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Manuel D Ortigueira

DEE of Faculdade de Ciências e Tecnologia da UNL
Campus da FCT da UNL, Quinta da Torre, 2825 – 114 Monte da Caparica, Portugal
mdortigueira@uninova.pt
Manuel Duarte Ortigueira was born in Seia, Portugal in 1949. He received the Electrical Engineer (Telecommunications) degree in 1975, the Ph. D. degree in Electrical Engineering in 1984, and the title of “Agregado” in Electrical Engineering all from the Instituto Superior Técnico, Lisbon, Portugal. He was Assistant Professor at Instituto Superior Técnico till 2001 when he joined the Electrical Engineering Department of Faculty of Sciences and Technology of Universidade Nova de Lisboa where he his Associate Professor. He currently teaches Signals and Systems Theory and Digital Signal Processing. In 1985 he joined the Research Institute in Systems Engineering and Computer Science (INESC), in Lisbon where he was with the Digital Signal Processing and with the Signal Processing Systems Groups (where he is invited researcher). In 1997 he joined UNINOVA being with the Centre for Technology and Systems (former Centre for Intelligent Robotics). From 1988 to 1990 he was on leave at the Telecommunication School of Catalonia Polytechnic University, Barcelona, Spain, where he worked in Radar simulation and moving source detection. His scientific interests include Digital Signal Processing (Spectral Estimation, modeling, and identification), Biomedical Signal Processing (ECG and EEG), and Fractional Signal Processing (fractional derivative definitions, system modeling, and identification)

Journal articles

2008
Valério, Ortigueira, S á da Costa (2008)  Identifying a Transfer Function from a Frequency Response   ASME Journal of Computational and Nonlinear Dynamics 3:  
Abstract: In this paper, the classic Levy identication method is reviewed and reformulated using a complex representation. This new formulation is able to solve the well known bias of the classic method at low frequencies. The formulation is generic, addressing both integer order and fractional order transfer functions. A new algorithm based on a stacked matrix and its pseudo-inverse is proposed to accommodate the data over a wide range of frequencies. Several simulation results are presented, together with a real system identication. This system is the Archimedes Wave Swing, a prototype of a device to convert the energy of sea waves into electricity.
Notes:
Ortigueira, Batista (2008)  On the relation between the fractional Brownian motion and the fractional derivatives   Physics Letters A 372: 958-968  
Abstract: The definition and simulation of fractional Brownian motion is considered from the point of view of a set of coherent fractional derivative definitions. To do it, two sets of fractional derivatives are considered: a) the forward and backward and b) the central derivatives, together with two representations: generalised difference and integral. It is shown that for these derivatives the corresponding autocorrelation functions have the same representations. The obtained results are used to define a fractional noise and, from it, the fractional Brownian motion. This is studied. The simulation problem is also considered.
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Rato, Ortigueira, Batista (2008)  On the HHT, its problems, and some solutions   Mechanical Systems and Signal Processing 22: 6. 1374-1394 8  
Abstract: The Empirical Mode Decomposition (EMD) is reviewed and some questions related to its effective performance are discussed. Its interpretation in terms of AM/FM modulation is done. Solutions for its drawbacks are proposed. Numerical simulations are carried out to empirically evaluate the proposed modified EMD
Notes:
Ortigueira (2008)  On the Fractional Central Differences and Derivatives   Journal of Vibration and Control 14: 9-10. 1255-1266  
Abstract: Fractional central differences and derivatives are studied in this paper. These are generalisations to real orders of the ordinary positive even and odd integer orders differences and derivatives and also coincide with the well known Riesz potentials. A study for coherence is done by applying the definitions to functions with Fourier transform. Some properties of these derivatives are presented and some particular cases studied.
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Ortigueira (2008)  An Introduction to the Fractional Continuous-Time Linear Systems   IEEE Circuits and Systems Magazine 2008: 3. 19-26 7  
Abstract: A brief introduction to the fractional continuous-time linear systems is presented. It will be done without needing a deep study of the fractional derivatives. We will show that the computation of the impulse and step responses is very similar to the classic. The main difference lies in the substitution of the exponential by the Mittag-Leffler function. We will present also the main formulae defining the fractional derivatives.
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2007
Ortigueira, Serralheiro (2007)  Pseudo-Fractional ARMA modelling using a double Levinson recursion   IET Control Theory and Applications 1: 1. 173-178 1  
Abstract: In this paper the modeling of Fractional Linear Systems through ARMA models is addressed. To perform this study a new recursive algorithm for Impulse Response ARMA modelling is presenting. This is a general algorithm that allows the recursive construction of ARMA models from the Impulse Response sequence. This algorithm does not need an exact order specification, since it gives some insights into correct orders. It is applied to modelling Fractional Linear Systems described by fractional powers of the backward difference and the bilinear transformations. The analysis of the results leads us to propose suitable models for those systems.
Notes:
Ricardo, Ortigueira, Gerald (2007)  New Quasi-orthogonal BCH-derived Sequences for CDMA Applications   European Transactions on Telecommunications 18: 803-810  
Abstract: Based on two methods recently proposed ï­ the âRanging Criterionâ and the âGenerators Ranging Criterionâ ï­ new (quasi-orthogonal) even BCH-derived sequences are generated which are very attractive for synchronous or quasi-synchronous CDMA systems. Numerical results show that the new family of BCH-derived sequences can contain a higher number of quasi-orthogonal sequences with lower correlation values and higher processing gains than the spreading sequences typically used in the third generation of mobile communications system, UMTS or in the recent LAS-CDMA technology. It is shown that the even BCH-derived sequences are easily generated by a linear shift register generator, allowing the construction of systems with receiver structures of low complexity as compared with those of quasi-synchronous systems using low correlation zone sequences, as for instance the LAS CDMA system.
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2006
Ortigueira, Serralheiro (2006)  A New Least-Squares Approach to Differintegration Modelling   Signal Processing 86: 10. 2582-2591 10  
Abstract: In this paper a new least-squares (LS) approach is used to model the discrete-time fractional differintegrator. This approach is based on a mismatch error between the required response and the one obtained by the difference equation defining the auto-regressive, moving-average (ARMA) model. In minimizing the error power we obtain a set of suitable normal equations that allow us to obtain the ARMA parameters. This new LS is then applied to the same examples as in [1] and [11] so performance comparisons can be drawn. Simulation results show that both magnitude frequency responses are essentially identical. Concerning the modeling stability, both algorithms present similar limitations, although for different ARMA model orders.
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Ortigueira (2006)  A coherent approach to non integer order derivatives   Signal Processing 86: 10. 2505-2515 10  
Abstract: The relation showing that the Grünwald-Letnikov and generalised Cauchy derivatives are equal is presented. This establishes a bridge between two different formulations and simultaneously between the classic integer order derivatives and the fractional ones. Starting from the generalised Cauchy derivative formula, new relations are obtained, namely a regularised version that makes the concept of pseudo-function appear naturally without the need for a rejection of any infinite part. From the regularised derivative, new formulations are deduced and specialised first for the real functions and afterwards for functions with Laplace Transforms obtaining the definitions proposed by Liouville. With these tools suitable definitions of fractional linear systems are obtained.
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Ortigueira (2006)  Riesz Potentials and Inverses via Centred Derivatives   International Journal of Mathematics and Mathematical Sciences 2006:  
Abstract: Fractional centred differences and derivatives definitions are proposed generalising to real orders the existing ones valid for even and odd positive integer orders. For each one, suitable integral formulations are obtained. The computations of the involved integrals lead to new generalisations of the Cauchy integral derivative. To compute this integral, a special two straight line path was used. With this the referred integrals lead to the well known Riesz potential operators and their inverses that emerge as true fractional centred derivatives, but that can be computed through summations similar to the well known Grünwald-Letnikov derivatives
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2005
Inácio, Gerald, Ortigueira (2005)  New PN Even Balanced Sequences for spread spectrum systems   EURASIP Journal of Wireless Communication and Networking 2005: 3. 447-458  
Abstract: In this paper a new class of Pseudo Noise Even Balanced (PN-EB) binary spreading sequences is derived from existing classical odd-length families of maximum length sequences, such as those proposed by Gold, by appending or inserting one extra zero element (chip) to the original sequences. The incentive to generate large families of PN-EB spreading sequences is motivated by analysing the spreading effect of these sequences from a natural sampling point of view. From this analysis, a new definition for PG is established, from which it becomes clear that very high Processing Gains (PGs) can be achieved in band-limited Direct Sequence Spread Spectrum (DSSS) applications by using spreading sequences with zero mean, given that certain conditions regarding spectral aliasing are met. To obtain large families of even balanced (i.e., equal number of ones and zeros) sequences, two design criteria are proposed, namely the Ranging Criterion (RC) and the Generating Ranging Criterion (GRC). PN-EB sequences in the polynomial range 3â¤nâ¤6 are derived using these criteria, and it is shown that they exhibit secondary auto-correlation and cross-correlation peaks comparable to the sequences they are derived from. The methods proposed not only facilitate the generation of large numbers of new PN-EB spreading sequences required for CDMA applications, but simultaneously offer high processing gains and good despreading characteristics in multi-user SS scenarios with bandlimited noise and interference spectra. Simulation results are presented to confirm the respective claims made.
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Ortigueira, Tenreiro-Machado, S á da Costa (2005)  Which Differintegration?   IEE Proceedings Vision, Image and Signal Processing 152: 6. 848-850  
Abstract: Despite the great advances in the theory and applications of fractional calculus, some topics remain unclear making a systematic use difficult. In this paper the fractional differintegration definition problem is studied from a systems point of view. Both local (Grünwald-Letnikov) and global (convolutional) definitions are considered. It is shown that the Cauchy formulation should be adopted since it is coherent with usual practice in signal processing and control applications.
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Ortigueira (2005)  Two new integral formulae for the Beta function   International Journal of Applied Mathematics 18: 1109-116.  
Abstract: Two new formulae for representing the Beta function in integral format are presented. They enlarge the argument domains for the inclusion of negative values. The proofs are based on the Mellin Transform.
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2004
Ortigueira, Coito (2004)  From Differences to Derivatives   Fractional Calculus & Applied Analysis 7: 4. 459-471  
Abstract: A relation showing that the Grünwald-Letnikov and generalized Cauchy derivatives are equal is deduced confirming the validity of a well known conjecture. Integral representations for both direct and reverse fractional differences are presented. From these the fractional derivative is readily obtained generalizing the Cauchy integral formula.
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Ortigueira, Batista (2004)  A Fractional Linear System View of the Fractional Brownian Motion   Nonlinear Dynamics 38: 295-303  
Abstract: A definition of the fractional Brownian motion based on the fractional differintegrator characteristics is proposed and studied. It is shown that the model enjoys the usually required properties. A discrete-time version based in the backward difference and in the bilinear transformation is considered. Some results are presented.
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2003
Ortigueira (2003)  A new symmetric fractional B-spline   Signal Processing 83: 11. 2311-2318 11  
Abstract: A new definition of a symmetric fractional B-spline is presented. This generalises the usual integer order B-spline, that becomes a special case of the new one.
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Ortigueira (2003)  On the initial conditions in continuous-time fractional linear systems   Signal Processing 83: 11. 2301-2309 11  
Abstract: The initial condition problem for fractional linear system initialisation is studied in this paper. It is based on the generalised initial value theorem. The new approach involves functions belonging to the space of Laplace transformable distributions verifying the Watson-Doetsch lemma. The fractional derivatives of these functions are independent of the derivative definition. This class includes the most important functions appearing in computing the Impulse Response of continuous-time fractional linear systems.
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2002
Ortigueira, Matos, Piedade (2002)  Fractional Discrete-Time Signal Processing: Scale Conversion and Linear Prediction   Nonlinear Dynamics 29: 173-190  
Abstract: A generalisation of the linear prediction for fractional steps is reviewed, widening well-known concepts and results. This prediction is used to derive a causal interpolation algorithm. A reconstruction algorithm for the situation where averages are observed is also presented. Scale conversion of discrete-time signals is studied taking as base the fractional discrete-time system theory. Some simulation results to illustrate the behaviour of the algorithms will be presented. A new algorithm for performing the zoom transform is also described.
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2001
Ortigueira (2001)  The comb signal and its Fourier Transform   Signal Processing 81: 3. 581-592 3  
Abstract: In this paper, we study the aperiodic comb signal from the point of view of the Fourier Transform. The comb is very important in the theory of ideal sampling. The knowledge of its properties is crucial for the establishment of suitable interpolation schemes. Here, we present sufficient conditions so that the Fourier Transform of an aperiodic comb is an aperiodic comb. We use this result to propose: 1) an alternative approach to the definition of an almost periodic signal and its anharmonic Fourier series; 2) a generalisation of the Shannon-Whittakker sampling/reconstruction for the irregular sampling case. Application of this theory to Pulse Duration Modulation and Pulse Position Modulation is also presented.
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2000
Ortigueira (2000)  Introduction to Fractional Linear Systems. Part 1: Continuous-time Systems   IEE Proceedings Vision, Image and Signal Processing 147: 1. 62-70 2  
Abstract: In this paper, the class of continuous-time linear systems is enlarged with the inclusion of the fractional linear systems. These are systems described by fractional differential equations. It is shown how to compute the impulse, step, and frequency responses from the transfer function. The theory is supported by definitions of fractional derivative and integral, generalisations of the usual. An introduction to fractal signals as outputs of fractional differintegrators is also done. It is shown how to define a stationary fractal.
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Ortigueira (2000)  Introduction to Fractional Linear Systems. Part 2: Discrete-time Systems   IEE Proceedings Vision, Image and Signal Processing 147: 1. 71-78 2  
Abstract: In this paper, the class of discrete linear systems is enlarged with the inclusion of the discrete-time fractional linear systems. These are systems described by fractional difference equations and fractional frequency responses. It is shown how to compute the impulse response and transfer function. Fractal signals are introduced as output of special linear systems: fractional differaccumulators, systems that can be considered as having fractional poles or zeros. The concept of fractional differaccumulation is treated generalising the notions of fractal and â1/f noiseâ, and introducing two kinds of fractional differaccumulated stochastic processes: hyperbolic, resulting from fractional accumulation - similar to the continuous-time case - and parabolic noises resulting from fractional differencing.
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Ortigueira, Gerald, Inácio (2000)  Communicación Sobre Lineas de Suministro de Energía   Información Tecnológica 11: 6.  
Abstract: Proponemos un sistema para la comunicación sobre líneas de la energia eléctrica usando DS/BPSK con un régimen binario de 300 bits/s. La señal de ensanchado tiene una secuencia máxima de longitud 7 como código de âchipâ, repetida 30 veces para cada dígito binario del código de la información BPSK. La sincronización entre la señal del receptor y la secuéncia PN localmente generada es obtenida por un nuevo acercamiento basado en un filtro adaptado.
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1996
Ortigueira (1996)  A New Algorithm for the Factorization and Inversion of Recursively Generated Matrices   Signal Processing 52: 65-73  
Abstract: In this paper we present a recursive algorithm for factorization and inversion of matrices generated by adding dyads ( elementary or rank-one matrices ) as it happens in recursive array signal processing. The algorithm is valid for any recursively generated rectangular matrix and it has two parts: one valid for rank deficient and the other for full rank matrices. This later case is used to obtain a generalization of the Sherman-Morrison algorithm for the recursive inversion of the covariance matrix. The proposed algorithm is used also to obtain a new one to compute the inverse (pseudo-inverse) of any matrix, assumed to be formed by joining columns. In a similar way, it can be used to invert simultaneously two matrices, by considering their product.
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Ortigueira (1996)  Matrices generadas por adición de díadas (matrices de Rango 1): Propiedades y Aplicaciones   Métodos Numéricos para Calculo y Diseño en Ingieneria 12: 4. 427-445  
Abstract: Se estudian las matrices elementales de rango 1 (diadas). Para estas matrices se presentan fórmulas para su factorizacion, inversion, descomposition en valores própios y valores singulares. Estos resultados son aplicados en análisis recursivo de qualquiera que sea la matriz, siempre que se descomponga en una suma de matrices de rango 1.
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1993
Ortigueira (1993)  ARMA Realization from the Reflection Coefficient Sequence   Signal Processing 32: 3. 6  
Abstract: The problem of ARMA modeling is focused in this paper. It is shown how to compute the ARMA parameters from a finite set of consecutive reflection coefficients in a completely recursive way. The algorithm consists on simple recursions for both the AR and MA parameters and it generalizes in a unifying framework other existing AR and MA algorithms.
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1991
Ortigueira, Lagunas (1991)  Eigendecomposition vs Singular Value Decomposition in Adaptive Array Signal Processing   Signal Processing 25: 35-49  
Abstract: Two important questions in array signal processing are adressed in this paper: the data matrix vs autocorrelation matrix alternative and the recursive implementation of subspace DOA methods. The discussion of the first question is done in face of the proposed class of recursive algorithms. These new algorithms are easily implementable and have a high degree of parallelism, suitable for on-line implementations. Algorithms for recursive implementation of the eigendecomposition (ED) of the autocorrelation matrix and SVD of the data matrix are described. The ED/SVD trade-off is discussed.
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1988
Ortigueira (1988)  A three step algorithm for ARMA modeling   Signal Processing 15: 23-30 7  
Abstract: Abstract. A new ARMA estimation algorithm is proposed. It is based on a fundamental relationship which shows that the AR polynomial of an ARMA(N, M) model belongs to the linear space spanned by the forward and backward linear predictors. This relationship allows us to construct an equivalent linear system with two inputs and the same output of the ARMA system. The inputs of this new system are the forward and backward linear prediction errors. As in this case the inputs and output are known, a least-squares identification algorithm is used to obtain the parameters of the system. These parameters define three polynomials. One of them is the AR polynomial. The other two converge asymptotically to the MA polynomial and to zero. Simple recursions are available to perform such a limiting operation.
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1984

Book chapters

2008
Rato, Ortigueira, Batista (2008)  The Empirical Mode Decomposition an Useful Tool for Signal Analysis   In: New Signal Processing Research Edited by:Takumi Maeda. Nova Science Publishers, Inc  
Abstract: The Empirical Mode Decomposition is a technique to decompose a given signal into a set of elemental signals called Intrinsic Mode Functions. The Empirical Mode Decomposition is the base of the so-called âHilbert-Huang Transformâ that comprises also a Hilbert Spectral Analysis and an instantaneous frequency computation. A modified improved algorithm for the Empirical Mode Decomposition is described. Its interpretation in terms of AM/FM modulation is done. Numerical simulations are carried out to empirically evaluate the algorithm.
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2006
Ortigueira (2006)  Riesz potentials as centred fractional derivatives   In: Advances in Fractional Calculus: Theoretical Developments and Applications in Physics and Engineering Edited by:Sabatier, Agrawal, Machado. 552 Springer  
Abstract: Generalised Fractional centred differences and derivatives are studied in this chapter. These generalise to real orders the existing ones valid for even and odd positive integer orders. For each one, suitable integral formulations are presented. The limit computation inside the integrals leads to generalisations of the Cauchy derivative. Their computations using a special path lead to the well known Riesz potentials. A study for coherence is done by applying the definitions to functions with Fourier transform. The existence of inverse Riesz potentials is also studied.
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Conference papers

2008
Ortigueira, Coito (2008)  The Initial Conditions of Riemann-Liouville and Caputo Derivatives   In: 6th EUROMECH Conference ENOC 2008  
Abstract: The initial condition problem for fractional linear system initialisation is restudied in this paper. A general formulation similar to the integer order is presented. The Riemann-Liouville and Caputo initial conditions are interpreted in terms of the general scheme.
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Ortigueira, Coito (2008)  Initial conditions: what are we talking about?   In: proceedings of the 3rd FAC Workshop on Fractional Differentiation and its Applications  
Abstract: The initial condition problem for fractional linear system initialisation is restudied in this paper. A formulation generalizing the integer order approach is presented. This is based on a fractional jump formula and it does not depend of any transform. It is also valid for the time variant case. The Riemann-Liouville and Caputo initial conditions are interpreted in terms of the general scheme.
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Coito, Ortigueira (2008)  Fractional Controller Design Trough Multi-Objective Optimization   In: 8th Portuguese Conference on Automatic Control – CONTROLO’2008  
Abstract: In this paper we propose a procedure for the design of closed-loop controllers able to simultaneously satisfy multiple specifications. The procedure is based on the solution of a multi objective control problem. A particle swarm optimisation algorithm is used to minimise an objective function. A fractional controller is designed according to the propose approach.
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Ortigueira (2008)  The Fractional Quantum Derivative and the Generalised Euler-Cauchy Equation   In: 2nd Conference on Nonlinear Science and Complexity, NSC08  
Abstract: Two formulations of the quantum fractional derivative are presented here: the analogue to the common Grünwald-Letnikov derivatives and their integral formulations. The Mellin transforms of these derivative formulations will be used to solve the Euler-Cauchy differential equation.
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Ortigueira (2008)  A fractional quantum derivative   In: proceedings of the 3rd FAC Workshop on Fractional Differentiation and its Applications  
Abstract: New derivative formulations analogue to the common Grünwald-Letnikov derivatives will be presented defining the quantum fractional derivative. The Mellin transform of this derivative will be used to obtain two integral formulations similar to the usual Liouville derivatives.
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Rato, Ortigueira, Batista (2008)  The EMD and its use to identify system modes   In: proceedings of the International Workshop on New Trends in Science and Technology  
Abstract: The Empirical Mode Decomposition (EMD) is a technique to decompose a given signal into a set of elemental signals called Intrinsic Mode Functions (IMF). Each IMF is an AM/FM modulated signal, but it is not a true mode, understood as an impulse response of a second order linear system. In this paper we show how to obtain a set of mode functions from the set of IMF.
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2007
Ktonas, Golemati, Xanthopoulos, Sakkalis, Ortigueira, Tsekou, Zervakis, Paparrigopoulos, Soldatos (2007)  Potential dementia biomarkers on the time-varying microstructure of sleep EEG spindles   In: Proceedings of the 29th Conference of IEEE Engineering in Medicine and Biology Society, Sciences and Technologies for Health  
Abstract: The time-varying microstructure of sleep EEG spindles may have clinical significance in dementia studies. In this work, the sleep spindle is modeled as an AM-FM signal and parameterized in terms of six parameters, three quantifying the instantaneous envelope (IE) and three quantifying the instantaneous frequency (IF) of the spindle model. The IE and IF waveforms of sleep spindles from patients with dementia and normal controls were estimated using the time-frequency technique of Complex Demodulation (CD). Sinusoidal curve-fitting using a matching pursuit (MP) approach was applied to the IE and IF waveforms for the estimation of the six model parameters. Specific differences were found in sleep spindle instantaneous frequency dynamics between spindles from dementia subjects and spindles from controls.
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Ortigueira, Batista (2007)  A new look at the fractional Brownian motion definition   In: Proceedings of The Sixth International Conference on Multibody Systems, Nonlinear Dynamics and Control (MSNDC), ASME IDETC07  
Abstract: A reinterpretation of the classic definition of fractional Brownian motion leads to a new definition involving a fractional noise obtained as a fractional derivative of white noise. To obtain this fractional noise, two sets of fractional derivatives are considered: a) the forward and backward and b) the central derivatives. For these derivatives the autocorrelation functions of the corresponding fractional noises have the same representations. The obtained results are used to define and propose a new simulation procedure.
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Ortigueira (2007)  A Non Integer Order Quantum Derivative   In: Symposium on Applied Fractional Calculus (SAFC07)  
Abstract: An alternative to the common Grünwald-Letnikov and Liouville derivatives will be presented: the quantum fractional derivative. A Grünwald-Letnikov like formulation called will be presented and its relation with the Mellin transform used to obtain two integral formulations similar to the usual Liouville derivatives.
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Ortigueira, Valério, S á da Costa (2007)  Identifying a Transfer Function from a Frequency Response   In: Proceedings of The Sixth International Conference on Multibody Systems, Nonlinear Dynamics and Control (MSNDC), ASME IDETC07  
Abstract: In this paper, the classic Levy identication method is reviewed and reformulated using a complex representation. This new formulation is able to solve the well known bias of the classic method at low frequencies. The formulation is generic, addressing both integer order and fractional order transfer functions. A new algorithm based on a stacked matrix and its pseudo-inverse is proposed to accommodate the data over a wide range of frequencies. Several simulation results are presented, together with a real system identication. This system is the Archimedes Wave Swing, a prototype of a device to convert the energy of sea waves into electricity.
Notes:
Ortigueira, Coito (2007)  Revisiting the Initial Conditions Problem in Fractional Linear Systems   In: Symposium on Applied Fractional Calculus (SAFC07)  
Abstract: The initial condition problem for fractional linear system initialisation is restudied in this paper. It is shown that it does not depend on the derivative definition or on the use of any transform but on the structure of the system at hand. A general formulation similar to the integer order is presented.
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2006
Ortigueira (2006)  Fractional Centred Differences and Derivatives   In: Proceedings of the 2nd IFAC Workshop on Fractional Differentiation and its Applications  
Abstract: Fractional central differences and derivatives are proposed in this paper. The positive even and odd integer orders differences and derivatives are generalised to real orders, obtaining two new different types of differences and derivatives. For each type, suitable integral formulation is obtained. Their computations of the integrals lead to generalisations of the well known Riesz potentials. A study for coherence is done by applying the definitions to functions with Fourier transform.
Notes:
Ortigueira, Batista (2006)  On the fractional derivative of stationary stochastic processes   In: Proceedings of the CST2006 & ECT2006 Conferences  
Abstract: The problem of the definition of a fractional derivative suitable for stationary stochastic processes is faced in this paper. To do it two sets of fractional derivatives are considered: a) the forward and backward and b) the central derivatives. It is shown that for these derivatives the corresponding autocorrelation functions have the same representations. The obtained results are used to define a fractional noise and, from it, the fractional Brownian motion. This is studied and it is shown that the proposed formulation leads to Mandelbrotâs definition.
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Xanthopoulos, Golemati, Sakkalis, Ktonas, Ortigueira, Zervakis, Paparrigopoulos, Tsekou, Soldatos (2006)  Comparative analysis of time-frequency methods estimating the time-varying microstructure of sleep EEG spindles   In: Proceedings of the Information Technology Applications in Biomedicine  
Abstract: Parameter estimation for an assumed sleep EEG spindle model (AM-FM signal) is performed by using four time-frequency analysis methods. Results from simulated as well as from real data are presented. In simulated data, the Hilbert Transform-based method has the lowest average percentage error but produces considerable signal distortion. The Complex Demodulation and the Matching Pursuit-based methods have error rates below 10%, but the Matching Pursuit-based method produces considerable signal distortion as well. The Wavelet Transform-based method has the poorest performance. In real data, all methods produce reasonable parameter values. However, the Hilbert Transform and the Matching Pursuit-based methods may not be applicable for sleep spindles shorter than about 0.8 sec. Matching Pursuit-based curve fitting is utilized as part of the parameter estimation process.
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2005
Ortigueira (2005)  Fractional Differences Integral Representation and its use to define Fractional Differintegrations   In: Proceedings of the ENOC-2005, Fifth EUROMECH Nonlinear Dynamics Conference  
Abstract: Integral representations for both direct and reverse fractional differences are presented. From these the fractional differintegration is readily obtained generalizing the Cauchy integral formula. Its shown that the Grünwald-Letnikov and generalized Cauchy differintegrations are equal.
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Rato, Ortigueira (2005)  A Modified EMD Algorithm for Application in Biomedical Signal Processing   In: Proceedings of the International Conference on Computational Intelligence in Medicine and Healthcare, CIMED 2005  
Abstract: The Empirical Mode Decomposition (EMD) is reviewed and some questions relating to its effective performance are discussed. Algorithmic variations, including a new parabolic interpolation method are presented. Numerical simulations are carried out to empirically evaluate the modified EMD. The modified EMD is used to conduct an eeg analysis. The attained results support the modified EMD.
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Pinto, Alves, Ortigueira, Batista (2005)  ECG Wave Detector and Delineation with Wavelets   In: Proceedings of the International Conference on Computational Intelligence in Medicine and Healthcare, CIMED 2005  
Abstract: Efficient R wave detection is a crucial preprocessing step of most of the ECG signal analysis . Likewis e, ECG delineation comprising P wave and QRS complex, is required for ECG processing, namely in High- Resolution Electrocardiography (HR-ECG). We have performed R wave detection using the Mexican Hat wavelet and achieved a sensibility Se=97.77 % and a positive prediction P+=99.46%. Our data is HR-ECG and the delineation process occurs in two steps: wavelet detection and application of the Task Force (Breithardt et al.) detection rules.
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Ortigueira (2005)  A new look at the differintegration definition   In: Proceedings of the ENOC-2005, Fifth EUROMECH Nonlinear Dynamics Conference  
Abstract: Starting from the generalised Cauchy formula for the differintegration, new relations are obtained, namely a regularised version. From this, forward and backward differintegrations are deduced and specialised for the case of functions with Laplace Transform. As a by-products of the algorithm generalisations of the Euler Beta function are presented.
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Ricardo, Gerald, Ortigueira (2005)  Analysis of New Quasi-Orthogonal BCH-Derived Sequences for CDMA Applications   In: Proceedings of the DCIS'05 XX Conference on Design of Circuits and Integrated Systems  
Abstract: Based on two recently proposed methods in the literature â the âRanging Criterionâ and the âGenerators Ranging Criterionâ â new (quasi-orthogonal) even BCH-derived sequences are generated, which are very attractive for (synchronous or quasi-synchronous) CDMA applications. These sequences can be easily generated by a linear shift register generator, adding one extra zero or by a cyclically read memory table. Simulation results showed that the new family of even BCH-derived sequences can contain a higher number of sequences with lower correlation values and higher processing gains than the spreading sequences typically used in the third generation of mobile communications system, UMTS.
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2004
Ortigueira, Serralheiro (2004)  New Insights into Pseudo-Fractional ARMA Modelling   In: Proceedings of the International Conference on Computational Cybernetics, Vienna University of Technology  
Abstract: In this paper the modeling of Fractional Linear Systems through ARMA models is addressed. This study is performed by using a recursive algorithm for Impulse Response ARMA modelling leading us to propose suitable models for this problem
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Ortigueira, Tenreiro-Machado, S á da Costa (2004)  Considerations about the choice of a differintegrator   In: “,Proceedings of the 2nd International Conference on Computational Cybernetics  
Abstract: Despite the great advances in the theory and applications of fractional calculus, some topics remain unclear making difficult its use in a systematic way. This paper studies the fractional differintegration definition problem from a systems point of view. Both local (Grünwald-Letnikov) and global (convolutional) definitions are considered. It is shown that the Cauchy formulation must be adopted since it is coherent with usual practice in signal processing and control applications..
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2003
Evans, Goes, Steiger- Garção, Ortigueira, Paulino, Sousa Lopes (2003)  Low-Voltage Low-Power CMOS Analogue Circuits for Gaussian and Uniform Noise Generation   In: Proceedings of IEEE, ISCAS 2003  
Abstract: A CMOS analogue circuit for Gaussian noise generution as well as a novel circuitfor transforming Gaussian noise into uniform noise, hoth.designed/or operating with a supply voltoge o/ I . S K arepresented. Both circuits are optimizedfor U 0 . 3 5st~an - dord CMOS technology using an equation-based design methodology based on generic algorithms. Electrical simulations demonstrate that high noise amplinrdes together with reasonable hondwidths can be achieved with relatively low power dissipation. Potential applications include self-calihrution and on-chip self-testing of video-rate analogue-to-digital converters.
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Ricardo, Inácio, Gerald, Ortigueira (2003)  New BCH-Derived Sequences for CDMA Systems   In: 46th IEEE - Midwest Symposium on Circuits and Systems  
Abstract: New BCH-derived PN-EB (pseudo-noise even balanced) sequences suitable to be used in CDMA systems are presented in this paper. It is assumed a new definition for Processing Gain, which better accounts for the system performance regarding the narrow band noise rejection, and it is shown how to obtain high processing gain values, namely, by using zero mean spreading signals in channels with selective noise. The new sequences have low autocorrelation levels, exist in large numbers and can provide higher processing gain with selective noise.
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Unterweissacher, Goes, Paulino, Evans, Ortigueira (2003)  Efficient Digital Self-Calibration of Video-Rate Pipeline ADCs using White Gaussian Noise   In: Proceedings of IEEE, ISCAS 2003  
Abstract: A digital-domain self-calibration technique for video-rate pipeline A/D converters based on a white Gaussian noise input signal is presented. The implementation of the proposed algorithm requires simple digital circuitv. An application design example of the self-calibration of a IZb. 40 MUS CMOSpipeline ADC is shown to illustrate that the overall linearity of the ADC can be highly improved using this technique.
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2002
Ortigueira (2002)  A new approach to the initial value problem   In: 5th Portuguese Conference on Automatic Control  
Abstract: The initial condition problem for fractional linear system initialisation is studied in this paper. A new approach that involves functions belonging to the space of Laplace transformable distributions is presented. It is based on the generalised initial value theorem and on reinterpretations of the most common differintegration definitions.
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2001
Batista, Rodrigues, Ortigueira (2001)  Time-Frequency and Time-Scale Characterisation of the Beat-by-Beat High-Resolution Electrocardiogram   In: Proceedings of the Sixth Portuguese Conference on Biomedical Engineering  
Abstract: Time-Frequency and Time-Scale distributions of sequential ST segments the High-Resolution Electrocardiogram, are represented using both the Morlet Wavelet based scalogram and the Zhao- Atlas-Marks representation. Resolution and representation readability, concerning the Ventricular Late Potentials are compared. The beat-tobeat variation of one VLP case is examined.
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Ortigueirta, Batista (2001)  A Signal Processing View of Differintegration   In: Proceedings of the “European Control Conference, ECC’01  
Abstract: The fractional differintegration problem is treated from the Signal Processing point of view. A brief review of the Laplace transform approach to differintegration is done. The continuous-time/discrete-time system conversion is discussed and presented a Grünwald-Letnikov integration
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Ortigueira, Matos (2001)  New Results on Fractional Linear Prediction   In: European Control Conference, ECC’01  
Abstract: A generalisation of the linear prediction for fractional steps is reviewed, widening well-known concepts and results. This prediction is used to derive a causal interpolation algorithm. A reconstruction algorithm for the situation where averages are observed is also presented.
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Ortigueira, Matos, Piedade (2001)  Fractional Signal Processing: Scale Conversion   In: Proceedings of the “ECCTD '01 - European Conference on Circuit Theory and Design”  
Abstract: Scale conversion of discrete-time signals are studied taking as base the fractional discrete-time system theory. Some simulation results to illustrate the behaviour of the algorithms will be presented. A new algorithm for performing the zoom transform is also described.
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2000
Ortigueira (2000)  Introduction to Fractional Linear Systems: Interconversions   In: Proceedings of the 5th Iberoamerican Symposium on Pattern Recognition, SIARP-2000  
Abstract: Fractional linear systems are introduced as well as their Transfer Functions and Impulse Responses that are computed in a way to emphasise their main similarities and differences. Special attention is paid to the interconversions between continuous-time and discrete-time, by showing procedures and relating problems
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Ortigueira, Matos (2000)  An Introduction to the Fractional Linear Prediction   In: Proceedings of the 5th Iberoamerican Symposium on Pattern Recognition, SIARP-2000  
Abstract: A generalisation of the linear prediction for fractional steps is presented, widening well-known concepts and results. This prediction is used to derive a causal interpolation algorithm useful in several applications.
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1999
Inácio, Cercas, Gerald, Ortigueira (1999)  Performance Analysis of TCH Codes and other New PN Even Balanced (PN-EB) Codes Suitable for High Processing Gain DS-SS Systems   In: Actas da II Conferência de Telecomunicações, Conftele99  
Abstract: In this paper we will focus our special attention into the DS-SS communication systems with band limited noise channel and no aliasing, using special sequences which ones allowed to achieve a higher Processing Gain (PG). The new type of sequences are all PN even balanced sequences (PN-EB), which means they present the same number of âzeroesâ and âonesâ, and so, have zero mean. The number of available PN-EB sequences is very large, when compared with the number of TCH sequences and others with the same length. They perform a very good secondary peak level of auto-correlation function.
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Inácio, Gerald, Ortigueira (1999)  A New PN Even Balanced (PN-EB) Sequences Suitable for High Processing Gain DS-SS Systems   In: MWSCAS'99 - 42nd Midwest Symposium on Circuits and Systems  
Abstract: The design of new sequence for high Processing Gain (PG) in DSSS systems with band limited noise channel and no aliasing, is shown. The PN-EB sequences present the same number of âzeroesâ and âonesâ, and zero mean. The number available is very large. All perform very good secondary peak level of autocorrelation.
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Inácio, Gerald, Ortigueira (1999)  Design of New PN Even Balanced (PN-EB) Sequences Suitable for High Processing Gain DS-SS Systems   In: ICSPAT'99 - 10th International Conference on Signal Processing Applications & Tecnology  
Abstract: In this paper we will focus on the design of new sequences, to achieve a high Processing Gain in DSSS communication systems with band limited noise channel and no aliasing. This sequences are PN even balanced (PN-EB) sequences, which means they present the same number of âzeroesâ and âonesâ, and so, have zero mean. The number of PN-EB sequences available is very large, when compared with other sequences of the same length. All of them perform a very good secondary peak level of the autocorrelation function.
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Ortigueira, Roma, Martins, Piedade (1999)  An Archetypal Based ECG Analysis System   In: III Congreso de Usuários de Matlab, MatLab'99  
Abstract: In this paper a description of a MatLab implementation of a new ECG analysis system based on the use of archetypal analysis for the study and interpretation of ECG signals is presented. The archetypal analysis is described together with new algorithms for the study of the heart frequency variability. Some features of the system are described and some results presented.
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2007
Ortigueira, Serralheiro (2007)  Frequency estimation by zooming   www.uninova.pt/~mdortigueira [site]  
Abstract: In this paper we deal with a frequency estimation problem using a zoom transform algorithm. In many a signal processing problems, it is of paramount importance the exact determination of the frequency of a signal. Some techniques derived from the FFT, just pad the signal with enough zeros in order to better sample its Discrete-Time Fourier Transform. Our approach, although FFT-based, does not rely on such method, using instead a âzoomâ function built around the well-known sinc(.) function, resulting in an exact and deterministic method. Its analytic formulation is presented and illustrated with some simulation results over short-time based signals
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2003
Ortigueira (2003)  A Route Towards Differintegration   www.uninova.pt/~mdortigueira [site]  
Abstract: The fractional differintegration problem is treated from the Signal Processing point of view. A review of the Laplace Transform approach to differintegration is done. The initial condition problem for fractional linear system initialisation is also presented.
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