manner. Moreover, a simple recursive equation is established, relating coefficients of two digital differentiators of adjacent ranks.
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  • 6
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    Circuits, systems and signal processing 14 (1995), S. 661-667 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract This paper presents a sequence of pseudorandom arrays with triangular symmetry. The sequence of arrays is also pseudorandom in nature, so it can be called a pseudorandom sequence of arrays, or PRSA. A circuit for the PRSA generator is given and some interesting properties of this type of PRSA are discussed. Also, some of the concepts presented in this paper are clarified with an example.
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  • 7
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    Circuits, systems and signal processing 14 (1995), S. 1-16 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract In this paper, we present a new class of two-dimensional FIR-median hybrid (FMH) filters, which we call separable FMH filters, for image noise smoothing. In a separable FMH filter, a one-dimensional FMH filter is applied to the rows and the columns of an image successively. The deterministic properties of separable and cross window FMH filters are discussed. Under certain assumptions, it is proved that a root of a separable FMH filter is a root of the corresponding cross window FMH filter. The noise attenuating properties of a separable FMH filter are studied and compared with those of the separable median filter, the two-dimensional median filter with a square window, and the cross window FMH filter.
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  • 8
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    Circuits, systems and signal processing 14 (1995), S. 69-85 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The Chinese remainder theorem is a fundamental technique widely employed in digital signal processing for designing fast algorithms for computing convolutions. Classically, it has two versions. One is over a ring of integers and the second is over a ring of polynomials with coefficients defined over a field. In our previous papers, we developed an extension to this well-known theorem for the case of a ring of polynomials with coefficients defined over a finite ring of integers. The objective was to generalize number-theoretictransforms, which turn out to be a special case of this extension. This paper focuses on the extension of the Chinese remainder theorem for processing complex-valued integer sequences. Once again, the present work generalizes the complex-number-theoretic transforms. The impetus for this work is provided by the occurrence of complex integer sequences in digital signal processing and the desire to process them using exact arithmetic.
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  • 9
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    Circuits, systems and signal processing 14 (1995), S. 145-166 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract A method to improve the stability of the forced response of a recursive digital filter with nonlinearities due to finite wordlength is presented. An analysis of applying a stability criterion on the nonlinear filter is performed by transforming the problem into a mathematical problem of finding a zero set. As a result of the mathematical analysis, one can find a maximal sector size that bounds an equivalent nonlinearity, so that stability is ensured. This sector size is influenced by the choice of the desired nonlinearity characteristic appropriate to the problem, and the amount of the input scaling needed. Another improvement of the filter's stability is obtained by adding a feedback gain, the value of which is determined by using the zero set-finding method. Simulation results showing the improved stability properties of a filter designed by this method are presented.
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  • 10
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    Circuits, systems and signal processing 14 (1995), S. 285-298 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract In this paper a fully systolic, bit level, three-port unconstrained adaptor, which constitutes the main nontrivial building block of ladder wave digital filters, is generated. The one-dimensional binary convolution is used as the underlying algorithm for the implementation of a multiplication. The Isb-first input data organization approach is adopted and thecanonical mapping methodology is used to fully systolize the unconstrained parallel three-port adaptor at the bit level with piplining period a=1. The technique is based on a transformation of the adaptor's signal-flow graph, so that unidirectional data flow takes place. A ring-systolic scheme is proposed for implementing communications among adaptors.
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  • 11
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    Circuits, systems and signal processing 14 (1995), S. 317-349 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract We consider the problem of robust stochastic adaptive control of not necessarily minimum phase systems in the presence of unmodelled dynamics. Stochastic gradient algorithms with parameter projection and modified gain sequence are used for the estimation of the unknown controller parameters. Global stability of the adaptive system is achieved without requiring the strictly positive real condition and the persistency exciting condition to be satisfied.
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  • 12
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    Circuits, systems and signal processing 14 (1995), S. 427-443 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract A theoretical framework for the investigation of the qualitative behavior of differential-algebraic equations (DAEs) near an equilibrium point is established. The key notion of our approach is the notion of regularity. A DAE is called regular locally around an equilibrium point if there is a unique vector field such that the solutions of the DAE and the vector field are in one-to-one correspondence in a neighborhood of this equilibrium point. Sufficient conditions for the regularity of an equilibrium point are stated. This in turn allows us to translate several local results, as formulated for vector fields, to DAEs that are regular locally around a given equilibrium point (e.g. Local Stable and Unstable Manifold Theorem, Hopf theorem). It is important that these theorems are stated in terms of the given problem and not in terms of the corresponding vector field.
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  • 13
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    Circuits, systems and signal processing 14 (1995), S. 495-524 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The classical notion of the λ-generalized nullspace, defined on a matrixA εR n×n,where λ is an eigenvalue, is extended to the case of ordered pairs of matrices(F, G), F, G ε R m×nwhere the associated pencilsF − G is right regular. It is shown that for every α εC ∪ {∞} generalized eigenvalue of (F, G), an ascending nested sequence of spaces {P α i ,i=1, 2,...} and a descending nested sequence of spaces {ie495-02 i=1, 2,...} are defined from the α-Toeplitz matrices of (F, G); the first sequence has a maximal elementM α * , the α-generalized nullspace of (F, G), which is the element of the sequence corresponding to the index τα, the α-index of annihilation of (F, G), whereas the second sequence has the first elementP α * as its maximal element, the α-prime space of (F, G). The geometric properties of the {M α i ,i=1, 2,...,τα and {P α i ,i=1, 2,...sets, as well as their interrelations are investigated and are shown to be intimately related to the existence of nested basis matrices of the nullspaces of the α-Toeplitz matrices of (F, G). These nested basis matrices characterize completely the geometry ofM α * and provide a systematic procedure for the selection of maximal length linearly independent vector chains characterizing theα-Segre characteristic of (F, G).
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  • 14
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    Circuits, systems and signal processing 14 (1995), S. 563-586 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The robustness problem of stability for large-scale uncertain systems with a class of multiple time delays is addressed in this paper. By applying the complex Lyapunov stability theorem, the matrix measure techniques, and norm inequalities, a new approach for solving a general case of the above problem is proposed. Several robust stability conditions, delay-dependent or delay-independent, are derived to guarantee the asymptotic stability and exponential stability of the uncertain large-scale time-delay systems. Moreover, these obtained results can also be applied to the stabilization design.
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  • 15
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    Circuits, systems and signal processing 14 (1995), S. 615-632 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Successful speech recognition is highly dependent on appropriate speech segmentation. The poor efficiency of the sequential detection of abrupt changes in the signals with relatively short stationary intervals, as is the case with speech signals, can be improved by the off-line maximum likelihood segmentation algorithm. In this paper the new segmentation algorithm is presented. For the a priori known number of segments, the algorithm determines such signal partitions for which the sum of segment distortion is minimal. The generalized maximum likelihood distortion measure has been introduced, and has proven to be particularly efficient on short signal segments. In the case of an unknown number of segments, its estimate is obtained comparing the reduction of the distortion. The asymptotic properties of the distortion sequence have been analyzed, which led to the definition of the presented segmentation algorithm. The introduced measure can be applied both to the AR and ARMA models. The segmentation algorithm is verified on test signals as well as on the natural speech signal, for which the pitch synchronous framing scheme is applied. The experimental results also include a comparison of the AR and ARMA model-based segmentations. The first results show that ARMA model-based segmentation gives somewhat better results than the AR model algorithm.
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  • 16
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    Circuits, systems and signal processing 14 (1995), S. 639-651 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract A new bit-serial architecture for implementation of high order FIR filters is introduced, as well as example FPGA and CMOS realizations. This structure exploits the simplicity of coefficients that consist of two power-of-two terms to yield efficient implementations. Quantization effects are discussed and a simple block scaling method for reducing rounding and truncation noise in high order filters is also presented.
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  • 17
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    Circuits, systems and signal processing 14 (1995), S. 39-55 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract This paper describes an implementation of 2-D FIR and IIR linear digital filters via VLSI array processors. The underlying realization structures are based on the matrix decomposition approach. The 2-D concurrent processing is used in order to implement the row and column delays within the cycle time. A high degree of concurrency is achieved by exploiting the pipelining of the array processors with the inherent parallelism of the matrix decomposition structure. The resulting structures are modular, and regular, use only local communication and internal local feedback loops, and achieve high throughput and sampling rates.
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  • 18
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    Circuits, systems and signal processing 14 (1995), S. 111-134 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract This article studies dynamical systems under perturbations. We prove an Equilibrium Equivalence Theorem that guarantees that the dynamics of the system remains unchanged under perturbations with certain fairly general assumptions. We also prove that a power system is robust with respect to parameter changes in generic situations. Concepts of equilibrium equivalence and equilibrium equivalence structural stability are developed and are applied to studies of bifurcations of vector fields on noncompact manifolds. A constructive approach to equilibrium equivalence structural stability verification is emphasized. General results on structural stability of vector fields on differential manifolds are established and important applications of this theory to stability analysis are considered.
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    Circuits, systems and signal processing 14 (1995), S. 167-185 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The problem of designing a stabilizing compensator for a control system to achieve prescribed initial value constraintsυ (i)(0+)=yi is considered. Indeed, modulo certain technical conditions, such a compensator exists if and only if yi=0;i= 0,1,...,r〈p〉 +r〈t〉 −2; wherer〈p〉 is the relative degree of the plant andr〈t〉 is the relative degree of the system input. This theorem is derived and a complete parameterization of the set of compensators that achieve the prescribed design constraints is formulated.
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    Circuits, systems and signal processing 14 (1995), S. 255-278 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Wideband source location in array signal processing has received much attention in the literature lately. Methods such as the Coherent Signal Subspace (CSS) method proposed by Wang and Kaveh [12], and the signal subspace method used by Cadzow [3], are typical of the approaches used to tackle the multiple wideband source location problem. Most of these methods are variations of the narrowband high-resolution methods. Grenier [5], on the other hand, has applied the idea of time-dependent Auto-Regressive (AR) modeling [7] for a nonstationary process to the frequency domain AR modeling of the sensor outputs in a linear array and has been able to produce good results for a wideband signal. The AR coefficients in the model are expanded in a set of frequency-dependent basis functions. The choice of the basis functions was deemed immaterial and the method works even when only one snapshot of the array output is available. In this paper, we re-examine this method and present an extension of the frequency-dependent AR modeling approach to a planar array. It is shown that the use of a set of sinc functions for representing the frequency-dependent AR coefficients accurately tracks their evolution in the frequency domain, and gives superior performance compared to that when power or Legendre functions are used. We also propose two methods for smoothing the spatial spectra, from which the source locations are determined. Comparison with the CSS method are also presented.
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    Circuits, systems and signal processing 14 (1995), S. 17-38 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Centralized methods for source location using sensor arrays have computational and communication burdens that increase significantly with the number of sensors in the array. Therefore, these methods may not be usable in the applications involving very large arrays. In such applications, the data processing may need to be decentralized. This paper introduces two methods for decentralized array processing, based on the recently proposed MODE algorithm. For prescribed nonoverlapping subarrays, both methods are shown to be statistically optimal in the sense that asymptotically they provide the most accurate decentralized estimates of source location parameters. The problem of subarray selection to further optimize the estimation accuracy is only briefly addressed. The two methods are intended for different types of applications: the first should be preferred when there exist significant possibilities for local processing or for parallel computation in the central processor; otherwise the second method should be preferred. The accuracy of the two decentralized methods is compared to the centralized Cramér-Rao bound, both analytically and numerically, in order to provide indications about the loss of accuracy associated with decentralized processing.
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    Circuits, systems and signal processing 14 (1995), S. 87-110 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract We present in this paper a recursive-in-order least-squares (LS) algorithm to compute efficiently the parameters of a 2-D Gaussian Markov random field (GMRF) model. The algorithm is based on the fact that the least-squares estimation of the parameters of a 2-D noncausal GMRF model is consistent and the coefficient matrix in the normal equation has near-to-block-Toeplitz structure. Hence, it has a Levinson-like form for the updating of model parameters by introducing auxiliary variables. Moreover, this paper proposes the concept ofrecursive path for 2-D recursive-in-order algorithms, and points out that there exists a tradeoff between fast computation of the parameters and accurate choice of model support; a compromise recursive path is then suggested where the orders change alternately in two directions. The computational complexity of the developed algorithm is analyzed, and the results show that the algorithm is more efficient when either the image size or the model support is larger. It is found that the total number of multiplications (mps) involved in the new algorithm is only about 14% of that in the conventional LS method when the image size is 512 × 512 and the neighbor set of the model is a 17 × 17 window. Computer simulation results using the recursive-in-order algorithm developed in this paper and the conventional LS method are given to verify the correctness of the new algorithm.
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    Circuits, systems and signal processing 14 (1995), S. 539-553 
    ISSN: 1531-5878
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Cross terms are an inherent consequence of the second order nature of Cohen's class TFDs (Time-Frequency Distributions) [5], [6]. They are manifest in a TFD of multicomponent signals as spurious artifacts arising from interactions between the various signal components, and they can often appear at times and/or frequencies inconsistent with the underlying physical nature of the signal, causing misinterpretation [2], [3], [4]. There are many time frequency distributions that avoid the cross term effect; the best are the Choi-Williams ED (Exponential Distribution) [1] and Levin's IPS (Instantaneous Power Spectrum) [9]. In this paper we combine the cross term reducing philosophy of the ED and IPS to obtain a new TFD that most effectively reduces the cross term effect. Surprisingly, the new TFD also satisfies most desired TFD properties.
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    Circuits, systems and signal processing 14 (1995), S. 669-674 
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The stochastic likelihood function [(STO)LF] associated with the narrowband signal processing problem can be concentrated with respect to the signal covariance matrix elements and the noise power. Although this is a known fact, no clear-cut derivation of the concentrated (STO)LF appears to be available in the literature. In this short paper we provide a simple, complete proof of the concentrated (STO)LF formula.
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    Circuits, systems and signal processing 14 (1995), S. 725-734 
    ISSN: 1531-5878
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract We give bounds on the self-sustained limit cycles in fixed-poin implementations of state-variable-form digital filters having general stable system matrices. These bounds, which are period independent and which concern limit cycles due to quantization errors in physical realizations of digital filters, are given in closed form for second-order sections. A numerical comparison shows that the bounds are smaller than corresponding ones reported in the literature. This is due to our use of real similarity transformations.
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    Circuits, systems and signal processing 14 (1995), S. 787-815 
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract We consider the concept reachability for Polynomial Matrix Descriptions (PMDs); i.e., systems of the form ∑: A(ρ)β(t)=B(ρ)u(t),y(t)=C(ρ)β(t), whereρ:=d/dt the differential operator,A(ρ)=A0+A1 ρ+...+ Av ρ v εR r×r[ρ], AiεR r×r,i=0, 1,..., ν ≥ 1 with rank R A v ≤r B(ρ) =B 0+B 1ρ+...+B σρσ εR r×m[ρ], Bi εR r×m,i=0,1,...,σ ≥ 0 C(ρ)=C0+C1 ρ+...+Cσ1 ρ σ1 εR m1×r[ρ],C i εR m1×r ,i=0, 1,..., σ1 ≥ 0, β(t): (0−, ∞) →R r is the pseudostate of (∑),u(t): [0, ∞) →R m is the control input to (∑), and y(t) is the output of the system (∑). Starting from the fact that generalized state space systems, i.e., systems of the form ∑1: Ex(t)=Ax(t)+ Bu(t), y(t)=Cx(t), whereE εR r×r, rank R E 〈r, A εR r×r,B εR r×m,C εR m1×r represent a particular case of PMDs, we generalize various known results regarding the smooth and impulsive solutions of the homogeneous and the nonhomogeneous system (∑1) to the more general case of PMDs (∑). Relying on the above generalizations we develop a theory regarding the reachability of PMDs using time-domain analysis, which takes into account finite and infinite zeros of the matrix A(s)=L.[A(ρ)]. The present analysis extends in a general way many results previously known only for regular and generalized state space systems.
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    Circuits, systems and signal processing 14 (1995), S. 445-463 
    ISSN: 1531-5878
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract This paper proposes a scaling and squaring geometric series method along with the inverse-geometric series method for finding discrete-time (continuous-time) structured uncertain linear models from continuous-time (discrete-time) structured uncertain linear systems. The above methods allow the use of well-developed theorems and algorithms in the discrete-time (continuous-time) domain to indirectly solve the continuous-time (discretetime) domain problems. Moreover, these methods enhance the flexibility in modeling and control of a hybrid composite system. It has been shown that the commonly used bilinear approximation model is a specific class of the proposed geometric series model.
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    Circuits, systems and signal processing 14 (1995), S. 525-538 
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Of concern is the undistorted propagation of pulse stress waves in long thin bodies. The primary signals are arbitrarily sharp pulses projected along thex-axis. It will be seen that these pulses travel close to the speed of sound in unbounded media. Secondary pulses follow representing interactive disturbances traveling along they-axis. But the process brings out certain anomalies in circuit realizability requirements. These issues, transmission parameters, and coupling between the various signals are studied in detail. The results are supported by experiments.
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    Circuits, systems and signal processing 14 (1995), S. 187-211 
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract This paper investigates circuit Markov processes under the standpoint of duality principle.
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    Circuits, systems and signal processing 14 (1995), S. 237-254 
    ISSN: 1531-5878
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract In this paper, a high resolution technique for estimating DOAs of spatially close source signals is presented. It is observed that the array manifold over a sector of interest is rank deficient and the dimension of the array manifold space, which is the range space of the array manifold, is less than the number of sensors in the array. The true signal subspace is a subspace in the array manifold space. A novel technique is provided that searches for the signal subspace in this array manifold space. The resulting estimated signal subspace has minimum principal angles with the data signal subspace generated by eigen-decomposing the covariance matrix of the array data vector. It is proved that the proposed estimator is asymptotically consistent and the estimated signal subspace is closer to the true signal subspace than the data signal subspace formed by MUSIC. The proposed novel technique has better performance than the MUSIC algorithm. Its performance is comparable to MLE and MD-MUSIC yet it requires only one-dimensional searches and is computationally much less intense. Simulation results are presented to show the effectiveness of the proposed technique, and comparisons with MUSIC, MLE, and MD-MUSIC algorithms are also included.
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    Circuits, systems and signal processing 14 (1995), S. 351-400 
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract High-level synthesis is becoming increasingly important in the area of VLSI CAD. During the last decade, much research has been carried out in this area and some results have been achieved. This paper presents a survey on the state-of-the-art of current high-level synthesis research and applications. Besides basic knowledge, a comprehensive overview has been conducted among the various theories, methods, algorithms, and applications of the existing systems. Their advantages, limitations, and feasibilities are also described. Finally, some points on current status and future directions of high-level synthesis are addressed.
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    Circuits, systems and signal processing 14 (1995), S. 401-414 
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    Notes: Abstract This paper introduces the running kernels that yield recursive structures for time-frequency distributions (TFDs). The running kernels offer important properties not possessed by the commonly used block distribution kernels. The introduced kernels allow an invariance in computations with respect to the extent of the kernel in the time or the lag variable. However, contrary to the wide class of block kernels that satisfy the desired timefrequency (t-f) properties, most recursive (running) time-frequency distributions (RTFDs) violate the marginal and the support properties. This paper considers both the direct and the indirect types of recursion and presents examples for illustration.
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    Circuits, systems and signal processing 14 (1995), S. 465-472 
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract In this paper the dual topics of robust signal detection and robust estimation of a random variable are considered, where the data may be both dependent and nonstationary. We note that classical saddlepoint techniques for robustness do not readily apply in the dependent and/or nonstationary situation, and thus our results have application in a larger domain than what was feasible heretofore. In addition, our methods make possible the quantitative measurement of robustness and admit essentially arbitrary perturbations in an underlying joint statistical distribution away from the nominal. In particular, our methods show that the presence of dependency can result in a reduction of the robustness of the linear detector by approximately 50% and that appropriate censoring can improve this situation. We also show that, somewhat surprisingly, a weak amount of censoring can actually reduce robustness rather than increase it, even with dependent data that is “almost” independent. This calls into question the common practice, inspired by classical saddlepoint results for independent data, of employing censoring in cases where residual dependency is conceded. When applied to estimation, our work shows that for nominally Gaussian data, the conditional expectation estimator is optimal not only in terms of performance but also robustness (under appropriate performance measures), thus reinforcing the appeal of this estimator. On the other hand, for other performance measures, we also note that the conditional expectation estimator can be completely unrobust, regardless of whether the data is nominally Gaussian or not. Finally, our results establish a bound on estimator robustness.
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    Circuits, systems and signal processing 14 (1995), S. 707-724 
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Selection of test nodes is an important phase of the fault dictionary approach. It is demonstrated in this paper that the techniques used for this purpose in other approaches of analog fault diagnosis like fault analysis and fault verification are not in general suitable for the fault dictionary approach. The ambiguity set is a simple and effective concept for choosing test nodes in the context of dictionaries. These sets are formed such that each faulty condition lies in only one ambiguity set. Deviating from this thinking, overlapping ambiguity sets are proposed in this paper, giving rise to a generalized fault dictionary. These sets use information more fully and hence reduce the number of test nodes. The concept of hashing is applied in this paper for selecting test nodes. This gives a linear time algorithm (linear in the number of fault voltage specificationsf′) and it isf′ times faster than the existing methods. It is not possible to select test nodes faster than this. This technique can also be used to select test nodes by the process of elimination of nodes. This is also linear inf′ per node elimination. Even a group of nodes can be eliminated or selected within the same computation. This freedom is not possible with the existing methods.
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    Circuits, systems and signal processing 14 (1995), S. 689-705 
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    Notes: Abstract A major reason for the success of linear autoregressive (AR) modeling is that Kolmogrorov proved that every linear system could be represented by a linear AR model of infinite order. The computation of a finite order AR approximation is, of course, the practical goal. In this paper, we prove that every nonlinear system with a Volterra series expansion can be represented as a nonlinear AR model of infinite order. Our method shows how an approximation to any desired order and degree can be achieved.
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    Circuits, systems and signal processing 14 (1995), S. 213-236 
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract This paper presents an improved direct control architecture for the on-line learning control of dynamical systems using backpropagation neural networks. The proposed architecture is compared with the other direct control schemes. In this scheme the neural network interconnection strengths are updated based on the output error of the dynamical system directly, rather than using a transformed version of the error employed in other schemes. The ill effects of the controlled dynamics on the on-line updating of the network weights are moderated by including a compensating gain layer. An error feedback is introduced to improve the dynamic response of the control system. Simulation studies are performed using the nonlinear dynamics of an underwater vehicle and the promising results support the effectiveness of the proposed scheme.
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    Circuits, systems and signal processing 14 (1995), S. 299-316 
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    Notes: Abstract The analysis of initial value problems associated with singular linear dynamic systems using spectral methods is discussed. These methods, though extensively studied in the literature, can be very sensitive to numerical errors and to the presence of noisy input data. Moreover, using piecewise constant functions, like Walsh or Block-Pulse, spectral methods have been shown to be unstable with respect to incorrect initial conditions. In this paper regularization techniques are used to reduce arbitrarily the sensitivity of the spectral methods, and at the same time to stabilize them. Although the regularized spectral methods maintain the structure of the original algorithm, they can also be rewritten in recursive form, leading to the definition of a class of discrete time filters. Finally, some applicative numerical examples are discussed in order to show the effectiveness of the proposed algorithms.
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    Circuits, systems and signal processing 14 (1995), S. 415-425 
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    Notes: Abstract A complete solution is given to the problem of determining when a certain iteration will converge to the minimum of an important type of quadratic functional. It is shown that convergence occurs whenever the minimum exists, and that the iterates produced by the iteration will be unbounded for every starting point if the minimum does not exist. Applications are given concerning an adaptive filtering algorithm and nonharmonic series expansions.
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    Circuits, systems and signal processing 14 (1995), S. 587-602 
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    Notes: Abstract The least mean square (LMS) algorithm is investigated for stability when implemented with two's complement quantization. The study is restricted to algorithms with periodically varying inputs. Such inputs are common in a variety of applications, and for system identification, they can always be generated as shown with an example. It is shown that the quantized LMS algorithm is just a special case of a quantized periodically shift-varying (PSV) filter. Two different sufficient conditions are obtained for the bounded input bounded output (BIBO) stability of the PSV filter. When the filter is BIBO stable, two different bounds on the filter output are also derived. These conditions and bounds are then applied to the quantized LMS algorithm. The results are illustrated with examples.
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    Circuits, systems and signal processing 14 (1995), S. 603-614 
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    Notes: Abstract An improved version of the Fast Polynomial Transform for the computation of two-dimensional cyclic convolutions is proposed in this paper. This new version of the Fast Polynomial Transform comes from the concept of splitting product-fields and gives more than a 20% to 50% reduction in the numbers of multiplications. An average of 20% improvement on overall computation time compared with the previous approach is also achievable.
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    Circuits, systems and signal processing 14 (1995), S. 653-660 
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    Notes: Abstract An algorithm is presented for the spectrum of experimentally obtained exponentially damped sinusoidal signals. The implementation of this algorithm is straightforward and requires only simple basic mathematical operations. Applications of the algorithm are illustrated by several examples.
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    Circuits, systems and signal processing 14 (1995), S. 675-688 
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    Notes: Abstract Exact analysis of second-order sigma-delta modulators with constant input is presented. Some properties of general limit cycles are established. Sufficient conditions for the existence of a general class of limit cycles of the state vectors of the modulator are determined. The conditions interrelate the input, an initial vector, and the period of the limit cycle. Also, sufficient conditions are determined for a class of initial condition vectors to converge to a limit cycle.
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    Circuits, systems and signal processing 14 (1995), S. 735-747 
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    Notes: Abstract A weighted median filter is a nonlinear digital filter consisting of a window of length 2N + 1 and a weight vector W=(W− N,..., W0,..., WN). A root signal of a median type filter is a signal that is invariant to the filter. However, not all weighted median filters possess the convergence property. In this paper, we shall study the root structures and the convergence behavior of a subclass of weighted median filters, calledclass- 1 filters, which is symmetric in its weight vector. We shall introduce an important parameter, calledfeature value, and show that any one-dimensional unappended signal of lengthL will converge to a root signal in at most $$3\left\lceil {\frac{{L - 2}}{{2(2N + 2 - p)}}} \right\rceil$$ passes of aclass −1 filter with window width 2N + 1 and thefeature value p.
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    Circuits, systems and signal processing 14 (1995), S. 771-786 
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    Notes: Abstract A multirate modeling theory of the ARMA stochastic signals is derived from a state-space viewpoint in this work. Its application to the signal reconstruction problem for the recovery of the complete ARMA signal from its noise-corrupted, missing-sample sequence is then developed in detail. The proposed estimation-interpolation problem can be resolved by using the multirate optimal state estimation scheme of this work. Theoretically, the multirate Kalman reconstruction filters derived in this paper produce the minimum variance estimation and interpolation of the original complete, clean ARMA signal. Practically, the numerical examples show that the multirate Kalman reconstruction filters illustrate good estimation/interpolation performances, not only for synthetic ARMA sequences but also for human speech signals.
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    Circuits, systems and signal processing 14 (1995), S. 817-825 
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    Notes: Abstract The necessary and sufficient conditions for the invariance of theU-function of a linear time-invariant two-port network under lossless nonreciprocal imbedding are given in terms of the anti-Hermitian property of the immittance matrix of the imbedding network.
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    Circuits, systems and signal processing 14 (1995), S. 749-770 
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    Notes: Abstract The mean-square error (MSE) of Capon estimate of the directions-of-arrival (DOA) is established in the narrowband array processing case. An improved Capon-like DOA estimator is proposed and its MSE is studied as well. Performance comparisons between the standard and improved Capon DOA estimates, and between these two estimates and the linear prediction DOA estimate, are performed. It is concluded that the improved Capon-like method introduced in this paper provides more accurate DOA estimates in most cases.
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    Mathematics of control, signals, and systems 8 (1995), S. 1-26 
    ISSN: 1435-568X
    Keywords: Extended Kalman filter ; Stability ; Discrete-time nonlinear systems ; Frequency tracking
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    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics , Technology
    Notes: Abstract The error dynamics of the extended Kalman filter (EKF), employed as an observer for a general nonlinear, stochastic discrete time system, are analyzed. Sufficient conditions for the boundedness of the errors of the EKF are determined. An expression for the bound on the errors is given in terms of the size of the nonlinearities of the system and the error covariance matrices used in the design of the EKF. The results are applied to the design of a stable EKF frequency tracker for a signal with time-varying frequency.
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    Mathematics of control, signals, and systems 8 (1995), S. 82-95 
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    Keywords: Linear systems ; Realization theory ; Polynomial matrix representations ; Continuity of systems
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    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics , Technology
    Notes: Abstract In this paper we consider in a behavioral setting the subclass of dis-crete-time, linear, finite-dimensional systems, which can be represented by autoregressive (AR) equations. It will be shown that, with respect to the convergence of all coefficients in an AR representation, there exist continuously dependent input-state-output (i/s/o) representations, under the condition that some specified degree remains constant. This continuous i/s/o representation is given by the Fuhrmann realization.
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    Mathematics of control, signals, and systems 8 (1995), S. 257-278 
    ISSN: 1435-568X
    Keywords: Model reduction ; Rational wavelets ; Matching pursuit ; Least squares ; Infinite-dimensional systems ; Hardy-Sobolev norm
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    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics , Technology
    Notes: Abstract The model approximation of transfer functions using rational wavelets (or molecular decompositions) is considered. By using techniques from Hardy-Sobolev spaces it is shown that Hilbert space methods such as a modified matching-pursuit algorithm and least-squares technique can be employed to obtain good approximations in bothH 2 andH ∞ norms. Several theoretical results are given on rates of convergence when the methods are applied to delay systems and fractional filters.
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    Mathematics of control, signals, and systems 8 (1995), S. 299-333 
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    Keywords: Feedback classification ; Canonical forms ; Singularities ; Affine modules ; Homotopy method
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    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics , Technology
    Notes: Abstract We consider nonlinear control-affine systems with two inputs evolving on three-dimensional manifolds. We study their local classification under static state feedback. Under the assumption that the control vector fields are independent we give complete classification of generic systems. We prove that out of a “singular” smooth curve a generic control system is either structurally stable and thus equivalent to one of six canonical forms (models) or finitely determined and thus equivalent to one of two canonical forms with real parameters. Moreover, we show that at points of the “singular” curve the system is not finitely determined and we give normal forms containing functional moduli. We also study geometry of singularities, i.e., we describe surfaces, curves, and isolated points where the system admits its canonical forms.
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    Mathematics of control, signals, and systems 8 (1995), S. 352-374 
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    Keywords: Gap metric ; Time-varying systems ; Robust control
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    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics , Technology
    Notes: Abstract The connection between the time-varying gap metric and two-block problems is utilized to obtain criteria for robust stabilization of linear, discretetime, time-varying systems. In particular we give a formula for the optimal minimal angle for a stabilizable linear time-varying system and show that it has a maximally stabilizing controller.
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    Mathematics of control, signals, and systems 8 (1995), S. 375-389 
    ISSN: 1435-568X
    Keywords: Two-dimensional (2-D) nonhomogeneous random fields ; Total-order ; Wold decomposition ; Purely-indeterministic random fields ; Deterministic random fields ; Evanescent random fields
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    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics , Technology
    Notes: Abstract Imposing atotal-order on a two-dimensional (2-D) discrete random field induces an orthogonal decomposition of the random field into two components: Apurely-indeterministic field and adeterministic one. The purely-indeterministic component is shown to have a 2-D white-innovations driven moving-average representation. The 2-D deterministic random field can be perfectly predicted from the field's “past” with respect to the imposed total-order definition. The deterministic field is further orthogonally decomposed into anevanescent field, and aremote past field. The evanescent field is generated by the columnto-column innovations of the deterministic field with respect to the imposed nonsymmetrical-half-plane total-ordering definition. The presented decomposition can be obtained with respect to any nonsymmetrical-half-plane total-ordering definition, for which the nonsymmetrical-half-plane boundary line has rational slope.
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    Mathematics of control, signals, and systems 8 (1995), S. 390-402 
    ISSN: 1435-568X
    Keywords: Discrete-time linear stochastic system ; Stochastic stability ; Markov process ; Robust stability ; Bounded uncertainties
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    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics , Technology
    Notes: Abstract This paper deals with the robustness of a class of discrete-time linear systems with Markovian jumping parameters and unknown but bounded uncertainties. Assuming that the Markovian jump process (disturbance) has finite state space and that there is complete access to the system's state and its mode, we establish necessary and sufficient conditions for stochastic stability of the autonomous nominal model. We also establish sufficient conditions for robust stability for this class of uncertain systems under matching conditions and with bounded uncertainties.
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    International journal of wireless information networks 2 (1995), S. 83-90 
    ISSN: 1572-8129
    Keywords: Fading channels ; joint source-channel coding ; finite-state channels ; channels with feedback
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The effect of the presence of a feedback channel on the transmission of information was first considered by Shannon, who showed that the capacity of a memoryless channel is not increased by the existence of a feedback link even if the feedback link is noiseless. Later it was shown that the information on a feedback channel can be used to improve considerably the performance of channel coding. In this work we study the transmission of an information source through a fading channel with feedback, modeled by a finite-state channel in the Gilbert-Elliot sense. We show that by employing the feedback information in the quantizer design for this finite-state channel, one can achieve lower overall distortion compared to the case where feedback is not available. The feedback channel is used to estimate the channel state using a hidden Markov model, and a quantizer matched to the channel state is chosen based on this information.
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    International journal of wireless information networks 2 (1995), S. 91-98 
    ISSN: 1572-8129
    Keywords: Slow-frequency-hopped TDMA ; high tier system ; hard-limited receiver
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract In recent years, various access technologies have been studied for wireless access to telephone networks and for personal digital radio communications. This paper discusses the performance of a slow-frequency-hopped time-division multiple access (SFH-TDMA) technique, which can be considered as a “high-tier” extension of a low-complexity TDMA architecture optimized for low-power pedestrian applications. A hard-limited receiver is employed, and a novel weighting factor for diversity combining is developed to achieve an effect similar to that of a maximal ratio combiner. Numerical performance results for frequency-selective and non-frequency-selective fading channels over a range of fading rates were obtained through computer simulations. Results indicate that, with the proposed weighting factor for diversity combining, the degradation due to receiver hard-limiting is less than 1.5 dB in a SFH-TDMA system.
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    International journal of wireless information networks 2 (1995), S. 99-120 
    ISSN: 1572-8129
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract HIPERLAN is the new European standard for radio LANs currently being formulated by ETSI RES10 for operation at 5 GHz and 17 GHz. It will be suitable for radio replacement of wired LANs and for ad hoc networking providing a user data rate of 10–20 Mbit/s. This paper is a review of the work of ETSI RES10 on the formulation of the HIPERLAN standard. The origins of ETSI RES10 are documented. The targets set for HIPERLAN and the problems in achieving these targets are discussed. The paper reviews the technical arguments for and against the proposed solutions to these problems, concentrating on the design of the transmission scheme and the channel access mechanism. The discussions leading to significant decisions about the standard are summarized. If the FCC in the United States of America allocate the equivalent 5 GHz band to unlicensed wireless LAN systems, many of these discussions will be revisited for future standards in this band. The paper will be of interest to anyone in academia or industry wishing to be brought quickly up to date with the state of the standard in order to focus their research or development activities. The paper also gives a general insight into the technical side of the standards formulation process in ETSI.
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    International journal of wireless information networks 2 (1995), S. 149-163 
    ISSN: 1572-8129
    Keywords: DS-CDMA ; fading channels ; multiple access ; mobile packet radio networks
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The probability of packet success for asynchronous direct sequence code division multiple access (DS-CDMA) over the multipath fading channel is investigated under different fading conditions and for different cases. We studied the bit-to-bit dependence caused by the multipath fading and by the multiple access interference (MAI) originating from the chip and phase offsets between the desired and interfering signals. We investigated the effect of using the Gaussian versus the improved Gaussian process on the computation of probability of packet success as well as the effect of the bit-to-bit dependence within a packet. The improved Gaussian approximation provides more accurate values for the probability of bit error for any number of simultaneous users, while it also allows us to incorporate the effects of bit-to-bit dependence into the calculations of the probability of packet success.
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    International journal of wireless information networks 2 (1995), S. 27-39 
    ISSN: 1572-8129
    Keywords: CDMA ; acquisition ; complex sequences
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract This paper investigates the acquisition performance of code-division multiple-access communication systems using deterministic complex signature sequences. Unlike most earlier studies, performances are obtained in the presence of multiple user interference, and the results can be applied to complex as well as binary signature sequences. Two serial search acquisition schemes are considered. The first scheme assumes that there is no data in the reference user's signal until the acquisition is achieved. The second scheme allows data modulation in the reference user's signal during the acquisition process. Probability of detection and probability of false alarm of both schemes are obtained without using the Gaussian approximation. Instead, a discrete approximation of the probability density function of the MUI is used in performance evaluations. Numerical results are presented for example sets of Gold, 4-phase, and FZC sequences. They show that complex and binary sequences have comparable acquisition performances. Results also show significant degradation of acquisition performances as the number of users increases.
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    ISSN: 1572-817X
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    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Results of modelling of light propagation in 45° self-aligned total internal reflection rib waveguide mirrors on InP substrate are compared. Six laboratories participated in the comparison with the following six modelling methods: the standard fast-Fouriertransform beam propagation method (BPM), the standard finite-difference (FD) BPM using the Crank-Nicholson scheme (two laboratories), the FD-BPM with the correction for the slowly varying envelope approximation, the method of lines, the eigenmode expansion and propagation method, and a simple method based on the field overlap. All the laboratories used the effective-index method to reduce the three-dimensional problem to two dimensions. The differences among the results obtained by different methods are briefly discussed and qualitatively compared to measured values.
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    Optical and quantum electronics 27 (1995), S. 943-950 
    ISSN: 1572-817X
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    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract An extension of a beam propagation algorithm based on the method of lines is presented. Cylindrical coordinates are used to analyse taper structures. The results are compared with those using Cartesian coordinates. The numerical effort is reduced and better convergence is obtained. The field distribution and the power flow for a slab guide taper are presented.
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    Optical and quantum electronics 27 (1995), S. 951-959 
    ISSN: 1572-817X
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    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract A finite difference beam propagation method (FD-BPM) algorithm using an adaptive mesh is proposed for optimal use of computer memory and minimization of CPU times in three-dimensional longitudinally varying guiding structures. Preliminary results confirming its feasibility and the advantages of the new algorithm both in CPU and computer memory requirements are described in a 2D case using a Y-junction.
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    Notes: Abstract Efficient interface conditions (EICs) are derived for the propagation equation using the slowly varying envelope approximation for the dominant electric field component. At the interface between two different media, the two lateral second derivatives in the discretized propagation equation are adapted such that the discretized modal field equation is correct up to second order in the lateral grid spacing. Since the error term is then of the order of the lateral grid spacing, our EICs are first-order EICs. These interface conditions are compared with well-known zero-order EICs derived by Stern and Kim and Ramaswamy. It is shown that the first-order EICs yield faster convergence to the exact effective index value as the lateral grid spacing is decreased than do the zero-order EICs. It turns out that our EICs are very much like those derived by Vassallo. Using essentially the same method, he derived EICs of second and first order for the field component respectively parallel and perpendicular, to the interface. Hence the accuracy of his EICs is one order higher for the field component parallel to the interface, although it introduces an extra asymmetry in the propagation matrix.
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    Optical and quantum electronics 27 (1995), S. 977-985 
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    Notes: Abstract In short waveguides used in integrated optical circuits, the radiation field is often present in the guiding region of a waveguide as well as the guided modes. We show that this can be investigated efficiently by extracting the radiation part from the total input field and simulating its evolution along the waveguide by use of the beam propagation method. The results are presented for a single-mode waveguide and a waveguide junction excited by a Gaussian beam. For a noncentred excitation, a considerable fraction of the radiation power is still in the guiding region after propagation over several tens of micrometres, and can be coupled to guided modes past the junction.
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    Optical and quantum electronics 27 (1995), S. 987-994 
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    Notes: Abstract By transforming the infinitex-y plane onto the unit square and using two-dimensional Fourier series expansions, the modal fields and propagation constants of dielectric waveguides can be accurately determined within the scalar (weak-guidance) regime. In particular, the method gives cutoffV-values directly. By applying the technique to symmetric and asymmetric rectangular-core couplers, the second-mode cutoffV-value is determined numerically as a function of both the centre-to-centre core separation and the core aspect ratios. Approximate analytical expressions are derived from the exact numerical data which give the cutoff values to within 2.5%.
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    Optical and quantum electronics 27 (1995), S. 1015-1026 
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    Notes: Abstract The even and the odd supermodes of nonlinear directional couplers are determined by means of a variational approach; both slab and channel nonlinear couplers are considered. The validity of the solutions given by the analytical method and their stability properties are analysed through numerical integration of the governing equations. It is shown that channel nonlinear directional couplers improve the stability of the eigenmodes.
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  • 66
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    Optical and quantum electronics 27 (1995), S. 1027-1051 
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    Notes: Abstract Rate equations with temporal, spectral and spatial dependences are derived to obtain the characteristics of the laser emission of dye molecules that undergo acid-base processes. The model has been tested to reproduce the experimental results of the laser emission due to the monoprotonated species of 4-amino-7-phenyl-8H-pyrazino[2,3-c]-1,2,6-thiadiazine 2,2-dioxide in acetonitrile. Numerical solution of the rate equations is performed to predict general lasing characteristics of acid-base dye laser systems. The temporal evolution and the superficial density of output energy of both a neutral and a base species are studied as a function of the pumping energy and the acidity of the solution. Cases of equilibrium or nonequilibrium in the excited state and overlap or nonoverlap between fluorescence and absorption bands of the different species are considered. The possibility of simultaneous lasing of both species at two different frequencies is discussed.
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  • 67
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    Notes: Abstract The effective mode locking of a CW solid-state laser with a nonlinear Fabry-Pérot interferometer with a period matched to that of the main cavity period (mode locking by additional cavity, or coupled-cavity mode locking) is considered. The main features of the ultra-short pulse generation obtained on the basis of numerical simulations in the framework of the fluctuation model are interpreted using an analysis of nonlinear reflectivity of the interferometer. A correlation between parameter ranges for effective mode locking and self-consistent solutions of the system describing the CW ultra-short pulse generation allows one to explain a ‘discrete’ range character. A new mode-locking regime has been described and the influence of interferometer bistability on the mode-locking efficiency has been evaluated.
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    Optical and quantum electronics 27 (1995), S. 1069-1074 
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    Notes: Abstract The scalar local-field wave equations in helical fibres are derived and, with the aid of a special mathematical treatment, solved approximately in a local coordinate system—the Serret-Frenet frame from the Maxwell's equations. Two basic results are obtained: (1) The local modes in a single-mode helical fibre are circularly polarized. (2) The difference of the propagation constants between the two fundamental modes is 2τ, where τ is the torsion. They agree well with the known experimental measurements.
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    Optical and quantum electronics 27 (1995), S. 1081-1086 
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    Optical and quantum electronics 27 (1995), S. 1181-1191 
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    Notes: Abstract New applications such as welding of material combinations and the ability to fill opening gaps between the workpieces offer new prospects for laser beam welding processes with filler wire. To guarantee good quality, vertical distance variations between wire tip and weld pool are, above all, not permissible as this causes globular metal transfer and would accordingly result in strongly rippled, unclean welds. A process-internal signal, recorded by a sensor, helps to solve this problem. The automatic tracking of the vertical wire position is possible on-line via a controller. In this way, the running process can be optimized and a consistently good weld quality can be achieved.
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    Optical and quantum electronics 27 (1995), S. 1089-1102 
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    Notes: Abstract An overview is given of the types of lasers dominating the field of laser materials processing. The most prominent lasers in this field are the CO2 and the Nd: YAG laser. The domain of CO2 lasers is applications which demand high laser powers (up to 30 kW are available at present), whereas the domain of Nd:YAG lasers is micro-machining applications. In the kilowatt range of laser output power, the two types of lasers are in competition. New diffusion-cooled CO2 laser systems are capable of output laser powers of several kilowatts, with good beam qualities, while still being quite compact. The output power and beam quality of Nd:YAG lasers has been improved in recent years, so that Nd:YAG lasers are now an alternative to CO2 lasers even in the kilowatt range. This is especially true for applications that demand optical fibre transmission of the laser beam, which is possible with Nd:YAG laser light but not with the longerwavelength light emitted by CO2 lasers. The main problem in solid-state lasers such as Nd:YAG is the thermal lensing effect and damage due to thermal stresses. In order to reduce thermal loading, cooling has to be enhanced. Several alternative geometries have been proposed to reduce thermal loading and, by this, thermal lensing effects. There are now slab and tube geometries which allow much higher output powers than the conventionally used laser rods. A very new scheme proposes a thin slab whose cooled side is also used as one of the laser mirrors, so that thermal gradients occur mainly in the direction of the beam propagation and not perpendicular to it, as is the case in the other geometries. As well as CO2 and Nd:YAG lasers, semiconductor laser diodes are very promising for direct use of the emitted light or as pump sources for Nd:YAG and other solid-state lasers. When packaging together thousands of single laser diodes, output powers of several kilowatts can be realized. Major problems are collimation of the highly divergent laser beams and cooling of the laser diode bars.
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    Optical and quantum electronics 27 (1995), S. 639-642 
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    Optical and quantum electronics 27 (1995), S. 699-704 
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    Optical and quantum electronics 27 (1995), S. 725-734 
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    Notes: Abstract The theoretical description of excitation of anisotropic colour centres under the action of ultrashort laser pulses in a birefringent crystal is considered. Peculiarities of spaceperiodic modulation phenomena caused by nonsteadiness of excitation have been studied. It is shown that space-periodic modulation is observed under conditions of overlap of ordinary and extraordinary pulse components. Possible ways of utilizing this phenomenon in new methods of ultrashort pulse duration measurement and determination of the orientation of anisotropic quantum systems in crystals are discussed.
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    Optical and quantum electronics 27 (1995), S. 747-753 
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    Notes: Abstract A novel, highly sensitive probe consisting of a uniform region sandwiched between two linear tapers for an evanescent wave fibre-optic absorption sensor is proposed. The diameter of the uniform region is chosen to be equal to the minimum allowed diameter of the taper's output end. The first taper is used to bring the angles of the rays in the input fibre close to the critical angle of the sensing region while the second taper reconverts the angles into their initial values so that they can propagate in the output fibre. It has been shown that the sensitivity of the sensor depends on the length of the uniform region of the probe, the refractive index of the fluid and the numerical aperture of the fibre. The sensitivity increases with increase in the length of the uniform region and also with decrease in the numerical aperture of the fibre. In addition, the sensitivity of the proposed probe can be further increased by launching the selected rays into the input fibre.
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    Optical and quantum electronics 27 (1995), S. 777-783 
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    Notes: Abstract The effects produced by a birefringent rod on the beam quality factor,M infr sup2 , of a Gaussian TEM00 laser beam are studied. Analytical expressions for the beam parameters at the output plane of a rod amplifier are derived. The theory predicts that the beam quality degrades rapidly with the heat power dissipated in the rod.
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    Optical and quantum electronics 27 (1995), S. 785-798 
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    Notes: Abstract The coupling between spatial and temporal parameters of ultrashort pulsed beams propagating through a dispersive system is treated by a 6×6 matrix formalism. A typical grating-pair system which is commonly used in pulsed beam compression is analysed in detail. Numerical examples show that the spatial-temporal coupling becomes severe under some conditions.
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    Optical and quantum electronics 27 (1995), S. 828-828 
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    Optical and quantum electronics 27 (1995), S. 1-13 
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    Notes: Abstract This review discusses the physical mechanisms of absorptive and dispersive nonlinearity in amplifiers resulting from interband and intraband electron transitions, with an assessment of the relative strengths and response times of these nonlinearities. Where appropriate, the potential applications of these nonlinearities in optical networks are also indicated.
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    Optical and quantum electronics 27 (1995), S. 15-33 
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    Notes: Abstract The transverse shift in the field distribution and the correction to the propagation constant of the fundamental and symmetric cladding modes on bent finite-clad single-mode fibres and slab waveguides are evaluated from perturbation theory for effective index values extending below the cladding index. Analytical results are derived in both geometries for the step-profile that are valid within the overall limitations of the theory. However, it is found that, for the fibre geometry only, the method breaks down at certain discrete wavelengths because of degeneracies that occur between the HE12 (LP02) and TE01 (LP11) mode propagation constants.
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    Optical and quantum electronics 27 (1995), S. 43-48 
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    Notes: Abstract An experimental study of two-step Ag+/Na+ ion exchange in BK-7 glass substrates in molten solutions consisting of AgNO3/NaNO3 mixtures is presented. This paper describes a method for evaluating the measured mode index spectra and the index profile of double-diffused semiburied waveguides. They are first diffused at 320°C in 2% melt and are then converted to semiburied waveguides by a second diffusion step consisting of a short-time reimmersion in a 0.25% melt.
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    Optical and quantum electronics 27 (1995), S. 35-41 
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    Notes: Abstract Theoretical and experimental investigations are reported on spatial frequency doubling of holographic gratings, recorded with two linear orthogonal polarizations in dyesensitized dichromated gelatin. This frequency doubling is caused by photoinduced isotropic changes in the optical parameters of the material that depend on the polarization state of the irradiating light. The dependence of the diffraction efficiency on the composition, exposure and spatial frequency of the recorded gratings is investigated in detail.
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    Optical and quantum electronics 27 (1995), S. 49-57 
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    Notes: Abstract Use of the variational approximation to describe soliton propagation in a nonlinear optical coupler is shown formally to reduce the problem to one with the same form as CW operation of a coupler, albeit with a different form of nonlinearity.
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    Optical and quantum electronics 27 (1995), S. 59-61 
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    Optical and quantum electronics 27 (1995), S. 829-829 
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    Optical and quantum electronics 27 (1995), S. 887-896 
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    Notes: Abstract A new design for a single-stage filter, calledsyn thesizedgrat ing structure (SYNGRAT), is presented and investigated theoretically. It is shown that, even for weak electrooptical effects, the tuning range can be as high as 450 channels for a channel separation of 1 nm. The electrooptically synthesized grating structure gives full control of the filter characteristics in terms of channel separation, side-lobe suppression and coupling efficiency over a very large tuning range. Efficient side-lobe suppression can be used to reduce the crosstalk. This concept gives new freedom of functionality and flexibility.
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  • 87
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    Notes: Abstract The design of a double-parallel amplified recirculating optical delay line utilizing an erbium-doped fibre amplifier for a fibre optical bus is described. This structure is used for amplifying, equalizing and filtering multicarrier signals coming from optoelectronic units attached to fibre couplers. The use of this structure for a hybrid multimode-singlemode fibre bus that includes optoelectronic units for sensors is also demonstrated.
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    Optical and quantum electronics 27 (1995), S. 831-846 
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    Notes: Abstract We show how to evaluate the performance of optical systems operating in long fibre links, focusing the study on the signal propagation. A comparison between the performance of single- and multichannel systems is reported, considering conventional and soliton signals. A study on the propagation in the presence of polarization mode dispersion is also reported, showing how the Kerr nonlinearity can also compensate this degradation.
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    Optical and quantum electronics 27 (1995), S. 859-869 
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    Notes: Abstract A fast and accurate method for numerical modelling of a cross-phase-modulated (XPM) fibre laser is introduced. The method can handle variations in the laser parameters and deal with detuning and loss in the laser, and is suitable for optimizing the performance of XPM lasers. For a given modulation pulse width, it is shown in numerical examples that the quality of signal pulses depends on the repetition rate of the external modulation and the length of the modulation path.
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    Optical and quantum electronics 27 (1995), S. 871-880 
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    Notes: Abstract Nonlinear effects in erbium-doped fibre amplifiers are analysed by a numerical approach based on the combined use of the finite element method and the Runge-Kutta algorithm. In particular, the variation of the refractive index due to the perturbation in the gain coefficient is evaluated through the nonlinear Kramers-Kronig relations. For the first time the nonlinear coefficients of the polynomial expansion of the refractive index variation have been calculated. Several kinds of perturbation, produced by signal and pump intensity, by amplifier length and by dopant concentration have been considered to investigate the refractive index change behaviour. The use of the finite element method allows the local evaluation of the pump and signal intensities without introducing any simplifying hypothesis on the electromagnetic field distribution.
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    Optical and quantum electronics 27 (1995), S. 881-886 
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    Notes: Abstract A modified noise effective bandwidth definition is proposed, leading to short computation times and accurate gain and noise figure computation. Compared to a reference solution, the largest discrepancy is less than 0.5 dB.
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  • 92
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    Notes: Abstract To simplify the driving conditions as well as to improve the wavelength response of multileg Mach-Zehnder interferometers (MZI) based on multimode interference (MMI) couplers we develop a new interpretation of these devices. Used for optical switching and for wavelength filtering, they are shown to work as active or passive integrated prisms. This analogy greatly facilitates the design of such components and simplifies the understanding of their functionality. The driving conditions of optical switches based on such multileg MZI are easility studied and will be reduced to basic active phase shifts. For wavelength filtering applications using such elements, a new design of the phased waveguide array is proposed which improves the characteristics of the filters.
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  • 93
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    Notes: Abstract Light transmission through an inhomogeneous waveguide whose refractive index profile can be represented by one-dimensional hyperbolic secant function is analysed from a geometrical optics point of view. Focusing and collimation conditions are obtained and the design of a light deflector is proposed. The possibility of obtaining almost perfect collimation for an off-axis point source by polishing the exit face of the waveguide is considered.
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    Optical and quantum electronics 27 (1995), S. 921-934 
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    Notes: Abstract We have developed a semianalytical approach to study the propagation characteristics of multimode interference (MMI) waveguide structures. The effect of transverse crosssection geometry on self-imaging length and optical power throughput are investigated. We present results confirming that the nonquadratic modal dispersion encountered in stripe-loaded waveguides impairs self-imaging performance. Results indicate that the image degradation is more predominant for asymmetrically fed MMI waveguides.
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    Optical and quantum electronics 27 (1995), S. 1009-1013 
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    Notes: Abstract A scalar Rayleigh-Ritz method using rational functions as an orthonormal basis is presented. This formalism is especially useful for the calculation of the fundamental eigenmode of optical waveguides near cutoff. An application for the optimum design of the large-spot output waveguide in tapered spot-size transformers is described.
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    Optical and quantum electronics 27 (1995), S. 1053-1059 
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    Notes: Abstract The dynamic optical distortion in a KrF amplifier was measured using a Sagnac interferometer. Maximum wavefront aberration at 249 nm was λ/3 using helium buffer gas, indicating the potential for transmitted beams very close to the diffraction limit.
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    Optical and quantum electronics 27 (1995), S. 995-1007 
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    Notes: Abstract The mode expansion propagation method is a modelling technique for a large variety of waveguide components. Until now only the case with TE modes has been reported; we give details on the mode expansion propagation method for TM modes. Instead of one type of overlap integral in the TE case, a TM analysis requires two kinds of overlap integrals. The inclusion of radiation modes in this method is discussed. The modifications in the algorithm to include waveguides with gain or loss structures are also considered. As an example, the method was used for analysing a grating-assisted codirectional coupler. In particular, the radiation loss is calculated and compared with calculations when TE modes, instead of TM modes, propagate through the codirectional coupler. The radiation losses are found to be much higher for TM modes.
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    Optical and quantum electronics 27 (1995), S. 1075-1079 
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    Notes: Abstract Optical low-coherence reflectometry (OLCR) has been applied for the first time to the ovaluation of optoelectronic integrated circuits (OEICs). Benefits are demonstrated for analysis and measurement of integrated structures. In particular reflection strengths of various integrated devices and interfaces on real fibred OEICs have been quantified.
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    Optical and quantum electronics 27 (1995), S. 1087-1087 
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    Optical and quantum electronics 27 (1995), S. 1127-1147 
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    Notes: Abstract This paper describes some of the results obtained in the United Kingdom within the Eureka Eurolaser Project EU194 — Industrial Applications of High-Power CO2 Lasers. Areas selected include laser welding of aluminium alloys, in particular with respect to the tensile and formability properties of the welds produced; a new technique, involving laser beam spinning, is described for thick section cutting of mild steel using only modest laser power; a comparison of thermal cutting processes is presented in terms of the kerf widths, speed, squareness, roughness and size of heat affected zone produced; a mathematical model for the welding of thin sheet material is described. along with the results of experimental verification of the predictive capabilities of the model; and finally, laser welding of tailored blanks for application in the automotive industry is described, with examples of two particular areas where this technology might be used to advantage.
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