Sharif Digital Repository

This thesis investigates methods to improve the utility of cellular spectrum to secondary users. Here, secondary users exploit the spectrum by hierarchical spectrum access. Since the information of the channels is available at the central controller, cellular networks can play an active role in spectrum use of the secondary users and increase their utility in this way. Transmitting the information available at the central controller to the secondary users and ordering the channels in use by primaries can help improve spectrum sensing. First, we consider the schemes which maximize secondary users’ utility by ordering the channel allocation process to primary users. We model the secondary... 

Increasing demand for higher data rates and more reliable communications along with growing applications of multimedia networks in everyday life on one hand, and limited resources on the other, have given rise to researches investigating spectrum efficiency enhancement techniques, among which multiuser diversity has always been of considerable importance. This thesis is a study on utilization of multiuser diversity in wireless LANs via scheduling. Where throughput and Quality of Service (QoS) in communication networks are being investigated, it is mostly advantageous to describe the traffic behavior of resources by greedy or non-greedy models. So far, there are plenty of investigations on... 

Network coding as an efficient approach for performance improvement of communication networks has been introduced by Ahlswede et al. Despite simple forwarding schemes, in network coding scenarios, intermediate nodes forward the encoded version of their received packets. In wireless Networks, resource allocation includes routing, network coding, flow control, scheduling and power control.In this dissertation a joint scheduling, routing, flow control and Inter-session NC formulation based on a convex optimization problem is studied. Our formulation integrates the optimal scheduling, flow control and Inter-session network coding with a modified version of minimum delay routing. We propose a way... 

Fair allocation of bandwidth between stations of a shared medium, is an important and challenging problem. It is even more challenging when there is no coordinator to arbitrate access to the medium and therefore, a distributed algorithm is needed to provide fairness. IEEE 802.11 DCF is the dominant distributed medium access control (MAC) protocol in a wireless LAN. However, DCF is not in general fair and suffers from short-term unfairness. In addition to that, it is desired to provide differentiated service for the stations of a LAN based on their demands. Since priority based protocols may lead to starvation of the low priority traffic, we are more interested in fair queuing. On the other... 

Generally to deal with errors in communication systems, two methods have been proposed: FEC and ARQ. In FEC, redundancy bits are added to the data that causes detection and correction of certain error patterns. ARQ method is based on retransmission of information if there is an error in the information. Another method, which is called Hybrid-ARQ, is the combination of these two methods; there are two types of TypeI and TypeII for this category. In typeI, number of redundancy bits in consecutive transmissions of one codeword is constant but in typeII, number of redundancy bits of a codeword increases in subsequent retransmissions.
To deal with burst errors, we can use interleaving along... 

In a data network, expressing an appropriate model for traffic and service is necessary for the guarantee of quality of service (QoS) parameters, e.g. delay and backlogged traffic. There are four main approaches for modeling the traffic and service. First approach is classic queuing theory. It is a useful tool for analyzing the networks with memory less input traffic. Second approach is deterministic network calculus. In this approach some deterministic bounds on delay and backlogged traffic can be derived. However, it is not accurate for analyzing the bursty traffic and wireless networks, because only trivial bounds can be derived in these scenarios. Third approach is effective bandwidth... 

In contrast to traditional Networks where a designer can specify an action plan for each agent, in a network with strategic agents, every agent acts selfishly and chooses his strategy privately so as to maximize his own objective. In this dissertation, we study problems arising in the design of static and dynamic networks with strategic agents. We consider two classes of design problems. In the first class, the designer utilizes her control over decisions and resources in the system to incentivize the agents via monetary incentive mechanisms to reveal their private information that is crucial for the efficient operation of the system. In this class of problems, we address two main challenges... 

There has been a sudden emergence of next generation networks in the past decade where the primary purposes are data ggregation/mining, distributed information and signal processing, and environmental control and monitoring. The distributed algorithms operating in such networks, should have simple structure and be robust against node failures or network dynamics.Extensive studies on designing and analyzing these algorithms have resulted in introducing different models of distributed systems with similar properties such as gossip algorithms, population protocols, and cellular automata-based systems. In this dissertation, we take first steps toward understanding the computational power of... 

There are some important technologies for the 5th generation of wireless communication (or 5G) and one of them is transmitting signals by millimeter waves or mmWave. Since mmWave antennas are so small and they have high frequencies, mmWave signals are highly directable. MmWave antennas can generate transmission patterns that have a number of beams directed in different directions with small beam width.In this thesis, we propose a scheduling scheme for the mmWave networks with directional antennas.Since using millimeter waves is one of the most important choices for 5G, designing an efficient transmission scheduling algorithm in these networks have been a quite big challenge. As there are... 

Scheduling algorithms are responsible for allocation of various types of resources, interference management, and choosing the times and paths through which messages are to be delivered in a communication network. We consider certain fundamental problems in network scheduling. Any scheduling algorithm, to be practical, must have tractable computational complexity and tolerable delay. In the first part of the thesis, we address the following question: “for a given class of networks, does there exist a scheduling algorithm that performs polynomially many computations (in terms of the network dimensions), while keeping the expected delay of the network polynomially bounded (again in terms of the... 

Intercellular coordination is a technique to mitigate intercell interference. In this technique, base stations of different cells coordinate with each other to send data for all users, so each user is serviced by several base stations. Nowadays, with the increasing use of wireless networks, the design of these networks is very important. Most problems in the design of wireless networks are due to limitations such as: interference, the lack of bandwidth and changes of channel. Using all channels in each cell is a suggested method to use the bandwidths efficiently; however it increases intercell interferences hence a method to control interference is needed. Coping with interference can be... 

The loss function form in deep networks has been the subject of numerous studies in machine learning communities in recent years. Despite the fact that the problem of optimization in these networks has high dimensions and usually non-convex, gradient-based algorithms have acceptable results in practical experiments. In other words, the convergence point is usually a low-cost local minimum, and conventional optimization algorithms do not get stuck in high-cost local minima. It has been shown that this phenomenon occurs if the number of network weights is much greater than the number of training samples, and it is referred to as the over parameterization region. Recently, several studies have... 

In the modern world today, the increasing need for transfering data in high speeds and long distances has encouraged the investors of this area to use more efficient technologies of this field. With regard to the need for extensive and scalable communication networks, reusing the bandwidth in closer distances seems to be inevitable. Hence, finding an optimal way of using resources like bandwidth and power is necessary. In OFDMA-based networks there is no intra-cell interference due to using orthogonal subbands and the most important limiting factor in designing these networks is inter-cell interference. Therefore in this thesis we are gonig to analyase the partial frequency reuse method... 

Technological developments in today’s world followed by the rapid rowth of energy consumption has given rise to global concerns about the environment. Hence, energy-efficiency is widely addressed in various areas of academic and industrial research. In communication technology too, energy-demanding systems such as wireless cellular networks have undergone immense and rapid developments, resulting in major concerns about their energy consumption.To address these concerns, the 4G generation of cellular standards provides an option for relay transmission in order to reduce the energy required for achieving a given communication rate in a certain area of coverage. In this thesis, we address the... 

According to the wide range of applications and numerous implementations, wireless networks are considered as a prominent means of providing data services, and have attracted the attention of many researches in recent years. Performance enhancement of these networks can be very beneficial to industry and wireless data service providers. Manipulation of relays, have been always considered as a proper means for improvement of network performance measures throughput and outage probability.In order to design relay networks efficiently, one should address two following problems: 1- determining transmission policy, and 2- relay location. In this research work, having in mind the optimality of... 

In this thesis, the individuals’ decision process to achieve consensus in complex networks is modeled. A set of experiments with volunteered participants were run, in which the person is a node in a network with 35 other nodes who behave based on a prescribed model. Each participant is tested twice, once with and once without reward. Performance of individuals in the two experiment types and the effect of structural properties of the underlying network on the results are studied. It is observed that increase in the characteristic path length, diameter, clustering coefficient and local efficiency of the network adversely affects consensus.On the other hand, global efficiency and closeness... 

Environmental pollution and depleting the resources of energy are two significant crises which has been caused by excessive Energy consumption in all over the world. One of the most appropriate and reproducible energies is the Biogas energy. Huge amount of sewages and dry wastes are produced in cities every day can causes devastated damages to environment. All types organic matter extracted from different processes of treating sewages are influenced by germ activities and will be transformed to mineral structures and the final product of these reactions would be gases which can be used as enormous sources of fuel and energy. There are different devices made all over the world to be utilized...