Background of the Invention In a satellite cellular communications system, a range of communication services can be provided to subscribers who roam outside of the bounds of their home systems. Thus, if the subscriber unit can communicate with an orbiting satellite, the satellite system can provide telecommunications services to the subscriber provided a roaming agreement with the subscriber's home cellular system is in place. Through roaming agreements, virtually any subscriber unit, which can communicate with an orbiting satellite, can place outgoing cellular telephone calls as well as receive incoming cellular telephone calls.

When a subscriber leaves the bounds of the home network and roams into an area served by a satellite cellular communications system, the home system is queried by the satellite cellular communications system in order to determine if the roaming subscriber is valid within the home system. This allows the satellite cellular system to bill the home system for any services which the satellite cellular communications system provides to the roaming subscriber. In a typical system, the satellite system places any information acquired from the subscriber's home network into a visitor location register.

One drawback of roaming arrangements is that the roaming subscriber must make use of the features of the satellite cellular communication system as these features are implemented in the system. Therefore, if there are features which exist only in the home subscriber system which do not exist in the satellite system, the roaming subscriber can make use of only those features offered by the satellite system. Additionally, in the event that the satellite system does offer features equivalent to those offered by the home subscriber network, these features present different user interfaces. These differences require the roaming subscriber to quickly

become accustomed to using the particular feature as it has been implemented within the satellite cellular communications system.

Therefore, what is desirable, is a global telecommunications system which allows a roaming subscriber to make use of the features implemented in the subscriber's home network while communicating by way of a satellite cellular communications system. What is also desirable, is a global telecommunications system which allows these features to be implemented in a manner identical to the implementation in the subscriber's home network in order to allow system differences to be transparent to the roaming subscriber unit.

Brief Description of the Drawings The invention is pointed out with particularity in the appended claims. However, a more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures, and: FIG. 1 illustrates a satellite access subsystem in a cellular communications environment which allows satellite cellular communications using home network services in accordance with a preferred embodiment of the invention; FIG. 2 illustrates another embodiment of a satellite access subsystem in a cellular communications environment which allows satellite cellular communications using home network services in accordance with a preferred embodiment of the invention; and FIG. 3 illustrates a method executed by a satellite access subsystem in a cellular communications environment in accordance with a preferred embodiment of the invention.

Detailed Description of the Drawings A method and system for satellite cellular communications using home network services allows a roaming subscriber to make use of the services provided by a home network while roaming within an area covered by a satellite cellular communications system. Additionally, services provided to a roaming subscriber are provided in a

manner which is substantially identical to the manner in which these services are provided in the subscriber's home cellular communications system. Therefore, as services become available in a subscriber's home network, these services can be extended to the satellite system when the subscriber roams into an area covered by the satellite cellular communications system. Furthermore, operators of terrestrial cellular systems who wish to extend their coverage areas through the use of satellite assets are not required to operate dedicated satellite networks. These operators can make use of existing satellite networks in order to allow their own unique features to be used by their own subscribers regardless of the location of those subscribers.

FIG. 1 illustrates a satellite access subsystem (80) in a cellular communications environment which allows satellite cellular communications using home network services in accordance with a preferred embodiment of the invention. In FIG. 1, subscriber unit 55 communicates with satellite communications node 45 through link 50. Although shown as a cellular telephone, subscriber unit 55 can be any communications device such as a hand-held telephone subscriber unit, portable fax machine, or other device which communicates by way of a satellite communications node to other voice or data terminals.

Messages to and from subscriber unit 55 are conveyed through satellite communications node 45 using link 50. Satellite communications node 45 is representative of a global telecommunications system which provides access to communications services through a network of orbiting satellites. These satellites can be placed in low earth orbits, or can be placed in other types of orbits such as medium earth orbits or geostationary orbits. Additionally, satellite communications node 45 may communicate with other, similarly equipped satellites, through one or more inter- satellite cross links. For this example, satellite communications node 45 communicates directly with earth terminal controller 35 through link 40.

Earth terminal controller 35 communicates with satellite communications node 45 through link 40. This communication includes satellite orbit adjustments, as well as message traffic intended for and originating from subscriber unit 55. In a preferred embodiment, satellite communications node 45 functions to provide roaming subscriber unit 55 with communications services by projecting a communications beam onto the geographic area occupied by subscriber unit 55. The management of satellite resources such as the activation of communications beams, subscriber unit location translations, and cellular telephone call handovers are performed by satellite

base station controller 70. Other examples of satellite resources include a portion of the available frequency bandwidth in a frequency domain multiple access (FDMA) system, a time slice in a time domain frequency access (TDMA) system, or a particular orthogonal code used in a code division multiple access (CDMA) system. Satellite base station controller 70 may be co-located with earth terminal controller 35, or may be remote from earth terminal controller 35. Additionally, although satellite base station controller 70 is shown as communicating with earth terminal controller 35 through routers 25, this need not be the case. Satellite base station controller 70 may interface directly with earth terminal controller 35 without loss of functionality.

Routers 25 function to convey message traffic from earth terminal controller 35 to terrestrial system interface 30 as well as functioning to convey control commands to satellite base station controller 70. Terrestrial system interface 30 interfaces with at least one network services system 14 in at least one public land mobile network (PLMN) 10 as shown in FIG. 1. Desirably, terrestrial system interface 30 interfaces with other networks such as PLMN 20 as shown in FIG. 1. It should be noted that PLMN 10 and network services subsystem 14 may represent any network services system in any network, including private land mobile networks, enhanced specialized mobile radio networks, public switched telephone networks or private branch exchanges supporting wireless local loop systems and packet-switched data networks.

As shown in FIG. 1, network services subsystem 14 may also interface with existing wireless base station subsystems 19, which provide subscribers of PLMN 10 with wireless access to the services of network services subsystem 14. In accordance with a preferred embodiment, satellite access subsystem 80 does not contain functionality, such as location registers or switching centers, normally found within a network services system.

Terrestrial system interface 30 supports connection of satellite access subsystem 80 to PLMN 10 through interface 12. Interface 12 comprises data communication links and protocols for carrying user information. This user information can be encoded speech, facsimile, or other data. Additionally, interface 12 comprises data communication links and protocols for carrying resource control signalling information such as channel assignments. Further, interface 12 comprises data communication links and protocols for carrying user services signalling information such as service requests and responses.

In a preferred embodiment, when subscriber unit 55 roams into the coverage area of satellite communications node 45, service signalling information and user information are delivered between network services subsystem 14 in PLMN 10 and subscriber unit 55 by way of satellite access subsystem 80. For example, when subscriber unit 55 desires to inform PLMN 10 of its location or make use of a particular cellular service provided by PLMN 10, the subscriber unit transmits the appropriate service instantiation request to satellite communications node 45. The service instantiation request is then conveyed to earth terminal controller 35 through link 40.

The request is then routed by way of one of routers 25 to terrestrial system interface 30. The request is then forwarded through interface 12 to network services system 14.

The response to the service instantiation request is then generated in network services subsystem 14 and conveyed through interface 12 back to terrestrial system interface 30. The response is then forwarded through router 25, through earth terminal controller 35, to subscriber unit 55 through satellite communications node 45.

In a preferred embodiment services signalling information between network services subsystem 14 and subscriber unit 55 is treated as a generic message.

Therefore, the content of such traffic is preferably not examined, interpreted, nor implemented by any part of satellite access subsystem 80. This aspect of the invention ensures that for those services which require only signalling to effect, such as location updating and subscription management, the subscriber using subscriber unit 55 experiences those services in substantially the same way when access is provided by satellite access subsystem 80 as when access is provided by base station subsystem 19 in PLMN 10.

Other types of service, such as call setup and release and packet-data session control, require additional access resources beyond the signalling path in order to effect a path for user information flow. In these cases, neither the services signalling nor the user information is examined by any part of satellite access subsystem 80. A resource control signalling protocol is used on interface 12 to establish resources in satellite access subsystem 80 for the user information flow. Using this protocol, network services subsystem 14 requests the necessary access resources, such as channel bandwidth and information adaptation functions. Preferably, the resource control protocol on interface 12 is the same as is used on interface 11 between network services subsystem 14 and base station subsystem 19 inside PLMN 10. The portion of satellite access subsystem 80 which is required to act on the resource

control signalling from network services subsystem 14 is satellite base station controller 70, which must ensure that adequate resources of satellite communications node 45 routers 25, and terrestrial system interface 30 are available in order to provide the particular service feature to subscriber unit 55. In an alternate embodiment suitable for situations in which the network services subsystem 14 cannot participate in a resource control protocol on interface 12, the resource control protocol is used between subscriber unit 55 and satellite base station controller 70. In either case, the service-independent resource control protocol coupled with the transparent passing of services signalling and user information by satellite access subsystem 80 ensures that the user of subscriber unit 55 experiences the network services in substantially the exact same way when access is provided by satellite access subsystem 80 as when access is provided by base station subsystem 19 in PLMN 10.

Through the use of satellite access subsystem 80, operators of public networks, such as PLMNs 10 and 20, can provide services to their subscribers through the existing satellite network. Additionally, these services are provided using the network services systems of the individual networks. Further, since communications between PLMNs 10 and 20 and satellite access subsystem 80 involves resource control, using interface 12, these service features can be implemented through the satellite access system in the precise manner as those services are implemented in each of PLMNs 10 and 20.

FIG. 2 illustrates another embodiment of a satellite access subsystem (80) in a cellular communications environment which allows satellite cellular communications using home network services in accordance with a preferred embodiment of the invention. FIG. 2 incorporates many of the identical elements of FIG. 1. In FIG. 2, the network elements of PLMN 110 include elements which comply with GSM standards.

Preferably, interface 112 and 113 interface satellite base station controllers 170 and 130 of satellite access subsystem 180 with PLMN 110 and 120 using the"A"interface.

As known to those familiar with current GSM standards, the"A"interface is used to communicate between base station subsystems and mobile services switching centers resident within a network subsystem. Thus, satellite base station controller 130 interfaces with mobile services switching center 214 through"A"interface 113.

Similarly, satellite base station controller 170 interfaces to mobile services switching center 114 through"A"interface 112. Additionally, mobile services switching center 114 performs the functions needed to provide service features to the subscriber units

within the coverage area of PLMN 110 based on subscription information stored in home location register 119.

In a similar manner, mobile services switching center 214 performs the functions needed to provide services to the subscriber units within the coverage area of PLMN 120, based on subscription information stored in home location register 219.

For both PLMN 110 and 120, the functionality for providing service features such as location updating, subscriber authentication, inter-access-system handovers, call routing and setup, and a variety of supplementary services such as call forwarding and multi-party calling, are provided within each PLMN. Those skilled in the art will recognize that this list cannot be completely enumerated, as the potential for differentiation based on service provider business needs is unbounded.

In a manner similar to that of the description of FIG. 1, services are provided to the user of subscriber unit 55 by mobile services switching center 114 and home location register 119, via satellite access subsystem 180, according to the principles of the invention."A"interface 112 and elements of satellite access subsystem 80 provide a transport layer which conveys user information and services signalling, between PLMN 110 and subscriber unit 55, without interpreting their contents."A"interface 112 also provides a standard resource control protocol, the"Base Station Subsystem Management Application Protocol"known as BSSMAP to those skilled in the art, for mobile services switching center 114 to request access to system resources from satellite base station controller 170.

As shown in FIG. 2, earth terminal controller 135 desirably interfaces with multiple satellite base station controllers, such as satellite base station controllers 130 and 170, thereby supporting multiple networks, such as PLMNs 110 and 120.

Additionally, for each PLMN supported by satellite access subsystem 80, the associated satellite base station controller may optionally be co-located either with earth terminal controller 135, or with the corresponding mobile services switching center, according to the business and technical circumstances particular to the PLMN.

In yet another embodiment, the network services system of FIG. 1 is a Public Switched Telephone Network. In this embodiment, the interface between satellite access subsystem 80 and the wireline switched telephone network is the V5.2 standard.

FIG. 3 illustrates a method executed by a satellite access subsystem in a cellular communications environment in accordance with a preferred embodiment of

the invention. In step 310, the satellite access subsystem receives a service instantiation request originated from a subscriber unit. In step 320 the service instantiation request is conveyed to a terrestrial communications system via a terrestrial system interface. Also in step 320, a network services system for the network in which the subscriber unit is home provides the response to the service instantiation request. In step 330, the satellite access subsystem receives the message traffic destined for the particular subscriber unit. In step 340, the particular response is conveyed to the subscriber unit through the satellite access system.

A method and system for satellite cellular communications using home network services allows a roaming subscriber to make use of the services provided by a home network while roaming within an area covered by a satellite cellular communications system. Additionally, services provided to a roaming subscriber are provided in a manner which is substantially identical to the manner in which these services are provided in the subscriber's home cellular communications system. Therefore, as services become available in a subscriber's home network, these services can be extended to the satellite system when the subscriber roams into an area covered by the satellite cellular communications system allowing any system differences to transparent to the user. Furthermore, operators of terrestrial cellular systems who wish to extend their coverage areas through the use of satellite assets are not required to operate dedicated satellite networks. These operators can make use of existing satellite networks in order to allow their own unique features to be used by their own subscribers regardless of the location of those subscribers.

Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.