Continuous Frequency Response Research Articles

UWB channel modeling and simulation with continuous frequency response

Ultra-wideband (UWB) technology is a prospective technology for high-rate transmission and accurate localization in the future communication systems. State-of-art channel modeling approaches usually divide the UWB channel into several sub-band channels and model them independently. By considering frequency-dependent channel parameters, a novel analytical UWB channel model with continuous frequency response is proposed. The composite effect of all frequency components within the UWB channel on the channel impulse response (CIR) of delay domain is derived based on the continuous channel transfer function (CTF) of frequency domain. On this basis, a closed-form simulation model for UWB channels and geometry-based parameter calculation method are developed, which can guarantee the continuity of channel characteristics on the frequency domain and greatly reduce the simulation complexity. Finally, the proposed method is applied to generate UWB channel with 2 GHz bandwidth at sub-6GHz and millimeter wave (mmWave) bands, respectively. The channel measurements are also carried out to validate the proposed method. The simulated CIR and power gain are shown to be in good agreement with the measurement data. Moreover, the comparison results of power gain and Doppler power spectral density (DPSD) show that the proposed UWB channel model achieves a good balance between the simulation accuracy and efficiency.

Optimal power sharing of wind farms for frequency response

This paper presents a uniform optimal power sharing strategy to coordinate the wind turbines (WTs) in a wind farm (WF) to provide occasional and continuous frequency response (FR). The coordination of WTs is formulated as an optimisation problem that takes into account the WT dynamics and tries to reduce the long term loss of energy yield caused by the provision of FR. This is achieved by maximising the total kinetic energy of the WF over time while reducing wear and tear of WTs. The proposed optimal power sharing strategy relies on periodic communication between each WT and a WF controller. Local linear approximations are employed to predict the system behaviour and the solution of the optimisation problem is obtained using the proposed centralised and/or distributed algorithm. The distributed algorithm only requires one-way communication between the WF controller and local WTs, reducing the communication overheads. Simulation studies are carried out on a WF model to demonstrate the effectiveness of the proposed strategy. The results show the strategy enables reduction of yield loss over previous methods while avoiding over torque operation during FR provision.

High thermal stability 40 GHz electro-optic polymer modulators

As a consequence of the urgent demand for an electro-optic (EO) polymer modulator with an elevated temperature stability, we have prepared a selection of EO polymers having glass transition temperatures of up to 194°C. The measured half-wave voltage characteristics of the fabricated Mach-Zehnder interference modulators revealed an excellent thermal resistance at 105°C for approximately 2,000 hours. By utilizing traveling-wave electrodes on the modulator, the high-frequency response at 10-40 GHz was evaluated under ascending temperatures. The advantage of such high-temperature stability EO polymer modulators was clearly demonstrated by the continuous frequency response up to 130°C.

Uncovering inner detached resonance curves in coupled oscillators with nonlinearity

Detached resonance curves have been predicted in multi-degree-of-freedom nonlinear oscillators, when subject to harmonic excitation. They appear as isolated loops of solutions in the main continuous frequency response curve and their detection may thus be hidden by numerical or experimental analysis. In this paper, an analytical approach is adopted to predict their appearance. Expressions for the amplitude-frequency equations and bifurcation curves are derived for a two degree-of-freedom system with cubic stiffness nonlinearity, and the effect of the system parameters is investigated. The interest is specifically towards the occurrence of closed detached curves appearing inside the main continuous frequency response curve, which may lead to a dramatic reduction of the amplitude of the system response. Both cases of hardening and softening stiffness characteristics are considered. The analytical findings are validated by numerical analysis.

A Framework for Economic Load Frequency Control Design Using Modified Multi-objective Genetic Algorithm

This article deals with the coordination of security-constrained economic dispatch and load frequency control in an interconnected power system. The realistic and performance optimization inherent of the load frequency control (LFC) and security-constrained economic dispatch are fully considered without simplifying assumptions. For this purpose, modeling security-constrained economic dispatch as a discontinuous control action in the continuous frequency response model of a power system is well addressed. Considering conflict behavior of LFC and security-constrained economic dispatch beside the powerfulness of the multi-objective genetic algorithm (GA) to solve high-dimensional problems with conflicted objective functions makes it attractive for the automatic generation control coordination problem. The employed security-constrained economic dispatch utilizes the advantages of dynamic economic dispatch to achieve more realistic results. The GA is used to compute the decentralized control parameters and centralized generation levels of the on-line units to achieve an acceptable operating point. A significant modification in convergence speed has been performed by using LFC model properties in corporation with the genetic algorithm, so the proposed method gives considerable promise for implementation in multi-area power systems. The efficiency of the proposed algorithm and modification is demonstrated on a three control area power system.

Uplink Resource Management for Multiuser OFDM Video Transmission Systems: Analysis and Algorithm Design

We consider a multiuser OFDM system in which users want to transmit videos via a base station. The base station knows the channel state information (CSI) as well as the rate distortion (RD) information of the video streams and tries to allocate power and spectrum resources to the users according to both physical layer CSI and application layer RD information. We derive and analyze a condition for the optimal resource allocation solution in a continuous frequency response setting. The optimality condition for this cross layer optimization scenario is similar to the equal slope condition for conventional video multiplexing resource allocation. Based on our analysis, we design an iterative subcarrier assignment and power allocation algorithm for an uplink system, and provide numerical performance analysis with different numbers of users. Comparing to systems with either only physical layer or only application layer information available at the base station, our results show that the user capacity and the video PSNR performance can be increased significantly by using cross layer design. Bit-level simulations which take into account the imperfection of the video coding rate control, the variation of RD curve fitting, as well as channel errors, are presented.

Micropolar Continuous Modeling and Frequency Response Validation of a Lattice Structure

A simple approach is employed here to determine an equivalent continuum representation of a lattice type structure with flexible joints. Kinetic and strain energy expressions are written in terms of the nodal velocities and strain components of the beam members, as well as the joints stiffness values. Necessary assumptions are made to reduce the order of the strain variables while retaining the effects of the microrotations that are coupled to the primary strain terms. As a result, an equivalent one-dimensional model has been found, which takes the assumptions of a micropolar continuum into account rather than an ordinary continuum. The frequency response function of the presented model has been validated experimentally and is shown to be in good agreement with the experimental results for a planar truss with Pratt girder configuration.

Measurements of acoustic scattering from zooplankton and oceanic microstructure using a broadband echosounder

Abstract Lavery, A. C., Chu, D., and Moum, J. N. 2010. Measurements of acoustic scattering from zooplankton and oceanic microstructure using a broadband echosounder. – ICES Journal of Marine Science, 67: 379–394. In principle, measurements of high-frequency acoustic scattering from oceanic microstructure and zooplankton across a broad range of frequencies can reduce the ambiguities typically associated with the interpretation of acoustic scattering at a single frequency or a limited number of discrete narrowband frequencies. With this motivation, a high-frequency broadband scattering system has been developed for investigating zooplankton and microstructure, involving custom modifications of a commercially available system, with almost complete acoustic coverage spanning the frequency range 150–600 kHz. This frequency range spans the Rayleigh-to-geometric scattering transition for some zooplankton, as well as the diffusive roll-off in the spectrum for scattering from turbulent temperature microstructure. The system has been used to measure scattering from zooplankton and microstructure in regions of non-linear internal waves. The broadband capabilities of the system provide a continuous frequency response of the scattering over a wide frequency band, and improved range resolution and signal-to-noise ratios through pulse-compression signal-processing techniques. System specifications and calibration procedures are outlined and the system performance is assessed. The results point to the utility of high-frequency broadband scattering techniques in the detection, classification, and under certain circumstances, quantification of zooplankton and microstructure.

A Closed-Form Description for the Continuous Frequency Response of Lagrange Interpolators

In this letter, we present a closed formula for the continuous frequency response of Lagrange interpolators. The continuous frequency response is one of the most useful characterizations of an interpolation system. The proposed formula holds for arbitrary interpolation orders and enables efficient and numerically stable evaluation. Until now, several properties of Lagrange interpolators such as zero locations were described only empirically. Due the concise form of the proposed formula, these properties can now be characterized in an exact way.

CONTINUOUS TIME SYSTEM IDENTIFICATION OF NONPARAMETRIC MODELS WITH CONSTRAINTS

Although structural constraints such as model order and time delay have been incorporated in the continuous time system identification since its origin, the constraints on the estimated model parameters were rarely enforced. This paper proposes a continuous time system identification approach with constraints. It shows that by incorporating physical parameter information known a priori as hard constraints, the traditional parameter estimation schemes are modified to minimize a quadratic cost function with linear inequality constraints. Using the structure of Frequency Sampling Filters as the vehicle, the paper shows that the constraints can be readily imposed on continuous time frequency response estimation and step response estimation. In particular, a priori knowledge in both time-domain and frequency domain is utilized simultaneously as the constraints for the optimal parameter solution. A Monte-Carlo simulation study with 100 noise realization is used to demonstrate the improvement of the estimation results in terms of continuous time frequency response and continuous time step response.

Hermite Interpolation in Loop Groups and Conjugate Quadrature Filter Approximation

A classical result of Weierstrass ensures that any continuous finite length trajectory in a vector space can be uniformly approximated by one whose coordinates are trigonometric functions. We derive an analogous result for trajectories in spheres and apply it to show that a continuous frequency response of a conjugate quadrature filter can be uniformly approximated by the frequency response of a finitely supported conjugate quadrature filter. We also extend this result, so as to preserve specified roots of the frequency response, and derive an approximation result for refinable functions whose integer translates are orthonormal. Our methods utilize properties of loop groups, jets, and the Brouwer topological degree.

Measurements of the response of gas-filled micropores to ultrasound by using PVDF transducers

Techniques that use hydrophobic polycarbonate thin sheets containing randomly spaced, fairly uniform small holes, immersed in water to trap air bubbles, have been found useful in biophysical experiments. Using such sheets, the scattering from a stable array of small air-filled holes exposed to ultrasound has been investigated [D. L. Miller, J. Acoust. Soc. Am. 71, 471–476 (1982)]. A set of piezoelectric crystal transducers of high-quality factor were used during the earlier study, which allowed measurements to be made at a discrete set of frequencies. In the present work, utilization of PVDF transducers, which have flat frequency response in the range of interest, makes it possible to study the continuous frequency response of the trapped bubbles. Results of the new experiments will be presented and discussed. [Work supported by institutional funds of University of Vermont.]

Time-delay spectrometry for ultrasonic transducer characterisation

The use of the technique of time-delay spectrometry in ultrasonic transducer characterisation is described for both transmitters and receivers. The principles of the technique, the choice of experimental parameters, and methods of instrumentation are outlined. Applications for measurement of the frequency response, linearity and directionality of miniature hydrophones, for transmitted field mapping, for finite receiver diffraction correction measurement, and alignment assessments are described and illustrated. Particular features of the technique are the continuous frequency response obtained, the high signal-to-noise ratio, and the ease of comparative measurements (e.g. for use in transducer batch production).

Digitalizing Existing Continuous-Data Control Systems Via "Continuous Frequency Matching"

A method for converting existing continuous-data control systems into digital control systems is presented in this paper. The digital controller is synthesized by matching the "continuous frequency response" of the digital control system to that of the continuous-data control system with a minimum weighted mean square error. Formulas for computing the parameters of the digital controller are obtained as a result.

Basilar membrane and middle-ear vibration in guinea pig measured by capacitive probe.

Measurements of amplitude and phase were made on the basilar membrane and middle ear of guinea pigs. A subminiature capacitive probe giving wide dynamic range and linearity was used in conjunction with a slave filter to provide continuous frequency response records and a lock−in amplifier to provide high−sensitivity measurements on the basilar membrane down to 40 dB SPL. The results confirm the general features of previous measurements using various optical and Mössbauer techniques but also demonstrate that the mechanical tuning properties of the basilar membrane are equally poor at low SPLs and under excellent physiological condition of the cochlea, as indicated by the threshold of the gross cochlear action potential to clicks. The basilar membrane response conforms to the differential cochlear microphonic rather than the much sharper single cochlear nerve−fiber threshold curve. Linearity of response at the peak of the mechanical curve was observed over an 80 dB range: significant nonlinearities were observed only for frequencies on the high−frequency slope, and then only for sound levels above 110 dB SPL. The existence of an amplitude and phase plateau was confirmed. The peak basilar membrane/incus ratio averaged 29 dB independent of cutoff frequency over a range from 16−43 kHz, with an rms vibration amplitude at the peak of 500 Å at 100 dB SPL over cutoff frequencies in the 16−26−kHz range. Subject Classification: 65.20, 65.26, 65.24.

Aspects of the frequency response testing of simple sampled systems†

The difficulties associated with identifying sampled-data systems using continuous frequency response techniques are examined. It is shown how, without taking certain precautions, gain and phase measurements vary in a random manner. This measurement scatter can be eliminated if certain synchronization techniques between the sampler and the start of the measurement are incorporated. Consideration of the mode of operation of the transfer function analyser in interpretation of the measured results is vitally important. The effect of variation of the sampling period as a function of the output is also investigated.

Application of the bilinear Z-transform method to ground motion studies

Abstract A recursive relationship in the time domain is derived for the response of a single degree of freedom oscillator to an arbitrary digitized ground motion. In developing the relationship the oscillator's continuous frequency response characteristics are expressed in terms of an equivalent sampled function Z-plane representation using the bilinear Z-transform method. Oscillator response time histories and response spectra for prescribed ground motions are compared with exact solutions. Application of the presented method to instrument correction studies is discussed.

Acoustical Environment for Calibration of Microphones

AbstractAcoustical environment consisting of a duct and an anechoic box to simulate free-field conditions for measurement of the performance characteristics of microphones is described. The duct simulates free-field conditions over the frequency region of 50–800 cps., and the anechoic box over the frequency region of 500–10,000 cps.Measurements of sound pressure were made to evaluate the free-field conditions in the duct and in the anechoic box. The responses of two Western Electric 640-AA microphones were compared in the duct and anechoic box by the substitution method. The results show that the response curves agree within ±0·5 db. of the data supplied by the manufacturers. The experimental arrangement for obtaining a continuous frequency response curve is described.