Mobile Satellite System

Until the late 1980s, most commercial mobile satellite systems used GEOs with bent pipe transponders. In the 1980s, extensive studies were undertaken in the UK and by the European Space Agency (ESA) to investigate the feasibility of deploying intelligent satellites in highly eccentric elliptical orbits for mobile communications in the European region. Studies also investigated the feasibility of deploying LEO satellites for mobile satellite systems. One such study concluded that non geostationary mobile satellite systems were feasible, offered advantages but required new technology which could be available by the end of the millennium (Richharia et al., 1990).

The trend in space segment architecture diverged at the beginning of 1990 when a number of commercial non geostationary satellite systems were proposed for hand held voice and data communication services. The architecture proposed LEOs or MEOs, as hand held services via geostationary satellites would require extremely complex spacecraft and suffer transmission delays. Their architecture varied widely in orbital choice, satellite complexity, transmission schemes, network routing and the market addressed.

By mid 1990 there was a re emergence of geostationary systems based on powerful satellites deploying several hundreds of spot beams. A comparison of various types of systems and architectures is given in Chapters 2, 8, 9 and in the Appendix.

Third generation mobile systems have been standardized on the premise that satellites will form an integral part of the network. International forums which have been instrumental in the process include the ITU forum called the International Mobile Telecommunication 2000 or IMT 2000 and the European Telecommunication Standardisation Institute (ETSI), a European forum where the third generation mobile systems are called universal mobile telecommunication systems or UMTS.

IMT 2000 proposes three data rates 144 kbps for car and train travellers, 384 kbps for people on foot and 2 Mbps for stationary users. The system therefore consists of pico cells covering buildings and small areas, micro cells of a few hundred metres covering streets, macro cells which cover areas of 1 10 km and overlay cells of 100 500 km made up of satellite spot beams. Components of each system will be connected through standard interfaces, as conceptualized in Figure 1. 12.

The services should be affordable and include:

*audio, video and data communication at high rates up to 2 Mbps;
*as far as possible, services available in the fixed network including the same range of multimedia services to the extent possible, considering the limitations of radio interface;
*value added services such as navigation, vehicle location, road traffic information, etc.;
*international roaming using a variety of satellite terminals.

Embedded in the concept are broadband mobile systems in the range 2 155 Mbps. The earliest applications of these broadband services are expected to be specialized. Examples may be movable broadcast studios, mobile studio cameras for live broadcasting, emergency services, etc.

While international standardization progresses, MSSs in the commercial world continue to evolve rapidly. Several systems were introduced in the late 1990s. The majority of them integrate with terrestrial systems to some extent, but without agreed standards integration with future universal systems can be rather limited. An interesting trend is the personal communications services being proposed by FSS in direct competition with the high throughput end of MSS.