Sharif Digital Repository

In this research, path planning in unknown environment is considered. It is assumed that the robot’s sensors are the only tools to collect information from the scene. Volume of the information gathered from the environment depends on the capability of the sensor. So, a detour from the optimal path is unavoidable. In this research, the basic robot is equipped with a minimal sensing system that only detects the iscontinuities in depth information (gaps). We present some online search strategies that guide such a robot to navigate unknown streets from a start point s to reach a target point t. Then, we empower the robot by adding a compass to patrol more general classes of polygons. We present... 

TIN (Triangulated Irregular Network) is a Data Structure for storing and manipulating surfaces like maps in GIS applications or 3D models in game engines or Vector Fields . A TIN contains a set of triangulated vertices (which has irregular distribution) and each vertex shows a point in space . We trying to find the smallest possible set (optimal solution) of vertices from input surface (which given as a TIN) to approximate input surface with epsilon error which is given as input. also, surface simplification problem is a member of NP-Hard class. The hardness of problem makes it infeasible to find an optimal solution in polynomial time. So we try finding an approximate algorithm for this... 

Optical metasurfaces are 2D structures of a repetitive arrangement of a number of nanoparticles that are artificially constructed and do not exist in nature. Important applications of optical metasurfaces are meta lenses, flexible solar cells and extremely small antennas. Fabricating surfaces with nanoparticles of proper geometry under light radiation along with elastic deformation, we can have optical metasurfaces with selective reflection or transparency in the visible spectrum. The aim of this project is modeling of an optical metasurface with elastic deformation leading to change of color and transparency. For this purpose, assuming vertical light radiation and linear polarization, we... 

Incoporating Gauge theory to study 4-dimensional manifolds,initiated by works of Donaldson,leading to resolve many problems of 4-dimensional topology.after Donaldson ,Seiberg and Witten introduced a novel Gauge theory that we will introduce and study some application of this theory.at beginning, we will establish Seiberg-Witten equation on these manifolds, following that we investigate moduli space of solutions of Seiberg Witten equations and later we define Seiberg - Witten invariants on these manifolds.at f we probe some applications of Seiberg-Witten invariants in Symplectic Geometry and we will show that Seiberg-Witten invariants are non-zero for Symplectic manifolds with canonical SpinC... 

As the core of this thesis, we express the relation between optimal transportation and convex geometry especially the variational approach to solve Alexandrov problem, which leads to a geometric interpretation to adversarial models and propose a novel framework for these models. Along the way, we peruse generative adversarial networks from optimal transportation view and show that generator calculates the transportation map and the discriminator computes Wasserestein distance, which is equivalent to Kantorovich potential. By using optimal mass transportation theory and choosing an especial cost function c, we see that the generator and discriminator are equivalent. Therefore, once the... 

In this research, we consider the problem of guarding orthogonal polygon with sliding cameras. This problem is originated from the well-known Art Gallery problem. In the Art Gallery problem the goal is to guard the polygon with minimum number of guards. Each guard is considered as a point and can guard all the points inside the polygon that are in its visibility area. In sliding camera version, each guard is modeld as a horizontal or vertical segment which is entirely in the polygon. The guarding area of each camera is defined by its segment and its power. In 0-transmitter model, the guarding area of each camera is limited by the edges of polygon, but in the k-transmitter model cameras can... 

The layout problem is directly related to the material handling system; therefore, these two concepts are always being considered together. The main criterion for evaluating the layout in the objective function, is the cost of controlling and moving the materials. In order to calculate the material handling cost, it is necessary to provide an accurate information on the distance between departments, number of goods transported, and the cost of moving a unit of goods per unit of distance; therefore, accurate calculation of the distance between departments is an issue of paramount importance. In the literature, the distance between the departments is measured in three ways: Euclidean distance,... 

In this thesis, we focus on a subset of geometric optimization problems (including k-center) in the Sliding Window model. The sliding window model is driven from the Data Stream model in which input points arrive one by one and the space is limited. The main diffrenece of these two models is that in the sliding window model we are interested in the N latest points not all of the arrived points. In this thesis, we study Minimum Enclosing Ball, 2-center, and Euclidean k-center in the Sliding Window model. We provide a (1 + ")-approximation algorithm for MEB in d-dimensions. To our knowledge there is no algorithm for MEB in d-dimensions where d >2. We also provide a (1 + ")-approximation... 

In Massive multiple input multiple output (MIMO) systems, due to utilizing a large antenna array in each base station, each user’s channel is orthogonal to one another. These systems have some valuable advantages such as the reduction of energy needed for transmission and significant improvement in spectral efficiency, which makes them be suitable for the next generation of wireless communication systems. In this thesis a massive MIMO working is considered on TDD mode, for wich the channel estimation is done in the base station using the pilot sequences recieved from users. At the beginning of each coherence block, users send their pilot sequences and the rest of the times is allocated to... 

The present paper proposes a micromechanical damage model for two and three dimensional Representative Volume Element (RVE) with randomly distributed cohesive rough slit-like and penny-shaped micro cracks (Barenblatt-Dugdale type). First, the influence of crack roughness on the crack opening under macro hydrostatic stress state is studied and then the energy release contribution to material damage process is estimated. Considering the fractality of the crack trajectories yields to lower values of the volume crack opening. This will result in lower energy release rate in RVE and higher levels of material resistance. Based on the energy release rate of RVE, the effective material properties... 

Broadcasting in wireless ad hoc networks serves many important tasks such as data dissemination and route establishment in which one node sends its data to all other network nodes. The most popular form of broadcasting adopted by many ad-hoc protocols is flooding. In flooding, each node receiving a packet from a neighboring node rebroadcasts it exactly once. Wide adoption of flooding for broadcasting traffic is mainly due to its simplicity. The importance of performance analysis of flooding lies on its aforementioned popularity. The parameters of interest in analysis of flooding are network coverage, energy consumption and broadcast throughput while network coverage is the average percentage... 

This research presents inverse isogeometric models based on the deformation theory of plasticity to predict the blank and strain distribution in sheet metal forming processes. In this study, the NURBS functions are utilized for both modeling of a final part and analysis of the forming process. Therefore, the developed models require only one modeling and analysis representation, in contrast to finite element models which deal with two separate modeling and analysis representations. In this research, a membrane one-step inverse isogeometric model is initially presented to analyze sheet metal forming processes. This method has various advantages such as solving the governing equations in... 

Multi Input Multi Output (MIMO) systems can be used in wireless communications in order to increase data rate. This increase in data rate becomes possible with multiplexing and diversity techniques. Indeed using MIMO systems we may achieve high spectral efficiency. High spectral efficiency is obtained by the fact that scattering environment causes each transmitted signal in each receiver antenna to be uncorrelated to a great extent with other transmitted signals. Many parameters play role in performance of a MIMO system. One of these parameters is position of each antenna in transmitter or receiver arrays relative to other antennas. In this thesis we study receiver array geometry effect on... 

Massive MIMO systems are systems, where the base station has many number of antenna so each user’s channel is orthogonal to one another.These systems have some valuable advantages such as reduction of energy needed for transmission and significant improvement in spectral efficiency, which makes them be suitable for next generation of wireless communication systems. Another advantage of Massive MIMO systems is inherent resistance against passive eavsdroppers due to having a directed power radiation in direction of the desired user. However another physical layer attack threat these systems: attacking in pilot phase then eavsdropping in downlink phase which must taken care of in physical... 

If we consider a point set on the plane as the vertices and straight-line segments between them as the edges of a graph, and weight of each edge is equal to its Euclidean length, from this kind of graphs, triangulation is the graph with maximum number of the edges. In a triangulation, dilation of any two vertices is equal to the ratio of their shortest path length to their Euclidean distance, and dilation of the triangulation is defined as the maximum dilation of any pair of its vertices. Dilation is a characteristic that shows how much a triangulation approximates a complete graph. In the minimum dilation triangulation problem, the objective is to find a triangulation of a given set of... 

Correct and timely extraction of smoke is very important in fire incidents. Statistics show that more than seventy percent of fire casualties are caused by breathing toxic gases. One of the destructive factors in the operation of the smoke exhaust system is the plugholing phenomenon. In this phenomenon, due to the improper design of the ventilation system, the exhaust vent extracts air instead of smoke. The plugholing phenomenon causes the accumulation of smoke in the structure, therefore it is essential to know this phenomenon to prevent it. In this research, the plugholing phenomenon in the smoke extraction process has been investigated by the numerical simulation method. To investigate... 

In this thesis, we considered several types of the visibility problem. These problems include computing the visibility polygon of a point observer inside a polygonal domain, maintaining visibility polygon of a moving point observer, visibility coherence in space, maintaining visibility polygon of a moving segment observer and visibility dependent simplification. Furthermore, we considered these problems in both offline and streaming settings. These problems arise in different practical areas, such as computer graphics, machine vision, robotics, motion planning, geographic information systems (GIS) and computational geometry. We obtained effective theoretical results as well as superior... 

In recent years, application of porous media is highlighted among researchers due to their wider use in micro-scale problems, such as in gas reservoirs, micro-filtering, and heat exchangers. In such applications, accurate description of flow behavior, using governing equations based on continuum assumption, is not valid, since mean free path has the same order as the characteristic length of the problem. In such cases, imposing appropriate slip condition on the fluid-solid interface of porous media (in pore-scale level), based on kinetics theory, is an appropriate approach. For this purpose, pore-scale simulation of flow inside porous media in slippery and transient regimes is carried out... 

Security for wireless communication networks has become a crucial issue because of the broadcast nature of wireless channels. On every cellular network Since it is possible for passive nodes to easily overhear the confidential information transmitted between transmitter and receiver, it is necessary to provide secure communication.When a cellular user wants to communicate with a node or base station on the cellular network, it expects that information will be transmitted and received without interference, with enough quality and without eavesdropping. On the other hand In practical scenarios, we know that cellular network nodes are not equipped with ideal equipment and some percent... 

An interesting theoretical and practical set of problems in computer science is concerned with the study of spatial relations among objects in a geometric space. Examples of such problems for a set of points P are finding the closest pair of the points P, partitioning space into regions such that all points of a region have minimum distance to the same point in P, and computing the Euclidean minimum spanning tree on P. Moreover, we need mechanisms to efficiently update these properties when the points P are allowed to move or may be inserted or deleted. This is to avoid re-computation of these properties from scratch. Here, we consider the Euclidean minimum spanning tree (EMST) of a set of...