Sharif Digital Repository

Cementing plays a crucial role in ensuring the long-term integrity of oil and gas wells, geothermal wells, and geologic sequestration (GS). The productivity of the well is directly impacted by the quality of cementing operations throughout its lifecycle, spanning drilling, production, and abandonment phases. This thesis focuses on modeling cement setting under varying pressure and temperature conditions within wells. It aims to accurately estimate the thickening time of cement slurry, and analyze the influence of thermodynamic factors like pressure, temperature, and heat generated by hydration reactions on the cement setting process through laboratory investigations. The study also delves... 

Dye-sensitized solar cell (DSSCS) is the third-generation of solar cells based on semiconductors formed between a photo sensitized anode and counter cathode which make a photo electrochemical system. They haven’t been commercially marketed due to their lower efficiency than the previous generations. In order to achieve the higher efficiency, the electron injection and light absorption must be increased. One way to increase electron injection is doping the semiconductor with an external ion to reduce lattice band gap. In this work, we made powder and Nano-structured film of titanium dioxide doped with various molar ratios of Mg by the sol-gel process. The effect of Mg:Ti molar ratio on... 

Energy Harvesting is one of the techniques which can be utilized in Green Communications and increase the wireless networks lifetime. In an EH system, each resource is supplied with the energy harvested from the environment. When we confront with a situation that the users of one channel want to increase their rates and this action negatively affects the other user’s rates, the user’s cooperation problem for obtaining a fair rate will be created. In this study, at first we consider a Multiple Access Channel and we assume that the energy resources of transmitters are harvested according to a known probability. In addition, we assume that the transmitters do not know the instantaneous amount... 

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... 

Neural systems are the threshold ones. It means if the electrical potential passed through a specific amount then they could spike and cause the activity of other cells. The models that really mimic the action of neurons are usually complicated and are not suitable when you put them on a network to study the collective behavior of the neurons. Simple threshold models have been designed for such purposes. One on most studied ones is the ’integrate and fire’ model, in which cells integrate the inputs until the threshold potential and then spike. usually, a network of these objects are simulated and different properties of such network are investigated.However, this model has some shortcomings... 

The increasing expansion of wireless communications and the increase in information traffic and the number of communication users, and on the other hand, the frequency and energy frequency limitations have led to the emergence of new generation communications. In the fifth generation of communications, one of the solutions to ensure the optimal use of bandwidth is the use of the non-orthogonal multiple access (NOMA) schema, which of course, the use of such a scheme in the system naturally increases the complexity of the receivers. Unlike most access methods in previous generations of communications, the goal of NOMA is the simultaneous use of multiple users from a frequency band for data... 

The problem of distributed binary hypothesis testing in the Gray-Wyner network with side information is studied in this dissertation. An observer agent has access to a discrete memoryless and stationary source and describes its observation to two detecting agents via one common and two private channels. The channels are considered error-free but rate-limited. Each detecting agent also has access to its own discrete memoryless and stationary source, i.e., the side information. The goal is to perform two distinct binary hypothesis testings on the joint distribution of observations at detecting agents. Additionally, the observing agent aims to keep a correlated latent source private against the... 

Collaborative edge computing (CEC) is a recently popular paradigm enabling sharing of computation resources among different edge devices. In CEC, multiple stakeholders (mobile users, mobile edge computing servers, cloud servers,...) collaborate to provide new computation capacity in order to perform computation-intensive tasks efficiently in the edge. In the thesis, each task, the data that should be computed in the cloud or edge, was modeled as a graph of dependent sub-tasks. Resource allocation for task offloading is an important problem to address in CEC as we need to decide when and where each subtask is executed. In this work, we mathematically formulate the problem of resource... 

The increase in mobile data traffic adversely affects the quality of service experienced by mobile users. Moreover, the cost incurred by developing infrastructure exceeds its profit. Therefore, there is a trend toward finding cost effective schemes to cope with increase in mobile data traffic. One of the recent elegant solutions is mobile data offloading based on D2D communications. In this thesis, we propose a new offloading scheme using D2D communications for delay-tolerant services. In this regard, we assume a common file is demanded by a subset of users in different time instances and we accumulate demands to serve them in a scheme where devices are divided into non-overlapping... 

Energy crisis has always been a crucial issue faced by the development of wireless communication techniques. As a more energy-efficient network architecture, green communications and networks (GCNs) have recently attracted significant attention from academia and industry. In order to enable the technical and economical GCNs, several emerging techniques have been proposed including energy-efficient and energy harvesting techniques. Although these emerging techniques have drawn considerable attention and have been studied recently, there are still many open theoretical and practical problems to be addressed. Energy harvesting (EH) is a key technology to achieve the objectives of GCNs. This... 

In the next generation of wireless networks, wireless network virtualization is considered as an efficient technology to enhance the network capacity. This technology enables the separation between wireless network infrastructure and the services it provides that enhances some of the existing capabilities and make new capabilities in the network. Spectrum sharing,infrastructure sharing, network sharing, resource slicing are some of these capabilities.Furthermore, the mentioned separation causes the lower entry barriers for small beneficiary entities. The increment in the number of beneficiary entities makes intense competitions among them leading to lower costs for different services. Also,... 

With the increasing use of wireless communications, the need for optimal utilization of bandwidth and energy has become undeniable. Reconfigurable Intelligent Surfaces (RIS) have emerged as a promising technology for the efficient use of telecommunications resources. One proposed model of these surfaces is the Simultaneously Transmitting and Reflecting RIS (STAR-RIS), which, unlike conventional reflective surfaces, can simultaneously reflect part of the incoming waves and transmit part of them. This study examines the potential use of these surfaces in simultaneous wireless information and power transfer networks. In the defined problem, energy receivers are not reliable, and there is a... 

Increasing demand of bandwidth in wireless communications systems led to excessive congestion in frequency spectrum. Since radio spectrum is an expensive and rare source, its efcient use is an important challenge. These limitations as well as increasing number of communications service users,necessitate the use of newer technologies that work efciently with numerous users. Device-to-Device (D2D) communications, full-duplex capability and cognitive radio systems are introduced for this reason. In the previous technologies, due to excessive self-interference (SI), full-duplex transmission has not been implemented. However nowadays technology allows self-interference to be attenuated up to 100... 

Sharing secret key is an essential prerequisite of symmetric key cryptography. So, we consider a source model for information theoretic secret key agreement based on the distance among the nodes. Secret key agreement based on information theory, unlike computational models, guarantee full information secrecy, eavesdropper receives no efficient information. The model is a basic system consisting of two legitimate users and an eavesdropper. The legitimate nodes try to agree on a reliable and secure key based on their noisy observation of distance between them. Eavesdropper observes the distance, too. First, we model distance estimation by the nodes to study performance of the system (secret... 

Due to the explosive growth of mobile data traffic and growth of the number of access points (APs), Wi-Fi data offloading is a promising solution to cope with the data explosion. In delayed WiFi offloading, mobile users (MUs) can intentionally bear some delay in order to increase the chance of offloading and remarkably improve offloading ratio. In this thesis, we propose a new decision method for delayed offloading in a network comprised of a base station (BS), some non-overlapping WLANs, and some MUs. We take into account the residual service time and the congestion level in WLAN as the key parameters for decision making in offloading process. Because these parameters are time-varying and... 

We study an LTE-WiFi-Aggregation (LWA) network where some of the users support the LWA function, first standardized in 3GPP Rel. 13, and can simultaneously receive data from a WiFi access point and a cellular base station. In LWA networks, a base station can communicate with one or more Wi-Fi access points and if need be, it can transmit some of the user packets through the access points and their Wi-Fi networks. We consider an LWA network where some of the users are in the coverage of a base station and an access point, and support the LWA function. We aim to provide a scheduling policy for routing the LWA packets such that their expected delay is minimized. Three methods are proposed for... 

Caching is the process of storing part of popular contents near the users, which not only results in better traffic balance, but also reduces the delay of content delivery. Due to limited cache capacities and time-varying nature of contents popularity, using a proper replacement policy for cached contents during the heavy traffic hours is necessary. Many of the existing methods have ignored the dynamics of requests. Also, most of the previous works have focused on parameters other than QoS characteristics as the performance metric.We propose a cache updating policy for wireless networks considering dynamic popularity for file requests. Our model is a wireless heterogeneous network consisting...