Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-05-14
2001-12-11
Nguyen, Lee (Department: 2746)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S436000, C455S439000, C370S331000
Reexamination Certificate
active
06330449
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to cellular communication systems. More particularly, the present invention relates to a recovery method to facilitate the completion of a soft handoff when a reverse link is faulty.
2. Description of the Related Art
FIG. 1
shows a schematic diagram of a conventional CDMA mobile communication system. As shown, the conventional system comprises: the base transceiver stations (BTSs) (
30
,
31
) which provide the mobile communication services with the mobile stations
40
,
41
; the BSCs (Base Station Controller)
20
,
21
which control BTSs
30
,
31
; the MSC (Mobile Switching Center)
10
which connects a plurality of the BSCs
20
,
21
with the PSTN (Public Switched Telephone Network)
11
.
The forward link is directed from the BTS to the mobile station, and the reverse link is directed from the mobile station to the BTS in the above CDMA system. All forward channels in each BTS share an identical PN (Pseudo random Noise) sequence offset. Therefore, the PN offset is an identification signal which distinguishes one BTS from another.
When a mobile station exits a present service region and enters a new region, the call is maintained by the handoff function in the cellular system. The CDMA mobile communication system provides various handoffs to guarantee the call continuity. The method for connecting a plurality of call paths without the need to switch frequencies by establishing a communication path with a new base station prior to terminating a communication path with an existing base station is called a soft handoff. U.S. Pat. No. 5,722,074 “Soft handoff in a cellular telecommunication system” discloses a soft handoff method in which the mobile station can communicate the wireless signals with at least 2 BTSs.
The system which supports the soft handoff manages a plurality of cells (or sectors), as an active set for connecting the mobile station to the channel. The mobile station changes the active set only by command of the BSC. That is, a new cell (or a sector) with a gradually increasing PN offset signal level is added to the active set, and the existing cell (or sector) with a gradually decreasing PN offset signal is removed from the active set.
FIG. 2
shows a conventional soft handoff process. When receiving a new PN offset signal, the mobile station
40
reports the change to the active set to the BSC
20
through the BTS (step
110
). The BSC
20
determines if handoff is performed or not and the kinds of handoff (step
120
) and commands the channel element of the BTS
30
to transmit a new wireless resource (step
130
).
When the BSC
20
instructs the mobile station
40
to perform the handoff (step
140
), the mobile station
40
changes the active set by performing the handoff, and reports the handoff completion to the BSC
20
through the BTS
30
.
As previously stated, the handoff process initiated from the mobile station
40
is performed by command of the BSC
20
.
In performing the handoff in the manner described above, when the quality of the
20
forward wireless link or the reverse wireless link is degraded because of the influences of the geographical features, artifacts, building, and climate, messages relating to the handoff may be lost. Further, the quality of the reverse link is equally likely to be degraded. When the HCM (Handoff Completion Message) of the mobile station
40
is lost (step
150
) because of the poor quality of the reverse link, the BTS doesn't know whether handoff is completed or not, so it may occur that the handoff is not correctly performed afterwards. As a result, a discontinuity phenomenon of the call occurs because the mobile station is not controlled for proper handoff according to its direction of motion.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method to prevent soft handoff failures due to a faulty reverse wireless link. In general, when a BSC does not receive a handoff completion message from a mobile station, the BSC commands the mobile station to report the change of the active set a second time. The second report is then compared to a previous report to determine whether there is any change of the active set. In the case there is no change in the active set, the BSC commands the mobile station to perform the handoff. This method is efficient in that the entire soft handoff process need not be repeated in its entirety in the event of a fault on the reverse link.
The method of the present invention is performed in a cellular mobile communication system including mobile stations which receive mobile communication services and manage active sets consisting of a plurality of received identification signals; identifying each service area a plurality of BTSs (Base Transceiver Stations) providing mobile communication services to the mobile stations moving between a plurality of service areas each a BSC (B
3
ase Station Controller) to connect the plurality of BTSs with the MSC (Mobile Switching Center) and to provide the mobile stations with the handoff.
When the mobile stations receive commands to perform handoff, handoff may fail because the BTSs have not received an acknowledgement signal due to a faulty reverse link. In this case, the method comprises the steps of: the BSC ordering the plurality of mobile stations to report a second time the change of pilot signal of each sector to the BSC; checking whether the contents of said second report is the same as a first report of the pilot signal, and if so, the BSC ordering the mobile stations to change the active set; and the mobile stations reporting to the BTS that said pilot signal change has been completed thereby completing the handoff.
In another aspect of the invention, the method comprises the steps of: the mobile station reporting said change of pilot signal of each cell to the BSC; determining by the BSC based on said report whether handoff is to be performed or not and which kind of handoff is to be performed; if handoff is to be performed according to said determination, the BSC orders the channel element of BTS to assign a new sector of a new cell according to said determined handoff; the BTS ordering the mobile stations by BTS to change the active set according to said transmitting order; if the mobile stations report the BSC the completion of said change of the active set in a predetermined time, BTS terminates handoff, or else BSC orders the mobile stations to report the change of the pilot signal of each cell; reporting BSC by the mobile stations said change of the pilot signal of each cell; checking whether said change is the same with those of the previous pilot signal; if not the same, determining by BSC whether handoff is to be performed or not and which kind of handoff is to be performed and ordering the release of the resources which began to transmit according to the previous handoff and transmitting the newly allocated resources; if the same or said transmitting order exists, ordering the mobile stations by BTS to change the active set; and reporting BTS by the mobile stations the completion of said change.
REFERENCES:
patent: 5884175 (1999-03-01), Schiefer et al.
patent: 5946621 (1999-08-01), Chheda et al.
patent: 5987013 (1999-11-01), Kabasawa
patent: 6049716 (2000-04-01), Jung
Dilworth & Barrese LLP
Nguyen Lee
Nguyen Simon
Samsung Electronics Co,. Ltd.
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