The present invention relates to a communication apparatus, a
wireless communication system and a communication method.
Background Art
[0002]
10 In Long Term Evolution (L TE), a small cell having a small cell
radius is defined as well as a macro cell having a large cell radius. In
order to increase communication speed in a small area while taking
advantage of a characteristic of covering a smaller area than a macro cell
covering a broad area, a plurality of small cells are formed in a high-load
15 region such as a downtown and a commercial building. In· addition, in
order to cover an indoor area within a macro cell where radio waves are
hard to reach, a small cell is also formed indoors. Because of such a use
application of a small cell, a macro cell and a plurality of small cells are
often in an adjacent relationship with respect to each other. However, a
20 plurality of small cells are not always in an adjacent relationship with
respect to each other.
[0003]
Fig. 1 illustrates an example of cell arrangement in an L TE wireless
communication system.
25 [0004]
In Fig. 1 ~ Cell X that is a macro cell and Cells Y and Z that are
small cells are in an adjacent relationship with respect to each other.
However, Cell Y and Cell Z are not in an adjacent relationship with respect
to each other.
2
[0005]
Incidentally, in L TE, a Physical Cell Identity (PCI) is used as a cell
identifier for identifying a cell. It is assumed herein that the term "cell
identifier" refers to the PCI. The PCI is a cell identifier locally used for
5 user equipment (UE) to identify a cell on a wireless section, and there are
five hundred and four PCI values that are repeatedly used in L TE.
[0006]
Thus, as illustrated in Fig. 1, there is a case in which Cells Y and Z
adjacent to Cell X may have duplicate PCI values(= 84). Such PCI value
10 duplication is referred to as PCI confusion.
[0007]
Herein, Cell X and Cells Y and Z are in an adjacent relationship
with respect to each other. Thus, there is a case in which small cell base
stations (hereinafter, referred to as Small cell base stations y and z)
15 respectively forming Cells Y and Z can acquire neighbor information
indicating neighboring cells adjacent to Cell X from a macro cell base
station (hereinafter, referred to as Macro cell base station x) forming Cell
X via a connection line or the like (e.g., X2 Interface). When Small cell
base stations y and z can acquire neighbor information from Macro cell
20 base station x, Small cell base stations y and z can determine that, on the
basis of the neighbor information, PCI Confusion is occurring, in which
Cells Y and Z have PCI values duplicate to each other. In this case, when
any of Small cell base stations y and z autonomously re-selects a PCI value,
the PCI confusion is eliminated.
25 [0008]
However, there is a case in which Small cell base stations y and z
cannot directly acquire neighbor information from Macro cell base station
x via a connection line or the like. In addition, there is a case in which,
even when Small cell base stations y and z can acquire neighbor
3
information from Macro cell base station x, some of neighboring cells
adjacent to Cell X cannot be recognized due to insufficiency of the
neighbor information. In addition, since Cell Y and Cell Z are not in an
adjacent relationship with respect to each other, Small cell base stations y
5 and z cannot acquire neighbor information from the other side's small cell
base station.
[0009]
Accordingly, Small cell base stations y and z cannot determine that
a PCI value of the own cell has caused PCI confusion in which the PCI
10 value of the own cell is duplicated with a PCI value of the other cell in any
15
of the cases described above. Small cell base stations y and z lose an
opportunity to autonomously re-select a PCI value, and the PCI confusion
is not eliminated.
[00 1 0]
Herein, think about a case in which a UE moves from Cell X to Cell
Y while PCI confusion is occurring, in which PCI values of Cells Y and Z
are duplicated to each other.
[0011]
In this case, the UE transmits, to Macro cell base station x forming
20 Cell X, a Measurement Report message including a PCI value "84" of Cell
Y that is a handover destination candidate.
[0012]
However, the PCI confusion state does not allow a value of an ECG I
(E-UTRAN Cell Global Identifier. E-UTRAN: Evolved Universal
25 Terrestrial Radio Access Network) corresponding to a PCI value to be
uniquely determined. Herein, the ECG I refers to a cell identifier for
uniquely identifying a cell throughout a communication network, unlike
the PCI.
[00 13]
4
Thus, there is a case in which Macro cell base station x may request
for handover not to Cell Y that is a handover destination candidate, but to
wrong Cell Z having the same PCI value as that of Cell Y. In this case,
there is a problem that handover fails and this results in a decrease in a
5 handover success rate.
[00 14]
Examples of a technique for solving the problem include a method
described in PTL 1. In the method described in PTL 1, while assuming a
case in which there are a plurality of small cells using the same PCI value
10 in a macro cell, UE adds, to a measurement report, a PCI value and a CGI
value of a handover destination candidate cell.
Citation List
Patent Literature
[0015]
15 PTL 1: Japanese Unexamined Patent Application Publication No.
2010-109664
20
25
Summary of Invention
Technical Problem
[00 16]
According to the method described in PTL 1, since a CGI value of a
handover destination candidate cell is uniquely determined, a probability
that a macro cell base station requests for handover to a wrong cell can be
lowered and a decrease in a handover success rate can be prevented.
[00 17]
However, in the method described in PTL 1, the UE needs to
receive broadcast information on a handover destination candidate cell in
order to know a CGI value of the cell. Thus, there is a problem that it
takes a long time for acquiring the CG I value and this causes a delay in
transmission of a measurement report.
5
[00 18]
Therefore, a decrease in a handover success rate needs to be
prevented by using a method different from the method described in PTL 1.
[00 19]
5 For the purpose of the above, it is an issue to eliminate PCI
10
confusion indicating duplication of a PCI value of a small cell with a PCI
value of another cell regardless of an acquisition status of neighbor
information in a small cell base station from a macro cell base station.
[0020]
In view of the above, an object to be achieved by the exemplary
embodiments disclosed herein is to provide a technique for solving the
above-described issue and capable of eliminating cell identifier
duplication between a cell identifier of a cell of a base station and a cell
identifier of another cell regardless of an acquisition status of neighbor
15 information in the base station. It should be noted that the object is
merely one of a plurality of objects to be achieved by the exemplary
embodiments disclosed herein. Other objects or issues and novel features
will be apparent from the following description or the accompanying
drawings.
20 Solution to Problem
[0021]
A first communication apparatus according to the present invention
is a communication apparatus in a wireless communication system
including a terminal, a first base station forming a first cell, and a second
25 base station forming a second cell adjacent to the first cell, including:
a communication status acquisition unit that acquires a
communication status in the second base station; and
a control unit that eliminates cell identifier duplication between a
cell identifier of the second cell and a cell identifier of another cell based
5
10
6
on the number of times that the second base station has received, from the
terminal, a first message requesting for re-establishment of an RRC
connection.
[0022]
A second communication apparatus according to the present
invention is a communication apparatus in a wireless communication
system including a terminal, a first base station forming a first cell, and a
second base station forming a second cell adjacent to the first cell,
including:
a communication status acquisition unit that acquires a
communication status in the second base station; and
a control unit that eliminates cell identifier duplication between a
cell identifier of the second cell and a cell identifier of another cell
adjacent to the first cell based on the number of times that hand over of the
15 terminal from the first cell to the second cell has failed.
[0023]
A first wireless communication system according to the present
invention is a wireless communication system including a terminal, a first
base station forming a first cell, and a second base station forming a
20 second cell adjacent to the first cell, including:
a communication status acquisition unit that acquires a
' communication status in the second base station; and
a control unit that eliminates cell identifier duplication between a
cell identifier of the second cell and a cell identifier of another cell based
25 on the number of times that the second base station has received, from the
terminal, a first message requesting for re-establishment of an RRC
connection.
[0024]
A second wireless communication system according to the present
7
invention is a wireless communication system including a terminal, a first
base station forming a first cell, and a second base stati"on forming a
second cell adjacent to the first cell, including:
a communication status acquisition unit that acquires a
5 communication status in the second base station; and
a control unit that eliminates cell identifier duplication between a
cell identifier of the second cell and a cell identifier of another cell
adjacent to the first cell based on the number of times that hand over of the
terminal from the first cell to the second cell has failed.
10 [0025]
A first communication method according to the present invention is
a communication method performed by a communication apparatus in a
wireless communication system including a terminal, a first base station
forming a first cell, and a second base station forming a second cell
15 adjacent to the first cell, including:
acquiring a communication status in the second base station; and
eliminating cell identifier duplication between a cell identifier of
the second cell and a cell identifier of another cell based on the number of
times that the second base station has received, from the terminal, a first
20 message requesting for re-establishment of an RRC connection.
[0026]
A second communication method according to the present invention
is a communication method performed by a communication apparatus in a
wireless communication system including a terminal, a first base station
25 forming a first cell, and a second base station forming a second cell
adjacent to the first cell, including:
acquiring a communication status in the second base station; and
eliminating cell identifier duplication between a cell identifier of
the second cell and a cell identifier of another cell adjacent to the first cell
5
8
based on the number of times that handover of the terminal from the first
cell to the second cell has failed.
Advantageous Effects of Invention
[0027]
The present invention can obtain an advantageous effect that a
communication apparatus is capable of eliminating cell identifier
duplication between a cell identifier of a cell of a base station and a cell
identifier of another cell regardless of an acquisition status of neighbor
information in the base station.
10 Brief Description of Drawings
[0028]
Fig. 1 is a diagram illustrating an example of cell arrangement in an
L TE wireless communication system.
Fig. 2 is a diagram illustrating an example of an overall
15 configuration of a wireless communication system according to the present
invention.
20
25
Fig. 3 is a sequence diagram illustrating an example of a sequence
in a case when handover succeeds in the wireless communication system
illustrated in Fig. 2.
Fig. 4 is a sequence diagram illustrating an example of a sequence
in a case when handover fails in the wireless communication system
illustrated in Fig. 2.
Fig. 5 is a diagram illustrating an example of an RRC Connection
Reestablishment Request message.
Fig. 6 is a block diagram illustrating an example of a configuration
of a small cell base station according to a first exemplary embodiment of
the present invention.
Fig. 7 is a block diagram illustrating an example of a configuration
of a small cell base station according to second and third exernplary
9
embodiments of the present invention.
Fig. 8 is a sequence diagram illustrating an example of a sequence
in a case when handover is attributed to Too Early HO in a wireless
communication system according to the third exemplary embodiment of the
5 present invention.
Fig. 9 is a sequence diagram illustrating an example of a sequence
in a case when handover is attributed to Too Late HO in a wireless
communication system according to the third exemplary embodiment of the
present invention.
10 Fig. 10 is a block diagram illustrating an example of a schematic
configuration of a communication apparatus according to the present
invention.
Description of Embodiments
[0029]
15 ( 1) Basic Concept of the Present Invention
First, an overall configuration of a wireless communication system
according to the present invention will be described.
[0030]
Fig. 2 illustrates an example of an overall configuration of a
20 wireless communication system according to the present invention.
[0031]
The wireless communication system illustrated in Fig. 2 is an L TE
wireless communication system that includes a UE 10, a macro cell base
station 20A, small cell base stations 20B and 20C, and a Mobility
25 Management Entity (MME) 30.
[0032]
The macro cell base station 20A is a first base station forming Cell
A (PCI value "142") that is a macro cell (first cell).
[0033]
10
The small cell base station 20B is a second base station forming
Cell B (PCI value "84") that is a small cell (second cell).
[0034]
The small cell base station 20C is a second base station forming
5 Cell C (PCI value "84") that is a small cell (second cell).
[003 5]
Cells B and C are provided within Cell A. Thus, Cell A and Cells
B and C are in an adjacent relationship with respect to each other.
However, Cell B and Cell C are not in an adjacent relationship with respect
10 to each other.
[003 6]
Herein, it is assumed that a cell adjacent relationship includes a
relationship between a certain cell and peripheral cells adjacent to the cell,
without limitation to a relationship between a certain cell and cells
15 included in the cell (a relationship between Cell A and Cells B and C) as
illustrated in Fig. 2.
[003 7]
In addition, the PCI values of Cells B and C are both the same, "84".
Thus, the PCI values of Cells B and C adjacent to Cell A are duplicate to
20 each other, which means that PCI confusion is occurring.
[003 8]
The UE 10 is a terminal subjected to handover between Cells A to C.
Note that handover when seen from a handover source and handover when
seen from a handover destination are referred to as Handout and Hand-in,
25 respectively, as appropriate.
[003 9]
The MME 3 0 is a core network device arranged in a core network
and metnaging movement of the lJE 10.
[0040]
5
11
Note that the configuration and the cell arrangement of the wireless
communication system in Fig. 2 are merely exemplary and the present
invention is not limited thereto.
[0041]
Next, as a technique that is a premise of the present invention,
sequences in respective cases when handover of the UE 10 succeeds and
fails in the wireless communication system illustrated in Fig. 2 will be
described. A handover scenario via an inter-base station line (X2
Interface) will be introduced as an example. However, the same applies
10 to a handover scenario using a line between a base station and a core
network (S 1 Interface).
[0042]
First, a sequence in a case where handover of the UE 10 succeeds
will be described.
15 [0043]
Fig. 3 illustrates an example of a sequence in a case where handover
of the UE 10 succeeds. Note that an operation in Fig. 3 starts from a state
in which Cell A is a serving cell of the UE 10.
[0044]
20 As illustrated in Fig. 3, the macro cell base station 20A forming
Cell A that is a serving cell of the UE 10 transmits, to the UE 10, a
Measurement Control message instructing measurement of reception
quality of neighboring cells adjacent to Cell A (Step A 1 ). The
Measurement Control message herein indicates a message including
25 information relating to measurement, and includes an RRC Connection
Reconfiguration message as an example.
[0045 J
Next, the UE 10 n1easures reception quality of neighboring cells
adjacent to Cell A and transmits, to the macro cell base station 20A, a
12
Measurement Report message reporting a PCI value of a handover
destination candidate cell with good reception quality out of these
neighboring cells (Step A2). It is assumed herein that the UE 10 reports a
PCI value "84" of Cell B. In addition, the macro cell base station 20A
5 stores a PCI value and an ECGI value of a cell in association with each
other, and it is assumed herein that the macro cell base station 20A stores
an ECG I value of Cell B in association with the PCI value "84".
[0046]
Thus, the macro cell base station 20A transmits, to the small cell
10 base station 20B forming Cell B, a Handover Request message requesting
for handover of the UE 10 to Cell B (Step A3 ).
[004 7]
In other words, a Handover Request message herein is transmitted
to the small cell base station 20B forming correct Cell B that is a handover
15 destination candidate.
[0048]
Next, the small cell base station 20B transmits, to the macro cell
base station 20A, a Handover Request Acknowledgement message
accepting handover of the UE 10 (Step A4 ). The message includes
20 information on an RRC connection between the UE 10 and Cell B that is a
handover destination candidate cell.
[0049]
Next, the macro cell base station 20A transmits, to the UE 10, an
RRC Connection Rcconfiguration message including information on the
25 RRC connection between the UE 10 and Cell B that is a handover
destination candidate cell (Step AS).
[0050]
lierein, the Handover Request tnessage at Step A3 has been
transmitted to the small cell base station 20B forming correct Cell B that is
13
a handover destination candidate cell of the UE 1 0.
[0051]
Thus, the UE 10 completes reconfiguration of the RRC connection
between the UE 10 and Cell B, and transmits, to the small cell base station
5 20B, an RRC Connection Reconfiguration Complete message notifying the
completion of the RRC connection reconfiguration (Step A6).
Accordingly, the serving cell of the UE 10 is switched from Cell A to Cell
B.
10
[0052]
Thereafter, the small cell base station 20B transmits, to the MME
30, a Path Switch Request message requesting for switching of a path from
the macro cell base station 20A to the small cell base station 20B (Step A 7).
Then, the MME 30 transmits, to the small cell base station 20B, a Path
Switch Request Acknowledgement message accepting switching of a path
15 (Step A8). Subsequently, the small cell base station 20B transmits, to the
macro cell base station 20A, a UE Context Release message instructing
release of a UE Context (Step A9).
[0053]
Next, a sequence in a case where handover of the UE 10 fails will
20 be described.
[0054]
Fig. 4 illustrates an example of a sequence in a case where handover
of the UE 10 fails. Note that an operation in Fig. 4 starts from a state in
which Cell A is a serving cell of the UE 10.
25 [005 5]
As illustrated in Fig. 4, first, transmission and reception of a
Measurement Control message and a Measurement Report message are
performed between the macro cell base station 20A and the UE 10 in the
same manner as Steps A1 and A2 in Fig. 3 (Steps B 1 and B2). However,
14
it is assumed herein that the UE 10 reports a PCI value "84" of Cell C as a
PCI value of a handover destination candidate cell. In addition, it is
assumed that the macro cell base station 20A stores an ECGI value of Cell
B in association with the PCI value "84".
5 [0056]
Thus, the macro cell base station 20A transmits, to the small cell
base station 20B forming Cell B, a Handover Request message requesting
for handover of the UE 10 to Cell B (Step B3 ).
[0057]
10 In other words, a Handover Request message herein is not
transmitted to the small cell base station 20C forming correct Cell C that is
a handover destination candidate, but is transmitted to the small cell base
station 20B forming wrong Cell B that is not a handover destination
candidate.
15 [0058]
20
Next, the small cell base station 20B transmits, to the macro cell
base station 20A, a Handover Request Acknowledgement message
accepting hand over of the UE 10 (Step B4 ).
[0059]
Next, the macro cell base station 20A transmits, to the UE 10, an
RRC Connection Reconfiguration message instructing reconfiguration of
an RRC connection between the UE 10 and a handover destination
candidate cell (Step B5).
[0060 J
25 Herein, the Handover Request message at Step B3 has been
transmitted to the small cell base station 20B forming Cell B that is not a
handovcr destination candidate of the U E 10.
[0061]
Thus, the UE 10 does not complete reconfiguration of the RRC
15
connection between the UE 10 and Cell C that is a handover destination
candidate, and transmits, to the small cell base station 20C, an RRC
Connection Reestablishment Request message requesting for
re-establishment of the RRC connection (Step B6).
5 [0062]
Next, the small cell base station 20C transmits, to the UE 10, an
RRC Connection Reestablishment Reject message rejecting
re-establishment of the RRC connection (Step B7).
[0063]
10 Thereafter, an Attach procedure in which the UE 10 is attached to
the small cell base station 20C is executed (Step B8). Accordingly, the
serving cell of the UE 10 is switched from Cell A to Cell C.
[0064]
Incidentally, in Fig. 4, the failure of handover of the UE 10 is able
15 to be detected by the macro cell base station 20A. Besides, the small cell
base station 20B and the small cell base station 20C are also able to detect
the failure of handover of the UE 10 in a manner as follows.
[0065]
For example, in Fig. 4, the small cell base station 20B receives a
20 Handover Request message from the macro cell base station 20A and
proceeds with the handover procedure, but does not receive an RRC
Connection Reconfiguration Complete message from the UE 10. Thus,
the small cell base station 20B is able to determine that handover
(Hand-in) of the UE 10 from Cell A to Cell B has failed although the macro
25 cell base station 20A has requested for the handover.
[0066]
On the other hand, in Fig. 4, the small cell base station 20C does
not receive a Handover Request message from the macro cell base station
20A, but receives an RRC Connection Reestablishment Request message
16 '
from the UE 10. Thus, the small cell base station 20C is able to
determine that handover of the UE 10 has failed. In addition, the small
cell base station 20C is also able to determine the failure of handover of
the UE 10 and a handover source cell of the UE 10 by referring to contents
5 of the RRC Connection Reestablishment Request message as illustrated in
Fig. 5. For example, the small cell base station 20C is able to determine
the failure of handover of the UE 10 from the fact that "handoverFailure"
and "reconfigurationFailure" are set in "Reestablishment Cause". In
addition, the small cell base station 20C is able to determine a handover
10 source cell of the UE 10 from a PCI value set in "physCellld" included in
""Reestab DE-Identity".
[0067]
The present invention is intended to eliminate PCI confusion
indicating duplication of a PCI value of a cell of the small cell base
15 stations 20B and 20C with a PCI value of another cell by using the fact that
the small cell base stations 20B and 20C are able to detect the failure of
handover of the UE 10.
20
(2) Exemplary Embodiments of the Present Invention
(2-1) First Exemplary Embodiment
The present exemplary embodiment is intended for the small cell
base station 20B to determine occurrence of PCI confusion in the wireless
communication system illustrated in Fig. 2.
[0068]
In view of the above, the following will describe a configuration of
25 the small cell base station 20B in detail.
[0069]
Fig. 6 illustrates an example of a configuration of the small cell
base station 20B according to the present exemplary embodiment.
[0070]
17
As illustrated in Fig. 6, the small cell base station 20B includes a
communication unit 21 B, a communication status acquisition unit 22B, and
a control unit 23B. Note that Fig. 6 illustrates only an excerpt of
essential components from the components within the small cell base
5 station 20B and omits other components.
[0071]
The communication unit 21 B communicates various messages and
the like with the UE 10, the macro cell base station 20A, the small cell
base station 20C, and the MME 30.
10 [0072]
15
The communication status acquisition unit 22B acquires a
communication status of various messages and the like in the small cell
base station 20B.
[0073]
The control unit 23B eliminates PCI value duplication (PCI
confusion) between a PCI value of Cell B and a PCI value of another cell
adjacent to Cell A on the basis of the number of times that handover of the
UE 10 from Cell A to Cell B has failed.
[0074]
20 Specifically, the control unit 23B counts the number of times that
handover of the UE 10 from Cell A to Cell B has failed. For example,
when the control unit 23B receives no RRC Connection Reconfiguration
Complete message from the UE 10 within a predetermined period of time
after receiving a Handover Request message requesting for handover
25 (Hand-in) of the UE 10 from Cell A to Cell B, the control unit 23B is able
to determine that the handover has failed.
[0075]
Then, when the uuruber uf tirues that han dover (Hand-in) of the UE
10 from Cell A to Cell B has failed is equal to or more than a
5
18
predetermined number of times, the control unit 23B determines that a PCI
value of Cell B is duplicated with a PCI value of another cell adjacent to
Cell A and has caused PCI confusion.
[0076]
Note that the control unit 23B is able to determine that Cell A is a
handover source of the UE 10 from the Handover Request message.
[0077]
In addition, the control unit 23B may determine PCI confusion only
in a case (A) or (B) below.
10 (A) When the small cell base station 20B is unable to directly acquire
neighbor information indicating neighboring cells adjacent to Cell A from
the macro cell base station 20A via a connection line such as an X2
Interface or the like
(B) When the small cell base station 20B is unable to recognize some of
15 neighboring cells adjacent to Cell A due to insufficiency in neighbor
information indicating neighboring cells adjacent to Cell A acquired from
the macro cell base station 20A
Note that the case (B) where neighbor information is insufficient
refers to, for example, a case where neighbor information acquired from
20 the macro cell base station 20A indicates only a neighboring cell
establishing an X2 interface out of neighboring cells adjacent to Cell A.
In addition, whether the case (B) is applicable or not can be determined by,
for example, whether the number of neighboring cells indicated by
neighbor information acquired from the macro cell base station 20A is
25 equal to or less than a predetermined number or not.
[0078]
In addition, the control unit ?. 3 R may determine PCT confusion with
arbitrary timing, for example, periodically. Alternatively, the control
unit 23B may determine PCI confusion at a point in time when the number
19
of times that hand over has failed is, as a result of counting the number,
equal to or more than a predetermined number of times.
[0079]
When determining that a PCI value of Cell B is duplicated with a
5 PCI value of another cell adjacent to Cell A and has caused PCI confusion,
the control unit 23B eliminates the PCI confusion. When eliminating the
PCI confusion, the control unit 23B autonomously re-selects a PCI value of
Cell B.
10
15
[0080]
At this time, for example, the control unit 23B excludes the current
PCI value and a PCI value having been selected several times in the past
from re-selection candidates, and re-selects a PCI value from the remaining
re-selection candidates.
[0081]
Note that changing a PCI value of Cell B so as to be different from
a PCI value of a neighboring cell adjacent to Cell B and a PCI value of a
neighboring cell adjacent to the aforesaid neighboring cell falls under a
general technique and is not directly relevant to the present invention. In
addition, it is necessary to avoid only PCI value duplication in an identical
20 frequency of an identical Radio Access Technology (RAT), and changing
in such a manner also falls under a general technique.
[0082]
Alternatively, the control unit 23B may request for allocation of a
new PCI value fron1 a11 external server rather than autonomously
25 re-selecting a PCI value of Cell B, and the external server accepting the
request may allocate a new PCI value to Cell B. In this case, the external
server desirably holds information on neighboring cells adjacent to Cell B
as a database.
[0083]
20
As described above, according to the present exemplary
embodiment, the control unit 23 B of the small cell base station 20B
eliminates PCI value duplication (PCI confusion) between a PCI value of
Cell Band a PCI value of another cell adjacent to Cell A on the basis of the
5 number of times that handover of the UE 10 from Cell A to Cell B has
failed.
[0084]
Thus, an advantageous effect of being capable of eliminating PCI
confusion can be obtained regardless of an acquisition status of neighbor
10 information in the small cell base station 20B. Accordingly, the macro
cell base station 20A is able to request for handover of the UE 10 to a
correct cell, and a decrease in a handover success rate of the UE 10 can be
prevented.
15
[0085]
Specifically, when the number of times that handover of the UE 10
from Cell A to Cell B has failed is equal to or more than a predetermined
number of times, the control unit 23B of the small cell base station 20B
determines that a PCI value of Cell B is duplicated with a PCI value of
another cell adjacent to Cell A and has caused PCI confusion.
20 [0086]
In addition, when determining that PCI confusion is occurring, the
control unit 23B eliminates the PCI confusion by autonomously
re-selecting a PCI value or requesting for allocation of a new PCI value
from an external server. Note that the control unit 23B may cause the
25 another cell adjacent to Cell A to re-select a PCI value.
(2-2) Second Exemplary Embodiment
In the first exemplary embodiment, the macro cell base station 20A
transmits a Handover Request message to a single base station forming a
handover destination candidate cell. However, it is also contemplated
21
that a plurality of cells (that may have different PCis) are handover
destination candidate cells and the macro cell base station 20A is
configured to have a mechanism of transmitting a Handover Request
message to a plurality of base stations respectively forming these plurality
5 of cells. In this case, in the first exemplary embodiment, a number of
base stations having received a Handover Request message result in
receiving no RRC Connection Reconfiguration Complete message. It is
then predicted that these base stations may erroneously determine that PCI
confusion is occurring even though in fact PCI confusion is not occurring.
10 [0087]
In order to solve the above-described problem in the first exemplary
embodiment, the present exemplary embodiment is intended for the small
cell base station 20C to determine occurrence of PCI confusion in the
wireless communication system illustrated in Fig. 2.
15 [0088]
In view of the above, the following will describe a configuration of
the small cell base station 20C in detail.
[0089]
Fig. 7 illustrates an example of a configuration of the small cell
20 base station 20C according to the present exemplary embodiment.
[0090]
As illustrated in Fig. 7, the small cell base station 20C includes a
communication unit 21 C, a communication status acquisition unit 22C, and
a control unit 23C. Note that Fig. 7 illustrates only an excerpt of
25 essential components from the components within the small cell base
station 20C and on1its other con1ponents.
[0091]
The communication unit 21 C communicates various messages and
the like with the UE 10, the macro cell base station 20A, the small cell
base station 20B, and the MME 30.
[0092]
22
The communication status acquisition unit 22C acquires a
communication status of various messages and the like in the small cell
5 base station 20C.
[0093]
The control unit 23C eliminates PCI value duplication (PCI
confusion) between a PCI value of Cell C and a PCI value of another cell
on the basis of the number of times that an RRC Connection
10 Reestablishment Request message has been received from the UE 10.
[0094]
Specifically, the control unit 23 C counts the number of times that
an RRC Connection Reestablishment Request message has been received
from the UE 10.
15 [0095]
Then, when the number of times that an RRC Connection
Reestablishment Request message has been received from the UE 10 is
equal to or more than a predetermined number of times, the control unit
23C determines that a PCI value of Cell C is duplicated with a PCI value of
20 another cell and has caused PCI confusion.
[0096]
Note that the control unit 23C may count the number of times of
reception only when a particular RRC Connection Reestablishment Request
message is received rather than all RRC Connection Reestablishment
25 Request messages.
[0097]
For example, the control unit 23C may count the number of times of
reception only for an RRC Connection Reestablishment Request message in
which a PCI value set in "physCellld" included in "ReestabUE-Identity"
23
indicates Cell A. "physCellld" indicates a PCI value of a cell to which
the UE 10 has been connected before handover failure. For example, in
Fig. 4, since Cell A is a cell to which the UE 10 has been connected before
handover failure, a PCI value of Cell A is written in "physCellld". In
5 this case, when the number of times of reception thereof is equal to or more
than a predetermined number of times, the control unit 23C determines that
a PCI value of Cell C is duplicated with a PCI value of another cell
adjacent to Cell A and has caused PCI confusion. In this ·case, the control
unit 23C may determine PCI confusion only in a case (A) or (B) below.
10 (A) When the small cell base station 20C is unable to directly acquire
neighbor information indicating neighboring cells adjacent to Cell A from
the macro cell base station 20A via a connection line such as an X2
Interface or the like
(B) When the small cell base station 20C is unable to recognize some of
15 neighboring cells adjacent to Cell A due to insufficiency in neighbor
information indicating neighboring cells adjacent to Cell A acquired from
the macro cell base station 20A
Note that the definition of "insufficiency" in (B) and the
determination method as to whether (B) is applicable or not are the same as
20 those in the first exemplary embodiment.

The present invention relates to a communication apparatus, a
wireless communication system and a communication method.
Background Art
[0002]
10 In Long Term Evolution (L TE), a small cell having a small cell
radius is defined as well as a macro cell having a large cell radius. In
order to increase communication speed in a small area while taking
advantage of a characteristic of covering a smaller area than a macro cell
covering a broad area, a plurality of small cells are formed in a high-load
15 region such as a downtown and a commercial building. In· addition, in
order to cover an indoor area within a macro cell where radio waves are
hard to reach, a small cell is also formed indoors. Because of such a use
application of a small cell, a macro cell and a plurality of small cells are
often in an adjacent relationship with respect to each other. However, a
20 plurality of small cells are not always in an adjacent relationship with
respect to each other.
[0003]
Fig. 1 illustrates an example of cell arrangement in an L TE wireless
communication system.
25 [0004]
In Fig. 1 ~ Cell X that is a macro cell and Cells Y and Z that are
small cells are in an adjacent relationship with respect to each other.
However, Cell Y and Cell Z are not in an adjacent relationship with respect
to each other.
2
[0005]
Incidentally, in L TE, a Physical Cell Identity (PCI) is used as a cell
identifier for identifying a cell. It is assumed herein that the term "cell
identifier" refers to the PCI. The PCI is a cell identifier locally used for
5 user equipment (UE) to identify a cell on a wireless section, and there are
five hundred and four PCI values that are repeatedly used in L TE.
[0006]
Thus, as illustrated in Fig. 1, there is a case in which Cells Y and Z
adjacent to Cell X may have duplicate PCI values(= 84). Such PCI value
10 duplication is referred to as PCI confusion.
[0007]
Herein, Cell X and Cells Y and Z are in an adjacent relationship
with respect to each other. Thus, there is a case in which small cell base
stations (hereinafter, referred to as Small cell base stations y and z)
15 respectively forming Cells Y and Z can acquire neighbor information
indicating neighboring cells adjacent to Cell X from a macro cell base
station (hereinafter, referred to as Macro cell base station x) forming Cell
X via a connection line or the like (e.g., X2 Interface). When Small cell
base stations y and z can acquire neighbor information from Macro cell
20 base station x, Small cell base stations y and z can determine that, on the
basis of the neighbor information, PCI Confusion is occurring, in which
Cells Y and Z have PCI values duplicate to each other. In this case, when
any of Small cell base stations y and z autonomously re-selects a PCI value,
the PCI confusion is eliminated.
25 [0008]
However, there is a case in which Small cell base stations y and z
cannot directly acquire neighbor information from Macro cell base station
x via a connection line or the like. In addition, there is a case in which,
even when Small cell base stations y and z can acquire neighbor
3
information from Macro cell base station x, some of neighboring cells
adjacent to Cell X cannot be recognized due to insufficiency of the
neighbor information. In addition, since Cell Y and Cell Z are not in an
adjacent relationship with respect to each other, Small cell base stations y
5 and z cannot acquire neighbor information from the other side's small cell
base station.
[0009]
Accordingly, Small cell base stations y and z cannot determine that
a PCI value of the own cell has caused PCI confusion in which the PCI
10 value of the own cell is duplicated with a PCI value of the other cell in any
15
of the cases described above. Small cell base stations y and z lose an
opportunity to autonomously re-select a PCI value, and the PCI confusion
is not eliminated.
[00 1 0]
Herein, think about a case in which a UE moves from Cell X to Cell
Y while PCI confusion is occurring, in which PCI values of Cells Y and Z
are duplicated to each other.
[0011]
In this case, the UE transmits, to Macro cell base station x forming
20 Cell X, a Measurement Report message including a PCI value "84" of Cell
Y that is a handover destination candidate.
[0012]
However, the PCI confusion state does not allow a value of an ECG I
(E-UTRAN Cell Global Identifier. E-UTRAN: Evolved Universal
25 Terrestrial Radio Access Network) corresponding to a PCI value to be
uniquely determined. Herein, the ECG I refers to a cell identifier for
uniquely identifying a cell throughout a communication network, unlike
the PCI.
[00 13]
4
Thus, there is a case in which Macro cell base station x may request
for handover not to Cell Y that is a handover destination candidate, but to
wrong Cell Z having the same PCI value as that of Cell Y. In this case,
there is a problem that handover fails and this results in a decrease in a
5 handover success rate.
[00 14]
Examples of a technique for solving the problem include a method
described in PTL 1. In the method described in PTL 1, while assuming a
case in which there are a plurality of small cells using the same PCI value
10 in a macro cell, UE adds, to a measurement report, a PCI value and a CGI
value of a handover destination candidate cell.
Citation List
Patent Literature
[0015]
15 PTL 1: Japanese Unexamined Patent Application Publication No.
2010-109664
20
25
Summary of Invention
Technical Problem
[00 16]
According to the method described in PTL 1, since a CGI value of a
handover destination candidate cell is uniquely determined, a probability
that a macro cell base station requests for handover to a wrong cell can be
lowered and a decrease in a handover success rate can be prevented.
[00 17]
However, in the method described in PTL 1, the UE needs to
receive broadcast information on a handover destination candidate cell in
order to know a CGI value of the cell. Thus, there is a problem that it
takes a long time for acquiring the CG I value and this causes a delay in
transmission of a measurement report.
5
[00 18]
Therefore, a decrease in a handover success rate needs to be
prevented by using a method different from the method described in PTL 1.
[00 19]
5 For the purpose of the above, it is an issue to eliminate PCI
10
confusion indicating duplication of a PCI value of a small cell with a PCI
value of another cell regardless of an acquisition status of neighbor
information in a small cell base station from a macro cell base station.
[0020]
In view of the above, an object to be achieved by the exemplary
embodiments disclosed herein is to provide a technique for solving the
above-described issue and capable of eliminating cell identifier
duplication between a cell identifier of a cell of a base station and a cell
identifier of another cell regardless of an acquisition status of neighbor
15 information in the base station. It should be noted that the object is
merely one of a plurality of objects to be achieved by the exemplary
embodiments disclosed herein. Other objects or issues and novel features
will be apparent from the following description or the accompanying
drawings.
20 Solution to Problem
[0021]
A first communication apparatus according to the present invention
is a communication apparatus in a wireless communication system
including a terminal, a first base station forming a first cell, and a second
25 base station forming a second cell adjacent to the first cell, including:
a communication status acquisition unit that acquires a
communication status in the second base station; and
a control unit that eliminates cell identifier duplication between a
cell identifier of the second cell and a cell identifier of another cell based
5
10
6
on the number of times that the second base station has received, from the
terminal, a first message requesting for re-establishment of an RRC
connection.
[0022]
A second communication apparatus according to the present
invention is a communication apparatus in a wireless communication
system including a terminal, a first base station forming a first cell, and a
second base station forming a second cell adjacent to the first cell,
including:
a communication status acquisition unit that acquires a
communication status in the second base station; and
a control unit that eliminates cell identifier duplication between a
cell identifier of the second cell and a cell identifier of another cell
adjacent to the first cell based on the number of times that hand over of the
15 terminal from the first cell to the second cell has failed.
[0023]
A first wireless communication system according to the present
invention is a wireless communication system including a terminal, a first
base station forming a first cell, and a second base station forming a
20 second cell adjacent to the first cell, including:
a communication status acquisition unit that acquires a
' communication status in the second base station; and
a control unit that eliminates cell identifier duplication between a
cell identifier of the second cell and a cell identifier of another cell based
25 on the number of times that the second base station has received, from the
terminal, a first message requesting for re-establishment of an RRC
connection.
[0024]
A second wireless communication system according to the present
7
invention is a wireless communication system including a terminal, a first
base station forming a first cell, and a second base stati"on forming a
second cell adjacent to the first cell, including:
a communication status acquisition unit that acquires a
5 communication status in the second base station; and
a control unit that eliminates cell identifier duplication between a
cell identifier of the second cell and a cell identifier of another cell
adjacent to the first cell based on the number of times that hand over of the
terminal from the first cell to the second cell has failed.
10 [0025]
A first communication method according to the present invention is
a communication method performed by a communication apparatus in a
wireless communication system including a terminal, a first base station
forming a first cell, and a second base station forming a second cell
15 adjacent to the first cell, including:
acquiring a communication status in the second base station; and
eliminating cell identifier duplication between a cell identifier of
the second cell and a cell identifier of another cell based on the number of
times that the second base station has received, from the terminal, a first
20 message requesting for re-establishment of an RRC connection.
[0026]
A second communication method according to the present invention
is a communication method performed by a communication apparatus in a
wireless communication system including a terminal, a first base station
25 forming a first cell, and a second base station forming a second cell
adjacent to the first cell, including:
acquiring a communication status in the second base station; and
eliminating cell identifier duplication between a cell identifier of
the second cell and a cell identifier of another cell adjacent to the first cell
5
8
based on the number of times that handover of the terminal from the first
cell to the second cell has failed.
Advantageous Effects of Invention
[0027]
The present invention can obtain an advantageous effect that a
communication apparatus is capable of eliminating cell identifier
duplication between a cell identifier of a cell of a base station and a cell
identifier of another cell regardless of an acquisition status of neighbor
information in the base station.
10 Brief Description of Drawings
[0028]
Fig. 1 is a diagram illustrating an example of cell arrangement in an
L TE wireless communication system.
Fig. 2 is a diagram illustrating an example of an overall
15 configuration of a wireless communication system according to the present
invention.
20
25
Fig. 3 is a sequence diagram illustrating an example of a sequence
in a case when handover succeeds in the wireless communication system
illustrated in Fig. 2.
Fig. 4 is a sequence diagram illustrating an example of a sequence
in a case when handover fails in the wireless communication system
illustrated in Fig. 2.
Fig. 5 is a diagram illustrating an example of an RRC Connection
Reestablishment Request message.
Fig. 6 is a block diagram illustrating an example of a configuration
of a small cell base station according to a first exemplary embodiment of
the present invention.
Fig. 7 is a block diagram illustrating an example of a configuration
of a small cell base station according to second and third exernplary
9
embodiments of the present invention.
Fig. 8 is a sequence diagram illustrating an example of a sequence
in a case when handover is attributed to Too Early HO in a wireless
communication system according to the third exemplary embodiment of the
5 present invention.
Fig. 9 is a sequence diagram illustrating an example of a sequence
in a case when handover is attributed to Too Late HO in a wireless
communication system according to the third exemplary embodiment of the
present invention.
10 Fig. 10 is a block diagram illustrating an example of a schematic
configuration of a communication apparatus according to the present
invention.
Description of Embodiments
[0029]
15 ( 1) Basic Concept of the Present Invention
First, an overall configuration of a wireless communication system
according to the present invention will be described.
[0030]
Fig. 2 illustrates an example of an overall configuration of a
20 wireless communication system according to the present invention.
[0031]
The wireless communication system illustrated in Fig. 2 is an L TE
wireless communication system that includes a UE 10, a macro cell base
station 20A, small cell base stations 20B and 20C, and a Mobility
25 Management Entity (MME) 30.
[0032]
The macro cell base station 20A is a first base station forming Cell
A (PCI value "142") that is a macro cell (first cell).
[0033]
10
The small cell base station 20B is a second base station forming
Cell B (PCI value "84") that is a small cell (second cell).
[0034]
The small cell base station 20C is a second base station forming
5 Cell C (PCI value "84") that is a small cell (second cell).
[003 5]
Cells B and C are provided within Cell A. Thus, Cell A and Cells
B and C are in an adjacent relationship with respect to each other.
However, Cell B and Cell C are not in an adjacent relationship with respect
10 to each other.
[003 6]
Herein, it is assumed that a cell adjacent relationship includes a
relationship between a certain cell and peripheral cells adjacent to the cell,
without limitation to a relationship between a certain cell and cells
15 included in the cell (a relationship between Cell A and Cells B and C) as
illustrated in Fig. 2.
[003 7]
In addition, the PCI values of Cells B and C are both the same, "84".
Thus, the PCI values of Cells B and C adjacent to Cell A are duplicate to
20 each other, which means that PCI confusion is occurring.
[003 8]
The UE 10 is a terminal subjected to handover between Cells A to C.
Note that handover when seen from a handover source and handover when
seen from a handover destination are referred to as Handout and Hand-in,
25 respectively, as appropriate.
[003 9]
The MME 3 0 is a core network device arranged in a core network
and metnaging movement of the lJE 10.
[0040]
5
11
Note that the configuration and the cell arrangement of the wireless
communication system in Fig. 2 are merely exemplary and the present
invention is not limited thereto.
[0041]
Next, as a technique that is a premise of the present invention,
sequences in respective cases when handover of the UE 10 succeeds and
fails in the wireless communication system illustrated in Fig. 2 will be
described. A handover scenario via an inter-base station line (X2
Interface) will be introduced as an example. However, the same applies
10 to a handover scenario using a line between a base station and a core
network (S 1 Interface).
[0042]
First, a sequence in a case where handover of the UE 10 succeeds
will be described.
15 [0043]
Fig. 3 illustrates an example of a sequence in a case where handover
of the UE 10 succeeds. Note that an operation in Fig. 3 starts from a state
in which Cell A is a serving cell of the UE 10.
[0044]
20 As illustrated in Fig. 3, the macro cell base station 20A forming
Cell A that is a serving cell of the UE 10 transmits, to the UE 10, a
Measurement Control message instructing measurement of reception
quality of neighboring cells adjacent to Cell A (Step A 1 ). The
Measurement Control message herein indicates a message including
25 information relating to measurement, and includes an RRC Connection
Reconfiguration message as an example.
[0045 J
Next, the UE 10 n1easures reception quality of neighboring cells
adjacent to Cell A and transmits, to the macro cell base station 20A, a
12
Measurement Report message reporting a PCI value of a handover
destination candidate cell with good reception quality out of these
neighboring cells (Step A2). It is assumed herein that the UE 10 reports a
PCI value "84" of Cell B. In addition, the macro cell base station 20A
5 stores a PCI value and an ECGI value of a cell in association with each
other, and it is assumed herein that the macro cell base station 20A stores
an ECG I value of Cell B in association with the PCI value "84".
[0046]
Thus, the macro cell base station 20A transmits, to the small cell
10 base station 20B forming Cell B, a Handover Request message requesting
for handover of the UE 10 to Cell B (Step A3 ).
[004 7]
In other words, a Handover Request message herein is transmitted
to the small cell base station 20B forming correct Cell B that is a handover
15 destination candidate.
[0048]
Next, the small cell base station 20B transmits, to the macro cell
base station 20A, a Handover Request Acknowledgement message
accepting handover of the UE 10 (Step A4 ). The message includes
20 information on an RRC connection between the UE 10 and Cell B that is a
handover destination candidate cell.
[0049]
Next, the macro cell base station 20A transmits, to the UE 10, an
RRC Connection Rcconfiguration message including information on the
25 RRC connection between the UE 10 and Cell B that is a handover
destination candidate cell (Step AS).
[0050]
lierein, the Handover Request tnessage at Step A3 has been
transmitted to the small cell base station 20B forming correct Cell B that is
13
a handover destination candidate cell of the UE 1 0.
[0051]
Thus, the UE 10 completes reconfiguration of the RRC connection
between the UE 10 and Cell B, and transmits, to the small cell base station
5 20B, an RRC Connection Reconfiguration Complete message notifying the
completion of the RRC connection reconfiguration (Step A6).
Accordingly, the serving cell of the UE 10 is switched from Cell A to Cell
B.
10
[0052]
Thereafter, the small cell base station 20B transmits, to the MME
30, a Path Switch Request message requesting for switching of a path from
the macro cell base station 20A to the small cell base station 20B (Step A 7).
Then, the MME 30 transmits, to the small cell base station 20B, a Path
Switch Request Acknowledgement message accepting switching of a path
15 (Step A8). Subsequently, the small cell base station 20B transmits, to the
macro cell base station 20A, a UE Context Release message instructing
release of a UE Context (Step A9).
[0053]
Next, a sequence in a case where handover of the UE 10 fails will
20 be described.
[0054]
Fig. 4 illustrates an example of a sequence in a case where handover
of the UE 10 fails. Note that an operation in Fig. 4 starts from a state in
which Cell A is a serving cell of the UE 10.
25 [005 5]
As illustrated in Fig. 4, first, transmission and reception of a
Measurement Control message and a Measurement Report message are
performed between the macro cell base station 20A and the UE 10 in the
same manner as Steps A1 and A2 in Fig. 3 (Steps B 1 and B2). However,
14
it is assumed herein that the UE 10 reports a PCI value "84" of Cell C as a
PCI value of a handover destination candidate cell. In addition, it is
assumed that the macro cell base station 20A stores an ECGI value of Cell
B in association with the PCI value "84".
5 [0056]
Thus, the macro cell base station 20A transmits, to the small cell
base station 20B forming Cell B, a Handover Request message requesting
for handover of the UE 10 to Cell B (Step B3 ).
[0057]
10 In other words, a Handover Request message herein is not
transmitted to the small cell base station 20C forming correct Cell C that is
a handover destination candidate, but is transmitted to the small cell base
station 20B forming wrong Cell B that is not a handover destination
candidate.
15 [0058]
20
Next, the small cell base station 20B transmits, to the macro cell
base station 20A, a Handover Request Acknowledgement message
accepting hand over of the UE 10 (Step B4 ).
[0059]
Next, the macro cell base station 20A transmits, to the UE 10, an
RRC Connection Reconfiguration message instructing reconfiguration of
an RRC connection between the UE 10 and a handover destination
candidate cell (Step B5).
[0060 J
25 Herein, the Handover Request message at Step B3 has been
transmitted to the small cell base station 20B forming Cell B that is not a
handovcr destination candidate of the U E 10.
[0061]
Thus, the UE 10 does not complete reconfiguration of the RRC
15
connection between the UE 10 and Cell C that is a handover destination
candidate, and transmits, to the small cell base station 20C, an RRC
Connection Reestablishment Request message requesting for
re-establishment of the RRC connection (Step B6).
5 [0062]
Next, the small cell base station 20C transmits, to the UE 10, an
RRC Connection Reestablishment Reject message rejecting
re-establishment of the RRC connection (Step B7).
[0063]
10 Thereafter, an Attach procedure in which the UE 10 is attached to
the small cell base station 20C is executed (Step B8). Accordingly, the
serving cell of the UE 10 is switched from Cell A to Cell C.
[0064]
Incidentally, in Fig. 4, the failure of handover of the UE 10 is able
15 to be detected by the macro cell base station 20A. Besides, the small cell
base station 20B and the small cell base station 20C are also able to detect
the failure of handover of the UE 10 in a manner as follows.
[0065]
For example, in Fig. 4, the small cell base station 20B receives a
20 Handover Request message from the macro cell base station 20A and
proceeds with the handover procedure, but does not receive an RRC
Connection Reconfiguration Complete message from the UE 10. Thus,
the small cell base station 20B is able to determine that handover
(Hand-in) of the UE 10 from Cell A to Cell B has failed although the macro
25 cell base station 20A has requested for the handover.
[0066]
On the other hand, in Fig. 4, the small cell base station 20C does
not receive a Handover Request message from the macro cell base station
20A, but receives an RRC Connection Reestablishment Request message
16 '
from the UE 10. Thus, the small cell base station 20C is able to
determine that handover of the UE 10 has failed. In addition, the small
cell base station 20C is also able to determine the failure of handover of
the UE 10 and a handover source cell of the UE 10 by referring to contents
5 of the RRC Connection Reestablishment Request message as illustrated in
Fig. 5. For example, the small cell base station 20C is able to determine
the failure of handover of the UE 10 from the fact that "handoverFailure"
and "reconfigurationFailure" are set in "Reestablishment Cause". In
addition, the small cell base station 20C is able to determine a handover
10 source cell of the UE 10 from a PCI value set in "physCellld" included in
""Reestab DE-Identity".
[0067]
The present invention is intended to eliminate PCI confusion
indicating duplication of a PCI value of a cell of the small cell base
15 stations 20B and 20C with a PCI value of another cell by using the fact that
the small cell base stations 20B and 20C are able to detect the failure of
handover of the UE 10.
20
(2) Exemplary Embodiments of the Present Invention
(2-1) First Exemplary Embodiment
The present exemplary embodiment is intended for the small cell
base station 20B to determine occurrence of PCI confusion in the wireless
communication system illustrated in Fig. 2.
[0068]
In view of the above, the following will describe a configuration of
25 the small cell base station 20B in detail.
[0069]
Fig. 6 illustrates an example of a configuration of the small cell
base station 20B according to the present exemplary embodiment.
[0070]
17
As illustrated in Fig. 6, the small cell base station 20B includes a
communication unit 21 B, a communication status acquisition unit 22B, and
a control unit 23B. Note that Fig. 6 illustrates only an excerpt of
essential components from the components within the small cell base
5 station 20B and omits other components.
[0071]
The communication unit 21 B communicates various messages and
the like with the UE 10, the macro cell base station 20A, the small cell
base station 20C, and the MME 30.
10 [0072]
15
The communication status acquisition unit 22B acquires a
communication status of various messages and the like in the small cell
base station 20B.
[0073]
The control unit 23B eliminates PCI value duplication (PCI
confusion) between a PCI value of Cell B and a PCI value of another cell
adjacent to Cell A on the basis of the number of times that handover of the
UE 10 from Cell A to Cell B has failed.
[0074]
20 Specifically, the control unit 23B counts the number of times that
handover of the UE 10 from Cell A to Cell B has failed. For example,
when the control unit 23B receives no RRC Connection Reconfiguration
Complete message from the UE 10 within a predetermined period of time
after receiving a Handover Request message requesting for handover
25 (Hand-in) of the UE 10 from Cell A to Cell B, the control unit 23B is able
to determine that the handover has failed.
[0075]
Then, when the uuruber uf tirues that han dover (Hand-in) of the UE
10 from Cell A to Cell B has failed is equal to or more than a
5
18
predetermined number of times, the control unit 23B determines that a PCI
value of Cell B is duplicated with a PCI value of another cell adjacent to
Cell A and has caused PCI confusion.
[0076]
Note that the control unit 23B is able to determine that Cell A is a
handover source of the UE 10 from the Handover Request message.
[0077]
In addition, the control unit 23B may determine PCI confusion only
in a case (A) or (B) below.
10 (A) When the small cell base station 20B is unable to directly acquire
neighbor information indicating neighboring cells adjacent to Cell A from
the macro cell base station 20A via a connection line such as an X2
Interface or the like
(B) When the small cell base station 20B is unable to recognize some of
15 neighboring cells adjacent to Cell A due to insufficiency in neighbor
information indicating neighboring cells adjacent to Cell A acquired from
the macro cell base station 20A
Note that the case (B) where neighbor information is insufficient
refers to, for example, a case where neighbor information acquired from
20 the macro cell base station 20A indicates only a neighboring cell
establishing an X2 interface out of neighboring cells adjacent to Cell A.
In addition, whether the case (B) is applicable or not can be determined by,
for example, whether the number of neighboring cells indicated by
neighbor information acquired from the macro cell base station 20A is
25 equal to or less than a predetermined number or not.
[0078]
In addition, the control unit ?. 3 R may determine PCT confusion with
arbitrary timing, for example, periodically. Alternatively, the control
unit 23B may determine PCI confusion at a point in time when the number
19
of times that hand over has failed is, as a result of counting the number,
equal to or more than a predetermined number of times.
[0079]
When determining that a PCI value of Cell B is duplicated with a
5 PCI value of another cell adjacent to Cell A and has caused PCI confusion,
the control unit 23B eliminates the PCI confusion. When eliminating the
PCI confusion, the control unit 23B autonomously re-selects a PCI value of
Cell B.
10
15
[0080]
At this time, for example, the control unit 23B excludes the current
PCI value and a PCI value having been selected several times in the past
from re-selection candidates, and re-selects a PCI value from the remaining
re-selection candidates.
[0081]
Note that changing a PCI value of Cell B so as to be different from
a PCI value of a neighboring cell adjacent to Cell B and a PCI value of a
neighboring cell adjacent to the aforesaid neighboring cell falls under a
general technique and is not directly relevant to the present invention. In
addition, it is necessary to avoid only PCI value duplication in an identical
20 frequency of an identical Radio Access Technology (RAT), and changing
in such a manner also falls under a general technique.
[0082]
Alternatively, the control unit 23B may request for allocation of a
new PCI value fron1 a11 external server rather than autonomously
25 re-selecting a PCI value of Cell B, and the external server accepting the
request may allocate a new PCI value to Cell B. In this case, the external
server desirably holds information on neighboring cells adjacent to Cell B
as a database.
[0083]
20
As described above, according to the present exemplary
embodiment, the control unit 23 B of the small cell base station 20B
eliminates PCI value duplication (PCI confusion) between a PCI value of
Cell Band a PCI value of another cell adjacent to Cell A on the basis of the
5 number of times that handover of the UE 10 from Cell A to Cell B has
failed.
[0084]
Thus, an advantageous effect of being capable of eliminating PCI
confusion can be obtained regardless of an acquisition status of neighbor
10 information in the small cell base station 20B. Accordingly, the macro
cell base station 20A is able to request for handover of the UE 10 to a
correct cell, and a decrease in a handover success rate of the UE 10 can be
prevented.
15
[0085]
Specifically, when the number of times that handover of the UE 10
from Cell A to Cell B has failed is equal to or more than a predetermined
number of times, the control unit 23B of the small cell base station 20B
determines that a PCI value of Cell B is duplicated with a PCI value of
another cell adjacent to Cell A and has caused PCI confusion.
20 [0086]
In addition, when determining that PCI confusion is occurring, the
control unit 23B eliminates the PCI confusion by autonomously
re-selecting a PCI value or requesting for allocation of a new PCI value
from an external server. Note that the control unit 23B may cause the
25 another cell adjacent to Cell A to re-select a PCI value.
(2-2) Second Exemplary Embodiment
In the first exemplary embodiment, the macro cell base station 20A
transmits a Handover Request message to a single base station forming a
handover destination candidate cell. However, it is also contemplated
21
that a plurality of cells (that may have different PCis) are handover
destination candidate cells and the macro cell base station 20A is
configured to have a mechanism of transmitting a Handover Request
message to a plurality of base stations respectively forming these plurality
5 of cells. In this case, in the first exemplary embodiment, a number of
base stations having received a Handover Request message result in
receiving no RRC Connection Reconfiguration Complete message. It is
then predicted that these base stations may erroneously determine that PCI
confusion is occurring even though in fact PCI confusion is not occurring.
10 [0087]
In order to solve the above-described problem in the first exemplary
embodiment, the present exemplary embodiment is intended for the small
cell base station 20C to determine occurrence of PCI confusion in the
wireless communication system illustrated in Fig. 2.
15 [0088]
In view of the above, the following will describe a configuration of
the small cell base station 20C in detail.
[0089]
Fig. 7 illustrates an example of a configuration of the small cell
20 base station 20C according to the present exemplary embodiment.
[0090]
As illustrated in Fig. 7, the small cell base station 20C includes a
communication unit 21 C, a communication status acquisition unit 22C, and
a control unit 23C. Note that Fig. 7 illustrates only an excerpt of
25 essential components from the components within the small cell base
station 20C and on1its other con1ponents.
[0091]
The communication unit 21 C communicates various messages and
the like with the UE 10, the macro cell base station 20A, the small cell
base station 20B, and the MME 30.
[0092]
22
The communication status acquisition unit 22C acquires a
communication status of various messages and the like in the small cell
5 base station 20C.
[0093]
The control unit 23C eliminates PCI value duplication (PCI
confusion) between a PCI value of Cell C and a PCI value of another cell
on the basis of the number of times that an RRC Connection
10 Reestablishment Request message has been received from the UE 10.
[0094]
Specifically, the control unit 23 C counts the number of times that
an RRC Connection Reestablishment Request message has been received
from the UE 10.
15 [0095]
Then, when the number of times that an RRC Connection
Reestablishment Request message has been received from the UE 10 is
equal to or more than a predetermined number of times, the control unit
23C determines that a PCI value of Cell C is duplicated with a PCI value of
20 another cell and has caused PCI confusion.
[0096]
Note that the control unit 23C may count the number of times of
reception only when a particular RRC Connection Reestablishment Request
message is received rather than all RRC Connection Reestablishment
25 Request messages.
[0097]
For example, the control unit 23C may count the number of times of
reception only for an RRC Connection Reestablishment Request message in
which a PCI value set in "physCellld" included in "ReestabUE-Identity"
23
indicates Cell A. "physCellld" indicates a PCI value of a cell to which
the UE 10 has been connected before handover failure. For example, in
Fig. 4, since Cell A is a cell to which the UE 10 has been connected before
handover failure, a PCI value of Cell A is written in "physCellld". In
5 this case, when the number of times of reception thereof is equal to or more
than a predetermined number of times, the control unit 23C determines that
a PCI value of Cell C is duplicated with a PCI value of another cell
adjacent to Cell A and has caused PCI confusion. In this ·case, the control
unit 23C may determine PCI confusion only in a case (A) or (B) below.
10 (A) When the small cell base station 20C is unable to directly acquire
neighbor information indicating neighboring cells adjacent to Cell A from
the macro cell base station 20A via a connection line such as an X2
Interface or the like
(B) When the small cell base station 20C is unable to recognize some of
15 neighboring cells adjacent to Cell A due to insufficiency in neighbor
information indicating neighboring cells adjacent to Cell A acquired from
the macro cell base station 20A
Note that the definition of "insufficiency" in (B) and the
determination method as to whether (B) is applicable or not are the same as
20 those in the first exemplary embodiment.