Abstract:
A phased antenna array ( 60 ) for use on a satellite, that employs a density tapering technique for positioning the antenna elements ( 62 ) in the array ( 60 ) to reduce co-channel interference between adjacent cells. Particularly, the spatial position of the various antenna elements ( 62 ) in the array ( 60 ) are spread out so that the center portion of the array ( 60 ) has the highest density of elements ( 62 ), and the outer portion of the array ( 60 ) has the lowest density of elements ( 62 ). Predetermined schemes are used to set the density of the elements ( 62 ) in the array ( 60 ). By providing fewer antenna elements ( 62 ) at the outer portion of the array ( 60 ), the beam side lobes are reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a satellite communications system employing a phased antenna array that provides reduced co-channel interference and, more particularly, to a satellite communications system that employs a phased antenna array having a plurality of antenna elements, where the spatial distribution of the elements has a density taper to reduce beam side lobes and co-channel interference. 
     2. Discussion of the Related Art 
     Various communications systems, such as certain cellular telephone systems, cable television systems, internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals, usually in the form of digital data modulated onto a carrier wave. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and then is retransmitted by the satellite to another satellite or to the Earth as a satellite downlink communications signal to cover a desirable reception area depending on the particular use. The satellite is equipped with an antenna system, such as a phased antenna array system, including one or more arrays of antenna elements or feed horns that receive the uplink signals and transmit the downlink signals to the Earth. 
     FIG. 1 is a schematic block diagram of a transmit phased antenna array system  10  that includes an antenna array  12  having a plurality of array elements  14  for use on a satellite. Each array element  14  includes a phase shifter  16 , a high power amplifier  18 , such as a traveling wave tube amplifier (TWTA) or a solid-state power amplifier (SSPA), a resistor  20 , and an antenna element  22 , such as a feed horn. Only seven antenna elements are shown in this example, but as will be appreciated by those skilled in the art, a typical antenna array will include many antenna elements configured in a predetermined geometric pattern, such as a hexagon or circle. The system  10  includes a source  24  that generates a signal to be transmitted. The signal is sent to a beam forming network (BFN)  26  that distributes the signal to each of the separate array elements  14 . The phase shifters  16  set each of the separated signals to a predetermined phase progression and the amplifiers  18  amplify the signals for transmission. The antenna elements  22  may also generate beams for other downlink signals. 
     Each feed horn directs a separate beam at a certain frequency and at a certain beam intensity. A predetermined combination of the feed horns directs a specific downlink signal to a predetermined coverage cell within a reception area. Each downlink signal will include a main lobe directed to the coverage cell and side lobes that may be directed towards the coverage cell of the main lobe of another downlink signal. If the frequency of the two downlink signals is the same, the side lobes may cause co-channel interference (CCI) with the other cell in the reception area depending on the intensity of the side lobes. The CCI needs to be controlled to minimize bit error rate and maximize the channel data rate and system capacity. By reducing the CCI, the isolation between adjacent cells can be increased. 
     To illustrate this situation, FIG. 2 shows a diagrammatic view of a satellite  30  emitting a plurality of downlink beams  32  from a satellite antenna system  34 , such as a transmit phased array (TPA) of the type discussed above, to a reception area  36  on the Earth. The downlink beams  32  include a main lobe  38  and side lobes  40 . The main lobes  38  are directed towards a particular cell  42  in the reception area  36 . The side lobes  40  may be directed towards the cell  42  for another main lobe. The shape of the combination of the antenna elements  22  transmitting the downlink signal determines the shape of the cell  42 . In this view, the cells  42  are circular shaped but other cell shapes can also be generated, as would be understood to those skilled in the art. 
     The downlink beams  32  are required to be within a particular frequency band based on FFA requirements. Within that frequency band, sub-frequency bands are used to transmit the various beams  32  carrying the digital data. It is desirable to make the sub-frequency bands as wide as possible so that they are able to carry more information, such as for multi-media applications. However, the side lobes  40  of one beam  32  may interfere with the beam  32  for another cell  42  if the beams are using the same sub-frequency band. By using different sub-frequency bands for cells that are adjacent or proximate each other, the CCI can be significantly reduced or eliminated. However, as the bandwidth of the various sub-frequency bands decreases, the amount of information that can be transmitted is limited. Therefore, it is desirable to suppress the side lobes  40  and provide more frequency reuse for adjacent or proximate cells. 
     For phased array antenna elements, the traditional or conventional technique for reducing beam side lobes and CCI is to employ an amplitude-tapering scheme. In amplitude tapering, the various antenna elements in each array have an output intensity or amplitude that is selected based on its location in the array. Particularly, the centrally positioned antenna elements have the highest intensity output, and as the elements get farther from the center of the array, their intensity output is decreased. Therefore, the elements at the outside of the array have less radiating energy, which reduces the energy of the side lobes, which in turn reduces the co-channel interference for those downlink signals using the same frequency band. Various amplitude tapering algorithms are known in the art for determining the actual intensity output of a particular feed horn depending on its location in the array for different applications. Additionally, by providing a tapered amplitude of the beam in this manner, the width of the main lobe increases. 
     Amplitude tapering of the type described above suffers from a number of drawbacks. In one amplitude tapering scheme, different power amplifiers are used for the antenna elements to generate the beams of different intensities to establish the amplitude tapering. Because different amplifiers are required for different amplitudes, a wide variety of amplifier designs are employed in each antenna array. However, the cost of the array increases as the number of amplifier designs increases. 
     In an alternate amplitude tapering scheme, resistors, for example the resistors  20 , are used to attenuate the power output of the particular antenna element to provide the amplitude tapering. In this design, each antenna element employs the same amplifier so that the design is consistent, thus realizing cost savings. However, because power on a satellite is an important resource, it is undesirable to throw away power by using attenuating resistors. If the resistor is positioned before the amplifier, the efficiency of the amplifier may be reduced because it does not operate at its saturation point as is desirable. 
     Although amplitude tapering has been effective for reducing CCI, the drawbacks discussed above have caused phased antenna array designers to investigate additional ways to reduce CCI. It is desirable that all of the power amplifiers be the same for cost efficiency reasons and it is desirable to operate all of the amplifiers in their saturation regions without throwing power away. It is therefore an object of the present invention to provide an improved antenna array to reduce CCI. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a phased antenna array for use on a satellite is disclosed that employs a density tapering technique for positioning the antenna elements in the array to reduce co-channel interference between cells. Particularly, the spatial position of the various antenna elements in the array are spread out so that the center portion of the array has the highest density of elements, and the outer portion of the array has the lowest density of elements. Predetermined schemes are used to set the spatial density of the elements in the array. By providing fewer antenna elements at the outer portion of the array, the beam side lobes are reduced. 
     Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a transmit phased antenna array; 
     FIG. 2 is a diagrammatic view of downlink beams being emitted from a satellite to a particular coverage area on the Earth; 
     FIG. 3 is an illustration of the layout of the antenna elements in a known uniform hexagonal phased antenna array; 
     FIG. 4 is an illustration of the layout of the antenna elements in a density-tapered hexagonal phased antenna array, according to an embodiment of the present invention; 
     FIG. 5 is an illustration of the layout of the antenna elements in a density-tapered circular phased antenna array, according to another embodiment of the present invention; 
     FIGS.  6 ( a ) and  6 ( b ) show boresight radiation patterns for a uniform-tapered, hexagonal array; 
     FIGS.  7 ( a ) and  7 ( b ) show boresight radiation patterns for a density-tapered, hexagonal antenna element array, according to the invention; 
     FIGS.  8 ( a ) and  8 ( b ) show boresight radiation patterns for a density-tapered, circular antenna element array, according to the invention; 
     FIGS.  9 ( a ) and  9 ( b ) show 9° scan radiation patterns for a hexagonal antenna element array having a uniform taper; 
     FIGS.  10 ( a ) and  10 ( b ) show 9° scan radiation patterns for a hexagonal antenna element array having a density taper, according to an embodiment of the present invention; 
     FIGS.  11 ( a ) and  11 ( b ) show 9° scan radiation patterns for a circular antenna element array having a density taper, according to an embodiment of the present invention; 
     FIG. 12 is an illustration of the partitioning of the antenna elements in a hexagonal antenna element array, where the array includes 270 elements separated into three identical sub-arrays; and 
     FIG. 13 is a schematic block diagram of a two-beam density-tapered, antenna element array, according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a density-tapered transmit phased antenna array is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. Particularly, the discussion below includes using the phased array in connection with a satellite communications system. However, the density tapered array of the invention may have applications for other communications systems. 
     FIG. 3 is an illustration of an array  50  of antenna elements  52  in a hexagonal pattern, where the elements  52  have a uniform taper in position. The array  50  is being viewed from a signal emitting end of the array  50 . The spatial pattern shows each of the elements  52  contiguous with each other where the density of the elements  52  is consistent across the entire array  50 . In this example, the side lobes are suppressed by reducing the amplitude of the signals emitted from the elements  52  at an outer parameter of the array  50 , as discussed above. 
     According to the invention, side lobe level (SLL) and co-channel interference is reduced in a phased antenna array by a density tapering technique, instead of the known amplitude tapering technique. FIG. 4 shows an illustration of an antenna array  60  including a plurality of antenna elements  62  arranged in a hexagonal pattern, as shown. The array  60  is density-tapered in that the spatial position of the elements  62  at the center of the array  60  are more closely spaced together than the elements  62  at the outer edge of the array  60 . In this embodiment, there are nine concentric hexagonal rings of elements  62  around a center element  64 , where the inner five rings are substantially contiguous with each other and the outer four rings get progressively farther apart moving from the center of the array  60  towards the outer edge. Other arrays may include more or less rings of elements within the scope of the present invention. 
     In this array configuration, each element  62  generates the same signal intensity, but the outer portion of the array  60  generates less signal intensity because there are less elements  62 , and the center portion of the array  60  generates a greater signal intensity because there are more antenna elements  62 . Therefore, the side lobe level of the combined beam generated by the array  60  is reduced without the need to provide amplitude tapering. Thus, common power amplifiers can be used for each element  62 , and attenuation resistors are not needed to reduce the signal intensity of the outer elements. The array  60  includes the same number of elements as the array  50 , and therefore takes up slightly more space. However, the benefits realized by the advantages discussed above outweigh the increased space requirements. 
     The density tapering of the invention can be extended to other phased arrays that are not hexagonal in shape. FIG. 5 is an illustration of a phased antenna array  70  including a plurality of antenna elements  72 , where the array  70  has a circular pattern. As with the array  60  above, the array  70  includes concentric circular rings of the elements  72  around a center element  74 , where the rings are spaced farther apart from each other moving from an inner portion of the array  72  to an outer edge of the array  70 . The inner five rings are tightly packed together, and then the ring spacing gets increasingly farther apart for the last four rings. Other array patterns can also be employed besides hexagonal and circular, including square arrays and elliptical arrays. 
     Various techniques can be used to determine the element spacing in the density tapered element configuration according to the invention. In one embodiment, the element spacing is determined in the following manner. First, a maximum allowable radius r max  is determined for the entire array and an initial spacing d for the elements is determined. The inner ring r 1  of elements is set to zero and the number of antenna elements is set to one. Then, the radius of each ring of elements is determined by:          r     n   +   1       =       r   n     +     d     f        (     r   n     )                                  
     where r n  is the radius of the n-th ring and f(r n ) is the Taylor amplitude distribution at r n . In this example, the number of the elements in the n-th ring is equal to 6×(n−1). The coordinates of each element is determined in either the hexagonal or circular arrangement. In the case of a circular array, the number of elements in the n-th ring is the same as that of a hexagonal array. 
     Table 1 compares the performance of uniform-tapered, amplitude-tapered and density-tapered TPAs that delivers the same 60 dBW EIRP. Each amplifier associated with each element is operated in the saturation region for maximum efficiency. For the amplitude-tapered TPA, both single SSPA and multiple SSPA approaches are provided. The uniform-tapered TPA has a maximum SLL of −16 dB that is improved by both the amplitude-tapered and density-tapered TPA. The single SSPA amplitude-tapered TPA, however, has poor power efficiency that consumes more spacecraft power and burdens thermal management systems. The multiple SSPA amplitude tapered TPA requires an SSPA that can deliver 2 dB higher power than the one used in the density-tapered TPA, in addition to the multiple SSPA designs required. On the other hand, the density-tapered TPA offers low side lobe radiation patterns, while maintaining a single design of SSPA with high power efficiency. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Amplitude 
                 Amplitude 
                   
               
               
                   
                   
                 Taper 
                 Taper 
               
               
                   
                 Uniform 
                 (Single 
                 (Multiple 
                 Density 
               
               
                   
                 Taper 
                 SSPA Design) 
                 SSPA Designs) 
                 Taper 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 EIRP 
                 60 dBw 
                 60 dBw 
                 60 dBw 
                 60 dBw 
               
               
                 Relative 
                 1.00 
                 1.09 
                 1.09 
                 0.99 
               
               
                 Size 
               
               
                 SLL 
                 −16 dB 
                 −23 dB 
                 −23 dB 
                 −29 dB 
               
               
                 Efficiency 
                 25.0% 
                 10.0% 
                 25.0% 
                 25.0% 
               
               
                 Max. SSPA 
                 30 dBm 
                 32 dBm 
                 32 dBm 
                 30 dBm 
               
               
                 Power 
               
               
                 # of SSPA 
                 1 
                 1 
                 9 
                 1 
               
               
                 Designs 
               
               
                   
               
             
          
         
       
     
     FIGS. 6-8 show boresight radiation patterns for a uniform-tapered hexagonal phased antenna array, a density-tapered hexagon phased antenna array and a density-tapered circular phased antenna array, respectively. Each of FIGS.  6 ( a )- 8 ( a ) show the boresight contour in θ x  and θ y  degrees, where the circle  76  represents the edge of the Earth as viewed from the satellite. FIGS.  6 ( b )- 8 ( b ) show the cut pattern signal radiation pattern in degrees on the horizontal axis and gain in dB on the vertical axis. 
     FIGS. 9-11 show 9° scan contours for a uniform-tapered hexagonal antenna array, a density-tapered hexagonal antenna array, and a density-tapered circular antenna array, respectively. FIGS.  9 ( a )- 11 ( a ) show the 9° scan contour in θ x  and θ y  degrees, and FIGS.  9 ( b )- 11 ( b ) show the cut pattern signal radiation pattern in degrees on the horizontal axis and gain in dB on the vertical axis. It is clear that the density-tapered array suppresses near-in sidelobes and spreads the energy outside the Earth field-of-view, where the side lobes will not interfere with adjacent co-channel regions. 
     FIG. 12 is an illustration of a hexagonal phased antenna array  80  arranged in a density tapered scheme according to the invention. The array  80  is the same as the array  60  with the center antenna element removed. The array  80  is separated into three identical sub-arrays  84 ,  86  and  88 . The array includes 270 elements, where each sub-array  84 - 88  includes 90 elements. Each sub-array  84 - 88  includes 10 antenna elements  82  on opposing sides of the sub-array and nine elements  82  on the other opposing sides of the sub-array  84 - 88 . The sub-arrays  84 - 88  are trapezoidal shaped arrays where the center space is triangular shaped. This design allows the array  80  to be manufactured into three identical sub-arrays to reduce manufacturing costs and the like. 
     FIG. 13 is a schematic block diagram of a multi-beam antenna system  94  that employs the phased array  80  and emits two separate downlink beams. Each of the sub-arrays  84 ,  86  and  88  are represented as sub-arrays  96 ,  98  and  100 , and each of the 270 elements  82  are represented as feed horns  102 . In this multi-beam application, each feed horn  102  emits part of each of the two beams that are combined with the beams from the other horns that generate the downlink signals. The two beams are separated from each other by carrier frequencies, and may be directed in different directions. 
     In order to distribute the signal in each beam to each of the 270 horns in the array, a power divider network is necessary. The first beam is sent to a driver amplifier  106  that amplifies the signal. A three-way power divider 108 divides the signal into three separate signals at the same power level. Each of the three signals from the power divider 108 are then sent to three separate 90-way power divider networks (PDN)  110 ,  112  and  114 . Each PDN  110 - 114  distributes the beam power into ninety separate signals, where one signal is sent to each separate horn  102  in each sub-array  96 ,  98  and  100 . Likewise, the second beam is sent to a driver amplifier  118 , a three-way power divider  120 , and three 90-way PDNs  122 ,  124  and  126  in the same manner as the first beam. The PDNs  122 - 126  also distribute the power to each of the 270 horns  102  in the separate sub-arrays  96 ,  98  and  100 . 
     Each horn  102  in each sub-array  96 - 100  includes two phase shifters  130  and  132 , a high power amplifier  134 , a filter  136  and a polarizer  138 . The phase shifter  130  receives the first beam signal from one of the PDNs  110 - 114  and the second phase shifter  132  receives the second beam signal from one of the PDNs  122 - 126 . The phase shifters  130  and  132  align the particular beam with the predetermined phase progression for that beam. The power amplifier  134  significantly increases the power of the beams for transmission. The filter  136  filters out harmonics and signal noise and the polarizer  138  converts a linearly polarized signal to a circularly polarized signal for transmission if desirable. In this manner, each antenna element  80  separately or simultaneously transmits one of the two signals to be combined with the signals from the other elements  80  in a density-tapered configuration to reduce co-channel interference. 
     The foregoing discuss discloses and describes merely embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.