Research On Cooperative Transmissions In Cognitive Radio Networks

Within the current spectrum regulatory framework, most of the spectrum bands areexclusively allocated to the primary users (PUs), which are not allowed to be used bythe secondary users (SUs). Thus, as wireless technologies and services continue to grow,the limited spectrum resources can not accommodate the ever-increasing demands ofspectrum for various wireless systems using the fixed spectrum allocation policy. In fact,the spectrum scarcity is mainly induced by low spectrum utilization efficiency. Toimprove the spectrum utilization efficiency, cognitive radio (CR) is proposed, which hasattracted much attentions over past dozen years.CR allows SUs to use the licensed spectrum as long as PUs are sufficientlyprotected. Two key issues are needed to be well addressed in CR: spectrum sensing (SS)and spectrum access (SA). Generally, in CR networks (CRNs), SUs should monitor thespectrum to find spectrum holes, including idle bands and interference-controlled bands,before they transmit secondary signals. Then, according to the sensing results, SUs willadjust their transmission parameters to access the licensed spectrum in the best manner.Due to the multi-path fading and shadowing, the performances of traditional SS and SAwould be severely degraded. To solve this problem, cooperative transmission (CT) hasbeen recently applied into CR, which is regarded as a promising way to improve theperformance of spectrum sharing.However, there still have many unsolved issues in CT based SS and SA, such ascooperative sensing overhead, mutual interference, continuous secondary transmissions,green cognitive CT. The performance of CT-based CRN is mainly determined bywhether the above four issues can be effectively solved or not. Thus, this dissertationinvestigates how to enhance the performances of SS and SA using CT, aiming tofacilitate the design of CR systems with low overhead, high reliblity and energyefficiency. The main contributions of this dissertation can be summarized as follows:First, this dissertation studies low-overhead cooperative sensing strategies. It iswell known that the sensing accuracy can be greatly improved by CT, and will increasewith the number of cooperative SUs growing. However, a lot of cooperative SUs mayinduce much sensing overhead, such as sensing delay, power consumption, mutualinterference, and so on. To reduce the overhead, this dissertation proposes a selective-reporting and sequential-detection based cooperative sensing strategy. Thesensing time allocation optimization problems are studied, and then two optimum fusionrules are designed to maximize the sensing accuracy and minimize the sensing delay,respectively. Simulation results show that this strategy can not only improve the sensingaccuracy but also reduce the sensing overhead. However, like most existing copperativesensing strategies, a dedicated common control channel (CCC) is required in thisstrategy, which will cause additional manage complexity. To solve this problem, thisdissertation proposes two user-selection based cooperative sensing strategies. In thesetwo strategies, the mutual interference between PUs and SUs is considered. Then, byselecting the best cooperative user and controlling its transmit power, the local decisionscan be directly reported over the sensed channel without requiring a dedicated reportingchannel. Simulation results show that the proposed user-selection based cooperativesensing methods can reduce the sensing overhead while maintaining the sensingaccuracy. Besides, the miss detection probability and sensing overhead can beminimized by optimizing the sensing time allocation in these two strategies.Second, this dissertation studies the interference-cancellation based cognitive CTstrategies. In practice, various PUs and SUs coexist with each other, which will causemutual interference. Such interference would degrade both the performances of primarytransmissions and secondary transmissions. To cope with this issue, the dissertationproposes an interference-cancellation and best-relay selection based underlay strategy,aiming to improve the performance of secondary transmissions under strict powerconstraints. In the proposed underlay strategy, the interference-cancellation is naturallyincorporated into secondary best-relay selection and diversity reception to suppress theinterference from PUs. Simulation results indicate that the proposed underlay strategycan lower the secondary outage probability and also increase the secondary diversitygain compared with traditional underlay case. Then, this dissertation proposes a powercontrol and interference-cancellation based overlay strategy. In the proposed overlaystrategy, SUs ensure the quality-of-seviece (QoS) of PUs by limiting their transmitpower and improve the performance of secondary transmissions by mitigating theinterference from PUs. Simulation results demonstrate that, with guaranteed PU QoS,the proposed overlay strategy can significantly promote the performance of secondarytransmissions as compared to traditional overlay case.Third, this dissertation studies the continuous secondary CT strategies. There are two main reasons for discontinuous secondary transmissions. One is that PUs reuse theunoccupied spectrum, and as a result, the SUs should stop secondary transmissions andwithdraw from the licensed spectrum immediately. The other is that a SU moves out ofthe transmission range of its source. To resolve the first issue, this dissertation proposesan adaptive cognitive CT strategy with power control, cooperative relaying andbeamforming. In this strategy, a SU can increase the transmit power to reach itsdestination directly when PUs are absent. On the other hand, when new PUs come in,the SU will reduce its power and choose an intermediate relay to reach its destination toavoid the interference to the primary transmissions; in this case, the SU destination willleverage beamforming to receive data from the SU and relay respectively. Simulationresults reveal that this strategy can not only lower the secondary outage probability butalso ensure the PU QoS. To solve the secondary issue, this dissertation proposes to usecognitive relaying to ensure the continuous connections for SUs. In this strategy, boththe interference between PUs and SUs as well as the interference among different SUsare investigated. Then, this strategy uses power control at SUs to reduce the inducedinterference to PUs, and, at the same time, utilizes interference-cancellation andtransmission time optimization to mitigate the mutual interference among differenceSUs. Simulation results show that this strategy can ensure the continuous connectionsfor SUs in both downlink and uplink cases, and also can reduce the outage probabilitycompared with traditional cases.Finally, this dissertation studies energy-efficient secondary CT in SA. Recently, CRis regarded as an excellent means to facilitate the green communications in CRNs inaddition to solving the spectrum scarcity problem. To explore the applications of CR ingreen communications, this dissertation proposes a cognitive CT based greencommunication strategy. In the proposed strategy, SUs are allowed to assist the datatransmissions of PUs in exchange for some spectrum. Then, the secondary traffic flow issplit between the released licensed spectrum and the unlicensed spectrum to reduce theenergy consumption by network cooperation. Extensive simulation studies demonstratethat the proposed strategy can remarkably reduce the energy consumption of secondarytransmissions while ensuring the QoS of primary transmissions.