INTRODUCTION
To make a satellite phone call today from a location that does not offer terrestrial wireline or wireless coverage requires the use of a large, costly terminal, and entails very high per minute charges. Further, the quality of service is relatively poor because of annoying echoes, large transmission delays, overtalk associated with satellite communications using geostationary satellites. The next generation of satellite communication systems will use advances in satellite systems, wireless technologies, and miniaturization, to provide global mobile satellite services that will make calls between any two locations on earth much easier, much more affordable and much more user friendly.
Even in the year 2000, the terrestrial cellular coverage is available to less than 60% of the world’s population and only about 15% of the earth’s total surface. More than 3 billion of the world’s population have no phone service. The waiting list of landline telephone service has over 50 million names with the average wait greater than 1.5 years. Rural areas, regions, that are sparsely populated in developed countries and large parts of the developing world are destined to be underserved or to remain out of reach of terrestrial mobile services altogether. Thus, in many parts of the world, the demand for communications mobility can be met effectively only through global mobile satellite services. Handheld satellite phones are therefore forecast as the emerging mobile communications frontier with growth that could parallel recent growth in cellular mobile industry. Regardless of how you look at the numbers, there is a significant amount of people without phone service throughout the world. Mobile Satellite communication services will solve the need of worldwide travelers and provide phone services to many areas of the world that currently do not have phone service. The emerging next generating mobile systems are generally referred as GMPCS, for Global Mobile Personal Communication by Satellites.
Until now Communication Satellites have operated using Geo-Stationery Orbits (GEO), lying above 36,000 kilometers above the earth’s surface. From this Orbit the satellite appears to be stationery (fixed) above a specific location from earth, thereby ensuring continuous, uninterrupted coverage to that location. The primary role of a geostationary communications satellite is to act as a wireless repeater station in space that operates in a broadcast mode and provides a microwave link between two remote locations on earth. The key components of a communication satellite include various transponders, transceivers, and antennas that are tuned to the allocated frequency channels. Although the Geostationary Satellites have a large footprint, so that the entire surface of the earth can be covered by few such satellites, their high altitude leads to very long roundtrip signal delays and resultant degradation in service quality.
There is a trend for mobile satellite system architectures aimed at the deployment of multi-satellite constellations in Non-Geostationary Earth Orbits (NGEOs).This allows the user terminals to be small size, low cost and having low power demand.To enhance the coverage and quality of service, Low Earth Orbiting (LEO) constellations are usually selected. To supports a wide range of services and to provide superior service quality comparable to that available from terrestrial wireless and wireline networks, constellations of satellites operating in Low Earth Orbits (LEO) or Medium Earth Orbits (MEO) are considered more suitable.
A number of various global mobile satellite communications systems have already been in development stages. With the first global mobile satellite services initiated in 1998. The four such systems that are in advanced stages of planning or early implementations are Iridium, Globalstar, ICO and Teledesic.
The era of satellite-based mobile communications systems started with the first MARISAT satellite which was launched into a geostationary orbit over the Pacific Ocean in 1976 to provide communications between ships and shore stations. The combination of high cost and unacceptably large equipment has kept mobile satellite communications (MSC) systems from appealing to the wider market of personal mobile communications. However, the progress made over the last ten years in digital voice processing, satellite technology, and component miniaturization has resulted in the viability of MSC systems in responding to the growing market in personal mobile
Communications.The system architectures of each system are presented along with a description of the satellite and user handset designs, the multi-access techniques employed, and an analysis of their respective cost structures.It is concluded that the technical feasibility of satellite-based mobile communications systems seems to be secure. It will be challenging however, for the vendors to actually develop and deploy these systems in a cost effective, timely, and reliable way that meets a continually evolving set of requirements driven by user expectations fueled by a rapidly changing technology base.
In order to guarantee the service quality and reliability for mobile satellite communication systems, we have to take into account outages due to obstruction of the line-of-sight path between a satellite and a mobile terminal as well as the signal fluctuation caused by interference from multipath radio waves. Thus, we need a good characterization for the satellite propagation channel. It is commonly accepted that satellite communications systems (in particular,low earth orbit LEO systems) are the de facto solution for providing the real personal communications services (PCS’s) to the users either stationary or on the move anywhere, anytime and in any format (voice, data,and multimedia).Satellite communication systems have provided international telecommunications services since the 1960’s. These systems were augmented in the 1970’s and 1980’s with regional satellite systems, national systems, and private network-based very small aperture terminals (VSAT’s). Throughout this period, systems have been based exclusively on satellites in geosynchronous orbit communicating with earth stations using high gain fixed antennas. As the systems have evolved, the original 30-m-diameter Intelsat earth stations have evolved into 1.2-m -band VSAT’s for business and home TV usage, but the basic system architecture explaining a geosynchronous spacecraft has not changed during this period. With the launch of the first Iridium spacecraft in 1997 and 1998, a significant new architecture has been introduced into the field of satellite communications. These systems are based upon the use of LEO and medium earth orbiting (MEO) systems. hese LEO and MEO systems have several advantages over geosynchronous systems. The most significant advantages are:
1) The reduction in range provides a large decrease in path loss resulting in much small receiving antennas and
2) The reduction in range provides a significant reduction in propagation delay making voice conversation more pleasing to the user and increasing the throughput of most data communication protocols. These systems can and will serve mobile and portable users
With small near omni antennas.
However, the use of the small antennas as well as the motion of the transmitter and the receiver introduces the possibility of multipath and path blockage into the link budget of these satellite systems. Moreover, the propagation channel will be time varying due to different shadowing and scattering phenomena, so traditional channel models may
not work well.This is concerned with the statistical modeling of the propagation characteristics of LEO and MEO systems. Since in these systems, satellites and mobile users are all allowed to move during communication sessions, the channel characteristics
will be different from the geostationary systems (GEO’s). Due to the movement of receivers or transmitters, the received signals may fluctuate very rapidly from time to time. This fluctuation results from the combining effects of random multipath signals and obstruction of the line-of-sight path, which induces various fading phenomena. The communication quality of service(QoS) parameters such as the word-error rate will be affected in great deal in such communication environment. For effective mobile satellite communications system design, we must quantitatively know the propagation characteristics such as signal fading due to reflection; shadowing from trees, buildings,utility poles, and terrain; Doppler effects due to movement of mobile terminals, mobile satellites, or the communication effects; and other effects such as the rainfall. Such characteristics can be studied by the statistical distribution of the received
signal envelope or received power in mobile communication systems.
