Joint Beamforming Algorithm Research For Ris-aided Communication System

Cell-Free network,unmanned aerial vehicle(UAV)communication,and reconfigurable intelligent surface(RIS)are the key architecture and technology to meet the needs of future mobile communications.In order to fully guarantee and improve the performance of the mobile communication system while reducing network cost,the combined applications of RIS and various parts of the system are studied.Based on this,the main research contents of this paper are determined to include:First,in response to the challenge that the signal quality is easily affected by the user's movement,a RIS beam tracking and coverage enhancement algorithm for mobile users is proposed.Based on the estimation of the user's motion area,the RIS unit is divided into multiple sub-arrays and the corresponding phase shift matrix are designed to achieve effective coverage of the target area,and the water filling power algorithm is used to allocate and optimize the transmission power of the base station.The studie has shown that the joint RIS phase shift and base station power allocation algorithm design can achieve a 10% increase in the average system throughput.In addition,in order to reduce the signal recovery delay and reduce the number of times of searching for better beam,RIS wide beam design based on codebook is proposed.The optimal beam direction is determined layer by layer to achieve the same received power as the exhaustive search algorithm,and the complexity of linear growth with the number of RIS units is reduced to logarithmic growth.Then,in response to the requirements of integrated air-space-ground communication,in order to improve the quality of UAV communication services to users while ensuring the continuity of ground user communication,a system model for RIS to assist UAV communication with ground users is proposed.Aiming at the communication quality of ground users that is easily affected by RIS phase shift and UAV trajectory,the optimization problem with the goal of maximizing UAV communication system spectrum efficiency is established.Then the problem is decomposed into two sub-problems of RIS phase shift design and UAV trajectory design,which are solved by alternate optimization algorithm.That is,the binary tree algorithm and tabu search algorithm are used to optimize the RIS phase shift and UAV trajectory respectively.The simulation results verify the effectiveness of the proposed algorithm.The optimal UAV flight path in single RIS and multiple RIS scenarios are obtained,and the spectrum efficiency of the system is increased by 8%-20%.Finally,in order to take advantage of the Cell-Free network in ensuring the fairness of users,a communication model in which a RIS assists multiple access points(AP)to serve multiple users is established.Under the limited AP transmit power and RIS phase shift accuracy constraint,the optimization goal is to maximize the minimum signal-tonoise ratio among multiple users.Then alternate optimization and semi-definite relaxation algorithms are used to optimize the joint beamforming of AP and RIS.The proposed joint optimization algorithm achieves a 5-9 times improvement in the spectrum efficiency of the worst user,and further guarantees the communication performance of the network balance.In summary,this paper studies the joint optimization design of RIS beamforming with base station power,UAV flight trajectory,and AP beamforming.The goals of beam tracking and coverage enhancement for mobile users,reduction of recovery delay,maximum of system spectrum efficiency,and user fairness guarantee are achieved.The study shows that RIS-assisted communication is helpful to further improve the performance of mobile communication system,and the study results provide a theoretical basis for the design of future mobile communication system based on RIS-assisted.