Research On Secure Transmission Technology For Cell-free Massive MIMO System

User-centric cell-free massive multiple-input multiple-output(MIMO)can effectively reduce path loss,improve system capacity and energy efficiency,and overcome inter-cell interference and frequent handover problems,it is expected to become one of the key technologies of the sixth generation mobile communication.However,due to the broadcasting and openness of wireless communications channel,cellfree massive MIMO communication networks are faced serious threat of pilot spoofing attack(PSA).Therefore,this thesis focus on the secure transmission in the cell-free massive MIMO system,deeply investigating the influence of factors such as PSA,hardware impairments and spatial correlation on the secrecy performance of the cell-free massive MIMO system.We also propose a pilot attack joint detection algorithm based on the distributed characteristics of cell-free network to improve its security.The main work of this thesis can be summarized as follows:Firstly,for the cell-free massive MIMO system in the spatially correlated Rayleigh fading channel,a closed-form expression of the downlink achievable rate is derived by using the minimum mean square error channel estimation method and the maximum ratio transmission precoding.In addition,based on the largescale fading(LSF)coefficient,a distributed power control scheme is designed.The simulation results show that the existence of spatial correlation will reduce the downlink achievable rate of the system,and the power control scheme based on LSF can effectively improve the achievable downlink rate of the system.Then,for the cell-free massive MIMO secure system with eavesdropper’s PSA,a closed-form expression of achievable secrecy rate is derived.The influence of factors such as PSA,hardware impairments,spatial correlation and the location information of eavesdropper on the system channel estimation and secrecy performance are investigated in detail.Besides,a secure transmission power control scheme based on LSF was proposed.The simulation results show that the existence of PSA and hardware impairments will reduce the achievable secrecy rate of the system,while the existence of spatial correlation can alleviate the negative impact of PSA,and thus improve secrecy performance of the system.Compared with the equal power allocation scheme,the LSF-based power control scheme can significantly enhance the secrecy performance of the system.Finally,in order to reveal the potential advantages of cell-free massive MIMO system against PSA and further improve the system secrecy performance,a pilot attack joint detection algorithm is proposed based on the pilot power difference at the access point(AP)under PSA or not and the distributed signal processing mechanism of cell-free massive MIMO.The simulation results show that the AP joint detection algorithm based on the distributed AP feature of cell-free network provides better detection performance than the single AP attack detection algorithm under low attack power.