Issues in CDMA applications for mobile LEO satellite communications

An overview of the main problems in CDMA applications for mobile LEO satellite communications are presented. The discussion starts with channel modelling including the latest theoretical and measurement results. These results should provide the basis for the study of multipath and multisatellite diversity reception. A range of values for the diversity combining gain is given. A review of several projects of CDMA based mobile LEO satellite communication system is given to demonstrate the ideas in practice. The contrast of the differences between terrestrial and satellite mobile communications based on CDMA will be presented with special emphasis on intercell versus interspot interference in land and multiple beam interference in satellite systems, respectively. Specific synchronisation issues due to high Doppler will be also addressed. This will include frequency compensation techniques based on pilot carrier. The reliability of Doppler estimation for different elevation angles as a function of the ratio of the pilot carrier to the overall signal power will be given. A review of the existing and new code acquisition techniques for very low signal to noise ratios will be also presented. Capacity issues and possible improvements by using multi-user detectors will be discussed in a realistic environment. The paper will conclude with several additional topics such as, CDMA spectrum sharing, design imperfections and future trends including smart receivers.