Modeling and Performance Analysis of Cognitive Radio Networks Using Stochastic Timed Colored Petri Nets

Cognitive radio (CR) is currently one of the most promising information transmission technologies to deal with the problem of spectrum scarcity and spectrum underutilization in wireless communications. CR networks aim to enhance spectrum efficiency to meet the ever-increasing demands of end users. The principle is to provide the opportunity for unlicensed users (secondary users, SUs) to temporarily and dynamically access the unused or sparsely used bandwidth while ensuring that it never interferes or degrades the performance of the incumbent license holders, commonly called primary users (PUs). This raises several challenges to be addressed in CR networks and performance of secondary users is one of the critical issues tackled in this paper. That is, we propose to devise CR networks as a retrial queueing system where PUs have preemptive priority over SUs. To calculate performance measures of the devised model under quite general assumptions about the model parameters, analytical methods are known to require hard calculations and the obtained results are generally not exploitable. For this reason, simulation modeling becomes the last resort to assess the dependability indicators. To this extend, we build the simulation model of the queueing system using Timed Stochastic Colored Petri Nets. Various useful results will be hence drawn while varying network conditions. Both exponential and Erlang distributions are considered for modeling service time of SUs. The obtained results with restrictive assumptions fit the analytical outcomes experienced for quite similar queuing models, which demonstrate the effectiveness of the proposed STCPN simulation model.

     

Ring‐opening bulk polymerization of five‐ and six‐membered cyclic phosphonates using maghnite,a nontoxic proton exchanged montmorillonite clay

A new method of preparation of poly(alkylene H‐phosphonate)s by ring‐opening bulk polymerization of the five‐ and six‐membered cyclic phosphonates monomers using the nontoxic Maghnite‐H⁺ as the initiator is described. Cyclic phosphonate monomers have been first synthesized. In particular, a new one‐step synthesis of 2‐hydro‐2‐oxo‐1,3,2‐dioxaphospholane is reported with a yield of 70%. The efficiency of the montmorillonite sheet silicate clay which exchanged with protons, called Maghnite‐H⁺, as cationic initiator has been proved and the resulting biomimetic poly(alkylene H‐phosphonate)s have been characterized. The Maghnite‐H⁺ regenerated after one turn‐over has showed to be still efficient as initiator for the ring‐opening polymerization. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Efficient Immobilization of Yeast Transketolase on Layered Double Hydroxides and Application for Ketose Synthesis

Transketolase (TK) from S. cerevisiae was successfully immobilized on layered double hydroxides (LDH) using simple, affordable and efficient adsorption and coprecipitation based immobilization procedures. Optimization of the preparation was performed using zinc aluminium nitrate (Zn₂Al‐NO₃) and magnesium aluminium nitrate (Mg₂Al‐NO₃) LDH as immobilization supports, and the protein‐to‐LDH weight ratio (Q) was varied. The highest immobilization yields (98–99%) and highest relative specific activities (4.2–4.4 U⋅mg⁻¹ for the immobilized enzyme compared to 4.5 U⋅mg⁻¹ for the free enzyme) were both achieved when using a protein‐to‐LDH weight ratio (Q) of 0.38. Efficient lyophilization of the LDH‐TK bionanocomposites thus synthesized was proven to allow easy use and storage of the supported TK with no significant loss of activity over a three‐month period. The kinetic parameters of the LDH‐TK enzyme were comparable to those of the free TK. The LDH‐TK enzyme was finally tested for the synthesis of L ‐erythrulose starting from hydroxypyruvate lithium salt (Li‐HPA) and glycolaldehyde (GA) as substrates. L ‐erythrulose was characterized and obtained with an isolated yield of 56% similar to that obtained with free TK. The reusability of the LDH‐TK biohybrid material was then investigated, and we found no loss of enzymatic activity over six cycles.     

Solid-Phase Synthesis of Substrate-Based Dipeptides and Heterocyclic Pseudo-dipeptides as Potential NO Synthase Inhibitors

More than 160 arginine analogues modified on the C-terminus via either an amide bond or a heterocyclic moiety (1,2,4-oxadiazole, 1,3,4-oxadiazole and 1,2,4-triazole) were prepared as potential inhibitors of NO synthases (NOS). A methodology involving formation of a thiocitrulline intermediate linked through its side-chain on a solid support followed by modification of its carboxylate group was developed. Finally, the side-chain thiourea group was either let unchanged, S-alkylated (Me, Et) or guanidinylated (Me, Et) to yield respectively after TFA treatment the corresponding thiocitrulline, S-Me/Et-isothiocitrulline and N-Me/Et-arginine substrate analogues. They all were tested against three recombinant NOS isoforms. Several compounds containing a S-Et- or a S-Me-Itc moiety and mainly belonging to both the dipeptide-like and 1,2,4-oxadiazole series were shown to inhibit nNOS and iNOS with IC₅₀ in the 1–50 μM range. Spectral studies confirmed that these new compounds interacted at the heme active site. The more active compounds were found to inhibit intra-cellular iNOS expressed in RAW264.7 and INS-1 cells with similar efficiency than the reference compounds L-NIL and SEIT.     

Chemical Synthesis of A Pore‐Forming Antimicrobial Protein,Caenopore‐5, by Using Native Chemical Ligation at a Glu‐Cys Site

The 2014 report from the World Health Organization (WHO) on antimicrobial resistance revealed an alarming rise in antibiotic resistance all around the world. Unlike classical antibiotics, with the exception of a few species, no acquired resistance towards antimicrobial peptides (AMPs) has been reported. Therefore, AMPs represent leads for the development of novel antibiotics. Caenopore‐5 is constitutively expressed in the intestine of the nematode Caenorhabditis elegans and is a pore‐forming AMP. The protein (82 amino acids) was successfully synthesised by using Boc solid‐phase peptide synthesis and native chemical ligation. No γ‐linked by‐product was observed despite the use of a C‐terminal Glu‐thioester. The folding of the synthetic protein was confirmed by ¹H NMR spectroscopy and circular dichroism and compared with data recorded for recombinant caenopore‐5. The permeabilisation activities of the protein and of shortened analogues were evaluated.     

N-3 Polyunsaturated Fatty Acids Modulate In-Vitro T Cell Function in Type I Diabetic Patients

In this work, we assessed the in-vitro effects of eicosapentaenoic acid (EPA; C20:5n-3) and docosahexaenoic acid (DHA; C22:6n-3) (final concentration, 15 microM) on T cell blastogenesis, interleukin-2 and -4 (IL-2, IL-4) secretion, fatty acid composition and intracellular oxidative status in type I diabetic patients with or without complications. Con A stimulated lymphocyte proliferation, glucose uptake, intracellular reduced glutathione levels and catalase activity were lower in diabetics as compared to controls, regardless to the presence of complications. EPA and DHA diminished T-lymphocyte proliferation and IL-2 production but enhanced IL-4 secretion in both diabetic and control groups. No changes in the levels of reduced glutathione and in the activities of catalase and SOD were observed in control T cells cultured in the presence of EPA and DHA. However, in diabetic patients, addition of n-3 PUFA to culture induced an increase in T cell levels of reduced glutathione and hydroperoxide, and in activities of catalase and SOD. Low levels of arachidonic acid (C20:4n-6) were found in plasma membrane phospholipids of lymphocytes from diabetic patients compared to controls. Incubation of lymphocytes with EPA and DHA was associated with an incorporation of these fatty acids in membrane phospholipids. In conclusion, the beneficial effects of n-3 PUFA on T cell functions in type I diabetes could be attributed to their suppressive action and modulation of cytokine secretion, and to the improvement of intracellular oxidative status.     

Perovskite Formation and Dielectric Responses in PZN Modified PMF-PZT Ceramics

PMF-PZN-PZT （0.01Pb（Mol/3Fe2/3）O3-xPb（Znl/3Nb2/3）O3-（O.99-x）P（Zro53Tio 47）03 piezoelectric ceramics）, where x = 0.00 0.01, 0.03, 0.05 and 0.07 were prepared by a conventional mixed-oxide method. The results show that the pure peroveskit phase forms in these ceramics. X-ray diffraction analysis indicated that the phase of the material is a MPB （morphotropic phase boundary） structure. The effects of PZN content on the crystal structure and electrical properties were investigated, optimal dielectric properties were achieved at composition x = 0.07 ceramics by calcination at 800 ℃ and sintering at 1,180 ℃, with a curie temperature of approximately 430 ℃. These results clearly show the significance of PZN in controlling the electrical responses of the PMF-PZN-PZT system.