Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1997-08-21
2003-09-23
Bost, Dwayne (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S427000, C455S011100
Reexamination Certificate
active
06625450
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a telecommunication system where remote user terminals are in receipt of access to a larger telecommunication system by means of direct access to a satellite which, in turn, is in communication with a ground station. It particularly relates to a system where, potentially, a user terminal can access more than one satellite and, via a satellite or satellites, can access more than one ground station.
BACKGROUND
First, it is necessary to describe the general terrestrial telephone system. This is the “larger telecommunication system” mentioned above.
The terrestrial telephone system comprises the sum of all of the wire, optical, point-to-point, microwave and any other means of conducting signals from one point to another via the surface of the earth, together with their associated switches, amplifiers, encoders, decoders, amplifiers and repeaters. On a national level, the network links all access points in a country. Country is linked to country via international trunk lines. Oceans are spanned by submarine cables. The terrestrial network is a transparent carrier where a signal may be introduced from an access point and routed to another, selected access point elsewhere on the surface of the earth. A subscriber can access the terrestrial system via various gateways. For example, the terrestrial system can be accessed from the public switched telephone network, a public switched data network, an integrated switched digital network, a cellular telephone system, or a satellite telephone system. Each gateway provides a plurality of different access points, which, depending upon the gateway, can range from a simple telephone to a complex data or image source. The subscriber's gateway, at the subscriber's request, commands the terrestrial system to route and carry the message from the subscriber to a selected destination. The message comes in through the subscriber's gateway, passes through the terrestrial system, and exits through the selected destination gateway to reach the access point required. Once the message path is established, two-way communication can commence. This is the way the terrestrial system works. Each gateway is connected to another single gateway. The terrestrial system is not adapted, readily, to switch, during the course of a message, between different source gateways or between different destination gateways.
Each gateway, in what is generally understood to be a telephone system, provides a plurality of access points whereby a subscriber may make or receive calls. Each access point is unique in its identity or location and definitely is unique to its particular gateway. In other words, one access point means one gateway.
Now, satellite systems provide a gateway to the terrestrial system via earth stations, capable of establishing two way contact with a satellite or satellites. In turn, the satellites can communicate with user terminals (portable handsets). The user terminals are thus put in contact with the earth stations which can act as a gateway to the terrestrial system.
Sadly, each earth station is a separate gateway. Generally, the earth stations are widely spread apart. There is envisaged an earth station density of one, two or just a few earth stations on each continent. Thus, in general, each user terminal will see satellites which are visible only to one earth station. This satisfies the terrestrial network requirement “one access point means just one gateway.” This situation is not a problem. However, there are circumstances, for example, in the middle of the Pacific ocean, in the middle of the Sahara desert, high atop the Andes, where a user terminal may be able to see a satellite or satellites which, in turn, are visible to more than one earth station. This now breaks the rule “one access point means just one gateway”. The user terminal is an access point. Each earth station is a gateway. Now there is the situation “one access point-many gateways”. The terrestrial network is not adapted to respond to multiple gateways. Further, the signals to and from the earth station, satellites and user terminals are forever changing. What was a perfectly good satellite link a moment ago is no longer functional because the satellite has moved, the user terminal has been shielded by a building or other structure, and a host of other reasons. However, one of the other user terminal to satellite to ground station links might still work. How can the switch between ground stations be effected without confusing the operation of the terrestrial network? The present invention seeks to solve the twin problem of multiple gateway access and operational gateway switching at minimal cost and with least disruption to the general functionality of the terrestrial network.
SUMMARY OF THE INVENTION
According to a first aspect, the present invention consists in a communication system wherein a user terminal can communicate with a plurality of satellites; wherein each of said plurality of satellites can communicate with a plurality of earth stations; wherein each of said plurality of earth stations is operable to provide a gateway into a terrestrial system; and wherein said user terminal is registered at a selected one of said plurality of earth stations as its gateway station; said system being characterised by said gateway station, in the event of being unable to establish contact with said user terminal, being operative to select and instruct another earth station to establish contact with said user terminal to act as an alternative earth station and being further operative to pass messages to and from said user terminal through said alternative earth station while acting as said gateway into said terrestrial system.
The present invention further provides a communication system wherein said gateway earth station is operative to employ a low capacity data link to instruct said alternative earth station to attempt to establish contact with said user terminal and, in the event of said alternative earth station establishing contact with said user terminal, is operative thereafter to employ a high capacity voice link to pass messages to and to receive messages from said alternative earth station.
The present invention further provides a communication system wherein said gateway earth station is operative to recall the last known geographical location of said user terminal and is further operative to select, as said alternative earth station, that earth station most likely to establish contact with a user terminal at said last known geographical location.
The present invention further provides a communication system wherein said gateway earth station is operative to recall and employ a plurality of timed previous geographical locations for said user terminal to estimate the instant position of said user terminal and is further operative to select, as said alternative earth station, that earth station most likely to establish contact with said user terminal at said estimated instant position.
The present invention still further provides a communication system wherein said gateway earth station, in the event of being incapable of establishing contact with said user terminal, is operative to select in turn, as said alternative earth station, each of those earth stations which are geographically most proximate to said gateway earth station until an alternative earth station is found capable of establishing contact with said user terminal.
The present invention still further provides a communication system wherein said gateway earth station is operative to execute a calculation to determine whether or not said gateway earth station is capable of establishing contact with said user terminal and to select said another earth station as said alternative earth station if and only if said calculation indicates that said gateway earth station is incapable of establishing contact with said user terminal.
According to a second aspect, the present invention consists in a method for establishing communication with a user terminal fo
Chomet Patrick
Mullins Dennis Roy
Munoz-Garcia Samuel G.
Bost Dwayne
International Mobile Satellite Organization
Moore James K
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