Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-10-25
2001-09-25
Nguyen, Lee (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S439000, C455S442000, C455S560000, C379S219000, C370S331000
Reexamination Certificate
active
06295452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mobile communication system and method for implementing soft handoff, and more particularly, to a mobile communication system and method that supports soft handoff between mobile switching stations.
2. Background of the Related Art
Generally, handoff is implemented in such a manner that a mobile station currently in service detects the strength of pilot signals from a serving base station and detected neighboring or remaining base stations. The serving base station is a base station providing service (e.g., a call) to the mobile station. The mobile station compares the strength of the pilot signals from the detected base stations with the strength of the pilot signal of the current serving base station to ensure readiness for obtaining a new traffic channel. If the strength of the pilot signal of the current serving base station becomes lower than a certain value, a target base station capable of maintaining a traffic channel currently in service is selected, and a call is switched into the selected target base station. Accordingly, the mobile station can continue to perform communication using the new traffic channel.
In a cellular/personal communication service (PCS) system, there are hard handoffs and soft handoffs. In the hard handoff, communication is temporarily disconnected during the call conversion. The hard handoff occurs between base stations to which different frequencies are assigned or between different mobile switching stations. In the soft handoff, communication is not disconnected. The soft handoff for a particular mobile station occurs between base stations to which the same frequencies are assigned.
FIG. 1
shows soft handoff between two base stations in a mobile communication system. As shown in
FIG. 1
, a first mobile station MS
1
currently in service is moving from a first base station BS
1
to a third base station BS
3
. A second base station BS
2
is a neighboring base station. In the soft handoff, traffic channels between the base station BS
1
and the mobile station MS
1
and between the base station BS
3
and the mobile station MS are respectively set at a boundary between the base station BS
1
and the base station BS
3
.
If a signal provided through a traffic channel between the base station BS
1
and the mobile station MS
1
becomes equal to or below a prescribed threshold value as the mobile station MS
1
moves from the first base station BS
1
toward the base station BS
3
, the traffic channel between the base station BS
1
and the mobile station MS
1
is disconnected while the traffic channel between the base station BS
3
and the mobile station MS
1
is connected to implement the soft handoff. In such a soft handoff, the base station BS
1
and the base station BS
3
are controlled by the same base station controller (not shown). The base station BS
1
and the base station BS
3
are physically connected to each other through the base station controller.
FIG. 2
shows hard handoff between two base stations in a mobile communication system. With respect to
FIG. 2
, the hard handoff between the base stations to which different frequencies are assigned will be described.
As shown in
FIG. 2
, a first mobile station MS
1
currently in service is moving from a third base station BS
3
to a fourth base station BS
4
. In the hard handoff, at the boundary the mobile station MS
1
drops a traffic channel with the base station BS
3
and implements handoff using a new traffic channel from the base station BS
4
. The base station BS
4
cannot detect the pilot signal of the base station BS
3
.
The related art hard handoff procedure between mobile switching stations will now be described with reference to FIG.
3
.
FIG. 3
is a block diagram of respective mobile switching stations.
Referring to
FIG. 3
, first and second access switching subsystems for wireless subscribers (ASS-W)
32
and
42
perform a call processing function and handoff and mobility management functions for the mobile station MS
1
. First and second access switching subsystems for wireless subscribers (ASS-T)
35
and
45
match first and second mobile switching stations
30
and
40
with a public switching telephone network
50
to perform call processing for a trunk, as shown in FIG.
3
.
The first and second interconnection network subsystems
33
and
43
perform call processing functions such as mobile station number translation, route control function and switching functions. First and second central control subsystems
34
and
44
perform management, maintenance, charging and statistic functions of the system, process location registration of mobile subscribers and store/manage subscribers' information. The first mobile switching station
30
and the second mobile switching station communicate with a home location register
60
to determine and register the location of the mobile station MS
1
.
In
FIG. 3
, it is assumed that a mobile station is moving from a service area of a first and third base station
10
of the first mobile switching station
30
to a service area of a second and fourth base station
20
, as shown in
FIGS. 1 and 2
, respectively, of the second mobile switching station
40
. Initially, the first base station BS
1
is the serving base station
10
of the mobile station MS
1
for
FIG. 1
, or the third base station BS
3
is the serving base station
10
of the mobile station MS
1
for FIG.
2
.
With reference to
FIGS. 2 and 3
, the mobile station currently in service periodically measures the strength of the pilot signal provided from the third base station
10
where a speech path is currently set and the strength of the pilot signal provided from a candidate base stations. The mobile station further compares the respective pilot signals provided from the candidate base stations with a preset threshold value. As a candidate base station, if the strength of the pilot signal provided from the fourth base station
20
is greater than the threshold value, the mobile station transmits the measurement results to its base station (i.e., base station
10
) together with a pilot strength measurement message (PSSM).
The mobile station currently in service periodically measures the strength of a pilot signal of an active set, which includes active base stations where a speech path is set, and the strength of a pilot signal of a candidate set that includes candidate base stations received in the mobile station that can completely be modulated as a forward traffic channel. The mobile station further periodically measures the strength of a pilot signal that is not included in the current active set or candidate set or the strength of a pilot signal of a neighboring set, which includes neighboring base stations that can become a handoff candidate set. Then, the mobile station transmits the measurement results to its base station together with the PSMM.
The third base station
10
detects the fourth base station
20
toward which the mobile station is moving in response to the strength of the pilot signal included in the PSMM. If a corresponding mobile station departs from the third base station
10
service area, the third base station
10
transmits the PSMM to the first base station controller
31
. However, if it is determined that the base station corresponding to the received PSSM is not under the control of the first base station controller
31
, the first base station controller
31
detects the fourth base station
20
and transmits a handoff request signal to the second mobile switching station
40
. The handoff request signal is transmitted to the second base station controller
41
and the fourth base station
20
through a first ASS-W
32
, a first interconnection network subsystem
33
, a first ASS-T
35
, a public switching telephone network
50
, a second ASS-T
45
, a second interconnection network subsystem
44
, and a second ASS-W
42
.
The mobile station releases a speech path set between the mobile station and the third base station
10
under the control
Fleshner & Kim LLP
LG Information & Communications Ltd.
Nguyen Lee
Nguyen Simon
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