Satellite mobile telephone cell departure prediction
A mobile station ( 4 ) in a satellite mobile telephone system predicts when it will move into another cell on the basis of one set of broadcast information thereby reducing the overall processing burden. The mobile station ( 4 ) can predict with a useful degree of certainty which broadcast control channel (BCCH) frequencies it should listen on when it wakes up on the basis of a stored map of the relative positions of cells. The broadcast information includes information about the current cell's ( 40 ) shape (V 1 , V 2 , V 3 , V 4 , V 5 , V 6 ) and translational (T) and rotational (R) motion.
1. A method of operating a satellite mobile telephone in which a plurality of cells move across the surface of the Earth as a satellite orbits, comprising:
subsequent to completion of a registration process, receiving data related to a position and movement of a cell relative to the Earth's surface;
determining a position of mobile telephone; and
while the mobile telephone is not actively processing a telephone call, using the received data and the position of the mobile telephone to determine, at the mobile telephone, a prediction for the time when the mobile telephone will leave the cell.
2. The method of claim 1 , further comprising initiating a cell selection operation at the predicted time when the mobile telephone will leave the cell.
3. The method of claim 1 , wherein the information is provided to the mobile telephone by broadcasting the information defining a geographical position of a selected cell from a satellite to the selected cell, and receiving the data for the selected cell at the mobile.
4. The method of claim 1 , wherein the data includes information regarding the extent of the cell.
5. The method of claim 1 , wherein the data comprises a set of translational vectors defining the traversing of the cell over the surface of the Earth during a plurality of predetermined sub-periods of a period during which the information remains unchanged.
6. The method of 5 wherein determining a prediction comprises the mobile telephone predicting a selected one of the plurality of sub-periods during which it will leave the cell on the basis of the members of the set of translational vectors.
7. The method of claim 6 wherein the mobile telephone predicts the time within the selected sub-period when it will leave the cell.
8. The method of claim 1 , further comprising selecting at the mobile telephone a control channel to be monitored.
9. The method of claim 1 , further comprising determining at the mobile telephone the correctness of the prediction and performing a network registration process for the mobile telephone in dependence on the determined correctness.
10. The method of claim 1 , wherein the received data related to the movement of the cell relative to the Earth's surface is received from the satellite.
11. The method of claim 10 wherein the received data is received from the satellite via a control channel.
12. The method of claim 1 , wherein the data related to the movement of the cell comprises data related to a position of a central portion of the cell and a translational vector indicating a movement of the central portion of the cell.
13. The method of claim 1 , wherein the data related to the movement of the cell is calculated for a predetermined period of time.
14. The method of claim 13 wherein the data related to the movement of the cell is calculated for a plurality of predetermined periods of time covering a time range that exceeds the prediction for the time when the mobile telephone will leave the cell.
15. The method of claim 1 , wherein mobile telephone is communicating with a selected satellite access node (SAN) via the satellite, the method further comprising receiving location data indicating a location of the mobile telephone from the SAN via the satellite.
16. The method of claim 15 wherein the mobile telephone uses the location data in combination with the received data related to the movement of the cell to determine at the mobile telephone the prediction for the time when the mobile telephone will leave the cell.
17. The method of claim 1 , wherein the satellite mobile telephone has an awake mode and a power-down mode wherein receiving data subsequent to completion of the registration process is performed during a period in which the satellite mobile telephone changes from the power-down mode to the awake mode.
18. The method of claim 1 , wherein the satellite mobile telephone has an awake mode and a power-down mode, the method further comprising during a period in which the satellite mobile telephone changes from the power-down mode to the awake mode, scanning frequency channels associated with a satellite with which the satellite mobile telephone was previously communicating and frequency channels associated with a satellite with which the satellite mobile telephone is expected to communicate upon leaving the cell.
19. The method of claim 18 , further comprising selecting the satellite with which the satellite mobile telephone was previously communicating or the satellite with which the satellite mobile telephone is expected to communicate based on the satellite with greatest signal strength.
20. The method of claim 18 , further comprising selecting the satellite with which the satellite mobile telephone was previously communicating or the satellite with which the satellite mobile telephone is expected to communicate based on the prediction for the time when the mobile telephone will leave the cell if the satellite mobile telephone was previously communicating and the satellite with which the satellite mobile telephone is expected to communicate have approximately equal signal strength.
21. The method of claim 18 , further comprising scanning all frequency channels if no communication is established with the satellite with which the satellite mobile telephone was previously communicating or the satellite with which the satellite mobile telephone is expected to communicate upon leaving the cell.