DESCRIPTION
Title of Invention
RADIO COMMUNICATION SYSTEM
Technical Field
The embodiments discussed herein are related to a
radio communication system such as a mobile
telecommunication system.
Background Art
A cellular gommunication system has become the
mainstream of mobile telecommunication systems such as
mobile phone systems. With the cellular communication
system a plurality of areas (cells) in each of which a base
station can perform transmission and receiving are combined
to perform communication in a wide area. When a mobile
station moves, a base station is switched to continue
communication.
At present third generation mobile
telecommunication services based on a CDMA (Code Division
Multiple Access) system have been begun. A next generation
mobile telecommunication system which enables higher-speed
communication is widely discussed.
In 3GPP (3rd Generation Partnership Project), on
the other hand, LTE (Long Term Evolution) and a high-speed
radio service referred to as LTE-advanced which is a
further developed version of LTE are discussed. With LTE-
advanced the introduction of a relay station is discussed
as a technique for enhancing throughput or improving
characteristics in a dead spot.
To make it impossible for a mobile station to
recognize the presence of a relay station is possible. In
3GPP, however, to make a relay station operate in the same
way as an ordinary radio base station operates is mainly
discussed. In this case, an upper base station to the
relay station functions from the relay station as a mere
connection point like a router.
A technique for reducing the number of times a
mobile station performs transmission in a radio
communication system in which a relay station is located by
reducing the amount of signaling is proposed as a prior art
(patent document 1).
Citation List
Patent Literature
PTL1: Japanese Laid-open Patent Publication No.
2009-81513 (Paragraphs [0035]-[0047], FIGS. 1 and 2)
Summary of Invention
Technical Problem
With LTE or LTE-advanced base stations may
communicate with each other for hand-over or interference
control. A communication interface between base stations
is prescribed as an X2 interface.
FIG. 17 illustrates interfaces among base stations.
A radio network 5a includes base stations eNBO through eNB4.
With the X2 interface in LTE base stations are connected by
wire. In the case of FIG. 17, the base station eNBO is
connected to the base stations eNBl' through eNB4 via wired
transmission lines X2-1 through X2-4 respectively.
For the sake of simplicity only interfaces between
the base stations eNBO and eNBl, between the base stations
eNBO and eNB2, between the base stations eNBO and eNB3, and
between the base stations eNBO and eNB4 are illustrated.
In reality, however, each base station is connected to
another base station. As a result, mesh-like connections
are made.
When the base station eNBO communicates with
another base station, the base station eNBO uses wire in
accordance with the X2 interface. For example, when the
base station eNBO communicates with the base station eNBl,
the base station eNBO uses the wired transmission line X2-1.
When the base station eNBO communicates with the base
station eNB2, the base station eNBO uses the wired
transmission line X2-2.
Usually communication between base stations based
on the X2 interface is performed in this way by wire.
However, if the above relay station which operates in the
same way as a base station operates is included, the relay
station and an upper base station are connected by radio.
As a result, a radio connection is made in a part of an
interval based on the X2 interface,
FIG. 18 illustrates interfaces among base stations
in a radio network including a relay station. A radio
network 5b includes base stations eNBO through eNB4, a
relay station RN, and a mobile station UE.
The relay station RN operates in the same way as an
ordinary base station operates. In addition, there is an
upper base station (base station eNBO), which is also
referred to as a donor, to the relay station RN. The relay
station RN is connected to the upper base station eNBO via
a radio transmission line X2-5. The mobile station UE is
under the control of the relay station RN.
The relay station RN communicates with the base
stations eNBl through eNB4 via the upper base station eNBO.
Accordingly, not only wired communication but also radio
communication is performed. For example, when the relay
station RN communicates with the base station eNBl, the
radio transmission line X2-5 and a wired transmission line
X2-1 are used.
FIG. 19 is a sequence diagram of hand-over. Hand-
over is indicated as an example of communication between
the relay station RN and another base station. It Is
assumed that the mobile station UE under the control of the
relay station RN in the radio network 5b performs hand-over
and that candidate hand-over destinations are the base
stations eNBl and eNB2.
(5101) When the mobile station UE performs hand-
over, the mobile station UE measures receiving levels of
radio waves transmitted from surrounding base stations as a
result of the movement, adds measurement results to
signaling referred to as a measurement report, and
transmits the signaling to the relay station RN.
(5102) When the relay station RN receives the
receiving level measurement results, the relay station RN
recognizes that the receiving levels of radio waves
transmitted from the base stations eNBl and eNB2 are high
and that the base stations eNBl and eNB2 are candidate
hand-over destinations. It is assumed that the relay
station RN first transmits an HO request (hand-over request
signaling) to the base station eNBl. The HO. request is
transmitted to the base station eNBl via the upper base
station eNBO.
(5103) When the base station eNBl receives the HO
request, the base station eNBl determines from its
congestion state and the like whether or not hand-over to
the base station eNBl is possible. If hand-over to the
base station eNBl is possible, then the base station eNBl
returns an HO OK {hand-over enable signaling). If hand-
over to the base station eNBl is impossible, then the base
station eNBl returns an HO fail (hand-over disable
signaling). In this case, the base station eNBl determines
that hand-over to the base station eNBl is impossible, and
returns an HO fail to the relay station RN. The HO fail is
transmitted to the relay station RN via the upper base
station eNBO.
(5104) When the relay station RN receives the HO
fail from the base station eNBl, the relay station RN
transmits an HO request to the base station eNB2 which is
the other candidate hand-over destination. The HO request
is transmitted to the base station eNB2 via the upper base
station eNBO.
(5105) When the base station eNB2 receives the HO
request, the base station eNB2 determines whether or not
hand-over to the base station eNB2 is possible. If hand-
over to the base station eNB2 is possible, then the base
station eNB2 returns an HO OK to the relay station RN. The
HO OK is transmitted to the relay station RN via the upper
base station eNBO.
(S1C6) The relay station RN determines from the
contents of the HO OK that the base station eNB2 is a hand-
over destination, and gives the mobile station UE notice to
that effect by an HO command (hand-over command) . The
mobile station UE then recognizes from the contents of the
HO command that the base station eNB2 is a hand-over
destination base station, and performs hand-over to the
base station eNB2.
When the relay station RN communicates with another
base station, radio communication is performed via the
radio transmission line X2-5 between the relay station RN
and the upper base station eNBO. However, if interference
from another station, interference with another station,
and the like are taken into consideration, it is desirable
that the amount of radio signaling on the radio
transmission line X2-5 should be reduced. That is to say,
it is desirable that the amount of radio signaling
exchanged between the relay station- RN and the upper base
station eNBO or the number of times radio signaling is
exchanged between the relay station RN and the upper base
station eNBO should be small.
In the above hand-over sequence, however, hand-over
is performed without taking the amount of radio signaling
into consideration. Accordingly, radio signaling is
frequently exchanged between the relay station RN and the
upper base station eNBO before a hand-over destination base
station is determined.
As a result, the amount of radio signaling on the
radio transmission line X2-5 increases, the level of
interference from another station or interference with
another station rises, and communication quality
deteriorates. In addition, with radio communication there
is a long delay in a process such as establishment of a
radio line. Accordingly, if radio communication is
performed frequently, processing delay increases.
In the above description hand-over is taken as an
example. However, the same problem may occur in
communication other than hand-over between the relay
station RN and another base station.
The present invention was made to solve the above
problem. An object of the present invention is to provide
a radio communication system which improves communication
quality by reducing the amount of radio communication
between a relay station and a base station.
Solution to Problem
In order to solve the above problem, there is
provided a radio communication system. This radio
communication system includes a first radio base station, a
second radio base station, and a relay station which is
connected to the first radio base station by radio and
which communicates with the second radio base station via
the first radio base station.
The relay station requests the first radio base
station to communicate with the second radio base station
for the relay station. The requested first radio base
station communicates with the second radio base station for
the relay station and transmits a communication result to
the relay station.
Advantageous Effects of Invention
Communication quality can be improved by reducing
the amount of radio communication between a relay station
and a base station.
The above and other obj ects, features and
advantages of the present invention will become apparent
from the following description when taken in conjunction
with the accompanying drawings which illustrate preferred
embodiments of the present invention by way of example.
Brief Description of Drawings
FIG. 1 illustrates an example of the structure of a
radio communication system;
FIG. 2 illustrates an example of the structure of a
relay station;
FIG. 3 illustrates an example of the structure of
an upper base station;
FIG. 4 is a sequence diagram of communication;
FIG. 5 illustrates an example of the structure of a
relay station;
FIG. 6 is a sequence diagram of communication;
FIG. 7 illustrates an example of the structure of
an upper base station;
FIG. 8 is a sequence diagram of communication;
FIG. 9 illustrates an example of the structure of a
radio communication system;
FIG. 10 is a sequence diagram of communication;
FIG. 11 illustrates an example of the structure of
a relay station;
FIG. 12 illustrates an example of the structure of
an upper base station;
FIG. 13 is a sequence diagram of communication;
FIG. 14 illustrates an example of the structure of
a relay station;
FIG. 15 illustrates an example of the structure of
an upper base station;
FIG. 16 is a sequence diagram of cononunication;
FIG. 17 illustrates interfaces among base stations;
FIG. 18 illustrates interfaces among base stations
in a radio network including a relay station; and
FIG. 19 is a sequence diagram of hand-over.
Description of Embodiments
Embodiments will now be described with reference to
the drawings. FIG. 1 illustrates an example of the
structure of a radio communication system, A radio
communication system 1 includes a relay station 10, an
upper base station (first radio base station) 20, base
stations (second radio base stations) 31 and 32, and a
mobile station 2. The base stations 31 and 32 are
connected to the upper base station 20 by wire. The relay
station 10 is connected to the upper base station 20 by
radio and communicates with the base stations 31 and 32 via
the upper base station 20.
When the relay station 10 communicates with the
base station 31 or 32, the relay station 10 requests the
upper base station 20 to communicate with the base station
31 or 32 for the relay station 10. The upper base station
20 requested to communicate with the base station 31 or 32
for the relay station 10 communicates with the base station
31 or 32 for the relay station 10 and transmits a
communication result to the relay station 10. In order to
communicate with the base station 31 or 32, the relay
station 10 may request the upper base station 20 to perform
communication with the base station 31 or 32 for the relay
station 10 or may perform ordinary, communication with the
base station 31 or 32 without requesting the upper base
station 20 to perform communication with the base station
31 or 32 for the relay station 10.
The structure and operation of the relay station 10
and the upper base station 20 included in the radio
communication system 1 will now be described in detail.
FIG. 2 illustrates an example of the structure of the relay
station. FIG. 2 indicates a function regarding control
information processing. Functions such as data
transmission and receiving will be omitted.
The relay station 10 includes a communication
control section 11 and a proxy request section 12. When
the communication control section 11 receives control
information transmitted from the mobile station 2, the
communication control section 11 determines whether or not
the control information is for making a proxy request to
the upper base station 20. If the control information is
for making a proxy request to the upper base station 20,
then the communication control section 11 transmits the
control information to the proxy request section 12. If
the control information is not for making a proxy request
to the upper base station 20, then the communication
control section 11 exercises determined communication
control and transmits a result to the mobile station 2 or
the upper base station 20. In addition, when the
communication control section 11 receives control
information transmitted from the proxy request section 12
or the upper base station 20, the communication control
section 11 exercises determined communication control and
transmits a result to the mobile station 2.
When the proxy request section 12 receives the
control information for making a proxy request to the upper
base station 20, the proxy request section 12 generates a
proxy request including the contents of the control
information, and transmits the proxy request to the upper
base station 20. In addition, when the proxy request
section 12 receives a proxy response which is information
regarding a response to a proxy request and which is
transmitted from the upper base station 20, the proxy
request section 12 transmits the proxy response to the
communication control section 11.
FIG. 3 illustrates an example of the structure of
the upper base station 20. FIG. 3 indicates a function
regarding control information processing. Functions such
as data transmission and receiving will be omitted. The
upper base station 20 includes a communication control
section 21 and a proxy control section 22.
Control information which is transmitted from the
relay station 10 and which is a proxy request is
transmitted to the proxy control section 22. Control
information which is transmitted from the relay station 10
and which is not a proxy request is transmitted to the
communication control section 21. The communication
control section 21 exercises determined communication
control for the control information which is not a proxy
request, and transmits a result to the mobile station 2 or
another base station. In addition, when the communication
control section 21 receives control information transmitted
from another base station (network including the base
station 31/ 32, or the like), the communication control
section 21 exercises determined communication control and
transmits a result to the relay station 10.
When the proxy control section 22 receives the
proxy request transmitted from the relay station 10, the
proxy control section 22 performs a proxy process with
another base station and transmits a result transmitted
from it to the relay station 10 as a proxy response.
FIG, 4 is a sequence diagram of communication.
When the relay station 10 communicates with the base
stations 31 and 32, the relay station 10 requests the upper
base station 20 to perform communication processes for the
relay station 10, and the upper base station 20 transmits
results obtained by performing the communication processes
for the relay station 10 to the relay station 10.
(SI) When the relay station 10 communicates with
the base stations 31 and 32, the relay station 10 transmits
a proxy request (proxy request signaling) to the upper base
station 20. By doing so, the relay station 10 requests the
upper base station 20 to perform communication with the
base stations 31 and 32 for the relay station 10.
(52) When the upper base station 20 receives the
proxy request, the upper base station 20 recognizes from
the contents of the proxy request that the upper base
station 20 communicates with the base stations 31 and 32.
First the upper base station 20 transmits a request
(request signaling) to the base station 31.
(53) When the base station 31 receives the request,
the base station 31 performs a determined process and
informs the upper base station 20 about a result by a
response (response signaling).
(54) The upper base station 20 transmits a request
to the base station 32.
(55) When the base station 32 receives the request,
the base station 32 performs a determined process and
informs the upper base station 20 about a result by a
response.
(56) When the upper base station 20 receives the
responses transmitted from the base stations 31 and 32, the
upper base station 20 adds the contents of the responses to
a proxy response (proxy response signaling) and transmits
the proxy response to the relay station 10.
As has been described, when the relay station 10
communicates with the base stations 31 and 32, the relay
station 10 requests the upper base station 20 to perform
communication processes for the relay station 10, The
upper base station 20 communicates with the base stations
31 and 32 for the relay station 10 and transmits the
results of the communication processes to the relay station
10. As a result, the amount of radio communication between
the relay station 10 and the upper base station 20 can be
reduced.
The case where in the radio communication system 1
the relay station 10 requests the upper base station 20 to
perform a communication process for the relay station 10
will now be described with concrete communication control
as an example. First control exercised in the case where
the relay station 10 requests the upper base station 20 at
the time of hand-over by the mobile station 2 under the
control of the relay station 10 to select a hand-over
destination base station will be described.
FIG. 5 illustrates an example of the structure of a
relay station. The structure of an upper base station 20
is the same as that of the upper base station 20
illustrated in FIG. 3. A relay station 10a includes a
communication control section 11-1 and a proxy request
section 12. The communication control section 11-1
includes a hand-over processing unit 11a.
When the communication control section 11-1
receives a measurement report transmitted from a mobile
station 2, the communication control section 11-1
recognizes that the mobile station 2 performs hand-over.
In addition to ID of the mobile station 2, setting
information for the mobile station 2, and the like, the
hand-over processing unit 11a passes receiving level
measurement information included in the measurement report
(information which the mobile station 2 obtains by
measuring receiving levels of radio waves transmitted from
surrounding base stations) to the proxy request section 12.
The proxy request section 12 generates a proxy request for
requesting the upper base station 20 to perform a hand-over
process for the relay station 10a, and transmits the proxy
request to the upper base station 20.
On the other hand, the upper base station 20 having
the same structure as that of the upper base station 20
illustrated in FIG. 3 determines whether control
information transmitted from the relay station 10a is
ordinary information or a proxy request. If the control
information transmitted from the relay station 10a is a
proxy request, then the proxy request is transmitted to a
proxy control section 22. If the control information
transmitted from the relay station 10a is not a proxy
request, then the control information is transmitted to a
communication control section 21. When the proxy control
section 22 receives a proxy request, the proxy control
section 22 selects a hand-over destination for the relay
station 10a and transmits information regarding the
selected hand-over destination to the relay station 10a as
a proxy response.
FIG. 6 is a sequence diagram of communication.
When the mobile station 2 under the control of the relay
station 10a performs hand-over, the relay station 10a
requests the upper base station 20 to select a hand-over
destination base station.
(511) When the mobile station 2 performs hand-over,
the mobile station 2 transmits a measurement report to the
relay station 10a.
(512) When the relay station 10a receives the
measurement report, the relay station 10a recognizes that
the mobile station 2 is to perform hand-over. The relay
station 10a generates a proxy request and transmits the
proxy request to the upper base station 20. By doing so,
the relay station 10a requests the upper base- station 20 to
perform a hand-over process for the relay station 10a.
The proxy request includes state information for
the mobile station 2. Information (context), such as
mobile station ID and a transmission rate, regarding the
state of the mobile station 2, receiving level measurement
information included in the measurement report transmitted
from the mobile station 2, and the like correspond to the
state information.
(513) When the upper base station 20 receives the
proxy request, the upper base station 20 recognizes that
the receiving levels of radio waves transmitted from base
stations 31 and 32 are high and that the base stations 31
and 32 are candidate hand-over destinations. It is assumed
that the upper base station 20 first transmits an HO
request to the base station 31.
(514) When the base station 31 receives the HO
request, it is assumed that the base, station 31 determines
from its congestion state and the like that hand-over to
the base station 31 is impossible. In this case, the base
station 31 returns an HO fail.
(515) When the upper base station 20 receives the
HO fail from the base station 31, the upper base station 20
transmits an HO request to the base station 32 which is the
other candidate hand-over destination.
(S16) When the base station 32 receives the HO
request, it is assumed that the base station 32 determines
that hand-over to the base station 32 is possible. In this
case, the base station 32 returns an HO OK to the upper
base station 20.
(517) The upper base station 20 determines from the
contents of the HO OK that the base station 32 is a hand-
over destination, and informs the relay station 10a about
this result by a proxy response.
(518) When the relay station 10a receives the proxy
response, the relay station 10a recognizes that a hand-over
destination base station is the base station 32, and gives
the mobile station 2 notice to that effect by an HO command.
As indicated in FIG. 19, radio signaling has
traditionally been exchanged frequently between a relay
station and candidate hand-over destinations before
determination of a hand-over destination base station.
With the above hand-over proxy control, however, the relay
station 10a requests the upper base station 20 at the time
of hand-over by the mobile station 2 to select a
destination base station. The relay station 10a receives a
result from the upper base station 20 and informs the
mobile station 2 about the result.
As a result, radio signaling exchanged between the
relay station 10a and the upper base station 20 is the
proxy request and the proxy response. Accordingly, the
amount of radio communication between the relay station 10a
and the upper base station 20 can be reduced.
If there is a possibility that the number of times
radio signaling is exchanged with a candidate hand-over
destination is small, such as if only receiving level
measurement information for one base station is included in
a measurement report transmitted from the mobile station 2,
then the relay station 10a can determine that the relay
station 10a does not exercise proxy control. If the relay
station 10a determines that it is unnecessary to request
the upper base station 20 to perform a hand-over process
for the relay station 10a, then the relay station 10a may
perform an ordinary hand-over process like that indicated
in FIG, 19.
Control exercised in the case where state
information, such as receiving level measurement
information, for a mobile station 2 is transmitted in
advance to a relay station and where hand-over is performed
will now be described.
FIG. 7 illustrates an example of the structure of
an upper base station. The structure'of a relay station is
the same as that of the relay station 10a illustrated in
FIG. 5. An upper base station 20a includes a communication
control section 21 and a proxy control section 22-1. In
addition, the proxy control section 22-1 includes a state
information holding unit 22a.
When a relay station 10a having the same structure
as that of the relay station 10a illustrated in FIG. 5
receives a measurement report from the mobile station 2,
the relay station 10a extracts receiving level measurement
information included in the measurement report. A proxy
request section 12 adds the receiving level measurement
information to a proxy request and transmits the proxy
request to the upper base station 20a.
On the other hand, when the upper base station 20a
receives the proxy request transmitted from the relay
station 10a, the proxy control section 22-1 extracts the
receiving level measurement information and holds the
receiving level measurement information in the state
information holding unit 22a. The proxy control section
22-1 then selects a hand-over destination for the relay
station 10a on the basis of the receiving level measurement
information and transmits information regarding the
selected hand-over destination to the relay station 10a as
a proxy response.
FIG. 8 is a sequence diagram of communication.
Receiving level measurement information which the mobile
station 2 obtains by measurement is "transmitted in advance
to the relay station 10a. After that, hand-over is
performed.
(S21a) The mobile station 2 transmits the receiving
level measurement information to the relay station 10a by a
measurement report.
(S21b) When the relay station 10a receives the
measurement report, the relay station IGa transmits the
measurement report to the upper base station 20a. The
upper base station 20a extracts and holds the receiving
level measurement information included in the measurement
report.
(S22a) The mobile station 2 continues to move, and
measures receiving levels of radio waves transmitted from
base stations at a new place to which the mobile station 2
has moved. The mobile station 2 then transmits new
receiving level measurement information again to the relay
station 10a by a measurement report.
(S22b) When the relay station 10a receives the
measurement report, the relay station 10a transmits the
measurement report to the upper base station 20a. The
upper base station 20a extracts and holds the receiving
level measurement information included in the measurement
report and updates the receiving level measurement
information.
(523) By transmitting a proxy request to the upper
base station 20a, the relay station 10a requests the upper
base station 20a to perform a hand-over process for the
relay station 10a. Receiving level measurement information
is not included in the proxy request. Accordingly, the
amount of information included in the proxy request is
small compared with the amount of information included in
the proxy request generated in the above step S12.
(524) The upper base station 20a recognizes from
the receiving level measurement information about which the
upper base station 20a is already informed by the
measurement report that the receiving levels of radio waves
transmitted from base stations 31 and 32 are high and that
the base stations 31 and 32 are candidate hand-over
destinations. When the upper base station 20a receives the
proxy request, the upper base station 20a recognizes that
the mobile station 2 is to perform hand-over, and transmits
an HO request to the base station 31.
(525) When the base station 31 receives the HO
request, the base station 31 determines from its congestion
state and the like that hand-over to the base station 31 is
impossible, and returns an HO fail.
(526) When the upper base station 20a receives the
HO fail from the base station 31, the upper base station
20a transmits an HO request to the base station 32 which is
the other candidate hand-over destination.
(527) When the base station 32 receives the HO
request, the base station 32 determines that hand-over to
the base station 32 is possible, and, returns an HO OK to
the upper base station 20a.
(528) The upper base station 20a determines from
the contents of the HO OK that the base station 32 is a
hand-over destination, and informs the relay station 10a
about this result by a proxy reaponse.
(529) When the relay station 10a receives the proxy
response, the relay station 10a recognizes that a hand-over
destination base station is the base station 32, and gives
the mobile station 2 notice to that effect by an HO command.
As has been described, the relay station 10a
informs the upper base station 20a in advance about
receiving level measurement information about which the
relay station 10a is informed by a measurement report
transmitted from the mobile station 2 under the control of
the relay station 10a. The upper base station 20a holds
the receiving level measurement information.
When the mobile station 2 performs hand-over, the
relay station 10a requests the upper base station 20a by a
proxy request to perform hand-over for the relay station
10a. Receiving level measurement information is not
included in the proxy request. Accordingly, the amount of
radio communication can be reduced further. In addition,
the relay station 10a can request the upper base station
20a in a short period of time to perform hand-over for the
relay station 10a. As a result, a delay caused by a hand-
over process can be reduced.
In the above description state information for the
mobile station 2 is receiving level measurement information.
The mobile station 2 informs the relay station 10a in
advance about the receiving level measurement information
by a measurement report. However, a measurement report may
include mobile station ID, a transmission rate, and the
like in addition to receiving level measurement information.
Accordingly, the mobile station 2 may also inform the relay
station 10a in advance about the mobile station ID, the
transmission rate, and the like by this measurement report.
By doing so, the amount of information included in a proxy
request can be reduced further.
Control exercised in the case where candidate
destinations include a relay station and where an upper
base station to the relay station transmits a hand-over
request for a base station will now be described.
FIG. 9 illustrates an example of the structure of a
radio communication system. A radio communication system
la includes an upper base station (first radio base
station) 20-1, a base station (second radio base station)
33, relay stations 10-1 and 10-2, and a mobile station 2.
The upper base station 20-1 and the base station 33 are
connected by radio. The relay stations 10-1 and 10-2 are
connected to the upper base station 20-1 by radio. In
addition, the mobile station 2 is under the control of the
base station 33.
The radio communication system la has the above
structure. When the base station 33 recognizes on the
basis of a measurement report from, the mobile station 2
that the mobile station 2 is to perform hand-over, the base
station 33 selects a hand-over destination base station.
It is assumed that candidate hand-over destinations are the
relay stations 10-1 and 10-2. The base station 33 requests
the upper base station 20-1 to the relay stations 10-1 and
10-2 to perform a hand-over process for the base station 33.
FIG. 10 is a sequence diagram of communication. If
candidate destinations include the relay stations 10-1 and
10-2, the upper base station 20-1 to the relay stations 10-
1 and 10-2 transmits a hand-over request for the base
station 33.
(S31) When the mobile station 2 performs hand-over,
the mobile station 2 transmits receiving level measurement
information to the base station 33 by a measurement report.
(S32I When the base station 33 receives the
measurement report, the base station 33 recognizes that the
mobile station 2 is to perform hand-over. The base station
33 generates a proxy request and transmits the proxy
request to the upper base station 20-1. By doing so, the
base station 33 requests the upper base station 20-1 to
perform a hand-over process for the base station 33.
(533) When the upper base station 20-1 receives the
proxy request, the upper base station 20-1 recognizes on
the basis of the receiving level measurement information
included in the proxy request that the receiving levels of
radio waves transmitted from the relay stations 10-1 and
10-2 are high and that the relay stations 10-1 and 10-2 are
candidate hand-over destinations. It is assumed that the
upper base station 20-1 first transmits an HO request to
the relay station 10-1.
(534) When the relay station 10-1 receives the HO
request, it is assumed that the relay station 10-1
determines from its congestion state and the like that
hand-over to the relay station 10-1 is impossible. In this
case, the relay station 10-1 returns an HO fail to the
upper base station 20-1.
(535) When the upper base station 20-1 receives the
HO fail from the relay station 10-1, the upper base station
20-1 transmits an HO request to the relay station 10-2
which is the other candidate hand-over destination.
(536) When the relay station 10-2 receives the HO
request, it is assumed that the relay station 10-2
determines that hand-over to the relay station 10-2 is
possible. In this case, the relay station 10-2 returns an
HO OK to the upper base station 20-1.
(537) The upper base station 20-1 determines from
the contents of the HO OK that the relay station 10-2 is a
hand-over destination, and informs the base station 33
about this result by a proxy response.
(S38) When the base station 33 receives the proxy
response, the base station 33 recognizes that a hand-over
destination base station is the relay station 10-2, and
gives the mobile station 2 notice to that effect by an HO
command.
By adopting the radio communication system la
described above, the amount of radio communication between
the base station 33 and the upper base station 20-1 can be
reduced.
Control exercised in the case where an upper base
station reports an interference amount measured by a relay
station to another station for the relay station and
receives a report of an interference amount from another
station for a relay station will now be described. Base
stations report interference conditions to one another in
order to exercise interference control. As a result, a
relay station also reports its interference amount to
another station and receives a report of an interference
amount from another station.
Accordingly, an interference amount is transmitted
or received by radio via an upper base station. This is
the same with hand-over. In the case of communication
control on interference amount transmission and receiving,
an upper base station transmits and receives an
interference amount for a relay station. By doing so, the
amount of radio communication between the relay station and
the upper base station is reduced.
FIG. 11 illustrates an example of the structure of
a relay station. FIG. 12 illustrates an example of the
structure of an upper base station. A relay station 10b
includes a communication control section 11-2 and a proxy
request section 12. The communication control section 11-2
includes an interference amount information processing unit
lib. The upper base station 20b includes a communication
control section 21 and a proxy control section 22-2. The
proxy control section 22-2 includes an interference amount
information processing unit 22b.
The interference amount information processing unit
lib of the relay station 10b measures and holds an
interference amount of the relay station 10b and passes
interference amount information to the proxy request
section 12. The proxy request section 12 adds the
interference amount information to a proxy request and
informs the upper base station 20b about the interference
amount information. The interference amount information
processing unit 22b included in the proxy control section
22-2 of the upper base station 20b informed by the relay
station 10b about the interference amount information
informs surrounding base stations about the interference
amount information for the relay station 10b. In addition,
the interference amount information processing unit 22b
totalizes interference amount information transmitted from
surrounding base stations and the proxy control section 22-
2 transmits a totalization result to the relay station 10b.
FIG. 13 is a sequence diagram of communication.
The upper base station 20b reports an interference amount
of the relay station 10b to another station for the relay
station 10b and receives a report of an interference amount
from another station for the relay station 10b.
(541) In order to request the upper base station
20b to report interference amount information to another
station for the relay station 10b and receive a report of
interference amount information from another station for
the relay station 10b, the relay station 10b transmits a
proxy request to the upper base station 20b. The proxy
request includes interference amount information (relay-
station-side interference amount information) obtained by
measurement by the relay station 10b.
(542) The upper base station 20b receives the proxy
request. The upper base station 20b transmits a report
request to a base station 31 in order to request the base
station 31 to report interference amount information (base-
station-side interference amount information) obtained by
measurement by the base station 31. The report request
also includes the interference amount information for the
relay station 10b. By doing so, the upper base station 20b
informs the base station 31 about the interference amount
information for the relay station 10b.
(543) When the base station 31 receives the report
request, the base station 31 adds interference amount
information for the base station 31 to an interference
report and returns the interference report to the upper
base station 20b.
(544) The upper base station 20b transmits a report
request to a base station 32 in order to request the base
station 32 to report interference amount information
obtained by measurement by the base station 32. The report
request also includes the interference amount information
for the relay station 10b. By doing so, the upper base
station 20b informs the base station 32 about the
interference amount information for the relay station 10b.
(545) When the base station 32 receives the report
request, the base station 32 adds interference amount
information for the base station 32 to an interference
report and returns the interference report to the upper
base station 20b.
(546) The upper base station 20b informs the relay
station 10b about the interference amount information for
the base stations 31 and 32 by a proxy response.
As has been described, the relay station 10b
requests the upper base station 20b to report interference
amount information obtained by measurement by the relay
station 10b to each base station or collect interference
amount information obtained by measurement by each base
station. By doing so, the amount of radio communication
between the relay station 10b and the upper base station
20b can be reduced.
Interference amount information may be reported if
a condition set in advance is met. Alternatively,
interference amount information may be reported
periodically. In these cases, the upper base station 20b
holds interference amount information transmitted from
other base stations. When the relay station 10b requests
the upper base station 20b to collect interference amount
information for the relay station 10b, the upper base
station 20b informs the relay station 10b about the
interference amount information which the upper base
station 20b holds.
Control exercised in the case where a plurality of
base stations transmit data to a mobile station in
cooperation and where an upper base station informs the
plurality of base stations about scheduling information
regarding data transmission for the plurality of base
stations will now be described.
With LTE-advanced the introduction of a technique
which is referred to as CoMP (Coordinated Multiple Point
transmission and reception) and by which base stations
transmit data to a mobile station in cooperation is
discussed.
With CoMP a central base station performs
scheduling for data transmission. The other base stations
perform transmission in accordance with scheduling
information. If the central base station is a relay
station and the relay station transmits the scheduling
information to each base station via an upper base station,
then the amount of radio communication increases.
Therefore, in order to reduce the amount of radio
communication between the relay station and the upper base
station, the upper base station informs each base station
about the scheduling information for the relay station.
FIG. 14 illustrates an example of the structure of
a relay station. FIG. 15 illustrates an example of the
structure of an upper base station. A relay station 10c
includes a communication control section 11-3 and a proxy
request section 12. The communication control section 11-3
includes a scheduling information processing unit lie. An
upper base station 20c includes a communication control
section 21-3 and a proxy control section 22. The
communication control section 21-3 includes a scheduling
information processing unit 21c.
The scheduling information processing unit lie of
the relay station 10c performs scheduling and passes
scheduling information to the proxy request section 12.
The proxy request section 12 adds the scheduling
information to a proxy request and informs the upper base
station 20c about the proxy request.
The proxy control section 22 of the upper base
station 20c receives the proxy request, extracts the
scheduling information from the proxy request, and
transmits the scheduling information to the scheduling
information processing unit 21c included in the
coinmunication control section 21-3. In addition, the proxy
control section 22 transmits the scheduling information to
base stations 31 and 32. The scheduling information
processing unit 21c included in the communication control
section 21-3 sets the scheduling information in the upper
base -station 20c.
FIG. 16 is a sequence diagram of communication.
The relay station 10c performs CoMP on a mobile station 2
under the control of the relay station 10c
(551) The relay station 10c performs scheduling for
transmitting data to the mobile station 2 by CoMP, and
generates scheduling information including data
transmission timing. The relay station 10c then requests
the upper base station 20c by a proxy request to inform the
base stations 31 and 32 about the scheduling information
for the relay station 10c. The proxy request includes the
scheduling information.
(552) When the upper base station 20c receives the
proxy request, the upper base station 20c extracts the
scheduling information and informs the base stations 31 and
32 about the scheduling information.
(553) In accordance with the scheduling information
the relay station 10c, the upper base station 20c, and the
base stations 31 and 32 transmit data to the mobile station
2.
As has been described, when data is transmitted to
the mobile station 2 by CoMP, the upper base station 20c
informs base stations about scheduling information
regarding data transmission for the relay station 10c. By
doing so, the amount of radio communication between the
relay station 10c and the upper base station 20c can be
reduced.
As has been described in the foregoing, with the
radio communication system 1 the relay station 10 requests
the upper base station 20 to communicate with the base
stations 31 and 32 for the relay station 10 and the
requested upper base station 20 communicates with the base
stations 31 and 32 for the relay station 10. The upper
base station 20 transmits a communication result to the
relay station 10.
As a result, the amount of radio communication
between the relay station 10 and the upper base station 20
is reduced. This enables a reduction in interference from
or with another station or a reduction in processing delay.
Accordingly, communication quality can be improved.
The foregoing is considered as illustrative only of
the principles of the present invention. Further, since
numerous modifications and changes will readily occur to
those skilled in the art, it is not desired to limit the
invention to the exact construction and applications shown
and described, and accordingly, all suitable modifications
and equivalents may be regarded as falling within the scope
of the invention in the appended claims and their
equivalents.
Reference Signs List
1 radio communication system
2 mobile station
10 relay station
20 upper base station (first radio base station)
31, 32 base station (second radio base station)
WE CLAIM:
1. A method used in a radio communication system including a first radio base
station, a second radio base station that communicates with the first radio base
station, a relay station which communicates with the second radio base station
via the first radio base station, and a mobile station, the method comprising:
transmitting a request, by the second radio base station, for requesting the first
radio base station to communicate with the relay station for the second radio
base station; and
communicating, by the requested first radio base station, with the relay station
for the second radio base station and transmits a communication result to the
second radio base station.
2. The method as claimed in claim 1, further comprising:
transmitting, by the second radio base station, control information to the mobile
station according to a reception of the communication result from the first radio
base station, and
receiving the control information by the mobile station.
3. A method used by a first radio base station which is connected by radio to a
relay station and via which communication is performed between the relay
station and a second radio base station, the method comprising:
receiving a request transmitted from the second radio base station for requesting
the first radio base station to communicate with the relay station as a substitute
for the second radio base station;
communicating with the relay station for the second radio base station according
to the reception of the request; and
transmitting a communication result to the second radio base station.
4. A method used by a second radio base station which communicates with a relay
station via a first radio base station, the method comprising:
transmitting a request to the first radio base station for requesting the first radio
base station to communicate with the relay station as a substitute for the second
radio base station.

ABSTRACT

A method used in a radio communication system including a first radio base station, a
second radio base station that communicates with the first radio base station, a relay
station which communicates with the second radio base station via the first radio base
station, and a mobile station, the method comprising: transmitting a request, by the
second radio base station, for requesting the first radio base station to communicate
with the relay station for the second radio base station; and communicating, by the
requested first radio base station, with the relay station for the second radio base station
and transmits a communication result to the second radio base station.