Research And Implementation Of Handover For LEO Satellite Mobile Communication

Among the satellite mobile communication systems,the Low Earth Orbit(LEO)mobile satellite systems have shown great prospect due to their global coverage,low propagation delay,large communication capacity and low power consumption.Therefore,they will play an important role in the future personal communication systems.However,users will frequently handover between satellites during their communication because of high-speed movement of LEO satellite relative to the earth.Thus,the research of handover technology is of great significance.This paper focuses on the research of handover procedure and handover algorithm in LEO satellite communication systems.Based on LTE handover technology and srs LTE that is a free and open-source LTE software suite developed by SRS,this paper deeply studies the handover procedure of S1 interface.Considering the characteristics of LEO satellite communication systems,some optimizations have been made,including the modification of measurement configuration transmission timing and measurement report trigger conditions.As a result,the number of signaling transmissions between the satellite and the ground can be reduced.Then we use C/C++ programming language to complete the software design of handover procedure,including three parts: UE,e NB and EPC.Then the semi-physical simulation platform of LEO satellite communication systems is built and handover function is tested and verified.The test results show that handover procedure is executed correctly and the service can be on-going.How to choose the target satellite is a key problem that needs to be solved in the handover decision phase of handover procedure.And it has a direct impact on the performance of LEO satellite communication systems.Therefore,this paper conducts an in-depth study on the inter-satellite handover strategy.Considering the probability that the user is covered by multiple satellites is relatively high and global traffic distribution is nonuniform for LEO satellite communication systems,the paper proposes a shortest path handover algorithm based on load balancing.The algorithm generates a directed graph by using the user's service satellite list information,weights the directed edges in the directed graph based on the number of handover hops and idle channels of target satellite.Then Dijkstra algorithm is used to find shortest path of the directed graph,eventually the handover path of the user in the future is obtained.The algorithm also proposes the idea of channel reservation to reserve channel resources in advance in each satellite which belongs to the user's handover path.In order to improve the utilization of the channel,the reserved channel is only locked during the period in which the user actually uses.Finally,the simulation platform of LEO satellite communication systems for handover scenes is built by using OPNET software.The performance of the inter-satellite handover algorithm is simulated and evaluated.The simulation results show that the algorithm can effectively balance the load between satellites and reduces the probability of handover failure.