Sharif Digital Repository

In this thesis we have studied slotted and unslotted ALOHA random access optical CDMA packet switching networks with a chip-level receiver structure. We used generalized optical orthogonal codes (GOOC) and Overlapping PPM (OPPM) signaling technique to improve system performance. The impact of physical layer parameters such as GOOC code cross correlation value and OPPM overlapping index on key performance benchmarks such as network throughput and delay are analyzed. We have also studied the stability of the network based on the number of backlogged users, using expected state drift. It has been shown that using GOOC instead of strict OOC improves the network average delay and throughput.... 

In this thesis, we consider a network creation game in which, each player (vertex) has a limited budget to establish links to other players. In our model, each link has a unit cost and each agent tries to minimize its cost which is its local diameter or its total distance to other players in the (undirected) underlying graph of the created network. Two variants of the game are studied: in the MAX version, the cost incurred to a vertex is the maximum distance between that vertex and other vertices, and in the SUM version, the cost incurred to a vertex is the sum of distances between that vertex and other vertices. We prove that in both versions pure Nash equilibria exist, but the problem of... 

In addition to investigating the overall situation of healthcare information systems (HIS) in Iranian hospitals, in this thesis the goal is to determine the key elements of success and failure in the implementation of HIS. With the advent of the computer systems in Iran and due to the successful and unsuccessful experiences of organizations, the necessity of detecting the influential elements in this field is perceived even more than before. Another importance of this research is because of the essential role of the cultural and social factors in the success of these systems; this claim can be justified by observing different results founded by researchers from different countries. In this... 

Power system frequency regulation entitled load frequency control (LFC) has been one of the important control problems in electric power system design and operation. Off-normal frequency can directly impact on power system operation and system reliability. A large frequency deviation can damage equipment, degrade load performance, cause the transmission lines to be overloaded and can interfere with system protection schemes, ultimately leading to an unstable condition for the power system . On the other hand, the increasing need for electrical energy in the 21st century, as well as limited fossil fuel reserves and the increasing concerns with environmental issues for the reduction of carbon... 

Controlling damage level of structures for an earthquake with certain intensity is the main purpose in seismic performance based design. This is done by estimating and confining the structure displacements and related global and local response parameters. Even though nonlinear time history analysis method is the most exact method for predicting seismic behavior of structures, but its application is limited because of being complex. Nonlinear static analysis procedure (Pushover analysis) which is based on analyzing the structure subjected to the incremental lateral load is an alternative method that has been recommended by recent structural codes. In order to consider the soil-structure... 

A series of wind tunnel tests were performed to examine the effects of wing sweep and wing tip vortices on the pressure distribution of the upper surface of three wings with sweep angles of 23,33 and 40 degrees. The wing section had a laminar flow airfoil similar to that of NACA 6-series airfoils. All tests, were conducted at a chord Reynolds number of 8×〖10〗^5, and for a range of angle of attack -2 to14 degree. Generation of Wing tip vortices are eliminated by placing an End-plate on the wing tip. Static pressure distributions over the upper surface of the wing at three chordwise direction and at one spanwise direction along the quarter chord line are obtained. The results show that the... 

User cooperation is an important concept in wireless networks to achieve best performance subject to low cost. Dynamic and broadcast natures of wireless networks are two indispensable characteristics of these networks. Therefore, when a user transmits his signal to the receiver, other users in his neighborhood may receive the signal as well. Hence, these neighbors can decide to cooperate with each other in transmitting their messages to the receiver. On the other hand, due to the dynamic nature of the wireless networks, one can assume that each user has some knowledge about the interference signals or the fading coefficient that are not known to the other users. Therefore by utilizing user... 

Accurate modeling of clutter and detection of low observable targets within clutter are the major problems in radar signal processing applications. Recently, fractal geometry is applied to the analysis of high range resolution radar sea clutters. The intrinsic difference in the fractal dimension between sea clutter and the echoes from targets is extracted and used to detect targets. The box-counting method is widely used to estimate fractal dimension but it has some drawbacks never or rarely considered in literature. We explain the drawbacks and propose a new fractal dimension based detector to increase detection performance in comparison with traditional detectors. Both statistically... 

In this thesis, we examine different forecasting methods to predict volatility in financial markets. Tehran Exchange Price Index (TEPIX) is adapted to forecast in short and long term periods. TEPIX is the most important index in Tehran Stock Market which is officially reported daily. Autoregressive Integrated Moveing Average models (ARIMA), Generalaized Autoregressive Heteroskedastic models (GARCH) and Artificial Neural Networks (ANN) are used for forecasting TEPIX. Spectral Analysis is also regarded as a completely new approach in financial mathematics to forecast TEPIX in short and long term periods. We consider different criteria to compare the performance of different methods of... 

In this thesis, three popular option pricing models, Black and Scholes Model, Heston Model and Bates Model are compared to each other in economic crises situation. We carry out a comparison between different option pricing models based on S&P500 Index options during 2008. First these data are classified regarding to moneyness and maturity time in nine categories and then the performance of the models, exerting two indexes, In Sample performance and Out Of Sample performance are compared in every category. To reach the prices via In Sample performance index, structural parameters of each model are estimated with the data of each day. Then, daily prices are computed by models and compared with... 

In this research, the numerical simulation of liquid jet ejecting for a series of elliptical orifices into gaseous phase with different aspect ratios, at Rayleigh flow regime and in a range of Weber numbers has been performed.
For simulations, the OpenFoam software was utilized, which its multiphase flow simulation solvers are based on volume of fluid (VOF) method. In order to achieve the most accurate results for axis-switching phenomenon and jet breakup length, the dynamic mesh refinement was used for all the examined cases. The results, which were validated with recent experimental and numerical works, indicate that jet breakup length raises by increasing Weber number from 20 to 300.... 

In this thesis, a computer program is produced to measure the activity of 137Cs and 60Co radionuclides in waste packages generated at Bushehr Nuclear Power Plant (BNPP). In this program, the activity of waste drum was calculated by dividing the waste matrix into very small identical cubic cells, and each cell was considered as a point source (in the center of the cell). The activity was calculated using the theory developed by Krings & Mauerhofer. Detection efficiencies were calculated by the developed program at different conditions of a) different detector distance from drum surface, b) different collimator length, c) different cell size and d) up to 4 segment number. The results were... 

Today energy sources, in any kind, have their special situation in human’s life and we must use our knowledge to manage them for effective usage by decreasing losses and finding modern renewable sources. One of the best way to achieve this purpose is increasing efficiency and improve energy generator performances under different environmental conditions. In spite of the fact that wind turbines stall dynamics behavior is under investigation for 6 years, study of dynamic stall phenomenon especially over series blades still needed. By addition to this phenomenon, improper wind turbine arrange decreases efficiency too. As a matter of fact, scrutiny of dynamics stall of wind turbines serial... 

Beam diagnostics is an essential constituent of any accelerator. The position information of the electron beam is obtained by a set of Beam Position Monitors (BPMs), which in addition to determining the position, are used to derive information concerning lattice functions and beam dynamics. To design of Button Beam Position Monitor(BPM) for a Synchrotron Light Source, to get beam stabilities on submicron levels, preliminary studies were done to have optimal design. We present an algorithem which caculate all required parameteres for different design of button BPMs. To calculate induced charge and voltage on each button, Poisson's equation has been solved by Green method. For sensivity... 

In the absence of appropriate control, several accidents in the nuclear power, lead to serious consequences including core melted and radiation leakage outside reactor containment. In all these events, when core melted, the reactor operation is always finished and all efforts are then directed to environmental impacts of radioactive materials to the external environment. However, the occurrence of events that would bypass reactor containment, have special significance, because all possible efforts in order to limit the leakage of radioactive materials to the external environment are ineffective. In this respect the occurrence of such incidents, as most of which is considered the worst... 

Today, the fractional-order systems are widely used in the system identification and design and implementation of control systems. By production of electrical elements with fractional-order relationship between voltage and current of their terminals, implementation of fractional-order functions with passive elements is possible. Fractional order capacitors are one of these elements. It was shown that necessary and sufficient condition for the implementation of an integer order function by passive elements is positive realness of such a function.
Like systems with integer-order, in this thesis the aim is to find corresponding conditions for implementation of fractional-order functions... 

Transonic flow study is important due to its vast applications. Supercritical airfoils are designed for transonic flow to postpone drag divergence and reduce the strength of shock. This type of airfoils are chiefly used in modern commercial airplanes because of their low drag in transonic regime and, as a result, reduction in their fuel consumption. Using supercritical airfoils needs accurate investigation of flow field due to several influential factors such as specific geometry, nonlinear nature of flow phenomena, and compressibility effects. However, the availability of experimental and numerical data for supercritical airfoils is less than that of the conventional ones. Thus,... 

Due to recent advances in laser technology and instruments, laser assisted machining processes has gained extra attention by scientists and industries. Naturally, effective modeling of the process is of great interest for researchers. Machining processes are generally modeled via FE tools and based on the requirements of model, it can be performed either mechanically or thermos-mechanically. About the LAM problems, it is basically meaningless to have a model without considering thermal effects. Hence, modeling is performed in two steps where through the first one, a thermal problem is solved to bring the temperature distribution of the work piece as the initial conditions of the next step.... 

This work describes the effect of hydroxyapatite with different morphology including nanorods and whisker on the properties of hydroxyapatite/polycaprolactone (HA/PCL) composite. Biodegradable polymer/Hydroxyapatite (HA) composites with the morphology of nanorods and whisker of (HA) as bone replacement scaffolds are synthesized by sol-gel and hydrothermal methods, respectively with a Ca/P ratio of 1.67 and 20:80 (HA/PCL).The crystallization behavior and porosity of HA/PCL composite are studied by scanning electron microscope and X-Ray diffraction. The response of bone marrow-derived human mesenchymal stem cells (hMSCs) in terms of cell proliferation and differentiation to the osteoblastic... 

The synthetic aperture radar (SAR) is a microwave imaging radar that achieves high resolution while taking advantage of pulse compression technology and the Doppler effect. In typical radar systems, in order to obtain a good azimuth resolution, one should use an antenna with big aperture. However this is not possible in many cases. SAR has overcome this problem by moving the antenna and so making a synthetic aperture. In inverse SAR (ISAR) the radar antenna is static and synthetic aperture is created by the movement of target. Typical SAR imaging aims at the static targets and when the target is moving, it does not operate well. When detecting moving targets by SAR, both radar and target are...