User Link Optimization For Cellular System With Underlaid D2D Communications

D2D communication is a short-distance communication technology that allows multiple terminal devices to communicate directly without the base station.It has the advantages of reducing base station load,reducing equipment energy consumption and communication delay.In order to further improve the performance of D2 D communication system,some literatures have proposed to adopt relay to assist D2 D transmission,so as to expand the scope of application of D2 D communication.This paper considers the optimization of related link parameters under the two scenarios where cellular system allowing relay to assist D2 D transmission and the cellular system allowing D2 D transmission with UAV as the base station.The main research work is as follows:Firstly,a D2 D underlaying system where fixed relays can be used to assist D2 D transmission with network coding is considered.Aiming to maximize the sum throughput of all D2 D users when the transmission power of cellular users are given,the transmission power control and channel allocation for D2 D users and relays is studied.The main work is as follows: 1)Given the corresponding mathematical expression of the problem,in order to solve the non-convex problem,it is decomposed into two mutually iterative subproblems which are D2 D user and relay power control subproblem,D2 D user and relay channel allocation subproblem;2)It is proved that the power control subproblem is convex,and the optimal transmission power of D2 D users and relays is solved based on the interior point method;3)For the channel selection subproblem,it is transformed into the equivalent variable relaxation problem,and the duality method is used to obtain the optimal channel allocation.Simulation results show that the performance of the proposed joint optimization algorithm is significantly improved compared with the traditional non-joint optimization algorithm.Secondly,a D2 D underlaying system where alterable relays can be used to assist D2 D transmission with network coding is considered.Aiming to maximize the sum throughput of all D2 D users when the transmission power of cellular users are given,the transmission power control and channel allocation for D2 D users and relays,D2 D mode and assisted-relay selection is studied.The main work is as follows: 1)The corresponding optimization problem is established.In order to solve this non-convex problem,the problem is divided into three mutually iterative subproblems for solving,namely,D2D user and relay transmission power control subproblem,D2D user and relay channel allocation subproblem,D2D mode and relay selection subproblem.2)When solving the power control subproblem,it is proved that the problem is convex,and the optimal transmission power solution of D2D users and relays is obtained based on the interior point method;3)Greedy algorithm is adopted to solve channel allocation subproblems;4)When solving the D2D mode and assisted-relay selection subproblem,the NP-hard problem is solved by using the coalition formation game,and the convergence analysis is given.Simulation results show that the joint optimization algorithm proposed in this paper has obvious advantages over the traditional non-joint optimization algorithm.Thirdly,the joint optimization of power control for D2 D users and ground terminals(GTs),the position optimization of UAV and the bandwidth allocation for GTs are considered in the UAV-aided communication underlaying a single D2 D user while ensuring the QoS demand of ground user.The main work is as follows: 1)Aiming at the mathematical optimization problem corresponding to the problem,the closed solution is given when the launching power of the uav to the ground user meets the optimal condition,and the equivalent conversion is made for the problem of maximizing the rate of D2 D user;2)The converted problem is still non-convex,and the problem is divided into three mutually iterative sub-problems for solution,namely,D2 D transmission power control subproblem,UAV position optimization subproblem and GTs' bandwidth allocation subproblem;3)When solving the D2 D transmission power subproblem,the corresponding closed-form expression is obtained;4)The closed solution of the optimal height is obtained by solving the subproblem of UAV position optimization,and the solution of plane coordinates is obtained by two-dimensional search method;5)When solving the subproblem of bandwidth allocation for GTs,it is proved that the subproblem is convex and the optimal solution is obtained by KKT method.The simulation results show that the algorithm proposed is similar to the approximate global optimal solution algorithm in performance and greatly reduces the algorithm complexity.Fourthly,the power control for D2 D users and GTs,resource block(RB)allocation for D2 D users and GTs,and position optimization of UAV are considered in the multi-D2 D users communication system under the condition of ensuring the QoS demand of ground user.The main work is as follows: 1)Aiming at the mathematical optimization problem corresponding to the problem,the optimal closed-form solution of D2 D user RB allocation and the transmission power of UAV to GTs are obtained,and the problem is transformed;2)the converted problem is still a non-convex optimization problem,which can be divided into three mutually iterative subproblems for solving,namely,the RB allocation subproblem for GTs,the subproblem of UAV position optimization and the subproblem of D2 D transmission power control;3)When solving the RB allocation for GTs,it is proved to be convex and the optimal solution is found through KKT method;4)When solving the subproblem of UAV position optimization,the optimal height of UAV is obtained and the plane coordinates of the UAV are obtained by two-dimensional search method;5)WMMSE method is adopted to solve the transmission power of D2 D users.Simulation results show that the joint optimization algorithm proposed in this paper has obvious advantages over the traditional non-joint optimization algorithm.