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Central features of future 4G mobile communication systems are high-speed data transmission (up to 1 Gb/s) and interactive multimedia services. For effective delivery of these services, the network must satisfy some stringent QoS metrics, defined typically in terms of maximum delay and/or minimum throughput. Mobile satellite systems will be fully integrated with the terrestrial cellular systems to provide ubiquitous global coverage to diverse users. The challenges for future broadband satellite systems, therefore, lie in the proper deployment of state-of-the-art satellite technologies to ensure seamless integration of the satellite networks into the cellular systems and its QoS frameworks, while achieving, as far as possible, efficient use of satellite link resources. The paper presents an overview of future high-speed satellite mobile communication systems, the technologies deployed or planned for deployment, and the challenges. Focusing in particular on nonlinear downlink channel behavior, shadowing and multipath fading, various physical channel models for characterizing the mobile satellite systems are presented. The most prominent technologies used in the physical layer, such as coding and modulation schemes, multiple-access techniques, diversity combining, etc., are then discussed in the context of satellite systems. High-speed and QoS-specific technologies, such as onboard processing and switching, mobility and resource management, IP routing and cross-layer designs, employed in the satellite systems are also discussed.