IAB node, IAB node control method, and program

Abstract

To provide a technique for quickly resuming communication after the occurrence of radio link failure.SOLUTION: A communication device functioning as a node for relaying the link between user equipment and a base station determines, in a state in which the user equipment is RRC (radio resource control) -connected with a first base station through the communication device, whether to switch the base station with which the user equipment is connected through 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, transmits, to the user equipment, a message to request handover including information on a cell identifier for the second base station.SELECTED DRAWING: Figure 4

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 a disconnection between IAB nodes and interrupting communication. When a BH RLF occurs, the IAB node can restore communication by switching the connection to another IAB node that can be connected and re-establishing backhaul communication. Even in cases other than the occurrence of a BH RLF, cases may arise where an already established route needs to be changed due to the influence of deterioration of communication quality between IAB nodes. Patent Document 2 discloses a handover procedure by user equipment from one base station to another base station, which enables more efficient use of spectrum. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2019-534625 [Patent Document 2] Special Publication No. 2017-526281 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, a UE connected to the IAB node starts an RLF detection timer after detecting that the connection with the IAB node has been stopped due to the DU of the IAB node being stopped. After the RLF detection timer expires, the UE transitions its RRC (Radio Resource Control) state to RRC_IDLE and starts a process of reconnecting to the IAB node to reestablish the connection with the IAB node.

[0007] Here, consider the case where the RLF detection timer expires in the UE before the IAB node establishes a connection with the new IAB donor. In this case, the UE starts searching for a base station, but the IAB node is not in a state where it can communicate with the new IAB donor and has not resumed its base station function, so the UE cannot find the IAB node. For this reason, the UE tries to connect to a discovered base station that is not an IAB node, or connects to a base station that can be connected. In such a case, even if the IAB node establishes a connection with the new IAB donor and resumes its base station function, there is a problem that it may take time for the UE to reconnect to the IAB node. In other words, there is a problem that when the IAB node completes switching of the IAB donor to which it is connected, the UE cannot quickly resume data communication, which reduces 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, the present invention IAB (Integrated Access and Backhaul) node The system has the following configuration: IAB Donors It acts as a relay node for the link between IAB Node And, a first other IAB node functioning as a node in a first link between a first IAB donor and the IAB node; a detection means for detecting a radio link failure (RLF) between the IAB node and the first other IAB node; a connection control means for establishing a connection with a second other IAB node functioning as a node in a second link different from the first link when the detection means detects the RLF; an acquisition means for acquiring, from the second other IAB node after establishing a connection with the second other IAB node, information of a cell identifier for an IAB donor to which the second other IAB node is connected; and if the cell identifier information acquired by the acquisition means is a cell identifier for a second IAB donor, The user equipment IAB Node Via The above 1. IAB Donors and the user equipment is in a state of being connected via RRC (Radio Resource Control) to the IAB Node Connect via IAB Donors The first IAB Donors from The above Second IAB Donors a determining means for determining to switch to a connection to which the user equipment is connected by the determining means; IAB Donors The second IAB Donors When it is determined to switch to the second IAB Donorsand a requesting means for transmitting a handover request message including information of a cell identifier for the selected cell. 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] 11 is a flowchart illustrating a process performed by a UE that receives a handover request. [Figure 6] 13 is a flowchart illustrating a process performed by a UE that has received an RRC connection release request. [Figure 7] 3 is a communication sequence according to the first embodiment. [Figure 8] 13 is a flowchart illustrating a process performed by an IAB node according to the second embodiment. [Figure 9] 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) 3 shows a configuration example of a communication system in this embodiment. Nodes 301, 302, 303, 304, and 305 are radio base stations operating as IAB (Integrated Access and Backhaul) nodes, and nodes 311 and 312 are radio base stations operating as IAB donors. The IAB nodes can support a New Radio (NR) access link to a UE in a 5G (5th Generation) mobile communication system and an NR backhaul link to a parent node or a child node.

[0014] A UE (User Equipment) 307 is located within a cell area 306 covered (managed) by an IAB node 305, and can be connected to the IAB node 305. In this embodiment, a path (adaptation layer route) of a backhaul network is managed by an IAB donor. For example, in FIG. 3, an IAB donor 311 manages a route A 308 consisting of IAB nodes 301, 303, and 305 to a UE 307. An IAB donor 312 manages a route B 309 consisting of IAB nodes 302, 304, and 305 to a UE 307.

[0015] 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.

[0016] In FIG. 3, when a BH RLF (Backhaul Radio Link Failure) occurs between the IAB node 305 and the IAB node 303 under the IAB donor 311, the IAB node 305 needs to change the IAB node (parent node) to which it is connected. The IAB node 305 detects the IAB node 304 of a neighboring cell, sets a new backhaul link, and establishes a new route B 309 (IAB donor 312 - IAB node 302 - IAB node 304 - IAB node 305). At this time, the IAB donor that establishes the route with the IAB node 305 is switched from the IAB donor 311 to the IAB donor 312. Therefore, the IAB node 305 and all UEs including the UE 307 connected to the IAB node 305, and further, if an IAB node is connected downstream of the IAB node 305, the downstream IAB node and all UEs connected to that node need to perform handover processing.

[0017] To perform handover, the IAB node 305 stops the active DU (Distributed Unit) connected to the 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 via 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 via 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.

[0018] (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. In FIG. 1, a control unit 101 controls the entire communication device by executing a control program stored in a 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.

[0019] 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.), and creates a measurement report.

[0020] 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 212 can request establishment and release of a connection with the 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.

[0021] (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).

[0022] 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 a broadcast signal transmitted from the IAB node 304 (note that in the following description, the PCI may be referred to as a cell identifier). Then, the switching decision unit 204 of the IAB node 305 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.

[0023] 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 connection control unit 202 determines that there is no IAB donor switching and ends the process. 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.

[0024] In S403, the transmitting / receiving unit 201 of the IAB node 305 acquires information on the number of stages (the number of relays) from the IAB node 304, which is the destination IAB node, to the IAB donor 312, which is the new IAB donor, from the IAB node 304. That is, the transmitting / receiving unit 201 of the IAB node 305 acquires information on the number of stages of nodes relaying between the IAB node 305 and the IAB donor (new IAB donor) to which the IAB node 305 connects. The information on the number of stages may be provided by the IAB node 304, or may be derived by the IAB node 305 based on information provided by nodes such as the IAB node 304. Then, the connection control unit 202 of the IAB node 305 judges whether the number of stages is equal to or greater than a specified value (S403).

[0025] If the number of stages is equal to or greater than the specified value (Yes in S403), the connection control unit 202 transmits a handover request message including a cell identifier to the UE 307 via the transmission / reception unit 201 (S404). In addition, when there are one or more nodes / UEs connected to the IAB node 305 under the control of the IAB node 305, the handover request message may be transmitted to the one or more nodes / UEs. For example, the handover request message may be transmitted to all UEs including the UE 307 connected to the IAB node 305, and when an IAB node is connected downstream of the IAB node 305, the handover request message may be transmitted to the downstream IAB node and all UEs connected to that node. In addition, the handover request message may be transmitted using an RRC Reconfiguration message standardized in the 3GPP specifications, or may be transmitted using a handover request message generated independently.

[0026] On the other hand, if the number of stages is smaller than the specified value (No in S403), the connection control unit 202 transmits a release request message of the RRC connection to the UE 307 via the transmission / reception unit 201 (S405). In addition, when there are one or more nodes / UEs connected to the IAB node 305 under the control of the IAB node 305, the release request message may be transmitted to the one or more nodes / UEs. For example, the release request message may be transmitted to all UEs including the UE 307 connected to the IAB node 305, and when an IAB node is connected downstream of the IAB node 305, the release request message may be transmitted to the downstream IAB node and all UEs connected to that node. In addition, the release request message may be transmitted using an RRC Release message standardized in the 3GPP specifications, or may be transmitted using a release request message generated independently. In addition, a list of frequencies to be preferentially connected may be added to the release request message. As a result, the UE after release can preferentially connect to the frequency set in the frequency list. 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 Figure 3) after sending an RRC connection release request message to UE 307, etc., the connection control unit 202 may notify UE 307 in a lower layer via the transceiver unit 201 that a connection with the new IAB donor has been established.

[0027] The process of S403 may be omitted, and if Yes is determined in S402, the process may proceed to S404 or S405. The condition in S405 may be replaced with another condition for determining whether or not to perform a handover.

[0028] The processing of the UE 307 upon receiving a handover request message including a cell identifier and an RRC connection release request message will be described later with reference to FIG. 5 and FIG. 6, respectively.

[0029] (UE processing) Next, a process of the UE when switching the IAB donor will be described. 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 in an RRC connection with the IAB node 305.

[0030] In the following description, 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 in the IAB node 305. In addition, the UE can transition from an RRC_IDLE (idle) state, which is a standby state, to an RRC_CONNECTED (connected) state (to establish an RRC connection with the base station) by reusing context information.

[0031] 5 is a flow chart showing a process performed when the UE 307 receives a handover request message during an IAB donor switch. The process may be initiated by the UE 307 receiving the handover request message sent by the IAB node 305 at S404 in FIG.

[0032] When the transceiver 211 of the UE 307 receives a handover request message including a cell identifier, the transceiver 211 transmits a message indicating that the message has been received (handover request receipt message) to the old DU of the IAB node 305 (S501). Next, the connection control unit 212 of the UE 307 releases the RRC connection with the old DU of the IAB node 305 that has been accessed (S502). Since the handover request message received by the transceiver 211 includes a cell identifier, the connection control unit 212 of the UE 307 promptly selects a new DU of the IAB node 305 (S503). Next, the connection control unit 212 establishes synchronization with the new DU and performs RACH processing (S504).

[0033] 6 is a flow chart showing a process performed when the UE 307 receives a handover request message during an IAB donor switch. The process may be initiated by the UE 307 receiving the handover request message sent by the IAB node 305 at S404 in FIG.

[0034] 6 is a flow chart showing a process performed when the UE 307 receives an RRC connection release request message during IAB donor switching. The process may be initiated by the UE 307 receiving the RRC connection release request message sent by the IAB node 305 at S404 in FIG.

[0035] When the transceiver 211 of the UE 307 receives an RRC connection release request message from the IAB node 305, the connection control unit 212 releases the RRC connection with the old DU of the IAB node 305 that was being accessed (S601). 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 (S602). 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. 8, 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 (F705 in FIG. 7). 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.

[0036] 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 (S603). 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 (S604). 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.

[0037] (Communication sequence in a communication system) 7 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 (D701, D702). 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 (F701).

[0038] The IAB node 305 loses the parent node of the connection destination due to a wireless failure in the backhaul, but establishes synchronization and performs RACH processing to connect to the IAB node 304 as the new parent node (F702). The synchronization establishment and RACH processing are similar to the procedure in which a normal UE finds a destination cell by cell search and establishes a connection.

[0039] The IAB node 305 starts an RRC connection reestablishment with the IAB donor 312, which is the new IAB donor (F703). 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 (F704). Next, the IAB node 305 starts an F1-C connection with the IAB donor 312 by the DU setup procedure (F705). As a result, the new DU of the IAB node 305 is connected to the IAB donor 312 and can access the UE 307. In addition, in the process up to this point, the IAB node 305 acquires information on the number of stages from the IAB node 304 to the IAB donor 312, and assumes that the number of stages is equal to or greater than a specified value (Yes in S403 in FIG. 4).

[0040] The IAB node 305 transmits a handover request message including the cell identifier to the UE 307 using an RRC Reconfiguration message (F706). The cell identifier may be included in a CellGroupConfig element. The UE 307 transmits a handover request acceptance message to the IAB node 305 using an RRC Reconfiguration Complete message (F707). The UE 307 executes an access (RRC connection) release process with the old DU of the IAB node 305 (F708).

[0041] Next, the IAB node 305 stops the old DU process through the connection with the IAB donor 311 (F709), and starts the new DU process through the connection with the IAB donor 312 (F710). At this time, the IAB node 305 may notify the UE 307 that the new DU process has started. The UE 307 detects (discovers) and selects the new DU of the IAB node 305 by the cell identifier information included in the handover request message (F711). After this, the UE 307 establishes synchronization with the IAB node 305 and performs RACH processing (F712). After that, the UE 307 executes the process of re-establishing the RRC connection with the IAB donor 312 (F713). When the process of re-establishing the RRC connection is completed, the UE 307 can resume data communication with the IAB donor 312 (D703, D704).

[0042] 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 (F714).

[0043] Thus, according to this embodiment, when the IAB node 305 detects a BH RLF, it establishes a connection in a new route (connection with a new IAB donor), then transmits a handover request message including a cell identifier to the UE 307, and stops the DU process. Meanwhile, the UE 307 performs an access release process with the DU of the IAB node 305, and can then promptly start a connection with the IAB node 305 based on the cell identifier included in the received handover request message. 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.

[0044] [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.

[0045] (IAB node processing) The process of the IAB node according to whether or not the IAB donor is switched will be described. Fig. 8 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.

[0046] 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) (S801). 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. 9.

[0047] 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 S802), it decides to switch the IAB donor, and the process proceeds to S803. In S803, the IAB node 305 transmits a handover request message including a cell identifier to the UE 307 after completing the connection with the new IAB donor. Note that the handover request message may be transmitted to all UEs including the UE 307 connected to the IAB node 305, and, if an IAB node is connected downstream of the IAB node 305, to the downstream IAB node and all UEs connected to that node. Also, the handover request message may be transmitted using an RRC Reconfiguration message standardized in the 3GPP specifications, or may be transmitted using a handover request message generated independently.

[0048] On the other hand, if 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 (No in S802), it ends the process.

[0049] (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.

[0050] (Communication sequence in a communication system) 9 is a communication sequence diagram in this embodiment. Assume that UE 307 is RRC connected on route A. UE 307 performs data communication with IAB donor 311 via route A (D901, D902). IAB node 305 transmits a measurement report message including measurement information such as RSSI to IAB node 303 (F901). The measurement report message may be transmitted at a predetermined timing or periodically, or may be transmitted 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 (F902).

[0051] Based on the measurement report received from the IAB node 305, the IAB donor 311 checks whether a link failure has occurred, and determines whether to perform a handover from the IAB node 303 to the IAB node 304. If it is decided to perform a 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 (F903). The IAB donor 312 transmits a UE context setup request message to the target IAB node 304, creates an MT (Mobile Termination (one function on the IAB node)) context, and sets up one or more bearers (F904). The IAB node 304 responds to the IAB donor 312 with a UE context setup response message (F905). The IAB donor 312 responds to the IAB donor 311 with an Xn handover request response message (F906).

[0052] The IAB donor 311 transmits 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 (F907). The IAB node 303 forwards the received RRC Reconfiguration message to the IAB node 305.

[0053] The IAB node 305 recognizes (determines) switching to another IAB donor based on the received RRC Reconfiguration message (F908). In addition, the IAB donor 311 transmits an SN status transfer message to the IAB donor 312 (F909).

[0054] The IAB node 305 detects the IAB node 304, establishes synchronization, and performs RACH processing (F910). Then, the IAB node 305 transmits an RRC Reconfiguration Complete message to the IAB node 304 (F911). The DU of the IAB node 304 transmits the RRC Reconfiguration Complete message to the IAB donor 312 via UL RRC transport (F912). The IAB donor 312 performs a path switch procedure for the CN 310 and the IAB node 305 (F913). After the path switch procedure is completed, the IAB donor 312 transmits a context release request message to the IAB donor 311 (F914).

[0055] The IAB donor 312 sets a new adaptation layer route on the wireless backhaul between the IAB node 305 to which the IAB donor 312 is to be transferred via the target IAB node 304. As a result, a new route B is established (F915). The DU on the IAB node 305 starts a new F1*-C connection to the IAB donor 312 (F916). As a result, the DU's service is resumed, and the new DU of the IAB node 305 becomes accessible to the UE 307. In addition, in the process up to this point, the IAB node 305 acquires information on the number of stages from the IAB node 304 to the IAB donor 312, and assumes that the number of stages is equal to or greater than a specified value (Yes in S403 in FIG. 4).

[0056] The IAB node 305 transmits an RRC Reconfiguration message to the UE 307 as a handover request message including a cell identifier (F917). The cell identifier may be included in a CellGroupConfig element. The UE 307 transmits an RRC Reconfiguration Complete message to the IAB node 305 as a handover request acceptance message (F918). The UE 307 executes an access (RRC connection) release process with the old DU of the IAB node 305 (F919).

[0057] Next, the IAB node 305 stops the old DU process by connecting with the IAB donor 311 (F920), and starts the new DU process by connecting with the IAB donor 312 (F921). The UE 307 selects a new DU of the IAB node 305 based on the cell identifier information included in the handover request message (F922). After this, the UE 307 establishes synchronization with the IAB node 305 and performs RACH processing (F923). After that, the UE 307 performs re-establishment processing of the RRC connection (F924). The IAB donor 312 executes a path switch of the UE 307 using the CN 310 (F925). After this, the UE 307 can resume data communication with the IAB donor 312 (D903, D904). The processing of F926 is the same as F714 in FIG. 7, and therefore will not be described.

[0058] 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 establishes a connection with the new IAB donor. Next, the IAB node 305 transmits a handover request message including a cell identifier to the UE 307 and stops the DU process. On the other hand, the UE 307 performs an access release process with the DU of the IAB node 305, and then can promptly start a connection with the IAB node 305 based on the cell identifier included in the received handover request message. As a result, the UE 307 can connect to the IAB node more quickly and resume data communication in response to changes in the radio conditions.

[0059] [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.

[0060] 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]

[0061] 310 CN (Core Network), 311-112 IAB donor, 301-305 IAB node, 306 cell area, 307 UE

Claims

1. An IAB node that functions as a node that relays a link between a user equipment and an IAB donor, a first other IAB node acting as a node in a first link between a first IAB donor and said IAB node, and detection means for detecting a radio link failure (RLF) between said IAB node and said other IAB node; a connection control means for establishing a connection with a second other IAB node functioning as a node in a second link different from the first link when the RLF is detected by the detection means; an acquiring means for acquiring, after establishing a connection with the second other IAB node, from the second other IAB node, information of a cell identifier for an IAB donor to which the second other IAB node is connected; a decision means for deciding to switch an IAB donor to which the user equipment is connected via the IAB node from the first IAB donor to the second IAB donor in a state in which the user equipment is RRC-connected to the first IAB donor via the IAB node, when the information on the cell identifier acquired by the acquisition means is a cell identifier for a second IAB donor; a requesting means for transmitting, when the determining means determines to switch the IAB donor to which the user equipment is connected to the second IAB donor, to the user equipment, a handover request message including information of a cell identifier for the second IAB donor; 1. An IAB node comprising:

2. measurement means for generating a measurement report indicative of received signal strength and / or quality measured on a link between said IAB node and said first IAB donor; a transmitting means for transmitting the measurement report to the first IAB donor; receiving means for receiving a reconfiguration message from the first IAB donor in response to transmitting the measurement report; 2. The IAB node according to claim 1, wherein the determination means determines to switch a connection between the user equipment and the IAB donor to the second IAB donor when the reconfiguration message recognizes that the IAB donor connected to the user equipment is to be switched to the second IAB donor.

3. 3. The IAB node according to claim 1, wherein the requesting means transmits the handover request message using an RRC reconfiguration message.

4. 4. The IAB node of claim 3, wherein in the handover request message, a cell identifier for the second IAB donor is included in a CellGroupConfig element.

5. 5. The IAB node according to claim 1, wherein when a number of stages of relay nodes between the IAB node and the second IAB donor is equal to or greater than a specified value, the requesting means transmits to the user equipment a handover request message including information of a cell identifier for the second IAB donor.

6. 6. The IAB node according to claim 1, wherein when there are a plurality of other IAB nodes connected to the IAB node under the control of the IAB node, the requesting means transmits the handover request message to the plurality of other IAB nodes.

7. The IAB node according to claim 5, characterized in that, when the number of stages is smaller than the specified value, the requesting means transmits, to the user equipment, a release request message for releasing an RRC connection with the IAB node.

8. 8. The IAB node according to claim 7, wherein, when there are a plurality of other IAB nodes connected to the IAB node under the control of the IAB node, the requesting means transmits a release request message for the RRC connection to the plurality of other IAB nodes.

9. A method for controlling an IAB node that functions as a node that relays a link between a user equipment and an IAB donor, comprising: a detection step of detecting a radio link failure (RLF) between a first other IAB node functioning as a node in a first link between a first IAB donor and the IAB node and the IAB node; a connection control step of establishing a connection with a second other IAB node functioning as a node in a second link different from the first link when the RLF is detected in the detection step; acquiring, after establishing a connection with the second other IAB node, from the second other IAB node, information of a cell identifier for an IAB donor to which the second other IAB node is connected; a determining step of determining, when the information on the cell identifier acquired in the acquiring step is a cell identifier for a second IAB donor, to switch an IAB donor to which the user equipment is connected via the IAB node from the first IAB donor to the second IAB donor in a state in which the user equipment is RRC-connected to the first IAB donor via the IAB node; a request step of transmitting, to the user equipment, a handover request message including information of a cell identifier for the second IAB donor, when it is determined in the determination step that the IAB donor to which the user equipment is connected is switched to the second IAB donor; A control method comprising the steps of:

10. A program for causing a computer to function as the IAB node according to any one of claims 1 to 8.

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