Multiple Access Control In Long-distance Wireless Networks

Space-air-ground integrated information network is an emerging network.It is integrated by satellite systems,air networks and terrestrial networks.It has the advantages of wide coverage,high flexibility and low cost,and is the development trend of wireless mobile communication in the future.As one of its key technologies,the access control method largely determines the communication efficiency of the space-air-ground information network.Different from the traditional mobile communications,there may be more long-distance links and fast changing wireless communication scenarios in the space-air-ground integrated network,which makes the existing access control protocols not directly applicable.Therefore,new methods are urgently needed to solve the efficient communications of long-distance wireless links such as aircraft and ground base stations,satellites and aircraft.This dissertation takes long-distance unmanned aerial vehicle(UAV)networks as the research objects.A long-distance UAV network includes a base station(BS)as the data receiver and a number of UAVs.These UAVs are usually for the purpose of reconnoiter and reconnaissance.They usually have long flight distances,wide coverages and high requirements for real-time communication.UAVs directly communicate with a BS without any relay.In this dissertation,resource requests and resource allocations in long-distance UAV networks are studied,and the corresponding solutions are proposed.The main contents include the following three parts.(1)This dissertation proposes a propagation-delay aware random access method to solve the problem of channel instability and signal conflicts in long-distance UAV wireless networks.When the network environment is unknown,the UAV that needs to apply for resources selects an appropriate time point to send the request in terms of its distance from the base station to reduce the conflict probability with other competing UAVs.Due to the inevitable conflict of random access and the instability of signals in long-distance link,competing UAVs choose to send multiple copies of requests at different time points in order to improve the success rate of requests.This dissertation proposes a propagation-delay aware time slot selection algorithm.The time slot selection algorithm and signal recovery algorithm are designed for the transmitter and the receiver respectively.And the success rate of request reception is analyzed theoretically.The simulation results show that the performance of the proposed algorithm is better than that of the random slot selection algorithm.At last,by the simulation results,the complex success rate of request is fitted with a curve,and a simple expression is obtained.(2)This dissertation proposes a propagation-delay aware slot allocation algorithm to solve the problem of low transmission efficiency and high re-transmission cost.The algorithm maximizes the system throughput under the premise of fairness.In shortdistance wireless networks,the propagation delay is usually ignored.It is generally considered that there is only one signal over the same channel at the same time,or multiple signals will conflict.While on the long-distance links,due to the large propagation delays,it is possible for multiple transmitters with different distances from the receiver to transmit signals at the same time without conflict.According to this idea,utilizing the distances between UAVs and base station,a propagation-delay aware slot allocation algorithm is designed for the UAVs to satisfy the requests and improve the system throughput.The problem of maximizing system throughput is formalized,and the propagation-delay aware time slot allocation method is utilized for local optimization.In this dissertation,the throughput of the network under saturated scenario and unsaturated scenario is analyzed theoretically.Besides the optimization of the proposed algorithm is proved.The simulation results show that the performance of the proposed algorithm is better than that of the traditional access methods.(3)This dissertation proposes a multi-channel,power constrained non orthogonal multiple access optimization algorithm to solve the problem of spectrum underutilization of single channel orthogonal multiple access in the long-distance wireless networks.Under the premise that the maximum UAVs transmitting over the same channel do not exceed a specific number of UAVs,and the transmission power differences of UAVs over the same channel is enough to decode,this dissertation addresses the system throughput maximization problem.First,according to the differences of the channel gains between UAVs and the BS,the UAVs are clustered.The UAVs with large difference would be divided into one cluster.That is,the signals in one cluster can be transmitted over the same channel.Then the transmission power of UAVs over the same channel is optimized.The mixed integer non-linear optimization(MINLP)problem of maximizing throughput is transformed into a convex optimization problem by using Lagrange multiplier method.Subsequently,the gradient descent method is used to solve the convex optimization problem,and the related algorithms are designed.The feasibility and performance of the algorithm are analyzed theoretically and verified by simulation experiments.