Sharif Digital Repository

In this thesis, we investigate the previously introduced Code-Shifted Reference (CSR) time hopping (TH) ultra-wideband (UWB) communication systems as a potential transmitted scheme reference technique. In this system, the orthogonality between the reference and data bearing signals is acquired in code domain. Also, we consider a simple scheme to clean to obtain a clean template to be used in the correlator receiver for the CSR –UWB, this method was used in UWB system before. In this method, which we call as averaged (Ave.) CSR-UWB, the template for each symbol is obtained by taking the average over all the frames of the symbol interval. It is shown that Ave-CSR system achieves higher... 

In recent years, UWB multiple access communication systems have received a great attention due to its many advantages. The most important advantages of a UWB system can be summarized as its high data rates, low power consumption, fading immunity and high precision routing. One of the important problems in a UWB system implementation is in its receiver part in which in order to use a coherent RAKE receiver, the channel coefficients have to be estimated. Since UWB channel is a dense multipath fading channel with a very high number of resolvable paths, the channel coefficient estimation based on using training sequence is very complicated. To resolve this problem, transmit reference (TR)... 

Providing security for digital information in a communication channel is a challenge in the recent years. This is due to the strength of high-speed processing resources that can threaten the security and correctness of communicating information. Using steganography method, the secret information can be hidden in an innocent media that does not attract the attention of third parties. This research discusses about steganography in images. The main goal of this research is presenting a universal steganography scheme with high steganography capacity. Using this scheme the steganographer can embed every large secret data in images securely. The reasons for selecting the subject of “Improving... 

In recent years, coded cooperation has been considered as one of the best methods to cope with fading and at the same time to achieve channel coding gain. On the other hand, the idea of network coding has been introduced for increasing the efficiency of cooperative networks. In addition, it has been shown that only by jointly designing of network coding and channel coding, the capacity will be achievable. Also, to adjust the rate to the channel capacity without requiring the channel state information at the transmitter, Rateless coding has been proposed. In this thesis above techniques are combined to gain their advantages and to improve system performance. To this end, first we consider a... 

With recent successes in laboratory tests, and in atmosphere and space demonstrations, there is no doubt that the free-space optical (FSO) communications is ready for operational deployment. Large bandwidth, unlicensed spectrum, excellent security, and quick and inexpensive setup are among its most attractive features. Despite their attractive features, FSO communications suffer from several challenges in practical deployment; the major of them is fading or scintillation and path loss. To overcome such limitations, Channel Coding along with MIMO and Cooperative techniques have been proposed. Although the promising effects of cooperative transmissions in RF communications have greatly been... 

Recently, demand for using variant applications in distributed networks has been increased largely. Most of these applications are based on group communication.In distributed group communication, two important factors need to be considered, group construction, and Group Key Management (GKM). The group construction specifies the process of constructing a new group by cooperation between distributed users. As well, GKM ensures re-freshness of the group key after each join/leave operation. Both of these tasks must be done by collaboration between members. This leads to imposing complexity and communication overhead to the system.This thesis proposes efficient methods for group construction and... 

In this dissertation, the statistical analysis of ultrashort light pulse detectors, in presence of different noise sources, is presented. By investigating performance degradation of ultrashort light pulses detection based on linear conventional detectors, the necessity of all optical preprocessors prior to the low speed photodetectors is approved. By using these preprocessors, we achieve high discrimination between noise and ultrashort light pulses at the photodetector input, for simplicity in detection of light pulses by conventional low speed photodetectors. Among various types of preprocessors, in this dissertation, we focus on optical crystals with dominant imposed nonlinear second... 

Studying data flows in conventional applications of Multi-Processor System-on-Chips (MPSoCs) denotes that most of these flows are the ones that transfer huge volume of data in inter-core communications. Previous works try to present architecture for interconnection network which some paths with low power and latency are reserved (statically or dynamically). However all of the presented methods are based on subnetworks or mechanism of transferring control messages (to establish a path and tear it down after transmission of data). Optical connections with low cost, low power and high bandwidth are good candidates to reduce power consumption of Network-on-Chips (NoCs). Therefore, using optical... 

Communications in organizations have been one of the most important issues. To run the business processes, the company needs communication between units and customers. We provide a tool which can be used as an aid for network designers by guiding them with a framework. The resulting QoS parameters help to provide more alignment between the network and business requirements. The framework contains values for delay, loss and availability as three QoS parameters of the network used in the business process. The organizations business processes are used as a basis for the method which used in the tool. We use UML as the tool to model the business processes in the organization and the process is... 

High volume domestic and international trade and business and their risks, force companies and business people to use financial services such as banks and insurance companies much more than ever, but heavy competition in this industry impel financial service managers to work hard on their marketing strategies and plans. Promotion and communication tools are one of the most important parts of each marketing campaign, but they are usually very costly and marketing managers have to find and use more effective techniques to manage their budget limitation. The purpose of this thesis is to find out the most important promotional and communication strategies in financial services industry and to... 

Swarm Robotics consists of homogonous robots with considerable abilities. In recent years, engineering applications, including multi-agent control and air vehicles have been existed and engineers have been interested more in multi-agent systems. Currently, progressive number of studies is being done on decreasing the expenses, developing applicability and compactness of robots. Exchanging some simple robots with a complicated robot results omitting over-complexity and expensive equipments. This project is looking for a method for emerging connectivity and consequently maintaining it for a swarm robotic suffering from limited communication. Limited communication means that agent distinguish... 

Most of the current routing protocols in the WMSNs only consider the inter-path interference and not necessarily intra-path interference factor which is considered as one of the main causes of energy consumption in these networks. Among those protocols that consider this factor, none of them has ever come up with an algorithm capable of performing the routing without the need to use either the special hardware in the nodes (referred to as the GPS or directional antennas) or with the unrealistic assumption that the interference range is equal to the transmission range. The previously mentioned factors are critical in the sense that the former assumption results in more expensive sensor nodes... 

Willingness to communicate (WTC) has recently received great attention in the area of second language learning. Thus, the concept of willingness to communicate and what would influence the students' willingness to communicate gain importance. The current study employed a hybrid design to probe Iranian students' willingness to communicate. First, self-report questionnaires were administered to 431 language learners to collect quantitative data. Then, interviews were conducted with 41 randomly selected learners who had already completed the questionnaires. Pearson correlation coefficient, one-way ANOVA, independent t-test through SPSS .16 and the Structural Equation Model (SEM) analyses... 

Today, several centuries after the time when science began to flourish of science, everybody is familiar with destroying consequences of modern science and technology, in addition to its achievments. Besides, domination and exclusive position of science in educational system and government organizations have marginalized alternative knowledge. Thus, some people found this trend dangerous for humankind’s fate and tried to change this situation. Nasr and Feyerabend are two philosophers that despite their different work area both criticise modern science and try to establish their own ideal scientific community.Nasr’s approach is regeneration of traditional science which was prevalent before... 

In this thesis, we study and analyze the performance of matched and partially-matched re-ceivers for a typical Ultra Wide-band Spectrally-Encoded Spread-Time (SE/ST) CDMA in the presence of multiple tone jammers and a coexisting OFDM system. We investigate a scenario in which there is not any information about the jammer and OFDM subcarrier frequencies in the transmitter side and we try to cancel jammers and interferences at the receiver. For this case, we demonstrate that by using a partially-matched filter at the receiver in an environment with multiple tone jammers and some coexisting systems the performance of the SE/ST system can be improved. We also assume K asynchronous users and... 

Communications is considered as a large part of project managers’ activities. Communications is one of success factors in the projects. A project manager in addition to have a good sight on project knowledge areas needs to know how to use them in order to reach a suitable point with integrating them by using communications. A survey on communications can predispose lots of dissertations in this field & subsequently causes improvement & continues development.
In this thesis, different areas of project body of knowledge have been explained & effect of communication on them has been described. In next step, this study depicts how to manage communications in projects & project-oriented... 

Complex networks have recently become a very noticeable subject because of their remarkable theory on real networks such as rumor spreading. This thesis contributes to a prevention strategy which uses a maximal clique algorithm to find overlapping communities and also uses inverse targeting immunization strategy to immune these communities. By the using maximal clique algorithm combined with inverse targeting strategy we have achieved that the TIK method (targeted immunization using k-clique percolation) obtains 10% of better immunization than HD (highest degree) method which is one of the best methods in targeted immunization strategies. The TIK also demonstrates that rumor spreading is... 

Opinion formation is one of the major topics in social networks analysis and mining. A number of methods have been introduced for describing and simulating this phenomenon. In this thesis, we proposed a novel optimization framework based on an opinion formation model that used the Deffuant’s model with some changes in neighbors’ selection policy. Its efficiency was compared with a number of benchmark optimization methods including genetic algorithms, differential evolution and particle swarm optimization.The proposed model demonstrated better performance than the others.In the proposed, each person changes his opinion based on one of his neighbors who has the best performance in terms of the... 

Design of a spiking neural network suitable for electronic hardware implementation has been a hot research topic for a long time, in recent years implementation of spiking neural network in real time and biological scale has become important for scientists. This is due to the proximity of these networks to human brain, and the efforts led to the creation of numerous biological models and hardware which has been proposed in this context. This research includes two main parts, the first part is effort to implement a greater number of neurons per device and the second part proposes a communication system among devices to achieve biological scale. This work investigates novel neural elements and...