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.

Satellite resource management efficiency has been identified as one of the key factors in the commercial success of mobile satellite systems, since optimisation of all link budget elements is crucially important in order to make the most out of the satellite limited resources (bandwidth, power) which in turn have a direct impact on the cost of the system. The compensation techniques used in order to overcome the fading effects experienced in the link are generally applied by considering the worst-case channel conditions, resulting in inefficient utilisation of the transmission power as well as frequency spectrum. There is no dynamic control adaption used in current mobile satellite systems except simple power control. In addition, a single scheme is neither capable of providing an optimum solution for fade mitigation nor closing the link budget at all times. Therefore, it is necessary to use more than one technique at a time which offers the best solution in terms of spectrum and power efficiency. The efficiency of conventional systems, therefore, can be improved if it has the ability to match the effective user bit rate to the channel conditions by using a hybrid scheme. Hence, the aim of our work is to develop a physical layer by using a hybrid scheme, which results in higher throughput under favourable channel conditions. This technique also introduces a reduction of the data rate during bad channel conditions without the need to increase the transmitted power significantly. The novelty of this research work is centred on the switching mechanism used for such adaptations. For the first time, we propose an adaptive system based on the Rice factor variation. The suitability of this parameter for the proposed adaptive system is investigated first by considering the real time variation in the environment. We are proposing a feed back type of system in which the receiver estimates the Rice factor and sends it to the transmitter. Upon receiving this information, the transmitter selects the optimum modulation and coding scheme for the transmission in order to improve spectral and power efficiency of the system. In order to develop an adaptive physical layer, the main issues related to mobile satellite systems should be identified. Therefore, the key differences between the terrestrial and satellite mobile communication systems are presented at the beginning. A brief description about T/S-UMTS and the air interfaces proposed for standardisation of S-UMTS are presented and compared in the following chapter. Subsequently, a suitable baseline model was chosen and the simulation aspects are presented. The following chapter presents some of the parameters that have to be estimated in order to develop an adaptive physical layer. The performance evaluation of adaptive modulation and coding is presented in the last chapter. Key words: S-UMTS, SW-CDMA, Adaptive modulation and coding. Rate matching, SNR estimation, modulation detection. Rice factor estimation. Histogram comparison. Re-encoding.