Multi-beam TDMA satellite mobile communications system

Abstract

An earth station ( 8 ) receives a return signal ( 3 ) via more than one satellite link ( 4 a , 4 b ) from a mobile terminal ( 2 ) using TDMA. The earth station ( 8 ) selects one or more of the satellite links ( 4 a ) for transmitting a forward signal ( 15 ) on the basis of the quality of signal ( 3; 15 ) received via each link. The earth section ( 8 ) allocates frequency channels to the mobile terminals ( 2 ) according to their location on the surface of the earth, so that the propagation time to and from those mobile terminals ( 2 ) which share the same frequency channel is approximately the same. The satellite ( 4 a; 4 b ) 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 ( 4 a ; 4 b ) 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 between a first earth station ( 18 ) and a second earth station ( 8 ) comprising: 
 receiving at the second earth station ( 8 ) information transmitted from the first earth station ( 18 ) within one or more time-division multiplexed time slots (t) and relayed via a plurality of beams ( 51 ) generated by one or more satellites ( 4 );    said method being characterised by the steps of: 
 selecting ore or more of said satellite beams ( 51 ) according to a property of the information received via said satellite beams ( 51 ); and  
 transmitting further information ( 15 ) from said second earth station ( 8 ) to the first earth station ( 18 ) such that the further information ( 15 ) is relayed via said selected one or more satellite beams ( 51 ).  
   
     
     
         2 . A method as claimed in  claim 1 , further comprising: 
 calculating variations in the delay in the transmission link between the first earth station ( 18 ) and the second earth station ( 8 ) via said selected one or more satellite beams ( 51 ), and transmitting the further information from the second earth station ( 8 ) to the one or more satellites ( 4 ) which generate the or each selected satellite beam ( 51 ) with a timing determined so as to compensate for said variations.    
     
     
         3 . A method as claimed in  claim 2 , wherein the further information is transmitted from the second earth station ( 8 ) to the first earth station ( 18 ) via two or more said selected beams ( 51 ), and the timings of the transmissions are determined so that the information is received substantially simultaneously at the first earth station ( 18 ) via the selected beams ( 51 ).  
     
     
         4 . A method as claimed in any preceding claim, further comprising measuring the Doppler shift from a predetermined frequency of the frequency of a signal ( 3   a ;  3   b ) containing the information received at the second earth station ( 8 ), and selecting the frequency of a signal ( 15 ) containing the further information so as to compensate for the measured Doppler shift.  
     
     
         5 . A method as claimed in any preceding claim, further comprising determining the position of the first earth station ( 18 ); wherein the further information is transmitted from the second earth station ( 8 ) with a frequency (f f ) determined according to the derived position of the first earth station ( 16 ).  
     
     
         6 . A method as claimed in  claim 5 , further comprising transmitting to the first earth station ( 18 ) a control signal for controlling the transmission and reception frequencies (f r ,f f ) of the first earth station ( 18 ), the control signal being generated according to the derived position of the first earth station ( 18 ).  
     
     
         7 . A method as claimed in  claim 1  or  2 , wherein the second earth station ( 8 ) receives the information more than once in different time slots (t 3 ,t 7 ) via a corresponding number of said satellite beans ( 51 ).  
     
     
         8 . A method as claimed in  claim 7 , wherein the second earth station ( 8 ) receives the information transmitted by the first earth station ( 18 ) within a first time slot (t 3 ) in a first frequency channel (f 1 ′) of a first satellite beam ( 51 ), and 
 receives the information which is transmitted again by the first earth station ( 18 ) during a second time slot (t 7 ) in a second frequency channel (f 2 ′) of a second satellite beam ( 51 ).  
 
     
     
         9 . A method as claimed in  claim 8 , further comprising: 
 comparing respective properties of the received information transmitted by the first earth station ( 18 ) during the first and second time slots (t 3 , t 7 ) and    transmitting the further information at a selected frequency corresponding to one of the first and second satellite beams ( 51 ) according to the result of the comparison.    
     
     
         10 . A method as claimed in any preceding claim, wherein if a property of the received information fails to satisfy a predetermined criterion, the second earth station ( 8 ) transmits the further information at a lower rate, and if the property of the received information satisfies said predetermined criterion, the second earth station ( 8 ) transmits the further information at a higher rate.  
     
     
         11 . A method as claimed in claim,  10 , wherein the information is transmitted by the steps of: 
 dividing the further information into first and second portions (Rx a , Rx b );    transmitting the first portion (Rx a ) to the first earth station ( 18 ) within a third time slot (t 3 ); and    transmitting the second portion (Rx b ) to the first earth station ( 18 ) within a fourth time slot (t 7 ).    
     
     
         12 . A method as claimed in any preceding claim, wherein said property relates to the quality of previous information previously received by the first earth station ( 18 ) from the second earth station ( 8 ).  
     
     
         13 . A method as claimed in any one of  claims 1  to  11 , wherein the property relates to the quality of said information received by the second earth station ( 8 ).  
     
     
         14 . A method of satellite communication using TDMA channels, comprising: 
 transmitting the same information from a first earth station ( 18 ) within each of a plurality of time slots (t) to a second earth station ( 8 ), at respective, different frequencies (f 1 ′, f 2 ′) so that the information is received in respective, different beams generated by one or more satellites ( 4 ).    
     
     
         15 . Apparatus for use in a satellite communications earth station ( 18 ) using TDMA channels, comprising: 
 a receiver ( 8   a ,  8   b ) arranged to receive information relayed by one or more satellites ( 6 ) from a remote earth station ( 18 ) within one or more time slots (t), the information being relayed via a plurality of beams ( 51 ) generated by said one or more satellites ( 6 );    beam selecting means ( 34 ) for selecting one or more of said satellite beams ( 51 ) according to a property of the information received therein, and a transmitter ( 12 ) arranged to transmit further information to the remote earth station ( 18 ) such that the information is relayed to the remote earth station ( 18 ) via the selected one or more satellite beams ( 51 ).    
     
     
         16 . Apparatus as claimed in  claim 15 , further comprising: 
 means ( 88 ) for calculating variations in the transmission delay to the remote earth station ( 18 ) via the selected one or more satellite beams ( 51 ), and control means ( 85 , 88 ) for controlling the timing of the transmitter ( 12 ) to compensate for the variations.    
     
     
         17 . Apparatus as claimed in  claim 16 , wherein the beam selecting means ( 14 ) is arranged to select two or more of said satellite beams ( 51 ), and the control means ( 85 , 88 ) is arranged to control the timing of the transmitter ( 12 ) so that the transmitted information is received substantially simultaneously at the remote earth station ( 18 ) via each of the selected beams ( 51 ).  
     
     
         18 . Apparatus as claimed in any one of  claims 15  to  17 , further comprising means ( 88 ) for measuring the Doppler shift in a received signal containing the information, and means ( 86 , 88 ) for adjusting the frequency of the transmitter ( 12 ) to compensate for the measured Doppler shift.  
     
     
         19 . Apparatus as claimed in any one of  claims 15  to  18 , further comprising means ( 88 ) for deriving the position of the remote earth station ( 18 ); and frequency selecting means ( 86 ,  88 ) for selecting the frequency of the transmitter ( 12 ) according to the derived position of the remote earth station ( 18 ).  
     
     
         20 . Apparatus as claimed in  claim 19 , including means ( 88 ) for generating a control signal for controlling the transmission and reception frequency of the remote earth station ( 18 ) according to the derived position thereof, the transmitter ( 12 ) being arranged to transmit the control signal to the remote earth station ( 18 ).  
     
     
         21 . Apparatus as claimed in  claim 15  or  16 , wherein the receiver ( 8   a ,  8   b ) is arranged to receive the information more than once sequentially via a corresponding number of said beams ( 51 ).  
     
     
         22 . Apparatus as claimed in  claim 21 , wherein the receiver ( 8   a ,  8   b ) is arranged to receive the information transmitted by the remote earth station ( 18 ) within a first time slat (t 3 ) in a first frequency channel (f 1 ′) of a first satellite beam ( 51 ), and to receive the information which is repeated by the remote earth station ( 18 ) during a second time slot (t 7 ) in a second frequency channel (f 2 ′) of a second satellite beam ( 51 ).  
     
     
         23 . Apparatus as claimed in  claim 21  or  22 , further comprising: 
 comparing means ( 88 ) for comparing a property of the information transmitted by the remote earth station ( 18 ) during the first and second time slots (t 3 ,t 7 ); wherein  
 the transmitter ( 12 ) is arranged to transmit a signal ( 15 ) in a frequency channel corresponding to one of the first and second beams ( 51 ) selected by the comparing means ( 88 ).  
 
     
     
         24 . Apparatus as claimed in any one of claims  15  to  23 , wherein the transmitter ( 12 ) is arranged to transmit at a lower rate if the received information fails to satisfy a predetermined criterion and to transmit at a higher rate if the predetermined criterion is satisfied.  
     
     
         25 . Apparatus as claimed in  claim 24 , wherein the transmitter ( 12 ) is arranged to divide the information to be transmitted into first and second portions (Rx a , Rx b ), to transmit the first portion (Rx a ) to the remote earth station ( 18 ) within a third time slot (t 3 ) and to transmit the second portion (Rx b ) to the remote earth station ( 18 ) within a fourth time slot (t 7 ).  
     
     
         26 . Apparatus as claimed in any one of  claims 15  to  25 , wherein said property relates to the quality of previous information previously received by the remote earth station ( 18 ) from the satellite communications earth station ( 8 ).  
     
     
         27 . Apparatus as claimed in any one of  claims 15  to  25 , wherein the property relates to the quality of the received information.  
     
     
         28 . A user station ( 18 ) for satellite communications, comprising a transmitter ( 2 ) arranged to transmit the same information within each of a plurality of time slots (t) to a base station ( 8 ) at respective different frequencies (f 1 ′,f 2 ′) so as to be receivable in respective different beams ( 51 ) generated by one or more satellites ( 4 ).  
     
     
         29 . A method of communication between a base station ( 8 ) and a plurality of user stations ( 18 ) each of which are located within the coverage area ( 50 ) of a beam ( 51 ) generated by a satellite ( 4 ), said beam ( 51 ) carrying a plurality of frequency channels, 
 characterised by allocating each of said frequency channels to a group of said user stations ( 18 ) which fall within one of a plurality of predetermined regions ( 52 ) within the beam ( 51 ) such that the variation in propagation delay to said satellite ( 4 ) among said group of said user stations ( 18 ) is limited, and communicating between the base station ( 8 ) and said user stations ( 18 ) within said beam ( 51 ) in said allocated frequency channels.    
     
     
         30 . A method as claimed in  claim 29 , wherein said group comprises ones of said user stations ( 18 ) which are approximately equidistant from said satellite ( 4 ).  
     
     
         31 . A method as claimed in  claim 30 , further comprising determining the positions of the user stations ( 18 ) relative to the earth's surface and allocating each of said frequency channels according to the determined positions of the user stations ( 18 ).  
     
     
         32 . Apparatus for communication with a plurality of user stations ( 18 ) which are located within the coverage area ( 50 ) of a beam ( 51 ) generated by a satellite ( 4 ), said beam ( 51 ) carrying a plurality of frequency channels, 
 characterised by frequency channel allocating means ( 88 ) for allocating each of said frequency channels to a group of said user stations ( 18 ) which fall within one of a plurality of predetermined regions ( 52 ) within the beam ( 51 ) such that the variation in propagation delay to said satellite ( 4 ) among said group of said user stations ( 18 ) is limited, and means ( 85 ,  86 ,  87 ) arranged to communicate with said user stations ( 18 ) within said beam in said allocated frequency channels.    
     
     
         33 . Apparatus as claimed in  claim 32 , including grouping means ( 88 ) for determining said groups of said user stations ( 18 ) such that each group comprises ones of said user stations ( 18 ) which are approximately equidistant from said satellite ( 4 ).  
     
     
         34 . Apparatus as claimed in  claim 33 , including position determining means ( 88 ) arranged to determine the positions of the user stations ( 18 ) relative to the earth's surface, the grouping means ( 88 ) being arranged to determine said predetermined regions ( 52 ) relative to the earth's surface.  
     
     
         35 . A method of satellite communication, comprising: 
 directing a plurality of beams ( 51   a - 51   m ) from a ion-geostationary satellite ( 4   a ) to a corresponding plurality of regions ( 52 ) which are fixed relative to the surface of the earth; and    determining when one of said beams ( 51   a - 51   m ) fails to satisfy a predetermined criterion, and redirecting said one of said beams ( 51   a - 51   m ) from the corresponding one of the fixed regions ( 52 ) to another fixed region ( 52 ) such that said one of said beams ( 51   a - 51   m ) satisfies said predetermined criterion; characterised in that the directions of other ones of said plurality of beams ( 51   a - 51   m ) which satisfy said predetermined criterion are maintained at the corresponding ones of the fixed regions ( 52 ) while said one of said plurality of beams ( 51   a - 51   m ) is redirected.    
     
     
         36 . A method as claimed in  claim 35 , wherein said directing and redirecting steps comprise sending a command signal to said satellite ( 4   a ) from an earth-based station ( 8 ), so as to control the direction of the beams ( 51   a - 51   m ) of said satellite ( 4   a ).  
     
     
         37 . A method as claimed in  claim 35  or  35 , wherein said predetermined criterion comprises a range of desirable beam directions whereat the satellite ( 4   a ) has an elevation angle greater than a predetermined minimum elevation angle with respect to the fixed region ( 52 ).  
     
     
         38 . Apparatus for satellite communication, comprising means operable to determine a plurality of beam directions, for a plurality of beams ( 51   a - 51   m ) projected by a non-geostationary satellite ( 4   a ), to corresponding fixed regions ( 52 ) of the earth's surface; 
 means operable to detect whether one or more of said beams ( 51   a - 51   m ) fail to satisfy a predetermined criterion; and    means operable to determine a new beam direction to a new fixed region ( 52 ) for each of said one or more of said beams ( 51   a - 51   m ), 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 ( 4   a ) so as to direct the beams ( 51   a - 51   m ) in the corresponding beam directions such that the directions of other ones of said beams ( 51   a - 51   m ) for which a new beam direction is not determined are maintained at the corresponding fixed regions ( 52 ).    
     
     
         39 . Apparatus as claimed in  claim 38 , wherein said control means includes means for sending a control signal to said satellite ( 4   a ).  
     
     
         40 . Apparatus as claimed in  claim 38  or  39 , wherein said predetermined criterion comprises a range of desirable beam directions to points (Ca-Cm) on the earth's surface whereat the satellite ( 4   a ) has an elevation angle greater than a predetermined minimum elevation angle.  
     
     
         41 . A terrestrial station including apparatus as claimed in any one of  claims 15  to  27 ,  32  to  34  and  38  to  40 .