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 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: 
 selected one 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 snore 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 ( 18 ).  
     
     
         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 beams ( 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 ( 14 ) 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 beam ( 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 slot (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 (t 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 ) curing 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 non-geostationary satellite ( 4   a ) to a corresponding plurality of regions ( 52 ) which are fixed relative to the surface of the earth; and    determining where 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    36   , 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 .