Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-02-25
2001-01-30
Le, Thanh Cong (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S435100
Reexamination Certificate
active
06181932
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of communications and more particularly to systems and methods for satellite communications.
BACKGROUND OF THE INVENTION
Mobile communication systems allow users access to a communications network through respective user terminals as the user moves within a geographic region. Some mobile communications systems partition the geographic region into areas wherein each area may be covered by a portion of the mobile communications system. Consequently, if a user moves from a first area to a second area within the serviced geographic region, the mobile communications system may stop providing service in the first area and start providing the service in the second area to maintain the user's access. The users may access the mobile communications system using a user terminal.
FIG. 1A
illustrates a satellite system which provides communication service to user terminals
200
located within areas
120
and
125
via respective spot beams
101
and
102
. The satellite system provides communication service to the user terminal
200
by tracking the location of the user terminal
200
within areas
120
and
125
and registering the user terminal
200
within that area. For example, if the user terminal
200
were located in area
125
, the satellite system would register the user terminal
200
as located in area
125
and communicate with the user terminal
200
via spot beam
102
. If, however, the user terminal
200
were to relocate to area
120
, the satellite system may re-register the user terminal
200
as located in area
120
and subsequently communicate with the user terminal
200
via spot beam
101
.
Spot beams
101
and
102
may provide respective communications channels for communications between the satellite
110
and the user terminal
200
. A spot beam may also provide respective control channels to user terminals within the area covered by the spot beam. A control channel may carry information concerning the service provided with the area covered by the respective spot beam. For example, spot beam
102
may broadcast information over a control channel which identifies spot beam
102
to the user terminals located within area
125
, and may also identify communications channels assigned to the spot beam.
The location of the user terminal
200
may be registered via a location update procedure. The location update procedure is a communications protocol whereby the user terminal
200
may notify the satellite system of the user terminal's location within the geographic region. A location update procedure may be performed when the user terminal
200
moves to a new area and requires re-registration in the new area. The user terminal
200
may initiate the location update procedure when the user terminal
200
detects that the signal strength of its present spot beam is less powerful than that of another spot beam. For example, if the user terminal
200
were located within area
125
serviced by spot beam
102
and subsequently moved to area
120
, the user terminal
200
may detect the greater signal strength of spot beam
101
. Upon detecting the greater signal strength of the spot beam
101
, the user terminal
200
switches from the control channel associated with spot beam
102
to the control channel associated with spot beam
101
and notifies the satellite system of its new location corresponding to area
120
using the information broadcast on the control channel associated with spot beam
101
. Thus the user terminal
200
re-registers with the satellite system in area
120
. Subsequently, the satellite system communicates with the user terminal
200
using spot beam
101
. The user terminal
200
may also perform periodic location update procedures if the user terminal
200
operates in a particular location for a long time.
Some satellites may exhibit a behavior known as an inclined orbit wherein the spot beams projected onto the geographic region periodically shift or oscillate. Such an oscillation is illustrated in FIG.
1
B. At a time t
1
, the satellite
110
services the area
120
via spot beam
101
and the area
125
via spot beam
102
. The user terminal
200
is located within the area
125
serviced by spot beam
102
. At a time t
2
, the inclined orbit causes an oscillation in the spot beams covering areas
120
and
125
. Specifically, spot beam
101
, formerly covering area
120
, now covers area
120
′. Similarly, the spot beam
102
, formerly covering area
125
, now covers area
125
′. Moreover, user terminal
200
, formerly located in area
125
, is now located within the area
120
′ serviced by spot beam
101
. Thus, the user terminal
200
has experienced a shift in spot beam service without moving. At a time t
3
, the coverage shown at time t
1
is restored, causing yet another shift in the coverage of the spot beams and the service to the user terminal
200
. The oscillation associated with an inclined orbit may be such that the shift in spot beam coverage described in
FIG. 1B
happens periodically.
As described above, the user terminal
200
may initiate location update procedures upon the detection of changes in the spot beam service. At time t
1
, for example, the user terminal
200
detects service via spot beam
102
. At time t
2
, however, user terminal
200
detects a change such that its service is provided by spot beam
101
. Consequently, the user terminal
200
may initiate a location update procedure. Subsequently, the user terminal
200
may detect another shift in spot beam service when the inclined orbit causes an oscillation in the reverse direction. Consequently, the user terminal may perform yet another location update. Moreover, a location update procedure may be requested for each periodic shift in the spot beam coverage.
The user terminal
200
may be a communications device, such as a radio telephone, which is capable of communicating with satellite system. The user terminal
200
may detect the signal strength of spot beams and a beam pair location update timer within the user terminal
200
may measure time associated with a change in spot beam service. A periodic location update timer, within the user terminal
200
, may measure the time elapsed since the present spot beam began servicing the user terminal
200
.
In some systems, the number of user terminals located within an area affected by the periodic oscillation resulting from the inclined orbit may be significant. For example, in some systems 10% of the user terminals serviced by the satellite system may be located in areas subject to oscillations in spot beam service. Consequently, 10% of the user terminals
200
serviced by satellite communications system may initiate location update procedures upon detecting each oscillation in spot beam service. Such a significant number of location update procedures may cause a significant strain on the satellite system's resources.
Existing systems may use a registration process to reduce the number of location updates produced by oscillations in spotbeam coverage. The registration process may require each spot beam to broadcast a single location area code (LAC) which uniquely identifies the spot beam within the satellite system and a list of neighboring spot beams called “beam pairs” on the corresponding control channel. The beam pairs may also be identified by a unique beam pair LAC. For example, spot beam
102
and spot beam
101
form a beam pair
115
. The control channel for spot beam
102
may therefore carry the single LAC corresponding to spot beam
102
and a list of LACs each of which correspond to a pair of spot beams. One of the LACs within the list of beam pair LACs would correspond to the beam
115
formed by spot beam
101
and spot beam
102
. The user terminal
200
may receive the single LAC and beam pair LAC list broadcast on the control channel and use the LACs to register with the satellite system. Subsequently, the satellite system locates the user terminal using the LAC with which the user term
Kolev Javor
Lindvall Hakan
Weiss Richard
Cong Le Thanh
Corsaro Nick
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
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