Outage probability of multihop transmission over Nakagami fading channels

We present a general analytical framework for the evaluation of the end-to-end outage probability of multihop wireless communication systems with nonregenerative relays over Nakagami fading channels. It is shown that the presented results can either be exact or tight lower bounds on the performance of these systems depending on the choice of the relay gain. More specifically, we obtain a closed-form expression for the moment generating function of the reciprocal of the end-to-end signal-to-noise ratio (SNR) and we then use this expression to calculate the outage probability via numerical inversion of the Laplace transform. Numerical examples show that regeneration is more crucial at low average SNR and for multihop systems with a large number of hops.     

On the optimal spreading sequence allocation in flat-fading channels

The sum capacity of optimally allocated spreading sequences with and without power control is studied for synchronous code-division multiple-access channels subject to frequency flat fading. Continuing upon the recently derived characterization of sum capacity for asymmetric received powers with no power control at the transmitter, we present an asymptotic analysis of the sum capacity in the wideband limit of a large number of users and evaluate possible gains compared with the random spread sequences. Next, we discuss group orthogonal optimal sequence allocation and analyze its potential to decrease the complexity of the receiver while achieving the sum capacity of optimally allocated equal-energy sequences. For the power controlled case, we give a general characterization of sum capacity and identify the optimal power-control law and sequence allocation that achieves this capacity. These results are also analyzed in the asymptotic limit of a large number of users.     

Efficient adaptive forward error control schemes for Ka-band satellite systems

Adaptive fade margin is required to counter the severe but varying rain attenuation in K_a-band satellite communications. In a search for a suitable rain countermeasure, the effectiveness of adaptive forward error control (AFEC) schemes is studied. Two AFEC schemes using convolution codes and concatenated codes are proposed and their performances analysed. The schemes can provide progressively adaptive fade margins of 10-1 and 10-4 dB, respectively, in excess of the system's fixed fade margin. To improve the efficient use of shared resources of the system, an AFEC scheme using double coding is introduced and its performance analysed. In this scheme a single codec is used repeatedly, resulting in reduced hardware cost and increased use. By using convolutional code and Golay code the above scheme can provide adaptive fade margins of 10°8 dB and 8°1 dB, respectively. Although concatenated codes have slightly better performance than double codes, the hardware implementation and decoding complexity of the latter are significantly less. Implementation complexity of AFEC resource sharing in the light of code complexity, time frame expansion, link condition monitoring and signalling is discussed in detail.     

Effect of extended defects in planar and pixelated CdZnTe detectors

We evaluated a spectroscopy-grade 15×15×7 mm³ CdZnTe (CZT) crystal with a high μτ-product, >10⁻² cm²/V, but impaired by microscopic extended defects, such as walls of dislocations, low-angle and sub-grain boundaries, and Te inclusions. First, we evaluated a planar detector fabricated from this crystal using a Micro-scale X-ray Detector Mapping (MXDM) technique. Then, we fabricated from the same crystal a pixel detector to study local non-uniformities of the electric field. The measured X-ray response maps confirmed the presence of non-uniformities in the charge transport, and they showed that the global- and local-distortions of the internal E-field correlated to the extended defects and space-charge buildup on the side surfaces.     

Gram-Schmidt orthogonalization for rapid reconstructions of Fourier transform infrared spectroscopic imaging data

Increasingly voluminous Fourier transform infrared (FT-IR) spectroscopic imaging data sets are being generated with the advent of both faster array detectors and the implementation of time-resolved imaging techniques, resulting in data processing becoming the limiting step in visualizing sample heterogeneity and temporal profile evolution. We report the application of a Gram-Schmidt vector orthogonalization procedure in interferogram space to provide a significant time saving advantage in processing of one to two orders of magnitude in comparison to conventional spectral processing. Illustrative data from human skin biopsies and from dynamic molecular reorganizations within liquid crystalline microdomains is employed to discuss the capabilities and limitations of this information-extraction approach.     

Modeling of high-k dielectric nanocomposites

In this paper, based on recent research on BaTiO₃ (BT) nanoparticles, BT/P(VDF-HFP) nanocomposites, frequency-dependent dielectric properties of such a material system with high energy density have been investigated as functions of the volume fraction of the nanoparticles at room temperature by several theoretical models. For single domain and single crystals of BT, a Debye type of dissipation and soft mode theory have been adopted to obtain a more precise frequency-dependent dielectric spectrum of BT. For nanodielectric composites, among the others, Wiener Rule, Lichtenecker model, Maxwell–Wagner model, Yamada, and modified Kerner model were applied to evaluate the frequency-dependent dielectric spectrum of nanocomposites. A simple rule of mixture for the dielectric loss tangent was obtained using Lichtenecker logarithmic rule. The results from theoretical calculations are compared with the experimental data. For the dielectric constant, Lichtenecker model, Maxwell–Wagner model, and Yamada model show reasonable agreements with the experimental data up to 50 % volume fraction of the nanoparticles. At the higher volume fraction of the nanoparticles, the experimental data show a decreasing trend of the dielectric constant of the composites due to an increase in porosity of the system. In this case, a three-phase model (nanoparticles/pores/matrix) was developed to predict dielectric properties of the system at higher volume fraction of the nanoparticles (up to 80 %). The results showed reasonable agreements for a wide range of frequency. This theoretical study provides an essential information on dielectric properties of polymer-based BT nanocomposites with a wide frequency range instead of the trial-and-error strategy of experiments and can be used for designing high energy density dielectric materials in the future.     

The Effects of Mn Substitution on Magnetization Reversal Properties in Gd0.7Ca0.3MnO3

Low temperature magnetization in polycrystalline Gd_0.7Ca_0.3Mn_1?x M _x O₃ (M=Cr, Ga, Ru; x=0, 0.2) has been investigated. The samples were prepared via the conventional solid state reaction method. For all the samples, the paramagnetic to ferrimagnetic (PM-FiM) transition temperature (say T _max) can be well defined from the temperature dependent ac susceptibility data. The negative magnetization suppresses due to 20 % Cr or Ga doping at the Mn site. On the other hand, below the compensation temperature (T _comp), the nature of the Ru doping sample is almost similar to that of undoped Gd_0.7Ca_0.3MnO₃. The nonmagnetic Ga doping drastically reduces magnetization and T _max shifts to a lower temperature. However, Ru and Cr doping increase the value of T _max. The network of canted Mn³⁺/Mn⁴⁺ moments changes with the substitution of Cr, Ga, and Ru at the Mn site giving rise to the variation of the internal magnetic moment. Thus, the antiparallel coupling of the Gd moments with Mn/M changes and affects the low temperature magnetization reversal properties.     

A uniform deformation gradient hexahedron element with artificial hourglass control

An 8-noded linear hexahedron element for large strain hyperelastic analysis is presented in this paper. The element is based on a constant deformation gradient interpolation and is formulated using a mixed variational principle of the Hu–Washizu type. Volumetric and isochoric components of the deformation are treated independently to ensure the correct evaluation of the element volume. A simple procedure to control the propagation of spurious hourglass deformation modes is also discussed. This is based on the addition of artificial hourglass forces which vanish under constant deformation gradient conditions, thereby ensuring that the element passes a non-linear version of the patch test. Applications in the field of neo-Hookean materials and superplastic forming processes are also considered.     

Transient Couette flow in a rotating non-Darcian porous medium parallel plate configuration: network simulation method solutions

The transient, viscous, incompressible, hydrodynamic Couette flow in a rotating porous medium channel is studied in this paper. The channel comprises a pair of infinitely long parallel plates which rotate with uniform angular velocity about an axis normal to the plates. The porous medium is simulated using a Darcy–Forchheimer drag force model which includes both bulk matrix porous drag (dominant at low Reynolds numbers) and second order inertial impedance (dominant at higher Reynolds numbers). The two-dimensional Navier–Stokes equations are reduced to a (z*, t*) coordinate system incorporating Coriolis terms, and appropriate initial and boundary conditions are prescribed. Separate porous drag body force terms are incorporated in both the primary and secondary flow momentum equations. Using a set of transformations, the model is rendered dimensionless and shown to be dictated by the Ekman number, Forchheimer number, Darcy number and Reynolds number in a (z, t) coordinate system. Numerical solutions are obtained for the transformed model using the Network Simulation Method. The influence of the hydrodynamic parameters are computed graphically and also the interaction of parameters on the velocity fields is discussed at length. Excellent agreement is found with earlier non-porous flow studies. The analysis has important applications in geophysics and also chemical engineering systems.     

Low temperature crystallization of amorphous silicon carbide thin films for p–n junction devices fabrication

Metal induced crystallization technique was used to crystallize hydrogenated amorphous silicon carbide (a-SiC:H) thin films at low temperatures. Two types of substrates, silicon and silicon carbide were considered and the substrate effects on the final crystallized film were studied. About 200 nm a-SiC:H films were deposited and crystallized successfully on n-type Si and n-type 6H SiC substrates at a temperature of 600 °C. Fourier Transform Infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) analysis confirmed the crystallization of a-SiC:H film. Current–voltage (I–V) and capacitance–voltage (C–V) measurement confirms the formation of p–n junction with rectification over five orders of magnitude from ?2 V to 2 V.