Sharif Digital Repository

Cognitive radio is a tool for optimal use of radio spectrum, not used by the primary users all the time. Two main features of a cognitive radio are cognitive capability and reconfigurability. An intelligent cognitive radio uses its cognition ability to identify the environmental parameters and makes future decisions based on this information. Then the reconfigurability is used to set the sending parameters to the calculated values in the cognition phase. Cogition phase consists of major steps; spectrum sensing, decision, mobility and handoff. Throughout this thesis we first propose a cooperative sensing scheme using algorithmic techniques based on sensors’ spatial information through... 

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... 

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... 

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... 

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... 

In this thesis, we consider the problem of spectrum sensing in cognitive radio networks. First, the collaborative energy detectors based spectrum sensing are investigated in the case of known noise variance for two models of primary user (PU) signal, i.e. random and unknown deterministic signals. Since the derived optimum collaborative energy detector requires the signal-to-noise ratio (SNR) of secondary users (SU) and it has complex structure, the generalized likelihood ratio (GLR) detector is proposed for both models of PU signal which leads to the same decision rules for both models. Simulation results show that the performance of the proposed GLR detector is near to that of optimal... 

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...