Telecommunications – Carrier wave repeater or relay system – Portable or mobile repeater
Reexamination Certificate
1997-08-11
2001-11-06
Eisenzopf, Reinhard (Department: 2682)
Telecommunications
Carrier wave repeater or relay system
Portable or mobile repeater
C455S429000
Reexamination Certificate
active
06314269
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and apparatus for communication via satellite, and in particular but not exclusively for voice or data communication using non-geostationary satellites and/or mobile terminals.
BACKGROUND ART
In communication systems which use non-geostationary satellites, the number and orientation of satellites in view of a ground-based terminal varies during a call. Thus, the communication link between the terminal and any one satellite may become weaker as the elevation angle of the satellite decreases and ultimately the link may become inoperable as the satellite moves out of sight of the terminal. It is therefore desirable to select another satellite for communication with the terminal, in a procedure known as “handover”. The document U.S. Pat. No. 3,349,398 describes one such method. However, handover between satellites may result in loss of part of the signal, or sudden variations in signal quality, which are unacceptable in voice or data communications.
Furthermore, the line of sight between the terminal and a particular satellite may become obstructed by buildings, trees or other obstacles as the terminal or the satellite moves during a call. This effect is known as “blockage”, and leads to fading in the received signal.
Signal fading may also occur when a signal transmitted by a satellite is reflected off the ground or buildings and the reflected signal is received at the terminal together with the direct signal. The phase difference between the direct and reflected signals may lead to destructive interference at the terminal, so that the received signal strength is reduced. This is known as “multipath” fading.
The document WO-A-93 09578 discloses a satellite communication system in which the satellites monitor the quality of signal received from a terminal and determine which one is best suited to handle the call to the terminal. One of the satellites re-transmits the signal received from the terminal to other satellites or gateways.
The conference paper “The Globalstar Mobile Satellite System for Worldwide Personal Communications” by Wiedeman and Viterbi, 3
rd
International Mobile Satellite Conference, Jun. 16-18 1993, Pasadena, Calif. discloses a communication system in which return link signals are received by two or three satellites; gateway stations measure the signal level of each of these alternate paths and control which signal paths are used. This system is exclusively designed for use with code-divided multiple access (CDMA).
However, CDMA suffers from a number of drawbacks when used for mobile communications. The mobile terminals are complex, since they require a separate decoder for each satellite path. Moreover, CDMA is inefficient in frequency re-use unless the users are evenly distributed, and power levels cannot be freely varied for each user without causing interference for other users. Furthermore significant interference takes place at peak levels of use.
STATEMENT OF THE INVENTION
According to one aspect of the present invention, there is provided a method of communication between a terrestrial station and a plurality of terminals using TDMA to address each of the terminals from the terrestrial station, in which diversity is provided in the link between the terrestrial station and each terminal, by sending the same information through two or more satellites.
The information may be sent in the same TDMA time slot through the two or more satellites, or may be sent in different time slots.
In this way, blockage may be reduced without the inherent disadvantages of CDMA.
The terrestrial station may either decode the best received signal from each terminal or may combine all of the received signals to reduce error in the received signal. The terrestrial station may then select a forward link to each terminal through one or more of the satellites according to the quality of signal received from the terminal through the satellites.
Thus, a smooth handover may be achieved and blockage and fading may be reduced.
In order that the selection of satellite for the forward link may be transparent to the terminal, the terrestrial station may calculate the delay in the transmission via the selected satellite and adjust the timing of its transmission accordingly so that the transmitted signal is received by the terminal in the same time slot throughout the call. The calculation may take into account both the variation in delay as the selected satellite moves relative to the earth, and the difference in delay when handing over from one satellite to another, so that the quality of communication is not impaired by handover and complex circuitry is not required in the terminal.
In addition, the terrestrial station may compensate for the Doppler shift in the signal received from the terminal and adjust the frequency of the transmitted signal accordingly so that the terminal receives a signal at a constant frequency throughout a call. The Doppler shift may be partially compensated for by the satellites.
According to another aspect of the present invention, in order to facilitate simultaneous communication with multiple users through one satellite, areas of the earth are divided into a number of fixed regions, with a frequency being assigned to a terminal both for transmission and reception of signals according to the region in which the terminal is located. The locations of the regions are determined according to their positions on the earth, rather than their positions relative to the satellite. Simultaneous communication between different terminals in the same region and at the same frequencies may be achieved by allocating different time slots within a repeating time frame to each of the terminals. Since the different terminals using the same frequencies are contained within a fixed region and the variation in propagation delays is therefore limited, interference between the adjacent time slots is avoided.
According to another aspect of the present invention, there is provided a method of controlling a non-geostationary satellite which generates a plurality of individually steerable beams to provide communications links, in which each beam is directed towards a fixed region of the earth's surface until the beam is no longer suitable for communication with that fixed region as a result of the progress of the satellite relative to the earth's surface. The beam is then redirected to a new fixed region with which satisfactory communication is possible. Calls to the previous fixed region may be routed through another satellite.
In this way, the frequency of beam-to-beam handover may be reduced, without affecting the frequency of satellite-to-satellite handover.
The present invention extends to a terrestrial station having means for performing the functions of one or more of the earth stations or terminals described above.
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European Search Report.
Haruo Kondo et al., “Fr
Bjornstrom Gunnar
Hart Nicholas
Banner & Witcoff , Ltd.
Bhattacharya Sam
Eisenzopf Reinhard
International Mobile Satelitte Organization
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