Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-05-01
2002-07-09
Urban, Edward F. (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S427000, C455S012100, C342S357490
Reexamination Certificate
active
06418314
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 communications 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 communications system which provides communication service to user terminals located within areas
120
and
125
via respective spot beams
101
and
102
. The satellite system provides communication service to user terminal
100
by registering the user terminal
100
within the spot beam corresponding to the area within which the user terminal is located. For example, if the user terminal
100
were located in area
125
, the satellite system would register the user terminal
100
as located in area
125
and communicate with the user terminal
100
via spot beam
102
. If, however, the user terminal
100
were to relocate to area
120
, the satellite system may re-register the user terminal
100
as located in area
120
and subsequently communicate with the user terminal
100
via spot beam
101
.
Spot beams
101
and
102
may provide respective communications channels for communications between the satellite
110
and the user terminal
100
. In particular, a spot beam may 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 to 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 user terminals located within area
125
, and may also identify traffic channels assigned to the spot beam. Spot beams may also be grouped in beam pairs so as to identify spot beams which are adjacent. For example, spot beam
102
and spot beam
101
form a beam pair
115
. Moreover, a user terminal
100
may be registered with spot beam
101
, spot beam
102
, or the beam pair
115
.
The user terminal
100
may be a communications device, such as a radio telephone, which is capable of communicating with the satellite communications system.
The location of the user terminal
100
may be registered via a location update procedure. The location update procedure is a communications protocol whereby the user terminal
100
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
100
moves to a new area and requires re-registration in the new area. The user terminal
100
may initiate the location update procedure when the user terminal
100
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
100
were located within area
125
serviced by spot beam
102
and subsequently moved to area
120
, the user terminal
100
may detect the greater signal strength of spot beam
101
. Upon detecting the greater signal strength of the spot beam
101
, the user terminal
100
switches from the control channel associated with spot beam
102
to the control channel associated with spot beam
101
and notifies the satellite communications 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
100
re-registers with the satellite system in area
120
. Subsequently, the satellite communications system communicates with the user terminal
100
using spot beam
101
. The user terminal
100
may also perform periodic location update procedures if the user terminal
100
operates in a particular location for some period of 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
100
is located within the area
125
serviced by spot beam
102
. At a time t
2
, the inclined orbit may cause 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
100
, formerly located in area
125
, is now located within the area
120
′ serviced by spot beam
101
. Thus, the user terminal
100
has experienced a shift in spot beam service without moving. At a time t
3
, the coverage shown at time t
1
may be restored, causing yet another shift in the coverage of the spot beams and the service to the user terminal
100
. 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
100
may initiate location update procedures upon detecting a change in the spot beam service. At time t
1
, for example, the user terminal
100
detects service via spot beam
102
. At time t
2
, however, user terminal
100
detects a change such that its service is provided by spot beam
101
. Consequently, the user terminal
100
may initiate a location update procedure. Subsequently, the user terminal
100
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
100
may also detect a change in spot beam service when the user terminal moves from one area to another. For example, if user terminal
100
were moved from area
125
to area
120
, the user terminal may detect the change in spot beam service from spot beam
102
to spot beam
101
. The user terminal
100
may then perform a location update procedure to register with spot beam
101
.
Existing systems may use a registration process to reduce the number of location updates produced by oscillations in spotbeam coverage by presuming that the original spot beam coverage will resume within a predetermined time. 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 a corresponding control channel. When the user terminal
100
detects a shift in its spot beam service, the user terminal
100
may determine whether a location update procedure is required or if a location update may possibly be unnecessary due to the oscillation behavior described above. The information broadcast on the control channel may require considerable system bandwidth for each spot beam.
If the user terminal
100
determines that the new servicing spot beam forms a beam pair with the registered spot beam, the user terminal
100
may register with the spot beam pair by performing a location update procedure so as to avoid repeatedly re-registering with a new spot beam every time the oscillation occurs. When the user terminal
100
registers with the servicing beam pair, the user terminal
100
starts a beam pair location update timer, wherein the duration of the beam pair location update tim
Kolev Javor
Lindvall Hakan
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
Smith Sheila
Urban Edward F.
LandOfFree
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