Study On The Transmission Strategy Of UAV-assisted Multi-user Two-way Relay System Based On Probabilistic LoS Channel

With the development of communication technology and diversification of application scenarios,Unmanned aerial vehicle(UAV)assisted communication is a rapidly developing communication technology in recent years,which uses UAVs as mobile communication relay stations to extend the communication range of ground users(GUs).Due to the unique features such as low deployment cost,low path loss,zero casualties and high mobility,UAVs are expected to become flexible flight relays for wireless communications.Despite the potential advantages of the UAV-assisted relay communication technology,there are still many technical challenges in practical applications.Compared with one-way transmission,two-way transmission can achieve higher data transmission rate and lower latency because it can transmit and receive data at the same time,and furthermore,two-way transmission can better adapt to changes in the communication environment,so it has high practical value in practical applications.Since UAVs can move freely in the air,for UAV-assisted relay systems,the three-dimensional(3D)trajectory design of UAVs has an important impact on communication quality and communication efficiency.A reasonable three-dimensional trajectory design can reduce the signal attenuation and interference in communication,and improve the reliability and transmission speed of communication.In addition,there are multiple channel models in wireless communication channels,and compared with the simple Line of Sight(Lo S)channel model,the probabilistic Lo S channel model considers the probability distribution of the two propagation modes of direct and reflected paths in the propagation process,so it can better describe the real channel environment and thus better guide the 3D trajectory design.Therefore,the study of 3D trajectory design for UAV-assisted multi-user two-way relay network under the probabilistic Lo S channel model has important practical significance.In this paper,an UAV-assisted two-way relaying framework is proposed,in which UAVs assist in information exchange between multiple pairs of GUs,and UAV’s 3D trajectories are designed to maximize the total system rate under a probabilistic Lo S channel model.In addition,this paper also investigates the importance of long-range user communication in UAV-assisted relay communication and explores a two-way multi-hop relay system with multiple UAV assistance to provide a more reliable and efficient solution for UAV-assisted relay communication in practical application scenarios.The innovation points and research of this thesis can be summarized as the following two points:1.Study on the transmission strategy of single UAV-assisted multi-user two-way relayFor a multi-user relay communication network under the probabilistic Lo S channel model,a time-slot pairing scheme is proposed based on the Physical Layer Network Coding(PNC)protocol,and an optimization problem is designed to maximize the overall rate of the two-way relay network by jointly optimizing the time-slot pairing,power allocation,and the 3D trajectory of the UAV.To address the proposed nonconvex problem,this paper first proposes an efficient offline algorithm that decomposes the problem into four low-complexity subproblems by using the successive convex approximation(SCA)technique and the block coordinate descent(BCD)method for iterative optimization.To fully utilize the instantaneous channel state information(CSI)of the two-way relay network during the UAV flight,we also propose a hybrid offlineonline algorithm to further improve the performance of the network based on the offline algorithm.Simulation results show that the proposed offline-online hybrid algorithm improves the rate of the network by about 20% compared to the offline algorithm,and verifies the advantage of UAV 3D trajectory optimization in improving the overall rate compared to the traditional UAV two-dimensional(2D)trajectory optimization.2.Study on the transmission strategy of multiple UAVs-assisted multi-user twoway multi-hop relayTo address the characteristics of a large communication range or the need to cross obstacles such as complex terrain or buildings,where a single UAV cannot provide an effective relay service,we use a Decode Forwarding(DF)strategy to study a multiuser two-way multi-hop relay system assisted by multiple UAVs under a probabilistic Lo S channel model.The downlink rate of the two-way relay system is maximized by jointly optimizing the time slot scheduling,power allocation and UAV 3D trajectory.This section also uses SCA technique,relaxed variable replacement method and BCD method for iterative optimization to solve the nonconvex optimization problem,and this method can effectively solve the problem and converge to a suboptimal solution.