Research On Transmission Performance And Physical Layer Security Of Wireless Cognitive Relay Networks In Fading Environment

Cognitive relaying network by integrating wireless relaying technique into cognitive radio systems not only can achieve diversity gain to improve the system capability by combating interference, extend wireless coverage area, but also improve the spectrum utilization, which plays important significance in the wireless communication. However, due to the open nature and broadcasting characteristics of wireless communication, which makes the communication system vulnerable to security threats such as eavesdropping attack and interference. The physical layer security technology is a new way to solve this problem. As well known, channel fading has significant impacts on the performances of wireless communication system. Therefore, the study of transmission scheme, transmission performance and physical layer security for various relay systems over fading channels is of high importance. In this thesis, some novel results are obtained for the cognitive relaying and cooperative relaying communication system over η-μ and Rayleigh fading channels in terms of performance evaluation.Firstly, some novel results on the outage probability of cognitive relay system under different transmit power constraints over η-μ fading channels are obtained. Specifically, considering two different transmit power constraints, i.e., the peak interference temperature, and the combined power constraints of the peak interference temperature and the maximum transmit power constraint, the closed-form lower bounds of outage probability for integer fading parameters are obtained while the lower bounds in integral form of the outage probability are derived for arbitrary fading parameters. Moreover, the asymptotic performance of the outage probability in high SNR (SNR) is analyzed by utilizing Taylor series expansion. Numerical and simulation results verify the correctness of the theoretical analysis. All the theoretical results are useful for guiding the performance optimization of cognitive relaying system in η-μ fading.Secondly, some exact theoretical results of intercept outage probability in cognitive multi-relaying systems in the presence of eavesdropping attack over independent and not necessarily identically distributed Rayleigh fading channels are obtained for two relay selection schemes. The first selection scheme (MinW scheme) is to guarantee the security of the system by minimizing the received signal-to-noise ratio (SNR) at the eavesdropper whereas the second one (MaxM scheme) is to ensure the reliability of the system by selecting the relay with the maximal received SNR at secondary destination. Closed-form intercept outage probability expressions as well as asymptotic results are derived for both two relay selection schemes, which are useful for guiding the security performance optimization of cognitive multi-relaying systems in Rayleigh fading.Finally, a new relay selection scheme based on threshold and secrecy capacity maximization is proposed. Consider cooperative multi-relaying communication system in presence of eavesdropping attack, and channel estimation is carried out before data transmission. Based on the channel estimates, the relay nodes which make the received SNR of the destination node greater than a certain SNR threshold γth are selected, denoting set Ω. And then the best relay node R* in set Ω will be selected, which can maximize the system security capacity. During the data transmission process, R* will be chosen to help transmission. Throughout the proposed relay selection scheme, the security of the whole communication system is ensured on the premise of ensuring the communication reliability of the main link, the best performance of the multi-relaying system can be obtained. The proposed scheme can be applied in multi-relaying to improve reliability and security performance.