Research On Joint Optimization Of UAV Communication Network Based On NOMA

In recent years,with the development of science and technology,the increasing maturity of UAV technology,the gradual reduction of manufacturing costs,flexible and convenient configuration,mobility,communication coverage expansion and other advantages,UAV-assisted wireless communication methods have gained widespread attention.In addition,while meeting the demand for massive communication nonorthogonal multiple access technology came into being,which can use the same frequency band resources to maximize the number of access users,enhance the access capacity of communication system,improve the transmission rate,greatly reduce the transmission delay,and improve the spectrum utilization.Therefore,the collection of UAV and NOMA will have more advantages.In recent years,an emerging technology that uses a large number of low-cost passive reflective elements to achieve intelligent signal reflection is considered as a potential key technology to build a new paradigm of future wireless communication,i.e.,intelligent reflective surface,which can effectively improve the quality of signal transmission and enhance communication quality,and also provide additional communication links for users whose communication is blocked,to solve the problem of blocked communication links in complex environments that cannot communicate directly.Therefore,this thesis adds intelligent reflective surfaces into UAVNOMA communication system to solve some problems in reality by complementing their respective characteristics.The main contributions of this thesis are as follows:1.In order to improve transmission flexibility,reduce transmission delay,and address emergency communication situations after disasters,a model of unmanned aerial vehicles carrying small base stations for downlink communication in NOMA mode was established.Maximize the maximum sum rate of the UAV-NOMA system by jointly optimizing the three-dimensional trajectory,pitch angle,and user clustering of the UAV.This thesis solves the problem by improving the cuckoo search algorithm and simulated annealing algorithm,and improves the global search ability of the algorithm by forcing small probability mutation to increase the population size.This overcomes the disadvantage of the original cuckoo search algorithm’s reduced search ability in the later stage.The simulation results show that the improved cuckoo algorithm in this thesis has obvious advantages compared to other algorithms.2.In order to solve the problem that the wireless channel is blocked by obstacles and the base station cannot communicate directly with the user,and to improve the transmission rate of the unmanned aerial vehicle communication system,a model of unmanned aerial vehicle(UAV)equipped with an intelligent reflector for downlink communication in NOMA mode is established,which forwards signals from the base station to the user.By jointly optimizing the IRS phase shift,the three-dimensional trajectory of the UAV,user scheduling,and power allocation,the sum rate of the system is maximized.Due to the nonconvex nature of the proposed problem,it cannot be directly solved,in this paper a block coordinate descent algorithm(BCD)is used to transform the original nonconvex problem into four subproblems.Firstly,the converted IRS phase shift subproblem is optimized using a semi positive definition relaxation(SDR)technique,followed by the introduction of relaxation variables The method of first order Taylor expansion and continuous convex approximation transforms the original non-convex subproblem into a convex problem and uses the CVX toolbox to optimize the threedimensional trajectory of the unmanned aerial vehicle.Then,by relaxing the processing constraints and using the CVX toolbox,the user scheduling subproblem is processed.Finally,the power allocation problem based on sum rate maximization is derived using the Lagrange multiplier method,The optimal power requirements for two users in a cluster and each cluster in NOMA mode are derived.The simulation results show that the proposed method can significantly improve the data transmission rate of users in the case of blocked wireless channels,and has good convergence.