Research On Network Assisted Full Duplex Secure Transmission Technology

China has officially entered the first year of 5G commercialization with the issuance of commercial licenses for the fifth generation(5G)to three major operators by the Chinese Ministry of Industry and Information Technology.As society enters the era of Internet of everything,wireless terminals and data volume are growing exponentially,and the demand for wireless communication resources is increasing.Large-scale distributed multiple-input and multiple-output(MIMO)technology has been widely studied and has advantages of significantly improving the flexibility of system resource allocation and substantially increasing the utilization of wireless resource.In order to meet the Quality of Service(Qo S)of different users,full duplex technology,which can send downlink data and receive uplink data in the same time and frequency resource block,has also received widespread attention.Network Assisted Full Duplex(NAFD)technology,which leverages the benefit of massive distributed MIMO and full duplex technology,has also emerged.Due to the broadcast nature of wireless communication,wireless communication system is vulnerable to eavesdropping and intrusion attacks.Physical layer security technology using wireless channel characteristic can effectively improve security and confidentiality of the system,so it has also become a current research hotspot.The problem of Cross Link Interferce(CLI)suppression in network-assisted full duplex and the secure transmission after CLI suppression is noticed.First,the CLI suppression problem of NAFD is studied.NAFD has two types of CLI:Downlink-to-Uplink interference(D2U)and Uplink-to-Downlink interference(U2D),and the suppression of CLI is essential to achieve flexible duplex and flexibly and efficiently utilize wireless resource.The optimization model for maximizing the total rate for both downlink and uplink in a NAFD-based massive distributed MIMO system with constraints on user Qo S,Remote Antenna Unit(RAU)energy,backhaul capacity and uplink user energy is developed.The SDR-BCD(Semi Definite Relaxation-Block Coordinate Descent,SDR-BCD)method is used to solve the original problem,that is,a semi-definite relaxation is introduced to the original problem and linearization is applied to transform the nonconvex problem into a convex one,then the problem is solved by the block coordinate descent method.The simulation results show that the SDR-BCD algorithm can effectively suppress the interference of CLI,and SDR-BCD NAFD has higher spectral gain than the traditional Time Division Duplex(TDD)and Cloud Radio Access Network(C-RAN)based Co-time Co-frequency Full Duplex(CCFD).Meanwhile,when the CLI is suppressed to a low level,the SDR-BCD NAFD and C-RAN CCFD can obtain superior performance gain compared to the TDD.Second,the secure transmission of NAFD is studied.After suppressing the CLI,The presence of eavesdropping nodes is considered in a large-scale distributed MIMO system based on NAFD under suppressed CLI,and the secure transmission model of the system is established.An optimization model is established with the objective of maximizing the total uplink and downlink security rate under the RAU transmit power constraint and uplink user power constraint,and jointly optimizing the RAU operating mode,precoding vector,uplink transmit power and artificial noise(AN).Since there is 0-1 integer programming and the optimization variables are coupled,the original problem is decomposed into two subproblems: secure transceiver design and duplex mode selection,and a two-stage optimization strategy is proposed.The first stage,the duplex mode selection problem is solved by a greedy algorithm,whose idea is that the different operating mode of the RAU which represents different channel matrice between the RAU and the user,provides the basis for the greedy algorithm.In the second stage,the non-convex optimization is changed to convex optimization by using equation transformation and linearization methods under the fixed RAU operation mode,the SCA(sequential convex approximation,SCA)iterative algorithm is proposed to design the secure transceiver.The simulation results show that the spectral efficiency of the system increases with the number of RAU antennas;the spectral efficiency of the system increases with the decrease of self-interference;the greedy algorithm achieves a compromise in time complexity and performance compared with the full-duplex algorithm and the exhaustive search algorithm.