Satellite Access

In an MSS environment, thousands of users share satellite resources and therefore satellite access efficiency is paramount. Demand assigned (DA) single channel per carrier (SCPQ frequency or TDMA schemes, where a pool of channels is shared by all users on a per call or packet basis, offer an effective solution for circuit or packet mode communication. The channel pool can be managed by either a central or distributed architecture. In a central architecture, a pool is managed centrally, whereas in a distributed architecture, separate pools are assigned to each participating fixed station for self management.

A CDMA scheme offers an advantage in terms of interference and multipath mitigation, soft handover and capacity. A number of modern systems have preferred CDMA.

Data traffic tends to exhibit a variety of characteristics ranging from sporadic bursts to continuous streams and therefore accessing schemes are matched specifically to traffic characteristics. Common accessing schemes used for data communications include Aloha, slotted Aloha, Reservation Time Division Multiple Access, etc. (see Section 4.4).

In recent years, with the proliferation of data applications, a scheme known as asynchronous transfer mode (ATM), which allows a flexible combination of various types of data at the link layer, has been introduced. As ATM was originally proposed for fixed terrestrial systems, considerable effort was expended in adapting the scheme for wireless access including satellite systems. Internet protocol (IP) which operates at the application layer is now widely used. Several future broadband systems intend to use IP over ATM in conjunction with TDMA or CDMA as the transport mechanism