Resource allocation in OFDMA networks based on interior point methods

This paper studies a joint optimization problem of sub‐carrier assignment and power allocation in orthogonal frequency division multiple access (OFDMA) wireless networks. A major challenge in solving the optimization problem is non‐convexity caused by the combinatorial nature of sub‐carrier assignment problem and/or non‐convex objective functions. To address the combinatorial complexity, we formulate the resource allocation problem as an optimization problem with continuous variables. We propose a novel approach based on a penalty function method and an interior point method (PM/IPM) to solve the problem. In specific, using a two‐step implementation, the penalty method is applied first to convert the non‐convex feasible region to a convex one. Then, the interior point method is deployed to solve the problem which is non‐convex only in the objective function. To evaluate the performance of PM/IPM, we apply a genetic algorithm (GA) that achieves near optimal solutions of the problem by iterative searching. Copyright © 2009 John Wiley & Sons, Ltd.     

Strain engineering of band dispersion and dielectric response of monolayer and bilayer AlN

The strain-induced modulation of the band dispersion and dielectric response of monolayer and bilayer AlN was studied using density functional theory. The results showed that, by applying strain on monolayer and bilayer AlN, the bandgap of these nanostructures can be controlled. Calculations of the dielectric response show that application of in-plane tensile (compressive) strain causes a red (blue) shift in the \(\varepsilon _2 (\omega )\) spectrum of both monolayer and bilayer AlN. Applying perpendicular strain on bilayer AlN leads to a shift of the valence-band maximum to the \({\Gamma }\)-point in the band structure and shift of the peak positions and variation of the peak intensities in the \(\varepsilon _2 (\omega )\) spectrum. The predicted tunability of the bandgap and dielectric response by strain application may lead to a wide range of applications for monolayer and bilayer AlN nanosheets in electronic and optoelectronic devices.     

Significance of thermodynamics and rheological characteristics of dope solutions on the morphological evolution of polyethersulfone ultrafiltration membranes

Herein, the effect of solvent type and polymer concentration on the thermodynamic and viscoelastic properties, and performance of polyethersulfone (PES) ultrafiltration (UF) membranes are investigated. The morphology and mechanical properties of the membranes are examined too. Rheological measurements indicated that solidification and instantaneous demixing occurred fast for PES/N-methyl-2-pyrrolidone (NMP) dope solution. This resulted in a dense skin layer with a finger-like structure confirmed by the scanning electron microscopy images. In comparison, PES/dimethylformamide (DMF) exhibited a delayed solidification and demixing with a sponge-like structure. Rheological characteristics and ternary phase diagrams of two systems are examined to gain insights and make correlations with the morphology of resultant membranes. Performance analysis revealed that membranes derived from PES/NMP system exhibited both improved pure water flux and bovine serum albumin rejection suggesting the superiority of NMP compared to DMF as the solvent of choice for the preparation of PES UF membranes.     

Facile route for synthesis of strontium hexaborate crystalline powders

In this paper, a solid-state reaction route was developed to synthesize SrB₆O₁₀ crystalline powders. Regarding the last preparation methods, a short process time of synthesis was achieved that has not reported before. The prepared sample was characterized using various methods, such as X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, and diffuse reflection spectroscopy. The obtained results proved that the prepared sample at temperature 840°C with 2.5 h duration consists of SrB₆O₁₀ crystalline powders with 4.96 eV bandgap.     

Production of argon free oxygen by adsorptive air separation on Ag‐ETS‐10

The purification of different components of air, such as oxygen, nitrogen, and argon, is an important industrial process. Pressure swing adsorption (PSA) is surpassing the traditional cryogenic distillation for many air separation applications, because of its lower energy consumption. Unfortunately, the oxygen product purity in an industrial PSA process is typically limited to 95% due to the presence of argon which always shows the same adsorption equilibrium properties as oxygen on most molecular sieves. Recent work investigating the adsorption of nitrogen, oxygen and argon on the surface of silver‐exchanged Engelhard Titanosilicate‐10 (ETS‐10), indicates that this molecular sieve is promising as an adsorbent capable of producing high‐purity oxygen. High‐purity oxygen (99.7+%) was generated using a bed of Ag‐ETS‐10 granules to separate air (78% N₂, 21% O₂, 1% Ar) at 25°C and 100 kPa, with an O₂ recovery rate greater than 30%. © 2012 American Institute of Chemical Engineers AIChE J, 59: 982–987, 2013     

Phase behavior of mixture of supercritical CO2 + ionic liquid: Thermodynamic consistency test of experimental data

Various models have been applied composed of the Peng‐Robinson equation of state (PR‐EoS) and the Soave‐Redlich‐Kwong equation of state (SRK‐EoS) associated with three mixing rules including the following: Wong‐Sandler (WS), van der Waals one (vdW1), and van der Waals two (vdW2) for phase behavior modeling of mixtures of supercritical CO₂ + different ionic liquids in vapor–liquid equilibrium (VLE) region. It has been found that the PR EoS implying the WS mixing rule can be used as a reliable thermodynamic model to perform a thermodynamic consistency test on the experimental data of phase behaviors of the supercritical CO₂ + ionic liquid systems (19 commonly‐used ionic liquids have been studied). The results show that 40% of the experimental data seem to be thermodynamically consistent, 55.5% seem to be thermodynamically inconsistent, and 4.5% seem to be not fully consistent. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3892–3913, 2013