Abstract
Future mobile broadband communication systems aim to provide a wide range of multimedia services (telephony, video conferencing, audio and video broadcasting, etc.) to a large number of users. In order to achieve a global coverage and reach remote users, satellite is foreseen to be a key component in the development of such networks. When compared to first-generation mobile satellite communication systems, these future satellite networks will need to provide significantly higher data rates whilst achieving very low bit error probability values. In order to keep the mobile user terminal as well as on-board processing complexity as low as possible, it is important to reach the required quality of service with very low values of the signal-to-noise ratio. Hence, particular attention must be paid to the design of the air interface of these future satellite systems. The use of CDMA presents a number of advantages for mobile communication systems, and hence CDMA technology has recently received a lot of attention for both second generation and future terrestrial multimedia systems. However, less effort has been put into the design of CDMA solutions for satellite communications. Moreover, good air interface designs for terrestrial systems do not necessarily translate into efficient solutions for the satellite environment. Hence, different techniques aimed at improving the performance of CDMA satellite systems are proposed in this thesis. First, the performance of S-CDMA, which reduces the level of MAI by co-ordinating the transmission of the different users, is analysed in satellite fading channels. Comparisons with the more conventional A-CDMA are provided and the impact on the transmission quality of imperfect power control, synchronisation errors and pulse shaping is assessed. In order to meet the very tight requirements set for multimedia services, FEC coding is required. Hence, the performance of S-CDMA in fading channels is presented for different channel coding techniques. The performance of the different schemes is characterised with both power and spectral efficiency and comparisons are made. In order to reduce the performance degradation due to the MAI, it is also possible to use multiuser detection algorithms at the receiver. In this thesis, the performance of MUD algorithms using an ANN architecture is analysed in satellite fading channels. Different training strategies and algorithms are considered and the performance of the ANN receivers is compared to that of the conventional correlator as well as other MUD algorithms. Finally, a number of different multi-rate spreading techniques are proposed and their respective complexity is discussed and compared. The power fluctuations caused by these different multi-rate spreading techniques are analysed in order to evaluate their performance in the satellite non-linear channel.