Management and Sensing of Spectrum in Cognitive Radio

Under the contemporary spectrum usage regulations, radio frequency bands are allocated statically to licensed users in a large geographical area and over a long period of time. Recent investigations revealed that such static spectrum allocation has led to very poor spectrum utilization. Cognitive radio has emerged as a new communication paradigm to improve spectrum utilization. It is defined as an intelligent communication system that allows coexistence of the unlicensed users with the licensed ones. Additionally, the users in this system adopt efficient communication protocols to enhance spectral efficiency. We employ cooperative mechanisms to accomplish two tasks: • Cooperative spectrum sensing: Licensed users do not in this task. Instead, unlicensed users monitor the activity of the licensed users and transmit only during their absence. • Cooperative spectrum management: Licensed and unlicensed users can benefit from cooperation with each other, e.g., can assist each other in transmission via relaying. Accordingly, they can save power or bandwidth and therefore, the whole network can accommodate more users. In the first part of this thesis, we focus on cooperative spectrum sensing. We i first study the asymptotic performance of optimal distributed detectors and identify the conditions under which the highest or lowest asymptotic performance is achieved. For each condition, we study several detectors and investigate their performancesensitivity-bandwidth tradeoffs. We then consider distributed detection of an OFDM signal. We propose several frequency-domain detectors that, despite their lower computational complexity, outperform state-of-the-art time-domain detectors. Finally, we consider spectrum sensing in fading environments and propose several novel detectors that significantly outperform traditional detectors. For all these detectors, we theoretically analyze their performance and prove that the proposed suboptimal detectors are asymptotically optimal. We then focus on cooperative spectrum management and study the problem of cooperative relay selection and power allocation. We determine the conditions, in terms of channel gains and network geometry, under which such cooperation leads to an increase in rate, or a reduction in power and bandwidth usage. Lastly, we propose cooperative protocols that use these results to enhance spectrum efficiency.