Research On UAV Cooperative Multiple-Input Multiple-Output Technology

The unmanned aerial vehicle(UAV)is low-cost,energy saving and has no danger of casualties,thus it is able to work flexibly in a variety of environments.The UAV data link communication has been widely studied so far.The single UAV shows out its limited ability with the task difficulty increasing,so it is a development trend to consider the cooperation of multiple UAVs to complete the task in the future.Multiple-Input Multiple-Output(MIMO)is one of the key technologies in the fourth generation mobile communication system(4G),it can effectively improve the data transmission rate and transmission reliability.It is not able to install too many antennas on a general single UAV in consideration of the size and energy constraints of UAV.Therefore,the application of traditional MIMO technology in UAV communication system is very limited.Cooperative communication technology is an extension technology of traditional MIMO,which overcomes lots of shortcomings of traditional MIMO technology.The basic principle of cooperative communication is the cooperation among multiple users and forming a multi-antenna system,in this way a virtual MIMO system is formed to get close to the performance of the traditional MIMO system.The in-depth study of UAV cooperative MIMO technology will greatly promote the development of UAV cooperative communication system and promote the use of UAV in military and civilian fields broader and more convenient.This paper studies the cooperative MIMO technology of UAV.Multiple time delays and multiple frequency offsets exist in the UAVs' distributed MIMO system,meanwhile,the large Doppler frequency offset exists when the UAV is in high dynamic scenarios,so it is necessary to study the time and frequency synchronization algorithm for UAV distributed MIMO system.The key to UAV cooperative MIMO technology is to establish a suitable cooperative model and choose an appropriate method of cooperative communication.In order to suppress the interference of signals from nonarrival wave directions,beamforming(BF)technology is introduced in the ground receiving station.In addition,UAV navigation and positioning error will cause the estimation error of the angle of arrival(AOA),so the influence of AOA error on beamforming system is analyzed and simulated.Based on the current research on cooperative MIMO technology and the development trend of UAV,The main contents of this paper include:Firstly,establishing an UAV air-ground channel model.According to the characteristics of UAV air-ground channel,a three-dimensional air-ground channel model for UAV formation is established by using Kronecker product channel model and the key factors are described in detail.Secondly,research on distributed MIMO synchronization algorithm for UAVs.According to the characteristics of multiple time delays and multiple frequency offsets in the UAVs' distributed MIMO system,a cooperative synchronization algorithm for UAVs is studied referring to the wireless frame structure and synchronous preamble sequence.The correction method of large Doppler frequency offset in the high dynamic environment of UAV is given.Thirdly,research on UAV cooperative communication mode.In view of the traditional two-user cooperative model and the three-point cooperative model,three cooperative communication modes are studied: full duplex adaptive distributed space-time code(DSTC)coded cooperation,half duplex adaptive DSTC coded cooperation and diversity cooperation.Their corresponding outage probabilities are derived and are compared with the corresponding non-cooperative systems respectively,and the influence of each parameter on the outage probability is studied by simulation.Finally,research on the beamforming technology.The problem of beamforming receiving of ground receiving station in UAV air-ground cooperative transmission system is studied,and the outage probability of beamforming system is derived and studied by simulation.Considering the navigation and positioning error,the influence of AOA error on the system is analyzed and the feasibility of beamforming at the receiver in UAV air-ground cooperative communication system is verified by simulation.