Global Mobile Satelite System

To make a satellite phone call today from a location that does not offer terrestrial wire line 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, over talk associated with satellite communications using geostationary satellites.

There is a trend for mobile satellite system architectures aimed at the deployment of multi-satellite constellations in Non-Geostationary Earth Orbits. This allows the user terminals to be small size, low cost and having low power demand. In present and next generation satellite systems, CDMA has been proposed as the multiple access technique for a number of mobile satellite communication systems. To enhance the coverage and quality of service, Low Earth Orbiting (LEO) constellations are usually selected. Here, we analyze the performance of the downlink of a LEO satellite channel. The provision of such a service requires that the user have sufficient link quality for the duration of service. To have sufficient link quality, the user must have an adequate power to overcome the path loss and other physical impairments to provide acceptable communication and improve the performance of the system.

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. 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).

The satellite segment is a network of GEO or LEO satellites arranged in orbital planes (i.e. different parts of the sky) in such a way that they have a communications link with end-user equipment, ground gateways and other satellites. The gateway connects the satellites to the local telephone network. The gateway also transmits signals to the satellites and receives transmission from the satellites. Due to the high mobility of low earth orbit (LEO) satellites, there is a significant number of handover attempts in a LEO-based mobile satellite communication system, causing a high handover failure rate. This paper proposes to extend the period of which a handover request is valid, and thus rendering higher probability of successful handover.

Satellite communication service can be provided by geostationary earth orbit (GEO), medium earth orbit (MEO) or low earth orbit (LEO) satellites. Because of its much shorter distance from earth, lower power requirement and thus smaller mobile terminal (MT) size, LEO satellite system is a preferable choice. Differences between satellite and terrestrial systems exist in spite of common objectives for high quality services and excellent spectrum efficiency. Some differences arise because:- user costs are closely related to satellite transmit power the satellite propagation channel is highly predictable satellite paths introduce significant propagation delays and Doppler shifts frequency co-ordination has to be on a global basis frequency re-use options are more limited, hence bandwidth is a tight constraint satellite beam shaping and sizing opportunities are limited.

The most significant attribute of any satellite communication system is the wide area coverage that can be provided with very high guarantees of availability and consistency of service. Satellite communication systems are designed to provide voice, data, fax, paging, video conferencing and internet services to users worldwide. Through satellite based systems, users will be able to make a phone call from an African safari or while sailing around the world. No matter where users are, they will be able to communicate with clients, customers, associates, friends, and family anywhere in the world. In addition, satellite communications will allow countries to provide phone services without large investments in landline or wireless systems. Satellite communications will be one of the fastest growing areas within the communications industry.

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