卷积码的线性系统

介绍了卷积码的线性离散时不变系统模型。从建立卷积码的状态变量方程出发,研究了卷积码的代数结构,并详细论述了基于状态变量方程描述的迭代译码算法。     

Dispersion-relation-preserving FDTD algorithms for large-scale three-dimensional problems

We introduce dispersion-relation-preserving (DRP) algorithms to minimize the numerical dispersion error in large-scale three-dimensional (3D) finite-difference time-domain (FDTD) simulations. The dispersion error is first expanded in spherical harmonics in terms of the propagation angle and the leading order terms of the series are made equal to zero. Frequency-dependent FDTD coefficients are then obtained and subsequently expanded in a polynomial (Taylor) series in the frequency variable. An inverse Fourier transformation is used to allow for the incorporation of the new coefficients into the FDTD updates. Butterworth or Chebyshev filters are subsequently employed to fine-tune the FDTD coefficients for a given narrowband or broadband range of frequencies of interest. Numerical results are used to compare the proposed 3D DRP-FDTD schemes against traditional high-order FDTD schemes.     

Modeling aviation baggage screening security systems: a case study

Aviation security protects vital national interests, as well as passengers and aircraft. Key components of an aviation security system include baggage and passenger screening devices and operations. Determining how and where to assign (deploy) such devices can be quite challenging. Moreover, even after such systems are in place, it can be difficult to measure their effectiveness. This paper describes how discrete optimization models can be used to address these questions, based on three performance measures that quantify the effectiveness of airport baggage screening security device systems. These models are used to solve for optimal airport baggage screening security device deployments considering the number of passengers on a set of flights who have not been cleared using a security risk assessment system in use by the Federal Aviation Administration (i.e., passengers whose baggage is subjected to screening), the number of flights in this set, and the size of the aircraft for such flights. Several examples are provided to illustrate these results, including an example that uses data available from the Official Airline Guide.     

Chemically induced residual stresses in dental composites

In several European countries, dental composites are replacing mercury-containing amalgams as the most common restorative materials. One problem with dental composites is residual stresses which may lead to poor performance of the restoration. In the present study, a combined modeling and materials characterization approach is presented and predictions compare well with experimental data on residual stresses. The model takes stress relaxation into account through the complete relaxation time spectrum of the resin. The approach allows for detailed parametric studies where resin and composite composition as well as cure conditions may be tailored with respect to residual stress generation.     

A new microcalorimeter for measurements in 3.5-mm coaxial line

A new microcalorimeter has been realized for implementing the national power standard up to 26.5 GHz in a 3.5-mm coaxial line at the Istituto Elettrotecnico Nazionale (IEN) Galileo Ferraris, Turin, Italy. The system is based on a dry thermostatic cell controlled by Peltier elements. The thermal load consists of a twin sensor system that is alternately supplied with high frequency test power and low frequency or direct current (DC) reference power through adiabatic coaxial lines. This microcalorimeter was originally designed for calibrating bolometric mounts. Recently, it has been modified for thermocouple power sensors, which can operate from DC to 26.5 GHz in a 3.5-mm coaxial line.     

Miniature and tunable filters using MEMS capacitors

Microelectromechanical system (MEMS) bridge capacitors have been used to design miniature and tunable bandpass filters at 18-22 GHz. Using coplanar waveguide transmission lines on a quartz substrate (/spl epsiv//sub r/ = 3.8, tan/spl delta/ = 0.0002), a miniature three-pole filter was developed with 8.6% bandwidth based on high-Q MEMS bridge capacitors. The miniature filter is approximately 3.5 times smaller than the standard filter with a midband insertion loss of 2.9 dB at 21.1 GHz. The MEMS bridges in this design can also be used as varactors to tune the passband. Such a tunable filter was made on a glass substrate (/spl epsiv//sub r/ = 4.6, tan/spl delta/ = 0.006). Over a tuning range of 14% from 18.6 to 21.4 GHz, the miniature tunable filter has a fractional bandwidth of 7.5 /spl plusmn/ 0.2% and a midband insertion loss of 3.85-4.15 dB. The IIP/sub 3/ of the miniature-tunable filter is measured at 32 dBm for the difference frequency of 50 kHz. The IIP/sub 3/ increases to >50 dBm for difference frequencies greater than 150 kHz. Simple mechanical simulation with a maximum dc and ac (ramp) tuning voltages of 50 V indicates that the filter can tune at a conservative rate of 150-300 MHz//spl mu/s.     

Gamma-irradiation-induced Ag/SiO2 composite films and their optical absorption properties

Small Ag particles or clusters dispersed mesoporous SiO₂ composite films were prepared by a new method: First the matrix SiO₂ films were prepared by sol-gel process combined with the dip-coating technique, then they were soaked in AgNO₃ solutions followed by irradiation of γ-ray at room temperature and in ambient pressure. The structures of these films were examined by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and optical absorption spectroscopy. It has been shown that the Ag particles grown within the porous SiO₂ films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With increasing the soaking concentration and an additional annealing, an opposite peakshift effect of the surface plasmon resonance (SPR) was observed in the optical absorption measurements.     

Spectral properties of diesel fuel droplets

Absorption spectra of several types of diesel fuel are studied experimentally. Index of refraction of these fuels is calculated using subtractive Kramers-Krönig analysis. The ageing process of fuels is simulated by prolonged boiling. Radiative properties of diesel fuel droplets are calculated using the Mie theory and a simplified approach, based on approximations of absorption and scattering efficiency factors. It is pointed out that the accuracy of the simplified approach is sufficient for practical applications in the visible and infrared ranges, for various types of diesel fuel, and for droplet radii in the range from 5 to 50 μm. The monodisperse approximation is shown to be applicable for the analysis of infrared radiative properties of realistic polydisperse diesel fuel sprays.     

Polarizational Frequency-Selective Multipath Interference Devices for Millimetric- and Submillimetric-Wave Receivers

Variants are considered of palarizational frequency-selective multipath interference devices for millimetric- and submillimetric- wave receivers. Main expressions are presented that describe their characteristics. Advantages are pointed out of the devices as compared with Fabry-Perot interferometers when solving problems of received frequency-band shaping, heterodyne noise rejection, and signal- and heterodyne-radiation transfer to mixer.     

Analysis of the interference immunity during reception of ultrawideband chaotic signals in an indoor multipath propagation environment

The models of indoor multipath propagation of wideband and ultrawideband (UWB) signals are considered. Application of these models is recommended by an IEEE working group. In the framework of one of the models corresponding to the line-of-sight conditions, the algorithms for reception of UWB chaotic signals in the presence of reflection from multiple surfaces (echo signals) are proposed and analyzed. The efficiency of reception techniques is estimated from the viewpoint of the error probabilities. Limitations caused by the specific properties of echo signals are revealed, and the methods for improvement of reception quality are discussed.