Patent Publication Number: US-9853716-B2

Title: Multibeam coverage for a high altitude platform

Description:
PRIORITY CLAIM 
     The present application is a divisional of, claims priority to and the benefit of U.S. patent application Ser. No. 14/510,790, now U.S. Pat. No. 9,401,759, filed on Oct. 9, 2014, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Satellites provide communication coverage over a specified surface area on Earth. As discussed herein, a cell is a geographical coverage area on the surface of the Earth and a spot beam is a radiation pattern of an antenna that illuminates a cell. A surface spectral density (Hertz per square kilometer (“Hz/km 2 ”) within the coverage area is typically increased by increasing the number of radiated spot beams to partition the coverage area into multiple cells and reusing the available spectrum many times. For instance, dividing an area previously covered by one broad beam into 19 cells covered by 19 narrow spot beams and splitting the frequency spectrum into four equal parts (and reusing the spectrum in smaller cells) results in a surface spectral density that is increased by a factor of 19/4 or nearly five-times. To provide broad and uniform coverage with a high surface spectral density, the satellite or telecommunications platform accordingly may use a plurality of antennas such that each antenna is configured to provide similar communication coverage (e.g., a spot beam) to a cell. It is also common practice to create multiple beams from a single antenna by using more than one duplex feed for each antenna. Phased array and beamforming techniques are also well-known. 
     Generally, antennas with identical dimensions and properties are chosen to reduce design variation. However, the variation of distances between a platform and various cells, resulting in part from the curvature of the Earth, leads to differently sized cells from identical antennas. For instance, cells directly below a platform are relatively smaller compared to cells at the edges of a specified coverage area. If the same amount of spectrum is used in each cell then the surface spectral density is greater for the smaller cells below the platform compared to the surface spectral density of the larger cells at the outer edges of coverage. Differences in surface spectral density between cells can result in service disruptions and/or service degradations or a difference in the perceived quality of service as a user moves between cells or as the cells move past the users. 
     In geostationary Earth Orbit (“GEO”) satellite systems, the effects of cell size differences are generally not severe or noticeable. For instance, at the geostationary orbit height of 35,786 km above the surface, Earth subtends an angle of only 20°. This means that except at the extreme edges of satellite coverage where the surface curves away from the satellite, cells within a coverage area are generally uniform. However, for satellites at lower altitudes, the differences between cell sizes are more pronounced. For instance, Earth subtends an angle of about 160° relative to a high altitude platform operating between 17 km and 22 km above the surface. This greater angle causes significant differences in cell areas covered by the same antennas. Such differences in cell size can affect the quality of service (“QoS”) because coverage in the cells located on the perimeter of the coverage area is subject to higher path loss and lower surface spectral density, thereby resulting in lower user bandwidth. 
     GEO satellites are stationary relative to a point on Earth but satellites in other orbits move relative to fixed points on Earth. For the latter systems, therefore even stationary user terminals will be served by different spot beams over time, i.e., the cells move with the telecommunications platform. If different cells use the same amount of frequency spectrum but have different sizes, the available bandwidth for each user terminal changes, which may cause service disruptions. 
     SUMMARY 
     The present disclosure provides a new and innovative system, method, and apparatus for providing multi-beam coverage to connect user terminals to the Internet via gateway stations using a telecommunications platform such as a high altitude platform (“HAP”) or a Low Earth Orbit (“LEO”) satellite platform. The example system, method, and apparatus disclosed herein use differently dimensioned antennas (e.g., antennas with differently sized apertures) to create substantially equal-sized cells (and/or cells with the same surface spectral density) within a specified user terminal coverage area. Communications links with the one or more gateway stations are provided by one or more stationary spot beams from a mechanically or electronically pointed antenna. The differently dimensioned/configured antennas provide corresponding different spot beams that compensate for the distance and subtended angle of each cell. This configuration of using differently dimensioned antennas produces a surface spectral density that is generally constant throughout a coverage area of the platform, which provides substantially uniform communication coverage among the different cells and a consistent perceived quality of service. Different sized antenna apertures may also be achieved with a single antenna with multiple feeds or a suitably designed phased array antenna. 
     In an example embodiment, a telecommunications platform or transceiver apparatus includes a plurality of antennas configured to provide communication links between a gateway station and a plurality of user terminals within a specified coverage area on the ground, each user link antenna being configured to communicate with a specified cell within the specified coverage area and each gateway link antenna being configured to communicate with a specific gateway. A system configuration management apparatus includes logical links to platforms, gateways and user terminals to receive status and other management information and to download configuration and provisioning parameters. The system configuration manager also includes system design planning and updating functions. As discussed herein, planning includes the design of cell numbers and sizes and an assigned frequency plan. Based on the cell sizes, the system configuration manager may design differently sized aperture antennas for the telecommunications platform (satellite or HAPS) apparatus to maintain a similar surface spectral density among the cells within the specified area to meet specified design criteria. 
     In another example embodiment, a method to provision a telecommunications apparatus includes determining an altitude at which the telecommunications apparatus will operate and determining a minimum elevation angle from the ground to the telecommunications apparatus. The method also includes determining a coverage area of the telecommunications apparatus based on the altitude and the minimum elevation angle, partitioning the coverage area into substantially equal-sized cells, and assigning an antenna to each of the cells. The method further includes determining a beamwidth and an elevation angle for each antenna to provide communication coverage to the corresponding cell, and determining an aperture for each of the antennas based on the beamwidth and the elevation angle. This method may also include determining an optimal design for a platform that does not maintain a constant altitude, which causes the cell sizes on the ground to vary. 
     Additional features and advantages of the disclosed system, method, and apparatus are described in, and will be apparent from, the following Detailed Description and the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a diagram of prior art equal-sized HAP antenna apertures and consequent coverage area on the surface of the Earth. 
         FIG. 2  shows a diagram of an example telecommunications communication system, according to an example embodiment of the present disclosure. 
         FIG. 3  shows a diagram of an example HAP coverage area for a HAP discussed above in conjunction with  FIG. 1 , according to an example embodiment of the present disclosure. 
         FIG. 4  shows a diagram of a coverage area of a prior art HAP that includes antennas of the same size. 
         FIG. 5  shows a diagram of a spherical hexagonal pattern incident on a u-v surface. 
         FIG. 6  shows a table of properties of the coverage area of  FIG. 4 . 
         FIG. 7  shows a diagram of an example coverage area that includes  19  similar sized cells, according to an example embodiment of the present disclosure. 
         FIG. 8  shows the example coverage area of  FIG. 7  with uniform cells, according to an example embodiment of the present disclosure. 
         FIG. 9  shows the uniform cells of  FIG. 7  as they project back onto a spherical u-v surface around the platform of  FIG. 2  and as they appear on the surface of the Earth, according to an example embodiment of the present disclosure. 
         FIG. 10  shows a table of properties of the cells and corresponding antennas discussed above in conjunction with  FIGS. 7 and 8 , according to an example embodiment of the present disclosure. 
         FIG. 11  shows a diagram of differently sized antennas of the HAP of  FIG. 2  used to produce the similarly sized cells of  FIGS. 7 and 8 , according to an example embodiment of the present disclosure. 
         FIG. 12  shows a diagram of parametric inputs to a design process implemented by the system configuration manager of  FIG. 2 , according to an example embodiment of the present disclosure. 
         FIG. 13  shows a diagram of a graph of representative atmospheric gas attenuation and rain attenuation for  28  GHz using an ITU reference atmospheric model, according to an example embodiment of the present disclosure. 
         FIG. 14  shows a diagram of modulation and coding schemes used in the DVB-S2 standard, according to an example embodiment of the present disclosure. 
         FIG. 15  shows a diagram of exemplary results of the example described in conjunction with  FIGS. 12 to 14 , according to an example embodiment of the present disclosure. 
         FIG. 16  shows a diagram of surface spectral density as a function of elevation angle throughout a coverage area specified by the example described in conjunction with  FIGS. 12 to 15 , according to an example embodiment of the present disclosure. 
         FIG. 17  shows a diagram of the coverage area including the location of the labeled points in the diagram of  FIG. 16 , according to an example embodiment of the present disclosure. 
         FIG. 18  shows a diagram of an example coverage area that includes different sized cells with the same spatial spectral density, according to an example embodiment of the present disclosure. 
         FIG. 19  shows a table of properties of the cells and corresponding antennas discussed above in conjunction with  FIG. 18 , according to an example embodiment of the present disclosure. 
         FIG. 20  shows a diagram of an example coverage area that includes  37  similar sized cells, according to an example embodiment of the present disclosure. 
         FIG. 21  shows a table of properties of the cells and corresponding antennas discussed above in conjunction with  FIG. 20 , according to an example embodiment of the present disclosure. 
         FIG. 22  shows a diagram of an example coverage area that includes sectored outer cells, according to an example embodiment of the present disclosure. 
         FIG. 23  illustrates a flow diagram showing an example procedure to configure antennas on a HAP to produce cells with a substantially uniform spatial spectral density, according to an example embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates in general to a method, apparatus, and system to provide multi-beam coverage by a telecommunications platform. As disclosed herein, the term ‘platform’ may refer to any Low Earth Orbit (“LEO”) satellite, Medium Earth Orbit (“MEO”) satellite, Geosynchronous Earth Orbit (“GEO”) satellite, and/or High Altitude Platform (“HAP”). A HAP may include any airship, airplane, balloon, etc. operating between, for example, 17 km and 22 km over the surface. 
     The example method, apparatus, and system disclosed herein use antennas with differently sized apertures to provide substantially consistent coverage among cells of a specified area. For instance, antennas may be configured to have different apertures so as to cover cells of relatively the same area and/or cells with substantially the same surface spectral density. The techniques described herein achieve uniform coverage using individual antennas per spot beam. However, the techniques are also extensible to phased arrays and/or multiple single feed per beam arrays. As discussed herein, antenna aperture is an effective area (orientated perpendicular to a direction of an incoming (or outgoing) radio wave) representative of how effective an antenna is at receiving (or transmitting) radio waves. Typically the effective area of an antenna is 55% of the actual physical area of the antenna aperture. 
     HAPs using antennas configured to support communications with the same sized aperture have been envisioned since the 1970s. However, technology to support high-speed and reliable wireless communication has not become available until recently. Additionally, technology to maintain HAPs within the air for extended periods of time has only recently become available. For instance, the energy density, weight, and size of batteries, fuel cells, and solar cells have become advanced enough to support continuous operation of an airship or blimp in the sky for 30 to 60 days or more. 
     HAPs have several potential advantages compared to higher altitude satellites. For instance, HAPs generally have a relatively low communication latency in the 100&#39;s of microseconds (“μsec”) compared to latencies of 100&#39;s milliseconds (“msec”) for GEO satellites and  10 &#39;s msec for LEO satellites operating over 500 km. Additionally, HAPs have a shorter product development cycle time compared to satellites, which require space qualification in addition to engineering design that ensures continuous operation for an extended period of time (e.g., ten years). Also, launching a few GEO satellites or a large constellation of LEO satellites can be very expensive and high risk. This means that HAPs may be developed with less upfront capital investment than satellites. HAPs may also be repaired and/or upgraded relatively easily by landing the HAPs for service. In comparison, satellites cannot generally be repaired or upgraded once launched into space. 
     Further, HAPs may be provisioned one at a time so that a HAP-based communication system can be rolled out to different geographic areas at different times without affecting performance of other HAPs within the system. In contrast to HAPs, satellites are expensive and generally take several years to design, build, qualify, and launch before service can begin. LEO satellite systems also generally require that all satellites be provisioned at the same time to provide system wide coverage. 
     Another disadvantage of satellites is that there is generally too much capacity provided in low usage areas. Satellites have coverage areas that are relatively large where a sizable portion of the coverage area includes oceans, lakes, deserts, forests, and protected lands that have few (if any) users. Additionally, some LEO satellites spend a significant amount of time orbiting over oceans and other uninhabited areas. Since a sizeable portion of the coverage area (and consequently bandwidth) is provided to sparsely populated areas, satellites have trouble providing enough capacity in relatively small high usage areas where the amount of bandwidth for that area is limited. In contrast, HAPs are deployed where there are large concentrations of users (e.g., cities), thereby providing service where there is the greatest demand/need. 
     A further disadvantage of satellites is the power and antenna size needed to provide high QoS communications. Satellites are generally thousands of kilometers above the surface, which requires high power output per antenna and larger antenna sizes to maintain acceptable QoS parameters. HAPs in contrast are much closer to the surface (e.g., 17 km to 22 km) and can provide the same (or better) QoS with lower power and smaller antennas. 
     Some HAPs have been proposed that use satellite communication technology including antennas and transceivers. These HAPs accordingly have antennas (or antenna elements) of the same size to provide spot beams to respective cells of a coverage area. These antennas provide spot beams with the same beam widths. However, the size of each cell under this antenna configuration varies significantly based on the location of the cell relative to the HAP. This cell area difference becomes larger for cells at the perimeter of the coverage area and becomes significantly pronounced for HAPs that operate at less than 30 km from the surface. 
     For example, a typical HAP that operates 20 km above the surface may have 19 antennas that provide communication coverage to 19 respective cells. In aggregate, the 19 cells define a coverage area of the HAP. In this example, if it is assumed that a surface coverage radius of 75 km, which corresponds to a minimum elevation angle at a user terminal at the edge of coverage of 15 degrees, the use of similar sized antennas results in the cell in the center of the coverage area having a surface area of 118 km 2  and the 12 outer cells each having a surface area of 1283 km 2 . This results in a cell size difference of 1165 km 2  between inner and outer cells of the coverage area. In other words, the inner cell is less than 9% the size of the larger outer cells. Assuming the density of users per square km within the coverage area is constant, users in the outer cells would have a factor of 11 less available bandwidth and significantly reduced QoS compared to users in the innermost seven cells.  FIG. 1  shows that a projection of equal aperture antenna radiation patterns (shown in chart  104 ) onto the surface results in some large beams at the intended edge of coverage (shown in chart  102 ). Some of these beams illuminate an area of nearly 3600 km 2  while others fall short of covering the intended area. The extended illumination at the edges of covers potentially causes interference with adjacent HAPs systems and may not be usable because the user terminal elevation angle would be very small and the path loss increases with distance from the HAP. This is not a problem for GEO satellites because the whole Earth subtends an angle of 20°, which is well inside the center HAP beam where the projection is better behaved. 
     Additionally, free space path loss experienced by the user terminals in the outer cells would be up to 11.6 dB greater than user terminals within the 7 inner cells because path loss is proportional to the square of the distance from the user terminal to the platform. Additionally, rain attenuation and atmospheric gas absorption both increase with the distance the signal propagates through the atmosphere or inversely with the user terminal elevation angle. These phenomena are location dependent as rain and water vapor depend on location of the coverage area on the surface of the Earth, which may vary from desert areas to high rain areas. User terminals that travel from an inner cell to an outer cell would experience a significant QoS degradation. 
     In instances where the HAP is mobile or the cells provided by the HAP are mobile, the coverage area and cells move as the HAP moves in the sky. A stationary user terminal within an inner cell at one point in time may be within an outer cell at another point in time. This stationary user terminal would be switched from a cell with high available bandwidth (or high QoS) to a cell with low available bandwidth (or low QoS). The user would accordingly perceive or detect a significant drop in performance while remaining in the same geographic location. 
     Additionally, in instances where the HAP position is substantially held constant by navigating in figure eights/circles, or flying into the wind, the cells may or may not move on the surface. The HAP may contain mechanisms for moving the on-board antennas to compensate for the HAP movement. The advantages of the uniform spectral density cells still apply in this case. This case, however, also allows for creating different spectral density cells if the users are not uniformly distributed over the coverage area. 
     The example platform (i.e., HAP) system disclosed herein uses differently sized antennas (or antenna elements) to provide substantially uniform QoS or surface spectral density across all cells within a coverage area. The differently sized antennas provide corresponding different size beam widths, which compensates for the angle at which Earth subtends at 17 km to 22 km resulting in substantially similarly sized cells. Such a configuration of differently sized antennas maintains a consistent QoS or available bandwidth throughout the cells of a coverage area so that a user does not experience service degradation when the user terminal moves between cells and/or the HAP moves relative to a user terminal. To maintain consistent cell areas, antennas covering the outer cells are relatively larger (and consequently have more gain) than those antennas coving the interior cells. The increased gain for the antennas covering the outer cells compensates, in part, for the increased path loss from the greater distance to reach those outer cells. Further, the consistent cell sizes means that link margins between user terminals and the HAP are similar, which means that antennas on the user terminals can be the same regardless of the location of the user terminal within the coverage area. 
     HAP Communication Environment 
       FIG. 2  shows a diagram of an example satellite communication system  200 , according to an example embodiment of the present disclosure. The example satellite communication system  200  includes a platform  202  (e.g., a HAP) configured to operate at a specified altitude above the Earth&#39;s surface  204 . For instance, the platform  202  may operate between 17 to 22 km above the surface of the Earth. In other examples, the platform  202  may be replaced by any other suitable communications satellite. 
     The example platform  202  includes antennas  206  in addition to hardware  207  (e.g., receiver, switch, transmitter, modem, router, filter, amplifier, frequency translator computing device, processor, memory/buffer, etc.) to facilitate the relay of communications between user devices  208  and a gateway  210 . For example, the platform  202  may have a transponder bent-pipe design for relaying communications signals between the gateway  210  and the user terminals  208  in multiple cells. In some embodiments, the platform  202  may include processing, switching or routing capability so that circuits may be switched or individual packets may be routed between different cells. The communications signals transmitted to/from the platform  202  can be any combination of standard or proprietary waveforms. Additionally, the gateway can be connected to any combination of communications networks such as the Internet. 
     The example hardware  207  may include a switch and/or processor that is configured to retransmit communications received from one cell back to the same cell or another cell. For instance, a switch may be configured to receive communication data from at least one of the gateway  210  and the user terminals  208  and determine a destination cell within a coverage area for the communication data. The switch then selects one of the plurality of antennas  206  corresponding to the destination cell to transmit the communication data and accordingly transmits the communication data via the selected antenna. In other embodiments the data could be sent to other HAPS, GEO/LEO satellites, or other aircraft. 
     The example user terminal  208  can be any terminal capable of communicating with the platform  202 . The user terminal  208  includes an antenna, transceiver, and processor to facilitate the transmission of data with the platform  202 . The user terminals  208  may be connected to any user communications equipment or device such as a router, switch, phone or computer  209 . The user terminal  208  may also include a mobile platform. 
     The example gateway  210  includes any centralized transceiver connected to a network  213  (e.g., the PSTN, Internet, a LAN, a virtual LAN, a private LAN, etc.). The gateway  210  may include one or more base stations, antennas, transmitter, receiver, processor, etc. configured to convert data received from the network  213  into signals for wireless transmission to the platform  202  and convert data received from the platform  202  into signals for transmission to the network  213 . In some instances, the platform  202  may be in communication with more than one gateway  210 . Additionally or alternatively, the gateway  210  may be in communication with more than one platform  202 . In these instances, the gateway  210  may select which platform  202  is to receive the data based on, for example, a destination of the data. 
     The example user terminals  208  and the gateway  210  are configured to communicate with the platform  202  via uplinks  214  downlinks  216 . The links  214  and  216  use spot beams provided by the platform  202  to cover specified cells containing the user terminal  208  and/or the gateway  210 . It should be appreciated that a spot beam may multiplex a plurality of signals on each uplink  214  and each downlink  216  based on the amount of user terminals  208  and/or gateways  210  transmitting or receiving data within a cell. Data is transmitted to the platform  202  from the user terminals  208  via the uplink  214   a  and data is received from the platform  202  at the user terminals  208  via the downlink  216   a.  Similarly, data is transmitted to the platform  202  from the gateway  210  via the uplink  214   b  and data is received from the platform  202  at the gateway  210  via the downlink  216   b.  The gateway  210  sends communication signals to the user terminal  208  via a forward link comprising the uplink  214   b  and the downlink  216   a  and the user terminal  208  sends communications signals to the gateway  210  via the return link comprising the uplink  214   a  and the downlink  216   b.    
     Mesh connectivity between user terminals  208  in the same or difference cells is also possible depending on the capabilities of the communications platform, i.e., the platform  202  or a satellite. While the disclosure is not limited to any frequency, certain frequency spectrums have been allocated for HAP communications by regulatory bodies. These allocated frequencies are used in the example discussed herein. The example embodiment assumes the uplink  214   b  may use a frequency band between 47.2 and 47.5 GHz, the downlink  216   b  may use a frequency band between 47.9 and 48.2 GHz, the uplink  214   a  may use a frequency band between 31.0 and 31.3 GHz and the downlink  216   a  may use a frequency band between 27.9 and 28.2 GHz. 
     Another possible embodiment assumes the uplink  214   b  may use a frequency band between 31.0 and 31.3 GHz and the downlink  216   a  may use a frequency band between 27.9 and 28.2 GHz and both the uplink  214   b  and the downlink  214   b  may use a frequency band between 47.2 and 47.5 GHz and 47.9 and 48.2 GHz. In the United States the allocation includes the entire band between 47.2 and 48.2 GHz. The advantage of the first embodiment is that rain attenuation on the user links is easier to close with higher data rates. The advantage of the second embodiment is that more spectrum is available for the user data links. The disclosure is not restricted to either of these frequency plans and in the future other frequencies may become available to HAP communications. If the methods and apparatus of disclosure are applied to LEO satellites, other spectrum is already available. 
     As discussed in more detail below, the antennas  206  of the example platform  202  are configured to have different sizes (e.g., different size apertures) to create cells of substantially the same size in order to achieve a constant surface spectral density throughout the coverage area. A system configuration manager  212  includes any processor or system tasked with designing, developing, and/or maintaining the antennas  206 , hardware  207 , and other features of the platform  202 . The system configuration manager  212  may determine a coverage area to be serviced by the platform  202  in addition to a number of antennas needed to provide acceptable bandwidth to user terminals and the size of the antennas to maintain spectral density uniformity among the cells. The system configuration manager  212  may also select the type of antenna including, for example, a reflector, array, open ended waveguide, dipole, monopole, horn, etc. The system configuration manager  212  may select the antenna type based on, for example, a desired spot beam size, bandwidth, gain, elevation angle relative to the surface, etc. The system configuration manager  212  may also select the size of the aperture of the antenna  206  based on the desired spot beam size, bandwidth, gain, elevation angle, etc. In some instances, the system configuration manager  212  may include a control link to configure the platform  202  based on a new set of coverage area and QoS parameters. Depending on the capability of the platform  202 , such parameters may include new frequency assignments, new spot beam forming coefficients or new routing tables. 
     In addition to configuring the platform  202 , the example system configuration manager  212  may also service and/or maintain the platform  202 . For example, the system configuration manager  212  may transmit software updates while the platform  202  is operational in the sky. The system configuration manager  212  may also instruct the platform  202  to move to a new geographical location. The system configuration manager  212  may further instruct the platform  202  to return to the ground for maintenance, upgrades, service, antenna reconfiguration, etc. The system configuration manager  212  may communicate with the platform  202  via the gateway  210  and/or a proprietary/private communication link. In some instances, the platform  202  may provide diagnostic and status information to the system configuration manager  212  via the proprietary/private communication link and/or through the gateway  210  multiplexed with communications traffic. 
     HAP Coverage Area Embodiment 
       FIG. 3  shows a diagram of an example HAP coverage area  300  for the platform  202  discussed above in conjunction with  FIG. 2 , according to an example embodiment of the present disclosure. In this example, the platform  202  operates at an altitude of r h  above the surface  204 . The altitude r h  may be any distance between, for example, 17 km and 22 km. The platform  202  provides communications for the coverage area  300  that falls within a maximum range  302 . The range  302  depends, on the altitude r h , a minimum acceptable elevation angle respective to a user device (“γ”)  303 , and desired link margin. The link margin is dependent on the available platform power, platform antenna gain, terminal antenna gain, link availability requirements, terminal gain, and other equipment and link limitations. For example, the platform  202  operating at 20 km may have a maximum range  302  of 75 km for a minimum elevation angle γ of 15 degrees, 110 km for a minimum elevation angle γ of 10 degrees and 195 km for a minimum elevation angle γ of 5 degrees. Assuming, for example, a maximum range  302  of 110 km and a minimum elevation angle γ of 10°, the coverage area  300  of the platform  202  would be 36,350 km 2 . Alternatively, assuming a maximum range  302  of 75 km for a minimum elevation angle γ of 15 degrees, the coverage area  300  of the platform  202  would be 16,675 km 2 . Note the distinction between range  302  in  FIG. 3  and coverage area radius  301  in  FIG. 3 . The range  302  is the RF line-of-sight propagation path between the platform  202  and the user terminal  208  and the coverage area radius  301  is a measure along the arc of the surface  204  from the sub-HAP point to the edge of coverage. 
       FIG. 3  also shows a spherical surface  304  which extends the u-v surface used in the design of GEO satellites. In a GEO system the u-v surface is nearly planar since the Earth subtends an angle of only 20 degrees with respect to this surface. For HAPs and LEO satellites, this surface cannot be approximated by a plane and must be considered spherical. 
       FIG. 4  shows a diagram of a coverage area  400  of a HAP  402  that includes antennas of the same size (or have the same aperture) similar to the prior art system described in connection with  FIG. 1 . The position of the antenna apertures in the HAP spherical u-v surface and the projection of the radiated beams are shown respectively in the charts  102  and  104  of  FIG. 1 . An example of the u-v surface  304  is shown in  FIG. 3 . Similar to  FIG. 3 , the HAP  402  of  FIG. 4  operates at an altitude of 20 km and has a range of 75 km. In this example the coverage area  400  is partitioned into  19  cells  404  including a central cell sounded by six cells, which themselves being surrounded by  12  cells. The HAP  402  includes  19  antennas of the same size to provide coverage respectively to the 19 cells. The 19 antenna patterns projected onto the u-v surface are hexagonal, as shown in the chart  102  of  FIG. 1 . These hexagons are each illuminated by an antenna which is pointed normal to the u-v surface. An equal-gain contour, for example the 3 dB contour, falls on the six vertices of the hexagon. The neighboring antenna illuminates an adjacent hexagon so that its 3 dB gain contour falls on the shared vertices. Along the line connecting the vertices both antenna radiation patterns have the same gain and this defines the boundary between the two cells. 
     A spherical hexagonal pattern incident on the u-v surface  304  is shown in the diagram of  FIG. 5 . In this figure, a cell  402  projects back onto a spherical hexagon  502 . In this example, the antennas are configured to have a beamwidth or central angle measured in the plane containing the antenna system boresight and the antenna Poynting vector of 29.7° and are assumed to have an efficiency η of 55% such that the gain of the center antenna and the antennas in the outer rank would be about 15.5 dBi. Note that the size of the spherical hexagons shown in  FIGS. 1 and 6  is determined by the desired coverage area and may not tessellate the spherical u-v surface. In this case the hexagons of  FIGS. 1 and 5  would not be regular and would need to be illuminated by antenna apertures with elliptical beam contours. 
     When these beams are projected onto the surface  204  the center beam illuminates a circular area and the inner and outer ring of beams project fan shaped coverage areas on the surface. The innermost cell  404   a  has a diameter of 12.2 km. In comparison, the 6 next innermost cells including the cell  404   b  are fan shaped and have an arc length in the radial dimension of 14.4 km, an arc length in the orthogonal dimension at an inner edge of 6.4 km, and an arc length in the orthogonal dimension at an outer edge of 21 km. Additionally, the 12 outermost cells including the cell  404   c  are fan shaped and have an arc length in the radial dimension from the coverage area center of 53 km and an arc length in the orthogonal dimension that varies from about 10.5 km at an inner edge to about 38 km at an outer edge. The use of the same antenna size for the different cells for the relatively low altitude HAP  402  results in significantly different sized cells as shown in  FIGS. 1 and 4 . Additionally, the difference in path loss between the edge of the coverage area  400  at the cell  404   c  and the center of the cell  404   a  is about 11.6 dB. Further, the outer cells would also experience more rain fading and atmospheric gas absorption as a result of the lower elevation angle and path length or range. This relatively high path loss difference between cells means that users at the edge of the coverage area  400  would experience a lower link margin and reduced bandwidth density, resulting in poorer QoS. 
       FIG. 6  shows a table  600  of properties of the coverage area  400  for the prior art example discussed above in conjunction with  FIG. 4 . The table  600  shows that the 12 outermost cells  402   c  each cover an area of 1283 km 2  while the innermost cell  404   a  only covers an area of 118 km 2 . Further, the cells  404   c  are further from the HAP  402  than cells  404   a  and  404   b,  which reduces the link margin. This increased path loss can be overcome by increasing the code rate but this further reduces the user data rates available. This means that the outermost cells  404   c  have lower available bandwidth per unit area by orders of magnitude compared to the innermost cell  404   a.    
     To overcome the issues of having differently sized cells, especially at the outermost cells, the antennas  206  of the example platform  202  of  FIG. 2  are configured such that the cells have a substantially uniform area and/or surface spectral density.  FIG. 7  shows a diagram of an example coverage area  700  that includes  19  similar sized cells. The coverage area  700  includes an innermost cell  702   a  surrounded by six next innermost cells  702   b,  which are surrounded by  12  outermost cells  702   c.  The antennas  206  of the platform  202  are sized differently to compensate for the different subtended angles. In some instances, the antennas corresponding to each ring of cells  702   a,    702   b,  and  702   c  may be configured to have the same dimensions since the cells within the same ring generally are the same distance from the platform  202 .  FIG. 7  also shows the cells as having a hexagonal-shape such that the entire coverage area  700  has a honeycomb shape. The spot beams which illuminate these cells generally have elliptical equal-gain cross sections so that the spot beams overlap. 
       FIG. 8  shows the example coverage area  700  with the uniform cells  702   a,    702   b , and  702   c.    FIG. 9  shows the uniform cells  702  as they project back onto the spherical u-v surface  304  around the platform  202  (chart  902 ) and as they appear on the surface  204  (chart  904 ). Similar to the prior art example discussed above in conjunction with  FIGS. 1 and 4 , the example platform  202  operates at 20 km above the surface  204  and has a range of 75 km for a user terminal elevation angle γ of 15°. However, since the antennas  206  are differently sized and/or have different sized apertures, the beamwidth of the antennas corresponding to different rings of cells  702   a,    702   b,  and  702   c  are also different. For instance, as shown in table  1000  of  FIG. 10 , the beamwidth of the antenna associated with the cell  702   a  is 79.7°, the beamwidth of the antennas corresponding to the cells  702   b  is 25.6° by 52°, and the beamwidth of the antennas corresponding to cells  702   c  is 8.9° by 28°. The central angle or beamwidth orthogonal to the radial angular dimension in the spherical u-v surface  304  for the inner and outer rings are determined by use of the cosine law from spherical trigonometry. This configuration yields cells  702  that all have an effective diameter of 33.4 km and an area of about 878 km 2 . Such a configuration accordingly provides cells  702  with a substantially similar surface spectral density and bandwidth, thereby providing a communications system that has a consistent QoS across the coverage area. The higher gain of the antennas covering the outer ring also helps to compensate for the additional path loss. In comparison, the system discussed in conjunction with  FIG. 1  has common beamwidths among the antennas but different sized cells. 
       FIG. 11  shows a diagram of differently sized antennas  206  of the platform  202  used to produce the similarly sized cells  702  of  FIGS. 7 and 8 . The example antenna  206   a  corresponding to the cell  702   a  is configured to have an aperture diameter of 0.35 inches and a gain of 7 dB to produce a beamwidth of 79.7°. In this example, the beamwidth of the antenna is selected based on the angle α of the cell  702   b  being less than 39.9°. The antenna  206   b  may be implemented by an open ended flared waveguide. 
     The example antenna  206   b  corresponding to the cell  702   b  is configured to have an elliptical aperture with dimensions of 1.07 by 0.47 inches and a gain of 13.4 dB to produce an elliptical beam of 25.6° by 52°. In this example, the radial beamwidth of the antenna  206   b  is selected based on the angle α of the cell  702   b  being between 39.9° and 65.5°. The antenna  206   b  may be implemented by a small feed horn. 
     The example antenna  206   c  corresponding to the cells  702   c  is configured to have an antenna aperture configured to produce the spot beam with a beamwidth of 8.9° by 28°. Such an antenna  206   c  would have a gain of 21 dB. Assuming 55% aperture efficiency, the antenna  206   c  is configured to have an elliptical aperture of 3.11 by 1.0 inches at 30 GHz (as shown in  FIG. 11 ). A small reflector may be used to provide an antenna with these dimensions. 
     It should be appreciated that all of these antennas  206  shown in  FIG. 11  may be implemented using phased arrays or other antenna technology. 
     Example Design Considerations 
     The design goal is to have a uniform user perceived QoS. The reasonable assumption that the user terminals  208  are uniformly distributed within a coverage area requires a uniform distribution of capacity in terms of available user data rates. It is further desirable that all of the user terminal antennas and transceivers are similar in design independent of their location within the coverage area. Since both HAPs and LEO satellites move, even stationary user terminals  208  will transition from one cell to the next cell and users will expect the same QoS without disruption. This motivates the allocation of the same amount of spectrum within each cell in conjunction with uniform cell sizes. 
       FIG. 12  shows a diagram of parametric inputs to the design process implemented by the system configuration manager  212  of  FIG. 2 . These inputs may change in time with improved technology and with new frequency spectrum allocations. Some of these design parameters are location dependent such as rain fall statistics and therefore rain fades. Some of these parameters are the choice of a service provider such as availability and minimum elevation angle or terminal costs. In any event, design process of the system configuration manager  212  remains the same. 
     Current estimates are that HAPs located 20 km above the surface  204  may move within a circle having a 0.25 km radius. At the sub-HAP point on the surface this corresponds to an angle of 0.7 degrees. If a non-tracking antenna is desired for the user terminal  208  then this constrains the antenna to a 3 dB beamwidth of 1.4 degrees and therefore constrains the gain of the user terminal antenna as well. However, new antenna designs with modest tracking capability are becoming available in the same time frame as HAP technology. This design example assumes an antenna with a 1 degree 3 dB beam width and the ability to track +/−0.5 degrees. This may be achieved with a reflector antenna illuminated by a small feed array of 16 elements, for example. This results in a standard user terminal gain of 45 dBi. It is further assumed that the user terminal receiver has a noise figure of 5 dB corresponding to a noise temperature of 627 degrees K. 
     Atmospheric attenuation due to rain, clouds and gas absorption is dependent on location. For the purposes of this example the standard atmosphere described in International Telecommunication Union (“ITU”) Recommendations P.676, P.618, P.840, and P.838 is assumed. For each HAP configuration, local atmospheric data, if available, is used. This additional loss is shown graph  1300  of  FIG. 13  as a function of elevation angle for the example embodiment. For example, the graph  1300  of  FIG. 13  shows there is a total attenuation of 99.5% (fade of 15 dB) for signals with a 28 GHz frequency due to rain, clouds, gas absorption, and scintillation. 
     In order to estimate the user data rate available at any possible user terminal location within the coverage area, link budgets are computed assuming the same modulation and coding waveforms as used in the DVB-S2 standard. This data is given by the table  1400  shown in  FIG. 14  where the modulation and coding schemes are ranked by Eb/No. The DVB-S2 waveform is use for the example embodiment but the described process is not limited to this particular waveform. 
     The results of the system configuration process include HAPs design parameters such as antenna apertures and required transmit power levels per cell. If the links do not close at the edge of coverage or if excessive transmit power is required then the input parameters must be re-evaluated. Design modifications include increasing the performance requirements on the user terminal  208  (i.e., more antenna gain), using a lower frequency with better rain attenuation performance, reducing the size of the coverage area (i.e., increasing the minimum user terminal elevation angle), and/or using an alternative coverage area embodiment by partitioning the coverage area into more cells. 
     In the example shown by table  1500  of  FIG. 15  and graph  1600  of  FIG. 16 , the output design has a uniform user data rate density of about 250 kb/sec/km 2 , which is about an order of magnitude higher than that achieved by many of the current operating satellite systems. In addition the total HAPs transmit power measured at the HAPs antenna flange is only 6.6 Watts, which is well within the expected available power for commercial HAPS. The locations within the coverage area of the points on the curve of  FIG. 16  are shown in coverage area  1700  of  FIG. 17 . In this case, these results are so good that the system configuration may be reevaluated with more relaxed user terminal performance requirements or the designer may consider extending the range of coverage or increasing the link availability above 99.5%. Another design possibility includes reducing the total number of cells within the coverage area from 19 to 7 (i.e., one center cell  702   a  and one outer ring of six cells  702   b ). However this reduces the total user data throughput of the system. 
     HAP Coverage Area Second Embodiment 
     In some instances, the system configuration manager  212  of  FIG. 2  may choose not to use a beamwidth of 80° for the center cell  702   a  because the gain for the antenna  206   a  is only 7 dB compared to gains of 13.4 dB and 21 dB respectively for the antennas illuminating cells  702   b  and  702   c.  Such an antenna is likely to have high side lobe radiation potentially interfering with the reuse of its frequency allocation in other beams. Also the difference in path loss at the outer cells  702   c  is only 11.6 dB. This means that the inner cell  702   a  may have the worst case link margins compared to the outer cells  702   c  in clear weather, excluding the attenuation due to atmospheric gas absorption. However, side lobes of the antenna  206   a  may be high enough to interfere with the reception of the other antennas  206   b  and  206   c,  thereby limiting frequency reuse. 
     To resolve this issue, the center cell  702   a  may further be divided into seven sub-cells  1802 , as shown in  FIG. 18 . To maintain the same surface spectral density among the cells  702   b,    702   c,  and  1802 , the spectrum or bandwidth available to each of the sub-cells is divided by seven from the total spectrum allocated to the center cell  702   a.  This configuration reduces the noise bandwidth and improves the link margins since the antenna gain would be 14.1 dB for the centermost cell  1802  and 16.3 dB for the surrounding cells  1802 , which is similar to the gains of 13.4 dB and 21 dB for the antennas illuminating cells  702   b  and  702   c.    
       FIG. 19  shows a table  1900  comparing the area and bandwidth of the cells  702   b  and  702   c  and the cells  1802 . While the coverage area of each of the cells  1802  is smaller at 125 km 2 , the surface spectral density is the same as the cells  702   b  and  702   c  because the spectrum for each of the cells  1802  is reduced to 21 MHz or 150/7 MHz. This configuration enables smaller beamwidths to be used for the center of the coverage area  702  rather than 80°, which provides consistent QoS between the outer cells  702   c  and the innermost cells  1802 . 
     HAP Coverage Area Third Embodiment 
       FIG. 20  shows a diagram of a coverage area  2000  of the example platform  202  of  FIG. 2  that includes 37 equal-sized cells  2002 . In this example, the cells  2002  are partitioned into four different rings with a first ring including cell  2002   a,  a second ring including six cells  2002   b,  a third ring including  12  cells  2002   c,  and a fourth ring including  18  cells  2002   d.  The antennas  206  corresponding to the cells  2002  of the same rings have similar dimensions while the dimensions between the antennas of the different rings differ. This configuration of antennas produces a surface spectral density that is roughly uniform throughout the coverage area  2002 . 
     As shown in table  2100  of  FIG. 21 , an antenna corresponding to the cell  2002   a  is configured to have an aperture diameter (and beamwidth) corresponding to an elevation angle of 59 degrees at the edge of cell coverage and an effective cell radius of 12 km. The six antennas corresponding to six cells  2002   b  are configured to have an aperture diameter (and beamwidth) corresponding to an elevation angle of 32.1 degrees at the edge of cell coverage and an effective cell radius of 12 km. The 12 antennas corresponding to 12 cells  2002   c  are configured to have an aperture diameter (and beamwidth) corresponding to an elevation angle of 20.7 degrees at the edge of cell coverage and an effective cell radius of 12 km. Additionally, the 19 antennas corresponding to 19 cells  2002   d  are configured to have an aperture diameter (and beamwidth) corresponding to an elevation angle of 15 degrees at the edge of cell coverage and an effective cell radius of 12 km. Such a configuration of antennas having different sizes produces cells  2002  with diameters of approximately 24 km. Further, since the cells  2002  are smaller than the cells  702 , the beamwidth for the center cell  2002   a  is reduced from 79.7° to 69.8° and the gain increased from 7 to 9.2 dB. 
     HAP Coverage Area Fourth Embodiment 
     In a fourth embodiment, a coverage area  2200  includes an outer ring of 12 cells, an inner ring of 6 cells, and a center cell. In this embodiment, the inner ring of 6 cells from the first embodiment described in conjunction with  FIGS. 7 to 11  may be divided or sectored each into multiple cells, as shown in  FIG. 22 . In the example shown, each cell in the outer ring is sectored into 3 separate cells. The beam necessary to illuminate each sector would be nearly circular with a beamwidth of 8.8° by 9.3° . The dimensions of the antenna aperture would be elliptical at 3.11 by 2.95 inches and would have a gain of 25.8 dB. This represents an improvement of about 5 dB over the design with non-sectored cells and all three beams can be produced by a single fixed phased array. 
     Flowchart of the Example Process 
       FIG. 23  illustrates a flow diagram showing an example procedure  2300  to configure antennas on a platform  202  to produce cells with a substantially uniform surface spectral density, according to an example embodiment of the present disclosure. Although the procedure  2300  is described with reference to the flow diagram illustrated in  FIG. 23 , it should be appreciated that many other methods of performing the steps associated with the procedure  2300  may be used. For example, the order of many of the blocks may be changed, certain blocks may be combined with other blocks, and many of the blocks described are optional. Further, the actions described in procedure  2300  may be performed among multiple devices. 
     The example procedure  2300  of  FIG. 23  operates on, for example, the system configuration manager  212  of  FIG. 2 . The procedure  2300  begins when the system configuration manager  212  receives a request  2301  to provision a HAP (e.g., the platform  202  of  FIG. 2 ) for a specified coverage area. The request  2301  may include, for example a latitude (e.g., geographic location) at which the proposed HAP will operate. The request  2301  may also include a season of the year in which the HAP will operate. Responsive to the request  2301 , the system configuration manager  212  determines an altitude at which the HAP will operate (block  2302 ). The altitude depends on the altitude of the tropopause, where the atmospheric winds are minimal. This altitude in turn depends on the latitude of the coverage area. A preliminary edge-of-coverage link analysis may be performed at this time. 
     The system configuration manager  212  also determines a minimum elevation angle (e.g., γ= 15 °) from the ground to the HAP and a maximum communication range for the HAP (block  2304 ). The system configuration manager  212  may use parametric inputs  2305  (e.g., terminal design constraints, HAP constraints, available spectrum, rain/atmospheric absorption fade statistics, etc.) described in conjunction with  FIGS. 12 to 17  to determine the minimum elevation angle and the maximum communication range. Using the altitude, the minimum elevation angle, and the maximum range, the system configuration manager  212  determines a coverage area for the specified area (block  2304 ). It should be noted that the costs of installation can increase with lower elevation angles since it becomes more difficult to site an antenna with visibility to the HAP when surrounded by buildings and trees. 
     The example system configuration manager  212  further determines bandwidth requirements and/or QoS requirements/parameters for the coverage area (block  2306 ). The bandwidth requirements may be based on inputs  2307  including, for example, a number of users or subscribers. The configuration manager  212  then partitions the coverage area into cells (block  2308 ). In some instances, the cells may be equal-sized hexagonal cells. After partitioning the coverage area, the example system configuration manager  212  is configured to assign an antenna to each of the cells (block  2310 ). 
     The system configuration manager  212  next determines a beamwidth, elevation angle, and/or gain for each antenna to provide communication coverage to the respective cell according to the bandwidth requirements and/or QoS requirements (block  2312 ). The system configuration manager  212  then determines an aperture size and/or an antenna type for each of the antennas based, for example, on the determined beamwidth, gain, elevation angle, etc. such that each cell has the same size and/or surface spectral density (block  2314 ). The aperture size and/or an antenna type for each of the antennas may also be based on link performance requirements  2315  and/or the availability of antenna designs. The system configuration manager  212  then performs a link analysis to determine if there is adequate link margin for the required level of service (block  2316 ). If there is sufficient margin, the HAP is provisioned (block  2318 ) and the procedure  2300  ends until another HAP is requested to be provisioned into service. The example procedure  1500  may also begin again if the HAP is returned from operation for an upgrade and/or modification. However, it should be noted that if a phased array is used instead of individual antennas, the system designer may configure the beam forming coefficients of the phased array to form new spot beams while the HAP is flying. 
     Returning to block  2316 , if there is insufficient margin, the example procedure  2300  returns to block  2304 . At this point the system configuration manager  212  determines a modified coverage area, minimum elevation angle, and maximum range that provides sufficient margin. For example, a smaller coverage area may be necessary or the number of cells may need to be increased from 19 to 37. 
     Conclusion 
     It will be appreciated that all of the disclosed methods and procedures described herein can be implemented using one or more computer programs or components. These components may be provided as a series of computer instructions on any computer-readable medium, including RAM, ROM, flash memory, magnetic or optical disks, optical memory, or other storage media. The instructions may be configured to be executed by a processor, which when executing the series of computer instructions performs or facilitates the performance of all or part of the disclosed methods and procedures. 
     It should be understood that various changes and modifications to the example embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims. 
     It should also be understood that a telecommunications platform providing substantially uniform surface spectral density using the methods described herein may be an element of a larger system. Examples of larger system include relays between platforms, relays between platforms and GEO satellites, relays between platforms to gateways shared by those platforms, relays between gateways and GEO satellites.