| Cognitive radio is emerging as a promising technology for future radio communications, which enables an unlicensed secondary user (also referred to as cognitive user) to identify (through spectrum sensing) and access a spectrum hole that is a radio frequency band licensed to primary users but not used at a particular time and/or a specific geographic location. Generally, ideal spectrum sensing without any false alarm of spectrum holes can not be realized, which results in mutual interference between the primary and secondary users. This would impair and limit the secondary user's data transmission performance under a target quality-of-service (QoS) requirement of the primary user (called primary QoS). Therefore, in cognitive radio systems, there are two fundamental concerns: (1) to devise an accurate spectrum sensing algorithm so as to detect spectrum holes as accurately as possible; and (2) to design an efficient secondary user transmission mechanism so as to utilize the detected spectrum holes as efficiently as possible.;As a consequence, this dissertation investigates the use of cooperative relay technology for cognitive radio systems and shows significant benefits to the above-mentioned two issues, i.e., the spectrum sensing and secondary transmissions, by using cooperative relay. By jointly considering the spectrum sensing and secondary transmissions, an optimal sensing-transmission tradeoff is developed in Rayleigh fading environment for two cognitive radio scenarios (with single relay and multiple relays, respectively), which aims to an optimization of secondary transmissions (e.g., minimization of the outage probability of secondary transmissions) under a protected primary QoS requirement. In addition, this dissertation also examines the mutual interference impact between the primary and secondary users, which arises from the fact that miss detection and false alarm of spectrum holes are unavoidable in spectrum sensing. It is shown that, given a target primary QoS requirement, the mutual interference greatly limits the secondary transmissions and causes a performance floor (e.g., outage probability floor). This dissertation proposes the use of cooperative relay techniques to reduce such performance floor, for which a fundamental diversity-and-multiplexing tradeoff is developed as a metric to evaluate the efficiency of proposed approaches. |
