Sharif Digital Repository

One of the key subjects in the area of mathematical optimization is a class of problems known as combinatorial optimization. We can find the optimal solution of continuous optimization problems feasible in time. But, in combinatorial optimization, we aim to obtain the optimal solution of the problem over a finite set. These problems are NP-hard and no polynomial-time solution has been proposed for this class of problems so far. Thus, in practical scenarios, we often use heuristic methods for solving NP-hard problems. There are lots of heuristic methods and choosing the best one in different situations might be challenging. In recent years, with the advances in deep neural networks,... 

One of the most important goals we pursue in telecommunications science is to send and receive information from telecommunication channels. By designing a powerful telecommunication system consisting of a transmitter and a receiver, we achieve this goal. Speed of data transmission, accuracy of received information and speed of data extraction are some of the criteria by which the performance of a telecommunication system can be evaluated. No telecommunication channel is free of noise. For this reason, additional information is added to the original information in the transmitter, which can still be extracted if the original information is noisy. This process is called coding. Following... 

In the communication systems, the raw signals of information are mainly encoded so as to prevent the detrimental effects of channel noises and distortions. After some processes, this encoded signal is passed through the channel. At the receiver side, the received signal has to be decoded to extract the information signal. In order to decode the received signal, the receiver require prior knowledge about the encoder parameters. The traditional approach is to send the encoder parameters along with the encoded signals. However, this transmission overhead might occupy a considerable amount of bandwidth since the type of coding may alter several times in a fraction of a second based on the... 

In some applications of distributed systems, agents use weak communication models instead of sending and receiving messages to each other. Beeping, Stone-age, and Population protocols are examples of weak communication models. For example, population protocols are currently used in a variety of areas, such as biological systems (like molecular programming).In this model, the agents are initialized based on their own values. These agents then wake up according to their local clocks and interact with other agents randomly. During these interactions and updating states repeatedly, the agents converge to their final states, which is the expected outcome of the problem.We consider the problem of... 

We study the problem of experiment design to learn causal structures from interventional data. We consider an active learning setting in which the experimenter decides to intervene on one of the variables in the system in each step and uses the results of the intervention to recover further causal relationships among the variables. The goal is to fully identify the causal structures with minimum number of interventions. We present the first deep reinforcement learning based solution for the problem of experiment design. In the proposed method, we embed input graphs to vectors using a graph neural network and feed them to another neural network which outputs a variable for performing... 

Given a social network modeled as a weighted graph G, the influence maximization problem seeks k vertices to become initially influenced, to maximize the expected number of influenced nodes under a particular diffusion model. The influence maximization problem has been proven to be NP-hard, and most proposed solutions to the problem are approximate greedy algorithms, which can guarantee a tunable approximation ratio for their results with respect to the optimal solution. The state-of-the-art algorithms are based on Reverse Influence Sampling (RIS) technique, which can offer both computational efficiency and non-trivial (1-1/e-ϵ)-approximation ratio guarantee for any ϵ>0. RIS-based... 

In this dissertation, we design channel access strategies for opportunistic spectrum access in cognitive radio networks. It is assumed that the secondary transmitter and receiver hop across spectrum opportunities synchronously. This synchronization between the transmitter and the receiver can be separated in two phases: In the first phase, two Secondary Users (SUs) meet each other for the first time in a spectrum opportunity and exchange control messages. In the second phase, two SUs hop across available channels to transfer data. Rendezvous algorithm can facilitate the first meeting of two SUs. For the first phase, we propose a blind rendezvous algorithm and study its features. We show that... 

In numerous scientific disciplines, analyzing a system often fails to reveal significant relationships between its various variables. However, in certain instances, changes in one variable can influence the behavior of one or more other variables. Systems where such underlying causal relationships exist are referred to as causal systems. The process of inferring the structure of these systems is known as causal learning. In causal learning, given observed samples from a system and under certain assumptions, the joint distribution of the variables can determine the equivalence class of their corresponding graphical model. Once the equivalence class is identified, interventions are employed to... 

This paper concerns the delay margin achievable using fractional order Proportional_Integral_Derivative controllers for linear time- invariant systems subject to variable, unknown time delays. The basic issue under investigation addresses the question: What is the largest range of time delay so that there exists a single fractional order Proportional_Integral_Derivative controller to stabilize the delay plants within the entire range? Delay margin is a fundamental measure of robust stabilization against uncertain time delays and poses a fundamental, longstanding problem that remains open except in simple, isolated cases. In this research, we develop explicit expressions of the exact delay... 

Cable-driven robots are referred to as parallel robots actuated with cables. In fact,in such robots rigid links are replaced by cables, which may be extended to desired lengths without making mechanism much heavy. This robot possesses a number of unique properties that makes it suitable for many industrial applications. The main factor which makes cable robots analysis different from other parallel robots is the incapability of cables to push objects. Hence, obtaining the workspace of a cable robot is one of the most important subjects associated with this type of robot.The goal of this thesis is to design a spatial translational cable driven robot, which may be used for object handling. For... 

Uncertain fuel prices and also global climate changes are accompanied by some state initiatives such as renewable portfolio standards (RPS). This has caused a fast growth in the amount of renewable energy installed worldwide especially wind energy over last decades. However, the intrinsic characteristics of wind farms output power, i.e. intermittency and volatility of wind speed along with being non-dispatchable in output generation of wind turbine raises many new technical and financial challenges for power system operators and planners. Up to these issues, large level of wind power penetration causes a growing concern in wind energy curtailment issue which the wind farm’s operator may be... 

In recent decades the use of FRP materials for strengthening structures has been developed extensively, thanks to their high strength, sufficient stiffness, high durablity, and ease of use. A great deal of research has been conducted on strengthening reinforced concrete beams and columns with FRP composites. By contrast, not many researches are performed on slabs, especially two-way slabs. In these few researches, the loading and boundary conditions are limited to simplest ones. Therefore, in this thesis, the focus of attention is on strengthening reinforced concrete slabs under various loads and boundary conditions using FRP composites.
In this thesis, first, FRP composites and their... 

Access to global memory is one of the bottlenecks in performance and energy in GPUs. Graphical processors uses multi-thread in streaming multiprocessors to reduce memory access latency. However, due to the high number of concurrent memory requests, the memory bandwidth of low level memorties and the interconnection network are quickly saturated. Recent research suggests that adjacent thread blocks share a significant amount of data blocks. If the adjacent thread blocks are assigned to specific streaming multiprocessor, shared data block can be rused by these thread blocks. However the thread block scheduler assigns adjacent thread blocks to different streaming multiprocessors that increase... 

In this thesis, various fractional order controllers (FOCs), e.g., FO-PID, FO-TID, CRONE and FO-Lead/Lag, are implemented using PLC. For this purpose first, several methods are used to approximate the differentiator and integral operators. Based on these methods, the equivalent rational transfer functions of the fractional order controllers are obtained. Then, using STEP7 software, the FOCs are implemented in Siemens PLC. Moreover, to facilitate the use of the implemented FOCs in the industry and to take full advantage of the controllers, WinCC software as an interface between human and machine (HMI) is used. The WinCC software enables the operator to access the PLC through a computer (PC)... 

In this study, some of robust and exact tuning methods for fractional order PD and PI controllers are investigated in order to extract their solution existence conditions. Using aforementioned methods needs solving a set of nonlinear equations to obtain desired controller parameters which satisfy the control objectives. Hence, one of the aims of this study is to find necessary and sufficient condition on tuning specifications which guarantee the applicability of the aforementioned methods. The achievable performance region of the methods is obtained through the extracted necessary and sufficient conditions. The other aim of this work is to investigate the uniqueness of the methods... 

Along with the development of electronic exchanges, accessibility to various data streams, increasing computing power, decreasing trading costs, and growing competition in financial investment industry, quantitative trading strategies or quantitative trading rules have developed rapidly in the recent decades. These strategies try to forecast the future price movements of risky assets from the historical market information in algorithmic ways or statistical ways and thus challenge the Efficient Market Hypothesis.
The increasing attention to these strategies and lack of related empirical studies in the financial markets of Iran, motivate the research in this area. Furthermore, despite its...