COMMUNICATION DEVICE, CONTROL METHOD FOR COMMUNICATION DEVICE, AND PROGRAM

Abstract

To quickly resume communication after the occurrence of radio link failure.SOLUTION: A communication device functions as a node that relays the link between user equipment and a base station, and in a state in which the user equipment and the communication device are RRC (radio resource control) connected and the communication device is connected with a first base station, the communication device determines whether to switch a base station with which the user equipment is connected via the communication device from the first base station to a second base station, and when determining to switch the base station with which the user equipment is connected to the second base station, requests the user equipment to release the RRC connection.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to wireless communication technology. [Background technology]

[0002] In the 3GPP (3rd Generation Partnership Project), standardization of IAB (Integrated Access and Backhaul) as a communication technology for backhaul is progressing. The IAB technology is a technology that simultaneously uses millimeter wave wireless communication such as 28 GHz band, which is used for access communication between base stations and user equipment (UE (User Equipment)), as backhaul communication (Patent Document 1). In backhaul communication using the IAB technology, a relay device called an IAB node relays communication from an IAB donor, which is a base station, by millimeter wave communication. By using the IAB technology, it is possible to expand the area coverage at a lower cost than conventional wired communication using optical fiber, etc.

[0003] When IAB technology is used, a radio link failure (BH RLF (Backhaul Radio Link Failure)) may occur in backhaul communication, causing the connection between IAB nodes to be cut off, resulting in a disruption of communication. When a BH RLF occurs, the IAB node can restore communication by switching the connection to another IAB node that can be reached and re-establishing backhaul communication. Even in cases other than the occurrence of a BH RLF, there may be cases where an already established route needs to be changed due to factors such as a deterioration in the quality of communication between IAB nodes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2019-534625 [Patent Document 2] International Publication No. 2017 / 078143 Summary of the Invention [Problem to be solved by the invention]

[0005] When switching the IAB node to which it is connected, an IAB node may connect to an IAB node that has established a connection with an IAB donor (new IAB donor) different from the IAB donor (old IAB donor) with which it had established a connection before the switch. In this case, the IAB node stops the operating DU (Distributed Unit) that was connected to the old IAB donor, and starts a new DU for connection with the new IAB donor. The newly started DU establishes a connection with the new IAB donor and restores communication.

[0006] On the other hand, in the UE connected to the IAB node, after detecting the disconnection with the IAB node due to the DU of the IAB node being stopped, the UE starts the RLF detection timer. After the RLF detection timer expires, the UE transitions its RRC (Radio Resource Control) state to RRC_IDLE, starts the process of reconnecting to the IAB node, and reestablishes the connection with the IAB node.

[0007] Here, consider the case where a connection with a new IAB donor is established in the IAB node before the RLF detection timer in the UE expires. In this case, even if the IAB node is in a state where communication with the new IAB donor is possible, the UE cannot reconnect to the IAB node until the RLF detection timer expires. In other words, when the IAB node completes switching of the IAB donor to which it is connected, the UE cannot quickly resume data communication, which causes a problem of degraded user usability.

[0008] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide a technique for quickly resuming communication after a wireless link failure occurs. [Means for solving the problem]

[0009] As one means for achieving the above object, a communication device of the present invention has the following configuration: That is, the communication device functions as a node that relays a link between a user equipment and a base station, ,before The communication device is connected to a first base station. In case The communication device Place A decision means for deciding whether to switch a base station to be connected from the first base station to a second base station, and when the decision means decides to switch a base station to be connected to the user equipment to the second base station, RRC (Radio Resource Control) CONNECTED state To the user equipment Send RRC Release message do send The present invention has a means and a method for performing the same. Effect of the Invention

[0010] According to the present invention, a technique for quickly resuming communication after a wireless link failure is provided. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a block diagram illustrating an example of a hardware configuration of a communication device. [Diagram 2] FIG. 2 is a block diagram illustrating an example of a functional configuration of a communication device. [Diagram 3] 1 shows an example of the configuration of a communication system. [Figure 4] 4 is a flowchart illustrating a process performed by an IAB node according to the first embodiment. [Diagram 5] 13 is a flowchart showing a process performed by a UE. [Figure 6] 3 is a communication sequence according to the first embodiment. [Figure 7] 13 is a flowchart illustrating a process performed by an IAB node according to the second embodiment. [Figure 8] 13 is a communication sequence according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0013] [First embodiment] (Communication System Configuration) Fig. 3 shows an example of the configuration of a communication system in this embodiment. UE (User Equipment) 307 is located in a cell area 306 managed by IAB (Integrated Access and Backhaul) node 305, and can be connected to the IAB node 305. In this embodiment, the path of the backhaul network (adaptation layer route) is managed by an IAB donor. For example, in Fig. 3, IAB donor 311 manages route A consisting of IAB nodes 301, 303, and 305 to UE 307. Also, IAB donor 312 manages route B consisting of IAB nodes 302, 304, and 305 to UE 307.

[0014] In addition, a routing table including routing information is set in each node on the route, and the routing table is set by the IAB donor. The routing information may include a destination address, a next hop node to which the packet is forwarded, a BH (Backhaul) link or a BH RLC (Radio Link Control) channel, and a cost metric. In FIG. 3, in each route, one IAB node is connected to the IAB donor, and the IAB donor is connected to the CN (Core Network) 310.

[0015] In Fig. 3, when a BH RLF (Backhaul Radio Link Failure) occurs between IAB node 305 and IAB node 303 under IAB donor 311, IAB node 305 needs to transition the IAB node (parent node) to which it is connected. IAB node 305 detects IAB node 304 of a neighboring cell, sets a new backhaul link, and establishes a new route (IAB donor 312 - IAB node 302 - IAB node 304 - IAB node 305). In this case, the IAB donor that establishes the route with IAB node 305 switches from IAB donor 311 to IAB donor 312, and therefore downstream IAB nodes including IAB node 305 and all UEs accessing the node need to perform handover processing.

[0016] When switching the IAB donor, the IAB node 305 stops the DU (Distributed Unit) that was in operation and connected to the old IAB donor (IAB donor 311) and starts a new DU for connection to the new IAB donor (IAB donor 312). The control between the IAB node 305 and each IAB donor is as follows. The IAB donor 311 and the IAB node 305 are connected by an F1-C association and an F1-U tunnel via another IAB node, but when a new route is established by the BH RLF, the IAB donor 312 and the IAB node 305 are connected by a new F1-C association and an F1-U tunnel. This process updates the connection destination of the DU of the IAB node 305 from the IAB donor 311 to the IAB donor 312. The IAB donor 312 updates the routing information by establishing a new route, so the IP address of each IAB node is updated.

[0017] (Configuration of communication device) FIG. 1 is a block diagram showing an example of a hardware configuration of a communication device (UE (user equipment), IAB node) in this embodiment. The control unit 101 controls the entire communication device by executing a control program stored in the storage unit 102. The storage unit 102 stores the control program executed by the control unit 101 and various information such as cell information (base station information), connected terminal information, and IAB routing information. Various operations described below can be performed by the control unit 101 executing the control program stored in the storage unit 102. The wireless communication unit 103 performs control for performing cellular network communication such as LTE (Long Term Evolution) and 5G that conform to the 3GPP (3rd Generation Partnership Project) standard. Note that the wireless communication unit 103 may be configured to perform control for performing communication that conforms to the IEEE802.11 series standard. The antenna control unit 104 controls an antenna 105 for wireless communication by the wireless communication unit 103. A plurality of antennas 105 may be provided to realize MIMO (Multi-Input Multi-Output) communication or the like.

[0018] FIG. 2 is a block diagram showing an example of the functional configuration of a communication device (UE (user equipment), IAB node) in this embodiment. FIG. 2(a) shows an example of the functional configuration of an IAB node. In FIG. 2(a), a transmitting / receiving unit 201 transmits and receives a message (frame / packet) to and from a communication partner device via a wireless communication unit 103, an antenna control unit 104, and an antenna 105 (FIG. 1). The transmitting / receiving unit 201 may also perform message generation processing (at the time of transmission) and analysis processing (at the time of reception). A connection control unit 202 controls a connection with a communication partner device. The connection control unit 202 can request establishment and release of a connection with the communication partner device via, for example, the transmitting / receiving unit 201. The RLF detection unit 203 detects (detects) a wireless failure in the physical layer occurring between the communication partner device and the communication partner device. The switching decision unit 204 decides to switch the IAB donor to be connected due to a change in the communication route. The measurement unit 205 measures (derives) the strength and / or quality of a received signal from a communication partner device (RSSI (Received Signal Strength Indicator / Indication), RSRQ (Reference Signal Received Quality), RSRP (Reference Signal Received Power), SINR (Signal to Interference and Noise Ratio), etc.).

[0019] FIG. 2(b) shows an example of the functional configuration of a UE. In FIG. 2(b), a transmitting / receiving unit 211 transmits and receives messages (frames / packets) to and from a communication partner device via a wireless communication unit 103, an antenna control unit 104, and an antenna 105 (FIG. 1). The transmitting / receiving unit 211 may also perform message generation processing (when transmitting) and analysis processing (when receiving). A connection control unit 212 controls a connection with a communication partner device. The connection control unit 202 can request establishment and release of a connection with a communication partner device via the transmitting / receiving unit 211, for example. A state control unit 213 controls the state, including state transition, of the UE.

[0020] (IAB node processing) Next, the process of the IAB node according to the presence or absence of IAB donor switching will be described. Fig. 4 is a flowchart showing the process executed by the IAB node according to this embodiment. Here, the process will be described as being executed by the IAB node 305 in the communication system as shown in Fig. 3. Also, it is assumed that the IAB node 305 is connected to the UE 307 via RRC (Radio Resource Control).

[0021] When the RLF detection unit 203 of the IAB node 305 detects a BH RLF between the IAB node 303 and the IAB node 303, the connection control unit 202 changes the IAB node (parent node) to be connected. Here, the connection control unit 202 of the IAB node 305 establishes synchronization with the IAB node 304, which is the new parent node (destination IAB node), and performs RACH (Random Access Channel) processing to connect. After that, the transmission / reception unit 201 of the IAB node 305 acquires cell information from the IAB node 304 (S401). For example, the IAB node 305 acquires a physical cell identifier (PCI) included in the broadcast signal transmitted from the IAB node 304. The switching decision unit 204 of the IAB node 305 then compares it with the PCI as cell information already acquired from the IAB node 303. Through this comparison, the switching decision unit 204 of the IAB node 305 determines whether the IAB donor to which the parent node is connected will switch (whether the route / cell is different) before and after the parent node change. In other words, it determines whether the destination IAB node is under a different IAB donor than before the transition.

[0022] For example, if the PCIs match and it is determined that the destination IAB node is under the same IAB donor as before the migration (No in S402), the process ends. On the other hand, if the PCIs do not match and it is determined that the destination IAB node is under a different IAB donor than before the migration (Yes in S402), the process proceeds to S403. At this time, the IAB node 305 recognizes that the UE 307 needs to perform handover processing.

[0023] In S403, the connection control unit 202 of the IAB node 305 requests the UE 307 to release the RRC connection. The request can be made by the connection control unit 202 transmitting an RRC connection release request message via the transmission / reception unit 201. The RRC connection release request message may be an RRC Release message standardized in the 3GPP specifications. A list of frequencies to be preferentially connected may be added to the message requesting the release of the RRC connection. This allows the UE after release to preferentially connect to the frequency set in the frequency list. The IAB node 305 may request the release of the RRC connection in another manner, such as transmitting a unique message to the UE 307 requesting the release of the RRC connection.

[0024] If an IAB node (child node) exists downstream of the section where the BH RLF occurred (after the determination of S403), the IAB node 305 may request the child node to release the RRC connection with the UE that is accessing it. In addition, when the connection control unit 202 of the IAB node 305 establishes a connection with a new IAB donor (IAB donor 312 in the example of FIG. 3) after requesting the UE 307 to release the RRC connection, the connection control unit 202 may notify the UE 307 in a lower layer via the transceiver unit 201 that a connection with the new IAB donor has been established.

[0025] (UE processing) Next, a process of the UE when switching the IAB donor will be described. Fig. 5 is a flowchart showing a process executed by the UE according to the present embodiment. Here, the process will be described as being executed by the UE 307 in the communication system as shown in Fig. 3. Also, it is assumed that the UE 307 is RRC-connected with the IAB node 305.

[0026] In the following description, in the IAB node 305, a DU for connecting to an old IAB donor is referred to as an old DU, and a DU for connecting to a new IAB donor is referred to as a new DU. As described in Patent Document 2, the UE can transition from an RRC_IDLE (idle) state, which is a standby state, to an RRC_CONNECTED (connected) state (establishing an RRC connection with the base station) by reusing context information.

[0027] When the transceiver 211 of the UE 307 receives an RRC connection release request message from the IAB node 305 by the process of S403 in FIG. 4, the connection control unit 212 releases the RRC connection with the old DU of the IAB node 305 that was being accessed (S501). After releasing the RRC connection, the state control unit 213 of the UE 307 transitions the state of the UE 307 from the RRC_CONNECTED state to the RRC_IDLE state, which is an IDLE standby state (S502). Since the UE is in the standby state, the standby state is maintained until, for example, the received power from the base station (the IAB node in this example) meets a certain reference value. As will be described later with reference to FIG. 6, while the UE is in the RRC_IDLE state, the IAB node 305 starts an F1-C connection with the IAB donor 312, which is a new IAB donor, by the DU setup procedure (F609 in FIG. 6). Once this connection is established, the new DU of the IAB node 305 is now connected to the IAB donor 312 and is therefore able to access the UE 307.

[0028] After the new DU of the IAB node 305 completes the connection with the IAB donor 312, the connection control unit 212 of the UE 307 quickly detects (discovers) and selects the new DU of the IAB node 305 without any problem in the radio unit because of the RRC_IDLE state (S503). After this, the connection control unit 212 of the UE 307 establishes synchronization with the new DU of the IAB node 305 and performs RACH processing (S504). Note that when the transmission / reception unit 211 of the UE 307 receives a notification from the IAB node 305 that a connection with the new IAB donor (IAB donor 312 in the example of FIG. 3) has been established, the UE 307 may perform the processes of S503 and S504.

[0029] (Communication sequence in a communication system) 6 is a communication sequence diagram in this embodiment. Assume that UE 307 is RRC connected in route A. UE 307 performs data communication with IAB donor 311 via route A (D601, D602). When IAB node 305 detects a wireless failure in the physical layer between IAB node 303 and itself, it starts a timer. After starting the timer, if the failure is not restored within the timer time (predetermined time), IAB node 305 detects and declares a BH RLF (F601).

[0030] Although the IAB node 305 loses the parent node of the connection destination due to a radio failure of the backhaul, it establishes synchronization and performs RACH processing to connect to the IAB node 304 as a new parent node (F602). The synchronization establishment and RACH processing are the same as the procedure in which a normal UE finds a destination cell by cell search and establishes a connection. After determining the IAB donor switching (Yes in S402), the IAB node 305 transmits an RRC connection release request message (for example, an RRC Release message standardized in the 3GPP specifications) to the UE 307 (F603, S403).

[0031] When the UE 307 receives an RRC connection release request message from the IAB node 305, it executes an access (RRC connection) release process with the old DU of the IAB node 305 (F604). Next, the IAB node 305 stops the DU process due to the influence of the BH RLF (F605). In the conventional method, the UE 307 continues to access the old DU of the IAB node 305, and a wireless problem occurs. In the conventional method, when a wireless problem occurs in the UE 307, a timer called a T1 timer is started from the time of the wireless problem occurrence, and when the T1 timer expires, the UE 307 attempts to connect to a connectable cell, but when the cell connection is not possible, it declares an RLF. On the other hand, in this embodiment, the UE 307 and the IAB node 305 operate as follows.

[0032] The IAB node 305 initiates an RRC connection reestablishment with the new IAB donor, the IAB donor 312 (F606). As a result, the IAB donor 312 establishes a new route B with the IAB node 305 via the IAB node 302 and the IAB node 304 (F607). Next, the IAB node 305 initiates an F1-C connection with the IAB donor 312 by a DU setup procedure (F608). As a result, the new DU of the IAB node 305 is connected to the IAB donor 312 and is therefore accessible to the UE 307.

[0033] Since the UE 307 is in the RRC_IDLE state (standby), there is no problem in the radio section, and the UE 307 quickly detects (discovers) and selects the DU of the IAB node 305 (F609). After this, the UE 307 establishes synchronization with the IAB node 305 and performs RACH processing (F610). After that, the UE 307 executes processing to re-establish an RRC connection with the IAB donor 312 (F611). Once the processing to re-establish the RRC connection is completed, the UE 307 can resume data communication with the IAB donor 312 (D603, D604).

[0034] Thereafter, the IAB donor 311 releases route A of the old adaptation layer route via the IAB node 303 and the IAB node 301 on the wireless backhaul between the IAB node 305 and the IAB donor 311. Furthermore, the IAB donor 311 releases the forwarding entry between the fronthaul on the old route on the wireless backhaul (F612).

[0035] Thus, according to this embodiment, when the IAB node 305 detects a BH RLF, it stops the DU process and requests the connected UE 307 to release the RRC connection. This allows the UE 307 to perform an access release process with the DU of the IAB node 305, and then, after the IAB node 305 establishes a route with a new IAB donor, it is possible to quickly detect and select a new DU of the IAB node (a DU whose connection destination has been updated to the IAB donor 312) and start a connection. As a result, compared to the conventional method, the UE 307 can connect to the IAB node and resume data communication more quickly after the BH RLF occurs.

[0036] [Second embodiment] Next, a second embodiment will be described. The configuration of the communication system and the configuration of each communication device in this embodiment are similar to those in the first embodiment, so the description will be omitted.

[0037] (IAB node processing) The process of the IAB node according to whether or not the IAB donor is switched will be described. Fig. 7 is a flowchart showing the process executed by the IAB node according to this embodiment. Here, the process will be described as being executed by the IAB node 305 in the communication system as shown in Fig. 3.

[0038] The measurement unit 205 of the IAB node 305 measures the strength and / or quality (RSSI, etc.) of the received signal. Next, the transmission / reception unit 201 of the IAB node 305 transmits a measurement report message including information of the measurement (measurement information) to the IAB node 303, which is the connecting IAB node (parent node) (S701). The transmitted measurement report is used by the IAB donor 311 to determine whether or not to perform a handover from the IAB donor 311 to the IAB donor 312. When the IAB donor 311 determines to perform a handover based on the measurement report, it transmits an RRC Reconfiguration message to the IAB node 305 via the IAB node 303. These processes will be described later with reference to FIG. 8.

[0039] When the switching decision unit 204 of the IAB node 305 recognizes the switching of the IAB donor by receiving an RRC Reconfiguration message from the IAB node 303 via the transmission / reception unit 201 (Yes in S702), it decides to switch the IAB donor, and the process proceeds to S703. The process of S703 is the same as S403 in FIG. 4 described in the first embodiment, and therefore a description thereof will be omitted. On the other hand, when the IAB node 305 does not recognize the switching of the IAB donor because it has not received an RRC Reconfiguration message within a predetermined time, for example (No in S702), it ends the process.

[0040] (UE processing) The processing of the UE when switching the IAB donor is similar to the processing shown in FIG. 5 described in the first embodiment, and therefore will not be described.

[0041] (Communication sequence in a communication system) 8 is a communication sequence diagram in this embodiment. UE 307 performs data communication with IAB donor 311 via route A (D801, D802). IAB node 305 transmits a measurement report message including measurement information such as RSSI to IAB node 303 (F801). The measurement report message may be transmitted at a predetermined timing or periodically, or in response to a request from IAB node 303. Next, IAB node 303 transmits the received measurement report message to IAB donor 311 by an RRC transfer message (F802).

[0042] The IAB donor 311 determines whether to perform handover from the IAB node 303 to the IAB node 304 based on the measurement report received from the IAB node 305. If it is determined to perform handover, the IAB donor 311 transmits an Xn handover request message to the IAB donor 312 and starts handover preparation. Also, handover preparation is performed for all access UEs and IAB nodes (F803). The IAB donor 312 transmits a UE context setup request message to the target IAB node 304, creates a MT (Mobile Termination (one function on the IAB node)) context, and sets up one or more bearers (F804). The IAB node 304 responds to the IAB donor 312 with a UE context setup response message (F805). The IAB donor 312 responds to the IAB donor 311 with an Xn handover request response message (F806).

[0043] The IAB donor 311 sends a UE context modification request message to the IAB node 303. This message includes an RRC Reconfiguration message generated for the MT of the IAB node 305 (F807). The IAB node 303 forwards the received RRC Reconfiguration message to the IAB node 305.

[0044] The IAB node 305 recognizes (determines) switching to another IAB donor based on the received RRC Reconfiguration message (F808). The IAB node 305 transmits an RRC connection release request message (for example, an RRC Release message standardized in the 3GPP specifications) to the UE 307 (F809, S703). As described in the first embodiment, the RRC connection release request message may include a list (information) of frequencies to be preferentially connected. This allows the UE after RRC release to preferentially connect to frequencies set in the frequency list.

[0045] When the UE 307 receives an RRC connection release request message from the IAB node 305, it executes a release process with the old DU of the IAB node 305 (F810). Also, the IAB donor 311 transmits an SN status transfer message to the IAB donor 312 (F811). The DU on the IAB node 305 loses the F1*-C connection to the IAB donor 311 and stops the service (DU process) (F812). In the conventional method, the UE 307 continues to access the old DU of the IAB node 305, so a wireless problem occurs. In the conventional method, when a wireless problem occurs in the UE 307, a timer called a T1 timer is started from the time of the wireless problem occurrence, and when the T1 timer expires, the UE 307 attempts to connect to a connectable cell, but if the cell connection is not possible, it declares an RLF. On the other hand, in this embodiment, the UE 307 and the IAB node 305 operate as follows.

[0046] The IAB node 305 detects the IAB node 304, establishes synchronization, and performs RACH processing (F813). Then, the IAB node 305 transmits an RRC Reconfiguration Complete message to the IAB node 304 (F814). The DU of the IAB node 304 transmits the RRC Reconfiguration Complete message to the IAB donor 312 via UL RRC transport (F815). The IAB donor 312 performs a path switch procedure for the CN 310 and the IAB node 305 (F816). After the path switch procedure is completed, the IAB donor 312 transmits a context release request message to the IAB donor 311 (F817).

[0047] The IAB donor 312 sets up a new adaptation layer route on the wireless backhaul between the IAB node 305 to be transitioned via the target IAB node 304. As a result, a new route B is established (F818). The DU on the IAB node 305 starts a new F1*-C connection to the IAB donor 312 (F819). As a result, the DU's service is resumed, and the new DU on the IAB node 305 becomes accessible to the UE 307.

[0048] Since the UE 307 is in the RRC_IDLE state (standby), there is no problem in the radio section, and the UE 307 quickly detects (discovers) and selects the DU of the IAB node 305 (F820). After this, the UE 307 establishes synchronization with the IAB node 305 and performs RACH processing (F821). After that, the UE 307 performs processing to re-establish the RRC connection (F822). The IAB donor 312 executes a path switch of the UE 307 using the CN 310 (F823). After this, the UE 307 can resume data communication with the IAB donor 312 (D803, D804). The processing of F824 is similar to F612 in FIG. 6, and therefore description thereof will be omitted.

[0049] Thus, according to this embodiment, when it is decided to switch the IAB donor based on the measurement information by the IAB node 305, the IAB node 305 requests the connected UE 307 to release the RRC connection. As a result, the UE 307 performs an access release process with the DU of the IAB node 305, and then, after the IAB node 305 establishes a route with a new IAB donor, it becomes possible to quickly select a new DU of the IAB node (the DU whose connection destination has been updated to the IAB donor 312) and start a connection. As a result, the UE 307 can connect to the IAB node more quickly in response to a change in the radio state and resume data communication.

[0050] [Other embodiments] The present invention can also be realized by a process in which a program for implementing one or more of the functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., ASIC) that implements one or more of the functions.

[0051] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0052] 310 CN (Core Network), 311-112 IAB donor, 301-305 IAB node, 306 cell area, 307 UE

Claims

1. A communication device that functions as a node relaying a link between a user equipment and a base station, comprising: a determination means for determining whether or not to switch a base station to which the communication device is connected from the first base station to a second base station when the communication device is connected to the first base station; a transmission means for transmitting an RRC Release message to the user equipment in an RRC (Radio Resource Control) Connected state when the determination means determines to switch the base station connected to the user equipment to the second base station; A communication device comprising:

2. detection means for detecting a Radio Link Failure (RLF) in said link between said first base station and a first other communication device acting as another node between said first base station and said communication device; a first connection control means for establishing a connection with a second other communication device functioning as a node in a link other than the link when the RLF is detected by the detection means; and acquiring means for acquiring information of a base station to which the second other communication device is connected from the second other communication device after establishing a connection with the second other communication device, The communication device according to claim 1, characterized in that, when the acquisition means acquires information indicating that the second other communication device is connected to the second base station, the decision means decides to switch the connection between the user equipment and the base station to the second base station.

3. 3. The communication device according to claim 2, wherein the information on the base station to which the second other communication device is connected is a physical cell identifier (PCI).

4. means for transmitting to the first base station a measurement report indicating a received signal strength or quality measured at the communication device on a link between the communication device and the first base station; receiving means for receiving a reconfiguration message from the first base station in response to transmitting the measurement report; The communication device according to claim 1, characterized in that, when it is recognized from the reconfiguration message that the base station connected to the user equipment is to be switched to the second base station, the decision means decides to switch the connection between the user equipment and the base station to the second base station.

5. 5. The communication device according to claim 1, wherein the transmission means transmits a predetermined message to request the user equipment to release the RRC connection.

6. The communication device according to claim 5 , wherein information on a frequency for preferentially connecting to the user equipment is added to the predetermined message.

7. a second connection control means for establishing a connection with the second base station after the RRC Release message is transmitted to the user equipment by the transmission means; 7. The communication device according to claim 1, further comprising: a notification means for, when a connection with the second base station is established by the second connection control means, notifying the user equipment that a connection with the second base station has been established.

8. A communication device that connects to a base station via another communication device that functions as a node, A receiving means for receiving an RRC Release message from the other communication device that has decided to change its connection from the first base station to a second base station in an RRC_CONNECTED state in which the other communication device is connected to a first base station via RRC (Radio Resource Control); a state transition means for changing a state of the communication device from RRC_CONNECTED to RRC_IDLE when the receiving means receives the RRC Release message; A connection control means for detecting the other communication device and initiating a connection in the RRC_IDLE state; A communication device comprising:

9. The communication device according to claim 8, characterized in that when the receiving means receives information from the other communication device indicating that the other communication device has established a connection with the second base station, the connection control means detects the other communication device and initiates a connection.

10. A method for controlling a communication device that functions as a node that relays a link between a user equipment and a base station, comprising the steps of: a determination step of determining whether or not to switch a base station to which the communication device is connected from the first base station to a second base station when the communication device is connected to a first base station; a transmission step of transmitting an RRC Release message to the user equipment in an RRC (Radio Resource Control) CONNECTED state when it is determined in the determination step that the base station connected to the user equipment is to be switched to the second base station; A control method comprising the steps of:

11. A method for controlling a communication device that connects to a base station via another communication device that functions as a node, comprising the steps of: a receiving step of receiving an RRC Release message from the other communication device that has decided to change a connection from the first base station to a second base station in an RRC_CONNECTED state in which the communication device connected to the first base station is RRC-connected; a state transition step of changing a state of the communication device from RRC_CONNECTED to RRC_IDLE when the RRC Release message is received in the receiving step; a connection step of detecting the other communication device and initiating a connection in the RRC_IDLE state; A control method comprising the steps of:

12. A program for causing a computer to function as the communication device according to any one of claims 1 to 9.

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