| Low Earth Orbit (LEO) satellite mobile communication system is a highlight in scientific research. Commonly, LEO system is consisted of space segment,user segment and ground segment. Space segment includes LEO satellites, which provide ground users and gateways with communication links; User segment includes a variety of mobile/fixed terminals; ground segment includes gateways and a number of operators. This dissertation focuses on simulation and analysis of access network segment which is between user segment and space segment. It is divides into 7 chapters, and the session from Chapter 2 and Chapter 7 is the most important part, which gives a detailed description of author's research work during the period of postgraduate. Chapter 1 is the preface, which introduces the development of satellite communication and the categories of satellite systems, and presents two typical LEO systems-Globalstar and Iridium, which are interpreted from the aspects of system construction,constellation parameters and main technologies. And the last part is the arrangement of whole dissertation. Simulation analysis and research on the key technologies of LEO satellite communication system is the main work of author. In Chapter 2, firstly, author gives a brief introduction of OPNET-the software, and lists its characteristics in satellite communication system simulating application; then, introduces modeling and simulating means according to project, and gives a brief overview of simulation responsibilities; presents system communication protocols and user location management; subsequently, proposes simplified signaling in a single section, a result of collaboration, which is merely regarded as system design reference and not discussed again; the last one is the procedure of originating and ending calls. Chapter 3 makes research on feeder link handover scheme. Since satellites of our interested LEO system have no inter-satellite links or capability of processing data, all of signals need to be delivered to ground for processing, and there is a demand to build up enormous ground gateways in order to support global real-time communication. Compared to other satellite communication system, user terminals will experience a lot of handovers between gateways. When it is related to gateway handover, the update of user location and route management needs a great deal of signaling exchange and ground route rebuild, which may make great pressure on network design and management design. This chapter discusses two feeder link handover schemes and two user link handover principles. According to Chinese situation, builds non-uniform distributed national traffic model of China, in addition, makes contribution to the choice principles of gateway's ground locations, providing reference. Under this background, author proposes four criteria parameter to measure system performance. The simulation results show, feeder link handover affects users'handover among gateways more than user link handover, and judge scheme with better performance, providing valuable reference to system design. Chapter 4 researches the means of user judging its own spotbeam in service satellite. Multiple spotbeam antenna is one of importance technologies in satellite mobile communication system, which has a direct effect on network structure,system investment ,capacity and so on. In simulation user estimating its spotbeam needs mathematic deduce with help of satellite coverage and spotbeam figure. This chapter makes use of satellite orbit space geometry to deduce mathematic model and formulas of judgment. At last, summarizes procedure flow chart of simulation algorithm, and gets simulation results approximately to real system parameters, consequently improves it. Meanwhile, provide simulation backbone and theoretical basement. The research emphasis in Chapter 5 is simulation of access and handover control schemes. Much similar simulation in the past cares only about inter-satellite handover or inter-spotbeam handover. Additionally, most of it is deduced through mathematic models, which is not similar to real situation. Based on former chapter, takes both of handovers into account, and discusses satellite handover principles: minimum distance principle and maximum time principle, and channel reservation scheme between spotbeams: random and fixed guard channel scheme. Lastly, makes analysis of handover performance by simulating. Chapter 6 researches Aloha protocols'application in system. Makes simulation analysis of delay performance of pure Aloha or slotted Aloha in certain cases equipped with... |
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