Schedule-Based Cooperative Multi-agent Reinforcement Learning for Multi-channel Communication in Wireless Sensor Networks

Wireless sensor networks (WSNs) have become an important component in the Internet of things (IoT) field. In WSNs, multi-channel protocols have been developed to overcome some limitations related to the throughput and delivery rate which have become necessary for many IoT applications that require sufficient bandwidth to transmit a large amount of data. However, the requirement of frequent negotiation for channel assignment in distributed multi-channel protocols incurs an extra-large communication overhead which results in a reduction of the network lifetime. To deal with this requirement in an energy-efficient way is a challenging task. Hence, the Reinforcement Learning (RL) approach for channel assignment is used to overcome this problem. Nevertheless, the use of the RL approach requires a number of iterations to obtain the best solution which in turn creates a communication overhead and time-wasting. In this paper, a Self-schedule based Cooperative multi-agent Reinforcement Learning for Channel Assignment (SCRL CA) approach is proposed to improve the network lifetime and performance. The proposal addresses both regular traffic scheduling and assignment of the available orthogonal channels in an energy-efficient way. We solve the cooperation between the RL agents problem by using the self-schedule method to accelerate the RL iterations, reduce the communication overhead and balance the energy consumption in the route selection process. Therefore, two algorithms are proposed, the first one is for the Static channel assignment (SSCRL CA) while the second one is for the Dynamic channel assignment (DSCRL CA). The results of extensive simulation experiments show the effectiveness of our approach in improving the network lifetime and performance through the two algorithms.

     

Photothermoelectric (PTE) Versus Photopyroelectric (PPE) Detection of Phase Transitions

The photopyroelectric (PPE) technique is one of the photothermal (PT) methods mostly used for phase transitions investigations. In this paper, we want to compare the PPE results with those obtained using another, recently developed PT method [the photothermoelectric (PTE) calorimetry] for the same purpose of detecting phase transitions. The well-known ferro-paraelectric phase transition of TGS, taking place at a convenient temperature (about \(49\, {}^{\circ }\hbox {C}\)), has been selected for demonstration. A comparison of the two PPE and PTE methods, both in the back detection configuration (in the special case of optically opaque sample and thermally thick regime for both sensors and sample) shows that they are equally suitable for phase transitions detection. Performing a proper calibration, the amplitude and phase of the signals can be used in order to obtain the critical behaviour of all sample’s static and dynamic thermal parameters.     

On the accurate determination of thermal diffusivity of liquids by using the photopyroelectric thickness scanning method

An enhanced accurate method of measuring the thermal diffusivity of liquids by the sample's thickness scan of the phase of the photopyroelectric signal is presented. The method, making use of the absolute values of the phase and sample thickness, leads to very accurate results for the room temperature values of thermal diffusivity (about +/-0.3%). The high accuracy of the method is due to a very precise control of the sample's thickness variation (0.1 microm step), to a proper localization of the thickness scan range, and to a new procedure of data analysis. The high accuracy of the method recommends it for the study of processes associated with small changes of the thermal parameters.     

HVOF sprayed WC–Co coatings: Microstructure,mechanical properties and friction moment prediction

This study aims at gaining a better understanding of the microstructural features that control the mechanical and the tribological performances of WC–12 wt.% Co coatings under High Velocity Oxygen Fuel (HVOF) spraying conditions. This paper looks at the influences of the HVOF process parameters for WC–12Co material on the microstructural and the tribological behaviours of the coatings. The correlation between the coating microstructure and the wear behaviour is investigated by observing and analysing the microstructure and by studying the friction moment using enhanced statistical tool based on neural computations. According to the experimental and the numerical results, it has been shown that the spray parameters affect the phase composition, hardness and porosity of HVOF sprayed WC–12Co coatings and the correlations with HVOF process parameters are fully predictable in the steady-state regime.     

Effect of pressure on the structural and elastic properties of ZnS and MgS alloys in the B3 and B1 phases

From the first-principles calculations, we have investigated the elastic stiffness coefficients C₁₁, C₁₂, C₄₄ and the bulk modulus B of the II-VI semiconductors ZnS and MgS under hydrostatic pressure. The calculations are based on the density functional theory within the generalized gradient approximation (GGA) for exchange-correlation interaction. For the structural properties we have shown that ZnS adopt the rocksalt (NaCl or B1) structure over 11.87 GPa pressure, the same character is adopted by MgS over 0.8 GPa. The elastic coefficients have the same behavior for the different structures of alloys; they increase with increasing pressure values. Our results for the structural parameters and equilibrium phase elastic constants are in good agreement with the available theoretical and experimental data.     

Maintainability defects detection and correction: a multi-objective approach

Software defects often lead to bugs, runtime errors and software maintenance difficulties. They should be systematically prevented, found, removed or fixed all along the software lifecycle. However, detecting and fixing these defects is still, to some extent, a difficult, time-consuming and manual process. In this paper, we propose a two-step automated approach to detect and then to correct various types of maintainability defects in source code. Using Genetic Programming, our approach allows automatic generation of rules to detect defects, thus relieving the designer from a fastidious manual rule definition task. Then, we correct the detected defects while minimizing the correction effort. A correction solution is defined as the combination of refactoring operations that should maximize as much as possible the number of corrected defects with minimal code modification effort. We use the Non-dominated Sorting Genetic Algorithm (NSGA-II) to find the best compromise. For six open source projects, we succeeded in detecting the majority of known defects, and the proposed corrections fixed most of them with minimal effort.     

Degenerate-wave mixing in new dithienylethylenes

We report the measurement of the degenerate fourth-wave mixing (DFWM) of new dithienylethylenes in chloroform solutions at λ=532 nm in ps regime with different numbers of π-conjugated bonds. From these measurements, we evaluated the values of the second order hyperpolarizabilities γ, which are about 10³ larger than the γ value of CS₂. The influence of π-conjugated bonds on the third-order susceptibilities and appropriate figures of merits is discussed. The more important seems to be the possibility of a simultaneous increase of the third-order susceptibilities, together with the decrease of the absorption coefficients that open a possibility of their use as promising materials for laser wavelengths mixing.     

Photopyroelectric Calorimetry of \hbox {Fe}_{3}\hbox {O}_{4} Magnetic Nanofluids: Effect of Type of Surfactant and Magnetic Field

Five types of magnetic nanofluids, based on \(\hbox {Fe}_{3}\hbox {O}_{4}\) nanoparticles with water as the carrier liquid, were investigated by using the two photopyroelectric (PPE) detection configurations (back (BPPE) and front (FPPE)), together with the thermal-wave resonator cavity (TWRC) technique as the scanning procedure. The difference between the nanofluids was the type of surfactant: double layers of lauric (LA–LA), oleic (OA–OA), and miristic (MA–MA) acids and also double layers of lauric–miristic (LA–MA) and palmitic-oleic (PA–OA) fatty acids were used. In both detection configurations, the information was contained in the phase of the PPE signal. The thermal diffusivity of nanofluids was obtained in the BPPE configuration, from the scan of the phase of the signal as a function of the liquid’s thickness. Using the same scanning procedure in the FPPE configuration, the thermal effusivity was directly measured. The influence of a 0.12 kG magnetic field on the thermal effusivity and thermal diffusivity was also investigated. Because of different surfactants, the thermal effusivity of the investigated nanofluids ranges from \(1530\,\hbox {W}\cdot \hbox {s}^{1/2} \cdot \hbox { m}^{-2}\cdot \hbox { K}^{-1}\) to \(1790\,\hbox { W}\cdot \hbox {s}^{1/2}\cdot \hbox { m}^{-2}\cdot \hbox { K}^{-1}\) , and the thermal diffusivity, from \(14.54~\times ~10^{-8}\,\hbox { m}^{2}\cdot \hbox { s}^{-1}\) to \(14.79~\times ~10^{-8}\,\hbox { m}^{2}\cdot \hbox { s}^{-1}\) . The magnetic field has practically no influence on the thermal effusivity, and produces a maximum increase of the thermal diffusivity (LA–LA surfactant) of about 4 %.     

Microcrystalline Bi2ZnB2O7-polymer composites with silver nanoparticles as materials for laser operated devices

A novel type of composite for optoelectronic which is operated by second harmonic generation in the Bi₂ZnB₂O₇ crystallites (with sizes varying within 1–30 μm) and Ag nanoparticles (NP) embedded in PMMA polymer composites is proposed. The substantial influence of the Ag NP on the bicolor induced second harmonic generation was established. The phototreatment was performed by bicolor beams of nanosecond Nd:YAG laser (1,064/532 nm) at angles between the fundamental and photoinducing beams varying within the 19°–21° range. The studies of the corresponding dependences of the SHG during illumination by the two coherent beams at 1,064/532 nm showed a maximal enhancement of the output SHG for the Ag NP average sizes equal to about 40 nm. The role of the excited plasmons may be here crucial. Additionally the time shift between the fundamental and the doubled frequency beam maxima was found, which shows strong sensitivity to illumination. The established time shift is sensitive to the pumping power.