Research On Handover Technology In High Altitude Platform Communication System

The High Altitude Platform Station(HAPS)is deployed in the stratosphere at the height of 17-22Km,and its height is between the terrestrial communication system and the satellite communication system.HAPS can be used as high-altitude base station or relay node.It has the characteristics of wide coverage,flexible deployment,low delay,high capacity and good channel conditions.So,it is widely used in the situation of insufficient capability of ground communication system or emergency communication.HAPS generally uses multi-beam antennas to achieve multi-cell coverage on the ground,and frequency reuse can be used between different cells to improve the capacity of HAPS system.The HAPS belongs to non-stationary base station,which is affected by atmospheric turbulence.The displacement movement changes the position and attitude of HAPS,the coverage of the terrestrial cells changed accordingly,resulting in the handover problem of the users at the edge of the cell.The main research work of this thesis includes the following two points:For the displacement movement characteristics of HAPS,the influence of different movement modes on the handover probability should be analyzed.In the existing analysis of handover probability in HAPS communication system,the equal beam width and equal coverage area model are often used,which can not reflect the influence of path loss change on cell coverage in HAPS unstable movement.Therefore,we establish a cell coverage model considering antenna directional gain and path loss,and give the mathematical expression of coverage.Based on the proposed cell coverage model,the handover probability of ground users in the process of horizontal,rotating,vertical and inclined movement of HAPS is analyzed.The analysis results show that the proposed ground model considering antenna directional gain and path loss is more in line with the actual situation and is suitable for guiding practical work.In the four kinds of unstable movement of HAPS,the influence of swing movement and rotation movement on the handover probability of the system is much greater than that of horizontal movement and vertical movement.In order to solve the problem that the signal strength changes caused by the unstable movement of the HAPS,which leads to the handover of users on the ground,a HAPS handover compensation and handover decision strategy is proposed.Firstly,according to the analysis conclusion of handover probability,an improved antenna center direction adjustment scheme is proposed,which only adjusts the antenna direction offset caused by rotating movement and swing movement.The mechanical and computational complexity of the platform can be greatly reduced,and the handover probability and link failure rate can also be reduced in good degree.Subsequently,we propose a handover decision algorithm based on Received Signal Strength(RSS)prediction assistant decision.Higher prediction accuracy and algorithm performance are obtained by performing second-order difference operations on the RSS sequence and Least Mean Squares(LMS)algorithm fitting.The simulation results show that the proposed algorithm has better performance in the same type of handover decision algorithm.The handover performance analysis method and conclusion proposed in this paper have strong reference significance for the research and application of high-altitude platform communication system.The proposed handover compensation and handover decision strategy has good application value in the scene of unstable movement of HAPS.