Sharif Digital Repository

With the proliferation of mass media such as smartphones and the use of social networks, the exchange of video data between users has increased, and according to forecasts in the coming years we will see significant growth in the amount of this data on mobile networks. In this dissertation, we examine the effect of sending video packets via LTE and WiFi and try to calculate two parameters that affect the quality of experience, namely the probability of interruption and the amount of delay. The modeling results show that if the packets are optimally routed to these two paths, there is about a 15% improvement in the probability of interruption. Sending packets from two separate routes Although... 

In wireless multi-user networks, choosing the right multiple access method plays a significant role in providing services to users considering network constraints. On the other hand, Intelligent Reflecting Surfaces (IRS) is a new technology that significantly improves the performance of multiple access methods in wireless multi-user networks using a number of cost-effective elements. By adjusting the phase shift, IRS can improve the received signal power for users, enhance physical layer security, reduce base station power consumption, and provide better coverage for users. In this thesis, we examine a scenario with two single-antenna base stations and one passive IRS. Each base station... 

In this contemporary era, distributed computing is a vital solution for many applications. The traditional computing scheme based on allocating one complete computation to only one server is not efficient anymore. Delay has a very important role in computation’s performance especially for the computation at the edge of the network. To alleviate the effect of big data and complex algorithms, distributed computing is a very promising solution. In this structure, all computations are divided into some small tasks or coded, based on the system's status and will be executed in parallel. In fully parallelized distributed computing, a task is broken into N smaller tasks and whenever all N subtasks... 

In recent years, cloud manufacturing as a successful business model has attracted a lot of attentions. Many papers and researches have discussed various aspects of cloud manufacturing. Despite of enourmous works in this area we can categorize them using the view to the manufacturing platform. According to the authors’ researches, almost all of the the researchers looked to the system in a centralized manner with private structure. A structure in which a central unit decides for all and seeks to optimize its intended objective function. In private platforms, a central unit which is usually the owner of the company, makes decision for its all subsideries. But, public platform has a different... 

This thesis presents the design of a novel mechanism for decentralized exchanges. In the introduction, decentralized exchanges and the reasons for their emergence in cryptocurrency-based systems are discussed. A fundamental limitation of these exchanges is the lack of information about the external market price, whereas tracking the external market price over time is crucial for them in various respects. There exist traders who are aware of the external market price and seek to profit from the difference between the exchange price and the external market price. Fortunately, the presence of such traders enables decentralized exchanges to track the external market price. The cost of this... 

OFDMA multi-hop cellular network (MCN) is the topic of a wide range of research today. In this thesis, we analyse this type of networks, regarding shadowing status, mobility patterns and different service requirements. In the multi-hop (MH) scenario, six fixed relays are set up in each cell, which split the cell into seven subcells. For mobility modeling, each subcell is split into several sub-regions. Mobile nodes are able to maintain at each sub-region with a random time distribution and migrate from it to one of its neighboring sub-regions. We also consider three types of services requiring specific number of subcarriers with specific BERs. The set of subcarriers is distributed among... 

In this thesis, we evaluate the maximum stable throughput of a cognitive multi-hop IEEE 802.11-based WLAN overlaid on a cellular network. In the considered scenario, the secondary users operate in the downlink or uplink frequency band of primary network and transmit their data in free primary channels using OFDM technique. The activity of primary nodes is modeled independently by ON-OFF alternating states. To model the scenario, each secondary user is modeled with an open queueing network. The queueing network can model the transmission of data as well as the effect of primary users. By writing the traffic equations of the queueing network and applying the stability conditions, we are able... 

In this thesis, we propose two new network coding-based packet relaying schemes suitable for multiple unicast scenarios in a random access network. In such networks the channels may have memory dependent upon the random access scheme. Thus, the optimal scheme for memoryless channels reported recently in the literature is not necessarily optimal in random access networks. In our proposed schemes, we convert a multiple unicast scenario to a combination of multiple anycast and a few multicast scenarios. By such an approach, we exploit the channels between relay node and the other nodes, instead of using only one channel at each packet transmission, for as many as possible. By simple analytical... 

Car accident damages take about 7 percent of GPD of Iran every year. As a result of car accidents, the economy of the country is damaging irreparably. The amount of road casualties equals casualties in a complete war. Considering the aforementioned conditions, the researchers are thinking about new technologies to reduce the cost of the accidents. So the use of Vehicular ad-hoc Networks (VANET) is inevitable in the near future. Content distribution service is one of the emerging services of VANETs. This service is used in both safety and comfort related applications. Content distribution describes delivery of media content such as Video, Audio, Image, Text, Software, and Video Games over... 

In this dissertation, we focus on two types of EH cooperative networks. In the first type, the source and relay nodes access a shared channel according to a random access protocol. Also, they harvest energy from non-RF environmental resources. The relay node has data and energy buffers to store the packets of the source and the harvested energy, respectively. Our main goal is to find the optimal relaying protocol at the relay node. In this regard, the first type of the EH cooperative networks are investigated in two different scenarios. In the first one, the MAC channels of the nodes are independent of the applied policy at the relay node, and thus the throughput of the network is fixed.... 

In this thesis, we investigate the age of information in multisource networks. To this end, in the first phase of this thesis, we evaluate the Age of Information (AoI) in slotted ALOHA network. Regarding the interactions among the nodes in such a network, we model the memory of this network with a Trellis and find the peak Age of Information (PAoI) by using this Trellis. Then, we evaluate Age of Information-constrained regions and achievable throughput regions in the slotted ALOHA network and optimize Age of information in a symmetric case. In the second phase, we propose an efficient prioritizing scheme from AoI point of view and find the distribution of AoI and PAoI in such a scheme with... 

Cooperative networks have attracted much attention in the last decade. Being introduced to mitigate the inefficiencies in the physical layer, cooperation can also be beneficial in upper layers, such as MAC layer. Using cooperative communications is specifically more challenging in the random access networks, where collision is a main concern. Delay, as one of the main performance metrics in random access networks, can decrease significantly by using cooperative communications. Transmission delay, i.e., the required time for delivering one packet form its source to destination, is an important metric in many communication networks, such as the networks in which each node generates and... 

Today, with the proliferation of smart devices and the advent of various innovative applications, there has been an exponential surge in the number of users in telecommunication networks. Consequently, the data traffic rate is escalating considerably. This has created a major predicament for wireless networks due to the limited frequency spectrum. Therefore, the conventional centralized architecture makes it difficult for the network to address these numerous simultaneous requests. This issue can be solved by designing new architectures. In a way, network information is disseminated to the network's periphery and the contents of the primary data center are provided temporarily in closer... 

OFDMA modulation method is a multiple access technique that is resistant to fading effect in cellular communication systems. In OFDMA, multiple access is performed both in time and frequency, so more users can be served with less delay than other protocols. In this thesis, based on queuing theory, we seek to present a traffic model for OFDMA system traffic analysis in single-cell mode and in the next section in multi-cell mode. Then, by examining and comparing the analytical results and the simulation results, we show that our proposed model for traffic analysis of the OFDMA system is a suitable and effective model. Also, since users in the multimedia environment need different types of... 

In data transmission from a transmitter to a receiver, in some applications receiving updated data is crucial. To this end, a metric called Peak Age of Information has been introduced, the minimum of which is equivalent to having more updated packets. In this thesis, a method for calculating the probability distribution function of this random variable in a random access network with a transmitter, a relay, and a receiver in which the error probability of the channels between the transmitter and the relay and also the relay and the receiver is less than the error probability of the channel between the transmitter and the receiver, is proposed. The transmitter and the relay send their packets... 

In many communication scenarios, dependent requests are commonly observed, such as caching networks. In this thesis, we propose a new analytical model to examine the impact of dependent arrivals on the average response time in a simple queue with exponential service time. We examine two dependency scenarios: batch arrivals and the arrival of one customer after another with a random delay with a known distribution. To accurately model the effect of dependency between arrivals, we define redundant customers in our model to compensate for the existing dependency effects. We demonstrate the validity of our modeling approach analytically in the first scenario and through simulation in the second... 

Edge computing networks are ideal for emerging applications that require computation-intensive and timely processing, as they enable complex computation tasks to be offloaded and computed in parallel and distributed manner. However, for many applications that the calculations are of one type, such as determining the location of an object, it is crucial to insure the freshness of the information. Age of Information (AoI) metric is employed to measure freshness. This research focuses on identifying an appropriate model for calculating the average AoI in distributed computing systems and determining the optimal decision-making policy to enhance it. In essence, the primary objective is to... 

As the global demand for data has continued to increase exponentially, researchers have begun exploring the concept of semantic communication as an effective tool to meet this demand. Semantic communication transcends the boundaries of conventional telecommunication systems, focusing on the transmission as well as accurate retrieval of meaning at the source and destination, respectively, rather than transmitting and recovering merely symbols. One approach in semantic communication emphasizes the significance of information, rather than its meaning, to achieve efficient and goal-oriented communication. One key metric in goal-oriented communication is the Age of Information (AoI). AoI is a... 

In this Ph.D dissertation, we investigate the information freshness in semantic communication networks. Information freshness plays a fundamental role in networked control systems. In fact, controlling decisions in such networks should be based on information generated recently. The quality of information freshness can be represented by the Age of Information (AoI) function, from which two well-known metrics, i.e., average AoI and average peak AoI (PAoI), are derived. In the semantic approach to information freshness, an effort is made to redesign the network structure to improve these metrics. However, this approach complicates the analysis of communication networks. Previously,... 

The activity status of primary users in a cognitive network scenario is a key parameter in saturation throughput of the secondary users. In this respect, usually a simple on-off traffic model with exponentially distributed intervals is exploited. For time-scheduled users in modern common standards like WiMAX or LTE this traffic model is not valid anymore. In this thesis, we focus on the saturation throughput analysis of a cognitive network scenario, comprised of a WLAN overlaid on a network with time-scheduled primary users like WiMAX. In this scenario, the cognitive nodes (CNs) hear downlink map (DL-MAP) and thus, know the frequency and time locations of all allocated slots at each downlink...