Patent Publication Number: US-6340949-B1

Title: Multiple beam phased array with aperture partitioning

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to active phased array antenna arrays for generating communications signals on multiple beams. More specifically, but without limitation thereto, the present invention relates to partitioning an active phased array antenna to reduce the peak signal power requirement of the solid state power amplifiers of each array element. 
     A typical active phased array antenna consists of many array elements arranged in a circular, square, or elliptical aperture. For a transmitting array, the distribution of signal amplitudes that drive the array elements may be tapered, with higher amplitude signals driving array elements near the center of the array to minimize sidelobes of the antenna pattern. The center array elements of the phased array antenna are generally the center elements for all beams. If different signals are transmitted on each beam, then the peak signal power output of the center array elements is approximately the sum of the peak signal power of all beams. If a large number of beams are used, then the maximum output power and average output power requirements of the array element power amplifiers may increase the cost of the array element power amplifiers. Also, because the center array elements are used to generate each beam, the number of phase shifters required at each of the center array elements is equal to the number of beams, and the complexity of the power combiner required to combine the output of the phase shifters at each array element is correspondingly high. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously addresses the problems above as well as other problems by providing a multiple beam phased array with aperture partitioning that minimizes the required maximum output signal power of the array element power amplifiers. 
     In one embodiment, the present invention may be characterized as a multiple beam phased array that includes a plurality of array elements partitioned into a plurality of array element groups for forming a plurality of beams wherein each array element group has a taper center located to minimize maximum array element power for the plurality of beams. 
     In another embodiment, the present invention may be characterized as a method of partitioning array elements of a multiple beam phased array that includes the steps of defining a group of array elements for each of a plurality of beams and locating a taper center of each group of array elements in the multiple beam phased array to minimize maximum array element power. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features and advantages of the present invention will be more apparent from the following more specific description thereof, presented in conjunction with the following drawings wherein: 
     FIG. 1 is a diagram of a transponder platform communications system;. 
     FIG. 2 is a diagram of a conventional phase control network for the multiple beam phased array transmitting antenna of FIG. 1; 
     FIG. 3 is a diagram of a circular array for the multi-beam phased array transmitting antenna of FIG. 1 illustrating a conventional array element grouping method; 
     FIG. 4 is an exemplary plot of peak signal power vs. array element number for the array element grouping method of FIG. 3; 
     FIG. 5 is a diagram of a circular array for the multi-beam phased array transmitting antenna of FIG. 1 illustrating an aperture partitioning method according to an embodiment of the present invention; 
     FIG. 6 is an exemplary plot of peak signal power vs. array element number for the aperture partitioning method of FIG. 5; 
     FIG. 7 is a diagram of a stepped beam taper for the aperture partitioning method of FIG. 5; and 
     FIG. 8 is a flowchart of the aperture partitioning method of FIG. 5; and 
     FIGS. 9A and 9B are a flowchart for calculating the total power to the array elements and the total number of phase shifters for the aperture partitioning method of FIG.  5 . 
    
    
     Corresponding reference characters indicate corresponding elements throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The following description is presented to disclose the currently known best mode for making and using the present invention. The scope of the invention is defined by the claims. 
     FIG. 1 is a diagram of a transponder platform communications system  100 . In this example, the transponder platform is illustrated as a satellite transponder platform, however, other spaceborne, airborne, and terrestrial transponder platforms may also be used in other embodiments to suit specific applications. Shown in FIG. 1 are a transponder platform  102 , a multi-beam receiving antenna  104 , a multi-beam phased array transmitting antenna  106 , ground transmitters  108 , ground receivers  110 , received beam signals  112 , and transmitted beam signals  114 . 
     In operation, the ground transmitters  108  transmit communications signals to the transponder platform  102  that are received by the multi-beam receiving antenna  104  as received beam signals  112 . The communications signals are then re-transmitted from the multi-beam phased array transmitting antenna  106  as the transmitted beam signals  114  to the ground receivers  110 . 
     FIG. 2 is a diagram of a phase control network  200  for the multi-beam phased array transmitting antenna  106  of FIG.  1 . Shown in FIG. 2 are beam input ports  201 , power dividers  202 , a phase shifter controller  205 , phase shifters  206 , array element power amplifiers  208 , array elements  210 , and power combiners  212 . 
     Each of the received signals  112  from the multi-beam receiving antenna  104  is coupled to one of the corresponding beam input ports  201 . The beam input ports  201  are coupled respectively to the power dividers  202 . The power dividers  202  are coupled respectively to phase shifters  206 . The outputs of the phase shifters  206  are connected to the power combiners  212 . The phase shifter controller  205  sets the amount of phase shift for each phase shift controller to generate each selected beam. The outputs of the power combiners are connected respectively to the array element power amplifiers  208 . The array element power amplifiers  208  may be, for example, solid state power amplifiers (SSPAs). The outputs of the array element power amplifiers  208  are connected respectively to the array elements  210 . The array elements  210  may be, for example, a circular array of uniformly spaced patch antenna elements. 
     In operation, the power dividers  202  split each of the input signals  112  at the beam input ports  201 . The phase coefficients that determine the beam pointing direction are implemented in this example by the phase shifters  206 . The phase shifters  206  are controlled by the phase shifter controller  205 . Phase-shifted signals output from the phase shifters  206  for each beam are summed by the power combiners  212  and amplified by the array element power amplifiers  208 . The outputs of the array element power amplifiers  208  are connected to the array elements  210 , which radiate the transmitted beam signals  114  to the ground receivers  110 . 
     FIG. 3 is a diagram of a single circular array aperture  300  for the multi-beam phased array transmitting antenna  106  of FIG. 1 illustrating conventional array element grouping for transmitting three beams. Shown in FIG. 3 are an array element group  312  for transmitting a beam A, an array element group  314  for transmitting a beam B, and an array element group  316  for transmitting a beam C. While only three beams are shown to simplify the illustration, there are generally many more beams used in a typical communications system. The array elements  210  are shown as the dense grid of squares in the circular array  300 , where each square represents one of the array elements  110 . Other arrangements of array elements may be used to suit specific applications. 
     A typical assignment or partitioning of the array elements  210  used for the beams A, B, and C is shown by the array elements  210  included within the conventional arrangement of array element groups  312 ,  314 , and  316 , respectively. Each of the array element groups  312 ,  314 , and  316  share a common center, so that the array elements  210  in the center of the array are used by all three beams A, B, and C, while the array elements  210  at the edge of the array are scarcely used at all. The peak signal power of the array elements  210  in the center of the array is therefore the sum of the peak signal power of all the beams. Also, the number of phase shifters required for the array elements  210  in the center of the array is equal to the total number of beams. 
     FIG. 4 is an exemplary plot  400  of peak signal power vs. array element number for the conventional array element grouping method of FIG. 3 for 600 array elements and 169 beams. The upper curve  402  shows the peak power for each element, and the lower curve  404  shows the envelope of the peak power for all elements. The maximum peak signal power for the center array elements is about 12 dBW, and the corresponding dynamic range required of the array element power amplifiers  208  is 31.7 dB. The array elements  210  at the center of the array require 169 phase shifters  206 , while those at the edge of the array only require one phase shifter  206 . 
     FIG. 5 is a diagram of a circular array  500  for the multi-beam phased array transmitting antenna  106  of FIG. 1 illustrating aperture partitioning. Shown in FIG. 5 are an array element group  512  for transmitting a beam A, an array element group  514  for transmitting a beam B, and an array element group  516  for transmitting a beam C. In contrast to the conventional array element grouping arrangement illustrated in FIG. 3, array element groups  512 ,  514  and  516  are arranged to minimize the peak signal power of each of the array elements  210 . As a result, the peak signal power from any one of the array elements  210  is the sum of the peak signal power of fewer than all the beams. Also, the number of phase shifters required for the array elements  210  in the center of the array is fewer than the number of beams. A beam uses an array element  210  if the array element  210  lies within the contour defining the array element group. Each array element  210  requires a separate phase shifter  206  and a power combiner input for each beam that uses that array element. The number of phase shifters N k  required for the kth array element  210  may be expressed as                N   k     =       ∑     i   =   1       N   beams                       {           1   ,           if                 beam                 i                 uses                 element                 k               0   ,         otherwise                     (   1   )                         
     The total number of phase shifters required is then given by                N   TOTAL     =       ∑     k   =   1       N   ELEMENTS                       N   k               (   2   )                         
     FIG. 6 is an exemplary plot  600  of peak signal power vs. array element number for the aperture partitioning method of FIG. 5 with 600 array elements and 169 beams. The upper curve  602  shows the peak power for each element, and the lower curve  604  shows the envelope of the peak power for all elements. The maximum peak signal power for any one of the array elements  210  is only about 4.0 dBW, and the corresponding dynamic range required of the array element power amplifiers  208  is only 16.1 dB. While the total number of phase shifters  206  required remains the same, the maximum number of phase shifters  206  required by any single array element  210  is only 67. Also, the maximum number of inputs required by the power combiner  212  is reduced by 60 percent. 
     FIG. 7 is a diagram of a stepped beam taper  700  for one of the groups of the aperture partitioning method of FIG.  5 . Shown in FIG. 7 are array elements radiating no power  702 , array elements radiating a first step power level  704 , array elements radiating a second step power level  706 , array elements radiating a third step power level  708 , and a taper center  710 . The power levels are stepped or tapered with distance from the center of the group, so the center of the group is called the taper center  710 . 
     The maximum array element power may be derived from the power radiated in each beam (P BEAM i ), where I is the beam index. The relative power between the steps of the taper is selected to meet the beam sidelobe requirements of the specific application. The relative power level for each taper step j, where j is the step index, may be defined as                α     STEP                 j       =       power                 level                 of                 elements                 in                 step                 j             maximum                 power                 level                 of                 any                 element                 used               for                 this                 beam                     (   3   )                         
     The power radiated in each beam may be expressed as a sum of the power in each step of the taper:                P     BEAM                 i       =       ∑     j   =   1       N   STEPS                       P       BEAM                 i     ,     STEP                 j                   (   4   )                         
     where P BEAM i, STEP j  is the power in the jth step of the ith beam. The power in each step of the taper is the product of the power per unit area in the step and the area of the array elements that make up the step: 
     
       
           P   BEAM i, STEP j   =W   BEAM i, STEP j   A   BEAM i, STEP j   (5) 
       
     
     where W BEAM i, STEP j  is the power per unit area or power density of the jth step of the ith beam and A BEAM i, STEP j  is the total area of the array elements in the jth step of the ith beam. The power radiated in the ith beam is then given by                P     BEAM                 i       =       ∑     j   =   1       N   STEPS                         W       BEAM                 i     ,     STEP                 j              A       BEAM                 i     ,     STEP                 j                     (   6   )                         
     The power density in the jth step of the ith beam is given by 
     
       
           W   BEAM i, STEP j =α STEP j   W   BEAM i, MAX   (7) 
       
     
     where α STEP J  is the relative power level in the jth step given by (3) and W BEAM i, MAX  is the maximum power density in the ith beam. The power in the ith beam may then be expressed as                P     BEAM                 i       =       W       BEAM                 i     ,   MAX              ∑     j   =   1       N   STEPS                         α     STEP                 j            A       BEAM                 i     ,     STEP                 j                       (   8   )                         
     The maximum power density in the ith beam is then                W       BEAM                 i     ,   MAX       =       P     BEAM                 i           ∑     j   =   1       N   STEPS                         α     STEP                 j            A       BEAM                 i     ,     STEP                 j                       (   9   )                         
     The amount of power radiated into the ith beam by an array element in the jth step of the ith beam is given by                      P     ELEMENT   ,     BEAM                 i     ,     STEP                 j         =       W       BEAM                 i     ,     STEP                 j              A   ELEMENT                   =       α     STEP                 j            W       BEAM                 i     ,   MAX            A   ELEMENT                     (   10   )                         
     where A ELEMENT  is the area of one array element. 
     The total power radiated by one array element is the sum of all the power that the array element radiates for every beam that uses that array element. In terms of the expression in (10), the total array element power is                P     ELEMENT   ,              TOTAL       =       ∑     i   =   1       N   BEAMS                         ∑     j   =   1       N   STEPS                         P     ELEMENT   ,     BEAM                 i     ,     STEP                 j         ·     {           1   ,           if                 array                 element                 is                 in                 step                 j                 of                 beam                 i               0   ,         otherwise                         (   11   )                         
     The maximum array element power is the maximum of the total power for all the array elements. 
     A cost function for minimizing the maximum array element power receives as input an array of taper center locations, evaluates the total array element power for each array element, and returns the maximum of the total power of all the array elements: 
     
       
         cost function=max( P   ELEMENT, TOTAL | all elements )  (12) 
       
     
     An optimization algorithm, such as “miOcin” from the Numerisk Institut described in “Non-Gradient Subroutines for Non-Linear Optimization”, may be used with the cost function (12) to calculate the array of taper center locations for each array element group that minimizes the maximum array element power for all beams. 
     FIG. 8 is a flowchart  800  for the aperture partitioning method of FIG.  5 . Step  802  is the entry point for the flowchart  800 . Step  804  initializes the array of taper center locations. Step  806  calculates the maximum array element power from the cost function. Step  808  tests whether the cost function has converged to a minimum. If yes, the flowchart  800  exits at step  810 . If no, Step  812  calculates the gradient that minimizes the maximum array element power. Step  814  calculates a new array of taper center locations from the gradient direction and transfers control to step  806 . 
     FIG. 9A and 9B are a flowchart  900  for calculating the total power to the array elements and the total number of phase shifters for the aperture partitioning method of FIG.  5 . Step  902  is the entry point for the flowchart  900 . Step  904  initializes the total power and phase shifter count to zero for every element k. Step  906  initializes the beam index i. Step  908  calculates the area of the array elements for each power level step according to 
     
       
           A   BEAM, STEP =( number of array elements in the step )· A   ELEMENT   
       
     
     Step  910  calculates the beam power density W BEAM,MAX  using (9). Step  912  initializes the array element index k. Step  914  determines which taper step array element k is in. Step  916  calculates array element power P ELEMENT,BEAM,STEP  using (10). Step  918  adds the result from step  916  to the total power. Step  920  increments the phase shifter count by one for element k. Step  922  increments the array element index k by one. Step  924  checks whether k exceeds the number of array elements. If no, control is transferred to step  914 . If yes, control is transferred to step  926 . Step  926  increments the beam index i by one. Step  928  checks whether i exceeds the number of beams. If no, control is transferred to step  908 . If yes, the flowchart  900  exits at step  930 . 
     The maximum array element power and corresponding dynamic range requirements of the array element power amplifiers are reduced by almost an order of magnitude using the aperture partitioning described above compared to conventional array element grouping methods. The reduced dynamic range requirement for the array element power amplifiers results in lower cost. Another advantage is that the number of phase shifters and the complexity of the power combiner for the center array elements are substantially reduced. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, other modifications, variations, and arrangements of the present invention may be made in accordance with the above teachings other than as specifically described to practice the invention within the spirit and scope defined by the following claims.