Multiuser Satellite Communications

Point-to-point communications occur between a single transmitter and a single receiver. In this chapter, we focus on multiuser communications, that occur between a network of fixed and/or mobile users and a common network center probably having access to a wireline network infrastructure. Multiuser communications in a satellite scenario usually refer to a star topology consisting of one or more gateways and a large number of small user terminals. Gateways communicate with the user terminals through the forward link and terminals may communicate with the Gateways through the return link, both links having a multiuser uplink channel and a multiuser downlink channel. Broadly speaking multiuser communications may refer either to a one-way communication as radio or TV broadcasting or to a two-way multiuser communication where each user is interested in a message specific to herself. In general, multiuser communications usually refer to the latter case. In order to support multiple users, a multiuser channel needs to be allocated to the different users. Given that an infinite number of possible allocations exist, multiuser channel capacity cannot be expressed by a single number but by a set of rates. The set of rates that can be supported simultaneously by the channel with an arbitrarily small error probability is called capacity region and its information theoretic limits are the subject of study of networking information theory. A multiuser system achieves a specific point in the capacity region depending on how the multiuser channel is shared by the users, which depends on the multiple access technique or MAC protocol used. MAC protocols are designed to coordinate multiuser transmissions as well as eventual retransmissions or resolution of collisions. In this chapter, we introduce a number of basic multiple access techniques and MAC protocols that are specific to the satellite scenario. This scenario poses some major constraints on a MAC protocol performance such as long propagation delay, remote control of on-board processing capabilities or power limitation that preclude the use of some protocols developed for a terrestrial scenario. A classification according to the most efficiently supported types of traffic is presented. The hybrid solution based on demand assignment is also discussed. We also present a brief introduction to some basic results of multiuser capacity discussing the effect of having channel side or state information. Finally, techniques able to mitigate or cancel the interference arising from spectrum sharing are also introduced and their applications to current satellite networks are presented.