Relay based spectrum sharing designs for cognitive radio networks

With ever-growing wireless data transmission requirements, there is a dramatic increase in the demand for radio spectrum. Within the current spectrum regulatory framework, all of the frequency bands are exclusively allocated to specific services. However, the licensed spectrum is often under-utilized across time and space dimensions. In this context, cognitive radio (CR) has emerged as a promising technology to deal with the stringent requirement and scarcity of the spectrum. Cooperative relaying can allow distributed terminals to collaborate so as to realize the space diversity to combat the detrimental effects of channel fading. This thesis focuses on designing the bandwidth efficient relay based spectrum sharing solutions to improve the capacity of CR networks while guaranteeing the performance of licensed users.A cognitive two-path successive relaying (TPSR) scheme is proposed to facilitate the spectrum sharing, where the superposition coding and successive interference cancellation techniques are used to realize the primary signal relaying and secondary signal transmission simultaneously. The optimal power allocation is determined to maximize the secondary success probability without violating the primary outage performance. For the multiuser CR network, an adaptive spectrum leasing scheme with the best secondary user (SU) selection is proposed. The SU can intelligently switch between the TPSR and decode-and-forward relay modes to assist the primary data transmission in exchange for the spectrum release for a fraction of time to transmit the secondary data. The cooperative diversity gain of primary system and the multiuser diversity of secondary system are studied.For large wireless network, it is nontrivial to select the relay in a coordinated fashion, as the heavy signaling overhead may negate the cooperation gains. Three uncoordinated relay selection schemes are proposed, where each relay can determine whether to cooperate or not independently according to the local channel or geo-location information. To address the uncertain interference problem in spatially random network, a cooperative spectrum sharing scheme between cellular network downlink and ad-hoc network is designed. The transmission capacity of ad-hoc network and the average throughput of cellular network are analyzed by using the stochastic geometry theory, based on which the optimal system parameters are determined