Research On Relaying Cooperative Communication System Assisted By RIS-Enabled UAV

Relay cooperation technology enables a single antenna device to form Multiple-Input Multiple-Output(MIMO)to enjoy spatial diversity gain.In some complicated or hard-to-reach areas,for example,the center of earthquake stricken area,it is difficult to deploy traditional fixed relays to support reliable communication.The unmanned aerial vehicle(UAV)is considered as one of the promising solutions thanks to the advantages of flexible deployment and large coverage.The UAV may work as a moving relay and cooperate with ground users to enhance the throughput and extend the coverage.Therefore,a new technology named Reconfigurable Intelligent Surface(RIS)works in full duplex mode without self-interference and has the merits of low hardware cost,low power consumption,flexible deployment,and intelligent reconfiguration.Combining the advantages of UAV and RIS,RIS-enabled UAV not only has the advantages of flexible deployment of UAV and large coverage area,but also has the merits of RIS,such as low hardware cost,low power consumption and light weight.This thesis considers relaying cooperative communication system assisted by RIS-enabled UAV and further maximizes Average Achievable Rate(AAR).The specific contributions are as follows:(1)this thesis proposes a novel amplify-and-forward(AF)relay-based cooperation communication scheme with RIS-enabled UAV enhancing.The UAV acts as a moving RIS in the sky which provides additional links to enhance the performance of the AF relay-based cooperation on the ground.Aiming to maximize the AAR of the proposed scheme,the scheme jointly optimizes the trajectory of UAV and the phase shift matrices of RIS.At first,the model of joint optimization problem which maximizes AAR is established;Secondly,the original intractable problem is divided into two subproblems of the phase shift matrices optimization and trajectory optimization;Subsequently,the optimal phase shift matrices are achieved by aligning the phases of received signals.Since the trajectory optimization subproblem is still complicated and nonconvex,the successive convex approximation(SCA)method is applied to reformulate it into a convex problem and further solve it by iterative method;The single-antenna scenario is further extended to a more general multi-antenna scenario.Simulation results demonstrate the proposed scheme significantly improves the AAR of the system.(2)On the basis of the above work,considering the advantage of Decode-and-forward(DF)relay performance,a RIS-enabled UAV assisting DF relay communication scheme is proposed.Multiple antennas are equipped at the transmitter and relay to form a Multi-input single-output(MISO)system to enhance communication.In order to further improve the AAR of the system,the precoding vectors of the transmitter and relay,the transmission power allocation,the phase shifts of RIS and UAV trajectory are jointly optimized and a joint optimization problem is established.Since the original problem is non-convex and difficult to solve,the alternating optimization(AO)method is applied to divide the original problem into four sub-problems,i.e.,precoding vector optimization,transmission power allocation optimization,RIS phase shifts optimization and UAV trajectory optimization.Maximum ratio transmission(MRT),triangle inequality and SCA method et al.are used to solve the problem respectively.Simulation results demonstrate the superiorities of the proposed scheme compared with the benchmark schemes,and also reveal the effects of various factors on the AAR performance.