Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-05-10
2003-05-13
Vo, Nguyen T. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S440000, C455S456500
Reexamination Certificate
active
06564057
ABSTRACT:
PRIORITY
This application claims priority to an application entitled “Handoff Device and Method For Mobile Communication System” filed in the Korean Industrial Property Office on May 8, 1998 and assigned Serial No. 98-16454, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to mobile communications systems, and in particular, to a system and method for determining a handoff target base station in a mobile communication system.
2. Description of the Related Art
A code division multiple access (CDMA) mobile communication system performs handoff to continuously provide a call service, even when a mobile station travels from a present cell to an adjacent cell during the call service. There are two types of handoff: a soft handoff and a hard handoff. In the soft handoff, the mobile station maintains a call using both a channel assigned by a handoff target base station to which the mobile station is to be handed off and a channel assigned by the present base station in service. Meanwhile, one of the two channels, whose channel condition has a value lower than a threshold, is released. In the hard handoff, a channel assigned by the present base station in service is first released and then, a connection to an adjacent base station is attempted.
In general, a mobile station, during a call, monitors the strength of pilot signals transmitted by base stations adjacent to or in proximity to the mobile station. The mobile station stores the pilot signals having a relatively higher strength in a table. The base stations included in the table are referred to as “adjacent (or handoff candidate) base stations”. As the mobile station travels, the pilot signals from the adjacent base stations vary in strength, and the mobile station changes the values stored in the table. The mobile station then provides information about the pilot signal strengths to the base station in service. Then, upon receipt of the pilot signal strength information, the base station in communication with the mobile station determines whether to perform a handoff operation. A base station having a pilot signal which has the highest strength, becomes a handoff target base station to which the mobile station is to be handed off. However, determining the handoff by simply measuring the strength of the received signal increases the frequency of the handoff operation resulting in an increase of a system load.
With reference to
FIG. 1
, there is shown a diagram illustrating a mobile communication system. A mobile communication network includes mobile switching center (MSC)
141
connected to multiple base station controllers (BSCs)
131
-
13
n
, each of which is connected to multiple base stations
121
-
12
n
. Mobile stations
111
-
11
n
belonging to one cell are wirelessly connected to corresponding base station
121
to provide a call service. Multiple base stations
121
-
12
n
are controlled by the BSC
131
. The BSCs
131
-
13
n are then controlled by the MSC
141
. If the mobile station
111
attempts to communicate with a wire subscriber
161
belonging to a public switched telephone network (PSTN)
151
, rather than with another mobile station belonging to the mobile communication network, the MSC
141
establishes a traffic channel by connecting the wire subscriber
161
to the mobile station
111
via the PSTN
151
. Alternatively, it is also possible to unify the base stations
121
-
12
n
and the corresponding BSC
131
into one body.
A description will now be made to a conventional handoff operation performed by the mobile communication system of
FIG. 1
with reference to FIG.
2
. When a mobile station MS
5
, which is being presently serviced by a base station BS
1
, travels to a cell area CELL
2
of an adjacent base station BS
2
, the handoff occurs at an overlapped area OL of a service area (or coverage) CELL
1
of the base station BS
1
and a service area CELL
2
of the base station BS
2
under the control of a BSC (not shown). That is, as the mobile station MS
5
being serviced by the base station BS
1
moves to the overlapped area OL, it measures a strength, T_add, of a signal received from the adjacent base station BS
2
and provides the measured strength information to the BSC. The BSC then determines whether the base station BS
2
can service the mobile station MS
5
, and provides the service to the mobile station MS
5
via both the base station BS
1
and the base station BS
2
when the base station BS
2
can provide the service.
If the strength of a signal received from the base station BS
1
decreases below a threshold T_drop as the mobile station MS
5
travels to the base station BS
2
, a radio link connected to the base station BS
1
presently in service is released and the service is provided via a radio link connected to the base station BS
2
. Here, the release of the radio link does not occur as soon as the signal strength of the base station BS
1
drops below the threshold T_drop, but the base station BS
1
is dropped when the signal strength is continuously maintained below the threshold T_drop for a guard time T
—Tdrop.
In some cases, the mobile station MS
5
may select a remote base station as the handoff target base station, instead of an adjacent base station or a base station which is the closest in proximity to the mobile station MS
5
. For example, referring to
FIG. 3
, when the mobile station MS
5
travels from the base station BS
1
to the base station BS
2
, the mobile station MS
5
determines a handoff target base station, depending on strengths of pilot signals transmitted from the base stations. Here, for handoff, it is preferable for the mobile station MS
5
to select a base station which is most adjacent thereto.
However, the signal strength of the nearest base station BS
2
may be lower than a signal strength of a remote base station BS
3
due to an obstacle such as a building. In this case, the mobile station MS
5
may be handed off to the remote base station BS
3
rather than the nearest base station BS
2
, depending on the signal strength. However, as the mobile station MS
5
continues to travel, the channel condition is changed such that the signal strength of the presently connected base station BS
3
decreases and becomes lower than the signal strength of the nearest base station BS
2
. Then, the mobile station MS
5
performs a handoff again. In a worst case scenario, an abrupt drop in the signal strength of BS
3
may cause the call service to be terminated, thereby disconnecting the call.
That is, when the signal strength of the base station BS
3
is higher than T_add in an instant due to the physical surroundings of the mobile station MS
5
, the mobile station MS
5
performs a handoff add process for adding the base station BS
3
to an active set. However, in the next instant, when the signal strength of the base station BS
3
decreases, the mobile station MS
5
performs a handoff drop process to drop the base station BS
3
from the active set. In this case, the control signals exchanged for the handoff may increase system load, and the base station BS
3
added to the active set may unnecessarily assign a traffic channel to the mobile station MS
5
, thereby reducing channel resources.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a handoff system and method for performing: handoff depending on strengths of signals received from base stations in proximity to a mobile station, and weights based on the relative location of the mobile station with respect to each of the base stations in proximity thereto.
It is another object of the present invention to provide a handoff system and method for performing handoff depending on the direction of travel of the mobile station.
It is still another object of the present invention to provide a handoff system and method for performing handoff depending on a traveling velocity of the mobile station.
To achieve these and other objects, a system and method for determining a handoff target b
Chun Kyong-Joon
Jung Kee-Sung
Lee Hyun-Woo
Yu Tae-Ho
Dilworth & Barrese LLP
Moore James K
Samsung Electronics Co,. Ltd.
Vo Nguyen T.
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