Sharif Digital Repository

In the present thesis we will design and study some rendezvous algorithms for cognitive radio networks. In cognitive radio networks, secondary users try to access the spectrum of primary users in an opportunistic manner. Each secondary user constructs a channel list of accessible resources by channel sensing. These resources are not necessarily the same for different users; therefore, in order to make a connection in absence of a fixed control channel, different secondary users should find each other properly and quickly after entering the network. This is known as the blind rendezvous problem. We will develop and examine a new sequence based algorithm for blind rendezvous. Moreover, some... 

In this thesis, the challenges of the PHY and MAC layers in cognitive radio networks are discussed and a plan which considers the cross layer concept between them is presented.In the first part of the thesis, a brief survey on various spectrum sensing techniques is presented. A detector which can improve the network performance is introduced and based on this detector, the important problem of trade-off between spectrum sensing period and the network throughput is considered. For this purpose, first of all, the truncated SPRT detector is presented and for low SNRs the optimal truncation time which leads to maximum network throughput is obtained. In the second part of the thesis,the... 

Cognitive radio (CR) can successfully deal with the growing demand and scarcity of the wireless spectrum. To exploit limited spectrum efficiently, CR technology allows unlicensed users to access licensed spectrum bands. Since licensed users have priorities to use the bands, the unlicensed users need to continuously monitor the licensed users activities to avoid interference and collisions. How to obtain reliable results of the licensed users activities is the main task for spectrum sensing. Based on the sensing results, the unlicensed users should adapt their transmit powers and access strategies to protect the licensed communications. The requirement naturally presents challenges to the... 

Today, with the expanding usage of wireless communication systems, the need for system which uses the spectrum efficiently is inevitable. Therefore, Cognitive Radio structures have be the center of attention, recently. In cognitive radio structures, the system tries to use the empty spectrum efficientlyin time, frequency, and space domain. Autonomous Vehicular Networks are interesting application, in which cognitive radio can be used. In such networks, due to high speed, the channel state changes drastically which negatively affects the system.Hence, in this thesis, an OFDM-based multiuser structure is designed which work properly in such condition. For designing the structure, a doubly... 

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... 

Cognitive Radio (CR) technology enables dynamic spectrum access (DSA) to overcome the problems of unlicensed bands which is becoming overcrowded. However, providing an acceptable quality of service (QoS) for cognitive users requiresmore sophisticated approachesdut to the existence of primary users (PU) with their high priority over licensed bands. Additionally, reducing interference with primary users so that they can communicate without interruption is of extreme importance. In order to meet cognitive user’s quality of service as much as possible and reduce interference with PUs, a new class-based multipath on-demand routing protocol for cognitive radio networks (CMORC) is proposed. CMORC... 

Dynamic spectrum access with the aid of cognitive radio (CR) technology is a viable solution to spectrum scarcity. There are many military and emergency applications that require the use of cognitive radio ad-hoc networks (CRAHNs). In these net- works, nodes must first find each other in a wide spectrum and then exchange their desired data. The process of finding each other in a multichannel environment is a challenging task and is usually known as rendezvous. Since rendezvous is the first step in communication, disrupting will disable the nodes from establishing proper communication sessions. That results in a drastic degrade in the performance of the network. Therefore, it is important to... 

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... 

Deployment of demands, increase in users and new services highlight the necessity of effective frequency allocation i.e. dynamic allocation instead of fixed allocation. Cognitive radio is a proper solution for dynamic frequency allocation. Cognitive radio can scan the spectrum and use the idle bands for transmitting and receiving data. The main part of this radio which is responsible for the most pivotal task is spectrum sensing. Recently, varied methods have been proposed to sense the spectrum. The scope of this research is on designing a very high-speed spectrum sensing system. In this thesis, in order to meet the required scan rate, a tunable complex filter with 40MHz pass-band has... 

In recent years, demand for frequency spectrum has increased rapidly. Cognitive radio technology, as a tool for better access to the frequency spectrum, opportunistically uses the bandwidth of a licensed frequency band to solve the problem of congestion in unlicensed frequency bands. The spectrum allocation is one of the first steps in the implementation of cognitive radio networks. Many studies have been done to solve the problem of optimal spectrum allocation in such networks. One of the most commonly used methods to solve the optimization problem is using meta-heuristic algorithms. Harmony search and artificial bee colony algorithms are two of the most successful methods for this purpose.... 

Ever increasing demand for higher data rate in wireless communication in the face of limited or underutilized spectral resources has motivated the introduction of cognitive radio for dynamic access to spectrum. In dynamic spectrum access a new type of users called secondary users measure the spectrum to see if it is occupied by licensed users (primary users or PU). When channel is empty secondary users can use it to transmit signal. This approach is called spectrum sensing. Hidden PU problem can severely defect detection ability of non-cooperativ spectrum sensing systems. Cooperative spectrum sensing (CSS) uses spatial diversity of spectrum sensors to tackle this problem. There are two kinds... 

The different methods of spectrum sensing to identify the presence of signal transmission are as follows: Energy detectors, Cyclostationary detectors and Matched-filter detectors. Energy detectors are simple and easy to implement but they are unable to differentiate interference from primary users and noise. They also have a very poor performance in low SNR. Matched-filtering is the optimum method for detection of primary users and can achieve a certain detection performance in a short time but its implementation complexity is too high and requires perfect knowledge of primary user’s signaling features that makes its usage in cognitive radios infeasible. Among these detectors Cyclostationary... 

Considering energy efficiency ways a key role in designing future wireless networks( 5G mobile networks). Moreover Spectrum efficiency is another critical issues in designing wireless networks. Cognitive radios can improve the spectrum efficiency. On the other hand, radio frequency (RF) energy harvesting has emerged as a promising technique to supply energy for wireless networks and thereby increase their energy efficiency. In this thesis, we propose a new technique for the RF-powered CRNs.To this end,We consider a cognitive radio network comprised of a primary user and a secondary user. The primary user, uses a typical frequency band for transmit data in a time slot basis and both the... 

In recent years, the Cognitive Radio Network has received significant attentions due to its high potential for better employment of the spectrum. One of the most important parts of this technology is the spectrum sensing that requires being fast and accurate. The conventional algorithms proposed so far encounter some fundamental challenges at low SNR regimes and in wideband sensing. On the other hand, the compressed sensing algorithms, which take advantages of the sparsity of the signal of interest and utilize measurements instead of the samples, can reduce the sampling rate and thus decrease the complexity associated with the wideband sensing. In this thesis, by exploiting the... 

The abstract is the most important section of the thesis because many readers limit most of their reading to abstracts, saving in-depth reading for specific theses. It should give the reader a "preview" of what's to come. The abstract should emphasize new and important aspects of the study or observations. The purpose of the abstract is to allow researchers to decide whether or not to read the whole thesis. The abstract is what researchers read first to decide if the thesis is important, interesting, and it allows them to assess the relevance of a thesis to their own research, without having to read the entire thesis. Thus, it is crucial that the abstract both summarize succinctly the key... 

In cognitive radio networks, secondary users try to use empty channels that primary users provide for transmitting data. First, the secondary users sense the target channels. If the selected channel is empty, they send their data. When the primary user returns, the secondary user has to find another empty channel for transmitting data. The process of changing channel is called hand off. In recent years, there are challenges in the spectrum mobility of for reducing delay time of secondary users packet. In this thesis, we assume that the secondary users network is an ad-hoc network. We use contestation based method for reducing collision between secondary users. In this thesis, we present an... 

The continous growth of wireless networks, the increasing numbers of users and devices, along with the demand for higher data rates have intensified the frequency band limitations as the communication medium. In search for more efficient methods to use this limited resource, the cognitive radio approach that allocates the local unused frequency bands to potential users has attracted considerable attension.
A fundamental and unseperable part of any cognitive radio system is its need to search, detect and understand the surrounding frequency spectrum. Although various methods have been already propose to overcome the challenges of this domain, simpler and more reliable methods are still in... 

In recent years, with the continuous growth and development of wireless communication systems, the demand for the use of radio spectrum has increased significantly, which has led to problems such as a shortage of spectrum. An effective solution to the spectrum shortage problem is to use cognitive radio. One of the main functions of cognitive radio networks is spectrum sensing and our focus in this study is to examine machine learning and deep learning methods to improve cooperative spectrum sensing. Since channel and noise parameters are not accurate in telecommunication systems, we need to use a method to estimate these parameters. In this regard, the structure of Bayesian with neural... 

Frequency scarcity has emerged the necessity of opportunistic utilization of frequency bands, which can be realized through a cognitive radio system. During an agile communication between unlicensed users, a cognitive radio system must avoid collision with licensed users. Hence it should continuously observe the band of interest and report the presence of licensed user signals. This task is fulfilled by a vital part of a cognitive radio system, called the spectrum sensing core. Recently several techniques have been proposed for the spectrum sensing in literature. Some of them like matched filtering, cyclostationarity based detection are based on primary user signal features. However energy... 

Cognitive radio faces specific challenges due to its special spectrum access scheme. Providing required QoS and preventing irrecoverable interference on the primary users is among the crucial requirements. Beam forming is one of the interference avoidance schemes utilized in MIMO networks. Most of the studies on this scheme assume that each symbol is transmitted via vectors in order to grant desired beam forming and specific combinations in the receiver. Since transmitting multiple symbols at every transmission can provide higher data rates, it seems necessary to exploit beam forming through matrixes instead of vectors. In this thesis, by neglecting the interference of the secondary user,...