Research On Spectrum Sharing Technologies Based On Cooperation For Cognitive Radios

As the wireless technology rapidly develops in the fields of mobile communications, broadcast TV and social security, the spectrum resource has played a more and more important role in modern society. As a new technology which can carry out a more efficient and flexible program for the usage of spectrum, cognitive radio (CR) has broad application prospects in the future wireless communication networks. The spectrum sharing technology, one of the key research topics of CR, aims to achieve efficient and reliable communications of secondary users (SUs) while guaranteeing the quality of service (QoS) of primary users (PUs).To achieve the aim of the spectrum sharing technology, this dissertation focuses on spectrum sharing technologies based on cooperation. We design and analyze several cooperative spectrum sharing schemes which are applicable to different spectrum sharing scenarios. The outline of this dissertation is described as follows:1) A complete system model of the underlay cognitive radio relay system, which takes into account the interference temperature constraint and spectrum sharing interference, is constructed. Based on this model, we investigate the outage performance and the symbol error probability performance.The existing system model ignores spectrum sharing interference, so we consider the interference temperature constraint and spectrum sharing interference and present more accurate and practical performance analyses for the secondary system with two different cooperative protocols. For the single-relay decode-and-forward cooperation, we derive the closed-form expression of the outage probability, as well as the asymptotically tight upper and lower bounds of the symbol error probability. And for the multi-relays selection cooperation, the closed-form expression of the outage probability is derived with consideration of the activities of the PUs.2) An overlay spectrum sharing scheme based on OFDM relay transmission is proposed, and an alternate and iterative optimization algorithm is presented to solve the problem of power allocation between the source and the relay.To solve the problem of adjacent-frequency interference in OFDM-CR systems, we introduce the cooperative relaying technique to avoid the interference by controlling transmit powers of the SUs while guaranteeing the QoS of the secondary transmission. An alternate and iterative optimization algorithm of power allocation is presented to maximize the total rate for the secondary system while keeping the instantaneous interference introduced into the PU below a certain threshold.3) An overlay spectrum sharing scheme based on cognitive relays, named the cognitive cooperative scheme, is proposed to improve the spectrum efficiency and achieve the efficient cooperative transmission.This scheme substitutes cognitive relays for conventional ones to utilize the detected spectrum holes to implement cooperative transmission, which can save the dedicated relay resources for the cooperative system and improve the transmission efficiency. We also propose two cognitive relay selection strategies, including the optimal instantaneous channel state information based strategy and the partial CSI based one, and we present the outage and throughput performance analysis. 4) A hybrid cooperative spectrum sharing scheme based on the spectrum sensing result is proposed to combine the advantages of underlay and overlay approaches.The hand-off between the underlay approach and the overlay approach is based on the spectrum sensing result,?.e., the SU can adapt its transmit strategy according to the spectrum sensing result. When switching to the underlay approach, the performance loss can be compensated by applying the cooperative relaying technique. Under this scheme, we consider the mutual interference between the PUs and the SUs, and then derive the closed-form expressions of the outage probability and the symbol error probability for the secondary network.5) In order to make the secondary users obtain spectrum opportunities more flexibly, we propose a compensative cooperative spectrum sharing scheme based on slot allocation.In such a scheme, the SU acts as the cooperative relay of the PU and helps to transmit the information of the PU in exchange for the spectrum opportunities. Firstly, we analyze the feasibility of this scheme and present the necessary preconditions. Secondly, we design a transmit protocol based on slot allocation to solve the joint scheduling problem of the primary and secondary transmissions. Finally, we evaluate the performance of the primary and secondary systems under the compensative spectrum sharing scheme, including the outage performance and the symbol error probability performance.