Multi-beam TDMA satellite mobile communications system

Abstract

An earth station receives a return signal via more than one satellite link from a mobile terminal using TDMA. The earth station selects one or more of the satellite links for transmitting a forward signal on the basis of the quality of signal received via each link. The earth section allocates frequency channels to the mobile terminals according to their location on the surface of the earth, so that the propagation time to and from those mobile terminals which share the same frequency channel is approximately the same. The satellite includes an antenna which generates an array of beams which are individually pointed to fixed regions of the earth, until the elevation of the satellite relative to a fixed region falls below a minimum value, in which case the corresponding beam is redirected to a new area, while the other beams remain pointed at the corresponding fixed areas. In this way, beam-to-beam handover is reduced, while maintaining the boresight of the antenna pointing at the nadir.

1 . A method of satellite communication, comprising: 
 directing a plurality of beams from a non-geostationary satellite to a corresponding plurality of regions which are fixed relative to the surface of the earth; and    determining when one of said beams fails to satisfy a predetermined criterion, and redirecting said one of said beams from the corresponding one of the fixed regions to another fixed region such that said one of said beams satisfies said predetermined criterion; characterised in that the directions of other ones of said plurality of beams which satisfy said predetermined criterion are maintained at the corresponding ones of the fixed regions while said one of said plurality of beams is redirected.    
     
     
         2 . A method as claimed in  claim 1 , wherein said directing and redirecting steps comprise sending a command signal to said satellite from an earth-based station, so as to control the direction of the beams of said satellite.  
     
     
         3 . A method as claimed in  claim 1  or  2 , wherein said predetermined criterion comprises a range of desirable beam directions whereat the satellite has an elevation angle greater than a predetermined minimum elevation angle with respect to the fixed region.  
     
     
         4 . Apparatus for satellite communication, comprising means operable to determine a plurality of beam directions, for a plurality of beams projected by a non-geostationary satellite, to corresponding fixed regions of the earth's surface; means operable to detect whether one or more of said beams fail to satisfy a predetermined criterion; and 
 means operable to determine a new beam direction to a new fixed region for each of said one or more of said beams, such that the predetermined criterion is satisfied by the beam in the new beam direction; characterised by control means operable to control said non-geostationary satellite so as to direct the beams in the corresponding beam directions such that the directions of other ones of said beams for which a new beam direction is not determined are maintained at the corresponding fixed regions.    
     
     
         5 . Apparatus as claimed in  claim 4 , wherein said control means includes means for sending a control means to said satellite.  
     
     
         6 . Apparatus as claimed in  claim 4  or  5 , wherein said predetermined criterion comprises a range of desirable beam directions to points on the earth's surface whereat the satellite has an elevation angle greater than a predetermined minimum elevation angle.