Bandwidth‐Efficient Selective Retransmission for MIMO‐OFDM Systems

In this work, we propose an efficient selective retransmission method for multiple‐input and multiple‐output (MIMO) wireless systems under orthogonal frequency‐division multiplexing (OFDM) signaling. A typical received OFDM frame may have some symbols in error, which results in a retransmission of the entire frame. Such a retransmission is often unnecessary, and to avoid this, we propose a method to selectively retransmit symbols that correspond to poor‐quality subcarriers. We use the condition numbers of the subcarrier channel matrices of the MIMO‐OFDM system as a quality measure. The proposed scheme is embedded in the modulation layer and is independent of conventional hybrid automatic repeat request (HARQ) methods. The receiver integrates the original OFDM and the punctured retransmitted OFDM signals for more reliable detection. The targeted retransmission results in fewer negative acknowledgements from conventional HARQ algorithms, which results in increasing bandwidth and power efficiency. We investigate the efficacy of the proposed method for optimal and suboptimal receivers. The simulation results demonstrate the efficacy of the proposed method on throughput for MIMO‐OFDM systems.     

Mixed convection boundary layer flow adjacent to a vertical surface embedded in a stable stratified medium

The steady mixed convection boundary layer flow through a stable stratified medium adjacent to a vertical surface is investigated. The velocity outside the boundary layer and the surface temperature are assumed to vary linearly from the leading edge of the surface. The transformed ordinary differential equations are solved numerically by the Keller-box method. It is found that dual solutions exist, and the thermal stratification delays the boundary layer separation.     

Show-through correction in scanned images using joint histogram

In scanning duplex printed documents, show-through effect is a common phenomenon where the text or image on one side of the duplex printed page is contaminated by that of the other side. In this paper, we propose an empirical model for show-through accounting for the dependency of back side absorptance weights on the back side reflectances to overcome the limitation of the space invariant point spread function as used in the previous model. The weight function of the proposed empirical model and other model parameters are calculated from a transfer function derived from the joint histogram of the reflectances of the two sides of the document. Show-through corrected images of the scanned documents are then obtained using the calculated parameters and the proposed empirical model. Simulation results are presented to demonstrate the effectiveness of the proposed method as compared to a popular method of show-through correction.     

OPTIMIZATION CONTROL OF COMPOSITE LAMINATES FOR MAXIMUM THERMAL BUCKLING AND MINIMUM VIBRATIONAL RESPONSE

The problem of maximizing the thermal buckling and minimizing the vibrational response of composite laminates is solved using optimal design and active control procedures. The problem is formulated based on a first-order shear deformation laminate theory with various cases of boundary conditions. The design objective is to maximize thermal buckling using ply thickness and the fiber orientation angle as design variables. The active control objective is to minimize the laminate vibrational response with the minimum possible expenditure of control energy. The vibrational response is expressed in terms of the total elastic energy of the laminate and a penalty functional of closed-loop control force. Liapunov-Bellman theory is used to obtain solutions for controlled deflections and optimal control force. Comparative examples are given for angle-ply antisymmetric laminates subjected to a uniform temperature distribution. A general representation for the design variables is presented such that the ply thickness is a function of the number of layers. Some of the obtained numerical results are compared with their counterparts in the literature.     

Physical properties of copper-treated cadmium telluride thin films by vacuum evaporation technique

Cadmium telluride thin films were prepared on microscopic glass substrates by vacuum evaporation technique. The morphology of the thin films as a function of substrate temperature and postdeposition heat treatment was investigated by atomic force microscopy. The grain size increases with the increase of substrate temperature and postdeposition annealing. Calculated amount of copper was evaporated to deposit on the top of the already deposited CdTe thin films. The whole assembly was then annealed at 450°C for about 4–6 min. These samples were characterized structurally, optically, and electrically by using XRD, spectrophotometer, and Hall Effect measurements, respectively. Doping with Cu has significantly changed the electrical properties of the CdTe films. The measurements have shown band gap of 1.475 eV and resistivity ranging from 0.132 to 0.002 Ω cm, depending on the copper concentration. Increase in the weight percentage of copper showed decreasing trends in resistivity up to 3 wt%. Mobility showed the opposite behavior than those of the resistivity. The carrier concentration showed a systematic increase.     

Stagnation flow of a micropolar fluid towards a vertical permeable surface

The steady stagnation flow towards a permeable vertical surface immersed in a micropolar fluid is investigated in this study. The external flow impinges normal to the heated plate and the surface temperature and velocity are assumed to vary linearly with the distance from the stagnation point. The governing system of partial differential equations is first transformed into a system of ordinary differential equations, and then they are solved numerically by a finite-difference scheme, namely the Keller-box method. The features of the flow and heat transfer characteristics for different values of the governing parameters are analyzed and discussed. Both assisting and opposing flows are considered. It is observed that dual solutions exist for the assisting flow, besides that usually reported in the literature for the opposing flow.     

Analysis and Performance of Three-Phase Phase-Controlled Thyristor AC Voltage Controllers

A simple generalized approach, based on numerical evaluation of the Fourier coefficients, is presented to evaluate the comprehensive performance of three-phase phase-controlled thyristor ac voltage controllers. Three-phase balanced resistive and inductive loads of different power factors are considered. The method of solution adopted is simple, accurate, fast, and does not require the derivation of Fourier coefficient equations. Various electrical properties are compared graphically for the different circuit configurations. The branch-controlled deltaconnected load is considered to have the best performance, giving the highest power factor per unit of power delivered.     

2.3 Investigations of metal contacts to amorphous evaporated Ge films

Amorphous Ge films are used as non-injecting ohmic contacts to high-resistivity n-silicon radiation detectors, but the function of this contact is not yet fully explored. One part problem is the role of the metal films used as external contacts to the amorphous film. In this paper we investigate the function of different contacting metals, such as Au, Al, Cr by measuring the I-V-characteristics of sandwich structures with two metals on both sides of the amorphous evaporated Ge film (of typical thickness 1 μm). It was found that while the symmetric CrGeCr structure (also AuGeAu) had low resistance (leading to resistivity values of ?4 × 10⁴ωcm for the Ge film), the unsymmetric structures e.g. AlGeCr and AlGeAu, showed higher resistance. This was again found for the AlGeAl structure, which also showed some polarity dependence. Rutherford backscattering was used to investigate diffusion effects between the layers; also the microstructure of the Ge films was investigated by electron microscopy.