Abstract
Mobile Satellite Communication Systems are the best, and probably the only solution to a full 'global communication coverage'. Despite the intense scrutiny and inherent difficulties in gaining acceptance by the telecommunications market and the recent switch back towards GEO satellites, mobile satellite systems in non-geostationary LEO (500-2000km) and MEO (10000-12000km) constellations still remain an attractive solution in an integrated satellite/terrestrial scenario as they offer lower delays and lower power requirements than GEO satellites (35800km). Since Quality of Service (QoS) and service availability are major subscriber concerns, the investigation of factors which influence them are of central importance to the design of such systems. In non-GEO mobile satellite systems both the QoS and the service availability are very much dependant on the changing dynamics of the constellation and on the time varying nature of the propagation environment. Motivated by the above issues, we present, analyse and evaluate the coverage and availability of first generation constellation proposals in terms of LOS and bit-error-rate (QoS) requirements. Handover management is also identified as an important issue affecting the QoS and therefore handover strategies and mechanisms for various satellite constellations are presented and analysed. Based on the statistical but predefined nature of the constellation dynamics as well as the influence of the propagation channel and its dependence on the constellation design we propose two new channel adaptive handover algorithms in an effort to reduce the handover signalling whilst maximising at the same time the QoS as perceived by the user in terms of reduced call dropping rate for a typical circuit mode telephone call. Finally, as the current trend of the telecommunication services is towards the provision of packet oriented services, we focus the final part of this study on the performance investigation, in terms of throughput versus delay characteristics, on the provision of GPRS-like services over mobile satellite systems and compare GEO and non-GEO delivery. It is concluded, based on the MAC protocol proposed in the last chapter, that depending on the type of traffic and on the network load, the LEO approach doesn't always give superior performance in terms of delay characteristics to that of a MEO. The results and findings presented in this thesis can be used as a reference for optimising and designing future mobile satellite systems. Key words: Availability, QoS, handover, MAC protocols, satellite channel.