Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-08-30
2004-03-30
Nguyen, Lee (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S226200, C455S441000, C455S456100
Reexamination Certificate
active
06714785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for performing a handoff.
2. Description of the Related Art
In general, for effective management of frequency resources, a mobile communication system divides a radio network into small cell areas, and a mobile station is provided with a service from a base station in a cell to which it belongs. When a mobile station travels from a cell in service to another cell, a handoff is performed for the mobile station to secure a continuous service.
As stated above, a mobile communication system performs a handoff to maintain a call, when a mobile station travels from one cell to another cell. Handoffs are generally divided into soft handoffs and hard handoffs. With regard to the soft handoff, a service is concurrently provided through a channel assigned from an adjacent handoff target base station and a channel assigned by a service base station and then, the channel from the service base station is disconnected when a channel quality of the service base station becomes lower than a predetermined threshold. With regard to the hard handoff, when a channel quality of the service base station becomes lower than a threshold during a call, the channel to the service base station is disconnected and thereafter, a new channel is assigned from an adjacent base station.
FIG. 1
illustrates an overall system structure of a mobile communication system. Referring to
FIG. 1
, a mobile station (MS)
111
from a plurality of mobile stations belonging to one cell communicate with a corresponding base station (BS). A plurality of base stations
121
-
12
n
are controlled by a corresponding base station controller (BSC)
131
. A plurality of base station controllers
131
-
13
n
are controlled by a mobile station controller (MSC)
141
. When a mobile station
111
attempts to call to a wire subscriber
161
in a public switched telephone network (PSTN)
151
through a radio network(BS, BSC, MSC), the mobile switching network
141
establishes a communication channel by connecting the mobile station
111
to the wire subscriber
161
through the PSTN
151
.
FIG. 2
illustrates the variation in signal power received from a service base station BS
1
and an adjacent base station BS
2
, when a mobile station travels from a position “a” to a position “b” in a overland area where a service area CELL
1
of the service base station BS
1
overlaps with a service area CELL
2
of the adjacent base station BS
2
.
A description will now be made regarding a conventional handoff procedure with reference to
FIG. 2
, on the assumption that a mobile station is presently being provided with a service from base station BS
1
, and travels to a cell area CELL
2
of the adjacent base station BS
2
.
The soft handoff is performed under the control of a base station controller (not shown) when the mobile station is situated in the overland area where the service area CELL
1
of the BS
1
and the service area CELL
2
of the BS
2
overlap each other. That is, the mobile station being presently provided with a service from the BS
1
detects the power of a signal from BS
2
, and if the detected value is determined to be greater than a predetermined threshold value T_ADD, sends the detected value to a base station controller. The base station controller then determines whether BS
2
is presently available to assign a channel between the BS
2
and the mobile station. When a channel is assigned for BS
2
, BS
1
and BS
2
simultaneously provide a service to the mobile station through the respective assigned channels. In the meantime, as the mobile station continuously moves toward BS
2
(i.e., from point a to point b), the mobile station eventually reaches a position where the signal power received from BS
1
drops below a previously determined drop threshold T_drop for a handoff. Upon reaching a signal from BS
1
at the drop threshold, the mobile station drops BS
1
, presently in service, and is provided with a service only from BS
2
. At this point, the mobile station does not drop BS
1
immediately after the received signal power drops below the threshold T_drop, but only after delay of a guard time T_Tdrop. That is, when the signal power has dropped below the threshold T_drop, the mobile station drops BS
1
after a lapse of the guard time T_Tdrop, so as to prevent a handoff from being erroneously performed due to an instantaneous drop of the signal power. However, in this case, the mobile station might erroneously select a farther base station rather than an adjacent base station as a handoff target base station.
This situation will be described in detail with reference to FIG.
3
. When a mobile station MS
5
moves from a cell serviced by base station BS
1
towards a cell serviced by base station BS
2
, the mobile station MS
5
determines a handoff target base station depending on pilot power. Preferably, the mobile station MS
5
should determine that the closest base station BS
2
as the handoff target base station, in a handoff area. However, in the case where the signal power from BS
2
becomes lower due to bad surroundings such as a building, a signal from BS
3
which is farther from MS
5
than BS
2
will transmit a higher power. In this case, MS
5
performs a handoff to the far base station BS
3
rather than the adjacent base station BS
2
. That is, when the signal power from the far base station BS
3
becomes instantaneously higher than T_add due to surroundings, MS
5
accepts BS
3
in the activation set and performs a handoff add process. However, before long, the signal power from BS
3
will become lower than the signal power from BS
2
, so that a handoff process will be performed again. Further, an abrupt drop of the signal power may completely disconnect a call.
The conventional handoff has the following problems:
(1) The mobile communication system will experience an increased load due to the unnecessary handoffs described above.
(2) When BS
3
is added to the activation set, a communication channel is assigned to MS
5
, thereby wasting channel resources.
(3) Handoff is performed depending only on the pilot signal power from the base stations, causing handoff errors according to topography.
(4) The traveling direction of the mobile station is not considered for the handoff. Therefore, when the adjacent base station has a low signal power, the mobile station continues to maintain a traffic channel to the present service base station and fails to assign a traffic channel to the adjacent base station, thus causing a call disconnection.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a device and method for performing a handoff taking into consideration a traveling direction of a mobile station in a mobile communication system.
It is another object of the present invention to provide a device and method for performing a handoff, in which a mobile station provides its present location and traveling direction to a base station during a handoff request.
It is a further object of the present invention to provide a device and method for performing a handoff depending on information about a location and a traveling direction, provided from a mobile station during a handoff request.
It is still another object of the present invention to provide a device and method for performing a handoff, which restrictively performs a handoff cancel upon receipt of a handoff cancel request from a mobile station.
To achieve the above objects, a device for performing a handoff in a mobile communication system includes a mobile station for transmitting a handoff request message including a measured signal power value and information about a traveling direction thereof, when signal power from an adjacent base station is determined to be higher than a predetermined threshold value; and a base station for performing a handoff depending on the measured signal power value and the traveling direction, upon receipt of the handoff request mess
Craver Charles
Dilworth & Barrese LLP
Nguyen Lee
Samsung Electronics Co,. Ltd.
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