Intelligent Reflecting Surface Assisted Secure Communication Based On Optical Centralized Beamforming

With the development of fifth-generation(5G)communication technology,indicators such as capacity,energy consumption,and security have attracted much attention.Under the Cloud Radio Access Network(C-RAN)architecture,active antenna unit(AAU)needs to be deployed in large numbers as the capacity demand increases dramatically.Moreover,the centralized optical hybrid beamforming based on centralized deployment has the advantages of energy consumption and cost.At the same time,there are security risks in the information transmission of 5G technology,and intelligent reflecting surface(IRS)can realize an intelligent and programmable wireless environment through software-controlled reflection,which is expected to solve the problem of physical layer security(PLS)communication.The better security and gain of the system can be obtained through the rational resource optimization configuration of the IRS and hybrid beamforming.For this reason,in the context of secure communication and considering the uncertainty of channel parameters,this thesis proposes two secure communication algorithms for the IRS-assisted system based on 5G optical concentrated beamforming.The specific work and contributions of this thesis are as follows:(1)In the case where the channel state information(CSI)of all users and multiple eavesdroppers are known,combined with the advantages of optical concentrated beamforming,an IRS-assisted multi-user secure communication algorithm based on optical concentrated beamforming is proposed.The sum secrecy rate maximization objectives are set,and the system's secure channel is established by using the singular value decomposition of the channel.The analog beamforming is realized by the optical codebook,which reduces the cost of the beamforming.By alternately optimizing the analog beamforming vector,the digital beamforming vector and the IRS reflection matrix,the secrecy rate of each user is improved,and multi-user secure transmission is realized.The research results verify that the security performance of the proposed algorithm is close to the full-digital scheme,which not only has the advantage of low cost,but also effectively guarantees the information security of the system.(2)The case where the passive eavesdropper's CSI is not available is studied,and the inter-user interference(IUI)management technology is innovatively introduced into the optical concentrated beamforming-based IRS-assisted non-orthogonal multiple access(NOMA)system to suppress the eavesdropping performance.Based on different assumptions of the secure user channel gain,two transmit power maximization problems are solved by jointly optimizing the active hybrid beamforming vector and the passive IRS reflection matrix.For a given successive interference cancellation(SIC)decoding order,the full-digital beamforming vector is optimized by applying semidefinite relaxation(SDR)and successive convex approximation(SCA),and then the analog beamforming vector is designed based on the full-digital beamforming vector and the optical codebook codeword selection scheme.SDR,SCA and Gaussian randomization are used to optimize the IRS reflection matrix,and finally the alternating iterative optimization of active hybrid beamforming and passive reflection matrix is realized until convergence is satisfied.The research results verify that the security of the optical concentrated beamforming-based IRS-assisted NOMA system can be effectively improved through reasonable IUI management,and the security performance of our proposed algorithm is close to the full-digital scheme.