Patent Publication Number: US-2006002347-A1

Title: Offset satellite communication cell arrays with orthogonal polarizations

Description:
FIELD OF THE INVENTION  
      The present invention relates to satellite communication systems and, in particular, to a communication satellite that provides offset arrays of geographical cells with communication signals of orthogonal polarizations.  
     BACKGROUND AND SUMMARY OF THE INVENTION  
      A conventional communication satellite in geosynchronous orbit has a communication signal receiving system and a communication signal transmitting system. The receiving system includes a satellite receiving reflector that receives multiple communication uplink signals from one or more terrestrial transmitting stations and concentrates the signals at corresponding ones of multiple receiving horns, which pass the communication uplink signals through an input filter system to a satellite low noise amplifier (LNA) and downconverter system.  
      A communication multiplexer system receives the low noise amplified and frequency converted uplink signals and channelizes and routes the signals to the transmitting system for transmission to terrestrial recipient stations. The transmitting system typically includes an amplifier system, which may include traveling wave tube (TWT) amplifiers, to provide high reliability, high power output amplification. The outputs of the high power amplifier system are connected through an output filter system to one or more transmit horns for transmission as downlink signals via a satellite transmit reflector.  
      The conventional communication satellite directs narrow zone communication signals to recipient stations in multiple cells over a satellite telecommunication region. The cells correspond to different geographic areas within the region and may form a dense-packed or “honeycombed,” optionally overlapping, arrangement that minimizes or eliminates the portions of region not covered by a cell. Next adjacent cells typically receive distinct communication signals or sub-bands. However, potentially interfering cells are typically within the telecommunication region. As a result, cells can have interference-induced signal-to-noise ratios (S/Nint) at a maximum of about 20 dB at the center of a cell with decreases to about 12 dB at the peripheries of the cells. A consequence of such a range of signal-to-noise ratios within a cell is that signal integrity and reliability is decreased at the cell peripheries.  
      Accordingly, the present invention includes a communication satellite having multiple communication signal amplifiers coupled to multiple transmit horns that transmit communication signals to multiple corresponding geographic cells. The satellite includes a first set of communication signal amplifiers and transmit horns that cooperate to deliver multiple distinct communication signals of a first polarization to a first array of multiple adjacent geographic cells having interstices or intersections between them.  
      The satellite also includes a second set of communication signal amplifiers and transmit horns that cooperate to deliver multiple distinct communication signals of a second polarization to a second array of multiple adjacent geographic cells having interstices or intersections between them. The first and second polarizations are orthogonal to each other, such as horizontal and vertical polarizations or right- and left-circular polarizations. In addition, the adjacent geographic cells of the first array are generally centered at the interstices between and overlap the cells of the second array, and the cells of the second array are generally centered at the interstices between and overlap the cells of the first array.  
      The satellite of the present invention provides reduced interference between cells that receive potentially interfering signals by interposing cells that receive non-interfering signals. As a result, communication signals are delivered with increased uniformity in the signal-to-noise ratios, which can noticeably improve signal reliability or allow the finite communication signal power to be allocated to more areas.  
      Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a prior art implementation of a communication satellite for geosynchronous orbit.  
       FIG. 2  is a prior art illustration of a satellite telecommunications region having multiple cells.  
       FIGS. 3 and 4  illustrate respective first and second cell patterns of geographic cells that receive communication signals with orthogonal polarizations according to the present invention.  
       FIGS. 5A and 5B  illustrate an overlapping arrangement of the cell patterns of  FIGS. 3 and 4 .  
       FIG. 6  illustrates a close-packed arrangement of central regions of cells in the overlapping arrangement of the cell patterns in  FIGS. 5A and 5B .  
       FIG. 7  is an illustration of an array of geographic cells arranged in a right-regular array with lower packing efficiency.  
       FIG. 8  is a circuit block diagram of a satellite communication signal transmitting system.  
       FIG. 9  is a circuit block diagram of a satellite communication signal receiving system. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  is a block diagram of a prior art implementation of a communication satellite  10  for geosynchronous orbit, as described in U.S. Pat. No. 6,275,479 and assigned to Spacecode LLC, the assignee of the present invention. Communication satellite  10  has a communication signal receiving system  12  and a communication signal transmitting system  14 . Receiving system  12  includes a satellite receiving reflector  16  that receives multiple communication uplink signals from one or more terrestrial transmitting stations and concentrates the signals at corresponding ones of multiple receiving horns  18 . Receiving horns  18  pass the communication uplink signals through an input filter system  20  to a satellite low noise amplifier (LNA) and downconverter system  22  having multiple individual receivers  24 . Each of the uplink communication signals may include multiple separate signals.  
      Low noise amplifier (LNA) and downconverter system  22  would typically include more individual receivers  24  than are necessary for the number of signals or channels to be handled by satellite  10 . The additional receivers  24 , or other components, provide redundancy and may be utilized upon the failure of any individual component. Such redundancy is typically utilized in satellite design and may be applied as well as in other systems within satellite  10  that are described below.  
      Accordingly, low noise amplifier (LNA) and downconverter system  22  includes switching arrays to route each channel of the uplink signal to the corresponding active receivers  24  that provide pre-amplification of the uplink communication signals and convert them to another (e.g., lower) frequency. For example, uplink signals may be Ku-band signals (i.e., about 14 GHz) or V-band signals (i.e. about 49-50 GHz), which may be converted to lower Ku-band frequencies (e.g., 11-12 GHz). A communication multiplexer system  26  receives the low noise amplified and frequency converted uplink signals and channelizes and routes the signals to appropriate ones of redundant high power amplifiers in a high power amplifier system  28  in transmitting system  14  for transmission to terrestrial recipient stations. In an implementation utilizing FDMA routing techniques, multiplexer  26  channelizes and routes the signals according to their carrier frequencies.  
      Amplifier system  28  may employ, for example, driver amplifiers  30  with associated traveling wave tube amplifiers  32 . Traveling wave tube amplifiers  32  provide high reliability, high power output amplification. The outputs of high power amplifier system  28  are connected through an output filter system  34  to one or more transmit horns  36  for transmission as a downlink signal via a satellite transmit reflector  38 . A control unit  40  is bus connected to various ones of these components to control their operation and interaction. The satellite includes power sources, orientation and position control systems, communication control systems, etc. as are known in the art.  
       FIG. 2  is a prior art illustration of a satellite telecommunications region  50  having multiple cells  60  (represented by circles) to which prior art satellite  10  directs narrow zone communication signals to recipient stations. Cells  60  correspond to different geographic areas within region  50 . Different groups of cells  60  receive downlink signals carried on different channels. In some applications, for example, the downlink signal carried on a single channel could be directed to a single cell  60 . As is known in the art, transmit horns  36  are arranged in relation to transmit reflector  38  to transmit particular communication signals to particular ones of cells  60 .  
      Prior art  FIG. 2  illustrates geographic cells  60  with recipient stations that receive narrow zone communication downlink signals carried on different channels with 1×3 multiplexing, as described below in greater detail. Cells  60  are designated by alpha-numeric indicators that correspond to particular multiplexed communication channel sub-bands. For example, cells  60  designated A- 1 , A- 2 , and A- 3  could receive respective communication sub-bands 12.200-12.367 Ghz, 12.367-12.533 Ghz, and 12.533-12.700 Ghz from a TWT amplifier  132 A ( FIG. 8 ). Similarly, each of the remaining cells  60  with the numeric suffices −1, −2, and −3 could receive respective communication sub-bands 12.200-12.367 Ghz, 12.367-12.533 Ghz, and 12.533-12.700 Ghz from a corresponding TWT amplifier  32  having a matching alphabetic designation of B-T. Accordingly, all of cells  60  having the same numeric suffix −1, −2, or −3 receive the same communication channel sub-band, although typically different communication signals.  
      The generally 50 percent lateral offset between successive rows of cells  60  provides a dense-packed or “honeycombed,” optionally overlapping, arrangement that minimizes (as shown) or eliminates the portions of region  50  not served by a satellite  10 . In addition, with at least 1×3 multiplexing of TWT power amplifiers  32 , cells  60  can be arranged to provide spatial separation between cells designated to receive the same channel sub-band. As a result, no two adjacent cells is designated to receive the same channel sub-band. This can be seen from the absence of any two adjacent cells having the same numeric suffix −1, −2, or −3. This eliminates interference between adjacent cells  60  because recipient stations in adjacent cells are tuned to receive different communication channel sub-bands.  
      With reference to an arbitrary cell I- 3  (outlined in bold), for example, the immediately adjacent cells G- 2 , I- 2 , and L- 2  and G- 1 , J- 1 , and L- 1  operate at different sub-bands that do not interfere with cell I- 3 . It will be appreciated, however, that the next-adjacent cells D- 3 , F- 3 , G- 3 , K- 3 , L- 3 , and N- 3  (illustrated as being centered about a dashed-line circle) receive the same sub-band and thereby can cause discernible interference with cell I- 3 . The signal-to-noise ratio (S/N) within cell I- 3  may be represented as: 
 
S/N=C/(N+Int), 
 
 where C is the carrier power of the sub-band directed to cell I- 3 , N is the noise, and Int is the interference from the next-adjacent cells (i.e., cells D- 3 , F- 3 , G- 3 , K- 3 , L- 3 , and N- 3 ) receiving the same sub-band. 
 
      With noise N being a generally the same across cells, the interference-induced signal-to-noise ratio (S/N int ) may be represented as: 
 
S/N int =C/Int. 
 
 Within this context, for example, the interference-induced signal-to-noise ratio (S/N int ) is a maximum of about 20 dB at the center of cell I- 3  and decreases to about 12 dB at the periphery of the cell. A consequence of such a range of signal-to-noise ratios within a cell is that signal integrity and reliability is decreased at the cell peripheries. 
 
       FIGS. 3 and 4  illustrate respective first and second cell patterns  80  and  82  of geographic cells  84  and  86  according to the present invention where recipient stations receive narrow zone communication downlink signals carried on different channels with 1×3 multiplexing, as described below in greater detail. Cells  84  and  86  are designated by alpha-numeric indicators that correspond to particular multiplexed communication channel sub-bands.  
      Cells  84  and  86  are drawn with hexagonal configurations to provide graphic illustration of the effective coverage areas provided by a close-packed arrangement of the cells. It will be appreciated, however, that the downlink beams transmitted to cells  84  and  86  would typically encompass generally circular geographical regions.  
      For example, cells  84  and  86  designated A- 1 , A- 2 , and A- 3  could receive respective communication sub-bands 12.200-12.367 Ghz, 12.367-12.533 Ghz, and 12.533-12.700 Ghz. Similarly, each of the remaining cells  84  and  86  with the numeric suffices −1, −2, and −3 could receive respective communication sub-bands 12.200-12.367 Ghz, 12.367-12.533 Ghz, and 12.533-12.700 Ghz. All of cells  84  and  86  having the same numeric suffix −1, −2, or −3 receive the same communication channel sub-band, although typically different communication signals. In addition, each of cells  84  and  86  includes a suffix X or Z indicating that the cell receives downlink signals with a first or a second polarization. The first and second polarizations are orthogonal to each other, such as vertical and horizontal polarizations or right- and left-circular polarizations.  
       FIGS. 5A and 5B  illustrate an overlapping arrangement  90  of cell patterns  80  and  82 . For purposes of clarity in the illustrations,  FIG. 5A  shows cell pattern  80  with overlapping cell pattern  82  drawn with dashed lines, and  FIG. 5B  shows cell pattern  82  with overlapping cell pattern  80  drawn with dashed lines.  
      In  FIG. 5A  cells  86  designated of cell arrangement  82  are labeled parenthetically, and in  FIG. 5B  cells  84  designated of cell arrangement  82  are labeled parenthetically. Overlapping arrangement  90  in  FIGS. 5A and 5B  represents the same combination of cell patterns  80  and  82 , but only one of cell patterns  80  and  82  is rendered in detail at a time to avoid excessive clutter in the drawings.  
      As illustrated in  FIG. 5A , cells  84  (in solid lines) of cell arrangement  80  ( FIG. 3 ) are positioned to be generally centered at about the intersections or interstices  92  between adjacent cells  86  (in dashed lines) of cell arrangement  82  ( FIG. 4 ). In the illustrated implementation, cells  84  (in solid lines) are generally centered at about the intersections or interstices  92  between a two-dimensional group  94  of adjacent cells  84 . Group  94  is two-dimensional in that the adjacent cells  84  are not co-linear with each other and, as a result, include more than just one adjacent pair of cells  84 .  
      For example, a cell  84  designated N- 1 X is centered at an interstice  92 A of a two-dimensional group  94 A of cells  86  designated E- 2 Z, M- 3 Z, and N- 1 Z. Likewise, a cell  84  designated N- 3 X is centered at an interstice  92 B of a two-dimensional group  94 B of cells  86  designated F- 1 Z, N- 2 Z, and N- 3 Z.  
      As illustrated in  FIG. 5B , cells  86  (in solid lines) of cell arrangement  82  ( FIG. 4 ) are positioned to be generally centered at about the intersections or interstices  96  between adjacent cells  84  (in dashed lines) of cell arrangement  80  ( FIG. 3 ). In the illustrated implementation, cells  86  (in solid lines) are generally centered at about the intersections or interstices  96  between a two-dimensional group  98  of adjacent cells  86 . Group  98  is two-dimensional in that the adjacent cells  86  are not co-linear with each other and, as a result, include more than just one adjacent pair of cells  86 .  
      For example, a cell  86  designated A- 2 Z is centered at an interstice  96 A of a two-dimensional group  98 A of cells  84  designated A- 2 X, A- 3 X, and H- 1 X. Likewise, a cell  86  designated B- 1 Z is centered at an interstice  96 B of a two-dimensional group  98 B of cells  84  designated B- 1 X, B- 2 X, and H- 3 X.  
      With reference to  FIG. 5A , cells  84  (in solid lines) centered at about interstices  92  between cells  86  (in dashed lines) have central regions  100  (illustrated by triangles). In central regions  100  cells  84  receive generally maximal power levels and cells  86  of each group  94  receive generally minimal power levels along their respective edges. As a result, central regions  100  are have relatively high signal-to-noise ratios and minimal interference due to the large power disparity between and the orthogonal polarizations of the corresponding cell  84  and overlapped group of cells  86 .  
      With reference to  FIG. 5B , cells  86  (in solid lines) centered at about interstices  96  between cells  84  (in dashed lines) have central regions  102  (illustrated by triangles). In central regions  102  cells  86  receive generally maximal power levels and cells  84  of each group  98  receive generally minimal power levels along their respective edges. As a result, central regions  102  are have relatively high signal-to-noise ratios and minimal interference due to the large power disparity between and the orthogonal polarizations of the corresponding cell  86  and overlapped group of cells  84 .  
       FIG. 6  illustrates a resulting close-packed arrangement of central regions  100  and  102  of respective cells  84  and  86 .  FIG. 6  corresponds, therefore, to the combination of central regions represented in  FIGS. 5A and 5B .  
      Central regions  100  of the cells  84  in array  80  are substantially surrounded by the central regions  102  of the cells  86  in array  82 . Likewise, central regions  102  of the cells  86  in array  82  are substantially surrounded by the central regions  100  of the cells  84  in the array  80 . For example, a central region  100  designated F- 1 X of a cell  84  in array  80  is substantially surrounded by central regions  102  designated H- 2 Z, E- 3 Z, and F- 1 Z of cells  86 .  
      As described above, the designations −1, −2, and −3 for cells  84  and  86  represent distinct communication channel sub-bands. As illustrated in  FIG. 3 , immediately adjacent geographic cells  84  of array  80  do not receive communication signals on the same communication sub-bands. As illustrated in  FIG. 4 , immediately adjacent geographic cells  86  of array  82  do not receive communication signals on the same communication sub-bands.  
      With these arrangements of communication channel sub-bands among the cells  84  and  86 , the central region of a cell (e.g., designated F- 1 X in  FIG. 6 ) of a selected polarization (e.g., polarization X) and a selected communication sub-band (e.g., sub-band −1) is separated from an adjacent central region of a cell of the same selected polarization and selected communication sub-band (e.g., cells designated N- 1 X, H- 1 X, B- 1 X, J- 1 X, and P- 1 X) by at least the full extent of the central region of a cell having a polarization or a sub-band different from the selected polarization and the selected communication sub-band (e.g., cells designated E- 3 Z, H- 1 Z, H- 2 Z, H- 3 Z, and F- 1 Z).  
       FIG. 6  illustrates that separation between a selected cell designated F- 1 X and cells of the same polarization and communication sub-band by a circle  104 . This separation of each cell central region  100  or  102  from a potentially interfering cell central region by the full extent of a non-interfering cell central region decreases interference between cells and thereby increases their signal-to-noise ratios. In contrast, prior close-packed arrangements of cells as illustrated in  FIG. 2  provide separation between potentially interfering cells of only about 70% of the extent of an interposed cell. For example, the cell  60  designated I- 3  in  FIG. 2  is separated from a cell designated K- 3  (and the other interfering cells) by about 70% of the extent or diameter of the cells designated I- 2  and L- 1 .  
      In one exemplary implementation, cells  84  and  86  could have a peak signal-to-noise ratio of about 20 dB within their respective central regions  100  and  102 , and decreased signal-to-noise ratios of about 12 dB at their peripheries or edges. Use of central regions  100  and  102  from first and second arrays of cells with orthogonal polarizations as illustrated in  FIG. 6  results in improved signal-to-noise ratios at the edges of central regions  100  and  102 . In one implementation, the signal-to-noise ratios at the edges of central regions  100  and  102  improve by about 1.5 dB over the signal-to-noise ratios of prior cell arrangements as illustrated in  FIG. 2 . Such an incremental improvement in signal-to-noise ratio uniformity can noticeably improve signal reliability or allow the finite communication signal power to be allocated to more areas.  
      The present invention has been described with reference to close-packed arrays  80  and  82  of respective cells  84  and  86 . It will be appreciated, however, that cells  84  and  86  could alternatively be arranged in arrays lower packing efficiencies. For example, cells  84  and  86  could each be arranged in a array with the centers of the cells aligned along perpendicular lines.  FIG. 7  is an illustration of one such array  110  with geographic cells  112  arranged in a right-regular array with lower packing efficiency.  
       FIG. 8  is a circuit block diagram of a communication signal transmitting system  114  including diplexers  129  that combine and deliver to horns  136  combined communication signals of first and second orthogonal polarizations, referred to as orthogonal communication signals. Communication signal transmitting system  114  includes multiplexed traveling wave tube (TWT) amplifiers  132  (only three shown) according to the present invention that receive the orthogonal communication signals. Each TWT amplifier  132  is multiplexed among three transmit horns  136 . Each transmit horn  136  transmits the orthogonal communication signals as downlink communication signals to corresponding cells  84  or  86  ( FIGS. 3 and 4 ). It will be appreciated, however, that the illustrated 1×3 multiplexing is merely exemplary and that greater degrees of multiplexing can be applied to TWT power amplifiers  132 .  
      With reference to TWT amplifier  132 A, for example, three output frequency filters  134 A- 1 ,  134 A- 2 , and  134 A- 3  pass different portions or segments of a given output frequency band for signals of each polarization to respective transmit horns  136 A- 1 ,  136 A- 2 , and  136 A- 3 . As one example, each TWT amplifier  132 , including TWT amplifier  132 A, is adapted to amplify and transmit all of the nominal 500 MHz bandwidth of a Ku-band downlink communication channel. Accordingly, output frequency filters  134 A- 1 ,  134 A- 2 , and  134 A- 3  pass signals with frequencies within different nominal 167 MHz sub-bands of the Ku-band channel. With a Ku-band downlink communication channel of 12.200-12.700 GHz, frequency filter  34 A- 1  could pass communication signals for frequencies in the sub-band 12.200-12.367 Ghz, frequency filter  134 A- 2  could pass communication signals for frequencies in the sub-band 12.367-12.533 Ghz, and frequency filter  134 A- 3  could pass communication signals for frequencies in the sub-band 12.533-12.700 Ghz. It will be appreciated that references to the KU-band downlink communication channel is only illustrative and is not a limitation on the scope of application for transmitting system  114 .  
      In an alternative implementation, a communication signal transmitting system could employ first and second separate sets of transmit horns and first and second reflectors to accommodate the respective communication signals of first and second orthogonal polarizations.  
       FIG. 9  is a circuit block diagram of a communication signal receiving system  120  with receivers  124  (only three shown) that are multiplexable and receive communications signals of first and second orthogonal polarizations, referred to as orthogonal communication signals.  
      In the illustrated implementation, each of receivers  124 A- 124 C is multiplexable among three receive horns  118 . Each receive horn  118  receives from a transmitting station a combined uplink communication signal that is separated into orthogonal communication signals by a diplexer  119  to be transmitted to different respective cells. It will be appreciated that such 1×3 multiplexing is merely exemplary and that different degrees of multiplexing, or no multiplexing at all, can be applied to receivers  124 A- 124 C.  
      With reference to receiver  24 A, for example, diplexers  119 A- 1 ,  119 A- 2 , and  119 A- 3  deliver to respective input frequency filters  120 A- 1 ,  120 A- 2 , and  120 A- 3  different portions or segments of given uplink frequency bans of the orthogonal polarizations. As one example, each of receivers  124 A- 124 C, including receiver  124 A, is adapted to receive and amplify all of the nominal 500 MHz bandwidth of a Ku-band uplink communication channel of each polarization. Accordingly, input frequency filters  120 A- 1 ,  120 A- 2 , and  120 A- 3  pass signals with frequencies within different nominal 167 MHz sub-bands of the Ku-band channel for each polarization. With a Ku-band downlink communication channel of 12.200-12.700 GHz, frequency filter  120 A- 1  could pass communication signals for frequencies in the sub-band 12.200-12.367 Ghz, frequency filter  120 A- 2  could pass communication signals for frequencies in the sub-band 12.367-12.533 Ghz, and frequency filter  120 A- 3  could pass communication signals for frequencies in the sub-band 12.533-12.700 Ghz. It will be appreciated that references to the KU-band uplink communication channel is only illustrative and is not a limitation on the scope of application for receiving system  120 .  
      In an alternative implementation, a communication signal receiving system could employ first and second separate sets of receive horns and first and second reflectors to accommodate the respective communication signals of first and second orthogonal polarizations.  
      The present invention has been described with respect to embodiments in which multiple signals of first and second orthogonal polarizations are transmitted to overlapping first and second arrays of cells. The overlapping arrays of cells receiving orthogonal polarizations decrease interference between adjacent cells. It will be appreciated that in alternative implementations multiple signals of distinct first and second signal bands or frequency ranges may be transmitted to overlapping first and second arrays of cells. In these alternative implementations the distinct or different signal bands or frequency ranges would provide non-interfering signal distinctions, rather than relying upon signal polarizations. These alternative implementations could employ substantially the same structural elements described above with reference to implementations utilizing orthogonal polarizations.  
      In view of the many possible embodiments to which the principles of our invention may be applied, it should be recognized that the detailed embodiments are illustrative only and should not be taken as limiting the scope of our invention. Accordingly, the invention includes all such embodiments as may come within the scope and spirit of the following claims and equivalents thereto.