Research On Precoding Optimization Technology Of Wireless IRS Communication Systems

Driven by the explosive growth of various wireless applications,the future wireless network have a higher demand on communication capacity.In order to achieve the goal,various advanced technologies such as massive antenna array,millimeter wave(mm Wave)communication and ultra-dense network have been deeply studied.However,these technologies generally have the disadvantages of high cost and high complexity.Intelligent reflecting surface(IRS)consists of massive low-cost passive reflecting elements,it can realize the intelligent configuration of wireless propagation environment,so as to significantly improve the performance of communication systems.Therefore,IRS has become an alternative technology for the next-generation mobile communication system.One of the key problems of IRS aided communication systems is the joint active and passive beamforming design,focusing on which,the dissertation conducts an intensive research on several different scenarios and obtains some research results.Firstly,aiming at the distributed IRS-assisted multiple-input multiple-output(MIMO)communication system,the joint optimization problem of the fully digital precoding at the base station(BS)and the passive precoding at the IRSs is studied.To maximize the achievable rate of the system,we propose an effective alternating optimization algorithm.Simulation results verify the effectiveness of our proposed algorithm and the improvement of system performance with deploying multiple IRSs.Secondly,for the IRS-assisted mm Wave communication system using subarray structure,the joint precoding design problem aiming to maximize the received signal power at the user for single-user scenario is studied,including the subarray-connected structure hybrid precoding at the BS and the passive reflection precoding at the IRS.For the optimization problem,we propose a low-complexity algorithm,in which the original problem is firstly converted to an approximate optimization problem by exploiting the rank one property of mm Wave channels and then the closed-form solution of the approximate problem is derived.Numerical results verify the feasibility of simplification,and also illustrate that the subarray-connected structure and the IRS can effectively enhance the performance of mm Wave communications.Finally,aiming at the physical layer secure transmission of the IRS-aided mm Wave communication system,the secrecy rate maximization problem for the single eavesdropper scenario is investigated and an efficient algorithm based on the alternating optimization framework is proposed.Concretely,we divide the joint optimization problem into three optimization subproblems: the digital baseband precoding,the analog radio frequency precoding and the passive precoding of the IRS,and then iteratively solve each subproblem.Simulation results show the significant gain of IRS in wireless communication security.