|Sunday, 12 November 2006|
Satellite telephone systems typically utilize many individual circuits that are routed through a constellation of satellites in low earth orbit (LEO) to effect communications. One important advantage of the satellite telephone system is that it provides ubiquitous coverage of large areas of the earth, without requiring the deployment of many small terrestrial cells. A satellite is a specialized wireless receiver/transmitter launched by a rocket and placed in orbit around the earth. Geostationary satellites, the most common type of satellite, orbit the earth directly over the equator remaining over the same spot at all times. A geostationary satellite can be accessed using an antenna aimed at the spot in the sky where the satellite hovers. A satellite can receive signals from and transmit signals to a user terminal provided the user terminal is within the "footprint" of the satellite. The footprint of a satellite is the geographic region on the surface of the Earth within the range of signals of the satellite. The footprint is usually geographically divided into "beams," through the use of beam-forming antennas. Each beam covers a particular geographic region within the footprint. In cellular satellite phone communications systems and methods, multiple beams are provided, each of which can serve distinct geographical areas in the overall service region, to collectively serve an overall satellite footprint. A satellite communication system can have a symmetrical or non-symmetrical air interface. In a symmetrical air interface, both the downlink and uplink carrier waveforms occupy the same channel bandwidth. A signal transmitted from the satellite to the user terminal is referred to as a satellite downlink signal or frequency. The satellite converts the satellite uplink frequency (user terminal reverse link) into a gateway-satellite system forward link frequency, transmitted from the satellite to the gateway. In a non-symmetrical air interface, the downlink carrier waveform occupies a first channel bandwidth and the uplink carrier waveform occupies a second channel bandwidth. In satellite communication systems, the base stations are located on a satellite that orbits the earth. Each satellite may function as a plurality of base stations and base station controller, which communicate with a satellite system controller that is on the ground.
Satellite-based cellular phone systems are similar to terrestrial cellular telecommunication systems in that they have a grid of service zones or cells. They differ in that the satellite systems use a constellation of satellites with communication transponders instead of fixed radio towers as in terrestrial cellular systems. Mobile satellite communication systems used for transmitting point-to-point voice and data communications may include a number of user terminals, several terrestrial ground stations or gateways, and a number of satellites for bi-directionally coupling the user terminals to terrestrial telecommunication networks. Satellite telecommunications systems typically use some type of digital voice encoder with and without compression. The encoder may be embodied as a codec and may include a vocoder for extracting spectral information from speech and compressing and encoding the extracted spectral information. After encoding and compressing, the speech is further modulated and then is transmitted to a receiver where a decoder reconstructs the original speech signal with the use of one or more digital signaling processors (DSP). In a satellite phone system, a user located in one of the cells communicates with the system through the satellite transponder covering a service zone in which the user is located. The satellite then communicates either with another satellite or with an earth station, to forward the call to a receiving party. A satellite phone generally comprises a handheld device together with an antenna which may be connected to the telephone via a suitable connection and which may be deployed in a position on the ground or other suitable structure for directing into the sky. The cellular architecture similar to that used in terrestrial cellular phone systems. A satellite phone communicates directly with an orbiting satellite thereby permitting the user of the phone to make or receive a phone call from virtually anywhere on earth.