Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-12-16
2001-06-12
Trost, William G. (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S439000, C455S440000, C455S013100
Reexamination Certificate
active
06246874
ABSTRACT:
(A) FIELD OF THE INVENTION
This invention relates generally to mobile satellite communication networks and, more particularly, to a method for predicting spot beam and satellite handover that utilizes the position of the mobile caller relative to the position of the satellite to estimate the time at which handover should occur.
(b) DESCRIPTION OF THE RELATED ART
Terrestrial cellular communication systems affect cellular communication in a geographic region via a plurality of stationary transmission towers each of which provides service to an individual service area commonly referred to as a cell. Each cell typically has a diameter in the range of several kilometers. To ensure continuous service to mobile subscribers traveling throughout the geographic region, the towers are positioned in a manner such that the cells are adjacent to and overlap with six other cells provided that the cells are not located on the boundary or edge of the service coverage area. Cells located on the edge of the service coverage area, i.e., an edge cell, may overlap with fewer than six other cells.
In land-based cellular communication systems, the decision to transfer a call from one service area to an adjacent service area (a procedure known as handover or handoff) primarily involves consideration of the signal quality of the on-going call. Typically, a stationary tower servicing a given service area (i.e., a cell) includes equipment to monitor the signal quality of the on-going call. When the measured signal quality decreases beneath a predetermined threshold, the tower transfers the call to the tower that services the adjacent cell into which the mobile subscriber is moving.
Although this method is acceptable in terrestrial cellular systems, the use of signal quality measurement as a means for determining when to effect handover can be subject to large errors in the satellite environment. These errors are typically caused by instability or other errors present in the signal that is transmitted by the earth station for subsequent relay to the mobile unit via the satellite. In particular, if an earth station based modem transmits control signals to a satellite beam at a power level that is different from the nominal level because of instability or other errors, the mobile unit that receives the transmitted signals may erroneously register its position as being inside the beam when in fact it may be outside of the beam or vice, versa.
A handover method has been proposed that involves tracking the mobile subscriber's position relative to the nominal cell boundaries to determine when handover should occur. In simplified terms, the method involves modeling the cells and cell boundaries on the earth's surface, then tracking the subscriber's movement relative to the earth based model. Thus, handover is performed when a subscriber crosses a cell boundary thereby moving out of a first cell and into an adjacent, second cell. The method becomes more complicated, however, because, to ensure continuous service coverage, adjacent cells slightly overlap each other. Due to the overlapping regions of adjacent cells, a mobile user may occupy a position that lies in two or three cells simultaneously. To determine when cell handoff should occur for a subscriber located in the overlapping region, it has been proposed that the cells be modeled using inscribed hexagons wherein the sides of the hexagons bisect the overlapping regions of the cells. The hexagons do not overlap but instead are placed in an abutting manner so that, when viewed together, they form a honeycomb-like grid. Thus, the hexagon boundaries are lines of demarcation used, in relation to the subscriber, to determine when handoff is to occur. For example, a call made by a subscriber located within the boundaries of a hexagon is serviced by the cell corresponding to that hexagon. When the mobile subscriber travels into an adjacent hexagon, then the call is handed over to the cell corresponding to this adjacent hexagon.
This proposed method of tracking a mobile subscriber's position relative to an earth based modeling system has several drawbacks when used in the non geo-stationary satellite environment. For example, in a non geo-stationary satellite communication system, a direct radiating antenna projects a plurality of circularly shaped spot beams onto the earth. Each spot beam represents a single service area or cell, such that a call made by a subscriber located within the boundaries of the cell are serviced by the corresponding spot beam. However, the spot beams, when mapped onto the surface of the earth, do not have a circular shape but instead have an elliptical shape due, in part, to the angle at which the satellite projects the beams onto the earth and also due to the spherical shape of the earth. Because of the elliptical shape of the spot beams, the inscribed hexagons used to model the spot beams and demarcate the cell boundaries are irregularly shaped, thereby making it very difficult to track the position of the subscriber relative to the irregular boundary. In addition, unlike the stationary hexagons used in a land-based system, the hexagons in the non geo-stationary satellite system are typically hundreds of kilometers in diameter and move over the surface of the earth in conjunction with the spot beam antenna on the satellite. To further complicate matters, various points on the boundaries of the hexagons move at different speeds. Thus, complex and time consuming geometrical procedures are required to model the rapid movement of the large, irregularly shaped hexagons. Moreover, the spherical shape of the earth causes the hexagons to be particularly distorted at the North and South poles so that the complex modeling procedures routinely lack precision in these areas.
Thus, there is a need in the art of satellite communications for a simplified method for predicting when handover should occur that is not subject to the polar sensitivity seen in the existing and proposed methods.
SUMMARY OF THE INVENTION
In one aspect, the present invention is directed to a method for performing handover in a satellite communication system. In particular, the method is used to determine when to transfer a call associated with a first spot beam to a second spot beam in a satellite communication system wherein the call is transmitted between a non geo-stationary satellite and a subscriber unit located in the first spot beam. The movement of the subscriber unit is tracked relative to a satellite based coordinate system and the movement of the subscriber is used to estimate a time at which the satellite will subsequently transfer the call.
In another aspect of the invention, a boundary is located between the first and the second spot beams and an interval of time during which the subscriber will cross over the boundary is estimated. The interval of time is repeatedly adjusted until the interval conforms to a desired level of accuracy.
To adjust the interval, the position of the subscriber within the satellite coordination system is calculated at the end of the interval and then compared to the location of the boundary to determine whether the subscriber has crossed over the boundary during the interval. If the subscriber has not passed over the boundary, then a new value is selected as the end of the interval. If instead, the subscriber has passed over the boundary, then the interval may be narrowed to obtain a more accurate estimate of the interval.
In yet another aspect of the present invention, the position of the subscriber and the position of the satellite are determined relative to an earth based coordinate system. The position of the subscriber is then converted using a transformational matrix such that the converted position is expressed relative to the satellite based coordinate system.
In yet another aspect of the present invention, a method is provided for determining when to transfer a call associated with a first satellite to a second satellite in a satellite communication system wherein the call is being transmitted from the firs
Hughes Electronics Corporation
Perez-Gutierrez Rafael
Sales Michael W.
Trost William G.
Whelan John T.
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