Research On Physical Layer Security And Covert Transmission Technologies Of Wireless Network

With the development of communication technology,long-distance communication and data transmission become more and more simple.People can enjoy the convenience brought by the high data rate wireless network anytime and anywhere.However,wireless communication is open and broadcast,so it is easy to be eavesdropped by the evedropper.People's privacy has a high risk of leakage.Data encryption technology at the application layer can alleviate the problem of privacy leakage to a certain extent,but with the continuous improvement of hardware computing ability,data encryption technology is facing the crisis of rapid cracking.The physical layer secure transmission technology which can use multi-antenna technology,artificial noise technology,random coding technology and other physical layer technologies,can realize the secure transmission of physical layer signals.It utilizes the characteristics of the channel without the assistance of the upper layer to fundamentally alleviate the problems caused by the openness of wireless communication.Furthermore,well-designed convert wireless transmission can further improve the concealment and security of the information transmission since it can make the third-party listening device cannot judge the existence of the communication.In this thesis,we focus on the research of physical layer secure transmission and covert transmission in wireless communication systems.The research involves many aspects such as beamforming technology in directional modulation network,hybrid analog and digital beamforming technology and covert transmission technology.Relevant research results can be applied to realistic engineering to improve the secure transmission rate of wireless system and realize the concealment of transmission behavior in specific scenarios.The main contributions of the thesis are:1)We considered the traditional channel estimation in the context of covert wireless communication,where the impact of a finite blocklength was examined.Specifically,we consider a Rayleigh fading communication channel for Additive White Gaussian Noise(AWGN)or Rayleigh fading detection channel.Based on a lower bound on the minimum detection error probability at Willie,we proved that equal transmit power for pilot and information signals can minimize Willie's detection performance under the case with AWGN detection channel.This result inspired us to consider the equal transmit power in the channel estimation for covert wireless communication in this case,where the number of channel uses allocated to channel estimation is optimized in order to maximize the effective signal-to-interference-plus-noise ratio(SINR)of the communication channel subject to the covertness constraint.This optimal number monotonically increases as the covertness constraint becomes tighter.For the case with Rayleigh fading detection channel,Willie's detection performance was analyzed for both the equal and unequal transmit power of pilot and information signals,based on which a general framework was presented to optimize resource allocation in covert communications.2)For the estimation error of arrival angel,we propose a robust hybrid ADB scheme.It is based on the diagonal loading method,where the NSP rule is adopted to design the total beamforming vector.The simulation results show that,in the case of perfect DOA available,the proposed scheme achieves a substantial beam gain over DL algorithm in the interference direction.In the presence of DOA measurement errors,the proposed robust hybrid ADB,with uniformly distributed angle errors,shows a good robustness compared to DL algorithm and the NSP hybrid method.As the maximum angle error becomes larger,its SINR performance over non-robust schemes such as DL becomes more significant.3)To enhance the secrecy rate of directional modulation,we proposed an alternating iterative structure between the beamforming and power allocation to maximum the secrecy rate.The proposed alternating iteration structure better than the traditional null space projection method in secrecy rate.And it can achieve the convergence state rapidly.Simulation results indicate that the SR performance of proposed AIS is much better than the null-space projection(NSP)based PA strategy in the medium and large signal-to-noise ratio(SNR)regions,especially when the number of antennas at the DM transmitter is small.In additional,the proposed scheme can rapidly converge to its rate ceil with only two or three iterations and increase the secrecy rate.4)In the integrated model of precise interference and wireless secure transmission,we propose two efficient amplitude beamforming schemes to futher improve the physical layer secrecy.One is the leakage-based method and the other is the maximizing receive power method.Making use of leakage criterion,we use the maximizing signal-to-leakage-noise ratio method to optimize the transmission vector.While we design the transmission vector by maximizing the receive power of confidential single at Bob in the maximizing receive power method.Simulation results show that the proposed maximizing receive power method and leakage-based methods perform much better than conventional equal amplitude beamforming method in terms of both bit error rate and secrecy rate at medium and high signal-to-noise ratio regions.