Investigation On Physical-Layer Security For Multi-antenna Cognitive Radio Systems

With the development of data volume of wireless communications,spectral resources have became especially scarce.However,the original fixed spectrum allocation policy has caused a large number of spectrum resources to be underutilized.To this end,cognitive radio has emerged as an effective mean to ease the pressure of spectrum scarcity.Additionally,due to the broadcast nature of wireless communications,cognitive wireless transmissions are extremely vulnerable to encounter eavesdropping attacks.As a promising technique to combat eavesdropping attacks,physical layer security fundamentally enhances secure transmission by using randomness,reciprocity and time varing characteristics of wireless channels.Therefore,this paper mainly focus on transmit antenna selection to enhance the physical layer security of cognitive transmission.The details are as follows:First,an optimal antenna selection(OAS)based on the global instantaneous channel state information(CSI)is proposed to protect eavesdropping attackers under the case of ensuring the quality-of-service of primary users.Additionally,the analysis of the effect of the antenna number of cognitive users and eavesdropper on the physical layer security and the secrecy diversity for OAS as well as round robin scheduling(RRS)scheme is carried out.The secrecy outage performance of OAS greately outperforms the conventional RRS scheme.Moreover,OAS scheme obtains upper bound performance for the multi-antenna cognitive radio systems in terms of physical layer security.Then,to deal with the CSI of wiretap channel is unavailable for cognitive users in some passive eavesdropping scenarios,a suboptimal antenna selection(SAS)scheme is proposed,which only bases on the CSI of main channel.The closed form expression of the secrecy outage probability for SAS scheme is provided.Additionally,in order to show an intuitive insight into the impact of the antenna number on cognitive serecy transmission,the secrecy diversity of SAS scheme is carried out.The simultaion results show that SAS importantly decrease the secrecy outage probaility compared with RRS.Finally,the security and reliability trade-off(SRT)of cognitive wireless transmissions is studied.We jointly consider energy harvesting and transmit antenna selection to achieve energy efficient,high security and reliability of cognitive wireless transmissions.In the case of the CSI of wiretap link is unknown for cognitive users,energy aware transit antenna selection(EATAS)scheme is proposed.For the purpose of comparison,RRS scheme is provided as a benchmark.Additional,we carry out the theoretical analysis of security and reliability for EATAS and RRS schemes.Moreover,the results are provided to show that EATAS significantly improve the SRT performance.