Research On The Mobility Management And Handover In LEO Satellite Mobile Communication System

LEO satellite mobile communication system has become one of the important development orientations of the next-generation mobile communication technology for its low delay,low transmission impairment and high channel bandwidth.Compared with the traditional terrestrial mobile communication system,the spaceborne base station moves around the earth at high speed which results in the frequent handover of connections between different beams.At the same time,the neighboring cell list of the terminal varies quickly with the movement of satellite,which makes neighboring cell measurement a difficult issue.As a consequence,great changes have happened in the mobility management and handover technology in LEO satellite mobile communication system compared with that of terrestrial mobile communication system.Besides the terminal receiving signal strength which has been widely used in traditional handover algorithm as the judgement,factors like the neighboring cell covering interval and the inter-satellite routing hops should be considered as well.At the same time,the real time sub-satellite point is usually used as the input parameter in the computation of neighboring cell covering interval.This thesis mainly makes researches on the handover algorithm between beams in LEO satellite mobile communication system.The main content of the thesis can be divided into two parts: the prediction of the trajectory of sub-satellite point of the LEO satellite and inter-beam handover algorithm based on the forecast ephemeris.Firstly,the traditional methods of predicting the trajectory of the sub-satellite point of the LEO satellite and the cell reselection and handover criterion in terrestrial communication systems have been analyzed.The basic principles and the corresponding implementation process of the handover criterion in LEO satellite mobile communication system are introduced in details.Then,the thesis concentrates on the prevalent problems in current researches,which includes the ambiguity of the neighboring cell list computation and the incorrectness which lies in that the single satellite instead of the single beam was used as a cell to compute the cell covering interval.The main contents of the thesis are as follows.Due to the fact that the real time location of the sub-satellite point is used as the input parameter in the terminal positioning and mobility management in the LEO satellite mobile communication system,the thesis proposes a method of predicting the trajectory of the sub-satellite point in the LEO satellite mobile communication system.The method divides the prediction into two parts: orbit extrapolation and computation of the location of the sub-satellite point.Weighing the computation complexity and precision,SGP4 algorithm is selected to solve the problem of orbit extrapolation and the projection method is selected to solve the problem of the computation of the location of the sub-satellite point.The perturbation considered includes the gravitation between satellite and the earth,the non-spherical shape of the earth,the lunisolar gravitation,atmospheric drag and so on.According to the simulation results,the error in orbit extrapolation is confined to the 10 km level in the three dimensions of the geocentric equatorial coordinates.The peak value of the error in the computation of the location of the sub-satellite point does not exceed 5km,with the average value of 1km in 40 hours,which is much shorter than the radius of the common beam.The time consumed per computation is confined to the level of microseconds.The algorithm could meet the requirements of the application in the computation precision and complexity.In order to solve the prevalent problem of the ambiguity of the neighboring cell list computation and the incorrectness which lies in that the single satellite instead of the single beam was used as a cell to compute the cell covering interval,the thesis proposes the handover algorithm between beams based on the forecast ephemeris.In the algorithm,the terminal predicts the handover triggered time according to the forecast ephemeris and computes the neighboring cell list as a further step.If the receiving signal strength is below the neighboring-cell-measurement triggered threshold,the terminal makes measurements of the receiving signal strength of the serving cell and neighboring cell at the same time.If the receiving signal strength is below the handover triggered threshold,the terminal selected the target cell based on beam-visibility time,handover propagation delay and neighboring-cell signal strength.Simulation results demonstrates that the algorithm could lower the frequency of handover obviously and avoids the Ping-pong handover.The higher the latitude of the terminal is,the better effect the algorithm has.At the same time,the total propagation delay of handover falls sharply.