Transport issues in S-UMTS

In principle, diversity could be used in S-UMTS to minimize the fading of the received signal. In a LEO system, when the user has two or more satellites in visibility, diversity can be used for constructive power combining and the reconstruction of the signal, or for the selection of the best link. In ICO constellations, it may be practical to exploit diversity by setting up 2 duplex links routed through different satellites where possible. Macrodiversity control can be of two types: soft and hard. The hard combining point can be located in the LE since the synchronization requirements and the network resources for hard combining are less critical than that in the soft combining. Soft combining can be located at the CSS level. Effective and reasonably straightforward soft diversity can be achieved by CDMA systems. It is possible that both types of macrodiversity control co-exist in the system. In the following sections a general description of the macrodiversity model is given for the S-UMTS according to different payload technologies. The proposed models do not refer to any specific systems or architectures. Finally, the protocol stack for each macrodiversity model is given.