Efficient utilization of the spectrum has become a fundamental requirement in modern wireless networks, due mainly to spectrum scarcity and the ever-increasing demand for higher data rate applications and internet services. A particularly interesting proposal to meet this requirement is the cognitive radio (CR) system which can adapt its transmission parameters according to the environment. CRs, as will be shown in later chapters, are very efficient in maximizing spectrum utilization due to their inherent spectrum sensing capability.The purpose of this dissertation is to investigate and analyze two main components of CR. First is the sensing or exploring component, which is the core of a CR device as it is the first stage to discover spectrum holes (SHs) in a spectrum band. For this component, a new algorithm to compute the detection probability in the case of odd degrees of freedom and a closed-form expression for the detection probability in Nakagami-m fading channels are presented, both for a local spectrum sensing scenario. For a cooperative scenario, the errors of CRs decisions which are caused by erroneous feedback channels are analyzed. In addition, the optimal number of CRs that are required to mitigate against such errors is derived. The second component is the access or exploiting component, i.e. how a CR device can exploit SHs efficiently. To study the second component, the interactions between the primary users (PUs) and secondary users (SUs) are modeled as a continuous time Markov chain (CTMC). Based on the CTMC model, the effect of two inevitable sensing errors (misdetection and false alarm) on the blocked call probability, the dropped call probability and system utilization is investigated for two access schemes. In the first scheme, the PUs are considered to access the system using a standard access policy. In the second scheme, the PUs use non-standard access policies. In both schemes, the overall (primary and secondary) system utilization is analyzed and compared under both perfect and imperfect sensing. The simulation results obtained concur with the analytical ones and it is determined that spectrum utilization can be improved by choosing a suitable non-standard access policy.
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Thesis advisor: Liang, Jie
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