Non-uniform Signal Research Articles

On the Relationship Between Uniform and Recurrent Nonuniform Discrete-Time Sampling Schemes

Recurrent nonuniform discrete-time signal samples can be regarded as a combination of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> mutual delayed sequences of uniform discrete-time signal samples taken at one <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> th of the Nyquist sampling rate. This paper introduces a new alternative discrete-time analysis model of the recurrent nonuniform sampling scenario. This model can be described by the analysis part of a uniform discrete Fourier transform (DFT) modulated filterbank from which the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> uniformly distributed and down sampled frequency bands are mixed in a very specific way. This description gives a clear relationship between uniform and recurrent nonuniform discrete-time sampling schemes. A side benefit of this model is an efficient structure with which one can reconstruct uniform discrete-time Nyquist signal samples from recurrent nonuniform samples with known mutual delays between the nonuniform distributed samples. This reconstruction structure can be viewed as a natural extension of the synthesis part of an uniform DFT modulated filterbank.

Dynamic Contrast-Enhanced Magnetic Resonance Images of the Kidney

In this article, an improved method to segment the renal cortex and medulla and to eliminate the influence of kidney motion produced by respiration is proposed. On the basis of an adaptive threshold estimated from the mean gray levels of the grown regions, a region-growing algorithm is presented to produce a three-dimensional (3-D) kidney contour and to segment the renal structures. Moreover, a shell mask of kidney margin is proposed to realize a coarse matching so as to eliminate the image translation, which makes the processing simple and direct in spatial processing without any image transform computations, and a correlation computation can be implemented with great efficiency. Then, a refined matching with a mask of cortex is completed through a 3-D correlation algorithm to ensure the accurate registration of the images in different phases. Comparing with the global mask including the whole kidney, both the shell mask and the cortex mask significantly contribute to decreasing the matching errors for images with nonuniform intensity signals, which much improves the registration quality of renal MR images. In this way, the effect of the respiration motions can be eliminated so that the intensity measurement in different phases becomes accurate within the respective structures.

Specifics of Constant Envelope Digital Signals

Features of digital signals, obtained in the case where the signal sample values are taken at crossings of an original analog signal and a sinusoidal reference function, are discussed. It is shown that spectrum analysis of this kind of nonuniform digital signals, with constant envelope not depending on the signal parameters, might be performed without massive multiplication of multi-digit numbers. Observed aliasing and cross-interference effects are considered and adapting signal processing to the specific signal-dependent sampling nonuniformities is suggested, including adapted spectrum analysis and waveform reconstruction. Ill. 4, bibl. 3 (anglų k.; santraukos anglų, rusų ir lietuvių k.).

Three-dimensional Solution Structure and Conformational Plasticity of the N-terminal Scavenger Receptor Cysteine-rich Domain of Human CD5

The lymphocyte receptor CD5 influences cell activation by modifying the strength of the intracellular response initiated by antigen engagement. Regulation through CD5 involves the interaction of one or more of its three scavenger receptor cysteine-rich domains present in the extracellular region. Here, we present the 3D solution structure of a non-glycosylated double mutant of the N-terminal domain of human CD5 expressed in Escherichia coli (eCD5d1m), which has enhanced solubility compared to the non-glycosylated wild-type (eCD5d1). In common with a glycosylated form expressed in Pichia pastoris, the [15N,1H]-correlation spectra of both eCD5d1 and eCD5d1m exhibit non-uniform temperature-dependent signal intensities, indicating extensive conformational fluctuations on the micro–millisecond timescale. Although approximately one half of the signals expected for the domain are absent at 298 K, essentially complete resonance assignments and a solution structure could be obtained at 318 K. Because of the sparse nature of the experimental restraint data and the potentially important contribution of conformational exchange to the nuclear Overhauser effect peak intensity, we applied inferential structure determination to calculate the eCD5d1m structure. The inferential structure determination ensemble has similar features to that obtained by traditional simulated annealing methods, but displays superior definition and structural quality. The eCD5d1m structure is similar to other members of the scavenger receptor cysteine-rich superfamily, but the position of the lone α helix differs due to interactions with the unique N-terminal region of the domain. The availability of an experimentally tractable form of CD5d1, together with its 3D structure, provides new tools for further investigation of its function within intact CD5.

S/N Ratio of 4-Channel A/D Geological Radar Non-uniform Sampling Signals

Using the quantitative error probability density method we studied the S/N ratio of alternately sampled signals digitized by a 4-channel A/D. A complete expression for the S/N ratio of a 4-channel A/D non-uniform sampling signal was deduced. First we obtained an expression for the S/N ratio of a 1-channel A/D uniform sampling signal when the sampling frequency was equal to or greater than 2 times the frequency of the sampled signal. Based on the S/N ratio of a 2-channel A/D, alternating, non-uniform sampling signal, we analyzed the distribution of quantitative error using the quantitative error probability density method and the distribution convolution formula. From this the S/N ratio expression of a 4-channel A/D sampling signal was deduced. The simulation result shows that the deduced expression is correct.

Efficient Evaluation of BER for Arbitrary Modulation and Signaling in Fading Channels

We present a numerically efficient method to evaluate uncoded bit-error rate for arbitrary two-dimensional constellation and nonuniform signaling in fading channels. Compared to the bounding techniques, treating the problem at the same level of generality, the proposed method is shown to be more accurate (by construction) and numerically more efficient: for moderate constellation size, the complexity reduction may reach two orders of magnitude. The operation of the proposed method is illustrated with numerical examples obtained in Nakagami-m fading.

Spectral Analysis and Reconstruction for Periodic Nonuniformly Sampled Signals in Fractional Fourier Domain

Nonuniform sampling occurs in many applications due to imperfect timebase or random events. Periodic nonuniform sampling is a special type of nonuniform sampling. The properties and applications of periodic nonuniform sampling signals in traditional Fourier domain have been extensively studied, but so far none of the research papers covering the spectral analysis and reconstruction of periodic nonuniformly sampled signals in fractional Fourier domain have been published. This correspondence is to explore the spectral properties of periodic nonuniformly sampled signals associated with the fractional Fourier transform. First, based on the uniform fractional Fourier transform kernel, the general spectral representation of periodic nonuniformly sampled signals has been derived. Second, detailed analysis of periodic nonuniformly sampled chirp signals in the fractional Fourier domain have been performed. The results can be used to estimate the chirp rate and the sampling offsets. Finally, a more simple relationship between the discrete fractional spectrum of periodic nonuniformly sampled signals and the continuous spectrum has been obtained. Based on this relationship, the original continuous spectrum can be reconstructed from periodic nonuniformly sampled signals in fractional Fourier domain. In addition, the simulations are carried out to verify the correctness of the results.

Estimation of Aliasing Effects Due to Periodical Nonuniform Individual Sampling in High-&lt;emphasis&gt;Q&lt;/emphasis&gt; Switched-Capacitor Filters

The periodical nonuniform individual sampling scheme has been shown suitable for capacitance spread and total capacitor area reduction in high quality (Q) factor switched-capacitor (SC) filters. However, the use of periodical nonuniform clock signals results in additional aliasing components in the output spectrum. This paper presents a simple model to analyze the generation of such alias components and gives practical expressions to estimate their power. The results are verified through circuit simulation of a 10.7-MHz second-order SC bandpass filter in a 0.35-mum CMOS technology. Implications on the use of this technique in the design of intermediate-frequency filters are discussed

Design and Development of a New Edge Sensor for Web Guiding

Existing edge sensors use the concept of blocking/unblocking to determine web lateral position. The most commonly used sensors employ either ultrasonic or optical signals to detect the web edge position by measuring the amount of signal attenuation due to blocking/unblocking of the signal. The main drawback of this sensing method is nonuniform signal attenuation due to web material variations and opacity. The research in this paper develops a new sensor which utilizes the phenomena of light scattering from the web edge and the directional sensitivity of optical fibers to determine the web lateral position. A collimated light beam is incident on the web edge and scattered light is collected by a linear array of fibers spatially positioned above the web edge. Based on the intensity of light received by each fiber in the fiber array, lateral position of the web is determined. The theory of operation and the development of the sensor is described. Experiments are conducted with different web materials to validate the proposed sensing method. A representative sample of the results are presented and discussed

Frequency Domain Analysis of Signals With Stochastic Sampling Times

In nonuniform sampling (NUS), signal amplitudes and time stamps are delivered in pairs. Several methods to compute an approximate Fourier transform (AFT) have appeared in literature, and their posterior properties in terms of alias suppression and leakage have been addressed. In this paper, the sampling times are assumed to be generated by a stochastic process. The main result gives the prior distribution of several AFTs expressed in terms of the true Fourier transform and variants of the characteristic function of the sampling time distribution. The result extends leakage and alias suppression with bias and variance terms due to NUS. Specific sampling processes as described in literature are analyzed in detail. The results are illustrated on simulated signals, with particular focus to the implications for spectral estimation.

Modeling microphysical influences on optical turbulence in complex areas

An earlier paper showed that there is a growing need for increasingly accurate and reliable numerical models to predict optical turbulence conditions, especially in complex (nonuniform) signal propagation environments. Thus, we present a finite-difference computer model to demonstrate a viable approach for predicting the microphysical (microclimate) influences on optical turbulence intensity (Cn2) around the ARL A_LOT Facility and its surroundings (which consist of multiple building arrays and forests). Our multi-dimensional prototype model begins to address optical turbulence conditions along more complex lines-of-sight and begins to account for inhomogeneities in Cn2 brought about by horizontal changes in landscape, wind flow, temperature, and humidity. For now, the model physics represent advection, pressure gradient, eddy diffusion, and vegetation drag force processes. Simple mechanisms to predict the heat and moisture source terms have also been incorporated. Initial model results have been quite encouraging. The model code is computationally efficient and extremely flexible with regard to modifications and debugging. We anticipate that this kind of computational research will be an important vehicle for investigating Cn2 and related laser-optic propagation effects in complex areas.

Exact Evaluation of Bit- and Symbol-Error Rates for Arbitrary 2-D Modulation and Nonuniform Signaling in AWGN Channel

Exact evaluation of bit- and symbol-error rates in a 2-D constellation is a fundamental problem of digital communications, which only for particular modulations and/or bits-to-symbol mapping has closed-form solutions. Here, we propose a general, numerically efficient algorithmic method, which yields the exact results for arbitrary modulation symbols' set and arbitrary bits-to-symbol mapping, and which deals with the case of nonuniform signaling. These three conditions define any digital transmission using memoryless modulation, so the proposed method is a general tool solving all problems tackled in the literature under constraints imposed on one or more of the parameters defining the modulation or signaling type. Such an evaluation tool is of practical importance during the design of the modulation. Our analysis and numerical simulations show the advantages offered by the new method when compared with the bounding techniques

Exact Evaluation of Bit- and Symbol-Error Rates for Arbitrary Two-Dimensional Modulation and Nonuniform Signaling in AWGN Channel

Exact evaluation of the bit- and symbol-error rates in a 2-D constellation has closed solutions for particular modulations and/or bits-to-symbol mapping. To solve this fundamental problem of digital communications, we propose a general method which yields the exact results for arbitrary modulation symbols' set, arbitrary bits-to-symbol mapping, and deals with the case of nonuniform signaling. These three conditions define any digital transmission using memoryless modulation, so the proposed method is a general tool solving all problems tackled in the literature, under constraints imposed on one or more of the parameters defining the modulation or signaling type. Such an evaluation tool is of practical importance during the design of the modulation. Our analysis and numerical simulations show the advantages offered by the new method when compared with the bounding techniques.

Series/Parallel Time-Multiplexed Switched-Capacitor Filters with Programmability Based on Non-Uniform Sampling

This paper presents an improved scheme for programmable time-multiplexed (TM) switched-capacitor (SC) filters. The proposed approach uses a novel sampling technique, which eliminates the need for resolution/area tradeoffs. The programmability of each processing channel is based on the use of non-uniform clock signals with noise-shaped sampling energy. No capacitor values are modified for programming frequency response parameters and, hence, the performance of the TM SC filter is not sacrificed for programmability. Such a sampling technique not only leads to an accurate frequency response control, but also allows the design procedures and the resulting SC circuit implementation to be simplified. A test-chip including a programmable second-order TM SC filter with a multiplexing order of four, which operates in series or in parallel mode, was fabricated in conventional CMOS technology. Measurement results demonstrate the effectiveness and versatility of the proposed technique.

Tight lower bounds on the symbol error rate of uncoded nonuniform signalling over AWGN channel

New Bonferroni-type lower bounds on the word error probability of uncoded systems are developed. The new family of bounds is based on a recent Bonferroni inequality proposed by Cohen and Merhav. These novel tight bounds are developed for optimal maximum a posteriori (MAP) coherent detectors with nonuniform signalling over additive white Gaussian noise channel. The results are compared to the state-of-the-art KAT lower bounds and it is shown that the superiority of one bound to another is dependent on the signal constellation, the amount of nonuniformity of the Bernoulli source to be communicated, and the SNR range of interest. For instance, for smaller deviations from the uniform case, which are in fact more plausible, and at low SNRs, the new bounds are tighter than KAT lower bounds for all the constellations studied.

Bonferroni-type bounds for CDMA systems with nonuniform signalling

This letter develops Bonferroni-type lower and upper bounds of the bit-error-rate (BER) performance for the optimal multiuser detector of code-division multiple access (CDMA) systems with nonuniform signalling. The bounds are shown to be very tight for all signal-to-noise ratios. The bounds are also tighter than the Verdu's bounds of the maximum-likelihood (ML) multiuser detector for the case of uniform signalling.

The role of random dendrites and inhibitory pathways in retinal neuron networks

It is shown theoretically that an apparent intellectual function of direction selectivity is naturally caused by neuronal threshold dynamics in the retinal neural network with non-uniform signal delays originating in the neuron's random dendrite geometry. By addition of information pathways via inhibitory cells, the selective function is enhanced by the effect of non-isotropic shielding and temporal sharpening of neural signals. A numerical simulation of retinal neuron network demonstrates the validity of this functional improvement by addition of the inhibitory pathways.

Investigation of non-uniform airflow signal oscillation during high frequency chest compression

BackgroundHigh frequency chest compression (HFCC) is a useful and popular therapy for clearing bronchial airways of excessive or thicker mucus. Our observation of respiratory airflow of a subject during use of HFCC showed the airflow oscillation by HFCC was strongly influenced by the nonlinearity of the respiratory system. We used a computational model-based approach to analyse the respiratory airflow during use of HFCC.MethodsThe computational model, which is based on previous physiological studies and represented by an electrical circuit analogue, was used for simulation of in vivo protocol that shows the nonlinearity of the respiratory system. Besides, airflow was measured during use of HFCC. We compared the simulation results to either the measured data or the previous research, to understand and explain the observations.Results and discussionWe could observe two important phenomena during respiration pertaining to the airflow signal oscillation generated by HFCC. The amplitudes of HFCC airflow signals varied depending on spontaneous airflow signals. We used the simulation results to investigate how the nonlinearity of airway resistance, lung capacitance, and inertance of air characterized the respiratory airflow. The simulation results indicated that lung capacitance or the inertance of air is also not a factor in the non-uniformity of HFCC airflow signals. Although not perfect, our circuit analogue model allows us to effectively simulate the nonlinear characteristics of the respiratory system.ConclusionWe found that the amplitudes of HFCC airflow signals behave as a function of spontaneous airflow signals. This is due to the nonlinearity of the respiratory system, particularly variations in airway resistance.

In vivo method for correcting transmit/receive nonuniformities with phased array coils

Phased array coils are finding widespread applications in both the research and the clinical setting. However, intensity nonuniformities with such coils can reduce the potential benefits of these coils, particularly for applications such as tissue segmentation. In this work, a method is described for correcting the nonuniform signal response based on in vivo measures of both the transmission field of body coil and the reception sensitivity of phased array coils, separately. For a uniform phantom, the reception sensitivity can be calculated using both Bloch equations and transmission field maps. For a heterogeneous object such as a brain, a minimal contrast acquisition must be obtained to map the receiver nonuniformities. This transmit field/receiver sensitivity (TFRS) approach is compared with the standard methods of using the body coil to obtain a reference scan and low-pass filtering. The quantitative comparison results shows that the TFRS approach provides superior results in correcting intensity nonuniformities for a uniform phantom. This approach reduces the ratio between signal intensity SD of an image and its mean intensity from approximately 21% before correction to 13% after correction. Results are also shown demonstrating the utility of this approach in vivo with human brain images. The method is general and can be applied with most pulse sequences, any coil combination for transmission and reception, and in any anatomic region.

Error Analysis for Nonuniform Signaling Over Rayleigh Fading Channels

We investigate the error performance of a communication system where a nonuniform memoryless binary source is transmitted via Gray-mapped M-ary phase-shift keying or quadrature amplitude modulation over memoryless Rayleigh fading channels, and demodulated via optimal maximum a posteriori detection. Using recently derived upper and lower bounds on the probability of a general union of events, which are tight and can be efficiently computed, the system symbol-error (P/sub s/) and bit-error (P/sub b/) rates are evaluated for a wide range of channel conditions. Since for nonuniform signaling, Gray mapping is not necessarily optimal for minimizing P/sub s/ or P/sub b/ (as was recently shown by Takahara et al.), we also evaluate the system performance under the map obtained by Takahara et al. and compare it with a Gray-mapped system.