U.S. patent application number 11/705247 was filed with the patent office on 2007-06-28 for mobile communications system.
Invention is credited to Atsushi Kanagawa.
Application Number | 20070149205 11/705247 |
Document ID | / |
Family ID | 14236876 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070149205 |
Kind Code |
A1 |
Kanagawa; Atsushi |
June 28, 2007 |
Mobile Communications system
Abstract
A base station device 31 provided in a general cell 41 is
accommodated in a base station controller 21, and a base station
device 32 provided in a general cell 42 is accommodated in a base
station controller 22. A base station device 33 provided in a
boundary cell 43 that is adjacent to both the general cells 41 and
42 is accommodated in both the base station controllers 21 and 22.
Frequencies RF#1 and RF#2 are allocated to each of the general
cells 41 and 42 and boundary cell 43. Communications conducted by
the base station device 33 using the frequency RF#1 is controlled
by the base station controller 21, and communications conducted by
the base station device 33 using the frequency RF#2 is controlled
by the base station controller 22.
Inventors: |
Kanagawa; Atsushi;
(Kawasaki, JP) |
Correspondence
Address: |
KATTEN MUCHIN ROSENMAN LLP
575 MADISON AVENUE
NEW YORK
NY
10022-2585
US
|
Family ID: |
14236876 |
Appl. No.: |
11/705247 |
Filed: |
February 12, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10080977 |
Feb 21, 2002 |
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11705247 |
Feb 12, 2007 |
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PCT/JP99/05389 |
Sep 30, 1999 |
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10080977 |
Feb 21, 2002 |
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Current U.S.
Class: |
455/446 |
Current CPC
Class: |
H04W 36/12 20130101;
H04W 36/18 20130101; H04W 16/14 20130101 |
Class at
Publication: |
455/446 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
1-13. (canceled)
14. A base station device, located adjacent to a base station using
a plurality of frequencies, that shares at least one of the
plurality of frequencies, comprising controlling means for
performing a soft hand-off process if there is a hand-off from this
base station to the adjacent base station when the shared frequency
is used, and performing a hard hand-off process between cells using
the shared frequency if there is a hand-off from adjacent base
station to this base station when a frequency other than the shared
frequency is used in the adjacent base station.
15. A base station that is adjacent to at least first and second
base stations, comprising: a wireless unit using a part or all of
frequencies used by the first base station as a first shared
frequency; a wireless unit using a part or all of frequencies used
by the second base station as a second shared frequency; and
controlling means for performing a soft hand-off using the first
shared frequency if there is a hand-off from the first base station
to this base station when the first shared frequency is used,
performing a soft hand-off using the second shared frequency if
there is a hand-off from the second base station to this base
station when the second frequency is used, performing a hard
hand-off process between cells for switching the frequency from the
second shared frequency to the first shared frequency if there is a
hand-off from this base station to the first base station when the
second shared frequency is used, and performing a hard hand-off
process for switching the frequency from the first shared frequency
to the shared second frequency if there is a hand-off from this
base station to the second base station when the first shared
frequency is used.
16. A base station controller, connected to a plurality of base
stations, for performing a soft hand-off with priority if there is
a hand-off between the connected base stations and if
communications conducted before and after the hand-off are
controlled by this base station controller, comprising controlling
means for controlling, for at least one of the plurality of base
stations, only communications conducted using a part of frequencies
used by the at least one of the plurality of base stations, and
allocating one of the part of the frequencies if there is a
hand-off when a mobile station using a frequency that is not
controlled by this base station controller in the at least one of
the plurality of base stations moves to an area controlled by
another base station to which this base station controller is
connected.
17. A base station device, that is used in a mobile communications
system including a first base station device provided in a first
wireless communications area to which at least a first frequency is
allocated, a second base station device provided in a second
wireless communications area to which at least a second frequency
is allocated, a first controller accommodating the first base
station device and a second controller accommodating the second
base station device, and that is provided in a third wireless
communications area which is adjacent to the first and second
wireless communications areas and to which the first and second
frequencies are allocated, wherein data are transmitted to and
received from a mobile station using the first frequency under the
control of the first controller which manages the first frequency
but does not manage the second frequency with respect to this base
station; and data are transmitted to and received from a mobile
station using the second frequency under the control of the second
controller which manages the second frequency but does not manage
the first frequency with respect to this base station.
18. A base station device, used in a mobile communications system
comprising a first base station device provided in a first wireless
communications area to which at least a first frequency is
allocated, a second base station device provided in a second
wireless communications area to which at least a second frequency
is allocated and a third base station device provided in a third
wireless communications area which is adjacent to the first and
second wireless communications areas and to which the first and
second frequencies are allocated, that accommodates at least second
and third base station devices, comprising: a base station
management table for registering base station devices to be
controlled for each frequency allocated to corresponding wireless
communications area; and controlling means for controlling the
second and third base stations based on information registered in
said base station management table.
19. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of International
Application No. PCT/JP99/05389, which was filed on Sep. 30,
1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a mobile communications
system with hand-off control, and in particular, relates to a CDMA
mobile communications system with hand-off control.
[0004] 2. Description of the Related Art
[0005] In a mobile communications system, usually, a communications
area is divided into many wireless communications areas, each
called a "cell", and base stations are provided in each area. Then,
a mobile station is accommodated in one of the base stations in the
communications area, and is connected to another communications
terminal through the base station.
[0006] FIG. 1 shows the configuration of a general mobile
communications system. In this specification, a CDMA mobile
communications system, which has recently been spread, is used as
an example.
[0007] In each cell, a base station device (BTS: base station
transceiver subsystem) is provided. The base station device can
simultaneously accommodate a plurality of mobile stations (MS), and
transmits/receives radio data to/from a mobile station. Each base
station device is accommodated in a base station controller
(BSC).
[0008] The base station controller accommodates a plurality of base
station devices and controls them. The base station controller is
also connected to a mobile services switching center (MSC) and
transmits/receives data to/from it, as requested. The mobile
services switching center is connected to another switching center
and a public switched telephone network (PSTN) is composed of the
plurality of switching centers.
[0009] In a CDMA mobile communications system, a different spread
code is assigned to each mobile station. Then, data that are
encoded using the spread code and transmitted between a mobile
station and a base station device.
[0010] A mobile station often moves from one cell to another cell
while communicating with another communications terminal. A
transition from a state where a mobile station is accommodated in a
base station device into a state where the mobile station is
accommodated in another base station device, when the mobile
station moves from a cell to another cell, is called "hand-off" (or
"hand-over").
[0011] A hand-off operation is largely classified into two
categories: a "soft hand-off" and a "hard hand-off". In the soft
hand-off, even when moving from one cell to another cell, a mobile
station is always connected to at least one base station device.
Therefore, in the soft hand-off, a channel between the mobile
station and base station device is never disconnected, and,
accordingly, communications are never interrupted. In the system
shown in FIG. 1, the soft hand-off occurs, if a mobile station
moves between cells that are controlled by the same base station
controller and if the same frequency can be allocated.
[0012] In the hard hand-off, when moving from one cell to another
cell, the mobile station is temporarily disconnected from the base
station device and then is connected to another base station
device. In this case, synchronization must be established again
between the mobile station and the base station device. Thus, in
the hard hand-off, a channel between a mobile station and a base
station device is temporarily disconnected and, accordingly,
communications are interrupted. Therefore, a user is often
disconcerted. In the system shown in FIG. 1, the hard hand-off
occurs when a mobile station moves between cells each of which is
controlled by a different base station controller. In other words,
the hard hand-off occurs when a mobile station crosses an
"accommodation boundary". The hard hand-off also occurs when a
mobile station moves between cells, to each of which a different
frequency is allocated.
[0013] FIG. 2 shows the configuration of the existing mobile
communications system, and the configuration covers the vicinity of
a boundary between areas each of which is controlled by a different
base station controller. In this example, a base station controller
1 (BSC#1) accommodates base station devices 11 (BTS#A) and 12
(BTS#B), and a base station controller 2 (BSC#2) accommodates a
base station device 13 (BTS#C). Here, it is assumed that the base
station devices 11 through 13 control the cell-a through cell-c,
respectively, and the same radio frequency is allocated to all of
the cells.
[0014] If a mobile station moves from cell-a to cell-b, or from
cell-b to cell-a, a soft hand-off occurs since both the base
station devices 11 (BTS#A) and 12 (BTS#B) are accommodated in the
base station controller 1 (BSC#1). However, if the mobile station
moves from cell-b to cell-a or from cell-c to cell-b, a hard
hand-off occurs since each of the base station devices 12 (BTS#B)
and 13 (BTS#C) is accommodated in a different base station
controller.
[0015] Therefore, if as shown in FIG. 3, the mobile station moves
back and forth between cell-b and cell-c several times while
communicating, many hard hand-offs occur, and the user will be
disconcerted and feel unassured.
[0016] As described above, in the existing mobile communications
system, a hard hand-off is easy to occur in the vicinity of a
boundary between particular communications areas, and a user feels
uncomfortable.
SUMMARY OF THE INVENTION
[0017] An object of the present invention is to reduce the
occurrence of hard hand-offs in a mobile communications system.
[0018] In the mobile communications system of the present
invention, first and second frequencies are allocated to each
wireless communications area. The mobile communications system
comprises a first base station device provided in a first wireless
communication area, a second base station device provided in a
second wireless communications area, a third base station device
provided in a third wireless communications area adjacent to the
first and second wireless communications areas, a first controller
for accommodating the first base station device and managing
communications conducted using the first frequency by the third
base station device and a second controller for accommodating the
second base station device and managing communications conducted
using the second frequency by the third base station device.
[0019] In the mobile communications system, the first controller
controls both the first and third base station devices. Thus, when
the mobile station moves between the first and third wireless
communications areas, a soft hand-off occurs. Similarly, the second
controller controls both the second and third base station devices.
Thus, when the mobile station moves between the second and third
wireless communications areas, a soft hand-off occurs.
[0020] The mobile communications system in another aspect of the
present invention comprises a first base station device provided in
a first wireless communications area to which at least a first
frequency is allocated, a second base station device provided in a
second wireless communications area to which at least a second
frequency is allocated, a third base station device provided in a
third wireless communications area that is adjacent to the first
and second wireless communications areas and to which the first and
second frequencies are allocated, a first controller for
accommodating the first base station device and managing
communications conducted using the first frequency by the third
base station device and a second controller for accommodating the
second base station device and managing communications conducted
using the second frequency by the third base station device.
[0021] In the mobile communications system of this aspect, when a
mobile station moves between the first and second wireless
communications areas or between the second and third wireless
communications areas, a soft hand-off occurs for the reason
described above.
[0022] The system in another aspect of the present invention
comprises a first base station for conducting wireless
communications using at least a first frequency, a second base
station for conducting wireless communications using at least a
second frequency different from the first frequency, a third base
station which is adjacent to the first and second base stations and
which conducts wireless communications using at least the first and
second frequencies, a first base station controller for managing
the first frequency used at least in the first and third base
stations and a second base station controller for managing the
second frequency used at least in the second and third base
stations. The first and second base station controllers further
comprise controlling means for allocating the same frequency if
there is a hand-off between base stations controlled by the
corresponding base station controllers.
[0023] In this system, for example, the first and second base
station controllers can also comprise instructing means for
instructing said first base station to use the first frequency when
there is a hand-off from said third base station to said first base
station, and instructing said second base station to use the second
frequency when there is a hand-off from said third base station to
said second base station.
[0024] The base station of the present invention is adjacent to a
base station using a plurality of frequencies and uses at least one
of the frequencies shared with the base station. The base station
device comprises controlling means for performing a soft hand-off
process if there is a hand-off from this base station to the
adjacent base station when the shared frequency is used, and
performing a hard hand-off process using the shared frequency if
there is a hand-off from adjacent base station to this base station
when a frequency other than the shared frequency is used in the
adjacent base station.
[0025] The base station in another aspect of the present invention
is adjacent to at least first and second base stations. The base
station comprises a first wireless unit using a part or all of
frequencies used by the first base station as a first shared
frequency; a second wireless unit using a part or all of
frequencies used by the second base station as a second shared
frequency; and controlling means for performing a soft hand-off
using the first shared frequency if there is a hand-off from the
first base station to this base station when the first shared
frequency is used, performing a soft hand-off using the second
shared frequency if there is a hand-off from the second base
station to this base station when the second frequency is used,
performing a hard hand-off process for switching the frequency from
the second shared frequency to the first shared frequency if there
is a hand-off from this base station to the first base station when
the second shared frequency is used, and performing a hard hand-off
process for switching the frequency from the first shared frequency
to the shared second frequency if there is a hand-off from this
base station to the second base station when the first shared
frequency is used.
[0026] The base station controller of the present invention is
connected to a plurality of base stations and performs with
priority a soft hand-off if there is a hand-off between the
connected base stations, and communications conducted before and
after the hand-off is controlled by this base station controller.
The base station controller comprises controlling means for
controlling, for at least one of the plurality of base stations,
only communications conducted using a part of frequencies used by
the at least one of the plurality of base stations, and allocating
one of the part of the frequencies if there is a hand-off when a
mobile station using a frequency that is not controlled by this
base station controller in the at least one of the plurality of
base stations moves to an area controlled by another base station
to which this base station controller is connected.
BRIEF DESCRIPTION OF DRAWINGS
[0027] FIG. 1 shows the configuration of a general mobile
communications system.
[0028] FIG. 2 shows the configuration of the existing mobile
communications system.
[0029] FIG. 3 shows the problems of the existing mobile
communications system.
[0030] FIG. 4 shows the basic configuration of the mobile
communications system of the embodiment.
[0031] FIG. 5 shows the hand-off operation in the mobile
communications system of the embodiment.
[0032] FIG. 6 is a flowchart showing the hand-off operation of the
embodiment.
[0033] FIG. 7 is a flowchart showing the hand-off operation in the
existing mobile communications system.
[0034] FIG. 8 is a flowchart showing another hand-off operation of
the embodiment.
[0035] FIG. 9 shows a hand-off operation in the mobile
communications system of the embodiment.
[0036] FIG. 10 is a sequence chart showing a hand-off
operation.
[0037] FIGS. 11A and 11B show the effect of the mobile
communications system in the embodiment. FIG. 11A shows a hand-off
operation in the existing mobile communications system and FIG. 11B
shows a hand-off operation in the mobile communications of the
embodiment.
[0038] FIG. 12 shows an example in which a base station device is
accommodated in a base station controller through a logical
path.
[0039] FIG. 13 shows a configuration of a base station device
provided in a boundary cell.
[0040] FIG. 14 shows another configuration of a base station device
provided in a boundary cell.
[0041] FIG. 15 shows the configuration and operation of a base
station controller that accommodates a base station device provided
in a boundary cell.
[0042] FIG. 16 shows the configuration of the mobile communications
system in another aspect of the present invention.
[0043] FIG. 17 shows a system in which three or more cells are
adjacent to one another.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] The embodiments of the present invention are described
below.
[0045] The entire configuration of the mobile communications system
in this embodiment is basically the same as that of the existing
mobile communications system shown in FIG. 1. Specifically, a base
station device (BTS) is provided for each cell, and each base
station device is controlled by a base station controller (BSC).
Then, each base station controller is accommodated in a mobile
services switching center (MSC). However, in the mobile
communications system of this embodiment, a hand-off operation in
the vicinity of a boundary between communications areas, each of
which is controlled by a different base station controller, is
different from that of the existing mobile communications
system.
[0046] FIG. 4 shows the basic configuration of the mobile
communications system in the embodiment and in particular shows the
configuration in the vicinity of a boundary between communications
areas each of which is controlled by a different base station
controller. Specifically, FIG. 4 shows the configuration in the
vicinity of a boundary between a communications area controlled by
a base station controller (BSC#1) 21 and a communications area
controlled by a base station controller (BSC#2) 22.
[0047] In this mobile communications system, two frequencies (RF#1
and RF#2) are allocated to each cell. General cells 41 and 42 are
ordinary cells. A boundary cell 43 is a cell located in a boundary
area between a communications area controlled by the base station
controller 21 and a communications area controlled by the base
station controller 22. If the general cells 41 and 42 already
overlap, a base station device (BTS) 33 can be provided in such a
way to cover the overlapped area.
[0048] A base station device (BTS#A) 31 and a base station device
(BTS#B) 32 are provided in the general cells 41 and 42,
respectively. The base station device 31 is accommodated in the
base station controller 21 and transmits/receives radio data
to/from a mobile station located in the general cell 41 using the
frequency RF#1 or RF#2. The base station device 32 is accommodated
in the base station controller 22 and transmits/receives radio data
to/from a mobile station located in the general cell 42 using the
frequency RF#1 or RF#2.
[0049] The base station device (BTS#3) 33 is provided in the
boundary cell 43. The base station device 33 is accommodated in
both the base station controllers 21 and 22. Specifically, among
communications in which the base station device 33 is participated,
communications conducted using the frequency RF #1 is managed by
the base station controllers 21, and communications conducted using
the frequency RF #2 is managed by the base station controllers 22.
In other words, the base station device 33 transmits/receives radio
data using the frequency RF#1 to/from a mobile station located in
the boundary cell 43 under the control of the base station
controller 21, and transmits/receives radio data using the
frequency RF#2 to/from a mobile station located in the boundary
cell 43 under the control of the base station controller 22.
[0050] As described above, in the mobile communications system of
the embodiment, a base station device provided in a general cell is
accommodated in one corresponding base station controller, while a
base station device provided in a boundary cell is accommodated in
a plurality of base station controllers.
[0051] FIG. 5 shows a hand-off operation in the mobile
communications system of the embodiment. Here a hand-off operation
in the case where a mobile station moves in the vicinity of a
boundary between areas, each of which is controlled by a different
base station controller, specifically in the case where a mobile
station moves between a general cell and a boundary cell. A
hand-off operation in the case where a mobile station moves between
general cells is the same as that of the conventional mobile
communications system.
[0052] In FIG. 5, physical transmission lines are provided between
the base station device 31 and the base station controller 21,
between the base station device 32 and the base station controller
22, between the base station device 33 and the base station control
21, and between the base station device 33 and the base station
controller 22, respectively. For the physical transmission lines,
for example, an optical fiber cable or a metallic cable and the
like are used. Physical transmission lines are also provided
between the mobile services switching center 51 and each of the
base station controllers 21 and 22. The physical transmission line
can be replaced by radio transmission path. An arrow symbol with
(1) through (6) and a narrow symbol with (11) through (13) indicate
the travel tracks of a mobile station.
[0053] As shown in FIG. 5, it is assumed that a mobile station is
located in the general cell 42 and accommodated in the base station
device 32 using frequency RF#2 (state (1)). Then, when this mobile
station moves up to an overlapped area between the general cell 42
and the boundary cell 43, a hand-off occurs (state (2)). This
hand-off is switching from a channel using the frequency RF#2 in
the general cell 42 to a channel using the frequency RF#2 in the
boundary cell 43. Here, among communications in which the base
station device 33 is participated, communications conducted using
the frequency RF #2 is managed by the base station controller 22.
Specifically, the base station controller 22 controls
communications conducted before and after the hand-off, and the
same frequency is used in both the communications. Therefore, this
hand-off is a soft hand-off without channel disconnection. Then,
this mobile station is accommodated by the base station device 33
(state (3)). In this communication, the frequency RF#2 is still
used.
[0054] Then, when this mobile station moves up to an overlapped
area between the boundary cell 43 and the general cell 41, a
hand-off occurs again (states (4) and (5)). This hand-off is
switching from a channel using the frequency RF#2 in the boundary
cell 43 to a channel using a frequency RF#1 in the general cell 41.
Here, the base station controller 22 controls the communications
conducted before the hand-off, while the base station controller 21
controls the communications conducted after the hand-off.
Therefore, this hand-off is a hard hand-off. Then, this mobile
station is accommodated by the base station device 31 (state (6)).
In this communication, the frequency RF#1 is used.
[0055] When a mobile station using the frequency RF#2 in the
general cell 41 moves up to an overlapped area between the general
cell 41 and the boundary cell 43, a hand-off from a channel using
the frequency RF#2 in the general cell 41 to a channel using the
frequency RF#2 in the boundary cell occurs (state (12)). In this
case, since a different base station controller performs control of
each of the channels before and after the hand-off, a hard hand-off
occurs.
[0056] As described above, in the mobile communications system, if
a mobile station moves from a general cell to a boundary cell, the
mobile station uses the same frequency used in the general cell
before a hand-off in the destination boundary cell. If a mobile
station moves from a boundary cell to a general cell, in the
example shown in FIG. 5, in the destination general cell, the
mobile station uses a frequency different from the frequency used
in the boundary cell before a hand-off. However, even if a mobile
station moves from a boundary cell to a general cell, in the
destination general cell, the mobile station sometimes uses the
same frequency as that used in a boundary cell before a hand-off. A
method for determining a radio frequency to be used in a
destination cell of a mobile station after a hand-off is described
below.
[0057] FIG. 6 is a flowchart showing the hand-off operation of the
embodiment. An "general state" means a state where a mobile station
communicates with another communications terminal in the mobile
communications system of the embodiment. When a mobile station
moves to the vicinity of a boundary between cells while holding
communications, processes in and after step S1 are performed.
[0058] In step S1, a hand-off process is started. Specifically,
when a mobile station moves to the vicinity of a boundary between
cells, the mobile station transmits a hand-off request to a base
station device that accommodates the mobile station. Then, on
receipt of this request, the base station device notifies a base
station controller that accommodates the base station device of the
request.
[0059] In step S2, the destination cell is detected. The base
station controller that receives the request recognizes this
destination cell. Since the processes in steps S1 and S2 are also
performed in the existing mobile communications system, the
detailed description is omitted here.
[0060] In step S3, it is judged whether the mobile station moves
from a boundary cell to its adjacent cell or from a general cell to
its adjacent cell. Information indicating whether a cell is a
general cell or a boundary cell is registered in a base station
controller.
[0061] If the mobile station moves from a boundary cell to its
adjacent cell, in step S4, it is judged whether a base station
controller that manages communications in which a frequency used by
the mobile station in the boundary cell is used and a base station
controller that manages the destination cell are the same. If these
two base station controllers are different, the flow proceeds to
step S5. In step S5, a channel with a frequency different from that
currently used by the mobile station is obtained in the destination
cell of the mobile station. Alternatively, a channel with a
frequency that can be shared by the base station devices in the
source cell and the destination cell, and that is managed by the
base station controller of the destination cell, is obtained.
[0062] If the mobile station moves from a general cell to its
adjacent cell (in step S3: No) or if the two base station
controllers are the same (in step S4: Yes), the flow proceeds to
step S6. In step S6, a channel using the same frequency as that
currently used by the mobile station is obtained in the destination
cell of the mobile station.
[0063] In step S7, a hand-off instruction is issued to the mobile
station. This hand-off instruction includes information about the
frequency of the channel obtained in steps S5 or S6. Then, the
mobile station is connected to a base station device provided in
the destination cell, according to the hand-off instruction. Then,
the mobile station is accommodated in the base station device of
the destination cell.
[0064] The characteristic point of the hand-off operation of this
embodiment is the steps S3 through S6. Specifically, as shown in
FIG. 7, in the existing mobile communications system, when a
hand-off operation is started and a destination cell is detected, a
channel to be used in the destination cell is simply obtained
regardless of whether the mobile station crosses the boundary of
communications areas controlled by a base station controller (step
S1). In the existing system, there is no concept that cells should
be categorized into general cells and boundary cells.
[0065] In addition, as shown in FIG. 2, in the conventional mobile
communications system, a hard hand-off is always performed
regardless of a used frequency if a mobile station moves between
cell-b and cell-c on a boundary between areas each of which is
controlled by a different BSC. Thus, a hard hand-off frequently
occurs in the specific area (a boundary between areas each of which
is controlled by a different BSC).
[0066] However, in the mobile communications system of this
embodiment, when a mobile station moves between general cells
controlled by the same base station controller, a soft hand-off
with the allocation of the same frequency is performed, and when
the mobile station moves from a general cell to a boundary cell, a
hand-off with the allocation of the same frequency is performed.
This operation provides the following effects.
[0067] Specifically, according to this frequency allocation, a soft
or hard hand-off is performed depending on a used frequency, when a
mobile station moves between a general cell 41 and a boundary cell
43 (BSC boundary (1)), or when the mobile station moves between a
general cell 42 and a boundary cell 43 (BSC boundary (2)), in FIG.
4. If RF#1 is used in a general cell 41 (if RF#2 is used in a
general cell 42), a hard hand-off is not performed even when the
mobile station crosses back and forth across the BSC boundary (1)
(BSC boundary (2)). Therefore, the occurrence of hard hand-offs in
the specific area (BSC boundaries (1) and (2)) can be reduced
compared with that of the conventional system, and there will be no
area where a hard hand-off frequently occur, such as between cell-b
and cell-c in FIG. 3.
[0068] This control can be obtained by performing the process shown
in the flowchart of FIG. 8. The flowchart shown in FIG. 8 can be
obtained by replacing steps S3 through S6 in the flowchart of FIG.
6 with steps S3a, S5a and S6a. In step S3a, it is judged whether a
mobile station moves from a general cell to its adjacent cell. If
the mobile station does not move from a general cell to its
adjacent cell, in step S5a, an empty channel is obtained. If the
mobile station moves from a general cell to its adjacent cell, in
step S6a, a channel with the same frequency is obtained.
[0069] Alternatively, RF#3 and RF#4 can also be further allocated
to the general cells 41 and 42, respectively. In this case, the
frequencies (RF#1 and RF#2) used in the adjacent general cells 41
and 42 can be allocated to the boundary cell 43, as shown in FIG.
5. It is preferable to allocate all the frequencies allocated to
the adjacent cells 41 and 42. Specifically, RF#1 through RF#4 are
allocated to the boundary cell 43. Simultaneously, BSC 21 controls
RF #1 and RF #3, and BSC 22 controls RF#2 and RF#4. In this case,
each BSC allocates the same frequency as that used in a general
cell to a mobile station, when the mobile station moves from the
general cell to the boundary cell.
[0070] Next, the hand-off operation of this embodiment is described
in detail with reference to FIGS. 9 and 10. In FIG. 9, an arrow
symbol with (11) through (18) indicates the travel track of a
mobile station. In this example, the mobile station moves back and
forth between the general cell 41 and boundary cell 43. The
sequence in FIG. 10 shows the operations of both base station
devices and base station controllers in the case where a mobile
station moves along a route from (11) through (18) shown in FIG.
9.
[0071] A mobile station (MS) is located in the general cell 41 and
is accommodated in the base station device 31 (state 11). Here, the
mobile station is connected to the base station device 31 through a
channel with a frequency RF#2.
[0072] When the mobile station moves up to an overlapped area
between the general cell 41 and the boundary cell 43 while holding
communications, the mobile station transmits a hand-off request to
the base station device 31 that accommodates the mobile station.
This movement is detected by the value of the receiving electric
field intensity of a pilot signal transmitted from BTS 33 and the
like. On receipt of this hand-off request, the base station device
31 notifies a base station controller 21 that accommodates the base
station device 31, of the request.
[0073] On receipt of the hand-off request, the base station
controller 21 performs steps S2 through S4 shown in FIG. 6. Here,
since the mobile station moves from a general cell to a boundary
cell, "No" is obtained in step S3. Then, "a frequency RF#2 is used
in the boundary cell 43" is obtained in step S6.
[0074] The base station controller 21 refers to a base station
management table and judges whether the base station controller 21
itself controls communications conducted using the frequency RF#2
in the boundary cell 43. In the example shown in FIG. 9, a base
station controller 22 controls the communications conducted using
the frequency RF #2 in the boundary cell 43. Therefore, the base
station controller 21 transmits a hand-off request to the base
station controller 22. This hand-off request includes information
indicating that "a frequency RF#2 is used in the boundary cell
43".
[0075] A variety of information is exchanged between base station
controllers through the mobile services switching center 51. The
base station management table is described later.
[0076] On receipt of the hand-off request, the base station
controller 22 returns a hand-off instruction to the base station
controller 21. This hand-off instruction includes information
indicating that "a frequency RF#2 is used in a boundary cell 43".
Then, the base station controller 21 notifies the mobile station of
the hand-off instruction through the base station device 31. When
the base station controller 22 generates the hand-off instruction,
the base station device 33 secures a channel with the frequency
RF#2, for this mobile station.
[0077] The mobile station is connected to the base station device
33 provided in the boundary cell 43 using the frequency RF#2
according to the received hand-off instruction. Thus, the hand-off
operation is terminated.
[0078] Communications of the mobile station is controlled by the
base station controller 21 before the hand-off, while the
communication is controlled by the base station controller 22 after
the hand-off. For this reason, communications between the mobile
station and a base station device is temporarily disconnected. In
other words, when the mobile station moves from the general cell 41
to the boundary 43 (11 through 13), a hard hand-off occurs.
[0079] After this hand-off, the mobile station is accommodated in
the base station device 33 (state 13). While being located in the
boundary cell 43, the mobile station uses the frequency RF#2.
[0080] Then, when moving up to an overlapped area between the
boundary cell 43 and the general cell 41 while holding
communications, the mobile station transmits a hand-off request to
the base station device 33 that accommodates the mobile station. On
receipt of this hand-off request, the base station device 33
recognizes that the mobile station uses the frequency RF#2 and
notifies the base station controller 22 of the hand-off
request.
[0081] On receipt of the hand-off request, the base station
controller 22 performs steps S2 through S4 in FIG. 6. Here, since
the mobile station moves from a boundary cell to a general cell,
"Yes" is obtained in step S3. On the other hand, communications
conducted using the frequency RF #2 in the boundary cell 43 is
controlled by the base station controller 22. However, the base
station device 31 provided in the general cell 41 is accommodated
in the base station controller 21. Therefore, "No" is obtained in
step S4. As a result, "a frequency RF#1 is used in the general cell
41", is obtained in step S5.
[0082] The base station controller 22 transmits a hand-off request
to the base station controller 21 that accommodates the base
station device 31 provided in a destination cell of the mobile
station. This hand-off request includes information indication that
"a frequency RF#1 is used in a general cell 41".
[0083] On receipt of the hand-off request, the base station
controller 21 returns a hand-off instruction to the base station
controller 22. This hand-off instruction includes information
indicating that "a frequency RF#1 is used in a general cell 41".
Then, the base station controller 22 notifies the mobile station of
the hand-off instruction through the base station device 33.
[0084] The mobile station is connected to the base station device
31 provided in the general cell 41 using the frequency RF#1,
according to the received hand-off instruction. Thus, the hand-off
operation is terminated. Then, the mobile station continues the
communications conducted using the frequency RF#1.
[0085] In the hand-off operation, communications of the mobile
station is controlled by the base station controller 22 before the
hand-off, while the communication is controlled by the base station
controller 21 after the hand-off. For this reason, communications
between the mobile station and a base station device is also
temporarily disconnected. That is, when the mobile station moves
from the boundary cell 43 to the general cell 41 (13 through 15), a
hard hand-off occurs.
[0086] Then, when moving up to the overlapped area between the
general cell 41 and boundary cell 43 while holding communications
again, the mobile station transmits a hand-off request to the base
station device 31 again. On receipt of the hand-off request, the
base station device 31 notifies the base station controller 21 of
the request as in the case described above.
[0087] On receipt of this hand-off request, the base station
controller 21 performs steps S2 through S4 shown in FIG. 6 again.
As in the case described above, "No" is obtained in step S3.
However, at this time, the mobile station uses the frequency RF#1.
Therefore, "a frequency RF#1 is used in the boundary cell 43" is
obtained in step S6.
[0088] Then, the base station controller 21 judges whether the base
station controller 21 itself controls communications conducted
using the frequency RF#1 in the boundary cell 43. In the example
shown in FIG. 9, the communications conducted using the frequency
RF#1 in the boundary cell 43 is controlled by the base station
controller 21. Thus, the base station controller 21 processes the
hand-off request without transmitting/receiving information to/from
the base station controller 22 and generates a hand-off instruction
by itself. This hand-off instruction includes information
indicating that "a frequency RF#1 is used in a boundary cell
43".
[0089] The base station controller 21 notifies the mobile station
of this hand-off instruction through the base station device 31.
Then, the mobile station is connected to the base station device 33
provided in the boundary cell 33 using the frequency RF#1,
according to the received hand-off instruction. Then, the hand-off
operation is terminated.
[0090] In this hand-off operation, a base station controller to
control the communication of a mobile station before and after the
hand-off is unchanged, and a frequency used between the mobile
station and a corresponding base station device before and after
the hand-off is unchanged. Therefore, there is no disconnection in
communications between the mobile station and base station devices.
That is, when the mobile station moves from the general cell 41 to
the boundary cell 43 (15-17), a soft hand-off is performed.
[0091] Then, when moving up to the overlapped area between the
boundary cell 43 and general cell 41 again while holding
communications, the mobile station transmits a hand-off request to
the base station device 33. When recognizing that the mobile
station uses the frequency RF#1 at this point, the base station
device 33 notifies the base station controller 21 of the hand-off
request.
[0092] On receipt of the hand-off request, the base station
controller 21 performs steps S2 through S4 shown in FIG. 6. In this
case, "Yes" is obtained in step S3, as described above. However,
communications conducted using the frequency RF#1 in the boundary
cell 43 is controlled by the base station controller 21. For this
reason, "Yes" is obtained in step S4. Specifically, "a frequency
RF#1 is used in a general cell 41", is obtained in step S6.
[0093] Then, since the communications conducted using the frequency
RF#1 in the boundary cell 43 is controlled by the base station
controller 21, the base station controller 21 processes the
hand-off request without transmitting/receiving information to/from
the base station controller 22 and generates a hand-off instruction
by itself. This hand-off request includes information indicating
that "a frequency RF#1 is used in a general cell 41".
[0094] The base station controller 21 notifies the mobile station
of this hand-off instruction through the base station device 33.
Then, the mobile station is connected to the base station device 31
provided in the general cell 41 using the frequency RF#1 according
to the received hand-off instruction. Then, the hand-off operation
is terminated.
[0095] In this hand-off operation, a base station controller to
control the communication of a mobile station before and after the
hand-off is unchanged, and a frequency used between the mobile
station and a corresponding base station device before and after
the hand-off is unchanged. Therefore, there is no disconnection in
communications between the mobile station and base station devices.
That is, when the mobile station moves from the boundary cell 43 to
the general cell 41 (17-18), a soft hand-off is performed.
[0096] Then, if the mobile station moves back and forth between the
general cell 41 and boundary cell 43, a soft hand-off is always
performed. That is, when the mobile station moves from the general
cell 41 to the boundary cell 43, a soft hand-off is performed
according to the process-A shown in FIG. 10. When the mobile
station moves from the boundary cell 43 to general cell 41, a soft
hand-off is performed according to the process-B.
[0097] In the example shown in FIGS. 9 and 10, it is assumed that a
frequency RF#2 is initially used in the general cell 41. However,
if a frequency RF#1 is initially used in the general cell 41, the
hand-off operation starts from the process-A shown in FIG. 10. In
this case, when the mobile station moves between the general cell
41 and boundary cell 43, there is no hard hand-off, and a soft
hand-off is always performed.
[0098] In the example described above, a case where a mobile
station moves between the general cell 41 and boundary cell 43 is
described. However, when the mobile station moves between the
general cell 42 and boundary cell 43, similarly, the occurrence
frequency of hard hand-offs can also be reduced.
[0099] FIGS. 11A and 11B show the effects of the mobile
communications system of the embodiment. FIG. 11A shows a hand-off
in the existing mobile communications system. FIG. 11B shows a
hand-off in the mobile communications system of the embodiment.
[0100] In the existing mobile communications system, each of base
station devices provided in a corresponding cell is accommodated in
one base station controller. In FIG. 11A, for example, a base
station device BTS#A is accommodated only in a base station
controller BSC#A, and a base station device BTS#B is accommodated
only in a base station controller BSC#B. For this reason, as shown
in FIG. 11A, when a mobile station moves back and forth between the
cell-a and cell-b, in a case where a base station device provided
in cell-a and a base station device provided in cell-b are
controlled by different base station controllers, a hard hand-off
is repeated.
[0101] However, in the mobile communications system of the
embodiment, when a mobile station moves back and forth between a
general unit and a boundary cell, as described with reference to
FIGS. 9 and 10, at most only two hard hand-offs occur, and then
soft hand-offs are performed. Therefore, even if the mobile station
repeatedly moves back and forth between the general cell 41 and the
boundary cell 43 in FIG. 11B, at most only two hard hand-offs
occur.
[0102] As described above, in the mobile communications system of
this embodiment, even if a mobile station moves in the vicinity of
a boundary between communications areas, each of which is
controlled by a different base station controller, the occurrence
frequency of hard hand-offs can be suppressed. As a result, a user
is not disconcerted by instantaneous disconnection due to a
hand-off.
[0103] As described with reference to FIG. 5, in this embodiment,
the base station device 33 provided in the boundary cell 43 is
connected to (accommodated in) both the base station controllers 21
and 22 by two independent physical transmission lines. Then,
information to be exchanged between the base station device 33 and
base station controller 21 and information to be exchanged between
the base station device 33 and base station controller 22 are
transmitted through respective independently-provided physical
transmission lines.
[0104] However, the mobile communications system of the embodiment
is not limited to this configuration. Specifically, a base station
controller does not always need to accommodate a base station
device provided in a boundary cell through a physical transmission
line. As shown in FIG. 12, the base station controller of the base
station can also accommodate the base station device through a
logical path (virtual path). In FIG. 12, physical paths and virtual
paths are expressed by solid lines and broken lines,
respectively.
[0105] In the example shown in FIG. 12, a base station device 33
provided in a boundary cell 43 is accommodated in a base station
controller 22 through a physical transmission line, and is also
accommodated in a base station controller 21 through logical paths
established in a transmission line for connecting the base station
device 33 and base station controller 22, a transmission line for
connecting the base station controller 22 and a mobile services
switching center 51 and a transmission line for connecting the
mobile services switching center 51 and base station controller 21.
Specifically, the logical paths are established in such a way that
the communications conducted by the base station device 33 using a
frequency RF#1 can be controlled by the base station controller
21.
[0106] With this system configuration, the number of physical
transmission lines required to connect a base station device
provided in a boundary cell and a base station controller can be
reduced, and the cost for establishing a mobile communications
system can be reduced accordingly. If base station controllers are
directly connected to each other by a physical transmission line or
a radio path without through switching center, the transmission
line and the like can also be used.
[0107] FIG. 13 shows the configuration of a base station device
provided in a boundary cell. In this example, a base station device
and a plurality of base station controllers are connected by a
plurality of independent physical transmission lines.
[0108] A base station device 33 is comprised of a wireless unit 61
and a wire unit 63. The wireless unit 61 transmits/receives radio
data to/from mobile stations. RF units 62a, 62b, . . . , are
provided for each frequency allocated to a boundary cell 43.
Signals from the base station device to a mobile station are
outputted from the RF units 62a, 62b, . . . , for each frequency,
are multiplexed and are transmitted from an antenna. Signals from a
mobile station to the base station device 33 are demultiplexed for
each frequency and are distributed to corresponding RF units.
[0109] The wire unit 63 transmits/receives data to/from a base
station controller. The line units 64a, 64b, . . . , are provided
for the RF units 62a, 62b, . . . , and terminate respective lines
(physical transmission lines) for connecting the base station
device and the plurality of base station controllers. A control
unit 65 controls the operation of a base station device.
[0110] In FIG. 13, it is assumed that the RF unit 62a is provided
for a frequency RF#1. In this case, data transmitted from a mobile
station using the frequency RF#1 are received by the RF unit 62a
and are transmitted to a base station controller BSC#1 through the
line unit 64a. Data transmitted from the base station controller
BSC#1 are transmitted to a mobile station using the frequency RF#1
in the boundary cell 43 through the line unit 64a and RF unit
62a.
[0111] FIG. 14 shows another configuration of a base station device
provided in a boundary cell. This base station device is provided
in a boundary cell in the mobile communications system shown in
FIG. 12.
[0112] In the system shown in FIG. 12, the base station device 33
provided in a boundary cell is connected only to a base station
controller 22 by a physical transmission line. Therefore, data
transmitted from the base station device 33 to a plurality of base
station controllers are multiplexed by a multiplex/demultiplex unit
66 and are outputted to the transmission line. Data from the
plurality of base station controllers to the base station device 33
are multiplexed and are transmitted through the transmission line.
The multiplex/demultiplex unit 66 demultiplexes the multiplexed
data and distributes demultiplexed data to corresponding line units
64a, 64b, . . . . The operations of the RF units 62a, 62b, . . . ,
and line units 64a, 64b, . . . , are basically the same as those
described with reference to FIG. 13.
[0113] FIG. 15 shows the configuration and operation of a base
station controller for accommodating base station devices provided
in a boundary cell. Here, a base station controller provided in the
mobile communications system in FIG. 12 is described.
[0114] The base station controller 21 is connected to the base
station device 31, and the base station controller 22 is connected
to the base station devices 32 and 33. However, in this mobile
communications system, logical paths are established in such a way
that communications conducted by the base station device 33 using
the frequency RF#1 can be controlled by the base station controller
21
[0115] A BTS interface unit 71 terminates transmission lines for
connecting a base station controller and the corresponding base
station devices. A control unit 72 controls this base station
controller. A BTS control unit 73 controls corresponding base
stations with reference to a base station management table 74. The
base station management table 74 registers information about all
base station devices accommodated in this base station controller.
The information for the base station devices is registered and
managed for each frequency allocated to each cell. This base
station management table 74 also registers information for
identifying a base station controller that controls communications
with base station devices provided in a boundary cell for each
frequency. For example, the base station management table 74
registers information indicating that "communications conducted by
the base station device 33 using a frequency RF#1 is controlled by
the base station controller 21" and information indicating that
"communications conducted by the base station device 33 using a
frequency RF#2 is controlled by the base station controller 22". An
MSC interface unit 75, which is connected to the BTS interface unit
71, terminates transmission lines for connecting this base station
controller and mobile services switching center 51.
[0116] In this system, information transmitted from the base
station device 32 is received by the base station controller 22 and
is processed by the base station controller 22 itself. On receipt
of information transmitted from the base station device 33, the BTS
interface unit 71 judges whether the received information will be
accepted or will be transferred to the mobile services switching
center 51, based on information registered in the base station
management table 74. In this embodiment, when the BTS interface
unit 71 receives information from the base station device 33, it
accepts information relating to communications conducted using the
frequency RF#2 but transfers information relating to communications
conducted using the frequency RF#1 to the mobile services switching
center 51. On receipt of information transmitted from a base
station controller, the mobile services switching center 51
transfers the information to a corresponding base station
controller according to the address attached to the
information.
[0117] In the above embodiment, as shown in FIG. 5, two frequencies
(RF#1 and RF#2) are allocated to each cell, however the mobile
communications system of the present invention is not limited to
this configuration. Specifically, for example, as shown in FIG. 16,
in the mobile communications system of the present invention, a
different frequency is allocated to each of adjacent cell groups,
and all the frequencies are allocated to a boundary cell located on
a boundary between those cell groups. In the example shown in FIG.
16, a frequency RF#1 is allocated to a cell group#1 including
general cells 101a and 101b, and a frequency RF#2 is allocated to a
cell group #2 including general cells 102a and 102b. Then, both the
frequencies RF#1 and RF#2 are allocated to a boundary cell 103
located in a boundary area between the cell groups #1 and #2.
[0118] A hand-off operation in the mobile communications system
with the configuration described above basically follows the
flowchart shown in FIG. 6. Therefore, when a mobile station moves
from a general cell to a boundary cell in this system, a soft
hand-off is always performed. For example, when a mobile station
moves from the general cell 101a to the boundary cell 103, the
mobile station uses in the boundary cell 103 the same frequency as
that used in the general cell 101a. In this case, since a mobile
station have used the frequency RF#1 in the general cell 101a, the
mobile station uses the frequency RF#1 in the boundary cell 103
too. Here, the base station controller 21 controls communications
using the frequency RF#1 in the boundary cell 103. Therefore, in
this case, a soft hand-off is performed.
[0119] If a mobile station moves from a boundary cell to a general
cell, whether a hard hand-off occurs or a soft hand-off is
performed depends on a frequency used in the boundary cell by the
mobile station.
[0120] For example, when a mobile station using a frequency RF#1 in
the boundary cell 103 moves to the general cell 101a, frequencies
used by the mobile station before and after the hand-off are the
same, and the communications conducted before and after the
hand-off is controlled by the base station controller 21.
Therefore, in this case, a soft hand-off is performed. However,
when a mobile station using a frequency RF#2 in the boundary cell
103 moves to the general cell 101a, respective frequencies used by
the mobile station before and after a hand-off are different. In
addition, the base station controller 22 performs control of
communications conducted before the hand-off, while the base
station controller 21 performs control of communications conducted
after the hand-off Therefore, in this case, communications between
a mobile station and a base station device are temporarily
disconnected. In other words, in this case, a hard hand-off
occurs.
[0121] If a mobile station moves from the general cell 102a to the
boundary cell 103, the same operations are performed.
[0122] Similarly, in the system shown in FIG. 16, a mobile station
moves to and from a boundary area, the occurrence frequency of hard
hand-offs can also be reduced. In addition, in the system shown in
FIG. 16, one frequency is allocated to each general cell, the
configurations of a base station device and a base station
controller can be simplified. Furthermore, every time a mobile
station moves from a general cell to a boundary cell, a soft
hand-off is performed. Therefore, the occurrence of hard hand-offs
can be further reduced.
[0123] Although in the embodiments described above, two
communications areas controlled by corresponding base station
controller are assumed to be adjacent, the present invention is
also applicable to a system that three or more communications areas
are adjacent to one another, as shown in FIG. 17. In this case,
three or more frequencies can be allocated to each cell.
Alternatively, different frequencies are allocated to adjacent
communications areas and all of the different frequencies are
allocated to a boundary cell adjacent to the communications areas.
In the example shown in FIG. 17, a cell controlled by BTS#4 is
adjacent to three cells (cells controlled by BTS#1-BTS#3,
respectively) In the cells respectively controlled by BTS#1-BTS#3,
frequencies RF#1-RF#3 are used, respectively, and in a cell
controlled by BTS#4, those three frequencies (RF#1-RF#3) are
used.
[0124] In the above embodiment, as shown in FIG. 1, a base station
device provided in a corresponding cell is accommodated by a base
station controller, and the base station controller is accommodated
by a mobile services switching center. However, the present
invention is not limited to this configuration. For example, the
present invention can be applied to a system in which a base
station device provided in a corresponding cell is directly
connected to a mobile services switching center. Specifically, the
present invention can be applied to a mobile communications system
in which a base station device is accommodated in a controller
(including a base station controller and a mobile services
switching center) that belongs to a higher layer above the base
station device.
[0125] Although the present invention is suitable for a CDMA
communications system, the present invention is also useful for
other communications systems (for example, TDM).
[0126] As described above, according to the present invention, when
a mobile station moves across a boundary between communications
areas, the occurrence frequency of hard hand-offs can be reduced.
Therefore, communications are seldom temporarily disconnected due
to a hand-off, and, accordingly, a user is rarely disconcerted.
[0127] The present invention can be used in a mobile communications
system, such as a CDMA communications system and the like.
* * * * *