Sharif Digital Repository

In this thesis, we study the role of cooperation and security in multi-user channel from an information theoretic point of view. First, we review the wiretap channel as a fundamental model for the study of confidentiality in information theoretic security. To do this, we first introduce two measures for measuring the level of secrecy of confidential message, namely, average equivocation rate (AER) and equivocation rate in probability (ERP). By using a new proof, based on random covering, we show that the capacity-equivocation region of wiretap channel under AER and ERP is the same. To study cooperation in multi-user channel, we introduce the cooperative network in which each user can... 

Two new frameworks for achievability proof in network information theory (NIT) are introduced . The first framework which we call it OSRB (Output Statistics of Random Binning) framework, uses random binning arguments and is based on a duality between channel and source coding problems. Furthermore, the OSRB framework uses pmf approximation arguments instead of counting and typicality which are used widely in conventional achievability proofs in NIT. This allows for proving coordination and strong secrecy problems, where certain statistical conditions on the distribution of random variables need to be satisfied. These statistical conditions include independence between messages and... 

Due to both dynamic and opportunistic spectrum access, Cognitive radio networks is a promising solution for spectrum scarcity problem. In cognitive radio networks, users need to have a control channels in order to cooperate for sharing the sensing information, channel sharing and etc. In Ad-hoc cognitive radio networks because of the absence of infrastructure and central nodes, every user runs a distributed algorithm to create a common control channel. If the control channel gets jammed, network’s performance degrades dramatically, therefore the jammer’s target in cognitive radio networks is the control channels to disturb the networks’ performance. The traditional solution to confront... 

In recent years, severe plastic deformation processes have been widely used to correct the grain size of metals. One of these methods is equal channel angular pressing. In this research, the improvement of mechanical property and structural changes of pure magnesium and AZ91 alloy after the ECAP process are investigated. The mold is made of H13 steel with a 90-degree channel crossing angle and a zero-degree external curve angle. The purpose of this study is to find the smallest average grain size at the optimum temperature on these materials. First, pure magnesium and AZ91 are subjected to the ECAP process at temperatures of 250, 300, 350, and 400 °C to calculate the best operating... 

Today steel tubular systems are used more than before, especially in recent years. A difficulty with this type of systems is making beam to column connections using bolts due to no access to holes of bolts from the inside of tube or hollow circular column. Reverse channel connection is devised and introduced to prevail this problem. Access to holes in this connection is from the inside of channel. In this connection, the web of the channel is connected to steel beam using bolts and the two legs of channel is welded to face of tube column.
This type of connection does include some innovations; not too many researchers work on different versions of this connection. This thesis provides an... 

In this thesis, we consider multiple-antenna K-user interference channels with backhaul collaboration in one side (among the transmitters or among the receivers) and investigate the trade-off between the rate in the channel versus the communication load in the backhaul. In this investigation, we focus on a first order approximation result, where the rate of the wireless channel is measured by the degrees of freedom (DoF) per user, and the load of the backhaul is measured by the entropy of backhaul messages per user normalized by log of transmit power, at high power regimes. This trade-off is fully characterized for the case of even values of K, and approximately characterized for the case of... 

The need for a fast, light and affordable payment method, was a reason to produce multiple payment channel network on various of networks. To deploy and expand these networks, we need to deploy a profit model for new customers on the betweenness they add to network. In this research we maximum profit solutions for network with fixed payment cost and network with reduced betweenness profit for longer distances.At first, we suggest for a limited fund, we decide to make channels with most betweenness profit and prove it's a np-hard problem. Then for limited charge of making new channels, we show that it's a np-hard too. In graph of a path, we prove there is a solution based on length of path,... 

Proton exchange membrane (PEM) fuel cell performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of channels. It has been understood that the flow field design has a deterministic role on mass transport and water management, and thus great efforts have been made for the optimal design of flow field such that high and stable cell performance can be achieved. This study concentrates on the improvement in the performance of PEM fuel cells through optimization of the channel dimensions and patterns in the velocity and pressure fields in bipolar plates. For design and optimization purposes, a 2D numerical... 

During the past years, microtechnologies have been extensively used in Chemical reactions. In these systems, using an optimal system for separation, and purification of Chemical Products is of great importance. In this study, we have focused on the separation of organic-phase and aqueous-phase Products in 26 fabricated microsystems. The fabrication method of the glass microseparators consists of Lithography, Acidization, and Glass-to-Glass Thermal Connection. We have also studied key parameters of organic and aqueous phase separation in continuous capillary microseparators like the width of the capillary channel, length of the capillary channel, number of the capillary channels, and flow of... 

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... 

The human brain can be described as a dynamic system with multiple subsystems interacting with one another and multi-channel observations of these subsystems are available. The modeling of a system from its observations allows us to gain insight into how its various components interact with one another and also provides intuition about the desired system. Hidden Markov Model (HMM) is a probabilistic model with hidden states that is suitable for modeling these types of systems. Multi-channel observations are available from several subsystems interacting with each other within a general system. In this case, it may be necessary to develop more comprehensive models incorporating multi-channel... 

Increasingly adopting of multiple channels by companies in order to achieve greater market coverage and more sale, has made managers to deal with a new challenge of avoiding conflict in multiple channels. The importance of the issue and lack of empirical research provided motivation to study approaches to minimize multiple channel conflict and explore how they reduce conflict. Therefore a qualitative study using a single case (Solico Group) was conducted. After reviewing the literature 7 approaches were identified to minimize multiple channel conflict: Superordinate goals (goals that achieving them requires efforts and resources of more than one channel), Brand (using products with different... 

Due to compability of Magnesium with the body, it is a suitable material for making biodegradable stents, Althogh, its mechanical properties are not desirable for stent application. Accordingly, lately, magnesium’s microstructure and mechanical properties have been improved using various methods, including the process of severe plastic deformation (SPD). Many severe plastic deformation methods have been introduced for the fabrication of ultrafine grain tubes till now, which the process of tubular channel angular pressing (TCAP) is the most effective one. In the first part of the research, Magnesium tubes with a thickness of 1 mm were processed using the TCAP with a trapezoidal channel, and... 

In this study, the effect of submerged vanes on the bed scouring and deposition pattern at a 90̊ open-channel junction was investigated. First, the bed formation without submerged vanes were studied under the effect of hydraulic variables like the discharge ratio of tributary to main channel and the ratio of mean approach velocity to the threshold velocity. The results show that scour holes enlarge with time while they move towards the downstream and the outer wall in the main channel. Also, by increasing the ratio of mean approach velocity to the threshold velocity about 0.5, the maximum scour depth occurred at the downstream junction corner with time. In the second stage, the proper vane... 

Nowadays, swift development of cellular network with the increased penetration rate, have made mobile voice channels widely available almost everywhere with higher priority comparing to the other conventional channels, e.g., GPRS, EDGE, HSDPA and etc. Leveraging on this fact two important utilization can be considered. The first one is related to urgent transmitting of vital data. As mentioned earlier, since this channel deems to be available everywhere and, recalling its priority, it is even accessible in congestion condition, it can be exploited as an efficient way to transmit small volume of emergence data urgently. Transforming the crash data on the road is an example of utilizing such... 

Understanding the flow of liquids and particularly water in nanochannels is important for scientific and technological applications, such as for filtration and drug delivery. In this study, we perform molecular dynamics simulations to investigate the transfer of single-file water molecules across straight or nonstraight single-walled carbon nanotubes (SWCNTs). In contrast with the macroscopic scenario, the nonstraight nanostructure can increase the water permeation. Remarkably, compared with the straight SWCNT, the nonstraight SWCNT with the minimal bending angle of 30° in the simulations can enhance the water transport up to 3.9 times. Also increasing length and diameter of carbon nanotubes... 

Micropumps are regarded as one of the devices used in microsystems, which are responsible for pumping working fluid. The Mercury Magneto Reciprocating (MMR) micropump, which has been introduced for less than a decade, is an innovative kind of micropumps which the pumping agent includes three liquid metal droplets (LMD) placed inside lateral chambers and reciprocated by the electromagnetic force inside the chambers. The working fluid located inside the main channel is pumped through due to the movement of these three LMDs. The equations governing this micropump are complex and their numerical solution is a time-consuming process, due to electromagnetic, hydrodynamic, and unsteady effects. The... 

In this thesis, the effect of cognition in wireless networks is studied. In a wireless communica-tion network, cognition is the ability of a device to sense the transmissions of neighboring nodes and adapt its own transmission to the sensed environment. Cognitive radios are very attractive be-cause of the increasing need for more efficient use of frequency spectrum. The goal of this thesis is the analysis of cognitive radio networks from an information theoretic perspective and to this end statistical models of the wireless medium with a limited and tractable number of devices are uti-lized. We consider three different models in three parts of the thesis. First, we consider a new... 

Time offset (TO) at an orthogonal frequency division multiplexing (OFDM) system destroys orthogonality of subcarriers and causes interchannel interference (ICI) and inter symbol interference (ISI). This problem is more challenging in multi-user multi-input multi-output (MU-MIMO) OFDM systems such as coordinated multi-point (CoMP) uplink transmission where joint processing is performed at base stations (BSs) and different delays form users to BSs cannot be compensated by timing strategies. In this thesis, we use delay-amplify and forward relays and post processing at receivers to decrease signal to interference plus noise ratio (SINR) degradation due to large TOs at insufficient cyclic prefix... 

One of the main challenges in the communication and storage of information is its evolution due to the environmental noises. There are various techniques in coding theory to decrease the probability of changing the message using redundancy. Among all of the possible codings, the maximum rate that can be achieved for reliable communication through a channel is called the capacity of that channel. In the classical information domain, this quantity has been investigated in the Shannon's second theorem and is equivalent to a finite convex optimization problem. This problem is much more complicated when dealing with quantum information and thus studying special common channels has been doubled in...