HK1036374B - Method and device for reducing location update procedures in satellite communication systems - Google Patents

Description

Method and apparatus for reducing location update procedure execution steps in a satellite communication system

Technical Field

The present invention relates to the field of communications, and more particularly to a system and method for satellite communications.

Background

A mobile communication system allows users to access a communication network through corresponding user terminals while moving within a geographical area. Some mobile communication systems divide this geographical area into a number of areas, each of which is covered by a part of the mobile communication system. Thus, if a subscriber moves from a first area to a second area within a served geographic area, the mobile communication system stops providing service in the first area and begins providing service in the second area to maintain subscriber access. A user may access the mobile communication system with a user terminal.

Fig. 1A illustrates a satellite system that provides communication services to user terminals 200 within regions 120 and 125 using respective spot beams 101 and 102. This satellite system tracks the location of user terminal 200 within areas 120 and 125 and registers user terminal 200 within this area to provide communication services to user terminal 200. For example, if the user terminal 200 is located within the area 125, the satellite system registers the user terminal 200 as being located within the area 125 and communicates with the user terminal 200 via the spot beam 102. If, however, the user terminal 200 re-enters the region 120, the satellite system will re-register the user terminal 200 as being within the region 120 and then communicate with the user terminal 200 using the spot beam 101.

Spot beams 101 and 102 provide respective communication channels for communication between satellite 110 and user terminal 200. The spot beam also provides the user terminal with a corresponding control channel in the area covered by the spot beam. The control channel may carry information about the service provided by the corresponding spot beam coverage area. For example, the spot beam 102 may broadcast information over a control channel indicating to the user terminals within the area 125 that the spot beam 102 is providing coverage and possibly the communication channel assigned to the spot beam.

The location of the user terminal 200 may also be registered through a location update procedure. The location update procedure is a communication protocol by which the user terminal 200 can communicate the location of the user terminal within the geographic area to the satellite system. When the user terminal 200 moves to a new area and requires re-registration in the new area, a location update procedure may be performed. When the user terminal 200 detects that the signal strength of its current spot beam is lower than the strength of another spot beam, the user terminal 200 may initiate a location update procedure. For example, if the user terminal 200 is located in the area 125 serviced by the spot beam 102 and then moves into the area 120, the user terminal 200 will detect that the signal strength of the spot beam 101 is higher. Upon detecting that the signal strength of spot beam 101 is higher, user terminal 200 switches from the control channel associated with spot beam 102 to the control channel associated with spot beam 101 and informs the satellite system of its new location corresponding to region 120 using information broadcast on the control channel associated with spot beam 101. Thus, the user terminal 200 is re-registered with the satellite system in the area 120. This satellite system then communicates with the user terminal 200 using spot beam 101. The user terminal 200 may also periodically perform a location update procedure if it is operated in a certain area for a long time.

Some satellites operate in orbits called inclined orbits, and their spot beams project onto the earth in geographic areas that are periodically shifted or swept. Such a wobble is illustrated in fig. 1B. In thatTime t₁Satellite 110 services region 120 via spot beam 101 and region 125 via spot beam 102. The user terminal 200 is located within the area 125 served by the spot beam 102. At time t₂The tilt of the tracks causes spot beam wobble for coverage areas 120 and 125. Specifically, spot beam 101, which previously covered area 120, now covers area 120'. Similarly, spot beam 102, which previously covered area 125, now covers area 125'. Further, the user terminal 200, which was previously located within the area 125, is now located within the area 120' served by the spot beam 101. Thus, the user terminal 200 does not move but experiences a drift of the spot beam service once. At time t₃And then resumes the time t₁Causing another drift in the spot beam coverage and service to the user terminal 200. The wobble associated with the inclined tracks causes the spot beam footprint shown in fig. 1B to drift periodically.

As described above, the user terminal 200 may initiate a location update procedure upon detecting a change in spot beam service. For example, at time t₁The user terminal 200 detects the service of the spot beam 102. And at a time t₂The user terminal 200 detects that it has changed and the service to it is provided by the spot beam 101. Thus, the user terminal 200 may initiate a location update procedure. Subsequently, the user terminal 200 detects another drift of the spot beam service when the tilted tracks cause a wobble in the opposite direction. Therefore, the user terminal can perform another location update again. Also, a location update procedure may be performed for each periodic drift request of the spot beam coverage area.

The user terminal 200 may be a communication device, such as a radiotelephone, capable of communicating with a satellite system. The user terminal 200 may detect the signal strength of the spot beam and the beam pair location update timer in the user terminal 200 may measure the time to change with the spot beam service area. The periodic location update timer in the user terminal 200 may measure the time elapsed since the current spot beam served the user terminal 200.

In some systems the number of user terminals may be very large in the area affected by the periodic wobble caused by the inclined tracks. For example, in some systems, 10% of the user terminals served by the satellite system may be located in the area affected by spot beam service area swings. Thus, 10% of the user terminals 200 served by the satellite communication system will initiate a location update procedure upon detecting each oscillation of the spot beam service area. Performing such a large number of location update procedures can place a significant strain on satellite system resources.

Existing systems may use a registration procedure to reduce the number of location updates caused by spot beam coverage area swings. This registration process may require each spot beam to broadcast a single Location Area Code (LAC) on the corresponding control channel that uniquely identifies the spot beam in the satellite system, as well as a list of adjacent spot beams called "beam pairs". This beam pair may also be determined with one unique beam pair LAC. For example, spot beam 102 and spot beam 101 form a beam pair 115. The control channel of the spot beam 102 may carry a list of the individual LACs corresponding to the spot beam 102 and the LACs that each correspond to a pair of spot beams. One LAC in the list of beam pairs LAC would correspond to beam 115 formed by spot beam 101 and spot beam 102. The user terminal 200 may receive the single LAC and beam pair LAC broadcast on the control channel and register with the satellite system using these LACs. The satellite system then uses the LAC registered by the user terminal 200 to determine the location of the user terminal. In addition, the user terminal 200 may store the single LAC broadcast for the spot beam it serves, as well as a list of spot beam pair LACs. When the user terminal 200 detects that its spot beam service area has drifted, this user terminal 200 may consult the stored list of individual LACs and beam pair LACs to determine whether a location update procedure needs to be performed.

If the stored list of spot beam pairs LAC indicates that the new spot beam providing service forms a beam pair with the registered spot beam, the user terminal 200 can register with the spot beam pair. For example, if the user terminal 200 were to move from area 125 to area 120, the user terminal 200 would detect that the coverage area of the spot beam 102 became the coverage area of 101. In addition, the ue 200 checks the LAC broadcast by the spot beam 102 and the stored list of LAC pairs to determine that the spot beam 101 is paired with the spot beam 102. Thus, user terminal 200 may register with the satellite system to provide service as a spot beam pair formed by spot beam 101 and spot beam 102. Then, all communication requests (i.e., paging information) to the user terminal 200 are broadcast in the spot beam 101 and the spot beam 102 at the same time.

When the user terminal 200 registers with the beam pair serving it, the user terminal 200 starts a beam pair location update timer. The timing length of the beam pair position update timer exceeds the wobble period of the inclined tracks. For example, if the wobble period of the inclined track is six hours, the beam pair position update timer 220 is longer than six hours. If the user terminal does not detect that its original spot beam coverage area has recovered before the beam pair location update timer expires, this user terminal 200 registers with the single spot beam currently serving. If the user terminal detects that it has returned to the original spot beam service area, the user terminal maintains its current registration with the beam pair and restarts the beam pair location update timer. By having the user terminal maintain the beam pair registration unchanged, the system has determined that the user terminal is located in an area where the wobble is caused by the inclined tracks. Furthermore, if the user terminal is always within this area, it may find its spot beam service area to be continuously swinging.

European patent application No. 0663736a1 entitled "user paging for mobile satellite communications" discusses a method of communication between a ground station and a mobile hand-note.

Despite the above-discussed communication systems and methods, there is still a need to further reduce the number of location update procedures performed in a satellite communication system.

Brief description of the invention

It is therefore an object of the present invention to provide an improved satellite communication method and system.

It is another object of the present invention to provide a satellite method and system that further reduces the number of location update procedures performed.

These objects and others are achieved by a method of registering a user terminal in a satellite communications system including a plurality of satellite spot beam transmitters, each of which defines a respective spot beam. The periodic oscillation of two adjacent satellite spot beam transmitters causes the geographic coverage of the corresponding adjacent spot beams to change, thereby enabling the user terminal to be alternately covered by the first and second spot beams while the user terminal is stationary. Upon determining that the user terminal is alternately covered by the first and second spot beams, the user terminal registers with a spot beam pair of the first and second adjacent spot beams, and registration information for the spot beam pair is stored in a memory of the user terminal. Communication operations between the user terminal and the satellite communication system may be terminated and re-established at a later time, at which point the user terminal re-registers with the spot beam pair indicated by the registration information stored in the memory.

Subsequent location update procedures may be dispensed with by re-registering the user terminal with the same spot beam pair that the user terminal previously registered with the communication operation terminated. In prior art systems, a user terminal would register with a single spot beam providing service when a communication operation is reestablished. Whereas if the user terminal is located in an alternate coverage area, the prior art end user terminal will re-register when the wobble is first detected. The method of the present invention is to establish service with this beam pair with a single location update when the communication operation is re-established.

The present invention may also reduce the number of location updates by prioritizing beam pair location update procedures and periodic location update procedures. A periodic update timer is used which initiates a location update procedure at a predetermined period to assist the satellite system in determining in time whether the user terminal can be accessed. The periodic location update timer has a period controlled by the satellite system. The location update may be performed when the periodic location update timer expires. This beam pair location update procedure is given priority over the location update procedure derived from the periodic location update timer by restarting the periodic update timer at the time the beam pair location update procedure is performed. The present invention can thus reduce the number of times the location update procedures are performed, which otherwise would be performed when the beam stops timing the location update timer. In other words, according to the present invention, the number of times of performing the location update procedure can be reduced by normalizing the operation of the beam pair and the periodic location update timer.

Reducing location update procedures can be critical to smooth operation of the satellite communication system. As described above, the present invention can reduce the traffic of the control channel by re-registering one beam pair that has served the user terminal before terminating the communication operation and by prioritizing the beam pair and the periodic location update. In particular, the location update procedure may occupy channel bandwidth. Repeated location updates can therefore cause system problems, especially when there are a large number of user terminals as in the prior art, and unnecessary location updates can be initiated.

Brief Description of Drawings

FIG. 1A is a block diagram of a prior art satellite communication system;

FIG. 1B is a schematic illustration of a prior art tilt of a satellite orbit resulting in a wobble spot beam footprint;

FIG. 2 is a block diagram of a satellite communication system in accordance with the present invention;

FIG. 3 is a block diagram of the user terminal shown in FIG. 2;

fig. 4A, 4B, and 4C are flowcharts illustrating the operation of the satellite communication system according to the present invention.

Detailed Description

The present invention will now be described in detail with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the invention to those skilled in the art. Like numbers refer to like elements throughout. As will be appreciated by one skilled in the art, the present invention may be embodied as a method or apparatus. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects.

Figure 2 shows a satellite system employing the present invention. Satellite 310 projects spot beams 301 and 302 to provide communication services to user terminals 500 in areas 320 and 325. Each spot beam may be identified by a unique single Location Area Code (LAC). The single LAC is broadcast in the control channel of each spot beam. This satellite system also pairs adjacent spot beams. For example, spot beam 301 and spot beam 302 may form a spot beam pair 315. The spot beam pair 315 also has a unique beam pair LAC. Each spot beam in the satellite system broadcasts a list of beam pairs LAC, wherein each beam pair LAC included in the list identifies a beam pair comprising the corresponding spot beam and an adjacent spot beam. For example, spot beam 302 may broadcast a list of beam pair LACs that include LACs corresponding to spot beam pair 315. The user terminal 500 registers with the satellite system using the spot beam LAC providing the service. For example, if the spot beam 301 is serving the user terminal 500, the user terminal registers with the satellite system using the LAC of the spot beam 301. Similarly, if the beam pair 315 is serving the user terminal 500, the user terminal registers with the LAC of the beam pair 315 in the satellite system.

Referring to fig. 3, a user terminal 500 may be a communication device, such as a radiotelephone, capable of communicating with a satellite communication system. The user terminal 500 may also be a personal computer, a personal digital assistant or other electronic device capable of communicating with a satellite communication system. The user terminal may also have dual functionality, thereby enabling the user terminal to communicate with a terrestrial communication system, such as a cellular radiotelephone system. The user terminal 500 may include an antenna 505 for transmitting and receiving communication signals between the satellite 310 and the user terminal 500. The transceiver 510 receives communication signals for transmission to a satellite and receives received communication signals. Detector 515 detects the signal strength of the spot beam received by transceiver 510. The beam pair location update timer 520 measures the time of change of the spot beam service area. The periodic location update timer 540 measures the time elapsed since the last location update procedure was performed. Processor 525 manages the operation of the user terminal and coordinates the operation of the components described herein. The user terminal memory 535 stores information for operating and managing the user terminal 500. User terminal memory 535 may be a non-volatile memory. I/O530 provides general-purpose input and output functions for user terminal 500. For example, I/O530 may provide a keypad, a display, and a speaker for user terminal 500.

By monitoring the control channels of the spot beams, the user terminal 500 detects the location area code of the spot beam broadcast currently serving the user terminal 500. The user terminal 500 then registers with the satellite system and stores a list of the individual LACs and beam pair LACs broadcast by the current spot beam. The user terminal 500 may receive a single LAC identifying the spot beam 302 and a list of beam pair LACs representing spot beam pairs including the spot beam 302, register with the single LAC representing the spot beam 302, and store this single LAC and the list of beam pair LACs. For example, spot beam 302 broadcasts a list of beam pairs LAC that includes the beam pairs LAC corresponding to beam pair 315. Spot beam 301 broadcasts a list of beam pairs LAC, which also includes LACs corresponding to beam pairs 315. Thus, if the spot beam coverage area of the user terminal 500 changes, the user terminal 500 can determine that it is currently being serviced by a spot beam that is a pair with the previously serviced spot beam by confirming that each of the beam pairs LAC includes an identical beam pair LAC, such as the beam pair LAC of spot beam pair 315.

The user terminal 500 may be served with spot beam or spot beam pair registration via a location update procedure. This location update procedure is a communication protocol that the user terminal 500 can use to inform the satellite communication system of the user terminal's location within the geographic area covered by a spot beam communication signal or spot beam pair. The location update procedure may be performed when the user terminal 500 moves to a new area and needs to re-register with a new area spot beam. The user terminal may initiate a location update procedure if the user terminal 500 detects that the signal strength of its current spot beam is weaker than the signal strength of another spot beam.

If the spot beam coverage has become a beam pair, the user terminal assumes that no location update is currently required. For example, if the comparison above indicates that the current spot beam coverage is provided by one spot beam included in a beam pair that includes the spot beam with which the user terminal is currently registered, then the drift in spot beam coverage detected by the user terminal 500 may be due to wobble associated with the tilted track. Alternatively, this change in coverage may be due to movement of the user terminal 500. If the change in coverage area is caused by movement of the user terminal 500, the user terminal 500 may re-register with the satellite system as being located in the area 320 serviced by the spot beam 301. If the change in coverage area is due to a wobble, however, user terminal 500 can register in spot beam pair 315 and request that a communication signal, such as a paging signal, be carried by both spot beam 301 and spot beam 302 for subsequent transmission to user terminal 500.

If the change in coverage area detected by the user terminal 500 originates from a tilted track, the user terminal 500 experiences a periodic wobble of the coverage area. The user terminal 500 then starts a beam pair location update timer 520 that counts a length longer than the satellite spot beam wobble period. Thus, if the user terminal 500 encounters a drift in spot beam coverage due to the wobble, subsequent spot beam coverage drift will occur before the beam pair position timer expires. If, however, the beam pair location update timer 520 stops counting before the user terminal 500 encounters the next spot beam coverage drift, the user terminal 500 assumes that the previous coverage drift was due to movement of the user terminal 500. When the user terminal 500 detects a drift in the spot beam coverage area before the beam pair location update timer expires, the user terminal 500 restarts the beam pair location update timer. This means that the user terminal may be in an area where the coverage area is wobbled due to the inclined tracks. If so, the coverage will return to being provided by the original spot beam within some predetermined time. Thus, the user terminal starts a beam pair location update timer that counts longer than the wobble period associated with the inclined track, and coverage returns to that provided by the originally served spot beam before the beam pair location update timer 520 stops counting. If the user terminal is not in the area affected by the wobble, the beam pair location update timer will stop counting before the user terminal detects a return to the original spot beam coverage area and the user terminal will register with the new spot beam. The user terminal maintains registration in the spot beam pair if the user is still in an area affected by coverage area fluctuations. The beam pair location update timer 520 is then restarted each time the coverage area changes between the spot beams that make up the spot beam pair.

The methods and systems of the present invention can register with a spot beam pair and then re-establish communication within a coverage area of the same spot beam pair to further reduce the number of location updates that can be made when a user terminal terminates communication with a satellite communication system. When re-establishing communication in prior art systems, the user terminal detects a single LAC for the servicing spot beam and registers with the single LAC. If the user terminal is located within an area of the swing coverage area, the user terminal may re-register within the beam pair at this point as described above.

In accordance with the present invention, the LAC of the spot beam pair 315 is stored in the memory of the user terminal as currently registered. When reestablishing communications, the user terminal 500 compares the LAC stored in the user terminal memory with the list of beam pairs LAC broadcast by the currently serving spot beam. If the LAC stored in the memory of the ue is the same as one of the LAC in the list of LAC pairs currently being broadcast by the servicing spot beam, the ue 500 re-registers with the same LAC pair. Thus, the present invention may be registered with a location update procedure. It should be understood that such an interruption may include any interruption in communications between the user terminal and the satellite communications system that results in a change in the registration information stored by the system, such as switching to a terrestrial system (e.g., a cellular system), or powering down.

The invention can set the priority for the position updating timer by the position updating program started by the periodic timer and the beam, thereby reducing the times of position updating. The two timer-initiated location update procedures are prioritized such that the location update beam obtains a higher priority over the timer-initiated location update procedures than the periodic location update timer-initiated location update procedures. Therefore, when a beam pair location update is performed because the beam pair location update timer stops counting, periodic location updates that have not been performed are ignored. The location update procedure may be suspended because the user terminal is unable to communicate with the satellite system due to the signaling state of the user terminal or due to radio conditions. After the beam pair location update is completed, the periodic location update timer 540 is restarted.

Alternatively, if the two timers stop counting at the same time, the ue 500 may perform the beam pair location update and restart the periodic timer. Then, when the location update periodicity timer 540 stops counting, the same LAC used for location update is updated with the previous beam. However, if the periodic location update timer 540 stops, and the beam pair location update timer 520 is still running, the location update can be performed without any effect on the state of the beam pair location update timer 520.

Fig. 4A, 4B, 4C are flow charts illustrating the operation of the present invention. It will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions. These program instructions may be provided to a processor in the satellite communication system and/or the user terminal so that the instructions which execute on the processor create means for implementing the functions specified in the flowchart block or blocks. These computer program instructions may be executed by a processor to cause a series of operational steps to be performed by the processor to produce a computer implemented process such that the instructions, which execute on the processor to provide steps for implementing the functions specified in the flowchart block or blocks.

Accordingly, blocks of the flowchart illustrations support combinations of means for performing the specified functions, combinations of steps for performing the specified functions and program instruction means for performing the specified functions. It will also be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or by combinations of special purpose hardware and computer instructions.

Referring now to fig. 4A, 4B, and 4C, a user terminal detects a change in coverage from a first spot beam offering to a spot beam offering served by a current offering (block 600). The user terminal reads the LAC of the list of spot beam and beam pair LACs currently providing service (block 602) and determines whether the registered LAC is equal to one LAC in the list of spot beam pair LACs providing service (block 606). If the registered LAC is equal to one LAC in the LAC list of beam pairs for the servicing spot beam, then the user terminal is currently registered with the beam pair comprising the first spot beam and the servicing spot beam. The user terminal starts a beam-to-location update timer and (block 608) gives the beam-to-location update procedure a higher priority than any simultaneous periodic location update procedure, see fig. 4C. Thereafter, the beam pair location update timer is monitored during normal operation of the user terminal (block 612). If the beam pair location update timer expires, the user terminal re-registers with the single LAC of the servicing spot beam. If the user terminal detects a change in the serving spot beam coverage before the beam pair location update timer expires, the process restarts at block 600.

Returning now to block 606, if the ue determines that the current ue is not registered with a beam pair, the ue determines whether the list of beam pair LACs stored in the ue has a same entry as the list of currently serving spot beam LACs (block 610). If the same beam pair is found, the user terminal is served by one spot beam that forms a pair with the first spot beam. The user terminal performs a location update with the LAC of the beam pair, registers with the beam pair (block 616), starts a beam pair location update timer for the serving spot beam LAC, and gives the beam pair location update procedure a higher priority than all simultaneous periodic location update procedures, see fig. 4C (block 618). The beam pair location update timer is then monitored during normal operation of the user terminal (block 619). If the beam pair location update timer expires, the user terminal re-registers with the single LAC of the servicing spot beam. If the user terminal detects a change in the serving spot beam coverage before the beam pair location update timer expires, the process restarts at block 600. The user terminal stores a Location Area Identity (LAI) (block 622) and a list of beam pairs LAC for the spot beams providing service (block 622) in the memory of the user terminal. Whereupon the user terminal continues normal operation (block 626).

Returning now to block 610, if the beam pair LAC stored in the user terminal does not include an entry for the location area code broadcast for the servicing spot beam, then the user terminal is serviced by a new spot beam that is not paired with the spot beam that previously serviced the user terminal. Thus, the user terminal performs a location update with the servicing spot beam by using the single LAC of the servicing spot beam, registers with the servicing spot beam (block 614), and clears the beam to the location update timer if it is counting (block 620). The user terminal then stores the registered LAI in the memory of the user terminal (block 622).

Referring now to fig. 4B, during normal operation of the user terminal (block 626), the user terminal may be instructed to terminate communication with the satellite system (block 628). Terminating the communication may include turning off the user terminal or switching to a terrestrial communication system, such as a cellular communication system. If the user terminal is turned off, the user terminal clears the beam pair location update timer if it is still counting (block 630) and terminates the run. If the user terminal switches to the terrestrial communication mode (block 628), the user terminal clears the beam pair location update timer (block 634) and waits for communication to be reestablished in the satellite mode (block 636). When communications are reestablished in satellite mode (block 636), the user terminal detects the spot beam providing the service and reads a list of individual LACs and beam pairs LACs (block 640). If the registered LAC stored by the user terminal is equal to one of the entries in the list of beam pair LACs (block 642), the user terminal performs a location update for the location area code with the same beam (block 616), starts a beam pair location update timer for the servicing spot beam LAC, and gives the location area update procedure a higher priority than all simultaneous periodic location update procedures, see fig. 4C (block 618). If, however, the registered LAC is different from any of the entries in the list of beam pair LACs (block 642), then the current service is provided by a spot beam for which the previous serving spot beam did not form a beam pair. Thus, the user terminal can perform a location update with a single LAC of the servicing spot beam (block 614).

Referring now to fig. 4C, the present invention prioritizes the beam pair location update timers and the periodic location update timer triggered location updates (block 644). If the beam pair or periodic location timer stops counting (block 646) and the radio conditions or signaling conditions do not allow a location update (block 650), the user terminal waits until the radio conditions or signaling conditions allow a location update. If the radio conditions and signaling status allow for location updates (block 650), a corresponding location update is made (block 654). But both timers will be suspended if the remaining timers stop counting (block 652) before radio conditions or signaling conditions allow a location update (block 658).

When the radio conditions or signaling state allow for the location update procedure to be performed, the user terminal performs the location update procedure with respect to the beam pair location timer (block 660). The user terminal then ignores the expiration of the periodic timer and the corresponding location update procedure (block 662). The user terminal then restarts the periodic location update timer (block 664).

In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims (15)

1. A method of registering a user terminal (500) in a satellite communications system including a plurality of satellite spot beam transmitters, each of which provides a respective spot beam (301, 302), wherein the user terminal (500) is alternately covered by first and second spot beams (301, 302) when the user terminal (500) is stationary, the method comprising the steps of:

detecting, at the user terminal (500), a change from being covered by a first spot beam (301) to being covered by a second adjacent spot beam (302);

registering the user terminal (500) with the spot beam pairs of the first and second adjacent spot beams (301, 302) if the spot beam pair list of the first spot beam (301) and the spot beam pair list of the second spot beam (302) include a same spot beam pair entry;

storing the spot beam pair record in a memory (535);

starting a beam pair location update timer (520), wherein the beam pair location update timer (520) is timed to be longer in length than the period of oscillation of the satellite spot beam transmitter; and

registering the user terminal with a single spot beam record when the beam times out a location update timer (520);

terminating communication operations between the user terminal (500) and the satellite communication system;

reestablishing communication operations between the user terminal (500) and the satellite communication system; and

the user terminal (500) is re-registered with the spot beam pair record stored in the memory (535).

2. The method of claim 1, wherein prior to the step of registering, comprising the steps of:

it is determined whether at least one of the plurality of spot beam pairs transmitted in a channel in the spot beams (301, 302) currently covering the user terminal (500) is the same as the stored spot beam pair record.

3. The method of claim 2, further comprising the steps of:

the beam pair location update timer (520) is cleared when communication between the user terminal (500) and the satellite communication system is terminated.

4. The method of claim 2, further comprising the steps of:

starting a periodic update timer (540), wherein a timing length of the periodic update timer (540) is longer than a timing length of the beam pair location update timer (520);

performing a periodic location update when the periodic update timer (540) times out; and

when the beam pair location update timer (520) times out, the periodic update timer (540) is restarted.

5. The method of claim 4, further comprising the steps of:

storing the spot beam pair records in a memory (535);

terminating communication operations between the user terminal (500) and the satellite communication system;

reestablishing communication operations between the user terminal (500) and the satellite communication system; and

the user terminal (500) is re-registered with the spot beam pair record stored in the memory (535).

6. The method of claim 4, wherein the user terminal (500) is a dual-mode user terminal (500) capable of operating in both a satellite communication system and a terrestrial communication system, and the communication operation between the user terminal (500) and the satellite communication system is terminated when the user terminal (500) transitions to a communication operation with the terrestrial communication system.

7. The method of claim 6, wherein the terrestrial communication system comprises a cellular communication system.

8. The method of claim 3, further comprising the steps of:

maintaining spot beam pair records for the user terminal (500) when a change in the user terminal coverage area is detected during operation of the beam pair location update timer (520); and

the beam pair location update timer is restarted (520).

9. The method of claim 3, further comprising the steps of:

storing the spot beam pair record in a user terminal memory (535) prior to terminating the communication operation;

after terminating the communication, reestablishing communication between the user terminal (500) and the satellite communication system; and

the user terminal is re-registered (500) with the stored spot beam pair record.

10. The method of claim 9, wherein the step of re-registering is preceded by the steps of:

it is determined whether at least one of the plurality of spot beam pairs currently transmitted in one channel of the spot beams (301, 302) covering the user terminal (500) is registered with a stored spot beam pair.

11. The method of claim 4, wherein the beam pair location update timer (520) is given a higher priority than the periodic update timer (540), such that when the periodic update timer (540) and the beam pair location timer (520) time out, single beam pair registration is performed and the periodic update timer (540) is restarted without a periodic location update.

12. A mobile user terminal (500) for communicating with a satellite communications system, the system comprising a plurality of satellite spot beam receivers and a plurality of satellite spot beam transmitters, wherein each satellite spot beam transmitter provides a respective spot beam (301, 302), the user terminal (500) being alternately covered by a first and a second spot beam when the user terminal (500) is stationary, the user terminal (500) comprising:

a transceiver (510) for receiving satellite communication signals from a spot beam transmitter and for transmitting the communication signals to a satellite spot beam receiver;

a detector (515) coupled to said transceiver, wherein said detector (515) detects that said user terminal (500) is alternately covered by first and second spot beams (301, 302);

a processor (525) coupled to the detector (515), wherein the processor (525) records the registered user terminal (500) with a spot beam pair of the first and second adjacent spot beams (301, 302) based on a detection of whether the user terminal (500) is alternately covered;

a beam pair location update timer (520) coupled to said processor (525), wherein said beam pair location update timer (520) has a length greater than a period of oscillation of a satellite spot beam transmitter, wherein said beam pair location update timer (520) is activated in response to said registration with said spot beam pair, wherein said beam pair location update timer (520) triggers said processor (525) to register a subscriber terminal (500) with a single spot beam when said beam pair location update timer (520) times out; and

a periodic update timer (540), wherein the timing length of the periodic update timer (540) is longer than the timing length of the beam pair location update timer (520), when the periodic update timer (540) times out, the periodic update timer (540) triggers the processor (525) to perform a periodic location update, wherein the periodic update timer (540) is restarted according to the time out of the beam pair location update timer (520).

13. The user terminal of claim 12, wherein the beam pair location update timer (520) is given a higher priority than the periodic update timer (540), such that when the periodic update timer (540) and the beam pair location timer (520) time out, the single spot beam registration is completed and the periodic update timer (540) is restarted without a periodic location update.

14. The user terminal (500) of claim 12, wherein the beam pair location update timer (520) is cleared when a communication operation between the user terminal (500) and the satellite communication system is terminated.

15. The user terminal (500) of claim 12 further comprising a user terminal memory (535) storing a record of said spot beam pairs, wherein said processor (525) re-registers said user terminal (500) with said record of spot beam pairs stored in said memory (535) after terminating communications operations between said user terminal and said satellite communications system and then re-establishing communications operations between said user terminal (500) and the satellite communications system.