Method and system for controlling a pilot measurement...

Details Method and system for controlling a pilot measurement... Method and system for controlling a pilot measurement...

1999-05-24

2001-02-27

Nguyen, Lee (Department: 2746)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C455S442000, C455S437000, C370S331000

Reexamination Certificate

active

06195552

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to cellular communications systems. More particularly, the present invention relates to a method and a system for controlling a pilot measurement request order (PMRO).
DESCRIPTION OF THE RELATED ART
As is well known in the art, a cellular mobile telecommunication system divides the entire service area into a plurality of base transceiver station (BTS) service areas which include smaller sized areas or cells. The cellular communication system controls each BTS using a switching system, thereby enabling users moving between cells to maintain communication with a BTS. In the cellular communication system, the handoff process allows a call to continue when a mobile station exits from a cell belonging to a servicing BTS into a cell of a neighboring BTS.
FIG. 1
shows a structure of a cellular system using a typical code division multiple access (CDMA) technique. As shown in
FIG. 1
, the cellular system comprises a plurality of mobile stations
400
,
410
, and
420
, a plurality of base transceiver stations (BTSs)
310
,
320
,
330
and
340
for providing communication service to the mobile stations, a plurality of base station controllers (BSCs)
200
and
210
for controlling the BTSs and a mobile switching center (MSC)
100
for connecting the BSCs with a public switched telephone network (PSTN).
The MSC
100
obtains information about the mobile stations
400
,
410
and
420
from a home location register (HLR)
110
and a visitor location register (VLR)
120
and provides a communication service.
A CDMA system provides various types of handoffs to allow a call to be maintained while a user moves from one service area to the next. For example, in a soft handoff, a mobile station is simultaneously connected with a plurality of base transceiver stations. A typical soft handoff method is disclosed in U.S. Pat. No. 5,640,414 entitled “Mobile Station Assisted Soft Handoff in a CDMA Cellular Communication System”.
In accordance with a conventional soft handoff method, a BTS measures the level of a received signal of a mobile station in communication with the BTS and monitors whether the measured signal level drops below a predetermined threshold value. When the received signal level drops below the predetermined threshold value, the BTS judges that the mobile station is located at or near the boundary of a cell and informs a base station controller (BSC) of a mobile switching center (MSC) of this, thereby causing the BSC to check whether a neighboring BTS is receiving a stronger signal from the mobile station.
When the signal level that the neighboring BTS receives from the mobile station exceeds the predetermined threshold value, the BSC transfers a message for requesting handoff from the mobile station to the neighboring BTS and commands the mobile station to communicate with the neighboring BTS, so that handoff to the neighboring BTS is performed.
FIG. 2
illustrates a typical soft handoff procedure. As a mobile station moves from a region controlled by a base transceiver station (BTS) A into a region controlled by BTS B, a pilot signal strength (Ec/lo) of the BTS A gets weaker and a pilot signal strength of BTS B gets stronger.
As the received pilot signal strength of BTS B gets stronger, the predetermined threshold value, T_ADD will eventually be exceeded, and at that time, the mobile station reports the growth of the pilot signal strength of BTS B to a BSC through a pilot strength measurement message (PSMM).
The BSC confirms that BTS B is registered in the neighbor list of BTS A and commands a handoff to the mobile station. That is, the handoff is initiated by a command of the BSC. This is referred to as an “ADD”. At that time, the mobile station is in simultaneous communication with both BTS A and BTS B.
As the mobile station moves closer to BTS B, the pilot signal strength of BTS A falls below a predetermined threshold value, T_DROP. When the pilot signal strength is continuously lower than the threshold T_DROP during a predetermined time period, T_TDROP, the mobile station requests a disconnection from BTS A. This is referred to as a “DROP”. A handoff area is defined from the geographical point where the pilot signal of the BTS B exceeds the threshold T_ADD to the geographical point where the pilot signal of the BTS A is continuously lower than T_DROP during a time period T_TDROP.
In a center of the city where cell-overlap is high, when a fault occurs in a BTS serving a cell the BTS does not generate a pilot signal, a mobile station as a consequence receives a pilot signal of a neighboring BTS and informs a BSC of the newly received pilot signal strength. Consequently, the BSC does not command a handoff if the reported BTS is not registered in the neighbor list.
FIG. 3
illustrates an exemplary movement of a mobile station moving within a multiple cell-structure. Assuming, for example, that a fault occurs in BTS D which supports cell D
40
. As a consequence of the fault, the mobile station which is currently being served by BTS A, does not receive the pilot signal from BTS D but instead receives a pilot signal from BTS B. However, since BTS B is not in the neighbor list of BTS A, the mobile station does not receive a handoff command from the BSC.
FIG. 4
illustrates a normal handoff operation in accordance with the prior art. As illustrated, because D cell operates normally, a mobile station
400
in an overlapped area of A cell
10
and D cell
40
, simultaneously communicates with BTS A and BTS D.
FIG. 5
illustrates an abnormal handoff operation in accordance with the prior art. When D cell
40
has a fault, signal receiving areas of A cell
10
and B cell
20
increase and are overlapped. A mobile station
400
perceives a signal of B cell
20
in the overlapped area and requests a handoff to B cell
20
. However, with reference to
FIG. 3
the neighboring BTSs of the A cell are G cell
70
, D cell
40
, and C cell
30
. Since the B cell is not registered in the neighbor list of BTS A, the BSC judges that the mobile station's request for handoff to B cell is mistaken.
FIG. 6
illustrates a method for controlling a pilot measurement request order (PMRO) of a handoff in accordance with the prior art. The handoff procedures will be described with reference to FIG.
6
.
A BSC
200
already knows that BTSs G, D, and C are the neighboring BTSs of A cell. A mobile station
400
within A cell
10
perceives pilot signals of A cell and B cell for the case where the D cell is faulty. The mobile station sends a pilot strength measurement message (PSMM) including the pseudorandom noise (PN) codes of A cell and B cell to a BSC
200
via BTS A of A cell at step
1
.
The BSC
200
sends a response to the PSMM (i.e., a BS_Ack_Order) to the mobile station
400
, at step
2
, and judges that the PSMM is bad since BTS B is not contained in the neighbor list of BTS A. The BSC
200
sends a pilot measurement request order (PMRO) to the mobile station
300
in order to request a PSMM again, at step
3
. The mobile station
400
sends the same PSMM (which includes the Pilot signals of A cell and B cell) if the mobile station still perceives the signal of BTS B, at step
4
. Then, the BSC
200
sends the same messages (BS_Ack_Order and PMRO) back to the mobile station in response to the second PSMM, at steps
5
and
6
, and steps
4
to
6
are repeated until the mobile station
400
exits from the service area
20
of BTS B.
If the mobile station
400
exits from the service area
20
of BTS B and enters into a service area
30
of BTS C, the mobile station
400
sends a PSMM containing Pilot signals of A cell
10
and C cell
30
to the BSC, at step
7
.
The BSC
200
transmits a response to the PSMM, at step
8
. After confirming that the BTS C is contained in the neighbor list of the BTS A, the BSC
200
sends a handoff direction message (HDM) to the mobile station
400
. The mobile station
400
performs handoff in response to the HDM and sends a handoff completion message (HCM) to the BSC
200
.
As stated above, wh

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