Asymptotic Behavior And Performance Optimization Of Secrecy Capacity In Artificial Noise Aided Communication System In The Presence Of Nonlinear Power Amplifier

Physical layer security technology makes it impossible for eavesdroppers to obtain accurate communication information through the heterogeneity of the channels so that a unique set of security issues presented by the broadcast nature of the wireless medium can be solved.The artificial noise(AN)method in the physical layer security technology realizes secure communication by simultaneously transmitting both the information-bearing signal and the AN to degrade the eavesdropper's channel without affecting the receiver.Since secrecy capacity depends on the signal-to-noise ratio(SNR)of the receiver and eavesdropper,the secure performance is sensitive to the nonlinear distortion of the power amplifier(PA).Therefore,this thesis analytically analyzes the effect of PA nonlinearity on the secrecy capacity of the AN-assisted secure communication system for the first time,and derives the closed-form expressions of the optimal point corresponding to the maximum of secrecy capacity under the condition of unconstrained and constrained total power of the transmitted signal to optimize the secrecy capacity.When the total power of the transmitted signal is unlimited,firstly,the closed-form expression of ergodic secrecy capacity with respect to the power of the useful signal and the power of AN is derived.Secondly,the asymptotic behavior of the ergodic secrecy capacity is analyzed to further investigate how secrecy capacity deteriorates with PA nonlinearity.According to the asymptotic behavior analysis,when nonlinear PA replaces the linear PA in the system model,the monotonic function of secrecy capacity dramatically changes to various non-monotonic function that reveals single-peak,single-valley,peakvalley,and valley-peak features.Different asymptotic behaviors correspond to different communication environment conditions.Finally,the closed-form expression of the optimal point corresponding to the upper bound of secrecy capacity function is derived in the low SNR environment.When the total power of the transmitted signal is constrained,the closed-form expression of ergodic secrecy capacity with respect to the total power of the transmitted signal and the fraction of total power allocated to the information signal is first derived,and the asymptotic behavior of the ergodic secrecy capacity is further analyzed.The asymptotic behavior shows that the function of secrecy capacity with respect to the total power presents five features,i.e.,single-peak,single-valley,peak-valley,valley-peak,and valleypeak-valley.When the fraction increases,the secrecy capacity shows three trends that are monotonically increasing,monotonically decreasing,and single-peak.Therefore,the upper bound of secrecy capacity exists.Finally,the closed-form expressions of the optimal total power and the optimal fraction corresponding to the maximum of the secrecy capacity are derived through the Lagrange multiplier method and the Hessian matrix in the low SNR environment.The above analytically analysis and the closed-form expressions of the optimal value corresponding to maximum secrecy capacity are verified through simulations.The secure performance can be improved by setting the operating point of the communication system to the optimal point.In addition,The analysis methodology and derived findings can be a useful reference for the designers of AN aided communication systems.