Research On Power Optimization And Precoding Design For Multi-UAVs Coopertive Transmission System

With the advances in manufacturing and technology,unmanned aerial vehicles(UAVs)have been widely used in various applications owing to their advantages of flexibility of movement,high mobility and low cost.with the aid of low-altitude UAVs,short-range line-of-sight(Lo S)communication links can be established in most scenarios,which potentially leads to significant performance improvement over direct communication between source and destination.This thesis focus on providing wireless communication service for users with no cellular network coverage by the multi-UAVs.In order to reduce interference between users,a multi-UAVs cooperative transmission scheme is proposed in this thesis.The UAVs uses precoding to eliminate interference in the received signal of users.Taking into account the capacity limitation of airborne backhaul,to avoid transmission a large amounts of non-payload information between UAVs.A cooperative transmission scheme that UAV only use local channel state information is presented in this thesis,and we optimize the corresponding resource allocation design.First,this thesis introduces the structure of the UAV cooperative transmission system,and details the workflow within a channel coherent time.Then,the channel model between the UAV and ground user in the single antenna scenario is analyzed.Base on the results,we further extended to the multi-antenna channel model.Then the influence of signal arrival delay difference on the channel model is analyzed.Based on the researched UAV channel model,the channel correlation between different users is analyzed in single antenna and multi-antenna scenarios,respectively,which is the basis for the subsequent theoretical analysis.Secondly,we research the system performance and power allocation issues in a single transmitting antenna scenario.In order to optimize the fairness of the proposed system,we optimize the power allocation scheme to maximize the minimum achievable rate.We transform the power allocation problem into a quadratic optimization problem and it can be solved efficiently by bisection.A non-cooperative transmission scheme based on user selection is introduced,and the corresponding power allocation problem is analyzed.Finally,the research in the single-antenna scenario is extended to the scenario of multiple transmitting antennas.In the scenario of using multiple antennas,the system can not only use the independence of the channels between the UAVs to obtain multiplexing gain,but also obtain the array gain of the antenna.In this thesis,the multi-antenna transmission model is analyzed,and the general expression of user reachable rate under this model is derived.In order to apply to different application scenarios,this thesis introduces channel matching,zero-forcing and local zero-forcing precoding schemes,and analyzes the user ratings of each scheme.And based on the derivation result of the user rate,a power allocation scheme is proposed in this thesis.