Sharif Digital Repository

Designing proper spectrum decision schemes for cognitive radios (CRs) improves spectral usage efficiency as well as CR network throughput. One of the key effecting factors of the CR network throughput is the spectrum sensing sequences of the secondary users (SUs). In this paper, the SUs throughput maximization through appropriately selecting the sensing sequences of the SUs, called sensing matrix (SM), is investigated. More specifically, first the average throughput of the CR network is evaluated for a given SM in the case of error-free channel sensing. Then, an optimization problem is formulated with the aim to maximize the network throughput by finding the optimal SM. In order to avoid... 

The Chern-Simons magnetohydrodynamics (CSMHD) is introduced using a Maxwell-Chern-Simons (MCS) Lagrangian including an axionlike field . The MCS equation of motion derived from this Lagrangian consists of a modified current, including a chiral magnetic (CM) and an anomalous Hall (AH) current, in addition to the ordinary Ohm current of resistive magnetohydrodynamics (MHD). The former consists of an axial chemical potential, which is given in terms of the temporal comoving derivative of , and the latter arises from the spatial gradient of . As it turns out, the existence of the axial chemical potential is a nonequilibrium effect that plays no role in the linear stability analysis, whereas the... 

Theoretical studies on the spin-dependent transmission and current-voltage characteristic in a mesoscopic system, which consists of two semi-infinite ferromagnetic (FM) leads (as source and drain) separated by a typical periodic quantum dot (QD) are presented. The calculations are based on the tight-binding model and transfer matrix method, and investigate the magnetoresistance (MR) and the spin polarization within the Landauer-Büttiker formalism. The spin-dependent transport behavior can be controlled via a gate voltage and an applied bias in the ballistic regime. The numerical results are shown for a periodic polymer chain with nonmagnetic (NM) and FM leads, and also, with two FM leads.... 

Powerful spectrum sensing schemes enable cognitive radios (CRs) to find transmission opportunities in spectral resources allocated exclusively to primary users. In this paper, the problem of maximizing the average throughput of a cognitive radio system through optimizing its spectrum sensing time is investigated, and a systematic neural network-based optimization approach is proposed which avoids challenges associated with the conventional analytical solutions. The proposed method exploits a novel learning and optimization cycle to enable an effective cooperation between two kinds of well-known artificial neural networks and finds the optimum value of the channel sensing time without any... 

Effective spectrum sensing strategies enable cognitive radios (CRs) to identify and opportunistically transmit on under-utilized spectral resources. In this paper, sequential channel sensing problems for single and multiple secondary users (SUs) networks are effectively modeled through finite state Markovian processes. More specifically, a model for single user case is introduced and its performance is validated through analytical analysis. Then, in order to address multiple SUs case, this model is extended to include the modified p-persistent access (MPPA) protocol. Since the scheme utilized experiences a high level of collision among the SUs, to mitigate the problem appropriately,... 

In this study, the average throughput maximisation of a secondary user (SU) by optimising its spectrum sensing time is formulated, assuming that a priori knowledge of the presence and absence probabilities of the primary users (PUs) is available. The energy consumed to find a transmission opportunity is evaluated, and a discussion on the impacts of the number of PUs on SU throughput and consumed energy are presented. To avoid the challenges associated with the analytical method, as a second solution, a systematic adaptive neural network-based sensing time optimisation approach is also proposed. The proposed scheme is able to find the optimum value of the channel sensing time without any... 

Copper sulfide deposited ZnO nanorods (ZnO NRs/CuS) have been applied as a new counter electrode material with high electrocatalytic activity towards polysulfide electrolyte, which results in the formation of a highly efficient counter electrode for QDSSCs. It was observed from the current density-voltage (J-V) characteristics that the short-circuit current density (Jsc), power conversion efficiency (PCE), and fill factor (FF) were enhanced from 7.63 mA cm-2 to 14.48 mA cm-2, 1.59% to 4.18%, and 0.29 to 0.38, respectively, when a bare CuS counter electrode was changed to a ZnO NRs/CuS counter electrode. Electrochemical impedance spectroscopy (EIS), Tafel polarization and cyclic voltammetry... 

Like-charge ion pairing is commonly observed in protein structures and plays a significant role in biochemical processes. Density functional calculations combined with the conductor-like polarizable continuum model were employed to study the formation possibilities of doubly charged noncovalently linked complexes of a series of model compounds and amino acids in the gas phase and in solution. Hydrogen bond interactions were found to offset the Coulombic repulsion such that cation-cation clusters are minima on the potential energy surfaces and neither counterions nor solvent molecules are needed to hold them together. In the gas phase the dissociation energies are exothermic, and the... 

In this paper, the project earthquake occurrence risk coefficient is determined for each construction project that is located in one of Iran's twenty seismic regions. This coefficient is allocated, regardless of the current situation of the project, being in the plan or execution phase or even completed. This coefficient indicates the possibility of an earthquake occurrence during a project's life time. To find this coefficient, the Gutenberg-Richter linear relationship has been applied, in conjunction with the Poisson distribution. The Gutenberg-Richter linear equation expresses the relationship between the magnitude of an earthquake and the number of occurrences, during a fixed time, of... 

In the present article, thermal behavior and heat transfer deterioration (HTD) of transcritical methane as well as the fluid state change in regenerative cooling with straight/curved rectangular channels are studied numerically. Simulations are conducted with a finite-volume based CFD solver utilizing reliable turbulence models and thermo-fluidic relations in transcritical conditions. The experimental and numerical results of hydrogen inside a heated tube in the literature are used for validation. The effects of mass flow rate, outlet pressure, wall temperature, surface roughness, and the channel geometry on the thermal behavior of the coolant fluid are studied in detail. According to the... 

In this article, the conjugate heat transfer of the methane coolant inside a rectangular channel is studied and the related Nusselt correlations are improved. The compressible methane flow enters the cooling channel at supercritical pressure and subcritical temperature. The coolant flow absorbs heat from the heated walls and exits the channel with a supercritical temperature. An in-house solver is developed employing Semi-Implicit Method for Pressure-Linked Equations-Consistent (SIMPLEC) algorithm accompanied by the appropriate thermodynamic and transport property relations for the supercritical conditions of the methane coolant. The solver is validated with the experimental data found in... 

This work explores the pyrolysis characteristics and kinetic behavior of paper-laminated phenolic printed circuit boards (PLP-PCBs) using thermogravimetric analysis under non-isothermal linear heating programs. The initial estimation of the kinetic parameters during the pyrolysis was obtained from the analysis of the experimental data by three isoconversional kinetic models, i.e. Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) integral methods, as well as the Friedman differential method. For all three methods, the apparent activation energy exhibited a strong dependence on the degree of the reaction conversion. To allow for the complexity of the reactions involved in the PLP-PCB... 

Plasma-catalytic conversion of formaldehyde, as a representative volatile organic compound, in a dielectric barrier discharge reactor over a NiOx/α-Al2O3 catalyst was studied. Effects of plasma treatment on the physicochemical properties of the catalyst were studied and XPS analysis showed that plasma treatment increased the surface contribution of more active sites (i.e. Ni2+ and adsorbed O). However, the physical properties of the catalyst, including BET surface area and porosity, did not significantly change after plasma exposure. Presence of oxygen in the plasma-catalytic process improved conversion efficiency. Individual roles of plasma and catalyst in the selective conversion of... 

Scheduling sports leagues has drawn significant attention to itself in recent years, as it involves considerable revenue as well as challenging combinatorial optimization problems. A particular class of these problems is the Traveling Tournament Problem (TTP) which focuses on minimizing the total traveling distance for teams. In this paper, an efficient simulated annealing approach is presented for TTP which applies two simultaneous and disparate models for the problem in order to search the solutions space more effectively. Also, a computationally efficient modified greedy scheme is proposed for constructing a favorable initial solution for the simulated annealing algorithm. Our... 

Residential and commercial sectors, share a significant portion of energy consumption in Iran; 37 percent of the total per capita consumption. Therefore, identification of the relation between revenue and energy consumption could be a great assistance to policy makers for clarification of policies employed in energy sector. This paper attempts to explore and analyze the relationship between revenue and energy consumption in residential and commercial sectors for the years 1969 to 2009 in Iran by Smooth Transition Regression Models. Smooth transition regression model is a nonlinear time series model which can be considered as a developed form of Switching Regression Model. The obtained...