Abstract:
Communications network architectures integrate communications using a geosynchronous satellite communications system including a geosynchronous communications satellite operating in the fixed satellite service (FSS) band and a low earth orbit satellite communications system including a plurality of low earth orbit communications satellites. A low earth orbit gateway having a terminal is used to communicate with the low earth orbit communications satellites using multiple access communication channels, and is used to communicate with a public switch telephone network. A geosynchronous gateway having a terminal is used to communicate with the geosynchronous communications satellite by overlaying multiple access communication channels onto fixed channel assignments and transmitting the overlayed channel assignments. The terminal in the geosynchronous gateway is also used to communicate with the public switch telephone network. A fixed terminal having a handset is used to selectively communicate with the low earth orbit communications satellite using the multiple access communication channels, and with the geosynchronous communications satellite by extracting the multiple access communication channels from the fixed channel assignments.

Description:
BACKGROUND 
     The present invention relates generally to satellite-based communication systems, and more particularly, to network architectures that provide for a low earth orbit and geosynchronous orbit satellite-based communications systems. 
     Currently, there are geosynchronous orbit satellite communication systems such as the Brazilsat system, for example, that provide fixed wide area communication (telephony) services. There are also low earth orbit satellite communication systems such as the GlobalStar and Iridium systems, for example, that provide mobile communication (telephony) services. However, heretofore, the fixed and mobile systems have not been coupled together to provide an integrated system. The present invention addresses this issue. 
     Accordingly, it is an objective of the present invention to provide for network architectures for low earth orbit and geosynchronous orbit satellite-based communications systems. 
     SUMMARY OF THE INVENTION 
     To accomplish the above and other objectives, the present invention is network architectures that provide telephony service for fixed and mobile satellite ground terminals using code division multiple access (COMA) access technology. The network architectures were developed to provide telephone service in areas of the earth that have relatively little or no land-based communication infrastructure, such as underdeveloped rural areas, for example. 
     The communications network architectures comprise a geosynchronous satellite operating in the fixed satellite service (FSS) band communications system including a geosynchronous communications satellite and a low earth orbit satellite communications system including a plurality of low earth orbit communications satellites. A low earth orbit gateway comprising a terminal is used to communicate with the low earth orbit communications satellites using multiple access communication channels. A geosynchronous gateway comprising a terminal is used to communicate with the geosynchronous communications satellite by overlaying multiple access communication channels onto fixed channel assignments and transmitting the overlayed channel assignments. The terminals in the geosynchronous and low earth orbit gateways communicate with a public switch telephone network (PST). A fixed terminal having a handset is used to selectively communicate with the low earth orbit communications satellite using the multiple access communication channels, or with the geosynchronous communications satellite by extracting the multiple access communication channels from the fixed channel assignments. 
     The LEO gateway communicates by way of the low earth orbit communications satellites to fixed terminals located at low telephone density sites or that can be part of a clustered village phone systems, for example. The LEO satellite communications system also has mobile terminals that are used by individuals in vehicles. The mobile terminals typically interface to the LEO communications satellites and to cellular sites in high telephone density areas. Thus, the mobile terminals may be used in both low and high telephone density areas. The mobile terminals interface by way of the cellular sites or LEO communications satellites and LEO gateway to the public switch telephone network. 
     Each communication site in the low telephone density areas has a fixed terminal and a public use handset that are used to communicate with both the geosynchronous communications satellite and the LEO communications satellites. Each clustered village phone system has a very small aperture terminal (VAST) that is coupled to either a wired or wireless loop interconnecting a plurality of conventional telephones. The very small aperture terminal also has a public use handset for backup purposes, in case of failure of the wired or wireless loops. 
     The present invention preferably uses code division multiple access (CDMA) communication technology in the fixed satellite service (FSS) band. The CDMA technology allows the fixed terminals to communicate with both the geosynchronous and LEO communications satellites. The present invention preferably uses GlobalStar terminals, handsets and gateway technology in conjunction with the fixed satellite service geosynchronous satellite to provide rural telephony. Thus, the capabilities of the GlobalStar system are expanded by the present invention to support fixed satellite service telephony traffic. 
     The present invention provides for a shared GEO and LEO telephony service gateway. The present invention uses low cost fixed and mobile satellite terminals. The architecture of the present invention provides for a scalable telephony solution that allows from one to several hundred users per clustered village terminal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing, wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 illustrates an exemplary hybrid communications network architecture in accordance with the principles of the present invention; 
     FIG. 2 illustrates an exemplary complementary communications network architecture in accordance with the principles of the present invention; 
     FIG. 3 illustrates an exemplary interface between a user terminal and a LEO/GEO gateway in accordance with the principles of the present invention; and 
     FIG. 4 illustrates an exemplary terminal overlay for the hybrid network architecture. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, FIG. 1 illustrates an exemplary hybrid communications network architecture  10  in accordance with the principles of the present invention. The hybrid communications network architecture  10  provides for system that permits communication between fixed and mobile users using geosynchronous (GEO) and low earth orbit (LEO) communications satellites  21 ,  31 . The hybrid network architecture  10  implements a system that provides communication (telephony) service by way of the geosynchronous and low earth orbit communications satellites  21 ,  31  to fixed and mobile user terminals  41 ,  51 ,  72  using code division multiple access (CDMA) access technology, for example. 
     More particularly, the network architecture  10  comprises a geosynchronous satellite communications system  20  including a geosynchronous communications satellite  21  and a geosynchronous satellite network control center  26 . The geosynchronous satellite network control center  26  controls the operation of the geosynchronous communications satellite  21 . 
     The hybrid network architecture  10  also comprises a low earth orbit satellite communications system  30 , such as the GlobalStar system developed by the assignee of the present invention, which includes a plurality of low earth orbit communications satellites  31 . Communication with the geosynchronous communications satellite  21  is provided by a low earth orbit/geosynchronous (LEO/GEO) gateway  22 . Communication with the low earth orbit communications satellites  31  is provided by a low earth orbit (LEO) gateway  32 . 
     Although two separate gateways  22 ,  32  are shown in FIG. 1, it is to be understood that a single gateway  22  containing the components of both the LEO/GEO and LEO gateways  22 ,  32  may be provided to implement the present invention. As such, the specific implementation shown in FIG. 1 should not be taken as limiting the scope of the present invention. 
     The LEO/GEO gateway  22  comprises a C-band terminal  24  and antenna  25  that communicates with a C-band antenna  23  and transponder  27  on the geosynchronous communications satellite  21 . The C-band terminal  24  interfaces by way of a public switch telephone network interface  61  to a public switch telephone network. The public switch telephone network is the land based telephone communications infrastructure provided by local telephone companies that are typically used in high telephone density areas. The LEO/GEO gateway  22  also communicates by way of a public switch telephone network (PST) interface  61  to the public switch telephone network. 
     Each low earth orbit communications satellite  31  comprises L- and S-band antennas  34 ,  35  for communicating with fixed user terminals  41  having a handset  71  and vehicle-mounted mobile terminals  72  having a mobile handset  71 . Transponders  36  onboard each satellite  31  transmit data in the L-, and S-bands. 
     The LEO gateway  32  comprises an L/S-band terminal  37  and antenna  38  that interfaces between the low earth orbit communications satellites  31  and the public switch telephone network. The L-band is used to communicate from the fixed terminals  41  and mobile terminals  72  to the low earth orbit communications satellites  31 . The S-band is used to communicate from the low earth orbit communications satellites  31  to the fixed terminals  41  and mobile terminals  72 . 
     Exemplary L- and S-band links used in the GlobalStar system are 1610-1626.5 MHz (terminal  41  to LEO satellite  31 ) and 2483.5-2500 MHz (LEO satellite  31  to terminal  41 ). Exemplary C-band links arc 5091-5250 MHz (gateway  22  to GEO satellite  21 ), and 6875-7055 MHz (GEO satellite  21  to gateway  22 ). 
     The satellite antenna configuration used in the GlobalStar system (L- and S-bands), for example, typically provides for a 16 beam fixed array. The antenna  73  employed in the mobile terminals  72  at mobile sites is an omnidirectional antenna  73 , and the antenna  42  used at the fixed sites  43 ,  53  is a switched-directional array. The antenna  38  in the LEO gateway  32  is a tracking antenna, for tracking the low earth orbit communications satellites  31 . 
     The geosynchonous communications satellite  21  comprises a C-handle antenna  23  and transponder  27  or communicating between the LEO/GEO gateway  22  and C-band very small aperature terminal; (VAST)  51 , via VAST antenna  52 , and between the LEO/GEO gateway  22  and fixed terminals  41  located at low telephone density sites  40 , and clustered village phone systems  50 . The LEO/GEO gateway  22  comprises at C-band terminal  24  that communicates with the C-band antenna  23  and transponder  27  onboard the geosynchronous communications satellite  21 . The C-band terminal  24  at the LEO/GEO gateway  22  also interfaces to the public switch telephone network. The LEO/GEO gateway  22  may also comprise L/S-band terminals  37  and antennas  38  for communicating with the LEO satellite  31 . 
     The LEO/GEO gateway  22  and LEO gateway  32  interconnects the geosynchronous and low earth orbit satellite based wireless networks and a public land mobile network (PLMN), such as Advanced Mobile Phone Service(AMPS) or Groupe Speciale MobileGSM), of the public switch telephone network. As such, the gateways  22 ,  32  provide a termination point for network transmission and network signaling. The gateways  22 ,  32  may be connected to the public switch telephone network using a standard E 1 /T 1  trunk supporting a variety of signaling protocols. To GSM networks, the gateways  22 ,  32  appear as a GSM base station subsystem. To mobile switches in an EIA/TIA environment, the gateways  22 ,  32  appear as another mobile switch supporting the IS- 41  Intersystem Operation Standard. Furthermore, the mobile terminals  72  are dual- or multi-mode terminals, and are compatible with AMPS, IS- 95 , and GSM standards. 
     Referring now to FIG. 2, it illustrates an exemplary complementary communications network architecture  10  in accordance with the principles of the present invention. The exemplary complementary communications network architecture  10  is configured in a manner substantially similar to the architecture  10  shown in FIG.  1 . However, the LEO/GEO gateway  22  used in the hybrid network architecture  10  is not employed. Instead, the complementary communications network architecture  10  employs a standard GEO gateway 
     Additional key aspects of the communications network architectures  10  shown in FIGS. 1 and 2 are discussed with reference to FIGS. 3 and 4. Referring now to FIG.  3 , it illustrates an exemplary interface between a fixed terminal  41  and the LEO/GEO gateway  22 . FIG. 4 illustrates an exemplary terminal overlay for the network architectures  10 . 
     As is shown in FIGS. 3 and 4, one possible implementation of the C-band terminal  24  in the LEO/GEO gateway  22  comprises C-band transponders that process 16.5 MHz traffic channels comprising thirteen (13) 1.23 MHz CDMA carriers. Each C-band transponder provides up to 3000 simplex voice circuits at 2.4 kbps. The traffic channels comprising the CDMA carriers are uplinked to the transponders  27  in the geosynchronous communications satellite  21 , which in turn downlinks them to selected fixed terminals  41  located at the low telephone density sites  40  and clustered village phone systems  50 . The LEO/GEO gateway  22  has an interface at S- and L-band to the C-band terminal  24  which is the complement of the interface for the handset  71  of each fixed and mobile terminal  41 ,  72 . 
     The fixed terminals  41  located at the low telephone density sites  40  and clustered village phone systems  50  include a C-band to L- and S- band block converter  44 . The block converter  44  is used to convert C-band signals transmitted by way of the geosynchronous communications satellite  21  into S-band signals processed by the fixed and mobile terminals  41 ,  72 . The block converter  44  is also used to convert L-band signals output by the fixed and mobile terminals  41 ,  72  into C-band signals that are uplinked to the geosynchronous communications satellite  21  for transmission to the LEO/GEO gateway  22 . Thus, it is possible to utilize similar satellite channel access means and terminal technology across both LEO and GEO systems. 
     The low earth orbit gateway  31  also communicates by way of the low earth orbit communications satellites  31  to low telephone density sites  40  and clustered village phone systems  50 . The low earth orbit satellite communications system  20  also has mobile terminals  72  that are used to communicate from vehicles. The mobile terminals  72  interface to the low earth orbit communications satellites  31  or to cellular sites  75  in high telephone density areas  60 . Thus, the mobile terminals  72  may be used in both low and high telephone density areas  40 ,  60 . The mobile terminals  72  interface by way of the cellular sites  75  to the public switch telephone network. 
     Each communication site in the low telephone density areas  40  has a terminal  41  and a public use handset  71  that are used to communicate with the geosynchronous communications satellite  21  and the low earth orbit communications satellites  31 . Each clustered village phone system  50  has a very small aperture terminal  51  (and VAST antenna  52 ) that is coupled to either a wired or wireless loop  53  interconnecting a plurality of conventional telephones. The very small aperture terminal  51  may also be coupled to a public use handset  71  for backup purposes, in case of failure of the wired or wireless loops  53 . 
     The CDMA technology used in the hybrid network architectures  10  is based upon the IS-95 CDMA standard to provide high-quality, digital voice, data, messaging and fax services. The IS-95 CDMA standard uses digital transmission methods in which users share time and frequency allocations and are assigned by unique assigned codes. The signals are separated at the terminals  41 ,  51 ,  37 ,  72  by using a correlator that accepts only signal energy from the desired circuit. Undesired signals are ignored as noise. The IS-95 CDMA technology allows a large number of wireless users simultaneously to access a single radio frequency channel orthogonally, thus reducing interference. This results in a manyfold increase in capacity when compared to analog systems, such as frequency division multiple access (FDMA) systems. 
     It is to be understood that the code division multiple access (CDMA) technology used in the exemplary architectures  10  is only one of many possible technologies that may be employed. Other possible system technologies include third generation mobile phone systems (sometimes referred to as 3G) now under development, frequency division multiplexed (FDM), time division multiple access (TDMA), and multi-frequency time division multiple access (MF-TDMA) technologies. 
     In operation, a call made via a user terminal  41 ,  51 ,  72  will first attempt to connect through the existing local cellular sites  75 , and failing that, via the geosynchronous satellite system  20  or the low earth orbit satellite system  30 . The call is then relayed via satellite  21 ,  31  down to the respective gateway  22 ,  32 , which then routes the call through the PST or PLMN system to its end destination. Thus, the network architectures  10  act as an extension of existing land-based systems. This minimizes capitalization costs required to interface (at the gateway  22 ,  32 ) to the PST or PLMN systems. The user terminals  41 ,  51 ,  72  are dual- or multi-mode terminals, and are compatible with other access standards such as AMPS, IS-95, and GSM protocols, for example. 
     FIGS. 3 and 4 illustrate several key innovative features provided by the present invention. The present invention allows communications signals generated using the low earth orbit communication system  30  to be communicated between the LEO/GEO gateway  22  and the fixed terminals  41  using the “bent pipe” transmission channel provided by the geosynchronous communications satellite  21 . Thus, the geosynchronous communication system  20  is caused to support data transmission having a CDMA or similar structure. The present invention thus interconnects the low earth orbit communication system  30  to the geosynchronous communications system  20 . 
     The block converters employed in the present invention map the low earth orbit CDMA channels into the GEO spectrum for transmission between the GEO satellite and the gateway. Thus, the infrastructure of the low earth orbit communication system  30  is not changed when using the geosynchronous communications system  20 . 
     There are significant benefits derived from using the network architectures  10  including provision of a common CDMA (or similar) platform shared by either LEO or GEO transport. The network architectures  10  has competitive FSS space segment costs and provides for capacity growth for fixed traffic using the FSS band. The network architectures  10  provides a cost effective solution for remote terminals having few voice circuits. The network architectures  10  uses a slightly modified GlobalStar low earth orbit gateway  32  to provide the geostationary gateway  22 , which minimizes development and implementation costs. 
     Thus, low earth orbit, geosynchronous orbit satellite-based communications architectures have been disclosed. It is to be understood that the above-described embodiment is merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.