Terminal device, base station device, and control method for efficiently updating security key

Abstract

To efficiently update parameters related to a security key in handover using LTM.SOLUTION: A terminal device conforming to a 3GPP cellular communication standard: receives, from a first base station device being connected, first configuration information of LTM, for each of a plurality of base station devices, of handover destination candidates including a second base station device and a third base station device, and second configuration information associated with the first configuration information and for updating a security key; holds the first configuration information and the second configuration information in association with each of the plurality of base station devices; executes handover to the second base station device; updates the security key by using the second configuration information associated with the first configuration information of the second base station device in response to the handover to the second base station device; and updates the second configuration information associated with the first configuration information while maintaining the held first configuration information of the third base station device after the execution of the handover.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a technique for updating security keys in a cellular communication system. [Background technology]

[0002] In cellular communication standards such as the fifth generation (5G) standard compliant with the 3rd Generation Partnership Project (3GPP (registered trademark)), a Layer 1 / Layer 2 Triggered Mobility (LTM) mechanism has been introduced to shorten the time required for handover. Conventional LTM is applied to handovers within a common gNB-Central Unit (CU), but not to handovers across gNB-CUs. For this reason, conventional LTM does not require updating parameters related to security keys for encryption and integrity protection. Summary of the Invention [Problem to be solved by the invention]

[0003] In the future, it is expected that LTM will be applied to handovers across gNB-CUs. In this case, it will be necessary to update parameters related to the security keys as described above. [Means for solving the problem]

[0004] The present invention provides an efficient technique for updating parameters related to security keys during handover via LTM.

[0005] A terminal device according to one embodiment of the present invention is a terminal device that complies with the cellular communication standard of the 3rd Generation Partnership Project (3GPP), and has: a receiving means for receiving, from a first base station device to which it is currently connected, first setting information for Layer 1 / Layer 2 Triggered Mobility (LTM) for each of a plurality of base station devices that are candidate handover destinations, including a second base station device and a third base station device, and second setting information associated with the first setting information for updating a security key; a holding means for associating and holding the first setting information and the second setting information for each of the plurality of base station devices; an execution means for executing a handover to the second base station device; and an update means for, in response to a handover to the second base station device, updating the security key using the second setting information associated with the first setting information of the second base station device, and, after the handover is executed, updating the second setting information associated with the first setting information while maintaining the held first setting information for the third base station device.

[0006] A base station device according to one embodiment of the present invention is a base station device that complies with the cellular communication standard of the 3rd Generation Partnership Project (3GPP), and has: a providing means for providing, to a first other base station device to which a terminal device is connected, as a candidate for a handover destination, first setting information for Layer 1 / Layer 2 Triggered Mobility (LTM) and second setting information associated with the first setting information for updating security keys; a connecting means for connecting to the terminal device through handover of the terminal device; a generating means for generating a security key to be used in communication with the terminal device using the second setting information; an acquiring means for acquiring, after the handover is executed, the first setting information and the second setting information for a second other base station device that is a candidate for a handover destination of the terminal device from the base station device; and a transmitting means for, if the terminal device holds the first setting information for the second other base station device, transmitting a message to the terminal device that does not include the first setting information but includes the second setting information for the second other base station device. [Effects of the Invention]

[0007] According to the present invention, parameters related to security keys can be updated efficiently during handover using LTM. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of a wireless communication system. [Figure 2] FIG. 1 is a diagram illustrating an example of a conventional process related to updating a security key. [Figure 3] FIG. 1 is a diagram illustrating an example of a conventional process related to updating a security key. [Figure 4] FIG. 10 is a diagram illustrating an example of processing related to updating a security key according to the present embodiment. [Figure 5] FIG. 10 is a diagram illustrating an example of processing related to updating a security key according to the present embodiment. [Figure 6]FIG. 2 is a diagram illustrating an example of the hardware configuration of a base station device and a terminal device. [Figure 7] FIG. 2 is a diagram illustrating an example of a functional configuration of a terminal device. [Figure 8] FIG. 2 is a diagram illustrating an example of a functional configuration of a base station device. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be arbitrarily combined. Furthermore, the same reference numerals are used for the same or similar components, and redundant explanations will be omitted.

[0010] Fig. 1 shows an example of the configuration of a wireless communication system according to this embodiment. This wireless communication system is a cellular communication system that complies with the cellular communication standard of the Third Generation Partnership Project (3GPP (registered trademark)), and is configured to include base station devices (e.g., base station device 101, base station device 102) and terminal devices (e.g., terminal device 111). It is assumed that terminal device 111 is currently connected to base station device 101, and then moves toward base station device 102 and executes handover to base station device 102. It is to be noted that, for the sake of simplicity, Fig. 1 shows only two base station devices and one terminal device, but it goes without saying that a large number of these devices may exist.

[0011] In such a wireless communication system, when the terminal device 111 performs a handover, the processing flow related to the security key may differ depending on whether the base station device 101 and the base station device 102 are associated with a common Access and Mobility Management Function (AMF) (or whether an Xn interface is established). This difference in procedure will be explained using the examples of Figures 2 and 3. Note that Figures 2 and 3 explain in detail the parts particularly related to this embodiment, and only provide an overview of other matters.

[0012] FIG. 2 shows an example of a processing flow in the case where a handover source base station device (base station device 101) and a handover destination base station device (base station device 102) are associated with a common AMF and an Xn interface is established between these base station devices. In this processing, for example, when the conditions for starting a handover are satisfied in the terminal device 111, a measurement report is notified from the terminal device 111 to the base station device 101 (S201). When the base station device 101 determines to hand over the terminal device 111 to the base station device 102 based on this measurement report, it uses the pre-handover parameter K NG-RAN * Based on parameters such as the nextHopChainingCount (NCC) and the like, the K NG-RAN * Then, the base station device 101 transmits a HANDOVER REQUEST message to the base station device 102 (S203). At this time, the HANDOVER REQUEST message contains the derived K NG-RAN *and the NCC value used when deriving that value. Note that, as with a normal handover, processing such as notification from the base station device 101 to the terminal device 111 of setting information for connection with the handover destination base station device 102 is performed between the terminal device 111 and the base station device 101 (not shown). Then, the terminal device 111 executes a random access procedure with the base station device 102 and establishes connection with the base station device 102. At that time, the base station device 102 sets keySetChangeIndicator to False and transmits an RRC Reconfiguration message including the NCC value to the terminal device 111 (S204). Note that RRC is an abbreviation for Radio Resource Control. As a result, the terminal device 111 can obtain the NCC value and the K value used to generate the security key for communication with the base station device 101 that is held within the terminal device. NG-RAN * Using parameters such as these, K used to generate a security key for communication with the base station device 102 is calculated. NG-RAN * In response to this, the terminal device 111 transmits an RRC Reconfiguration Complete message to the base station device 102 (S205). Then, the base station device 102 transmits an NGAP PATH SWITCH REQUEST message to the AMF associated with the base station device 102 (S206). In response to this message, the AMF transmits an NGAP PATH SWITCH REQUEST ACKNOWLEDGE message to the base station device 102 (S207). This message contains the K NG-RAN * Next Hop (NH) and NCC are included as parameters that enable the derivation of K. The base station device 102 stores the received NH and NCC (S208). In this way, when the base station device 101 and the base station device 102 are associated with a common AMF, the terminal device 111 uses the value of NCC acquired from the base station device 102 and the stored KNG-RAN * and K for generating a security key to be used in communication with the base station device 102. NG-RAN * can be updated.

[0013] FIG. 3 shows an example of a processing flow when a handover source base station device (base station device 101) and a handover destination base station device (base station device 102) are associated with different AMFs. In this processing, similar to the example of FIG. 2, a measurement report is transmitted from the terminal device 111 to the base station device 101 (S301), and handover processing is initiated based on the measurement report. The base station device 101 transmits an NGAP HANDOVER REQUIRED message to the AMF with which the base station device 101 is associated, and the handover request is transferred to another AMF with which the handover destination base station device 102 is associated (S302). Then, the handover destination AMF transmits an NGAP HANDOVER REQUEST message to the handover destination base station device 102 (S303). Note that this message includes K NG-RAN * The information for newly generating the NAS includes a NAS container, an NH, and an NCC. NAS is an abbreviation for Non-Access Stratum. The base station device 102 stores the received NH and NCC (S304), and generates the K based on the received information. NG-RAN * In other words, in the process of FIG. NG-RAN * is newly generated and the K used before the handover is NG-RAN * is not used to generate a new key. After that, the base station device 102 transmits an NGAP HANDOVER REQUEST ACKNOWLEDGE to the AMF (S306), and in response, an NGAP HANDOVER COMMAND is transmitted to the base station device 101 via the AMF of the handover source (S307). At this time, K NG-RAN *is transmitted to the terminal device 111 via the base station device 101 via an RRC Reconfiguration message (S308). Thereafter, the terminal device 111 executes a random access procedure or the like with the base station device 102 to complete the connection (S309).

[0014] In addition, the same K NG-RAN * is generated and its K NG-RAN * , and then the key K for concealing the user data is UPsec is generated.

[0015] Here, the terminal device 111 according to this embodiment changes the base station device to which it is connected using Layer 1 / Layer 2 Triggered Mobility (LTM) for shortening the time required for handover. LTM may be interpreted as an abbreviation for Lower Layer Triggered Mobility. Conventionally, LTM has been applied only to handovers between base station devices connected to a common gNB-Central Unit (CU). For this reason, in a handover using conventional LTM, configuration information (parameters) related to security keys are not updated. In contrast, in a handover across gNB-CUs, it is necessary to update the configuration information related to security keys. This embodiment provides a procedure for efficiently updating configuration information (parameters) for generating security keys in a handover using LTM across gNB-CUs.

[0016] Figure 4 is a diagram showing an example of a handover procedure across gNB-CUs according to this embodiment. Figure 4 shows an example in which there are two candidate base station devices as handover destinations. These candidate base station devices as handover destinations are assumed to be associated with an AMF different from that of the connection destination base station device. For this reason, although the AMF is omitted in the example of Figure 4, each base station device is assumed to perform processing involving communication with the AMF as in Figures 2 and 3.

[0017] In this process, a base station device (connected to a terminal device) that is the handover source transmits a HANDOVER REQUEST message to multiple other base station devices that are candidate handover destinations, for example, based on measurement results by the terminal device (S401). The candidate base station devices that are candidate handover destinations then return a HANDOVER REQUEST ACK message in response to the message (S402, S403). This HANDOVER REQUEST ACK message includes configuration information for LTM (LTM configuration), which includes MasterKeyUpdate, which is information for updating security keys. This is just an example, and the LTM configuration may not include MasterKeyUpdate, and each may be notified and managed as separate information. However, the LTM configuration and MasterKeyUpdate are assumed to be associated with each other. Here, MasterKeyUpdate includes a KeySetChangeIndicator (set to True), a NAS container, and an NCC, for example, when an associated AMF is changed. Furthermore, if the associated AMF is not changed, the MasterKeyUpdate includes a KeySetChangeIndicator (set to False) and an NCC. The handover source base station device transfers the LTM configuration including the MasterKeyUpdate to the terminal device (S404). Then, when the terminal device receives the information, it transmits a response message to the handover source (connected) base station device (S405). Note that the transfer of the LTM configuration in S404 can be performed using, for example, an RRC Reconfiguration message, and the response to that message can be performed using an RRC Reconfiguration Complete message.

[0018] Thereafter, based on the measurement results (L1 measurement) of Layer 1 (physical layer) measured by the terminal device, the base station device to which the terminal device is connected decides to execute handover, and the base station device transmits a Cell Switch Command instructing the terminal device to execute handover (S406, S407). The terminal device executes a random access procedure etc. with the base station device of the handover destination instructed by the command (S408, S409), and establishes a connection with the base station device using the LTM configuration for that base station device. At this time, the terminal device also uses the MasterKeyUpdate information included in the LTM configuration to NG-RAN * For example, when a terminal device receives a MasterKeyUpdate containing a KeySetChangeIndicator (set to True), a NAS container, and an NCC, it updates the K NG-RAN * Regardless of the information received, a new K NG-RAN * In addition, when the terminal device receives a MasterKeyUpdate containing a KeySetChangeIndicator (set to False) and an NCC, it generates the K NG-RAN * and NCC to be used after handover. NG-RAN * Generate.

[0019] Here, a method for updating the setting information related to the security key after handover will be described. First, after connection with the handover destination base station, if the candidate base station for handover from that base station is associated with a common AMF, that is, if the handover destination base station (the base station that will be the communicating base station after handover) is associated with K NG-RAN *An example of processing for generating the RRC Reconfiguration Complete message will be described below. When the handover destination base station device receives the RRC Reconfiguration Complete message from the terminal device, it transmits a HANDOVER SUCCESS message indicating that the handover has been successful to the handover source base station device (S410). At this time, the HANDOVER SUCCESS message contains the K NG-RAN * and NCC (i.e., K generated based on the MasterKeyUpdate included in the LTM configuration of the device) NG-RAN * The handover source base station notifies another candidate base station (candidate base station #2 in the example of FIG. 4) that the base station to which the terminal device is connected has changed (in the example of FIG. 4, the connection destination has changed to candidate base station #1). At this time, the handover source base station notifies the other ... K included in HANDOVER SUCCESS has changed. NG-RAN * The candidate base station transmits the information of the MasterKeyUpdate and NCC to the other candidate base stations and requests that the LTM configuration including the updated MasterKeyUpdate be transferred to the target base station (S411). In response to the request, the candidate base station updates the MasterKeyUpdate and transmits the LTM configuration including the MasterKeyUpdate information to the target base station (S412).

[0020] Next, after connection with the handover destination base station device, if the candidate base station device of the handover destination from that base station device is associated with a different AMF, that is, if the candidate base station device after the handover (the base station device that will continue to be the candidate base station device after the handover) is K NG-RAN *An example of processing for generating the RRC Reconfiguration Complete message will be described below. When the handover destination base station device receives the RRC Reconfiguration Complete message from the terminal device, it transmits a HANDOVER SUCCESS message indicating that the handover has been successful to the handover source base station device (S410). At this time, the HANDOVER SUCCESS message contains the K NG-RAN * and NCC may not be included. The handover source base station device notifies the candidate base station device that the terminal device is connected to that has changed (in the example of FIG. 4, the connection destination has changed to candidate base station device #1) via the first AMF associated with the handover source base station device and the second AMF associated with the base station device that was not selected as the handover destination and will remain a candidate base station device after the handover. At this time, the second AMF associated with the candidate base station device is NG-RAN * The candidate base station notifies the candidate base station of the NAS container, NH, and NCC as information for newly generating the MasterKeyUpdate. The candidate base station then updates the MasterKeyUpdate using the NAS container, NH, and NCC, and transmits the LTM configuration including the MasterKeyUpdate information to the target base station via the second AMF associated with the candidate base station and the third AMF associated with the target base station (candidate base station #1 in the example of FIG. 4).

[0021] The candidate base station determines the content of the MasterKeyUpdate, for example, by determining whether it is connected to a common AMF with the handover destination base station (i.e., whether an Xn interface is established). For example, if the candidate base station is associated with a common AMF with the handover destination base station, it may generate a MasterKeyUpdate that includes a keySetChangeIndicator set to False and the notified NCC. If the candidate base station is associated with an AMF different from that of the handover destination base station, it may acquire a NAS container, NH, and NCC by, for example, querying the AMF with which it is associated, and generate a MasterKeyUpdate that includes a keySetChangeIndicator, NAS container, and NCC that are set to True. The candidate base station then transmits the generated MasterKeyUpdate to the handover destination base station.

[0022] When the handover destination base station (candidate base station #1 in the example of FIG. 4) receives MasterKeyUpdate update information from the candidate base station, it transmits the received MasterKeyUpdate information to the terminal device in a format in which it associates it with the LTM configuration of the candidate base station (S413). When the terminal device receives the MasterKeyUpdate, it updates the MasterKeyUpdate information included in the LTM configuration of the candidate base station (candidate base station #2 in the example of FIG. 4) with the received information. Then, after completing the MasterKeyUpdate update, the terminal device transmits a response message to the handover destination base station (S414). In one example, an RRC Reconfiguration message is used when transmitting the information in S413, and the terminal device can transmit RRC Reconfiguration Complete to the handover destination base station as the response message in S414. However, this is just an example, and other messages may be used.

[0023] In this processing example, the terminal device receives parameters (MasterKeyUpdate) related to security keys as part of the LTM configuration. Then, when the terminal device performs a handover using LTM, it can reuse the LTM configuration of a candidate base station that was not selected as the handover destination. However, considering that the setting information related to security keys in the LTM configuration should be updated, the setting information can be acquired via the handover destination base station. This allows the terminal device to efficiently update the setting information related to security keys for candidate base stations that should still be treated as handover destination candidates after the handover, while maintaining most of the LTM configuration.

[0024] While FIG. 4 illustrates an example in which only one candidate base station device is not selected in the handover, it is naturally assumed that there are multiple such candidate base station devices. In this case, when the handover destination base station device notifies the configuration information related to the security key (MasterKeyUpdate), information indicating which candidate base station device the configuration information relates to is also notified. For example, if identification information (LTM configuration ID) is assigned to each LTM configuration configured in the terminal device, the configuration information related to the security key may be associated with the identification information and notified. In one example, when the MasterKeyUpdate of only some candidate base station devices is updated, the LTM configuration ID of the candidate base station device and the MasterKeyUpdate of the candidate base station device are associated and notified to the terminal device. Note that the identification information of the base station device may be associated with the configuration information related to the security key and notified. In other words, any identification information may be used as long as it can identify the configuration information related to the security key to be updated. Furthermore, when the configuration information related to all candidate base station devices is notified, the configuration information may be arranged and notified in a predetermined order, for example, ascending or descending order of the LTM configuration ID. In this case, the order in which the configuration information is notified can be used to identify which candidate base station device (LTM configuration id) the configuration information relates to, so there is no need to explicitly notify the identification information.

[0025] Although the above description describes processing related to a master cell group (MCG), similar processing may also be performed for a secondary cell group (SCG). That is, for a base station device that provides a candidate cell for a secondary cell, information on a parameter SK-Counter related to a security key for the SCG may be included in the LTM configuration and notified to the terminal device. Then, after a connection destination cell is changed by a Cell Switch Command, updated information related to the SK-Counter in the LTM configuration may be notified to the terminal device from the base station device that provides the new connection destination cell. Also, the LTM configuration may include both a MasterKeyUpdate for the MCG and an SK-Counter for the SCG. In this case, after a handover based on the LTM configuration is performed, the terminal device and the base station device will communicate using Dual Connectivity using both the master cell and the secondary cell, and MasterKeyUpdate is used to generate a security key for the master cell, and SK-Counter is used to generate a security key for the secondary cell. Furthermore, after the handover is executed, if there is an LTM configuration that was not selected and that LTM configuration includes both a MasterKeyUpdate for the MCG and an SK-Counter for the SCG, the update information for both the MasterKeyUpdate and the SK-Counter is notified to the terminal device after the handover.

[0026] 4 shows an example in which a handover source base station transmits a message to a candidate base station that was not selected as the handover destination, requesting the transfer of an LTM configuration to the handover destination base station. However, this is just one example, and for example, the handover destination base station may request the candidate base station to transmit an LTM configuration. An example of the processing flow in this case is shown in FIG. 5. In the example of FIG. 5, the same processes as those in FIG. 4 are assigned the same reference numerals, and their explanations will be omitted.

[0027] In this process, when the handover source base station receives a HANDOVER SUCCESS message from the handover destination base station, it transfers the LTM configuration of the candidate base station that was not selected as the handover destination, which was received in S403, to the handover destination base station (S501). Note that this transfer of the LTM configuration can be performed by a HANDOVER SUCCESS ACK message, which is a response to the HANDOVER SUCCESS message. However, this is just one example, and the LTM configuration can be transferred by another message. The handover destination base station then requests setting information for updating the security key from the candidate base station that was not selected as the handover destination and that corresponds to the received LTM configuration (S502). The handover destination base station can request MasterKeyUpdate, for example, using a HANDOVER REQUEST message. Note that this message also includes the K NG-RAN *, may include information about the NCC held by the base station device. The message may also include information indicating that the LTM configuration corresponding to the configuration information for updating the security key to be acquired has already been notified to the terminal device (by the handover source base station device). The candidate base station device recognizes that the configuration information for the security key needs to be updated for the LTM configuration previously notified to the handover source base station device, updates it, and transmits the updated configuration information to the handover destination base station device (S503). At this time, information included in the LTM configuration other than MasterKeyUpdate does not need to be transmitted to the handover destination base station device. The candidate base station device may transmit the updated configuration information to the handover destination base station device, for example, using a HANDOVER REQUEST ACK message. Upon acquiring MasterKeyUpdate, the handover destination base station device transfers the information to the terminal device (S413).

[0028] In this way, in the process of Fig. 5, the terminal device can be notified of updated setting information related to the security key, as in the case of Fig. 4. In the above embodiment, when the base station device of the handover destination and the candidate base station device after the handover are associated with a common AMF, that is, when the base station device of the handover destination (the base station device that will be the communicating base station device after the handover) is associated with K NG-RAN * The process for generating K has been described. However, if the candidate base station after the handover (the base station that will continue to be the candidate base station after the handover) is K NG-RAN *In other words, the handover destination base station device and the candidate base station device after handover may be associated with different AMFs. In this case, the handover destination base station device notifies the candidate base station device (candidate base station device #2 in the example of FIG. 5) that the base station device to which the terminal device is connected has changed (in the example of FIG. 5, the connection destination has changed to candidate base station device #1) via the first AMF to which that base station device is associated and the second AMF to which the candidate base station device that will continue to be the handover destination is associated. At this time, the second AMF is NG-RAN * The candidate base station device notifies the candidate base station device of the NAS container, NH, and NCC as information for newly generating the MasterKeyUpdate. Thereafter, the candidate base station device (candidate base station device #2 in the example of FIG. 5) updates the MasterKeyUpdate using the NAS container, NH, and NCC, and transmits the MasterKeyUpdate information or an LTM configuration including the MasterKeyUpdate information to the handover destination base station device (candidate base station device #1 in the example of FIG. 5) via the second AMF and the first AMF. Then, upon acquiring the MasterKeyUpdate, the handover destination base station device transfers the information to the terminal device (S413).

[0029] In the above-described embodiment, an example has been described in which the LTM configuration is transmitted including configuration information (MasterKeyUpdate) for updating a security key. However, this is not limiting. For example, the LTM configuration may not include the configuration information, and the configuration information may be included in the Cell Switch Command of S407. For example, when the handover source base station device and the handover destination base station device are associated with a common AMF, as shown in S204 of FIG. 2, the NAS container may not be notified to the terminal device, and NCC information may be notified to the terminal device. In such a case, the handover source base station device may transmit the LTM configuration without including the configuration information, and may transmit the Cell Switch Command including the configuration information (NCC). In this case, the keySetChangeIndicator may be omitted. Furthermore, when a handover is performed using a Cell Switch Command, the LTM configuration ID or information capable of identifying the LTM configuration ID is included in the Cell Switch Command. The terminal device identifies the handover destination base station device using the LTM configuration ID or information capable of identifying the LTM configuration ID. The terminal device uses the NCC included in the Cell Switch Command to communicate with the base station device of the handover destination. NG-RAN *is updated. Note that the Cell Switch Command may include the NCC value itself, or may include other information that can identify the NCC. For example, identification information may be assigned in advance to each of multiple NCCs by the LTM configuration, and the identification information may be included in the Cell Switch Command. According to this, identification information for identifying the NCC is transmitted in the LTM configuration that is notified by RRC layer signaling in an environment where encryption and the like are performed, and only the identification information is transmitted by the Cell Switch Command that is not encrypted and the like, so that even if the Cell Switch Command is intercepted, the NCC can be prevented from being identified by an interceptor.

[0030] Note that, for some candidate base station devices, the LTM configuration may include configuration information for security key update as described above, and for other candidate base station devices, the LTM configuration may not include the configuration information for security key update and the configuration information may be notified by the Cell Switch Command. Furthermore, even if the LTM configuration includes the configuration information for security key update, (at least a part of) the configuration information may be transmitted by the Cell Switch Command. In this case, the configuration information included in the LTM configuration may be updated and used by the configuration information specified by the Cell Switch Command. In one example, a first base station device, which is a handover source, and a second base station device, which is a handover destination, are associated with different AMFs, and the second base station device and a third base station device, which is a candidate and not selected as the handover destination, are associated with a common AMF. In this case, before handover from the first base station device to the second base station device, MasterKeyUpdate is associated with the LTM configurations for the second base station device and the third base station device and notified to the terminal device, as described above. In this case, the MasterKeyUpdate includes the keySetChangeIndicator set to True, the NAS container, and the NSS. After that, when a handover from the first base station to the second base station is performed, the MasterKeyUpdate of the third base station becomes outdated and needs to be updated. On the other hand, when a handover from the second base station to the third base station is performed, information such as the NAS container is not required to update the security key; only the NCC information is required. Therefore, after a handover from the first base station to the second base station is performed, the MasterKeyUpdate notification for the third base station may be omitted, and information that can identify the NCC may be notified by a subsequent Cell Switch Command.With this, the terminal device will continue to retain old information about the third base station device, but will be able to identify the NCC using the Cell Switch Command and use that NCC to update the security key without using the old information.

[0031] Note that if the LTM configuration includes only the configuration information of the SCG, when a secondary cell corresponding to that LTM configuration is added, the security key for the MCG does not need to be updated, and only the security key for the secondary cell needs to be generated / updated. In this case, the configuration information (SK-Counter) related to the security key for the secondary cell may be updated using a procedure different from the above-described process.

[0032] For example, the base station device may include a list of configuration information (SK-Counter) related to the security key of the SCG for each base station device that provides a candidate cell for the MCG in the LTM configuration (using an RRC Reconfiguration message) and transmit the list to the connected terminal device. For example, a separate SK-Counter list is configured for each of candidate base station device #1 and candidate base station device #2 in FIG. 4 or FIG. 5, and the list is transmitted to the terminal device. The terminal device also holds this SK-Counter list for the connected base station device that provides the MCG cell. For example, the terminal device may hold SK-Counter A1, SK-Counter A2, . . . , SK-Counter A16 for the connected base station device, and SK-Counter B1, SK-Counter B2, . . . , SK-Counter B16 for candidate base station device #1. In other words, a separate SK-Counter list is defined for each candidate base station device that can provide an MCG cell. Then, when the terminal device receives a Cell Switch Command indicating the addition of a secondary cell while communicating with the connected base station device, it generates a security key for the secondary cell using the SK-Counter A1 at the top of the list. If the terminal device subsequently receives a Cell Switch Command instructing the terminal device to use another cell as a secondary cell while communicating with the connected base station device, it updates the security key for the changed secondary cell using the SK-Counter A2. At this time, the terminal device deletes the SK-Counter A1 that had been used up until then from the list. In this way, when the SCG is changed without changing the MCG, the terminal device can update the security key for the SCG sequentially using the SK-Counters included in the list notified in advance. That is, a list of SK-Counters can be provided in common for each MCG for multiple SCG candidates that can be set together with that MCG.Therefore, as long as there is no change in the MCG, the terminal device can repeatedly update the security key for the SCG using the SK-Counter included in the list every time the SCG is changed. Therefore, there is no need to make an inquiry to the base station device (which provides SCG candidates) as shown in Figures 4 and 5 every time the SCG is changed.

[0033] Thereafter, the terminal device receives a Cell Switch Command instructing it to change the MCG (handover) and is assumed to have handed over to candidate base station device #1. When the terminal device receives a Cell Switch Command instructing it to add an SCG, it generates a security key for that SCG using SK-Counter B1 at the top of the list associated with candidate base station device #1. The terminal device then deletes the previously used SK-Counter A2 from the list. In this way, by separately setting a list of SK-Counters for each MCG, it is possible to efficiently generate a security key for the secondary cell after handover.

[0034] A base station device currently connected to a terminal device may instruct the terminal device to delete part or all of the configured SK-Counter list. This instruction may be transmitted from the base station device to the terminal device, for example, using an RRC Reconfiguration message. The list may be deleted by an explicit instruction from the base station device, or may be deleted without an instruction from the base station device when a predetermined condition is met. For example, a list associated with the MCG of the handover source held in the terminal device may be deleted on the condition that the MCG has been changed by a Cell Switch Command or a conventional handover. If the MCG of the handover source is treated as a candidate for the handover destination after the handover, it is assumed that the LTM configuration for the MCG and the SCG corresponding to the MCG are not deleted. In this case, the list may not be deleted either. Furthermore, a list configured for a candidate MCG that was not selected as the handover destination for the terminal device may be deleted on the condition that the MCG has been changed. In this case, whether or not the list is deleted may also be determined depending on whether or not the LTM configuration is maintained. For example, a list set for an MCG candidate that is excluded from the handover destination candidates may be deleted together with the LTM configuration, and lists set for other MCG candidates may not be deleted. Furthermore, whether or not a list is to be deleted may be determined depending on the base station device of the handover destination. In this case, setting information for this determination may be notified to the terminal device in advance. Furthermore, the base station device may notify the terminal device of information specifying criteria for deleting the list, and the terminal device may delete the list according to the criteria.

[0035] In the above example, the handling of the LTM configuration when a terminal device receives an LTM configuration from a base station device to which it is currently connected and then performs a handover has been described. This process also applies to the LTM configuration maintained before the handover. That is, the terminal device receives configuration information for updating the LTM configuration and security keys from the base station device that was connected before the currently connected base station device. Then, upon handover to the currently connected base station device, the terminal device may update the portion of the configuration information for updating the security keys while maintaining the LTM configuration for the candidate base station device that was not selected as the handover destination. Thereafter, when the terminal device performs another handover, it may execute the above process. That is, the terminal device may maintain the LTM configuration received and stored from the base station device to which it is not currently connected, and update only the configuration information for updating the security keys included in the configuration or stored in association with that information.

[0036] FIG. 6 shows an example of the hardware configuration of a base station apparatus and a terminal apparatus according to this embodiment. In one example, the base station apparatus and the terminal apparatus include a processor 601, a ROM 602, a RAM 603, a storage device 604, and a communication circuit 605. The processor 601 is a computer including one or more processing circuits, such as a general-purpose CPU (Central Processing Unit) or an ASIC (Application Specific Integrated Circuit), and executes the overall control process of the apparatus and each of the above-mentioned processes by reading and executing programs stored in the ROM 602 or the storage device 604. The ROM 602 is a read-only memory that stores information such as programs and various parameters related to the processes executed by the base station apparatus and the terminal apparatus. The RAM 603 functions as a workspace when the processor 601 executes the programs and is a random access memory that stores temporary information. The storage device 604 is, for example, a removable external storage device. The communication circuit 605 is, for example, a circuit for wireless communication of 5G or its successor standards. Although FIG. 6 illustrates one communication circuit 605, the base station apparatus and the terminal apparatus may have multiple communication circuits. For example, the base station apparatus and the terminal apparatus may have wireless communication circuits for 5G and its successor standard, and a common antenna for these circuits. The base station apparatus and the terminal apparatus may have separate antennas suitable for each standard. The base station apparatus may also have a wired communication circuit used when communicating with other base station apparatuses or nodes in the core network. The terminal apparatus may also have a communication circuit conforming to a wireless communication standard other than the cellular communication standard, such as a wireless local area network (LAN) or Bluetooth (registered trademark). The base station apparatus and the terminal apparatus may have separate communication circuits 605 for each of multiple available frequency bands, or may have a common communication circuit 605 for at least some of these frequency bands.

[0037] FIG. 7 shows an example of the functional configuration of a terminal device. The terminal device includes, for example, a setting information receiving unit 701, an LTM setting storage unit 702, a key information management unit 703, a key generation unit 704, and a communication unit 705. Note that FIG. 7 only shows functions particularly related to this embodiment, and does not illustrate various other functions that the terminal device may have. For example, the terminal device naturally has other functions that terminal devices compliant with 5G or subsequent standards generally have. The functional blocks in FIG. 7 are shown schematically, and the respective functional blocks may be integrated or further subdivided. Each function in FIG. 7 may be realized, for example, by the processor 601 executing a program stored in the ROM 602 or the storage device 604, or by a processor within the communication circuit 605 executing predetermined software. Since the details of the processing performed by each functional unit are as described above, only the general functions of the terminal device will be outlined here.

[0038] The setting information receiving unit 701 receives various setting information from the currently connected base station device. The setting information receiving unit 701 receives, for example, pre-configuration information for LTM for a candidate base station device that is a handover destination. The pre-configuration information is, for example, setting information for establishing an RRC layer connection with a candidate base station device, and is setting information that is notified to a terminal device when a handover has not actually been performed. The pre-configuration information corresponds to the above-mentioned LTM configuration. The setting information receiving unit 701 also receives setting information (for example, MasterKeyUpdate and SK-Counter) for generating and updating security keys. The LTM setting holding unit 702 holds the LTM pre-configuration information received by the setting information receiving unit 701. The LTM setting holding unit 702 is configured, for example, to be able to continue to hold pre-configuration information (LTM configuration) for a candidate base station device that was not selected as a handover destination after a handover is performed. The LTM setting holding unit 702 can delete the held pre-configuration information, for example, in response to an instruction from the handover source base station device or the handover destination base station device. Alternatively, the LTM setting storage unit 702 may set an expiration date for the preset information and delete the preset information upon expiration of the expiration date. In this case, the expiration date may be notified by the base station device currently connected when the preset information is received.

[0039] The key information management unit 703 maintains and manages configuration information, such as security keys or parameters for generating the security keys, for each of the currently connected base station and the candidate base station for handover. The key information management unit 703 manages, for example, security key configuration information received together with the LTM configuration from the currently connected base station, and manages the configuration information so that it is up to date. For example, when the key information management unit 703 receives configuration information (Master Key Update) for at least some of the candidate base stations not selected as the handover destination from the target base station, the key information management unit 703 replaces the configuration information it holds with the received configuration information. The key information management unit 703 may also obtain configuration information for generating security keys using a Cell Switch Command. This allows the key information management unit 703 to maintain the security key configuration information in an up-to-date state. Furthermore, the key information management unit 703 may hold, for each MCG, a list of SK-Counters for SCGs that can be used as secondary cells when that MCG is used, and may extract SK-Counter values ​​from the list in order each time a destination SCG is changed or added, and generate a security key for that SCG. Furthermore, when an SCG is changed or deleted, the key information management unit 703 may delete from the list the SK-Counter value that was used to generate a security key for the SCG that is no longer in use.

[0040] The key generation unit 704 generates a security key using the setting information (MasterKeyUpdate, SK-Counter) maintained and managed by the key information management unit 703. The communication unit 705 uses the security key generated by the key generation unit 704 to communicate with the destination base station device.

[0041] FIG. 8 shows an example of the functional configuration of a base station device. The base station device includes, for example, a setting information acquisition unit 801, a setting information provision unit 802, a setting information notification unit 803, a key generation unit 804, and a communication unit 805. Note that FIG. 8 only shows functions particularly related to this embodiment, and does not illustrate various other functions that the base station device may have. For example, the base station device naturally has other functions that base station devices compliant with 5G and subsequent standards generally have. The functional blocks in FIG. 8 are shown schematically, and the respective functional blocks may be integrated or further subdivided. Each function in FIG. 8 may be realized, for example, by the processor 601 executing a program stored in the ROM 602 or the storage device 604, or by a processor within the communication circuit 605 executing predetermined software. Since the details of the processing performed by each functional unit are as described above, only the general functions of the base station device will be outlined here.

[0042] The setting information acquisition unit 801 acquires setting information from another base station device. For example, while connected to the terminal device, the setting information acquisition unit 801 acquires an LTM configuration for LTM to another base station device and setting information (MasterKeyUpdate and SK-Counter) for generating or updating a security key in communication with the other base station device. Note that the setting information acquisition unit 801 may, for example, transmit a request message (e.g., HANDOVER REQUEST) to a base station device that is a candidate for a handover destination from the terminal device to acquire setting information for generating or updating the LTM configuration and the security key. Furthermore, for example, when the terminal device connects to the terminal device after being handed over from another base station device, the setting information acquisition unit 801 acquires setting information for generating or updating the LTM configuration and the security key for the base station device that is a candidate for a handover destination from the terminal device. Here, the setting information related to the LTM configuration and the security key may be acquired from the candidate base station device that is a handover destination in response to a request from the handover source base station device to transfer setting information related to the LTM configuration and the security key to the candidate base station device. Alternatively, the LTM configuration may be received from a base station device of a handover source, and configuration information related to a security key may be received from a base station device that is a candidate for a handover destination. The configuration information providing unit 802 provides the LTM configuration and configuration information related to a security key to another base station device. That is, the information provided by the configuration information providing unit 802 of the first base station device is acquired by the configuration information acquiring unit 801 of the second base station device.

[0043] The setting information notification unit 803 notifies the connected terminal device of the acquired setting information. For example, for a base station device for which the LTM configuration is not held in the terminal device, the setting information notification unit 803 notifies both the LTM configuration and setting information related to the security key. On the other hand, for a base station device for which the LTM configuration is held in the terminal device, the setting information notification unit 803 may notify only setting information related to the security key without notifying the LTM configuration. For example, during handover, the setting information notification unit 803 may receive an RRC Reconfiguration Complete message from the terminal device in handover processing, and after connection is established, transmit setting information related to the security key (and the LTM configuration) by an RRC Reconfiguration message. Furthermore, the setting information notification unit 803 may transmit setting information (NCC) related to the security key of the handover destination base station device in a Cell Switch Command. In this case, the setting information notification unit 803 can retain the setting information previously acquired by the setting information acquisition unit 801 until the time of handover, and can notify the terminal device of the setting information when handover is performed.

[0044] The key generation unit 804 holds, for example, setting information (such as MasterKeyUpdate) for generating a security key for communication with a terminal device, and generates a security key using the information when the terminal device is connected by handover. Furthermore, the key generation unit 804 updates the setting information in response to a request from another base station device (for example, a first other base station device or a second other base station device when the terminal device hands over from a first other base station device to a second other base station device) or an AMF, and holds the updated setting information. The updated setting information can be provided to the other base station device by the setting information providing unit 802, and can be notified to the terminal device from the other base station device. The communication unit 805 establishes a connection with the terminal device, and performs communication using the security key generated by the key generation unit 804.

[0045] As described above, in this embodiment, security keys can be efficiently shared and updated between a terminal device and a network (base station device) that perform handover using LTM. This makes it possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs), which is to "Develop resilient infrastructure, promote sustainable industrialization, and foster innovation."

[0046] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention.

Claims

1. A terminal device conforming to a cellular communication standard of the Third Generation Partnership Project (3GPP), a receiving means for receiving, from the currently connected first base station device, first setting information of Layer 1 / Layer 2 Triggered Mobility (LTM) for each of a plurality of candidate base station devices for handover, including the second base station device and the third base station device, and second setting information associated with the first setting information for updating a security key; a storage means for storing the first setting information and the second setting information in association with each other for each of the plurality of base station devices; an execution means for executing a handover to the second base station device; an update means for updating the security key by using the second setting information associated with the first setting information of the second base station device in response to a handover to the second base station device, and for updating the second setting information associated with the first setting information of the third base station device after the handover is performed while maintaining the first setting information held by the third base station device; A terminal device comprising:

2. the receiving means receives, after handover to the second base station device, the updated second setting information for the third base station device from the second base station device; the updating means updates the second setting information by replacing the second setting information held for the third base station device with the received second setting information.

2. The terminal device according to claim 1, wherein:

3. the receiving means receives identification information that enables identification of the second setting information for a fourth base station device that is at least a part of the plurality of third base station devices, and the updated second setting information for the fourth base station device; the update means replaces the second setting information identified by the identification information with the received second setting information.

3. The terminal device according to claim 2.

4. 4. The terminal device according to claim 3, wherein the first setting information is an LTM configuration, and the identification information is an LTM configuration id.

5. a predetermined order is assigned to the plurality of third base station devices; the receiving means receives information in which the updated second setting information is arranged in the predetermined order for all of the third base station devices; the update means identifies, in the predetermined order, to which of the plurality of third base station devices each of the received plurality of pieces of second setting information corresponds, and replaces the second setting information held for each of the plurality of third base station devices with the received second setting information according to the identification result.

3. The terminal device according to claim 2, wherein:

6. 3. The terminal device according to claim 2, wherein the receiving means receives the updated second setting information from the second base station device by an RRC Reconfiguration message after an RRC Reconfiguration Complete message is transmitted in the handover by the executing means.

7. the second setting information is MasterKeyUpdate, The MasterKeyUpdate is a keySetChangeIndicator set to True and a nextHopChainingCount (NCC) and Non-Access Stratum (NAS) container; or Contains NCC with keySetChangeIndicator set to False, 2. The terminal device according to claim 1, wherein:

8. 2. The terminal device according to claim 1, wherein the second setting information includes an SK-Counter.

9. the execution means executes the handover when receiving a Cell Switch Command instructing a handover to the second base station device; the updating means, when the Cell Switch Command includes the second setting information related to the second base station device and when the second setting information is held for the second base station device, updates the security key using the second setting information included in the Cell Switch Command; 2. The terminal device according to claim 1, wherein:

10. the receiving means further receives the first setting information for each of a plurality of fifth base station devices that provide a cell that is a candidate for a secondary cell when a cell provided by a currently connected base station device is used as a master cell, and a list of the second setting information common to the plurality of fifth base station devices; When the secondary cell is changed while the master cell is not changed, the update means deletes from the list the second setting information used before the secondary cell was changed, and updates a security key for communication of the changed secondary cell using one piece of second setting information included in the list.

2. The terminal device according to claim 1, wherein:

11. the receiving means further receives the first setting information for each of the plurality of fifth base station devices and a list of the second setting information common to the plurality of fifth base station devices when a cell provided by each of the plurality of base station devices becomes a master cell; the list of second setting information for the second base station device is defined separately from the list of second setting information for the third base station device; The terminal device according to claim 10 .

12. The terminal device according to claim 11, characterized in that the holding means holds a list of the second setting information for when a cell provided by a connected base station device and each of the plurality of base station devices becomes a master cell.

13. 13. The terminal device according to claim 12, wherein said holding means deletes the list of said second setting information based on an instruction from a connected base station device.

14. The terminal device according to claim 12, characterized in that the holding means deletes the list of second configuration information for the case where, when the terminal device is handed over to the second base station device, a cell provided by a base station device that is no longer a candidate for handover after the handover becomes a master cell.

15. A base station device conforming to a cellular communication standard of the Third Generation Partnership Project (3GPP), a providing means for providing, to a first other base station device to which the terminal device is connected, first setting information of Layer 1 / Layer 2 Triggered Mobility (LTM) as a candidate for a handover destination, and second setting information associated with the first setting information for updating a security key; a connection means for connecting to the terminal device by handover of the terminal device; a generating means for generating a security key to be used in communication with the terminal device using the second setting information; an acquisition means for acquiring the first setting information and the second setting information for a second other base station device that is a candidate handover destination of the terminal device from the base station device after the handover is executed; a transmitting means for transmitting, when the terminal device holds the first setting information for the second other base station device, a message to the terminal device that does not include the first setting information but includes the second setting information for the second other base station device; A base station device comprising:

16. 16. The base station apparatus according to claim 15, wherein the transmitting means transmits an RRC Reconfiguration message including the second setting information for the second other base station apparatus to the terminal apparatus after receiving an RRC Reconfiguration Complete message from the terminal apparatus during the handover.

17. 16. The base station device according to claim 15, wherein the transmitting means transmits a Cell Switch Command including the second setting information for the second other base station device when instructing the terminal device to perform handover to the second other base station device.

18. the providing means further provides the updated second setting information to the third other base station device when the terminal device performs handover from the first other base station device to a third other base station device; The base station device according to claim 15, characterized in that the generation means generates the security key using the updated second setting information provided to the third other base station device when the terminal device is handed over from the third other base station device to the base station device.

19. The base station device according to claim 15, characterized in that the acquisition means acquires the first setting information and the second setting information for the second other base station device transmitted by the second other base station device in response to a request from the handover source base station device.

20. The base station device according to claim 15, characterized in that the acquisition means receives a message including the first setting information for the second other base station device from the handover source base station device, and acquires the first setting information and the second setting information for the second other base station device by transmitting a message requesting the second setting information to the second other base station device based on the message.

21. 1. A control method executed by a terminal device conforming to a cellular communication standard of a Third Generation Partnership Project (3GPP), comprising: receiving, from the currently connected first base station device, first setting information of Layer 1 / Layer 2 Triggered Mobility (LTM) for each of a plurality of candidate base station devices as handover destinations, including the second base station device and the third base station device, and second setting information associated with the first setting information for updating security keys; storing the first setting information and the second setting information in association with each other for each of the plurality of base station devices; performing a handover to the second base station; In response to the handover to the second base station device, updating the security key using the second setting information associated with the first setting information of the second base station device, and after the handover is performed, updating the second setting information associated with the first setting information while maintaining the first setting information for the third base station device that is held; A control method comprising:

22. A control method executed by a base station device conforming to a cellular communication standard of a Third Generation Partnership Project (3GPP), comprising: providing, to a first other base station device to which the terminal device is connected, as a candidate for a handover destination, first setting information for Layer 1 / Layer 2 Triggered Mobility (LTM) and second setting information associated with the first setting information for updating a security key; connecting to the terminal device by handover of the terminal device; generating a security key to be used in communication with the terminal device using the second setting information; After the handover is executed, acquiring the first setting information and the second setting information for a second other base station device that is a candidate handover destination of the terminal device from the base station device; If the terminal device holds the first setting information for the second other base station device, transmitting a message to the terminal device that does not include the first setting information but includes the second setting information for the second other base station device; A control method comprising:

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