Method to minimize interference into legacy sdars reception by varying overlay modulating as a function of satellite position

Abstract

In exemplary embodiments of the present invention, a degree of hierarchical modulation can be varied as a function of satellite position, which can be related to time of day, and where such hierarchical modulation can be a combination of phase and amplitude shifts. In exemplary embodiments of the present invention, as each satellite in a multiple satellite system broadcasting to North America moves towards a D-node or an A-node position, the offset angle of a phase-based hierarchical modulation scheme can be varied. Thus, in exemplary embodiments of the present invention, the lowest satellite position in the sky can have the lowest offset angle for overlay bits, which offset angle can, for example, progressively increase as the position of the satellite in the sky increases. At a satellite's highest point in the sky, the overlay offset angle can, for example, thus be at its maximum. To aid a receiver to receive such varying overlay modulated data, the value of a varying overlay offset angle (for each satellite) can be embedded in an Overlay Identification Marker (OIM) in the bit stream broadcast by that satellite. In exemplary embodiments of the present invention each satellite broadcast receiver can thus decode each satellite's signal, knowing its respective instantaneous overlay offset angle. The receiver can then use this information to appropriately correct MRC weightings (weighting more heavily the (higher lying) satellite with the higher degree of Layer 2 modulation) before it is MRC combined across various received signal streams. Similar methods can be implemented for other overlay modulation techniques, and the method can be extended to a broadcast system using N satellites, each having a varying degree of Layer 2 modulation.

1 . A method of minimizing interference in satellite communications, comprising:
 providing an I,Q symbol bit stream modulated with legacy data; and   further modulating the I,Q bit stream with a layer 2 modulation scheme to encode a second layer of data; and   broadcasting the further modulated I,Q bit stream from a satellite,   wherein said layer 2 modulation scheme is varied as a function of the satellite's position in the sky.   
     
     
         2 . The method of  claim 1 , wherein at least one of:
 (i) the layer 2 modulation scheme comprises a phase shift, and wherein said phase shift varies instantaneously as a function of the satellite's position in the sky;   (ii) the instantaneous value of the overlay phase shift is transmitted in one of the overlay bit stream, the legacy bit stream and a service channel bit stream;   (iii) the instantaneous value of the overlay phase shift is transmitted in one of the overlay bit stream, the legacy bit stream and a service channel bit stream, and the value of the overlay phase shift is embedded in an Overlay Identification Marker (OIM);   (iv) N satellites are used, each having a varying amount of layer 2 modulation, and the signals received from the N satellites are combined using maximal ratio combining, where MRC weightings are corrected by a relative layer 2 modulation correction factor;   and   (v) N satellites are used, each having a varying amount of layer 2 modulation, the signals received from the N satellites are combined using maximal ratio combining, where MRC weightings are corrected by a relative layer 2 modulation correction factor, and wherein the signals received from the N satellites are combined using maximal ration combining, where MRC weightings are corrected by a relative layer 2 modulation correction factor.   
     
     
         3 - 4 . (canceled) 
     
     
         5 . The method of any of  claim 1 , wherein the overlay phase shift is at a maximum at the satellite's highest position in the sky and is at a minimum at the satellite's lowest position in the sky. 
     
     
         6 . (canceled) 
     
     
         7 . A receiver, comprising:
 a first receiving stage to receive a first bit stream of I,Q symbols that have been further modulated with a first varying overlay phase shift;   a first overlay offset angle detection stage to detect the value of the first offset angle as transmitted;   a first overlay demodulation stage to detect a direction of the first overlay phase shift;   a second receiving stage to receive a second bit stream of I,Q symbols that have been further modulated with a second varying overlay phase shift;   a second overlay offset angle detection stage to detect the value of the second offset angle as transmitted;   a second overlay demodulation stage to detect a direction of the second overlay phase shift; and   a maximal ratio combining stage that combines the first and second overlay bit streams using the following correction factors to MRC weightings:
     C 1=sin(offset angle Sat1)/sin(offset angle Sat2); and 
     C 2=1.0; 
   wherein C1 is the weight for the first overlay bit stream, C2 is the weight for the second overlay bit stream, offset angle Sat1 is the first offset angle as transmitted and offset angle Sat2 is the second offset angle as transmitted, and wherein Sat2>Sat1.   
     
     
         8 . The receiver of  claim 7 , wherein the offset angles are embedded in an Overlay Identification Marker (OIM) within each bit stream. 
     
     
         9 . The receiver of  claim 8 , wherein the overlay offset angle detection stage detects the value of an offset angle by processing the OIM. 
     
     
         10 . The receiver of  claim 7 , wherein after the weight W1 is applied to the first overlay bit stream the noise power for the first satellite can also, for example, be scaled by the same factor. 
     
     
         11 . The receiver of  claim 7 , further comprising a first and a second de-rotation stage to remove a defined angular offset from the received I,Q symbols, wherein each received I,Q symbol is overlay demodulated and de-rotated prior to being input to legacy demodulation stages. 
     
     
         12 . The receiver of  claim 7 , further comprising a de-rotation stage to remove a defined angular offset from the received I,Q symbols, wherein each received I,Q symbol is overlay demodulated and de-rotated prior to being input to legacy demodulation stages. 
     
     
         13 . The method of  claim 5 , wherein the angular offset associated with the overlay phase shift is between 0 and 22.5 degrees. 
     
     
         14 . The receiver of  claim 11 , wherein the angular offset associated with each overlay phase shift is between 0 and 22.5 degrees. 
     
     
         15 . The receiver of  claim 7 , wherein the angular offset associated with the overlay phase shift is between 0 and 22.5 degrees. 
     
     
         16 - 17 . (canceled) 
     
     
         18 . A program storage device readable by a processing unit, tangibly embodying a program of instructions executable by the processing unit to implement a method of minimizing interference in satellite communications, said method comprising:
 receiving a first bit stream of I,Q symbols that have been further modulated with a first varying overlay phase shift;   detecting the value of the first offset angle as transmitted;   detecting a direction of the first overlay phase shift;   receiving a second bit stream of I,Q symbols that have been further modulated with a second varying overlay phase shift;   detecting the value of the second offset angle as transmitted;   detecting a direction of the second overlay phase shift; and   maximal ratio combining the first and second overlay bit streams using the following relative layer 2 modulation correction factors to MRC weightings:
     C 1=sin( a 1)/sin( a 2); and 
     C 2=1.0; 
   wherein C1 is the correction factor for the first overlay bit stream, C2 is the correction factor for the second overlay bit stream, offset angle a1 is the first offset angle as transmitted and offset angle a2 is the second offset angle as transmitted, and wherein a2>a1.   
     
     
         19 . The program storage device of  claim 18 , further comprising de-rotating each I,Q symbol by its varying overlay phase shift and passing the symbol to legacy decoding stages after said de-rotating. 
     
     
         20 - 33 . (canceled)