| Flexibility and controllability have enabled the unmanned aerial vehicle(UAV)to play a significant part in future wireless communications,such as emergency communications and internet of things(IoT)data forwarding.Compared with the fact that the ground communication link blocked by buildings suffers greater signal attenuation,the UAV equipped with transceiver antennas can be used as the aerial wireless access point(AP)to transmit wireless information through the air-to-ground(AtG)link,which not only enhances the reconfiguration of network node deployment,but also improves the wireless channel capacity through the advantage of the AtG line-of-sight(LoS)transmission.From the perspective of large-scale fading,the LoS link helps to reduce the path loss of wireless signal propagation;from the perspective of small-scale fading,the LoS main component causes the AtG link to produce the Rician fading channel.The above shows that the AtG communication link is different from the ground communication link in terms of average channel gain and instantaneous channel gain.In order to comprehensively explore how large-scale fading and small-scale fading affect the quality of the AtG channel,this thesis aims to evaluate the reliability of communication systems after the introduction of UAVs from the perspective of the AtG link outage performance.Specifically,this thesis studies three basic scenarios in which UAVs are used for relaying communications,namely UAV static relaying,UAV mobile relaying with predefined trajectory and UAV mobile relaying with three-dimensional(3D)degree of freedom.Combined with full-duplex(FD)and non-orthogonal transmission technologies,three algorithms with joint UAV static deployment and resource allocation,dynamic resource allocation under the time varying channel as well as joint UAV 3D trajectory and dynamic resource allocation are proposed,improving the reliability of UAV relaying systems by reducing the outage probability of the AtG link.The main contents and innovations of this thesis are as follows:In view of the problem that the UAV deployment position affects the LoS propagation quality of the AtG link,a strategy for deployment and resource allocation of UAV relay in an emergency communication scenario is proposed,aiming at achieving reliable AtG propagation through a high LoS link probability.Considering the LoS probability-based AtG fading model and the FD relaying mode,the joint optimization of UAV deployment altitude,power control and bandwidth allocation is carried out to minimize the maximum outage probability of multiple relaying links.The formulated original problem is firstly decoupled into the altitude optimization subproblem and the resource optimization subproblem that can be handled using the successive convex approximation(SCA)method.Then,an iterative algorithm based on the block coordinate descent(BCD)is proposed to realize the global optimization.Simulation results reveal the performance gain of the proposed algorithm under three urban environments,and show that the reasonable increase of UAV altitude can reduce the link outage probability by taking the LoS propagation advantage,and the system reliability can be further improved by reducing the self-interference(SI)of FD relaying through power control.In view of the problem that the UAV mobility causes time-varying channel characteristic,a dynamic resource allocation strategy when the UAV performs emergency relaying communications under a preset trajectory is proposed,aiming at improving the reliability of AtG transmission by matching time-varying channel conditions.Introducing the non-orthogonal multiple access(NOMA)technique to downlinks,the bandwidth and power allocation optimization problem based on the instantaneous location of the UAV is constructed under the successive interference cancellation(SIC)constraints,with the objective of minimizing the maximum outage probability of multiple relaying links.The equivalent substitution relations of variables are firstly designed to change the original problem into a more tractable form,based on which a SCA iteration algorithm is proposed to solved the reformulated problem.Finally,the feasible solution to the original problem is restored according to the designed equivalent substitution relations.Simulation results analyze how bandwidth and power resources match the time-varying channel characteristics,and show that non-orthogonal transmission can improve the system reliability by the SIC.In view of the problem that the UAV flying trajectory affects the LoS main component of the AtG link,a 3D trajectory and dynamic resource allocation strategy when the UAV performs the IoT data forwarding task is proposed,aiming at realizing reliable data forwarding by perceiving the Rician channel characteristics.Considering the UAV mobility control and power allocation constraints,the formulated problem minimizes the average outage probability along the 3D UAV trajectory under the time-varying Rician fading channel.The 3D trajectory optimization subproblem and the power allocation optimization subproblem are firstly considered after decoupling the original problem.The 3D trajectory optimization subproblem is solved by the SCA under relaxation constraints,and the power allocation optimization subproblem is solved by the standard convex optimization.Finally,the proposed BCD algorithm alternately optimizes the 3D trajectory and power to reduce the system average outage probability.In order to reveal the reliability gain obtained by designing the 3D UAV trajectory under the time-varying Rician fading channel,a case where the UAV flies along the lowest altitude is also considered,jointly optimizing the UAV two-dimensional(2D)flying trajectory and power allocation to minimize the system average outage probability.Simulation results show that compared with the UAV flying along the 2D trajectory,the UAV flying along the 3D trajectory under the time-varying Rician fading channel can improve the reliability of the mobile relaying system. |
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