Showing papers on "Impulse response published in 2008"

Data-driven simulation and control

Abstract: Classical linear time-invariant system simulation methods are based on a transfer function, impulse response, or input/state/output representation. We present a method for computing the response of a system to a given input and initial conditions directly from a trajectory of the system, without explicitly identifying the system from the data. Similarly to the classical approach for simulation, the classical approach for control is model-based: first a model representation is derived from given data of the plant and then a control law is synthesized using the model and the control specifications. We present an approach for computing a linear quadratic tracking control signal that circumvents the identification step. The results are derived assuming exact data and the simulated response or control input is constructed off-line.

Compressed channel sensing

Abstract: Reliable wireless communications often requires accurate knowledge of the underlying multipath channel. This typically involves probing of the channel with a known training waveform and linear processing of the input probe and channel output to estimate the impulse response. Many real-world channels of practical interest tend to exhibit impulse responses characterized by a relatively small number of nonzero channel coefficients. Conventional linear channel estimation strategies, such as the least squares, are ill-suited to fully exploiting the inherent low-dimensionality of these sparse channels. In contrast, this paper proposes sparse channel estimation methods based on convex/linear programming. Quantitative error bounds for the proposed schemes are derived by adapting recent advances from the theory of compressed sensing. The bounds come within a logarithmic factor of the performance of an ideal channel estimator and reveal significant advantages of the proposed methods over the conventional channel estimation schemes.

Underwater Acoustic Communications: Design Considerations on the Physical Layer

Abstract: Acoustic propagation is characterized by three major factors: attenuation that depends on the signal frequency, multipath propagation, and low speed of sound (1500 m/s). The channel has a sparse impulse response, where each physical path acts as a time-varying low-pass filter, and motion introduces additional Doppler spreading and shifting. Because propagation is best supported at low frequencies, acoustic communication systems are inherently wideband. The way in which these facts influence the design of signal processing methods is considered for single-carrier and multi-carrier systems. Moreover, the facts that the available bandwidth and transmission power depend heavily on the distance, and that channel latency is high, bear important implications on the design of network architectures and related protocols.

Blind estimation of reverberation time based on the distribution of signal decay rates

Abstract: The reverberation time is one of the most prominent acoustic characteristics of an enclosure. Its value can be used to predict speech intelligibility, and is used by speech enhancement techniques to suppress reverberation. The reverberation time is usually obtained by analysing the decay rate of (i) the energy decay curve that is observed when a noise source is switched off, and (ii) the energy decay curve of the room impulse response. Estimating the reverberation time using only the observed reverberant speech signal, i.e., blind estimation, is required for speech evaluation and enhancement techniques. Recently, (semi) blind methods have been developed. Unfortunately, these methods are not very accurate when the source consists of a human speaker, and unnatural speech pauses are required to detect and/or track the decay. In this paper we extract and analyse the decay rate of the energy envelope blindly from the observed reverberation speech signal in the short-time Fourier transform domain. We develop a method to estimate the reverberation time using a property of the distribution of the decay rates. Experimental results using simulated and real reverberant speech signals demonstrate the performance of the new method.

Analysis of Oscillator Injection Locking Through Phase-Domain Impulse-Response

Abstract: This paper presents a novel approach to the analysis of oscillator injection locking due to weak external signals. From the intuitive concept of impulse-sensitivity function, a phase-domain macromodel is deduced which is able to capture high-order synchronization effects. Novel closed-form expressions for the synchronization regions are thus presented. The proposed phase-domain macromodel and the expressions derived for synchronization-regions are very general since they apply to any oscillator topology.

Modeling time series of ground water head fluctuations subjected to multiple stresses.

Abstract: The methods behind the predefined impulse response function in continuous time (PIRFICT) time series model are extended to cover more complex situations where multiple stresses influence ground water head fluctuations simultaneously. In comparison to autoregressive moving average (ARMA) time series models, the PIRFICT model is optimized for use on hydrologic problems. The objective of the paper is twofold. First, an approach is presented for handling multiple stresses in the model. Each stress has a specific parametric impulse response function. Appropriate impulse response functions for other stresses than precipitation are derived from analytical solutions of elementary hydrogeological problems. Furthermore, different stresses do not need to be connected in parallel in the model, as is the standard procedure in ARMA models. Second, general procedures are presented for modeling and interpretation of the results. The multiple-input PIRFICT model is applied to two real cases. In the first one, it is shown that this model can effectively decompose series of ground water head fluctuations into partial series, each representing the influence of an individual stress. The second application handles multiple observation wells. It is shown that elementary physical knowledge and the spatial coherence in the results of multiple wells in an area may be used to interpret and check the plausibility of the results. The methods presented can be used regardless of the hydrogeological setting. They are implemented in a computer package named Menyanthes (www.menyanthes.nl).

Continued fractions approximation of the impulse response of fractional-order dynamic systems

Abstract: Noninteger (fractional)-order controllers are recognised to guarantee better closed-loop performance and robustness with respect to conventional integer order controllers. But irrational transfer functions make time-domain analysis and simulation much difficult. A systematic approach is proposed here for inverting the transfer function of fractional systems of commensurate order. The approach is based on the continued fractions expansion to approximate irrational transfer functions by minimum-phase, stable, rational functions, which can be easily transformed in the time-domain. The accuracy of the designed low-order approximation is shown by simulation results.

Information Criteria for Impulse Response Function Matching Estimation of DSGE Models

Abstract: We propose new information criteria for impulse response function matching estimators (IRFMEs). These estimators yield sampling distributions of the structural parameters of dynamic stochastic general equilibrium (DSGE) models by minimizing the distance between sample and theoretical impulse responses. First, we propose an information criterion to select only the responses that produce consistent estimates of the true but unknown structural parameters: the Valid Impulse Response Selection Criterion (VIRSC). The criterion is especially useful for mis-specified models. Second, we propose a criterion to select the impulse responses that are most informative about DSGE model parameters: the Relevant Impulse Response Selection Criterion (RIRSC). These criteria can be used in combination to select the subset of valid impulse response functions with minimal dimension that yields asymptotically efficient estimators. The criteria are general enough to apply to impulse responses estimated by VARs, local projections, and simulation methods. We show that the use of our criteria significantly affects estimates and inference about key parameters of two well-known new Keynesian DSGE models. Monte Carlo evidence indicates that the criteria yield gains in terms of finite sample bias as well as offering tests statistics whose behavior is better approximated by first order asymptotic theory. Thus, our criteria improve on existing methods used to implement IRFMEs.

ACOUSTICS2008/460 Reconstruction of Rayleigh-Lamb dispersion spectrum based on noise obtained from an air-jet forcing

Abstract: The time-domain cross-correlation of incoherent and random noise recorded by a series of passive sensors contains the impulse response of the medium between these sensors. By using noise generated by a can of compressed air sprayed on the surface of a plexiglass plate, we are able to reconstruct not only the time of flight but the whole waveforms between the sensors. From the reconstruction of the direct A0 and S0 waves, we derive the dispersion curves of the flexural waves, thus estimating the mechanical properties of the material without a conventional electromechanical source. The dense array of receivers employed here allow a precise frequency-wavenumber study of flexural waves, along with a thorough evaluation of the rate of convergence of the correlation with respect to the record length, the frequency, and the distance between the receivers. The reconstruction of the actual amplitude and attenuation of the impulse response is also addressed in this paper [Larose et al, J. Acoust. Soc. Am 122(2007)].

Optimal Output Regulation for Discrete-Time Switched and Markovian Jump Linear Systems

Abstract: First, three different but related output regulation performance criteria for the linear time-varying system are defined in the discrete-time domain, namely, the peak impulse response, peak output variance, and average output variance per unit time. Then they are extended for switched linear systems and Markovian jump linear systems, and characterized by an increasing union of finite-dimensional linear matrix inequality conditions. Finally, the infinite-horizon suboptimal LQG control problem, which aims to maintain the average output variance below a given level subject to the uniform exponential stability of the closed-loop system, is solved for both switched linear systems and Markovian jump linear systems; the solution is given by a dynamic linear output feedback controller that not only perfectly observes the present mode but also recalls a finite number of past modes.

Super-Resolution Time of Arrival for Indoor Localization

Abstract: We present the results of a large number of measurements of the indoor radio channel at 5.8 GHz taken in a typical office environment. The time of arrival (TOA) for the measurements are analyzed using two recently proposed super- resolution algorithms, and compared with traditional approaches utilizing the inverse FFT. We find that the performance of the FFT is equal to or better than the super-resolution techniques. Based on a statistical analysis of our measurements, we show that the information content of the signal relating to the TOA is concentrated in just a few coefficients on the leading edge of the impulse response. Drawing on this result, we propose a pattern matching approach for the determination of the TOA which performs significantly better than existing techniques.

Calculation of impulse responses and acoustic parameters in a hall by the finite-difference time-domain method

Abstract: Impulse responses in a hall were calculated by the finite-difference time-domain (FDTD) method, and typical room acoustic parameters were obtained from the responses. The calculated parameters were compared with those actually measured in the hall. In the FDTD calculation, the impedance boundary condition was modeled by an equivalent mechanical system comprising masses, springs, and dampers. To calculate the impulse responses, the normal acoustic impedance of the interior finishing materials of the various surfaces in the hall were measured by applying the impedance-tube method, and the model of the room boundary condition was determined for the respective parts. A comparison between the calculated and measured values showed that the values of reverberation time RT, definition D50, clarity C80, and center time Ts were in good agreement in the middle-frequency bands. However, in low-frequency bands, large discrepancies were observed because of the difficulties in determining and modeling the boundary conditions.

A Data-Driven ${\cal H}_{2}$ -Optimal Control Approach for Adaptive Optics

Abstract: Adaptive optics (AO) is used in ground-based astronomical telescopes to improve the resolution by counteracting the effects of atmospheric turbulence. Most AO systems are based on a simple control law that neglects the temporal evolution of the distortions introduced by the atmosphere. This paper presents a data-driven control design approach that is able to exploit the spatio- temporal correlation in the wavefront, without assuming any form of decoupling. The approach consists of a dedicated subspace-identification algorithm to identify an atmospheric disturbance model from open-loop wavefront sensor data, followed by H2-optimal control design. It is shown that in the case that the deformable mirror and wavefront sensor dynamics can be represented by a delay and a two taps impulse response, it is possible to derive an analytical expression for the H2-optimal controller. Numerical simulations on AO test bench data demonstrate a performance improvement with respect to the common AO control approach.

A solution to the fundamental linear fractional order differential equation.

Abstract: This paper provides a solution to the fundamental linear fractional order differential equation, namely, (sub c)d(sup q, sub t) + ax(t) = bu(t). The impulse response solution is shown to be a series, named the F-function, which generalizes the normal exponential function. The F-function provides the basis for a qth order "fractional pole". Complex plane behavior is elucidated and a simple example, the inductor terminated semi- infinite lossy line, is used to demonstrate the theory.

Impulse response solution to the three-dimensional vector radiative transfer equation in atmosphere-ocean systems. I. Monte Carlo method.

Abstract: We have developed a powerful 3D Monte Carlo code, as part of the Radiance in a Dynamic Ocean (RaDyO) project, which can compute the complete effective Mueller matrix at any detector position in a completely inhomogeneous turbid medium, in particular, a coupled atmosphere-ocean system. The light source can be either passive or active. If the light source is a beam of light, the effective Mueller matrix can be viewed as the complete impulse response Green matrix for the turbid medium. The impulse response Green matrix gives us an insightful way to see how each region of a turbid medium affects every other region. The present code is validated with the multicomponent approach for a plane-parallel system and the spherical harmonic discrete ordinate method for the 3D scalar radiative transfer system. Furthermore, the impulse response relation for a box-type cloud model is studied. This 3D Monte Carlo code will be used to generate impulse response Green matrices for the atmosphere and ocean, which act as inputs to a hybrid matrix operator-Monte Carlo method. The hybrid matrix operator-Monte Carlo method will be presented in part II of this paper.

Fluctuations of correlations and Green's function reconstruction: role of scattering

Abstract: Correlations of ambient seismic or acoustic vibrations are now widely used to reconstruct the impulse response between two passive receivers as if a source was placed at one of them. This provides the opportunity to do imaging without a source, or \textsl{passive imaging}. Applications include terrestrial and solar seismology, underwater acoustics, and structural health monitoring, to cite only a few. Nevertheless, for a given set of data, correlations do not only yield the Green's function between the sensors. They also contain residual fluctuations that result from an imperfect time or source averaging that might eventually blur the images. In this article, we propose a heuristic model to describe the level of fluctuations of the correlations in the case of non-stationary wavefields, and more particularly in the case of scattering media. The work includes theoretical derivations and numerical simulations. The role of multiple scattering is quantitatively evaluated. The level of fluctuations decreases when the duration and intensity of the diffuse waves increase. The role of absorption is also discussed: absorption is properly retrieved by correlation, but the level of fluctuations is greater, thus degrading the Green's function reconstruction. Discrepancies of our simple model in the case of strong multiple scattering ($k\ell^*\leq 18$) are discussed.

Performance evaluation of left-handed delay lines for RFID backscatter applications

Abstract: The operation principle of a passive RFID tag on the basis of delay lines is presented and the performance of left- and right-handed lines for this application is compared. Additionally, limitations due to the dispersion properties of left-handed lines for the maximum number of usable cells are discussed. In conclusion, an expression relating the maximum number of usable bits for the tag impulse response to system parameters like bandwidth, centre frequency etc. is given. A proof of concept is realized with a 100 cell four bit backscatter tag. The set up of the tag is described and the performance analysed.

A Least-Squares Filter Design Technique for the Compensation of Frequency Response Mismatch Errors in Time-Interleaved A/D Converters

Abstract: This paper introduces a least-squares filter design technique for the compensation of frequency response mismatch errors in M-channel time-interleaved analog-to-digital converters The overall compensation system is designed by determining M filter impulse responses analytically through M separate matrix inversions The proposed technique offers an alternative to least-squares techniques that determine all filters simultaneously Several design examples are included for illustration

Reliability of estimating the room volume from a single room impulse response

Abstract: The methods investigated for the room volume estimation are based on geometrical acoustics, eigenmode, and diffuse field models and no data other than the room impulse response are available. The measurements include several receiver positions in a total of 12 rooms of vastly different sizes and acoustic characteristics. The limitations in identifying the pivotal specular reflections of the geometrical acoustics model in measured room impulse responses are examined both theoretically and experimentally. The eigenmode method uses the theoretical expression for the Schroeder frequency and the difficulty of accurately estimating this frequency from the varying statistics of the room transfer function is highlighted. Reliable results are only obtained with the diffuse field model and a part of the observed variance in the experimental results is explained by theoretical expressions for the standard deviation of the reverberant sound pressure and the reverberation time. The limitations due to source and receiver directivity are discussed and a simple volume estimation method based on an approximate relationship with the reverberation time is also presented.

Coherence effects in digital in-line holographic microscopy

Abstract: We analyze the effects of partial coherence in the image formation of a digital in-line holographic microscope (DIHM). The impulse response is described as a function of cross-spectral density of the light used in the space-frequency domain. Numerical simulation based on the applied model shows that a reduction in coherence of light leads to broadening of the impulse response. This is also validated by results from experiments wherein a DIHM is used to image latex beads using light with different spatial and temporal coherence.

Dispersion Limitations of Ultra-Wideband Wireless Links and Their Compensation Via Photonically Enabled Arbitrary Waveform Generation

Abstract: In this study, we present compression of ultra-wideband RF waveforms via photonic synthesis of phase pre-compensated waveforms. By exciting a dispersive wireless link (employing Archimedean spiral antennas) with variable-bandwidth excitation waveforms, we first demonstrate that such links exhibit a dispersion-limited output pulse duration. Subsequently, we utilize the RF spectral phase extracted from the impulse response of the link to create signals designed to negate the nonuniform phase response of the spiral antennas utilized in the link. Such waveforms remove the dispersion limitation and enable bandwidth-limited operation. We achieve compression to within a factor of two of the bandwidth limit for signals with bandwidths of 1-10 GHz at a center frequency of ~6 GHz. To our knowledge, this represents the first demonstration of dispersion pre-compensation for signals with fractional bandwidths exceeding 100%. Our technique is reprogrammable and may be extended to larger bandwidths and higher operational frequencies, making it an enabler for future radar and communication systems.

Impulse Response Time Measurements in Hg0.7Cd0.3Te MWIR Avalanche Photodiodes

Abstract: The response time of front-sided illuminated n-on-p Hg0.7Cd0.3Te electron avalanche photodiodes (e-APDs) at T = 77 K was studied using impulse response measurements at λ = 1.55 μm. We measured typical rise and fall times of 50 ps and 800 ps, respectively, at gains of M ≈ 100, and a record gain-bandwidth (GBW) product of GBW = 1.1 THz at M = 2800. Experiments as a function of the collection width have shown that the fall time is strongly limited by diffusion. Variable-gain measurements showed that the impulse response is first-order sensitive to the level of the output amplitude. Only a slight increase in the rise time and the fall time was observed with the gain at constant output amplitude, which is consistent with a strongly dominant electron multiplication. Comparisons of the experimental results with Silvaco finite element simulations confirmed the diffusion limitation of the response time and allowed the illustration of the transit time and RC effects.

Fluctuations of correlations and Green's function reconstruction : Role of scattering

Abstract: Correlations of ambient seismic or acoustic vibrations are now widely used to reconstruct the impulse response between two passive receivers as if a source was placed at one of them. This provides the opportunity to do imaging without a source, or passive imaging. Applications include terrestrial and solar seismology, underwater acoustics, and structural health monitoring, to cite only a few. Nevertheless, for a given set of data, correlations do not only yield Green’s function between the sensors. They also contain residual fluctuations that result from an imperfect time or source averaging that might eventually blur the images. In this article, we propose a heuristic model to describe the level of fluctuations of the correlations in the case of nonstationary wavefields, and more particularly in the case of scattering media. The work includes theoretical derivations and numerical simulations. The role of multiple scattering is quantitatively evaluated. The level of fluctuations decreases when the duratio...

An improved mu-law proportionate NLMS algorithm

Abstract: In this paper, we propose an algorithm to improve the performance of the MU-LAW PNLMS algorithm (MPNLMS) for non-sparse impulse responses. Although the existing MPNLMS algorithm was recently proposed to achieve optimal proportionate step size for both large and small tap weights, it converges even slower than conventional NLMS algorithm for dispersive channels. The proposed approach adaptively estimates the sparsity of the impulse response to be identified. Then the estimation of this sparsity is incorporated into the IPNLMS algorithm to accordingly adjust its parameters. Simulation results verify the effectiveness of the proposed algorithm.

Double-talk robust VSS-NLMS algorithm for under-modeling acoustic echo cancellation

Abstract: Most of the adaptive algorithms used for acoustic echo cancellation (AEC) are designed assuming an exact modeling scenario (i.e., the acoustic echo path and the adaptive filter have the same length) and a single-talk context (i.e., the near-end speech is absent). In real-world AEC applications, the adaptive filter works most likely in an under-modeling situation, i.e., its length is smaller than the length of the acoustic impulse response, so that the under-modeling noise is present. Also, the double-talk case is almost inherent, so that a double-talk detector (DTD) is usually involved. Both aspects influence and limit the algorithm's performance. Taking into account these two practical issues, a double-talk robust variable step size normalized least-mean- square (VSS-NLMS) algorithm is proposed in this paper. This algorithm is nonparametric in the sense that it does not require any information about the acoustic environment, so that it is robust and easy to control in practice.

Distant talking robust speech recognition using late reflection components of room impulse response

Abstract: We propose a robust and fast dereverberation technique for real-time speech recognition application. First, we effectively identify the late reflection components of the room impulse response. We use this information together with the concept of spectral subtraction (SS) to remove the late reflection components of the reverberant signal. In the absence of the clean speech in actual scenario, approximation is carried out in estimating the late reflection where the estimation error is corrected through multi-band SS. The multi-band coefficients are optimized during offline training and used in the actual online dereverberation. The proposed method performs better and faster than the relevant approach using multi-LPC and reverberant matched model. Moreover the proposed method is robust to speaker and microphone locations.

A new kernel-based approach for system identification

Abstract: We propose a new-kernel based approach for linear system identification The impulse response is modeled as realization of a Gaussian process which includes information on smoothness and BIBO-stability The corresponding minimum- variance estimate belongs to a Reproducing kernel Hilbert space which is given a spectral characterization and shown to be dense in the space of continuous functions The approach may prove particularly useful in order to obtain reduced order models and assess the corresponding bias error in the context of robust identification Several benchmarks taken from the literature demonstrate the effectiveness of the proposed approach