Magnetostatic Surface-Wave Transducers

Magnetostatic surface-wave (MSSW) transducer theory is extended and generalized. A Fourier transform relation is established between MSSW field amplitudes and transducer spatial current distribution. Expressions are developed for the radiation resistance of periodic meander and grating transducers, spatial harmonic amplitudes, and radiation resistance for uniform and nonuniform current distribution models. An expression is given for the radiation resistance of apodized transducers. The results enable one to predict transducer frequency response for a specified weighting of transducer element width, Iength and spacing.     

Formation of a periodic magnetic field by a sequence of rings with different magnetic and electric properties

The electrodynamic analysis of the topography of magnetic fields generated in a spatially periodic structure is performed. The structure is a solenoid containing periodically spaced rings made from materials with different electric and magnetic properties. The system of equations relating the amplitudes of spatial harmonics of the magnetic field is derived and numerically analyzed. Dependences of the amplitudes of longitudinal and transverse components of the modulated magnetic field on the ring dimensions, the current frequency, and the magnetic and electric properties of the ring materials are investigated. The field amplitudes obtained during the complete electrodynamic analysis are compared with the amplitudes of a periodic magnetic field obtained in the impedance approximation.     

Eccentric spheres models of the head

Equations are derived for electric potentials (electroencephalograms) and magnetic fields (magnetoencephalograms) produced by dipolar sources in three eccentric spheres models of the head. In these models, I) the thickness of the layer representing the skull varies around the model, II) the thickness of the scalp layer varies, and III) the electrical conductivity of an eccentric spherical "bubble" in the brain region varies. Using these equations, it was found that variations in these features of the models have at most only small effects on the general spatial patterns of the electric potentials and the radial component of the magnetic fields. However, some significant effects on the amplitudes were found. The effects of the variations in the skull and scalp layer thicknesses on the field amplitudes were found to be significantly smaller than on the potential amplitudes. The effects on the field amplitudes of the variations in the bubble conductivity were found to be only somewhat smaller than on the potential amplitudes. It was also found that the effects of variations in these features of the models on source localization accuracy were significantly smaller for inverse solutions using fields than for solutions using potentials.     

Digital processing for improvement of ultrasonic abdominal images

Digital processing that increases resolution by spatial deconvolution and histogram-based amplitude mapping has been used to improve ultrasonic abdominal image quality. The processing was applied to pulse-echo ultrasound data obtained from clinical imaging instrumentation modified to permit digital recording of signals in either RF or video forms for subsequent off-line analysis. Spatial deconvolution was accomplished both along the axis and across the width of the ultrasonic beam. Axial deconvolution was carried out on RF data with a point spread function derived from the echo of a wire target. Lateral deconvolution was performed on the video envelope placed in a matrix by an inverse filter with parameters that adjust themselves to the spatial frequency content of the image being processed. Resultant image amplitudes were mapped into a hyperbolic distribution to increase image contrast for improved demonstration of low amplitudes. The combination of processing produced resolution improvements to show boundaries more sharply and contrast changes to demonstrate more detail in the images.     

Simultaneous intracochlear stimulation based on channel interaction compensation: analysis and first results

A simultaneous paradigm for electric stimulation of the acoustic nerve based on a monopolar electrode configuration and sign-correlated pulses is presented. Simultaneous pulse amplitudes are determined by taking into account parameters of spatial channel interaction. The computation of simultaneous amplitudes requires the solution of linear systems of equations in an iterative procedure. The computation amount can be reduced significantly, if the spatial impulse responses in individual electrodes can be approximated by two exponentially decaying branches with decay constants alpha toward apex and beta toward base. Generally, the associated inverse of the channel interaction matrix is tridiagonal. Preliminary vowel and consonant identification tests with four cochlear implant patients have been conducted for sequential and simultaneous processor settings. For equal overall pulse repetition rates, comparable speech perception scores were obtained, when the decay constants alpha and beta were set accordingly. Theoretically, the pulse rate of an N-channel system can be increased up to a factor of N as compared to the standard sequential paradigm, and pulses with technically reasonable phase durations can be utilized.     

A novel stochastic millimeter-wave indoor radio channel model

The paper introduces a model for the stochastic millimeter-wave indoor radio channel. This model relates the stochastic properties of the radio channel to the underlying geometry of the investigated environment. The geometric simplicity of the millimeter-wave channel allows examining fundamental deterministic properties of the wave propagation behavior in environments of predefined randomness, i.e., environments whose dimensions and properties are described by various probability distributions. The influence of the randomness on the radio channel is studied for the down-link of a wireless local area network at 60 GHz. Joint amplitudes of path lengths, angles of departure, and amplitudes, as well as spatial power densities, average power of the direct paths, and k factors are investigated.     

A Model of 60 GHz Indoor Radio Channel

A novel model of millimeter-wave (MMW) indoor radio channel is presented in this paper. The model is related the random properties of the MMW radio channel to the underlying geometry of the environment. The geometric simplicity of the MMW channel is allowed examining fundamental deterministic properties of the wave propagation behavior in environments of predefined randomness. The dimensions and properties of environments are described by various probability distributions. Stochastic influence on the radio channel is studied for the down-link of a wireless local area network at 60 GHz. Other related factors, such as amplitudes of path lengths, angles of departure, and amplitudes, as well as spatial power densities, average power of the direct paths are investigated.     

Erratum to “Retinal Vessel Centerline Extraction Using Multiscale Matched Filters, Confidence and Edge Measures”

In order to detect vessels at a variety of widths, we apply the matched filter at multiple scales (i.e., compute for multiple values and then combine the responses across scales). Unfortunately, the output amplitudes of spatial operators such as derivatives or matched filters generally decrease with increasing scale. To compensate for this effect, Lindeberg introduced gamma-normalized derivatives. We use this notion to define a gamma-normalized matched filter     

Simulation of free-electron lasers in the presence of correlatedmagnetic field errors

The authors address the numerical simulation of field errors that possess statistical correlations. The inevitable errors of actual magnetic wigglers yield a degraded performance of the free-electron laser (FEL) with respect to the performance obtained from ideally modeled magnetic wigglers. The impact of these errors has been theoretically and computationally investigated for simple error modes, in which the field errors have uniform or sinusoidal spatial extent and amplitudes that are statistically independent for each magnet pole piece. These simple models have been recently extended to include more complicated spatial structures and statistically correlated field errors in the analysis of FEL performance. Numerical simulations of the FEL are presented that verify the analytic predictions of the recently extended model     

Statistical analysis of measured impulse response functions of 2.0GHz indoor radio channels

The measurements of the impulse response of a 2.0 GHz indoor radio channel are reported. A statistical analysis of the characteristics of the amplitude of multipath components is presented. In particular, the spatial correlation of the single multipath components and the cross-correlation between the amplitudes of adjacent multipath components have been determined. Conclusions are drawn with regard to the adequacy of the wide sense uncorrelated scattering model as a consistent model for the indoor radio channel     

Multislot estimation of fast-varying space-time communication channels

In mobile communications, coherent detection requires the estimate of an increasing number of channel parameters due to the promising performance of systems that deploy multiple antennas. However, the estimate of space-time channels requires a number of training symbols that grows with the number of unknowns. To overcome this problem, we propose a subspace-based estimation method that exploits the different varying rates in the structure of the space-time channel for moving terminals. Since the channel has some fast-varying (faded amplitudes of the paths) and slowly varying (delays and directions of arrival) features, the multislot (MS) estimate is composed of two terms: the slowly varying spatial and temporal bases estimated from L consecutive slots and the fast-varying amplitudes estimated on a slot-by-slot basis. Performance analysis and simulations confirm the expected benefits of the multislot approach and demonstrate that for large L, the mean square error (MSE) on the channel estimate depends only on the number of fast-varying parameters.     

Complex-valued receiver for multiuser collaborative coding schemes

The authors describe a practical technique of receiving multiuser collaborative coded signals with complex-valued chips and spatial diversity to enable a radio receiver to simultaneously receive a number of signals of differing phases and amplitudes. This technique could provide means of achieving the theoretical benefits of collaborative coding multiple-access (CCMA) techniques on a radio channel leading to higher capacity and bandwidth efficient digital radio systems     

High-resolution magnetoneurography

Low noise dc-SQUID systems allow noninvasive measurements of magnetic fields generated by electric currents in human peripheral nerves. High-resolution magnetoneurography techniques were used to detect the magnetic fields of stimulated leg nerves with amplitudes of only 10–20 fT over the lower spine. A signal-to-noise ratio of about 10 was achieved after a special designed signal processing routine which was sufficient for an analysis of the source current, i.e. in particular the estimation of its location, strength, and spatial extent.     

Tapered beamforming for concentric ring arrays

In this paper, some conventional filtering windows are modified and applied to uniform concentric circular antenna arrays (UCCA) for spatial smoothing and sidelobe reduction. The modified windows are applied to individual rings of the array that will taper the corresponding current amplitudes. The resulted sidelobe level, beamwidth and stability for amplitude errors are discussed for the different proposed tapering windows where it shows a sidelobe reduction to about 49 dB as in the case of Binomial UCCA while the Hamming window shows the most immunity to tapered amplitude errors.     

Statistics of the scattering cross-section of a small number ofrandom scatterers

Complex radar targets are often modeled as a number of individual scattering elements randomly distributed throughout the spatial region containing the target. While it is known that as the number of scatterers grows large the distribution of the scattered signal power or intensity is asymptotically exponential, this is not true for a small number of scatterers. The authors study the statistics of measured power or intensity, and hence scattering cross section, resulting from a small number of constant amplitude scatterers each having a random phase. They derive closed-form expressions for the probability density function (pdf) of the scattered signal intensity for one, two, and three scatterers having arbitrary amplitudes. For n>3 scatterers, they derive expressions for the pdf when the individual scatterers have identical constant amplitudes and independent random phases; these expressions are Gram-Charlier type expansions with weighting functions determined by the asymptotic form of the intensity pdf for a large number of scatterers n. The Kolmogorov-Smirnov goodness-of-fit test is used to show that the series expansions are a good fit to empirical pdfs computed using Monte-Carlo simulation of targets made up of a small number of constant amplitude scatterers with random phase     

Complex images for electrostatic field computation in multilayeredmedia

A rapidly convergent algorithm to find the spatial simulated images of a point charge in multilayered media is presented. The simulated images turn out to be complex; i.e. they have complex amplitudes and are located at complex positions. Surprisingly, these complex images give the static field in multilayered media very accurately (errors ~0.1%). The examples of two- and three-layered media are examined, together with the available exact image solutions of singly or doubly infinite series     

Combining spatial filters for the classification of single-trial EEG in a finger movement task

Brain-computer interface (BCI) is to provide a communication channel that translates human intention reflected by a brain signal such as electroencephalogram (EEG) into a control signal for an output device. In recent years, the event-related desynchronization (ERD) and movement-related potentials (MRPs) are utilized as important features in motor related BCI system, and the common spatial patterns (CSP) algorithm has shown to be very useful for ERD-based classification. However, as MRPs are slow nonoscillatory EEG potential shifts, CSP is not an appropriate approach for MRPs-based classification. Here, another spatial filtering algorithm, discriminative spatial patterns (DSP), is newly introduced for better extraction of the difference in the amplitudes of MRPs, and it is integrated with CSP to extract the features from the EEG signals recorded during voluntary left versus right finger movement tasks. A support vector machines (SVM) based framework is designed as the classifier for the features. The results show that, for MRPs and ERD features, the combined spatial filters can realize the single-trial EEG classification better than anyone of DSP and CSP alone does. Thus, we propose an EEG-based BCI system with the two feature sets, one based on CSP (ERD) and the other based on DSP (MRPs), classified by SVM.     

Combining spatial and scale-space techniques for edge detection to provide a spatially adaptive wavelet-based noise filtering algorithm

New methods for detecting edges in an image using spatial and scale-space domains are proposed. A priori knowledge about geometrical characteristics of edges is used to assign a probability factor to the chance of any pixel being on an edge. An improved double thresholding technique is introduced for spatial domain filtering. Probabilities that pixels belong to a given edge are assigned based on pixel similarity across gradient amplitudes, gradient phases and edge connectivity. The scale-space approach uses dynamic range compression to allow wavelet correlation over a wider range of scales. A probabilistic formulation is used to combine the results obtained from filtering in each domain to provide a final edge probability image which has the advantages of both spatial and scale-space domain methods. Decomposing this edge probability image with the same wavelet as the original image permits the generation of adaptive filters that can recognize the characteristics of the edges in all wavelet detail and approximation images regardless of scale. These matched filters permit significant reduction in image noise without contributing to edge distortion. The spatially adaptive wavelet noise-filtering algorithm is qualitatively and quantitatively compared to a frequency domain and two wavelet based noise suppression algorithms using both natural and computer generated noisy images.     

The indoor radio propagation channel

In this tutorial survey the principles of radio propagation in indoor environments are reviewed. The channel is modeled as a linear time-varying filter at each location in the three-dimensional space, and the properties of the filter's impulse response are described. Theoretical distributions of the sequences of arrival times, amplitudes and phases are presented. Other relevant concepts such as spatial and temporal variations of the channel, large-scale path losses, mean excess delay and RMS delay spread are explored. Propagation characteristics of the indoor and outdoor channels are compared and their major differences are outlined. Previous measurement and modeling efforts are surveyed, and areas for future research are suggested     

Analysis of the properties of slow-wave structures based on coaxial resonators coupled by two arc-shaped waveguides

The dispersion and the coupling resistance of spatial harmonics of slow-wave structures formed by a chain of coaxial resonators coupled by arc-shaped waveguides are theoretically analyzed. A 3D vector boundary value problem is solved in the rigorous electromagnetic formulation with the use of projection methods. The 2D problems of excitation of a coaxial resonator are considered and amplitudes of excited waves are determined and used for solution of the 3D problem of the junction of arc-shaped waveguides and a coaxial resonator. The dispersion equation for the considered slow-wave structure is obtained. The results of calculation of the dispersion and the coupling resistance for different numbers and different configurations of coupling apertures are analyzed.