JP5801973B2 - Radio relay station and control method - Google Patents

Description

  The present invention relates to a radio relay station and a control method for realizing relay transmission.

  Standardized by 3GPP (3rd Generation Partnership Project), the standardization organization for wireless communication systems, as a next-generation wireless communication system that realizes higher-speed communication than third-generation and 3.5th-generation wireless communication systems currently in operation There is LTE (Long Term Evolution). In LTE Advanced, which is an advanced version of LTE, adoption of relay transmission using a radio relay station called a relay node is planned (for example, see Non-Patent Document 1).

  The radio relay station is a low-power relay base station that can be connected to a radio base station (macro base station) via a radio link. A wireless terminal connected to the wireless relay station communicates with the wireless base station via the wireless relay station. A radio relay station is installed at a cell edge or a coverage hole of the radio base station, and the radio terminal communicates indirectly with the radio base station via the radio relay station, so that the radio terminal directly communicates with the radio base station. Communication can be performed under better conditions than communication.

3GPP TR 36.814 V0.4.1, Chapter 9, “Relaying functionality”, February 2009

  Since the radio relay station relays communication between the radio base station (or other radio relay station) and the radio terminal, the communication capacity between the radio terminal and the radio base station is the same as that between the radio base station and the radio relay station. Depends on the communication capacity between. For this reason, when the load of the connection destination of the radio relay station is high (for example, when there is a lot of traffic handled by the radio base station), the communication capacity between the connection destination of the radio relay station and the radio relay station is reduced. The communication capacity between the radio relay station and the radio terminal is also reduced.

  Therefore, a wireless terminal connected to a wireless relay station has a problem that throughput is lowered even when the wireless quality with the wireless relay station is good when the load at the connection destination of the wireless relay station is high. .

  Therefore, an object of the present invention is to provide a radio relay station and a control method that can improve the throughput of a radio terminal by achieving appropriate load distribution.

  In order to solve the above-described problems, the present invention has the following features. First, the first feature of the present invention is that a first communication unit (first communication unit 210) that communicates with a radio base station (radio base station 100A) or another radio relay station as a connection destination and at least one radio A second communication unit (second communication unit 220) that communicates with a terminal (for example, wireless terminals 300E to 300G), and relays data transmitted and received between the connection destination of the first communication unit and the wireless terminal A relay unit (relay unit 230); and a control unit (control unit 240) that controls the first communication unit and the second communication unit. The control unit switches the connection destination of the first communication unit. A handover request for controlling the first communication unit to transmit a relay station handover request for executing a relay station handover or a terminal handover request for executing a terminal handover for switching the connection destination of the wireless terminal When the first communication unit receives the (handover request unit 242) and a response message indicating that the relay station handover is permitted or instructed, the first communication unit is controlled to execute the relay station handover. When the first communication unit receives a handover control unit (handover control unit 243) and a response message for permitting or instructing the terminal handover, the second communication unit transmits a terminal handover instruction to the wireless terminal. The gist is that the wireless relay station (relay node 200) includes a handover instruction unit (handover instruction unit 244) that controls the communication unit.

  A second feature of the present invention relates to the first feature of the present invention, wherein the handover request unit is configured such that a relay station load, which is a load of communication performed by the second communication unit with the wireless terminal, is the first communication. The relay station handover request or the terminal handover request is transmitted when the load allowable amount determined according to the load of the connection destination of the unit is exceeded, or when the relay station load is predicted to exceed the load allowable amount Thus, the gist is to control the first communication unit.

  A third feature of the present invention relates to the first or second feature of the present invention, wherein the control unit notifies a load of a relay station handover candidate that is a new connection destination candidate of the first communication unit. The first communication unit is controlled to transmit a load notification request for requesting a relay station load notification and a terminal load notification for notifying a load of a terminal handover candidate that is a new connection destination candidate of the wireless terminal. A notification requesting unit (notification requesting unit 245), a handover determining unit (handover determining unit 249) that determines whether or not to perform the relay station handover based on the relay station load notification received by the first communication unit; The handover request unit controls the first communication unit to transmit the relay station handover request when it is determined to execute the relay station handover. And effect.

  A fourth feature of the present invention is according to the third feature of the present invention, wherein the handover determining unit performs the relay station handover from the relay station load notification and the relay station load to the relay station handover candidate. The gist is to calculate a load of the relay station handover candidate when it is performed, and to determine whether or not to execute the relay station handover based on the calculated load.

  A fifth feature of the present invention relates to the third or fourth feature of the present invention, wherein the handover request unit transmits the terminal handover request when it is determined not to execute the relay station handover. In summary, the first communication unit is controlled.

  A sixth feature of the present invention relates to any one of the first to fifth features of the present invention, wherein the control unit is based on the relay station load and a terminal load that is a load for each wireless terminal. The gist of the present invention is to further include a terminal determination unit (terminal determination unit 248) that determines at least one wireless terminal that executes the terminal handover so that the relay station load is maintained below the allowable load.

  According to a seventh aspect of the present invention, there is provided a first communication unit that communicates with a radio base station or another radio relay station as a connection destination, a second communication unit that communicates with at least one radio terminal, and the first A method for controlling a radio relay station having a relay unit that relays data transmitted and received between a connection destination of a communication unit and the wireless terminal, wherein the first communication unit executes a relay station handover for switching the connection destination Controlling the first communication unit to transmit a relay station handover request for performing a terminal handover request for executing a terminal handover request for switching a connection destination of the wireless terminal, or permitting the relay station handover or A step of controlling the first communication unit to execute the relay station handover when the first communication unit receives a response message to the effect of instructing; And a step of controlling the second communication unit so as to transmit a terminal handover instruction to the wireless terminal when the first communication unit receives a response message to permit or instruct a handover. To do.

  ADVANTAGE OF THE INVENTION According to this invention, the radio relay station and control method which can improve the throughput of a radio | wireless terminal by aiming at appropriate load distribution can be provided.

It is a figure which shows the structure of the radio | wireless communications system which concerns on 1st Embodiment. It is a figure for demonstrating a relay handover. It is a block diagram which shows the structure of the relay node which concerns on 1st Embodiment. It is a sequence diagram which shows operation | movement of the radio | wireless communications system which concerns on 1st Embodiment. It is a figure for demonstrating a terminal handover. It is a block diagram which shows the structure of the relay node which concerns on 2nd Embodiment. It is a sequence diagram which shows operation | movement of the radio | wireless communications system which concerns on 2nd Embodiment. It is a block diagram which shows the structure of the relay node which concerns on 3rd Embodiment. It is a sequence diagram which shows the whole operation | movement of the radio | wireless communications system which concerns on 3rd Embodiment. It is a block diagram which shows the structure of the relay node which concerns on 4th Embodiment. It is a sequence diagram which shows the whole operation | movement of the radio | wireless communications system which concerns on 4th Embodiment. It is a flowchart which shows the detail of step S404 of FIG. It is a flowchart which shows the detail of step S409 of FIG.

  Next, a first embodiment to a fourth embodiment of the relay node corresponding to the wireless relay station of the present invention, and a wireless communication system including the relay node, and other embodiments will be described with reference to the drawings. In the description of the drawings in the following embodiments, the same or similar parts are denoted by the same or similar reference numerals.

(1) First Embodiment In the first embodiment, (1.1) the configuration of a radio communication system, (1.2) the configuration of a relay node, (1.3) the operation of the radio communication system, (1.4 The effect of the first embodiment will be described.

(1.1) Configuration of Radio Communication System FIG. 1 is a diagram showing a configuration of the radio communication system 1 according to the present embodiment. The wireless communication system 1 has a configuration based on LTE-Advanced, which is positioned as a fourth generation (4G) mobile phone system, for example.

  The radio communication system 1 includes a radio base station 100A that forms a cell C1, and a radio base station 100B that forms a cell C2. The cell C1 is a communication area connectable to the radio base station 100A, and the cell C2 is a communication area connectable to the radio base station 100B. The radio base stations 100A and 100B are macro base stations that form cells C1 and C2 each having a radius of about several hundred meters, for example.

  The cells C1 and C2 are adjacent to each other in a partially overlapping state, and hereinafter, the radio base station 100B is appropriately referred to as “adjacent base station of the relay node 200”. In addition, although two radio base stations are illustrated in FIG. 1, other radio base stations may be installed adjacent to the two radio base stations.

  The radio base station 100A and the radio base station 100B are connected via a backhaul network (not shown) that is a wired communication network, and can directly perform communication between base stations. In LTE, such an inter-base station communication interface is referred to as an X2 interface.

  Wireless terminals 300A to 300D and relay node 200 are connected to wireless base station 100A via a wireless link. The radio terminals 300A to 300D communicate directly with the radio base station 100A. The wireless terminal in the present embodiment is configured to be movable. The relay node 200 is installed at the end of the cell C1 and in the vicinity of the cell C2.

  Wireless terminals 300E to 300G are connected to the relay node 200 via wireless links. The relay node 200 is a radio relay station that relays communication between the radio terminals 300E to 300G and the radio base station 100A. In the present embodiment, it is assumed that the relay node 200 is a fixed type.

  The radio terminals 300E to 300G communicate indirectly with the radio base station 100A via the relay node 200. By such relay transmission, the radio terminals 300E to 300G can communicate with the radio base station 100A even outside the cell C1. Hereinafter, the wireless terminals 300E to 300G connected to the relay node 200 are appropriately referred to as “wireless terminals under the relay node 200”.

  Wireless terminals 300H and 300I are connected to the wireless base station 100B via wireless links. The radio terminals 300H and 300I communicate directly with the radio base station 100B. In the following, when the radio base stations 100A and 100B are not distinguished from each other, they are simply referred to as “radio base station 100”, and when the radio terminals 300A to 300I are not distinguished from each other, they are simply referred to as “radio terminals 300”.

  In the example of FIG. 1, there are seven wireless terminals that communicate directly or indirectly with the wireless base station 100A, and there are two wireless terminals that communicate with the wireless base station 100B. For this reason, the radio base station 100A has a higher load than the radio base station 100B.

  When the load on the radio base station 100A is high (for example, when the traffic handled by the radio base station 100A is large), the radio resources allocated to the relay node 200 are reduced. And the communication capacity between the relay node 200 and the wireless terminals 300E to 300G is also reduced. For this reason, the throughput of the wireless terminals 300E to 300G is lowered.

  In the first embodiment, when the load on the radio base station 100A is high, as shown in FIG. 2, the relay node 200 switches the connection destination from the radio base station 100A to the adjacent base station (radio base station 100B). By executing (hereinafter referred to as relay handover), the throughput of the wireless terminals 300E to 300G is avoided from being lowered.

(1.2) Configuration of Relay Node Next, the configuration of the relay node 200 according to the first embodiment will be described. FIG. 3 is a block diagram illustrating a configuration of the relay node 200 according to the first embodiment.

  As illustrated in FIG. 3, the relay node 200 includes a first communication unit 210, a second communication unit 220, a relay unit 230, a control unit 240, and a storage unit 250.

  The first communication unit 210 is configured using an RF circuit, a BB circuit, and the like, and transmits and receives radio signals to and from the radio base station 100. The first communication unit 210 encodes and modulates transmission data to convert it into a transmission signal, amplifies and upconverts the transmission signal to convert it into a radio signal, and sends it out from the antenna 211. The first communication unit 210 amplifies and downconverts the radio signal received by the antenna 211 to convert it into a received signal, and demodulates and decodes the received signal to convert it into received data.

  In addition, the first communication unit 210 measures the radio quality of the radio signal received by the first communication unit 210. The radio quality is, for example, a received electric field strength (RSSI) of a reference signal periodically transmitted by the radio base station 100, or a carrier-to-interference noise ratio (CINR) of the reference signal.

  The second communication unit 220 is configured using an RF circuit, a BB circuit, or the like, and transmits / receives a radio signal to / from the wireless terminal 300. The second communication unit 220 encodes and modulates transmission data to convert it into a transmission signal, amplifies and upconverts the transmission signal to convert it into a radio signal, and sends it out from the antenna 221. The second communication unit 220 amplifies and downconverts the radio signal received by the antenna 221 to convert it into a received signal, and demodulates and decodes the received signal to convert it into received data.

  The relay unit 230 relays data between the first communication unit 210 and the second communication unit 220. Specifically, the relay unit 230 relays the reception data from the first communication unit 210 to the second communication unit 220 as transmission data, and the reception data from the second communication unit 220 to the first communication unit 210. Relay as.

  The control unit 240 is configured using, for example, a CPU, and controls various functions included in the relay node 200. The storage unit 250 is configured using, for example, a memory, and stores various types of information used for controlling the relay node 200 and the like.

  The control unit 240 controls each of the first communication unit 210, the second communication unit 220, and the relay unit 230. The control unit 240 includes a load calculation unit 241, a handover request unit 242, and a handover control unit 243.

  The load calculation unit 241 periodically calculates a relay node load that is a load of communication performed by the second communication unit 220 with the wireless terminal 300 under the relay node 200. Here, the relay node load is, for example, a necessary amount of radio resources (resource blocks) in the second communication unit 220 or a data amount (traffic amount) to be transmitted / received by the second communication unit 220. The relay node load is the overall load of the radio terminal under the relay node 200, and it does not matter whether the load is on the uplink or the load on the downlink.

  Also, the load calculation unit 241 periodically calculates a base station load that is a load of communication performed by the first communication unit 210 with the radio base station 100A. The base station load is, for example, a usable amount of radio resources (resource blocks) in the first communication unit 210 or a data amount (traffic amount) that can be transmitted and received by the first communication unit 210. The base station load depends on the load state of the radio base station 100A, and it does not matter whether it is an uplink load or a downlink load.

  When the relay node load is relatively high compared to the base station load, the throughput of the radio terminal 300 under the relay node 200 reaches a peak. Therefore, in the present embodiment, when the relay node load is higher than the load allowable amount determined according to the base station load, the handover request unit 242 transmits the relay handover request for executing the relay handover. 1 The communication unit 210 is controlled. The allowable load amount may be the base station load value itself, or may be a value slightly smaller than the base station load value.

  In addition, the handover request unit 242 controls the first communication unit 210 to transmit the relay node load information indicating the relay node load included in the relay handover request. As a result, the transmission destination of the relay handover request can grasp the required load based on the received relay handover request.

  There are the following two methods for specifying the transmission destination of the relay handover request.

  In the first transmission destination designation method, the handover request unit 242 controls the first communication unit 210 to transmit a relay handover request to a relay handover candidate that is a handover candidate of the first communication unit 210. The relay handover candidate may be a predetermined radio base station or a radio base station with good radio quality measured by the first communication unit 210 (except for the radio base station 100A). . In the case of the first destination designation method, the handover request unit 242 includes identification information (DeNB PCID) for identifying a relay handover candidate in the relay handover request. When receiving the relay handover request, the radio base station 100A transfers the relay handover request to the radio base station identified by the identification information included in the relay handover request.

  The second transmission destination designation method is a method of designating an arbitrary adjacent base station of the radio base station 100A rather than designating a specific radio base station. In this case, the radio base station 100A that receives the relay handover request determines the transfer destination of the relay handover request. In the case of the second transmission destination designation method, the handover request unit 242 includes the wireless quality information measured by the first communication unit 210 in the relay handover request. When the radio base station 100A receives the relay handover request, the radio base station 100A identifies a radio base station with good radio quality for the relay node 200 from the radio quality information included in the relay handover request, and sends the relay handover request to the identified radio base station. Forward.

  The handover control unit 243 controls the first communication unit 210 to execute the relay handover when the first communication unit 210 receives a handover instruction for instructing relay handover from the radio base station 100A. For example, the handover instruction includes information (handover destination identification information, channel information, etc.) used for communication with a new connection destination (hereinafter referred to as a handover destination).

  The first communication unit 210 ends communication with the radio base station 100A and starts communication with the handover destination under the control of the handover control unit 243. When the first communication unit 210 communicates with the handover destination, the relay unit 230 relays data transmitted and received between the handover destination and the radio terminal 300 under the relay node 200.

(1.3) Operation of Radio Communication System Next, the operation of the radio communication system 1 according to the first embodiment will be described. FIG. 4 is a sequence diagram showing an operation of the wireless communication system 1 according to the first embodiment. Here, an example in which the relay node 200 performs a relay handover from the radio base station 100A to the radio base station 100B will be described.

  In step S101, the first communication unit 210 of the relay node 200 uses the radio base station 100A as a connection destination and performs communication with the radio base station 100A.

  In step S <b> 102, the second communication unit 220 of the relay node 200 is a connection destination of the radio terminal 300 under the relay node 200 and performs communication with the radio terminal 300 under the relay node 200.

  In step S103, the handover request unit 242 of the relay node 200 compares the relay node load with the load allowable amount. When the relay node load exceeds the allowable load amount, in step S104, the handover request unit 242 of the relay node 200 controls the first communication unit 210 so as to transmit the relay handover request to the radio base station 100A. The relay handover request transmitted from the first communication unit 210 is received by the radio base station 100A via the radio link.

  In step S105, the radio base station 100A transmits a relay handover request to the radio base station 100B in accordance with the first transmission destination designation method or the second transmission destination designation method described above. The radio base station 100B receives the relay handover request via the X2 interface.

  In step S106, the radio base station 100B determines whether to accept the relay node 200 based on the received relay handover request. For example, the radio base station 100B determines to accept the relay node 200 when the result of adding the relay node load grasped from the relay handover request to the load of the radio base station 100B is equal to or less than a predetermined threshold. To do.

  When it is determined that the relay node 200 is accepted (step S107; YES), in step S108, the radio base station 100B transmits a handover command for instructing relay handover to the radio base station 100A. The radio base station 100A receives the handover command via the X2 interface.

  In step S109, the radio base station 100A transmits to the relay node 200 a handover instruction for instructing relay handover. The first communication unit 210 of the relay node 200 receives the handover instruction via the radio link.

  In step S110, the handover control unit 243 of the relay node 200 controls the first communication unit 210 so as to execute a relay handover to the radio base station 100B.

  In step S111, the first communication unit 210 of the relay node 200 performs communication with the radio base station 100B using the radio base station 100B as a new connection destination (handover destination).

(1.4) Effects of First Embodiment As described above, according to the first embodiment, the relay node 200 performs relay handover when the relay node load exceeds the allowable load amount. A handover request is transmitted to the radio base station 100A.

  When the relay handover is accepted by the radio base station 100B, the relay node 200 executes the relay handover to the radio base station 100B in response to the handover instruction. The relay node 200 relays data transmitted and received between the handover destination radio base station 100B and the radio terminal 300 under the relay node 200.

  As a result, the relay node 200 can perform handover to the radio base station 100B having a smaller load than the radio base station 100A. Therefore, after the handover, the communication capacity of the radio terminal 300 under the relay node 200 can be increased. And the throughput of the wireless terminal 300 under the relay node 200 can be improved.

  In addition, since the load on the radio terminal 300 under the relay node 200 is reduced in the radio base station 100A, the throughput of the radio terminals 300A to 300D that communicate directly with the radio base station 100A is also improved.

  In this embodiment, the relay node 200 transmits a relay handover request addressed to a relay handover candidate. For this reason, the relay handover request can be transmitted to the relay handover candidate, and an acceptance decision by the relay handover candidate can be requested.

  In the present embodiment, the relay node 200 transmits relay node load information indicating the relay node load included in the relay handover request. As a result, the relay handover candidate can make an acceptance decision in consideration of the relay node load, and the handover destination load after the relay handover can be prevented from becoming too high.

  Since a plurality of radio terminals 300 can be connected to the relay node 200, the relay handover has a great effect of reducing the load on the radio base station 100A. Further, the relay node 200 is assumed to be installed at a cell edge or a coverage hole, and has a high-level wireless communication function. Therefore, there is a high possibility that good communication can be realized even after handover.

(2) Second Embodiment Next, regarding the second embodiment, (2.1) the configuration of the relay node, (2.2) the operation of the wireless communication system, (2.3) the effect of the second embodiment. explain. However, a different point from 1st Embodiment is demonstrated and the overlapping description is abbreviate | omitted.

  In the second embodiment, as shown in FIG. 5, a terminal handover in which the radio terminal 300 under the relay node 200 switches the connection destination from the radio base station 100A to an adjacent base station (such as the radio base station 100B) of the radio base station 100A. Is mainly described.

(2.1) Configuration of Relay Node FIG. 6 is a block diagram illustrating a configuration of the relay node 200 according to the second embodiment.

  As shown in FIG. 6, the relay node 200 according to the second embodiment is different from the first embodiment in the configuration of the control unit 240. In addition, the second communication unit 220 receives a measurement result notification indicating the measurement result of the radio quality measured by each of the radio terminals 300 under the relay node 200.

  The control unit 240 includes a load calculation unit 241, a handover request unit 242, and a handover instruction unit 244.

  In addition to the functions described in the first embodiment, the load calculation unit 241 has a function of calculating a terminal load that is a load for each wireless terminal 300 under the relay node 200. The terminal load is an individual load for communication between each of the wireless terminals 300 under the relay node 200 and the second communication unit 220.

  In the present embodiment, the handover request unit 242 transmits a terminal handover request for at least one of the wireless terminals 300 under the relay node 200 to execute a terminal handover when the relay node load is higher than the load allowable amount. The first communication unit 210 is controlled to transmit to 100A.

  The number of wireless terminals that execute terminal handover may be determined according to the relay node load. For example, terminal handover can be executed by a minimum number of wireless terminals whose relay node load is lower than the allowable load amount. Such a method will be described in a fourth embodiment.

  In addition, the handover request unit 242 controls the first communication unit 210 so that terminal load information indicating a terminal load corresponding to a wireless terminal that executes terminal handover is included in the terminal handover request and transmitted.

  Note that there are the following two methods for specifying the transmission destination of the terminal handover request, as in the first embodiment.

  In the first transmission destination designation method, the handover request unit 242 controls the first communication unit 210 to transmit a terminal handover request to a terminal handover candidate that is a handover candidate of the radio terminal 300 under the relay node 200. . The terminal handover candidate may be a predetermined radio base station, and is a radio base station (excluding the radio base station 100A) with good radio quality indicated by the measurement result notification received by the second communication unit 220. There may be. In the case of the first destination designation method, the handover request unit 242 includes identification information (DeNB PCID) for identifying a terminal handover candidate in the terminal handover request. When receiving the terminal handover request, the radio base station 100A transfers the terminal handover request to the radio base station identified by the identification information included in the terminal handover request.

  The second transmission destination designation method is a method of designating an arbitrary adjacent base station of the radio base station 100A rather than designating a specific radio base station. In this case, the radio base station 100A that receives the terminal handover request determines the transfer destination of the terminal handover request. In the case of the second transmission destination designation method, the handover request unit 242 includes the wireless quality information indicated by the measurement result notification received by the second communication unit 220 in the terminal handover request. Upon receiving the terminal handover request, the radio base station 100A identifies a radio base station with good radio quality for the radio terminal 300 under the relay node 200 from the radio quality information included in the terminal handover request. The terminal handover request is transferred to the station.

  When the first communication unit 210 receives a handover instruction for instructing terminal handover from the radio base station 100A, the handover instruction unit 244 transmits a terminal handover instruction corresponding to the received handover instruction to the radio terminals under the relay node 200. The second communication unit 220 is controlled to transmit to 300. For example, the handover instruction includes information (such as handover destination identification information and channel information) used for communication with the handover destination.

  The wireless terminal 300 that has received the terminal handover instruction ends communication with the relay node 200 and starts communication with the handover destination.

(2.2) Operation of Radio Communication System Next, the operation of the radio communication system 1 according to the second embodiment will be described. FIG. 7 is a sequence diagram showing an operation of the wireless communication system 1 according to the second embodiment. Here, an example in which the radio terminal 300E under the relay node 200 performs a relay handover from the relay node 200 to the radio base station 100B will be described (see FIG. 5).

  Steps S201 and S202 are the same as in the first embodiment.

  In step S203, the handover request unit 242 of the relay node 200 compares the relay node load with the load allowable amount. When the relay node load exceeds the allowable load amount, in step S204, the handover request unit 242 of the relay node 200 performs the first operation so as to transmit the terminal handover request including the load information of the wireless terminal 300E to the wireless base station 100A. The communication unit 210 is controlled. The terminal handover request transmitted from the first communication unit 210 is received by the radio base station 100A via the radio link.

  In step S205, the radio base station 100A transmits a terminal handover request to the radio base station 100B according to the first transmission destination designation method or the second transmission destination designation method described above. The radio base station 100B receives the terminal handover request via the X2 interface.

  In step S206, the radio base station 100B determines whether to accept the radio terminal 300E based on the received terminal handover request. For example, the radio base station 100B accepts the relay node 200 when the result of adding the load of the radio terminal 300E grasped from the terminal handover request to the load of the radio base station 100B is equal to or less than a predetermined threshold. Is determined.

  When it is determined that the radio terminal 300E is accepted (step S207; YES), in step S208, the radio base station 100B transmits a handover command for instructing terminal handover to the radio base station 100A. The radio base station 100A receives the handover command via the X2 interface.

  In step S209, the radio base station 100A transmits a handover instruction for instructing a terminal handover to the relay node 200. The first communication unit 210 of the relay node 200 receives the handover instruction via the radio link.

  In step S210, the handover control unit 243 of the relay node 200 controls the second communication unit 220 so as to transmit a terminal handover instruction corresponding to the received handover instruction to the radio terminal 300E. The radio terminal 300E receives the handover instruction via the radio link and performs terminal handover according to the handover instruction (step S211).

  In step S212, the radio terminal 300E communicates with the radio base station 100B using the radio base station 100B as a new connection destination (handover destination).

(2.3) Effect of Second Embodiment As described above, according to the second embodiment, the relay node 200 is a terminal for executing terminal handover when the relay node load exceeds the allowable load amount. A handover request is transmitted to the radio base station 100A. The radio base station 100A receives the terminal handover request and transmits it to the radio base station 100B.

  When the terminal handover is accepted by the radio base station 100B, the relay node 200 receives the handover instruction instructing the terminal handover and transmits the handover instruction to the radio terminal 300 under the relay node 200. The wireless terminal 300 that has received the handover instruction performs terminal handover.

  Thereby, since the radio terminal 300 under the relay node 200 can perform handover to the radio base station 100B with a small load, the communication capacity of the radio terminal 300 can be increased after the terminal handover and throughput can be improved. .

  In the present embodiment, the relay node 200 transmits a terminal handover request addressed to a terminal handover candidate. For this reason, the terminal handover request can be transmitted to the terminal handover candidate, and an acceptance decision by the terminal handover candidate can be requested.

  In the present embodiment, the relay node 200 transmits terminal load information indicating a load for each wireless terminal 300 in a terminal handover request. As a result, the terminal handover candidate can make an acceptance decision in consideration of the terminal load, and the handover destination load after the terminal handover can be prevented from becoming too high.

(3) Third Embodiment Next, regarding the third embodiment, (3.1) the configuration of the relay node, (3.2) the operation of the wireless communication system, (3.3) the effect of the third embodiment. explain. However, differences from the first embodiment and the second embodiment will be described, and redundant description will be omitted.

(3.1) Configuration of Relay Node FIG. 8 is a block diagram showing the configuration of the relay node 200 according to the third embodiment.

  As illustrated in FIG. 8, the relay node 200 according to the third embodiment is different from the first embodiment and the second embodiment in the configuration of the control unit 240.

  The control unit 240 includes a load calculation unit 241, a notification request unit 245, a handover determination unit 249, a handover destination determination unit 246, a handover request unit 242, a handover control unit 243, and a handover instruction unit 244.

  The load calculation unit 241 has the same function as the first embodiment and the second embodiment.

  The notification request unit 245 controls the first communication unit 210 to transmit a load notification request for requesting a load notification of each of the terminal handover candidate and the relay handover candidate. The first transmission destination designation method described above is applied to the load notification request.

  The handover determining unit 249 determines whether to perform terminal handover or relay handover based on the load notification received by the first communication unit 210. For example, the handover destination determining unit 246 determines not to execute either the terminal handover or the relay handover when the loads of the terminal handover candidate and the relay handover candidate are higher than a predetermined amount.

  The handover destination determining unit 246 determines a handover destination based on the load notification received by the first communication unit 210. For example, based on the load notification, the handover destination determining unit 246 determines a handover destination having the smallest load or a smaller load than a predetermined amount from the terminal handover candidates and the relay handover candidates.

  The handover request unit 242 transmits a handover request when the relay node load exceeds the allowable load amount and when it is determined to execute the terminal handover or the relay handover. The handover request unit 242 controls the first communication unit 210 to transmit a handover request to the handover destination determined by the handover destination determination unit 246. Specifically, the handover request unit 242 includes identification information for identifying the determined handover destination in the handover request.

  The handover control unit 243 controls the first communication unit 210 to execute the relay handover when the first communication unit 210 receives a handover instruction for instructing relay handover.

  When the first communication unit 210 receives a handover instruction for instructing terminal handover, the handover instruction unit 244 causes the second communication unit 220 to transmit a terminal handover instruction to the radio terminal 300 under the relay node 200. Control.

  About another structure, it is the same as that of 1st Embodiment and 2nd Embodiment.

(3.2) Operation of Radio Communication System FIG. 9 is a sequence diagram showing the overall operation of the radio communication system 1 according to the third embodiment.

  Steps S301 and S302 are the same as in the above-described embodiment.

  When the relay node load exceeds the allowable load amount (step S303; YES), in step S304, the notification request unit 245 of the relay node 200 requests load notification for requesting the load notification of each of the terminal handover candidate and the relay handover candidate. The first communication unit 210 is controlled to transmit the request. The first communication unit 210 transmits a load notification request to the radio base station 100A. The radio base station 100A receives the load notification request via the radio link.

  In step S305, the radio base station 100A transmits a load notification request to the radio base station 100B based on the identification information included in the received load notification request. The radio base station 100B receives the load notification request via the X2 interface.

  In step S306, the radio base station 100B transmits a load notification for notifying the load of the radio base station 100B to the radio base station 100A. The radio base station 100A receives the load notification via the X2 interface.

  In step S307, the radio base station 100A transmits a load notification to the relay node 200. The first communication unit 210 of the relay node 200 receives a load notification via a wireless link.

  In step S308, the handover determining unit 249 and the handover destination determining unit 246 of the relay node 200 determine a handover destination from each of the terminal handover candidate and the relay handover candidate. Here, it is assumed that the radio base station 100B is determined as the handover destination.

  In step S309, the handover request unit 242 of the relay node 200 controls the first communication unit 210 so as to transmit a handover request to the determined handover destination. The first communication unit 210 transmits a handover request to the radio base station 100A. The radio base station 100A receives the handover request via the radio link.

  In step S310, the radio base station 100A transmits a handover request to the radio base station 100B based on the handover destination identification information included in the received handover request. The radio base station 100B receives the handover request via the X2 interface.

  In step S311, the radio base station 100B transmits a handover command for accepting a handover to the radio base station 100A. The radio base station 100A receives the handover command via the X2 interface.

  In step S312, the radio base station 100A transmits a handover instruction to the relay node 200. The first communication unit 210 of the relay node 200 receives the handover instruction via the radio link.

  When the handover instruction received by the first communication unit 210 of the relay node 200 is a handover instruction for the relay node 200 (step S313; YES), in step S314, the handover control unit 243 of the relay node 200 The first communication unit 210 is controlled to perform a relay handover from the station 100A to the radio base station 100B.

  When the handover instruction received by the first communication unit 210 is a handover instruction for the radio terminal 300 under the relay node 200 (step S313; NO), in step S315, the handover instruction unit 244 of the relay node 200 notifies the corresponding radio terminal. On the other hand, the second communication unit 220 is controlled to transmit a handover instruction. The second communication unit 220 transmits a handover instruction. The radio terminal 300 under the relay node 200 that has received the handover instruction executes terminal handover from the radio base station 100A to the radio base station 100B.

  In this operation sequence, the relay node 200 transmits a load notification request at YES in step S303, but the load notification request may be transmitted periodically. In this case, the load status of the handover candidate can be grasped in advance, and the processing related to the handover can be completed early.

(3.3) Effect of Third Embodiment As described above, the relay node 200 determines a handover destination based on the load notification received by the first communication unit 210, and makes a handover request addressed to the determined handover destination. Send. Accordingly, it is possible to determine an appropriate handover destination by determining the handover destination in consideration of the load of the handover candidate.

  In the present embodiment, the relay node 200 determines a handover candidate having the smallest load or a smaller load than a predetermined amount as a handover destination from among the handover candidates. Thereby, it is possible to prevent the load at the handover destination after the handover from becoming too high.

  In the present embodiment, the relay node 200 determines whether to execute a handover based on the load notification received by the first communication unit 210. Thereby, in the state where the load of all the handover candidates is high, the handover to the handover candidate can be stopped.

(4) Fourth Embodiment Next, regarding the fourth embodiment, (4.1) the configuration of the relay node, (4.2) the operation of the wireless communication system, (4.3) the effect of the fourth embodiment. explain. However, differences from the first embodiment to the third embodiment will be described, and redundant description will be omitted.

(4.1) Configuration of Relay Node FIG. 10 is a block diagram showing the configuration of the relay node 200 according to the fourth embodiment.

  As shown in FIG. 10, the relay node 200 according to the fourth embodiment is different from the first to third embodiments in the configuration of the control unit 240. The control unit 240 includes a load calculation unit 241, a handover candidate determination unit 247, a notification request unit 245, a handover determination unit 249, a terminal determination unit 248, a handover request unit 242, a handover control unit 243, and a handover instruction unit 244. Here, differences from the third embodiment will be briefly described.

  The handover determining unit 249 calculates the load of the relay handover candidate when the relay handover to the relay handover candidate is performed from the load notification and the relay node load, and determines whether to execute the relay handover based on the calculated load To decide. The handover request unit 242 controls the first communication unit 210 to transmit the relay handover request when it is determined to execute the relay handover.

  When it is determined that relay handover is not to be performed, the terminal determination unit 248 keeps the relay node load below the allowable load based on the relay node load and the terminal load that is a load for each wireless terminal. At least one wireless terminal to execute terminal handover is determined.

(4.2) Operation of Radio Communication System Next, regarding the operation of the radio communication system 1 according to the fourth embodiment, (4.2.1) overall operation, (4.2.2) handover candidate determination process, ( 4.2.3) The handover destination determination process will be described in this order.

(4.2.1) Overall Operation FIG. 11 is a sequence diagram showing the overall operation of the wireless communication system 1 according to the fourth embodiment.

  Steps S401 to S403 are the same as those in the above-described embodiment.

  In step S404, the handover candidate determination unit 247 of the relay node 200 determines a handover candidate. Here, it is assumed that the radio base station 100B is determined as a handover candidate. Details of step S404 will be described later.

  In step S405, the notification request unit 245 of the relay node 200 controls the first communication unit 210 to transmit a load notification request for requesting a load notification for notifying a load of a handover candidate. The first communication unit 210 transmits a load notification request to the radio base station 100A. The radio base station 100A receives the load notification request via the radio link.

  In step S406, the radio base station 100A transmits a load notification request to the radio base station 100B based on the identification information included in the received load notification request. The radio base station 100B receives the load notification request via the X2 interface.

  In step S407, the radio base station 100B transmits a load notification for notifying the load of the radio base station 100B to the radio base station 100A. The radio base station 100A receives the load notification via the X2 interface.

  In step S408, the radio base station 100A transmits a load notification to the relay node 200. The first communication unit 210 of the relay node 200 receives a load notification via a wireless link.

  In step S409, the handover determining unit 249, the handover destination determining unit 246, and the terminal determining unit 248 of the relay node 200 select a node (radio terminal or relay node) to be handed over from the relay node 200 and the wireless terminal 300 under the relay node 200. ) And a handover destination. Here, it is assumed that the radio base station 100B is determined as the handover destination. Details of step S409 will be described later.

  In step S410, the handover request unit 242 of the relay node 200 controls the first communication unit 210 to transmit a handover request addressed to the determined handover destination. The first communication unit 210 transmits a handover request to the radio base station 100A. The radio base station 100A receives the handover request via the radio link.

  In step S411, the radio base station 100A transmits a handover request to the radio base station 100B based on the handover destination identification information included in the received handover request. The radio base station 100B receives the handover request via the X2 interface.

  In step S412, the radio base station 100B transmits a handover command for accepting a handover to the radio base station 100A. The radio base station 100A receives the handover command via the X2 interface.

  In step S413, the radio base station 100A transmits a handover instruction to the relay node 200. The first communication unit 210 of the relay node 200 receives the handover instruction via the radio link.

  When the handover instruction received by the first communication unit 210 of the relay node 200 is a handover instruction for the relay node 200 (step S414; YES), in step S415, the handover control unit 243 of the relay node 200 The first communication unit 210 is controlled to perform a relay handover from the station 100A to the radio base station 100B.

  When the handover instruction received by the first communication unit 210 is a handover instruction for the radio terminal 300 under the relay node 200 (step S414; NO), in step S416, the handover instruction unit 244 of the relay node 200 notifies the corresponding radio terminal. On the other hand, the second communication unit 220 is controlled to transmit a handover instruction. The second communication unit 220 transmits a handover instruction. The radio terminal 300 under the relay node 200 that has received the handover instruction executes terminal handover from the radio base station 100A to the radio base station 100B.

(4.2.2) Handover Candidate Determination Processing Next, details of the handover candidate determination processing, that is, step S404 in FIG. 11 will be described. FIG. 12 is a flowchart showing details of step S404 in FIG.

  The first communication unit 210 of the relay node 200 measures the radio quality of the received radio signal, and notifies the handover candidate determination unit 247 of the measurement result. In step S501, the handover candidate determining unit 247 determines a radio base station other than the radio base station 100A having the best radio quality as a first candidate for a handover candidate (relay handover candidate). However, if the radio quality cannot be measured for a radio base station other than the radio base station 100A, or if it can be measured but is below a predetermined quality, the first candidate is absent.

  The second communication unit 220 of the relay node 200 periodically receives a measurement result notification indicating the measurement result of the radio quality of the radio signal received by the radio terminal 300 under the relay node 200 from the radio terminal 300 under the relay node 200. ing. In step S <b> 502, the handover candidate determining unit 247 determines each handover candidate of the radio terminal 300 under the relay node 200 based on the measurement result notification received by the second communication unit 220. Specifically, the radio base station with the best radio quality is set as the handover candidate for each radio terminal 300 under the relay node 200. If the radio quality of a radio base station cannot be measured, or if it can be measured but is below a predetermined quality, there is no handover candidate for that radio terminal.

  In step S503, the handover candidate determination unit 247 assigns priorities to the handover candidates listed in step S502 as the second candidate, the third candidate,.

  The first to Nth candidates are determined by the processing from step S501 to step S503.

(4.2.3) Handover Destination Determination Processing Next, details of the handover destination determination processing, that is, step S409 in FIG. 11 will be described. FIG. 13 is a flowchart showing details of step S409.

  In step S601, the load calculation unit 241 calculates each load (terminal load) of the radio terminal 300 under the relay node 200.

  In step S602, the load calculation unit 241 and the handover determination unit 249 have executed the handover from the relay node 200 to the first candidate based on the first candidate load and the relay node load corresponding to the load notification received in step S408. In this case, the load of the first candidate is calculated.

  In step S603, the handover determining unit 249 determines whether or not the relay node 200 can be handed over to the first candidate based on the handover destination load calculated in step S602. For example, the handover determining unit 249 makes a handover possible when the load of the first candidate is equal to or less than a predetermined value, and makes a handover impossible otherwise.

  When it is determined that the first candidate can be handed over, the handover destination determining unit 246 determines the first candidate as the handover destination of the relay node 200. When it is determined that the first candidate cannot be handed over, the handover of the relay node 200 is abandoned, and the handover is determined for each radio terminal 300 under the relay node 200 in step S604 and subsequent steps.

  In step S605, n indicating a candidate is set to 2.

  In step S606, the load calculation unit 241 and the handover determination unit 249 determine the terminal load of the wireless terminal corresponding to the nth candidate (here, the second candidate) and the load of the nth candidate corresponding to the load notification received in step S408. Based on the above, the load of the nth candidate when the wireless terminal corresponding to the nth candidate hands over to the nth candidate is calculated.

  In step S606, the handover determining unit 249 determines whether or not the wireless terminal corresponding to the nth candidate can be handed over to the nth candidate based on the load of the nth candidate calculated in step S656. For example, the handover control unit 132 makes a handover possible when the calculated nth candidate load is equal to or less than a predetermined value, and otherwise makes a handover impossible. When it is determined that the handover to the n-th candidate is possible, the handover destination determining unit 246 determines the n-th candidate as the handover destination of the wireless terminal.

  In step S607, the terminal determination unit 248 determines whether or not the relay node load of the relay node 200 is equal to or less than a predetermined value when the wireless terminal hands over to the handover destination (nth candidate). When the relay node load is equal to or less than the predetermined value, the terminal determination unit 248 determines the node to which the wireless terminal is to be handed over, and the handover destination determination unit 246 determines the nth candidate as the handover destination and ends the process.

  On the other hand, when it is determined that the nth candidate cannot be handed over, the handover determining unit 249 sets n = n + 1 in step S608, and proceeds to the determination process for the next candidate.

  In step S609, it is determined whether or not processing has been completed for all candidates. If there is a candidate for which processing has not been completed, the processing returns to step S605.

  When the processing is completed for all candidates (step S609; NO), and when the handover destination is determined in the processing up to that point, the terminal determination unit 248, the wireless terminal corresponding to the determined handover destination As a node to be handed over. If the processing has been completed for all candidates (step S609; NO), and if the handover destination has not been determined in the processing up to that point, the processing ends with no handover destination.

  In addition, when the process is finished with YES in step S607, the load on the relay node 200 is equal to or less than the predetermined value. However, when the process is finished with NO in step S607, the load on the relay node 200 is predetermined. It is not less than the value. When the radio terminal 300 under the relay node 200 is handed over to a terminal handover candidate and the load on the relay node 200 is reduced, the relay node 200 can be handed over to the relay handover candidate in this way. Also good.

(4.3) Effects of the Fourth Embodiment As described above, according to the fourth embodiment, the relay node 200 calculates the relay handover candidate load after the relay handover in advance, and the calculated load is allowed. The relay handover can be executed if the load is a certain load, and the relay handover can be prevented if the load after the relay handover is an unacceptable load.

  Relay handover leads to a sudden increase in load at the handover destination. Therefore, in a situation where the load on the radio base station 100B is high, the relay node 200 performs a terminal handover for each radio terminal 300 under the relay node 200 instead of a relay handover to the radio base station 100B.

  At that time, the relay node 200 hands over at least one radio terminal from among the radio terminals 300 under the relay node 200 so that the relay node load becomes a predetermined value or less. Thereby, the radio terminal to be handed over can be determined so that the balance between the load of the relay node 200 and the load of the handover destination after the terminal handover is optimal.

(5) Other Embodiments As described above, the present invention has been described according to the embodiment. However, it should not be understood that the description and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art. For example, each of the first to fourth embodiments and other embodiments may be implemented in combination with each other.

  Below, the example of a change of each embodiment mentioned above is explained.

(5.1) Modification 1
In each embodiment described above, in steps S103, S203, S303, and S403, it is determined whether or not the relay node load exceeds the allowable load amount. However, the determination is not limited to this, and the following determination may be performed.

  For example, when the relay node load increases rapidly, it is predicted that the relay node load will exceed the load allowance even if the relay node load exceeds the load allowance, and the handover request or You may transfer to processing such as a load notification request. For example, when the increase amount of the relay node load per unit time exceeds a predetermined increase amount, it can be predicted that the relay node load exceeds the allowable load amount. By such a method, the handover process according to the above-described embodiment can be started earlier.

  Alternatively, as a method for predicting that the relay node load exceeds the allowable load amount, the following method can also be adopted. The data received by the relay node 200 from the first communication unit 210 is temporarily stored in the storage unit 250 as a transmission buffer before being transmitted from the second communication unit 220, but the wireless data that can be used by the second communication unit 220 is used. When there are few resources and the amount of data that can be transmitted and received by the second communication unit 220 is small, data is accumulated in the storage unit 250 as a transmission buffer. For this reason, the relay node load may be predicted to exceed the allowable load amount based on a prediction criterion that the data accumulation amount of the transmission buffer exceeds a predetermined amount.

(5.2) Modification 2
In the second to fourth embodiments described above, the processing related to terminal handover is performed in consideration of the load and radio quality of each radio terminal 300 under the relay node 200. However, the radio terminal 300 under the relay node 200 is used. The processing related to terminal handover may be performed in consideration of the remaining amount of each battery.

  Specifically, the second communication unit 220 receives a battery notification indicating the remaining battery level of the wireless terminal 300 under the relay node 200 for each wireless terminal. Based on the battery notification received by the second communication unit 220, the terminal determination unit 248 determines a wireless terminal with a high remaining battery power as a wireless terminal that performs terminal handover with priority.

  For example, the terminal determination unit 248 may prioritize handover candidates based on the remaining battery level in step S503 of FIG. The wireless terminal 300 with a small remaining battery capacity can be connected to the relay node 200 closer to the wireless base station 100A, so that the transmission power can be reduced. Therefore, the wireless terminal 300 with a small remaining battery capacity can be connected to battery saving. Can do.

(5.3) Modification 3
In the above-described embodiment, the relay node 200 has been described as a fixed type, but may be configured to be movable. In the above-described embodiment, the handover candidate or the handover destination of the relay node 200 is determined in consideration of the radio quality of the relay node 200. However, for the fixed relay node 200, the handover candidate or the handover destination is determined in advance. You may keep it.

(5.4) Modification 4
In each of the above-described embodiments, the case where the relay node 200 and the radio terminal 300 under the relay node 200 perform a handover to the adjacent base station (radio base station 100B) is illustrated. However, the relay node 200 and the radio under the relay node 200 are illustrated. The handover destination of terminal 300 is not limited to a radio base station, and may be a relay node. When the relay node 200 performs a handover to another relay node, the first communication unit 210 of the relay node 200 communicates with the other relay node using the other relay node as a connection destination.

(5.5) Modification 5
In each of the above-described embodiments, the case where the relay node 200 receives a handover instruction from the radio base station 100A has been exemplified. However, instead of such a highly compulsory handover instruction, a message indicating that handover is permitted may be used. .

  Thus, it should be understood that the present invention includes various embodiments and the like not described herein. Therefore, the present invention is limited only by the invention specifying matters in the scope of claims reasonable from this disclosure.

  DESCRIPTION OF SYMBOLS 1 ... Wireless communication system, 100, 100A, 100B ... Wireless base station, 132 ... Handover control part, 200 ... Relay node, 210 ... 1st communication part, 211 ... Antenna, 220 ... 2nd communication part, 221 ... Antenna, 230 ... Relay unit, 240 ... Control unit, 241 ... Load calculation unit, 242 ... Handover request unit, 243 ... Handover control unit, 244 ... Handover instruction unit, 245 ... Notification request unit, 246 ... Handover destination determination unit, 247 ... Handover candidate Determining unit, 248 ... terminal determining unit, 249 ... handover determining unit, 250 ... storage unit, 300, 300A to 300I ... wireless terminal

Claims (4)

Communicating with the first wireless base station as a connection destination and relaying data transmitted and received between the first wireless base station and the wireless terminal by communicating with at least one wireless terminal A wireless relay station,
A load notification request for requesting notification of a load of the second radio base station is transmitted to the first radio base station, and based on the load of the second radio base station notified from the first radio base station A control unit for controlling handover of the wireless terminal
The control unit transmits a handover request for the radio terminal to the second radio base station to the first radio base station, and sends a response message to permit the handover request from the first radio base station. If received, instruct the wireless terminal to hand over to the second wireless base station,
The load notification request is a radio relay station including a cell identifier of the second radio base station. Communicating with the first wireless base station as a connection destination and relaying data transmitted and received between the first wireless base station and the wireless terminal by communicating with at least one wireless terminal A wireless relay station,
A load notification request for requesting notification of a load of the second radio base station is transmitted to the first radio base station, and based on the load of the second radio base station notified from the first radio base station A control unit for controlling handover of the radio terminal so as to adjust a load balance between the radio relay station and the second radio base station,
The control unit transmits a handover request for the radio terminal to the second radio base station to the first radio base station, and sends a response message to permit the handover request from the first radio base station. A radio relay station that, when received, instructs the radio terminal to perform handover to the second radio base station. Communicating with the first wireless base station as a connection destination and relaying data transmitted and received between the first wireless base station and the wireless terminal by communicating with at least one wireless terminal A control method for a radio relay station,
Transmitting a load notification request for requesting notification of a load of the second radio base station to the first radio base station;
Controlling handover of the radio terminal based on the load of the second radio base station notified from the first radio base station;
Transmitting a handover request of the radio terminal to the second radio base station to the first radio base station;
Instructing the radio terminal to perform handover to the second radio base station when a response message indicating that the handover request is permitted is received from the first radio base station,
The load notification request includes a cell identifier of the second radio base station. Communicating with the first wireless base station as a connection destination and relaying data transmitted and received between the first wireless base station and the wireless terminal by communicating with at least one wireless terminal A control method for a radio relay station,
Transmitting a load notification request for requesting notification of a load of the second radio base station to the first radio base station;
Based on the load of the second radio base station notified from the first radio base station, the radio so as to adjust the load balance between the radio relay station and the second radio base station Controlling the handover of the terminal;
Transmitting a handover request of the radio terminal to the second radio base station to the first radio base station;
Instructing the radio terminal to perform handover to the second radio base station when receiving a response message to permit the handover request from the first radio base station;
A control method comprising:

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