| The continuous development of wireless communication technology brings convenience to people,but at the same time,it increases the risk of information leakage,thus the pressure of communication security regulation supervision also increases accordingly.In the research of traditional physical layer security,due to the broadcasting characteristics of the wireless communication transmission medium,it is easy for illegal eavesdropping and attacking.Therefore,how to improve the information reliability in t he transmission process and ensure a low outage probability is an important link in the research.However,with the lower requirements for wireless network infrastructure,criminals may also use wireless communication equipment to carry out illegal activities such as stealing commercial or military secrets so as to harm enterprises and even national security.In order to prevent the occurrence of such incidents,it is necessary to deploy a reasonable and efficient regulatory system.Referring to the communi cation scene in the traditional physical layer security research and aiming at the situation that criminals may use for illegal communication,this thesis makes a deep discussion and research on the scheme design of proactive eavesdropping model and the optimization and improvement of lawful eavesdropping performance.Firstly,this thesis introduces the background of this research based on the existing research results,including the introduction of traditional physical layer security and common technologie s used to guarantee communication quality,as well as their application in proactive eavesdropping system.In addition,although the research of proactive eavesdropping system is similar to the traditional physical security research,it also has its unique performance indicators,such as the non-outage probability of eavesdropping,average eavesdropping rate,etc.,which will be introduced in the first two chapters in this thesis.Secondly,after introducing the basic knowledge,the concrete communication model will be calculated and analyzed.In order to avoiding the situation that the suspicious users communicate by simple mobile communication equipment for illegal behavior,a proactive eavesdropping system of Rayleigh channel was studied for legal departm ent to monitor suspicious users.The system adopts multiple eavesdropping users to conduct proactive eavesdropping.Considering that full duplex active eavesdropping users can interfere with suspicious users while eavesdropping and the self-interference cannot be completely eliminated,three different options was proposed.Besides,the approximate closed-form expression of eavesdropping non-outage probability was derived under these kinds of schemes.The theoretical analysis and numerical simulation results show that,the proactive eavesdropping performance of these three schemes has its own advantages and disadvantages in the case of high or low jamming power due to different emphases.But in general,the performance of the second scheme is the best under the condition of low jamming power,while the third scheme is obviously superior under the condition of high jamming power,and these three schemes are better than traditional scheme with single eavesdropper.Finally,an optimal jamming scheme for proactive eavesdropping on suspicious communication in decode-and-forward relay network is proposed.When the suspicious receiver is disturbed,the transmission rate of the sender is forced to change in order to maintain a constant outage probability.Take advantage of this,the average eavesdropping rates and the optimal jamming power of the two time slots are analyzed by different signal processing methods.And besides,the average eavesdropping rate and the optimal jamming power under two different signal combina tion schemes at the eavesdropping receiver are analyzed.Finally,three different methods of jamming including optimal jamming,constant jamming and zero jamming were compared.The expression and simulation results show that the scheme of maximal ratio combining with optimal jamming performs best when adopting maximal ratio combining. |
