Terminal, control method, and program for efficient connection destination change processing

Abstract

To efficiently configure a conditional cell change for a combination of a primary cell and a secondary cell.SOLUTION: In the cellular communication system, the preconfiguration information for the terminal UE to autonomously select and switch a cell as a connection destination includes first information indicating a condition for connecting to the cell and communication parameters to be used when connecting to the cell, and second information indicating a combination of the first cell as the candidate for the primary cell of the handover destination and the second cell as the candidate for the secondary cell, for each of the first cell as the candidate for the primary cell and the second cell as the candidate for the secondary cell of the handover destination; In the second information, when one cell is commonly included in two or more different combinations of the primary cell and the secondary cell, and the communication parameters of one cell are partially different, as the first information, the common first part is omitted and only the non-common second part is included in the preconfiguration information.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a technique for improving the efficiency of connection destination change processing. [Background technology]

[0002] In cellular communication systems, handover techniques are used to ensure communication continuity by switching the connection destination of a terminal device in response to factors such as the movement of the terminal device. A standard handover technique involves initiating handover preparations and other processes on the core network side after the communication quality of the currently connected wireless link in the terminal device deteriorates, and then transmitting an instruction message from a base station device to the terminal device. However, this technique requires time for processing from the actual deterioration of communication quality until the transmission of the handover instruction, which can result in a prolonged period of time during which communication is unavailable or communication quality is insufficient. In response to this, the Third Generation Partnership Project (3GPP (registered trademark)) is discussing a technique in which this preparation process is performed in advance, and when a predetermined condition is satisfied in the terminal device, the terminal device switches the connection destination cell without receiving an instruction message from a base station device (see Non-Patent Document 1). The predetermined condition includes, for example, when the wireless quality of a neighboring cell becomes better than the wireless quality of the currently connected cell by a predetermined level or more, or when the wireless quality of the currently connected cell falls below a predetermined value. This method significantly reduces the time during which communication is unavailable or communication quality is insufficient because the core network is ready to perform the handover before a deterioration in communication quality occurs that would require the handover to be performed. This type of handover is called conditional handover, or CHO.

[0003] Similarly, in multi-connectivity (MC) or the like in which a terminal device is connected to multiple cells, processing such as preparation is performed in advance, and when a predetermined condition is satisfied in the terminal device, the terminal device can add or change a secondary cell without receiving an instruction message from the base station device. Note that, hereinafter, this conditional addition or change of a secondary cell may be referred to as CPAC. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] 3GPP TR38.874, V16.0.0, December 2018 Summary of the Invention [Problem to be solved by the invention]

[0005] A secondary cell can be configured only in combination with a primary cell. Therefore, when CHO is performed, the secondary cell may also need to be changed. Therefore, pre-configuration may need to be performed for the combination of the primary cell and the secondary cell. However, a specific method for pre-configuring such a combination of the primary cell and the secondary cell has not been considered.

[0006] The present invention provides a technique for efficiently configuring a conditional cell change for a combination of a primary cell and a secondary cell. [Means for solving the problem]

[0007] A terminal device according to one aspect of the present invention includes a receiving means for receiving, from a base station device currently connected to the terminal device, preconfiguration information for the terminal device to autonomously select and switch a connection destination cell, the preconfiguration information including, for each of a first cell that is a candidate for a primary cell at a handover destination or a second cell that is a candidate for a secondary cell when the first cell is the primary cell, first information indicating conditions for connecting to the cell and communication parameters to be used when connecting to the cell, and information indicating the first cell that is a candidate for a primary cell at a handover destination and communication parameters to be used when connecting to the cell, and information indicating communication parameters for the first cell that is a candidate for a primary cell at a handover destination and a second cell that is a candidate for a secondary cell when the first cell is the primary cell. and second information indicating a combination of the candidate primary cell and the second cell, and if in the second information one cell is commonly included in two or more different combinations of a primary cell and a secondary cell and communication parameters of the one cell are partially different in each of the two or more combinations, as the first information for some of the two or more combinations, a first part of the communication parameters of the one cell that are common to each of the two or more combinations is omitted and only a second part of the communication parameters that are not common is included in the pre-configured information. [Effects of the Invention]

[0008] According to the present invention, it is possible to efficiently set a conditional cell change for a combination of a primary cell and a secondary cell. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 illustrates an example of the configuration of a communication system. [Figure 2] A figure showing an example of information notified to a terminal device for conditional handover and conditional change or addition of a secondary cell. [Figure 3] A figure showing an example of information notified to a terminal device for conditional handover and conditional change or addition of a secondary cell. [Figure 4]A figure showing an example of information notified to a terminal device for conditional handover and conditional change or addition of a secondary cell. [Figure 5] FIG. 2 illustrates an example of a hardware configuration of the apparatus. [Figure 6] FIG. 2 is a diagram illustrating an example of a functional configuration of a base station device. [Figure 7] FIG. 2 is a diagram illustrating an example of a functional configuration of a control device. [Figure 8] FIG. 10 is a diagram illustrating an example of a processing flow. [Figure 9] FIG. 10 is a diagram illustrating an example of a processing flow. [Figure 10] FIG. 10 is a diagram illustrating an example of a processing flow. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, 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 combined in any desired manner. Furthermore, the same reference numerals are used to designate identical or similar components, and redundant descriptions will be omitted.

[0011] (System Configuration) 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 cellular communication standards such as Long Term Evolution (LTE) and 5th Generation (5G) defined by the Third Generation Partnership Project (3GPP (registered trademark)). Note that this is just one example, and the following discussion can be applied to any system in which a terminal device can continue communication while switching the cell to which it is connected.

[0012] In the wireless communication system of FIG. 1, a base station device provides wireless communication services to a terminal device 141. Note that in the example of FIG. 1, base station devices 101 and 102 that provide cells 111 and 112 that cover relatively wide areas, respectively, and base station devices 121 to 124 that provide cells 131 to 134 that cover relatively narrow areas, respectively, are shown as base station devices. The terminal device 141 is configured to receive wireless communication services by connecting to at least one of these base station devices and transmitting and receiving wireless signals to and from the base station device to which it is connected. Note that the terminal device 141 connects to a base station device that configures a cell that includes the location where the terminal device 141 itself is located, and hereinafter, this may be referred to as "connecting to a cell (provided by that base station device)."

[0013] The terminal device 141 can be connected to two or more cells in parallel. Of these two or more cells, the cell that controls the communication of the terminal device 141 is called a primary cell, and the cell that connects to the terminal device 141 under the control of the primary cell is called a secondary cell. As an example, the terminal device 141 is connected to the cell 111 or the cell 112 as a primary cell, and to the cells 131 to 134 as secondary cells.

[0014] In this embodiment, for example, when the terminal device 141 establishes a connection with any cell, or when the wireless quality in the currently connected cell deteriorates to a certain degree, pre-configuration for cell switching of the terminal device 141 is performed. The pre-configuration state here refers to a state in which the settings of parameters and the like for the case in which the terminal device 141 connects to a candidate cell to which the connection is changed have been completed, and the terminal device 141 does not actually switch the destination base station device. The pre-configuration includes conditions for connecting to a cell for switching the connection from another cell or adding a cell, and when the terminal device 141 determines that the conditions are satisfied, it establishes synchronization with the new destination cell and attempts to connect. Note that when switching the destination cell, the terminal device 141 performs a process of disconnecting from the source cell. The process of autonomously switching the destination primary cell using such pre-configuration and when the terminal device 141 determines that the conditions are satisfied is called conditional handover (CHO). Furthermore, a process in which such a pre-setting is used and the terminal device 141 autonomously switches or adds a secondary cell in response to determining that the conditions are met is called conditional secondary cell addition or change (CPAC).

[0015] When CHO is performed, the base station device of the currently connected primary cell transmits a handover request message (HANDOVER REQUEST) including information on parameters currently used by the terminal device 141 to another base station device that provides a cell that is a candidate for the primary cell to which the terminal device 141 is connected, to another base station device that provides a cell that is a candidate for the primary cell to which the terminal device 141 is connected. Then, the candidate base station device of the handover destination transmits a response message (HANDOVER REQUEST ACKNOWLEDGE) to the message, including information on communication parameters to be used when the terminal device 141 is handed over, to the base station device of the currently connected primary cell to which the terminal device 141 is connected. Here, the candidate base station device of the handover destination holds the transmitted information on communication parameters and the like as preconfigured information. Furthermore, the currently connected base station device or the candidate base station device of the handover destination determines conditions that should be satisfied when handing over to the candidate base station device of the handover destination. Then, the currently connected base station device notifies the terminal device 141 of preconfigured information including information on communication parameters to be used when handing over to the candidate base station device of the handover destination and the handover conditions. This notification can be performed using, for example, a radio resource control (RRC) message. In one example, the pre-configuration information is notified to the terminal device 141 by an RRC Reconfiguration message.

[0016] When CPAC is performed, the base station device of the currently connected primary cell transmits a secondary cell addition request message (SgNB Addition Request) to another base station device that provides a cell that is a candidate for the secondary cell to be added or changed. Note that this secondary cell addition request may be initiated, for example, in response to a predetermined message (e.g., SgNB Change Required) being notified to the base station device of the currently connected primary cell from the base station device that provides the secondary cell to which the terminal device 141 is currently connected. In this case, the base station device of the currently connected primary cell may transmit the above-mentioned secondary cell addition request in response to receiving the predetermined message. Then, the other base station device that provides a cell that is a candidate for the secondary cell to be added or changed (hereinafter, may be referred to as the other base station device as a connection destination candidate) transmits a response message (SgNB Addition Request Acknowledge) to the message to the base station device of the currently connected primary cell to the terminal device 141, including information such as communication parameters to be used when the terminal device 141 adds or changes the secondary cell. Here, the other base station device as a connection destination candidate stores the transmitted information such as communication parameters as pre-configured information. Furthermore, the base station device of the currently connected primary cell or another base station device of a connection candidate determines conditions that must be satisfied when adding or changing the secondary cell. Then, the base station device of the currently connected primary cell notifies the terminal device 141 of preconfiguration information including information on communication parameters to be used when connecting to another base station device of a connection candidate and adding or changing the secondary cell, and conditions for such connection. This notification can be performed, for example, using a radio resource control (RRC) message. In one example, the preconfiguration information is notified to the terminal device 141 using an RRC Reconfiguration message.

[0017] Upon receiving the preconfiguration information, the terminal device 141 stores the preconfiguration information and begins monitoring whether the CHO or CPAC conditions are satisfied. If the conditions are satisfied, the terminal device 141 performs processing such as disconnecting from the currently connected base station device and establishing synchronization with the newly connected base station device in order to actually switch the connection destination. Note that there may be multiple candidate cells for the connection destination by CHO or CPAC. Therefore, conditions and communication parameters may be set separately for each of one or more candidate cells for the connection destination by CHO or CPAC. Furthermore, when one base station device configures multiple cells, conditions and communication parameters may be set separately for the multiple cells. The terminal device 141 monitors whether the conditions corresponding to each of the multiple cells are satisfied, and performs a change of connection destination, etc., using communication parameters corresponding to the satisfied conditions. In this way, the terminal device 141 completes CHO or CPAC and can continue communication even after changing the cell.

[0018] The secondary cell is configured under the control of the primary cell. Therefore, when the primary cell is changed by CHO, the secondary cell needs to be reconfigured. In contrast, after CHO is performed, the configuration of the secondary cell associated with the previous primary cell is discarded, so the secondary cell needs to be configured under the control of the new primary cell. Here, if the secondary cell is configured using the normal procedure (rather than CPAC), it may take a long time to complete the configuration of the secondary cell. Therefore, preconfiguration for CPAC of the secondary cell may be performed in combination with preconfiguration for CHO of the primary cell, so that when the primary cell is changed by CHO, the secondary cell may be added or changed by CPAC using information on the preconfigured secondary cell associated with the new primary cell. In this case, for example, a first base station device providing a cell that is a candidate for the primary cell may obtain preconfiguration information for CPAC from a second base station device providing a cell that can be used as a secondary cell when the first cell is set as the primary cell. Then, the first base station device may notify the base station device to which the terminal device 141 is connected of the CHO conditions and communication parameters and the CPAC conditions and communication parameters for each combination of the primary cell and the secondary cell when the cell provided by the first base station device is the primary cell, and the base station device may notify the notified information to the terminal device 141.

[0019] 1, when cell 111 is the primary cell, cells that can be used as secondary cells are cells 131 to 133. Therefore, for the combination of cell 111 and cell 131, the CHO conditions and communication parameters of cell 111 and the CPAC conditions and communication parameters of cell 131 can be notified to the terminal device 141. Furthermore, for the combination of cell 111 and cell 132, the CHO conditions and communication parameters of cell 111 and the CPAC conditions and communication parameters of cell 132 can be notified to the terminal device 141, and for the combination of cell 111 and cell 133, the CHO conditions and communication parameters of cell 111 and the CPAC conditions and communication parameters of cell 133 can be notified to the terminal device 141. Similarly, when cell 112 is set as the primary cell, the CHO conditions and communication parameters and the CPAC conditions and communication parameters may be notified to the terminal device 141 for the combination of cell 112 and cell 131, the combination of cell 112 and cell 132, and the combination of cell 112 and cell 134. In the example of FIG. 1, an example has been described in which a cell whose geographical range overlaps with the candidate primary cell is set as a candidate secondary cell, but this is not limited to this. For example, depending on the combination of available frequency bands, only a cell that uses a frequency band that can be used in parallel with the frequency band used by the primary cell may be set as a candidate secondary cell. In other words, even if the cell has an overlapping geographical range, a cell that uses a frequency band that cannot be used in combination with the frequency band used by the primary cell may be excluded from the candidate secondary cell.

[0020] In such notification, the CHO conditions and communication parameters for cell 111 or cell 112 are notified for each secondary cell candidate. In this case, for example, pre-configuration information such as that shown in FIG. 2 is notified to the terminal device 141. However, it is assumed that the hatched portions are overlapping information, resulting in inefficient information notification. For this reason, the present embodiment provides a method for reducing the amount of information notified to the terminal device 141 about pre-configuration information related to the combination of the CHO of the primary cell and the CPAC of the secondary cell, thereby enabling efficient pre-configuration.

[0021] In this embodiment, conditions and communication parameters for each cell are notified only once, thereby avoiding repeated transmission of the same information. For example, information as shown in FIG. 3(A) is notified to the terminal device 141 as pre-setting information when cell 111 is set as the primary cell. In this information, information indicating a combination of a primary cell and a secondary cell, and information on conditions and communication parameters for each cell are notified to the terminal device 141. When cell 111 is set as the primary cell, cells 131, 132, and 133 can be set as secondary cells. Therefore, information on cells 111 and 131, cells 111 and 132, and cells 111 and 133 is notified to the terminal device 141 as information on possible combinations. Furthermore, since the cells included in these combinations are cell 111 and cells 131 to 133, the conditions and communication parameters for these are notified. When the information as shown in FIG. 2 is notified, communication parameters for cell 111 are notified a total of three times for each combination. In contrast, in the information of FIG. 3(A), the communication parameters for cell 111 are notified only once.

[0022] Here, as an example, it is assumed that two bytes are required to notify one combination of a primary cell and a secondary cell, one byte to notify the conditions for one cell, and one kilobyte to notify the communication parameters for one cell. When notifying the information as shown in FIG. 2, it is not necessary to notify the combination of a primary cell and a secondary cell, but the conditions and communication parameters are notified for each of the three combinations of cell 111 and cell 131, cell 111 and cell 132, and cell 111 and cell 133. Therefore, the condition information and communication parameters are notified for six cells (for three combinations of two cells), resulting in 6 kilobytes + 6 bytes. On the other hand, in the information as shown in FIG. 3(A), 6 bytes are used for information specifying the three combinations, 4 bytes for information indicating the conditions for each cell, and 4 kilobytes for information indicating the communication parameters for each cell, resulting in a total of 4 kilobytes + 10 bytes. As such, the information notifying method of this embodiment can significantly reduce the size of the pre-configuration information. Similarly, information such as that shown in FIG. 3(B) is notified to the terminal device 141 as pre-setting information when the cell 111 is set as the primary cell.

[0023] Note that a first base station device that provides a primary cell to which the terminal device 141 is currently connected may acquire information such as that shown in FIG. 3(A) or 3(B) from a second base station device that provides a cell that is a candidate for the primary cell to which the terminal device 141 is connected via CHO, and may transfer the information to the terminal device 141. In this case, the second base station device transmits a message (e.g., an SgNB Addition Request) requesting information on conditions for performing CPAC and communication parameters to a third base station device that provides a cell that can be used as a secondary cell when the cell provided by the second base station device operates as the primary cell. The second base station device then receives a response message (e.g., an SgNB Addition Request Acknowledge) to the request message and may acquire information on conditions and communication parameters for connecting to the secondary cell provided by the third base station device, which are included in the response message. The second base station device then aggregates the information on the candidate secondary cells acquired from the third base station device and the information on the candidate primary cells provided by the second base station device, and notifies the first base station device of the aggregated information. For example, the base station device 101 that forms the cell 111 acquires information on connection conditions and communication parameters as candidates for secondary cells from the base station devices 121, 122, and 123 that respectively form the cells 131, 132, and 133. Then, the base station device 101 may generate information such as that shown in FIG. 3A as pre-configuration information based on the acquired information and notify the base station device that provides the primary cell to which the terminal device 141 is currently connected. Similarly, the base station device 102 that forms the cell 112 may acquire information on connection conditions and communication parameters as candidates for secondary cells from the base station devices 121, 122, and 124 that respectively form the cells 131, 132, and 134, and may generate information such as that shown in FIG. 3B as pre-configuration information and notify the base station device that provides the primary cell to which the terminal device 141 is currently connected.

[0024] Furthermore, a first base station device providing a primary cell to which the terminal device 141 is currently connected may acquire only information about a cell available as a secondary cell from a second base station device providing a cell that is a candidate for the primary cell by CHO, for example. In this case, the first base station device may transmit and receive a message similar to the above-described message for adding or changing a secondary cell to a third base station device providing a cell that is available as a secondary cell, and may acquire connection conditions and communication parameters for when the third base station device is used as a secondary cell directly from the third base station device. That is, the second base station device providing a cell that is a candidate for the primary cell may aggregate information about the third base station device providing a cell that is a candidate for the secondary cell and notify the first base station device of the aggregated information, or the first base station device may acquire information directly from each of the second base station device and the third base station device.

[0025] 3(A) and 3(B) to be transmitted to the terminal device 141, the information of FIG. 4(A) that aggregates these may be notified to the terminal device 141 as preconfiguration information. That is, the conditions and communication parameters for the cells 131 and 132 overlap between FIG. 3(A) and FIG. 3(B). For this reason, the base station device that provides the primary cell to which the terminal device 141 is connected may aggregate information on the cell combinations, generate preconfiguration information that includes only one piece of overlapping condition and communication parameter information, and notify the terminal device 141 of the information. Furthermore, the base station device that provides the primary cell to which the terminal device 141 is connected may, for example, concatenate the information of FIG. 3(A) and FIG. 3(B) and replace the information on the conditions and communication parameters in any of the concatenated information with a reference (pointer) to the same conditions and communication parameters in the other information. 4(B), information about cells 131 and 132 when the primary cell is cell 112 may be replaced with pointers to information about cells 131 and 132 when the primary cell is cell 111. This allows the information about conditions and communication parameters to be replaced with pointers that require information of a sufficiently small size, thereby making it possible to sufficiently reduce the size of the preconfiguration information.

[0026] In the above example, the connection conditions and communication parameters of cells 131 and 132 are the same when cell 111 is the primary cell and when cell 112 is the primary cell. However, this is merely an example, and different connection conditions and communication parameters may be used. In this case, the common parts may be omitted and only the different parts may be separately notified. In one example, when the connection conditions of a secondary cell differ for each primary cell to be combined with it, the connection conditions may not be omitted, and only overlapping communication parameters may be omitted to prevent the same information from being notified multiple times. In another example, certain information such as connection conditions may not be omitted regardless of whether they are common.

[0027] Upon acquiring the above-described information, the terminal device 141 measures the radio quality of the primary cell candidates and starts CHO when the radio quality of any of the candidates satisfies the connection condition. Furthermore, if the connection condition for any of the secondary cell candidates is satisfied at that time, the terminal device 141 starts CPAC for that secondary cell. In CHO and CPAC, the terminal device 141 executes a random access procedure (RACH process) to the cell to be connected to, and performs communication using communication parameters included in the preconfiguration information after the RACH process is completed. Note that, if the terminal device 141 successfully connects to the primary cell but fails to connect to the secondary cell, it may attempt to connect to a secondary cell candidate notified as a combination with the primary cell, for example. On the other hand, if the terminal device 141 successfully connects to the secondary cell but fails to connect to the primary cell, it may attempt to connect to another primary cell candidate. However, there may be cases where the primary cell candidate is a cell that cannot be used in combination with the secondary cell to which it has successfully connected. In this case, the terminal device 141 needs to disconnect the connection with the secondary cell to which it has successfully connected, as a result of executing the CHO to the primary cell candidate, and needs to attempt to establish a connection with one of the secondary cell candidates again after the CHO is completed. Therefore, when this state occurs, the terminal device 141 may attempt a connection by CHO, giving priority to a primary cell candidate that can be used in combination with the secondary cell to which it has successfully connected. For example, the terminal device 141 may add a predetermined offset to the measured wireless quality or a threshold value thereof for the prioritized cell, so that the cell is more likely to be selected as a handover target. Furthermore, the terminal device 141 may preferentially measure the wireless quality of the prioritized cell. Note that the terminal device 141 may not attempt to establish a connection with a primary cell candidate that cannot be used in combination with the secondary cell to which it has successfully connected. Furthermore, the terminal device 141 may be allowed to attempt to establish a connection with a primary cell candidate that cannot be used in combination with the secondary cell to which it has successfully connected, if the wireless quality of the prioritized cell is sufficiently higher than that of other primary cell candidates.In this case, the terminal device 141 may disconnect the connection to the secondary cell to which it has successfully connected at the time of attempting to establish a connection to a candidate primary cell.

[0028] As described above, when CHO to the primary cell and CPAC to the secondary cell are configured simultaneously, if the radio quality of any of the primary cell candidates satisfies the connection condition and the connection condition for any of the secondary cell candidates is satisfied, the terminal device 141 performs connection to the primary cell candidate that satisfies the connection condition and the secondary cell candidate associated with the primary cell and that satisfies the connection condition. In one example, the terminal device 141 may perform CHO and CPAC when the radio quality of the primary cell candidate is better (by a certain offset value or more) than the radio quality of the currently connected primary cell and the radio quality of the secondary cell candidate is better (by a certain offset value or more) than the radio quality of the currently connected secondary cell. Note that an event for performing such CHO and CPAC may be newly defined. Here, for convenience, this event may be referred to as "event X." In this case, for example, the primary cell may notify the terminal device 141 of this event X (together with information such as preconfiguration information) as a condition for performing CHO and CPAC. In another example, the terminal device 141 may perform CHO and CPAC when the radio quality of the currently connected primary cell and the radio quality of the currently connected secondary cell both fall below a first predetermined value, and the radio quality of the candidate primary cell and the radio quality of the candidate secondary cell both exceed a second predetermined value. An event for performing such CHO and CPAC may be newly defined. Here, for convenience, this event may be referred to as "event Y." The first predetermined value related to the radio quality of the currently connected primary cell may be different from the first predetermined value related to the radio quality of the currently connected secondary cell. The second predetermined value related to the radio quality of the candidate primary cell may be different from the second predetermined value related to the radio quality of the candidate secondary cell. In this case, for example, the primary cell may notify the terminal device 141 of event Y (together with information such as preconfiguration information) as a condition for performing CHO and CPAC.Furthermore, conventionally, an event has been defined as event A3 in which the wireless quality of a candidate cell to be connected to (a primary cell or a secondary cell) is better than the wireless quality of the currently connected cell (a primary cell or a secondary cell) by a certain offset value or more. Furthermore, conventionally, an event has been defined as event A5 in which the wireless quality of the currently connected cell falls below a first predetermined value and the wireless quality of the candidate cell to be connected to exceeds a second predetermined value. Here, the terminal device 141 may be configured to perform CHO and CPAC when the condition of event A3 is satisfied for the primary cell and the condition of event A5 is satisfied for the secondary cell. Furthermore, the terminal device 141 may be configured to perform CHO and CPAC when the condition of event A5 is satisfied for the primary cell and the condition of event A3 is satisfied for the secondary cell.

[0029] Furthermore, for example, when the conditions of the above-described event A3 or event A5 are satisfied with respect to the primary cell, the terminal device 141 may change the connection destination to the primary cell candidate while maintaining the conditions and preconfigured information regarding the secondary cell candidate associated with the primary cell candidate. Then, after changing the primary cell, the terminal device 141 may determine whether the CPAC conditions (e.g., event A3 or event A5) of the secondary candidate are continuously satisfied based on the measurement results of wireless quality, and may perform CPAC when it is determined that the conditions are satisfied. Thereafter, the terminal device 141 may perform CPAC based on the information regarding the maintained secondary cell candidate.

[0030] Furthermore, the terminal device 141 may simultaneously execute CHO and CPAC even in a state where the radio quality of any of the primary cell candidates satisfies the connection condition (for example, event A3 or event A5) while the radio quality of none of the secondary cell candidates satisfies the connection condition. In this case, the terminal device 141 may determine the connection destination of the secondary cell by CPAC by selecting, for example, a cell with the best radio quality from among the secondary cell candidates associated with the primary cell candidates to which the connection is to be changed. Note that, among the secondary cell candidates associated with the primary cell candidates to which the connection is to be changed, a candidate to be connected when CPAC is executed in a state where the connection condition is not satisfied may be specified, and the terminal device 141 may connect to the specified candidate cell. Furthermore, the terminal device 141 may determine, as the secondary cell to which the connection is to be made by CPAC, a secondary cell candidate that has radio quality exceeding a radio quality threshold that is lower than the connection condition. In addition, when multiple primary cell candidates satisfy the connection condition but none of the secondary cell candidates associated with each of them satisfy the connection condition, the terminal device 141 may select a cell with the best radio quality from among the secondary cell candidates associated with those primary cell candidates and perform CHO with the primary cell associated with that cell as the connection destination. For example, assume that the CHO condition is satisfied for both a first primary cell candidate and a second primary cell candidate. Here, assume that a first secondary cell candidate is associated with the first primary cell candidate, and a second secondary cell candidate is associated with the second primary cell candidate. In this case, if the radio quality of the first secondary cell candidate is higher than the radio quality of the second secondary cell candidate, the terminal device 141 may determine to perform CHO to the first primary cell and CPAC to the first secondary cell. In this way, even if CHO is performed when the CPAC condition for the secondary cell is not satisfied, the secondary cell can be made usable after the primary cell is changed.This makes it possible to perform large-volume data communication using the secondary cell immediately after the primary cell is changed by the CHO.

[0031] Furthermore, if the radio quality of any of the secondary cell candidates satisfies a connection condition (for example, event A3 or event A5), the terminal device 141 may change the secondary cell by CPAC. In this case, the terminal device 141 may maintain the conditions and preconfigured information related to the primary cell candidate associated with the changed secondary cell. Thereafter, the terminal device 141 may determine whether to perform CHO based on the maintained information on the conditions related to the primary cell candidate, and if the condition for performing CHO is satisfied, may perform CHO and establish a connection with the primary cell candidate.

[0032] Furthermore, the terminal device 141 may simultaneously execute CHO and CPAC even in a state where the radio quality of one of the secondary cell candidates satisfies the connection condition (for example, event A3 or event A5) while the radio quality of none of the primary cell candidates satisfies the connection condition. In this case, the terminal device 141 may determine the connection destination of the primary cell by CHO by selecting, for example, a cell with the best radio quality from among the primary cell candidates associated with the secondary cell candidates to which the connection is to be changed. Note that, among the primary cell candidates associated with the secondary cell candidates to which the connection is to be changed, a candidate to be connected when CHO is executed in a state where the connection condition is not satisfied may be specified, and the terminal device 141 may connect to the specified candidate cell. Furthermore, the terminal device 141 may determine, as the primary cell to which CHO is to be connected, a primary cell candidate that has radio quality exceeding a radio quality threshold that is lower than the connection condition. In addition, when multiple secondary cell candidates satisfy the connection conditions but none of the primary cell candidates associated with them satisfy the connection conditions, the terminal device 141 may select a cell with the best radio quality from among the primary cell candidates associated with those secondary cell candidates, and perform CPAC with the secondary cell associated with that cell as the connection destination. In this way, even if CPAC is performed in a state where the CHO condition of the primary cell is not satisfied, it is possible to make the primary cell usable after changing the secondary cell.

[0033] Furthermore, when performing CHO to a certain primary cell candidate, the terminal device 141 may prioritize a secondary cell candidate that can be used in combination with other primary cell candidates as a target for establishing a connection, among secondary cell candidates that can be used in combination with the primary cell. For example, in the example of FIG. 1, when connecting to the cell 111, the terminal device 141 may attempt to establish a connection with the secondary cell candidate cells 131 to 133 by CPAC. At this time, the cell 133 cannot be used in combination with the cell 112, while the cells 131 and 132 can be used in combination with the cell 112. For this reason, the terminal device 141 may prioritize the cell 131 or the cell 132 over the cell 133 and attempt to connect to the cell as a secondary cell candidate. According to this, if the connection to the primary cell candidate fails and only the connection to the secondary cell candidate is successful, it is possible to maintain the connection and attempt a connection to another primary cell candidate. Then, if the connection to another primary cell candidate is successful, it is not necessary to reconfigure the secondary cell.

[0034] (Device configuration) FIG. 5 shows an example of the hardware configuration of a base station apparatus and a terminal apparatus. In one example, the base station apparatus and the terminal apparatus include a processor 501, a ROM 502, a RAM 503, a storage device 504, and a communication circuit 505. The processor 501 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 programs stored in the ROM 502 or the storage device 504 to perform overall processing of the apparatus and each of the above-mentioned processes. The ROM 502 is a read-only memory that stores information such as programs and various parameters related to the processing executed by the base station apparatus and the terminal apparatus. The RAM 503 functions as a workspace when the processor 501 executes the program and is a random access memory that stores temporary information. The storage device 504 is, for example, a removable external storage device. The communication circuit 505 is, for example, a circuit for LTE or 5G wireless communication. Although FIG. 5 shows one communication circuit 505, the base station apparatus and the terminal apparatus may have multiple communication circuits. For example, a base station apparatus and a terminal apparatus may have a common antenna as well as wireless communication circuits for LTE and 5G. The base station apparatus and the terminal apparatus may also have separate antennas for LTE and 5G. 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 for other wireless communication systems, such as a wireless LAN. The base station apparatus and the terminal apparatus may have separate communication circuits 505 for each of a plurality of available frequency bands, or may have a common communication circuit 505 for at least some of these frequency bands.

[0035] FIG. 6 shows an example of the functional configuration of a base station device. The base station device has, for example, an information acquisition unit 601, an information aggregation unit 602, and an information notification unit 603 as its functions. Note that FIG. 6 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 LTE and 5G base station devices generally have. The functional blocks in FIG. 6 are shown schematically, and the respective functional blocks may be integrated or further subdivided. Each function in FIG. 6 may be realized, for example, by the processor 501 executing a program stored in the ROM 502 or the storage device 504, or by a processor within the communication circuit 505 executing predetermined software. The details of the processes performed by each functional unit will not be described here, and only their general functions will be outlined.

[0036] First, a case will be described in which the base station apparatus is a base station apparatus that provides a primary cell to which the terminal apparatus is connected, that is, a case in which the base station apparatus is a handover source base station apparatus.

[0037] The information acquisition unit 601 acquires information on connection conditions and communication parameters for CHO from a first other base station device that provides a primary cell that is a candidate for the handover destination. At this time, the information acquisition unit 601 may acquire, for example, information on connection conditions and communication parameters for CPAC for a cell that can be used as a secondary cell in combination with a cell provided by the first other base station device as the primary cell from the first other base station device. For example, the information acquisition unit 601 transmits a HANDOVER REQUEST message including information on communication parameters currently used by the terminal device to the first other base station device. Then, the information acquisition unit 601 receives a HANDOVER REQUEST ACKNOWLEDGE message including information on connection conditions and communication parameters for CHO and CPAC from the first other base station device. In this case, the information acquisition unit 601 may acquire, for example, information such as that shown in FIG. 3(A) or 3(B). Alternatively, the information acquisition unit 601 may acquire, for example, unaggregated information such as that shown in FIG. 2.

[0038] The information acquisition unit 601 may acquire, from a first other base station device, information on the connection conditions and communication parameters of the CHO and information for identifying a cell that can be used as a secondary cell in combination with a cell provided by the first other base station device as a primary cell. In this case, the information acquisition unit 601 may request information on the connection conditions and communication parameters for CPAC from a second other base station device that provides the cell, and acquire the information on the connection conditions and communication parameters in response to the request. In one example, the request and response in this case may be performed by an SgNB Addition Request and an SgNB Addition Request Acknowledge, respectively. That is, the information on the connection conditions and communication parameters for the secondary cell candidate may be acquired via the first other base station device that provides the cell that is a candidate for the primary cell, or may be acquired directly from the second other base station device that provides the cell that is a candidate for the secondary cell.

[0039] The information aggregating unit 602 aggregates the information acquired by the information acquiring unit 601 and performs processes such as deleting duplicate communication parameters and replacing them with pointers. For example, when the information aggregating unit 602 acquires non-aggregated information such as that shown in FIG. 2, the information aggregating unit 602 may generate information such as that shown in FIG. 3(A) or 3(B) in which information is aggregated for each primary cell, or may generate information such as that shown in FIG. 4(A) or 4(B) in which information about multiple primary cells is aggregated. The information notifying unit 603 notifies the terminal device of the aggregated connection conditions and communication parameters for CHO and CPAC generated by the information aggregating unit 602.

[0040] Next, a case where the base station apparatus is a base station apparatus that provides a primary cell to which the terminal apparatus is connected, that is, a handover source base station apparatus, will be described.

[0041] For example, when the terminal device itself provides a cell that is a candidate for the primary cell, the information acquisition unit 601 acquires information on connection conditions and communication parameters for CPAC for a cell that is a candidate for a secondary cell that can be used in combination with the cell. For example, the information acquisition unit 601 may collect information on connection conditions and communication parameters for CPAC in response to a request for information on connection conditions and communication parameters for CHO from a base station device that provides a primary cell to which the terminal device is currently connected. When the information acquisition unit 601 receives a HANDOVER REQUEST including information on communication parameters and the like being used by the terminal device from a first other base station device that provides a primary cell to which the terminal device is currently connected, the information acquisition unit 601 transmits an SgNB Addition Request including information corresponding to the communication parameters to a second other base station device that provides a cell that is a candidate for the secondary cell. Then, the information acquisition unit 601 may receive an SgNB Addition Request Acknowledge including information on connection conditions and communication parameters from the second other base station device.

[0042] The information aggregation unit 602 aggregates the information acquired by the information acquisition unit 601. For example, the information aggregation unit 602 of the base station device 101 generates information such as that shown in Fig. 3(A). The information notification unit 603 notifies the base station device that provides the primary cell to which the terminal device is currently connected of the information generated by the information aggregation unit 602. Note that the information acquired by the information acquisition unit 601 may be provided as is to the base station device that provides the primary cell to which the terminal device is currently connected, in which case the information aggregation unit 602 may be omitted.

[0043] A base station device that provides a cell that is a candidate for a secondary cell is configured to notify other base station devices that have requested the information of connection conditions and communication parameters for CPAC.

[0044] In one example, each base station device may have all or some of the above capabilities. That is, a base station device may have all or some of the functions of a base station device providing a primary cell to which a terminal device is currently connected, a base station device providing a candidate cell for the primary cell, and a base station device providing a candidate cell for the secondary cell.

[0045] FIG. 7 shows an example of the functional configuration of a terminal device. The terminal device has, as its functions, for example, a presetting unit 701, a condition determination unit 702, and a connection processing unit 703. Note that FIG. 7 only shows functions particularly related to this embodiment, and various other functions that the terminal device may have are omitted from the illustration. For example, the terminal device naturally has other functions that LTE and 5G base station devices generally have. Also, the functional blocks in FIG. 7 are shown schematically, and each functional block may be realized by being integrated or further subdivided. Also, each function in FIG. 7 may be realized, for example, by the processor 501 executing a program stored in the ROM 502 or the storage device 504, or may be realized, for example, by a processor within the communication circuit 505 executing predetermined software. Note that the details of the processing performed by each functional unit will not be described here, and only their general functions will be outlined.

[0046] The preconfiguration unit 701 acquires information on connection conditions and communication parameters for CHO and CPAC from the base station device providing the currently connected primary cell, and performs configuration for CHO and CPAC. The preconfiguration unit 701 receives, for example, an RRC Reconfiguration message and acquires information such as that shown in FIG. 3(A) or 3(B) or that shown in FIG. 4(A) or 4(B) included in the message. The preconfiguration unit 701 then preconfigures CHO for a primary cell candidate, and preconfigures CPAC for a secondary cell candidate that can be used in combination with the primary cell candidate. The condition determination unit 702 determines whether connection conditions are satisfied for each of the primary cell and secondary cell candidates configured by the preconfiguration unit 701. The connection processing unit 703 performs connection processing for a cell for which the condition determination unit 702 determines that the connection conditions are satisfied, using communication parameters preconfigured for that cell. For example, if the connection processing unit 703 succeeds in connecting to a secondary cell but fails in connecting to a primary cell, the condition determination unit 702 may adjust the connection conditions so that a connection to another primary cell candidate that can be set together with the successfully connected secondary cell is attempted. Also, the condition determination unit 702 may adjust the connection conditions so that a secondary cell candidate that can be used in combination with multiple primary cell candidates is preferentially connected to.

[0047] (Processing flow) Next, an example of the processing flow will be described. Fig. 8 shows a procedure in which a base station device (S-MN) that provides a primary cell to which a terminal device is currently connected provides information for CHO and CPAC to a terminal device 141. S-MN is an abbreviation for Source Master Node. Here, a base station device that provides a candidate for the primary cell is called a Target Master Node (T-MN). In the example of Fig. 1, the base station device 101 and the base station device 102 are T-MNs. Also, a base station device that provides a cell that is a candidate for the secondary cell is called a Target Secondary Node (T-SN). In the example of Fig. 1, the base station devices 121 to 124 are T-SNs.

[0048] First, the S-MN selects as the T-MN, from among the base station devices that provide cells that are candidates for the primary cell, a base station device whose reception quality of signals transmitted from those base station devices is equal to or higher than a predetermined value, and transmits a HANDOVER REQUEST (S801). The HANDOVER REQUEST includes, for example, information that enables the terminal device 141 to determine whether a communication service can be provided with communication performance corresponding to the communication parameters currently used. The HANDOVER REQUEST here may also include information requesting the provision of information on secondary cell candidates that can communicate with communication performance corresponding to the communication parameters of the secondary cell to which the terminal device 141 is currently connected. Information on the wireless quality measurement value of the terminal device 141 for each secondary cell candidate may also be provided. Upon receiving this information, the T-MN transmits an SgNB Addition Request to the T-SN that provides a cell that can be used as a secondary cell when the cell provided by the T-MN itself is set as the primary cell (S802, S804). The SgNB Addition Request here includes, for example, information that can identify the required communication performance. If the T-SN can provide a communication service with the required communication performance, it transmits to the T-MN an SgNB Addition Request Acknowledge including information indicating that the connection is permitted and the connection conditions and communication parameters in CPAC (S803, S805). When the T-MN receives the connection conditions and communication parameters in CPAC from the T-SN, it transmits to the S-MN a HANDOVER REQUEST ACKNOWLEDGE including the information and the connection conditions and communication parameters in CHO to the T-MN (S807). At this time, the T-MN may aggregate the information as shown in Figure 3(A) or 3(B) (S806) and then transmit it to the S-MN.

[0049] When the S-MN receives the information on CHO and CPAC from the T-MN, it notifies the terminal device 141 of an RRC Reconfiguration message including that information (S809), and the terminal device 141 transforms an RRC Reconfiguration Complete message into the S-MN (S810), completing the pre-configuration. Note that the S-MN may aggregate the information received from the T-MN (S808) and then provide the information to the terminal device 141. In this way, the pre-configuration of CHO and CPAC is completed in the terminal device 141.

[0050] Next, a processing flow when a terminal device changes its connection destination will be described with reference to Figures 9 and 10. In these examples, it is assumed that T-MN-A to T-MN-C exist as base station devices that provide candidates for the primary cell. It is also assumed that the primary cell provided by T-MN-A can be used in combination with a cell provided by T-SN-D as a secondary cell, and that the primary cells provided by T-MN-B and T-MN-C can be used in combination with a cell provided by T-SN-E as a secondary cell. Here, it is assumed that the terminal device attempts to establish a connection to T-MN-B by CHO and then attempts to establish a connection to T-SN-E by CPAC, but fails to connect to T-MN-B but succeeds in connecting to T-SN-E (S901, S902). In this case, in order to maintain the connection to T-SN-E, the terminal device determines to preferentially connect to T-MN-C, which can provide a primary cell that can be used in combination with a secondary cell provided by T-SN-E (S903). Then, the terminal device attempts to establish a connection with the T-MN-C while maintaining the connection with the T-SN-E (S904). This eliminates the need to retry the secondary cell connection, making it possible to establish a connection efficiently.

[0051] The example of FIG. 10 illustrates a case in which, before performing handover, the terminal device selects a T-MN that provides a usable primary cell together with a common secondary cell as a T-MN to which the terminal device should preferentially connect. In this example, there are two T-MNs, T-MN-B and T-MN-C, that provide primary cells that can use a cell provided by a T-SN-E as a secondary cell. On the other hand, there is only one T-MN, T-MN-A, that provides a primary cell that can use a cell provided by a T-SN-D as a secondary cell. Therefore, the terminal device determines that the secondary cell associated with more primary cells is a cell provided by the T-SN-E, and determines T-MN-B and T-MN-C, that provide primary cells that can be used together with this secondary cell, as targets to which the terminal device should preferentially connect (S1001). Then, based on this determination, the terminal device performs, for example, CHO to the base station device with better radio quality between T-MN-B and T-MN-C, and CPAC to the T-SN-E (S1002, S1003).

[0052] In this embodiment, it is possible to efficiently execute CHO and CPAC in this way, which makes it possible to contribute to Goal 9 of the Sustainable Development Goals (SDGs) led by the United Nations, which is to "build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0053] 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, a receiving means for receiving, from a base station device currently connected to the terminal device, pre-setting information for the terminal device to autonomously select and switch a cell to which the terminal device is connected; the preconfigured information includes, for each of a first cell that is a candidate for a primary cell at a handover destination or a second cell that is a candidate for a secondary cell when the first cell is set as the primary cell, first information indicating a condition for connecting to the cell and communication parameters to be used when connecting to the cell, and second information indicating a combination of the first cell that is a candidate for a primary cell at a handover destination and the second cell that is a candidate for a secondary cell when the first cell is set as the primary cell; In the second information, when one cell is commonly included in two or more different combinations of a primary cell and a secondary cell, and communication parameters of the one cell are partially different in each of the two or more combinations, a first part of communication parameters common to each of the two or more combinations among the communication parameters of the one cell is omitted, and only a second part of communication parameters that are not common to each of the two or more combinations is included in the first information for some of the two or more combinations. A terminal device characterized by:

2. a determination means for determining whether a first condition for connecting to a first cell that is a candidate for the primary cell and a second condition for connecting to a second cell that is a candidate for the secondary cell are simultaneously satisfied; an execution means for executing a connection between the first cell and the second cell when the determination means determines that the first condition and the second condition are satisfied; 2. The terminal device according to claim 1, further comprising:

3. The terminal device according to claim 2, characterized in that, when a first condition regarding the primary cell is satisfied, the terminal device connects to the handover destination while maintaining a second condition and setting information regarding a candidate secondary cell associated with the primary cell.

4. The terminal device according to claim 1, characterized in that when the second information indicates a combination of one of the first cells that is a candidate for a primary cell with each of multiple second cells that are candidate for a secondary cell, and when communication parameters of the one of the first cells are common to each of the combinations, the communication parameters of the one of the first cells are indicated only once in the first information.

5. 2. The terminal device according to claim 1, wherein, in the second information, a combination of each of a plurality of first cells that are candidates for a primary cell with one of the second cells that are candidates for a secondary cell is indicated, and when communication parameters of the one of the second cells are common to each of the combinations, the communication parameters of the one of the second cells are indicated only once in the first information.

6. 2. The terminal device according to claim 1, wherein the receiving means receives the preconfiguration information from the base station device by a radio resource control (RRC) message.

7. A control method executed by a terminal device, comprising: receiving pre-setting information from a base station device currently connected to the terminal device for the terminal device to autonomously select and switch a cell to which the terminal device is connected; the preconfigured information includes, for each of a first cell that is a candidate for a primary cell at a handover destination or a second cell that is a candidate for a secondary cell when the first cell is set as the primary cell, first information indicating a condition for connecting to the cell and communication parameters to be used when connecting to the cell, and second information indicating a combination of the first cell that is a candidate for a primary cell at a handover destination and the second cell that is a candidate for a secondary cell when the first cell is set as the primary cell; In the second information, when one cell is commonly included in two or more different combinations of a primary cell and a secondary cell, and communication parameters of the one cell are partially different in each of the two or more combinations, a first part of communication parameters common to each of the two or more combinations among the communication parameters of the one cell is omitted, and only a second part of communication parameters that are not common to each of the two or more combinations is included in the first information for some of the two or more combinations. A control method comprising:

8. A program for causing a computer provided in a terminal device to receive pre-setting information from a base station device currently connected to the terminal device, for the terminal device to autonomously select and switch a cell to which the terminal device is connected, the preconfigured information includes, for each of a first cell that is a candidate for a primary cell at a handover destination or a second cell that is a candidate for a secondary cell when the first cell is set as the primary cell, first information indicating a condition for connecting to the cell and communication parameters to be used when connecting to the cell, and second information indicating a combination of the first cell that is a candidate for a primary cell at a handover destination and the second cell that is a candidate for a secondary cell when the first cell is set as the primary cell; In the second information, when one cell is commonly included in two or more different combinations of a primary cell and a secondary cell, and communication parameters of the one cell are partially different in each of the two or more combinations, a first part of communication parameters common to each of the two or more combinations among the communication parameters of the one cell is omitted, and only a second part of communication parameters that are not common to each of the two or more combinations is included in the first information for some of the two or more combinations. A program characterized by:

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