On The Physical Layer Security Of Cognitive Radio Networks

Cognitive radio(CR)technology has rekindled enormous interest in the wireless community due to the fact that it can solve the spectrum scarcity problem by exploiting the existing wireless spectrum opportunistically.Due to the broadcast nature of transmission channels,security and privacy are one of the key problems for wireless communications.The main idea of physical layer secrecy(PLS)is to exploit the characteristics of the physical wireless channels,such as fading,noise and/or interference,which can achieve absolute security by using channel coding and signal processing technologies.Multi-antenna technology can improve signal-noise ratio and channel capacity by utilizing multiple antennas at nodes.Cooperative communications have emerged as a powerful spatial diversity technology that can effectively combat channel fading and increase array gain.In this thesis,the secrecy performance of multi-antenna CR systems and CR cooperative system are modeled and analyzed.First,a single-input-multiple-output(SIMO)cognitive wiretap system over Generalized-K channels are considered.Generalized-K distribution is one of the relatively new precise models use to describe the statistical behavior of path loss and shadowing fading effect.However,the probability density function(PDF)and the cumulative distribution function(PDF)of Generalized-K include the complex function and cause a difficulty in proceeding to further steps for performance analysis.Motivated by this context,a mixture gamma(MG)distribution is adopted to approximate Generalized-K distribution in this thesis.The closed-form expression for the secrecy outage probability is derived.Simulations are presented to validate the accuracy of proposed analytical results.Second,the secrecy outage performance of an underlay cognitive decode-and-forward(DF)relay network over independent but not necessarily identical distributed Nakagami-m fading channels is investigated.Based on whether the channel state information(CSI)of the wiretap links are available or not,the secrecy outage performance with optimal relay selection(ORS)and conventional relay selection(CRS)schemes are analyzed,and multiple relay selection scheme(MRS)scheme is considered for comparison purpose.The exactly and asymptotic closed-form expressions for the secrecy outage probability with three different relay selection schemes are derived and verified by Monte-Carlo simulations.Numerical results illustrate that ORS scheme always outperforms CRS and MRS schemes,and CRS scheme is better than MRS scheme in the lower fading parameters scenario.Furthermore,through asymptotic analysis,I find that these three different schemes achieve the same secrecy diversity order,which is determined by the number of the relays,and the fading parameters between the relay and the destination.In this thesis,the PLS of multi-antenna CR networks and multi-relay cooperative CR network is analyzed and modeled.The closed-form expressions build the relationship between the secrecy performance and the parameters of the related system which are beneficial for designing practical cognitive relay systems,especially when the PLS issues are considered.