Handover Performance Analysis Based On Stochastic Geometry In 3D UAV Networks

Thanks to the high-probability line-of-sight(LoS)link and controllable mobility of unmanned aerial vehicle(UAV),UAV acting as aerial base stations(BSs)to serve the ground users becomes an important solution for coverage enhancement and capacity enhancement in future networks.However,since UAV has high dynamic mobility and the link transform between LoS and NLoS,the time-varying channel leads to high probability of unnecessary handovers,which makes the handover performance in UAV network attract much attention.UAV distributed in 3D space makes cell boundary affected by UAV altitude,which is the essential different from terrestrial network.Using stochastic geometry,by modeling the 3D distribution of UAV networks,this thesis analysis the impact of UAV altitude on handover probability in multi-tier UAV networks,and charecterzes the handover failure and ping-pong effect with handover parameters by considering channel fading,so that the deployment altitude and handover parameters for UAV networks can be guided theoretically.The contributions of this thesis is divided into the following two parts:The handover probability for UAV networks in 3D spaec has not been studied in the existing works.By modeling UAV network as an independent K-tier Poisson point process,this thesis unifies handover standard for UAVs with different altitude based on the equivalent model,and derived the compact expressions of overall handover probability.Specifically,equivalent horizontal distance is introduced to eliminate the information loss brought by projection process,which is a nonlinear transform from Euclidean distance.Hence,the transmit power and UAV altitude lead to the unknown relation between equivalent horizontal distances of UAVs in different tiers when calculating handover probability.The results show that there is an appropriate deployment range of UAV altitude and density,which can reduce the handover probability with a tolerable user association probability.The channel fading in UAV networks leads to handover failure and ping-pong effect,which has not been analyzed in literature.This thesis considers a path-loss-plus-fading model for UAV networks by modeling the channel fading as Nakagami-m distribution,and quantizes the impact of handover parameters on handover performance.Taking handover parameters(time-to-trigger and offset)into account,a discrete time Markov chain is utilized where the user's states along the trajectory is discretized into multiple handover states by coherence time,in order to analytically characterize the transition probability among the handover states.Based on the discretization,handover failure and ping-pong probabilities are derived by calculation the handover state probabilities during the time-to-trigger duration.The results give insights into the tradeoff between HOF and PP probabilities under different HO parameters.In view of the lack of modeling and analysis of the mobility model for UAV serving as base station in the existing literature,in this thesis,the random waypoint mobility model is utilized to model the mobility of the UAVs,and the theoretical expressions of the coverage probability is derived,where the distribution of the ground users are model as cluster process with a serving UAV moving above them and the range of the UAV movement is determined by the user cluster radius and the movement factor.Specifically,the received signal strength of the user is determined by the statistical distribution of the distance from the corresponding serving UAV to the ground user,and the SIR distribution of the user is derived by calculating the Laplace transform of the cumulative interference of the network.The numerical results show that there is an optimal UAV altitude(about 10 m)and an optimal UAV movement radius factor(about 1.2),which makes the network coverage reach the maximum value.