Research On Uav-aided Wireless Coverage Enhancement

Unmanned aerial vehicle(UAV)enabled wireless communications is especially suitable for emergent communications or limited endurance communications scenarios due to their fast response capabilities,flexible mobility,low cost,on-demand deployment and the ease of establishing strong line-of-sight communications links.It is therefore of great importance to quickly deploy UAV-enabled base station(BS)/relay/access point for developing and utilizing the adjacent space resources and building an air-ground integrated communication network,e.g.,by improving the performance over the conventional terrestrial communications systems,extending the coverage area of ground BS and enhancing the connectivity of ground users.In this thesis,by fully exploiting the unique characteristics of fixed-wing and rotary-wing UAVs,we focus on several scenarios of UAV-aided coverage enhancement,such as UAV-enabled multicasting in a large area,energy-efficient multi-antenna fixed-wing UAV-enabled half-duplex mobile relay,optimal altitude analysis for multi-antenna rotary-wing UAV-enabled half-duplex static relay with channel estimation error,and joint optimization of beamforming and power allocation for multi-antenna fixed-wing UAV-enabled full-duplex mobile relay.In these scenarios,the UAV trajectory,resource allocation and multi-antenna beamforming can be jointly designed to improve the coverage performance.First,we respectively investigate the path planning problem for fixed-wing UAV-enabled multicasting in the rectangular area and rotary-wing UAV-enabled multicasting in the convex polygonal area,by assuming that the location information of all the ground users is unavailable at the UAV.Then the mission completion time and the UAV's energy consumption are respectively minimized by jointly optimizing the flight altitude,the antenna beamwidth and the flight speed of UAV.For the fixed-wing UAV-enabled multicasting in a rectangular area,we propose a fly-and-communicate protocol by exploiting the characteristic that a fixed-wing UAV must maintain a forward motion to remain aloft,where the UAV flies following a zigzag trajectory and disseminates some common information to all the ground users in its covered area.For the rotary-wing UAV-enabled multicasting in the convex polygonal area,we adopt a fly-hover-andcommunicate protocol by exploiting the characteristic that a rotary-wing UAV can hover at fixed locations,and propose a practical and easy-to-implement path planning algorithm to completely cover the convex polygonal area.For fixed-wing UAV-enabled multicasting in a rectangular area,numerical results show that the optimal flight altitude for minimizing the completion time is always the maximum allowable flight altitude,while the optimal altitude for minimizing the UAV's energy consumption can be obtained by numerical calculation.In contrast,for rotarywing UAV-enabled multicasting in the convex polygonal area,the optimal flight altitude is the maximum allowable flight altitude for minimizing both the completion time and UAV's energy consumption.Such insights provide key guidelines for practical design and operation of UAV-enabled multicasting systems.Then,we study the maximization of energy efficiency for multi-antenna fixed-wing UAVenabled decode-and-forward(DF)half-duplex mobile relay,subject to the sum transmit power constraint over both the source node and relay node.We maximize the energy efficiency through joint optimization of beamforming,statistical channel statement information(CSI)based power allocation,circular radius and flight speed.From the derived ergodic energy efficiency over a circular period,we observe that the denominator of energy efficiency does not depend on beamforming and power allocation,while the numerator only depends on the circle radius and flight speed.Therefore,the original problem can be decomposed into two sub-problems: the beamforming and power allocation optimization sub-problem,and the circular radius and flight speed optimization sub-problem.For the former sub-problem,the optimal transmitting beamforming vector is that for maximum ratio transmission(MRT)and the optimal receiving beamforming vector is that for maximum ratio combining(MRC),and then based on the obtained beamforming vectors,the power allocation results of source node and UAV relay node are obtained by leveraging the statistical CSI.For the latter sub-problem,we first derive the semi-closed-form expression of energy efficiency,and then obtain the optimal flight speed by taking the derivative of the denominator of energy efficiency and setting it equal to zero,as the flight speed only affects the denominator of energy efficiency.In addition,the optimal circular radius can be effectively obtained by numerical computation.Numerical results confirm that the proposed joint optimization can significantly enhance energy efficiency,where the optimal flight speed increases nonlinearly with the number of UAV antennas,and the optimal energy efficiency increases approximate linearly with the number of UAV antennas.Next,we analyze the optimal hover altitude for multi-antenna rotary-wing UAV-enabled DF half-duplex static relay under perfect and imperfect channel estimation,which respectively aim for minimizing the outage probability,minimizing the transmit power,and maximizing the coverage radius.We propose a general framework for multi-antenna air-to-ground channel,and derive the end-to-end outage probability based on the proposed channel framework.In order to analyze the optimal hovering altitude of the above three strategies,we propose a highly accurate approximation of the inverse function of the generalized Marcum Q function on its second parameter.With the help of the approximated inverse function,we study the effect of perfect and imperfect channel estimation on the optimal hovering altitude for the above three strategies respectively.It can be shown that the optimal altitude can be obtained numerically by solving an implicit function with respect to the elevation angle.For all three strategies,numerical results show that the optimal flight altitude increases with the number of UAV antennas and channel estimation error.Especially,for the coverage radius maximization strategy,as the antenna number increases,the optimal elevation angle tends to a constant for perfect and imperfect channel estimation.These insights greatly reduce the complexity for practical deployment of UAV-enabled static relay.Finally,we study the joint design of beamforming and instantaneous CSI based power allocation to improve the end-to-end performance for fixed-wing multi-antenna UAV-enabled fullduplex DF mobile relay,under both the individual and the sum power constraints over the source node and relay node.We propose a block-coordinate descent algorithm by iteratively updating the beamforming and power allocation alternately,where the optimal beamforming can be obtained via semi-definite relaxation technique together with bisection search.To reduce computational complexity of the optimal beamforming scheme,we propose two sub-optimal linear beamforming schemes.Furthermore,a closed form cumulative distribution function and outage probability for linear beamforming schemes with both optimal and uniform power allocation are respectively derived.And then,the high SNR approximations for outage probability expressions are derived.Numerical results show that the joint optimization of beamforming and power allocation effectively improve the end-to-end performance,e.g.,throughput and outage probability.Finally,we demonstrate that a fixed-wing UAV-enabled mobile relay adopting the circular trajectory is much more energy-efficient than the rotary-wing UAV-enabled static relay.