Showing papers by "Xidian University published in 1997"

Dielectric behavior of lead magnesium niobate relaxors

Abstract: The dielectric behavior of a solid solution, 10 mol % lead titanate in lead magnesium niobate, is measured at different frequencies from 100 Hz to 100 kHz in the temperature range from -100 to 120 \ifmmode^\circ\else\textdegree\fi{}C. A standardizing method is introduced to analyze the curve of the dielectric constant vs temperature. It results a master curve behavior between the dielectric constant and temperature at temperatures higher than the temperature of the dielectric constant maximum. The dielectric relaxation behavior is analyzed with various models. The best way to characterize the degree of the dielectric relaxation for relaxor ferroelectrics is established using the experimental data. It is indicated that the temperature dependence of the static dielectric constant can be well described by an exponential function, while the temperature dependence of the relaxation time is described by a superexponential function. Based on the specialty of the relaxor ferroelectrics, a distribution function for the relaxation times is introduced and a model is introduced to simulate the dielectric behavior of the relaxor ferroelectrics. The model can express well both the temperature and frequency dependence of the dielectric behavior for a relaxor ferroelectrics. All of the parameters in the fitting formula can be experimentally determined. The model shows that in the low-temperature range, there are two simple relationships about the dielectric frequency spectrum: \ensuremath{\varepsilon}\ensuremath{''}(\ensuremath{\omega},T)=(-\ensuremath{\pi}/2)\ensuremath{\partial}\ensuremath{\varepsilon}\ensuremath{'}(\ensuremath{\omega},T)/\ensuremath{\partial} ln \ensuremath{\omega} and \ensuremath{\varepsilon}\ensuremath{'}=B(T)(ln ${\mathrm{\ensuremath{\omega}}}_{0}$-ln \ensuremath{\omega}). These relationships are verified by the experimental results. A way to obtain the accurate value of ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\infty}}}$ in the low-temperature range is described.

The minimum entropy criterion of range alignment in ISAR motion compensation

Abstract: The envelope correlation algorithm is widely used in range alignment of ISAR motion compensation, however, this method often make the error accumulation, and what is more, a jump error will make when the reflection of target has strong scintillation. To reduce the error of range alignment a minimum entropy criterion of range alignment in ISAR compensation is proposed. Using this method the range bin can be alignment correctly which is verified by real radar data.

Space-time adaptive processing for airborne radar with various array orientations

Abstract: Space-time adaptive processing (STAP) is a crucial technique for the new generation airborne radar with high air-to-ground performance. The authors study this technique applied to airborne early warning (AEW) radars with various array orientations, which include the sideways looking array radar (SLAR) and non-sideways looking array radar. First, the ground clutter is characterised by showing the angular-Doppler spectra and Doppler-range relation for various array orientations. Secondly, the impact of various array configurations and pulse repetition frequencies (PRFs) on the STAP performance by the use of a multiple Doppler channels joint adaptive processing (M-CAP) approach is analysed. Thirdly, a 3-Doppler-channel processing scheme, in coordination with the various PRF modes, is recommended to solve the STAP problem for practical AEW radars with various array orientations. Finally, some examples are given by simulation to show the performance of the 3-Doppler-channel processing scheme for various cases of interest and to validate the theoretical analysis.

Silicon-on-insulator asymmetric optical switch based on total internal reflection

Abstract: Based on the large cross-section single-mode rib waveguide condition, total internal reflection (TIR) and the plasma dispersion effect, a silicon-on-insulator (SOI) asymmetric optical waveguide switch with transverse injection structure has been proposed and fabricated, in which the SOI technique utilizes silicon and silicon dioxide thermal bonding and back-polishing. The device performance is measured at a wavelength of 1.3 /spl mu/m. It shows that the extinction ratio and insertion loss are less than -18.1 and 6.3 dB, respectively, at an injection current of 60 mA. Response time is 110 ns.

Far-field radiation of planar gaussian sources and comparison with solutions based on the parabolic approximation

Abstract: Previous research on the Gaussian beam was restricted in scope because of the paraxial assumption. Based on a rigorous solution of the Helmholtz equation, the radiation properties of a Gaussian source are reexamined. The results are different from what previous paraxial theory predicted. It is shown that the commonly used concept of the Gaussian beam and the separability of the field distribution along the two transverse axes may be misleading in certain conditions. Furthermore the theoretical upper limit on the beam divergence angle is shown to be 65.5 degrees .

Defect structure and oxygen sensing properties of Mg-doped SrTiO3 thick film sensors

Abstract: The preparation of oxygen sensors based on Mg-doped SrTiO 3 thick film is described briefly. The oxygen sensing properties and the defect structure of the samples have been investigated as a function of the Mg content. The results show that all samples exhibit p-type semiconduction in the P O 2 region 3.8 × 10 −4 − 2.6 × 10 −1 atm and temperature range 400–1000 °C. A sensor prepared from the pre-calcined powder of Sr(Mg 0.4 Ti 0.6 )O 3 − δ revealed the best oxygen sensing properties. When the temperature is between 500 and 800 °C, its sensitivity to the change of P O 2 is 7–8.5 and its response and recovery times are 3 and 28–32 s, respectively. It can be used for monitoring and controlling the combustion processes under excess air (lean-burn) conditions. In addition, its approximate dependence of σ ∝ P O 2 1/3 was observed at intermediate temperature. To explain this result a new defect structure is proposed, in which a double ionized oxygen vacancy, V " o , double ionized acceptor of Mg, Mg″ Ti , and an association defect formed by Mg″ Ti , trapping one hole, (Mg″ Ti h . ), are the important defects.

Metal-semiconductor ohmic contact of SnO2-based ceramic gas sensors

Abstract: The realisation of an ohmic contact between metal electrodes and SnO2 semiconducting ceramics by a metal-n+ -n structure can cancel harmful effects in SnO2 gas sensors that are caused by the unmodified contact. This contact structure does not only increase the sensitivity of SnO2 gas sensors, but also decreases the scatter in gas-response properties of these sensors. Especially, it is necessary to adopt the metal-n+-n electrode structure when ceramic SnO2 gas sensors operate at lower temperatures.

The magnetic levitation of the projectile in coilguns

Abstract: An approach to the analysis of the magnetic levitation of the projectile in the coilgun's interior trajectory is proposed. The magnetic force acting upon the armature has the nature of a restoring force but its profile changes in every instant. So the transverse movement of the projectile can not be described by an ordinary oscillation caused by a restoring force. Since the transit process is calculated numerically step by step and all the necessary data are obtained at each step the analysis of the levitation motion can be implemented in the same way conveniently. The character of the magnetic forces acting upon the armature is analyzed in derail. The computation of the magnetic levitation during the whole transit process of a 5-stage gun is presented.

Some new methods for precision time interval measurement

Abstract: Using quantified time delay method to measure time interval, frequency, phase difference and period, high precision can be obtained. A method is described which is based on the phenomenon of stable and fast transmission of time and frequency signals. The method can be used in for both periodic and nonperiodic signals. Through a number of experiments, it has been shown that is possible to obtain ps level precision. Accuracy limitation is determined mainly by the stability of time delay of the components. Because of its high precision, simplicity and low price, this method can replace many digital frequency and time measuring techniques. The authors have applied a Chinese patent to the new method. This paper describes some time interval measurement techniques based on the method described. They include one time delay chain method and two time delay chains combined with coincidence detection method. Some methods which enable reduction of the number of components and achieve higher accuracy are also introduced. The methods can be used not only in measurements, but also in time control.

Closed-form solutions for one-dimensional inhomogeneous anisotropic medium in a special case. II. Inverse scattering problem

Abstract: For pt I see ibid, vol45, no5, p936, 1997 An analytical method is presented for the inverse scattering problem of a one-dimensional (1-D) inhomogeneous anisotropic medium in a special case Using the closed-form formulations for the reflection coefficients derived in the first part of this paper, reconstruction formulas are obtained for the wave impedance profiles, permittivity profiles or permeability profiles of the anisotropic medium, all of which are given in closed form In the meantime, a partial inverse scattering method for the electric parameters at the interface of the medium with free space is also investigated by using a Wedtzel-Kramers-Brillouin (WKB) approximation Numerical examples show the validity of the methods

Six-point synthetic method to estimate fundamental matrix *

Abstract: A new method, named the 6-point synthetic method, is developed to estimate fundamental matrix ( F matrix) based on two uncalibrated cameras and an 8-parameter model. First two parameters are found in the model by the new constraint developed, and they are the affine coordinates of an epipole. Then the rest 6 parameters are obtained by solving a set of linear equations. Finally, this method is tested by some real images and shows the advantages of obvious geometrical meaning, fewer matching pairs needed for calculation and high accuracy F matrix.

A framework of rank-reduced space-time adaptive processing for airborne radar and its applications

Abstract: A new equivalent formulation of the joint domain space-time optimum processor for airborne phased array radar application is derived. Then a new framework of space-time adaptive processing (STAP) for airborne radar systems which includes most of suboptimum algorithms in the literature is proposed. The performance of two typical rank-reduced time-space joint-domain processors based on Doppler pre-filtering is analyzed in detail based on the proposed framework.

Wavelet based adaptive equalization algorithm

Abstract: Two equalizer structures based on orthogonal wavelet bases are proposed, named the wavelet based linear equalizer (WBLE) and wavelet based decision feedback equalizer (WBDFE). The equalizer is represented by a set of orthogonal wavelets and the corresponding coefficients. The paper also gives the adaptation algorithm. Simulation results show that the WBLE and WBDFE converge faster than the conventional LMS based linear equalizer and decision feedback equalizer, while the increase in the computational complexity is very low.

Study of frame-based PRMA protocol under time-division duplex

Abstract: A new frame-based packet reservation multiple access (F-PRMA) protocol under time-division duplexing is proposed for microcellular mobile communications, the simulation model of which is built. By extensive simulations, the performances of the multiple access protocol including packet dropping probability P/sub drop/, channel capacity M/sub 0.01/ and system throughput /spl rho/ are analyzed under different speech service loads, and the effects of the system parameters and protocol variables on the efficiency of the protocol are examined throughout. The result of the study shows that the F-PRMA protocol is highly efficient under time-division duplexing (M/sub 0.01/=36, multiple access gain /spl eta/=1.6), and has almost the same performance as the original PRMA protocol under frequency-division duplexing.

Decoding of the Golay code

Abstract: A decoding algorithm based on revised syndromes to decode the binary (23, 12, 7) Golay code is presented. The algorithm strongly depends on the algebraic properties of the code. For the algorithm, the worst complexity is ~576 mod 2 additions, which is less than that of the algorithms available for the code; the average complexity is approximately 224 mod 2 additions, which is also less than that of the known algorithms for the code.

Group-normalized wavelet packet signal processing

Abstract: Since the traditional wavelet and wavelet packet coefficients do not exactly represent the strength of signal components at the very time(space)-frequency tilling, group- normalized wavelet packet transform (GNWPT), is presented for nonlinear signal filtering and extraction from the clutter or noise, together with the space(time)-frequency masking technique. The extended F-entropy improves the performance of GNWPT. For perception-based image, soft-logic masking is emphasized to remove the aliasing with edge preserved. Lawton's method for complex valued wavelets construction is extended to generate the complex valued compactly supported wavelet packets for radar signal extraction. This kind of wavelet packets are symmetry and unitary orthogonal. Well-defined wavelet packets are chosen by the analysis remarks on their time-frequency characteristics. For real valued signal processing, such as images and ECG signal, the compactly supported spline or bi- orthogonal wavelet packets are preferred for perfect de- noising and filtering qualities.

On wavelet variance

Abstract: Frequency stability of an atomic clock can be characterized by wavelet variance. Wavelet variance puts a limit on the influence of nonlinear and non-stationary processes. The character of wavelet variance is analyzed by using Fractal Brownian Motion function as an example of atomic clock signal. As in the cases of Allan variance and Hadamard variance, we can choose a suitable basis function. It is proved that Allan variance is a special case of wavelet variance at Haar basis. Hadamard variance is a special case of wavelet variance where the scale is much large than 1. It is concluded that frequency stability of atomic clocks is characterized by wavelet variance.

Cultural transplantation and transformation: Metaphor translation of a dream of red mansions

Abstract: This article deals with metaphor translation of A Dream of Red Mansions by Cao Xueqin. To retain to the utmost extent the unique artistic devices and techniques, the flavor, the tone, and the cultural implications of the original, the first choice should be transplantation of images whenever possible, which can be regarded as cultural transplantation. For metaphors that may be entirely maimed through transplantation of images in translation owing to the vast distance between the two cultures and languages, cultural transformation, such as image substitution, explanatory notes or total abandonment of the figure of speech should be resorted to in order to make the meaning and function of the original metaphors accessible to the readers of the translated version. It is concluded that selection of a good translation for a metaphor depends on an overall consideration of all elements and on differentiating what is primary from what is secondary in the translator's endeavour to minimize losses of the or...

Wavelet based decision feedback equaliser

Abstract: A decision feedback equaliser (DFE) structure based on wavelets (WBDFE) is presented. The WBDFE shows faster convergence than the conventional DFE, with a very small increase in computation. In addition, the WBDFE has good time-frequency localisation, and is suitable for time-varing channels.

Discounted semi-markov decision process in a semi-markov environment

Abstract: This paper presents the discounted semi-Markov decision process (SMDP) with Borel state space in a semi-Markov environment. It describes a system which behaves like a SMDP except that the system is influenced by its semi-Markov process environment. Following each state transition of the environment, the parameters of the SMDP changes. After present me the model, we show the validity of the optimality equation and the existence of ∊-optimal policies

Silicon raised strip waveguides based on silicon and silicon dioxide thermal bonding

Abstract: Si raised strip waveguides on SiO/sub 2/ have been proposed and fabricated, which are based on silicon-on-insulator (SOI) material. In the waveguides, the SOI technique utilizes silicon and silicon dioxide thermal bonding and back-polishing. An anisotropic etchant is used to produce the trapezoidal Si raised strip waveguides by etching the Si film down to the SiO/sub 2/ etch-stop buried layer. The transmission losses of the Si waveguides are measured to be less than 0.2 dB/cm at the 1.3 /spl mu/m wavelength for the lowest mode TE-like mode.

Genetic algorithm applied to inverse scattering: two examples

Abstract: The Genetic Algorithm (GA) is an optimization technique, applying the nature philosophy to searching for the best fit solution. We apply the GA to treat inverse scattering problems. Two examples are presented: the detection of a buried circular conducting cylinder and the inversion of a dielectric profile. The examples show the convergence and robustness.

Fast image coding of interval interpolating wavelets

Abstract: We present the interval interpolating wavelet transform for fast image compression. Comparing with the common used wavelet coding, this method is more systematically stable, with less computing complexity and the inherent parallel processing. In theory, it has the nearly optimal minimax compression characteristics. The simulation shows that the interval interpolating wavelet transform are more qualified and remove the artificial blocking effects. The interval wavelet is introduced to deal with the boundary points of the finite localized image. This method does not only improve the compress rate, but also deletes the quantization aliasing of the boundary pixels.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Group-normalized processing of complex wavelet packets

Abstract: Linear phase is not possible for real valued FIR QMF, while linear phase FIR biorthogonal wavelet filter banks make the mean sqared error of the constructed signal exceed that of the quantization error. W. Lawton’s method for complex valued wavelets construction is extended to generate the complex valued compactly supported wavelet packets that are symmetrical and unitary orthogonal; then well-defined wavelet packets are chosen by the analysis remarks on their time-frequency characteristics. Since the traditional wavelet packets transform coefficients do not exactly represent the strength of signal components, a modified adaptive wavelets transform, group-normalized wavelet packet transform (GNWPT), is presented and utilized for target extraction from formidable clutter or noises with the time-frequency masking technique. The extended definition oflp-norm entropy improves the performance of GNW-PT. Similar method can also be applied to image enhancement, clutter and noise suppression, optimal detection and radar imaging.

To increase the accuracy of the range alignment in ISAR imaging by super-resolution technique

Abstract: Envelope drift and jump error caused by using the envelope amplitude correlation method pulse by pulse may be as large as the length of several range bins. These errors decrease the quality of the image heavily. By using the super-resolution technique, the accuracy of the range alignment is increased, and the envelope drift error and jump error in the range alignment are decreased. Imaging results of real ISAR data show that the quality of the image is improved clearly by the super-resolution technique.

A new model of generalized invertor and its applications

Abstract: A new model of generalized invertor is presented, The model is generalized from the conventional K-, J-invertor. So it is easy to be applied directly to design microwave circuits such as impedance transformers, filters, and couplers of any complex discontinuities, when the symmetrical and asymmetrical networks are involved. The theory based on the new model is developed to enrich the microwave theory on network synthesis. Examples are given to show the effectiveness and convenience in application.

Nonlinear ECG filtering by group normalized wavelet packets

Abstract: The group normalized wavelet packet transform is presented to extract ECG components from the additive noise on time-frequency tilings together with the time-frequency masking technique. FIR filters with linear phase is the natural requirement for anti-aliasing. However, linear phase compactly supported real wavelet packets are non-orthogonal so that coefficients at different scale have correlated redundancy and destroy the energy conservation. We design group-normalization to make our entropy definition follow a comparable standard.