Base station, control method, and program for suppressing congestion in dual connectivity

Abstract

To suppress congestion of a secondary node in a mobile communication system capable of using dual connectivity.SOLUTION: A base station capable of operating as a secondary node for dual connectivity defined in the cellular communication standard of the Third Generation Partnership Project (3GPP), based on the satisfaction of a first condition for restricting addition of a base station as a secondary node, notifies another base station of a first notification including information for causing it to execute a determination process of whether to make a secondary node addition request to the base station.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a technique for controlling dual connectivity.

Background Art

[0002] In the cellular communication standard of the 5th generation mobile communication system (5G) of the 3rd Generation Partnership Project (3GPP), a specification of a technique called Dual Connectivity has been established, which bundles and simultaneously uses wireless connections between each of a plurality of base stations connected by a backhaul and a terminal. By using dual connectivity, a user's terminal can communicate with both a main base station (master node) and a secondary base station (secondary node) at the same time. Therefore, effects such as improvement of throughput in the terminal and offloading of communication traffic in the main base station are expected. Non-Patent Document 1 defines a Secondary Node Addition procedure for adding another base station as a secondary node of dual connectivity when the terminal is connected to the master node.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the 3GPP cellular communication standard, a mechanism for restricting the connection of a terminal to a specific cell is defined. For example, a base station can perform such a restriction to prevent or alleviate the congestion of communication traffic in the cell provided by the base station itself. However, when dual connectivity is used, the addition of a congested base station as a secondary node may accelerate the congestion state. The present invention provides a technique for suppressing the congestion of a secondary node in a mobile communication system capable of using dual connectivity.

Means for Solving the Problem

[0005] A base station according to an aspect of the present invention is a base station operable as a secondary node of dual connectivity defined in the cellular communication standard of the Third Generation Partnership Project (3GPP), and based on the satisfaction of a first condition that the addition of the base station as a secondary node should be restricted, it has notification means for sending a first notification including information for causing another base station to execute a determination process as to whether to request the addition of the base station as a secondary node.

[0006] A base station according to an aspect of the present invention is a base station operable as a master node of dual connectivity defined in the cellular communication standard of the Third Generation Partnership Project (3GPP), and has receiving means for receiving a first notification including information for the base station to execute a determination as to whether to request the addition of another base station as a secondary node, which is transmitted from the other base station based on the satisfaction of a first condition that the addition of the other base station as a secondary node should be restricted in the other base station, determination means for executing the determination for the addition of the other base station as a secondary node based on the first notification, and control means for controlling to execute the request for the addition of the secondary node if it is determined to request the addition of the secondary node according to the result of the determination, and not to execute the request for the addition of the secondary node if it is determined not to request the addition of the secondary node.

Advantages of the Invention

[0007] According to the present invention, in a mobile communication system capable of using dual connectivity, congestion of secondary nodes can be suppressed.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0009] Hereinafter, 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 the features described in the embodiments are essential for the invention. Two or more of the plurality of features described in the embodiments may be arbitrarily combined. Also, the same or similar configurations are given the same reference numerals, and duplicate descriptions are omitted.

[0010] (System Configuration) FIG. 1 shows a configuration example of a mobile communication system according to the present embodiment. The mobile communication system is, for example, a cellular communication system compliant with the cellular communication standard of the 3rd Generation Partnership Project (3GPP (registered trademark)). However, it is not limited thereto, and the following discussion can be applied to a mobile communication system compliant with any wireless communication standard. The mobile communication system includes, for example, a terminal 101, a base station 111, a base station 112, and a 5th Generation Core Network (5GC) 120. The terminal 101 is a terminal used by a user and exchanges wireless signals with the base station 111 and the base station 112 via a wireless medium. The terminal 101 can be called a User Equipment (UE). The terminal 101 includes, for example, a smartphone, a mobile phone, a personal computer, a tablet terminal, a wearable terminal, an IoT (Internet of Things) terminal, and the like. In FIG. 1, an example is shown in which the terminal 101 communicates with the base station 111 and the base station 112 via a wireless medium, but two or more terminals 101 may be connected to one base station 111 or base station 112. The base station 111 exchanges wireless signals with the terminal 101 via a wireless medium. The base station 111 is a so-called base station and includes, for example, a gNB (next Generation Node B), an eNB (evolved Node B), and the like. On the other hand, similarly to the base station 111, the base station 112 exchanges wireless signals with the terminal 101 via a wireless medium. The base station 112 also includes, for example, a gNB (next Generation Node B), an eNB (evolved Node B), and the like. The range in which communication with the base station 111 or the base station 112 can be performed provided by the base station 111 and the base station 112 is called a cell. For example, in FIG. 1, a cell 131 and a cell 132 are provided by each of the base station 111 and the base station 112. Each of the base station 111 and the base station 112 is connected to the 5GC 120. In addition, the base station 111 and the base station 112 are connected to each other, and for example, communication using the X2 protocol or the Xn protocol is performed. The 5GC 120 is a core network having various network functions.The 5GC 120 includes network functions such as, for example, an Access and Mobility Management Function (AMF), a Session Management Function (SMF), and a User Plane Function (UPF). The AMF performs procedures and management such as registration, connection, and mobility of the terminal 101 to / from the network. The UPF provides connection point functions such as routing for transferring user plane data (data) transmitted from the terminal 101 to the data network and transferring data received from the data network to the destination terminal 101. The SMF provides a session management function when the terminal 101 transmits data to the data network. For example, the SMF performs IP address allocation for the terminal 101, selection of the destination UPF, session management of the PDU (Packet Data Unit), etc. The functions provided by the AMF and SMF can be called control plane (CP) functions. On the other hand, the functions provided by the UPF can be called user plane (UP) functions. In this embodiment, an example where the core network is the 5GC is used for explanation, but the core network may be an Evolved Packet Core (EPC) or the like.

[0011] A mode in which the terminal 101 is simultaneously connected to the base stations 111 and 112 and performs transmission and reception using a plurality of component carriers is called dual connectivity. For example, in the case where the base station 111 (which is a master node in this embodiment) is an eNB and the base station 112 (which is a secondary node in this embodiment) is a gNB, dual connectivity can be used in a non-standalone (NSA) type system that uses a wide-coverage Long-Term Evolution (LTE) / LTE-Advanced area as an anchor and provides 5G New Radio (NR) services. On the other hand, dual connectivity in which both the base stations 111 and 112 are gNBs is used in a standalone (SA) type system and can be used, for example, for the purpose of increasing the throughput of the terminal 101 or offloading the traffic of the base station 111 to the base station 112. In any mode, the control plane signals are communicated with the terminal 101 via the base station 111. User plane signals (i.e., data) transmitted from the data network can be transferred to the terminal 101 via the UPF of the 5GC 120 and the base station 111. At this time, the base station 111 can transfer a part of the data to the base station 112 and transfer it to the terminal 101 via the base station 112. On the other hand, data transmitted from the data network can be transferred to the terminal 101 via the UPF of the 5GC 120 and the base station 112. At this time, the base station 112 can transfer a part of the data to the base station 111 and transfer it to the terminal 101 via the base station 111.

[0012] Figure 2 shows an example of a sequence when dual connectivity using base station 111 and base station 112 is established based on the request of terminal 101. In Figure 2, it is assumed that terminal 101 has established a connection with base station 111. After establishing the connection with base station 111, terminal 101 performs cell search and attempts to detect other base stations that satisfy a predetermined condition for enabling dual connectivity. Cell search means that terminal 101 searches for cells that it can connect to. Terminal 101 can measure the received power of the cell (which can also be called Reference Signal Receive Power, RSRP) and the received quality (which can also be called Reference Signal Receive Quality, RSRQ) using synchronization signals and notification signals transmitted in each cell, and can also obtain the physical ID (PCI) of the cell. Also, the predetermined condition for enabling dual connectivity can be provided from base station 111 to terminal 101. When terminal 101 detects base station 112 that satisfies the predetermined condition, it reports to base station 111. Terminal 101 can report, for example, by transmitting a Measurement Report. When base station 111 receives the measurement report from terminal 101, it starts the procedure for setting up dual connectivity with base station 112. First, base station 111 transmits a dual connectivity (DC) setup request (SN Addition Request) to base station 112 (S201). In response to the DC setup request, base station 112 transmits a response signal (SN Addition Request Acknowledge) to base station 111 (S202). The response signal may include radio parameter information in the cell provided by base station 112. When base station 111 receives the response signal, it transmits a radio resource setup signal (RRC reconfiguration) to terminal 101 (S203). This radio resource setup signal may include radio parameter information in the cell provided by base station 112. Terminal 101 transmits a completion notification (RRC reconfiguration complete) for the setup signal to base station 111 (S204).Also, the terminal 101 performs a random access procedure with respect to the base station 112 to establish a connection (S205). The base station 111 transmits a DC configuration completion notification (SN reconfiguration complete) to the base station 112 (S206). Thereby, the establishment of dual connectivity is completed. Then, the base station 111 performs a notification regarding the data transfer status (SN Status Transfer) (S207), and starts transferring the data received from the 5GC 121 to the base station 112 (S208). This data is transmitted to the terminal 101 via the base station 112.

[0013] Base stations 111 and 112 can perform access control when the traffic in the cells provided by their own devices converges, etc. For example, base stations 111 and 112 can execute access control for the purpose of ensuring the connectivity of important communications such as emergency calls, or for the purpose of suppressing a sudden increase in access when their own devices recover from a failure. As an example of access control, Unified Access Control (UAC) regulation can be used. In UAC regulation, base stations 111 and 112 can perform access control by specifying the type of communication (Access Category and Access Identity) and the transmission rate. Access Category is an identifier of the service type, and can identify, for example, emergency transmissions, voice calls, video calls, etc. Also, Access Identity is an identifier of the call type, and can distinguish Multimedia Priority Service, Mission Critical Service, priority calls, etc. from other calls. Also, base stations 111 and 112 can provide information regarding access control (regulation information) using the Master Information Block (MIB), System Information Block (SIB), etc. that are notified to the cells provided by their own devices. For example, barringInfoSet of SIB1 can be used. In this case, terminal 101 recognizes the regulation information by receiving the MIB and SIB, and determines whether or not it is subject to access control based on the type of communication each time a communication request occurs. If it is subject to access control, terminal 101 determines whether or not to perform access using a random number. In this embodiment, unless otherwise specified, the random number can take a value in the range of 0 to 1. For example, when the transmission rate is 30%, terminal 101 performs access if the value of the random number is 0.3 or less, and does not perform access if it exceeds 0.3.

[0014] In this way, the base stations 111 and 112 can suppress the number of accesses from terminals existing within the cell provided by the own device by allowing access only to terminals that satisfy the conditions set for access control. On the other hand, when a connection request is made via a communication protocol between base stations such as the above-described dual connectivity, the above access control does not function. For example, even if access control is set for the base station 112, the base station 111 that does not recognize this can transmit a DC setting request to the base station 112. The base station 112 can reject this request. However, if the request itself from the base station 111 cannot be stopped, for example, as the number of base stations arranged on the network increases, the communication volume between base stations due to connection requests from these base stations and signaling for rejecting these requests will increase. Also, the fact that access control is set for the base station 112 highly likely means that the processing load in the base station 112 is increasing, and in addition, the load for processing signals received from the base station 111 may also increase, which can also be a problem. On the other hand, in response to the setting of access control for the base station 112, it is also conceivable that the base station 112 disconnects communication with other base stations including the base station 111. However, in order to disconnect communication, it is necessary to execute a disconnection procedure for each base station, and also, when communication is disconnected, the cooperative operation between base stations including the operation as a secondary node becomes impossible. For example, for a dual connectivity request, it is not possible to perform stepwise regulation as in the access control within the cell.

[0015] In view of such circumstances, in this embodiment, based on the satisfaction of the first condition that the addition of the base station 112 as a secondary node should be restricted, a first notification including information for executing a determination process as to whether to request the addition of the base station 112 as a secondary node is sent to the base station 111. Further, the base station 112 may further send a second notification including expiration value information for specifying the expiration value of a timer started at the base station 111 based on the satisfaction of the first condition. The expiration value information can be used for a timer that measures the deadline of the period during which the base station 111 is prohibited from requesting the addition of the base station 112 as a secondary node when it is determined not to add the secondary node as a result of the determination process for adding the secondary node for the base station 112 performed by the base station 111. On the other hand, the base station 111 is configured to receive a first notification including information for the base station 111 to execute a determination as to whether to request the addition of the base station 112 as a secondary node, which is sent from the base station 112 based on the satisfaction of the first condition at the base station 112. Then, based on the first notification, the base station 111 executes a determination for adding the base station 112 as a secondary node. If it is determined to request the addition of the secondary node, the base station 111 executes the request for adding the secondary node. If it is determined not to request the addition of the secondary node, the base station 111 does not execute the request for adding the secondary node. Further, when the base station 111 determines not to execute the request for adding the secondary node as a result of executing the determination process for adding the secondary node for the base station 112, the base station 111 starts a predetermined timer. The predetermined timer is a timer that measures the period during which the base station 111 is prohibited from requesting the addition of the base station 112 as a secondary node, and the base station 111 uses the expiration value information included in the second notification received from the base station 112 to specify the deadline of this period. That is, when the base station 111 requests the addition of the secondary node, if it executes the determination process and as a result determines not to request the addition of the secondary node, it sets a timer for measuring the deadline of a predetermined period, and during this period, it does not request the addition of the base station 112 as a secondary node.Thus, when the addition of the base station 112 as a secondary node of dual connectivity should be restricted, the secondary node request from the base station 111 to the base station 112 can be suppressed. In the present embodiment, a series of operations performed by the base station 111 based on the above notification from the base station 112 may be referred to as restricted operations.

[0016] For example, when access control is set for the base station 112 and it is assumed that a condition (first condition) for restricting the addition of the base station 112 as a secondary node is satisfied, the base station 112 can notify the base station 111 of the restriction. Then, upon receiving this notification, the base station 111 can recognize that it is necessary to execute a determination process in order to add the base station 112 as a secondary node. For example, after receiving the above notification, the base station 111 can execute the determination process when the base station 112 satisfies a predetermined condition in the measurement report from the terminal 101. In addition, after receiving the above notification, the base station 111 may execute the determination process based on the connection of the terminal 101 to the own device. The base station 111 can perform a determination process by drawing a random number within a predetermined range (for example, a range of 0 to 1) and determining whether to request the addition of a secondary node based on the value of the random number. The random number can be called Rand. For example, the notification from the base station 112 may include a predetermined threshold value when the base station 111 executes this determination. The predetermined threshold value may be called a transmittance or a throughput rate, and is not limited thereto, and may be called by other names. In this embodiment, this predetermined threshold value may sometimes be referred to as a transmittance or the like. When the random number drawn by the base station 111 is equal to or greater than the threshold value, the base station 111 can determine to execute a request for adding a secondary node, and when the random number drawn by the base station 111 is less than the threshold value, the base station 111 can determine not to execute a request for adding a secondary node. Also, when the random number drawn by the base station 111 is equal to or greater than the threshold value, the base station 111 can determine not to execute a request for adding a secondary node, and when the random number drawn by the base station 111 is less than the threshold value, the base station 111 can determine to execute a request for adding a secondary node. When the base station 111 determines to request the addition of a secondary node, the base station 111 makes a request to the base station 112. The base station 112 can return a response to this request. On the other hand, when the base station 111 determines not to request the addition of a secondary node, the base station 111 does not execute a request for adding a secondary node and starts a timer for measuring the expiration of the period during which a request for adding a secondary node to the base station 112 is prohibited.In this way, based on the fact that access control is set for base station 112, a notification is sent to base station 111. Before requesting the addition of a secondary node to base station 112, base station 111 determines whether to make such a request based on a random number value subtracted from a predetermined range. As a result, when access control is set, base station 112 can suppress communication to itself not only within the cell but also in the connection between base stations. Also, base station 112 can accept addition as a secondary node based on a predetermined threshold value. For example, even after access control is set, base station 112 can flexibly adjust the ratio of operating as a secondary node by adjusting a predetermined threshold value based on the traffic situation, etc. in the cell provided by the own device. Furthermore, if base station 111 determines not to execute the request for adding a secondary node, it starts a timer and does not make a request for adding a secondary node to base station 112 during the period until the expiration. As a result, base station 112 can further reduce the frequency of being requested to add a secondary node from base station 111. Also, by adjusting the expiration value that base station 112 should use at the expiration of this timer based on the traffic situation, etc. provided by the own device and providing it to base station 111, the frequency of receiving a request for adding a secondary node can be flexibly adjusted.

[0017] In the notification to base station 111, base station 112 may apply different transmittance rates, etc. according to the type of communication executed by the terminal. For example, when the communication handled by terminal 101 is a communication with high urgency or priority, base station 112 may set a high transmittance rate. For this purpose, base station 112 may notify base station 111 of the transmittance rate, etc. set for each access category or access identity. Base station 111 selectively uses the transmittance rate, etc. for determining whether to request the addition of a secondary node according to the type of communication handled by terminal 101 connected to its own device. Note that base station 112 may set a transmittance rate only for a specific type of communication and may not accept requests for adding a secondary node for other types of communication. In this case, base station 112 may identify the communication for which the transmittance rate is set and notify base station 111. In this case, base station 111 does not execute requests for adding a secondary node for communications other than the specified type of communication. In this way, even after access control is set, base station 112 can flexibly execute operations as a secondary node according to the type of communication handled by terminal 101.

[0018] In addition, the base station 112 may notify, in the notification to the base station 111, a predetermined reference value for specifying the expiration value to be used for the timer activated by the base station 111. Then, the base station 111 may calculate the expiration of the activated timer by using this reference value in a predetermined calculation formula. For example, the base station 111 may use the value of T390 calculated by the formula T390 = (0.7 + 0.6 × rand) × uac - BarringTime as the expiration of the timer. Here, rand is a random number in a predetermined range (for example, 0 to 1), and uac - BarringTime is the reference value notified from the base station 112 to the base station 111. In this case, after activating the timer, the base station 111 does not make a secondary addition request to the base station 112 during the period from the activation of the timer to the expiration indicated by the value of T390. In this way, by using a random number in the calculation of the expiration value of the timer activated by the base station 111, the expiration times of the timers activated by each base station 111 will be different from each other. As a result, in a situation where a large number of terminals 101 attempt to connect intensively, such as at the time of recovery after a network failure, it is possible to disperse the secondary node addition requests from multiple base stations.

[0019] FIG. 3 shows an example of a sequence between base station 111 and base station 112 when a first condition that base station 112 should be restricted from being added as a secondary node is satisfied. This sequence can be executed using, for example, an X2 interface or an Xn interface established between base station 111 and base station 112. Note that the protocol for executing this sequence is not limited to the X2 interface or the Xn interface, and can be applied to any interface used for notification between base station 111 and base station 112. For example, before an X2 interface or an Xn interface is established between base station 111 and base station 112, assume that at base station 112, the first condition is satisfied. In this case, base station 112 can notify that the first condition is satisfied (for example, access restriction is set) using an establishment request or an establishment response of the X2 interface or the Xn interface exchanged with base station 111 (S301, S302). In the example of FIG. 3, base station 112 makes the notification in an Xn Setup Response as a response to the Xn Setup Request received from base station 111. When base station 112 transmits an Xn Setup Request, the notification can be made in the Xn Setup Request. Note that this notification may include, as restriction information (ac-BarringInfo), a notification that it is a notification for restricting a secondary node request in dual connectivity (ac-BarringForDualConnectivity), and parameters as restriction information. As an example, the parameters as restriction information may include a transmittance (ac-BarringFactor), expiration value information to be used for a timer (ac-BarringTime), and information indicating whether a specific access category (Special Access Category) is subject to restriction (ac-BarringForSpecialAC). For example, in FIG. 3, the transmittance p05 may indicate that 95% of the secondary requests are restricted and 5% are permitted. Also, the expiration value s16 may indicate that 16 seconds should be set as the expiration of the timer. Note that 16 seconds may be used as a reference value for calculating T390.The transmittance and other information and the completion value information may be indicated in the same notification or in individual notifications respectively.

[0020] When an X2 interface or an Xn interface is established between base station 111 and base station 112, if a first condition is satisfied at base station 112, base station 112 may notify base station 111 using a message for notifying an update of its own device settings (S303). For example, NG-RAN NODE CONFIGURATION UPDATE may be used as this message. In this case, the master node may respond with NG-RAN NODE CONFIGURATION UPDATE ACKNOWLEDGE (S304). Then, when base station 111 makes a secondary node addition request to base station 112 based on a measurement report or the like from terminal 101, it executes a condition determination (determination process) (S305). If it is determined to make a request in the condition determination, base station 111 transmits a request (SN Addition Request) (S306). In response to this, base station 112 transmits a response (SN Addition Request ACK) (S307). Thereby, in the dual connectivity of terminal 101, base station 112 is added as a secondary node. On the other hand, if it is determined not to make a request in the condition determination, base station 111 sets a prohibition period for the secondary node addition request without transmitting a request (S308).

[0021] In the notification to base station 111, base station 112 may use the same regulation information as the regulation information used for access control of the cell provided by the own device. For example, when access control is set, base station 112 may notify the regulation information using system information (MIB, SIB1, SIB2, etc.). In this case, the system information may include the type of communication and the transmission rate as parameters related to access control. For example, in the notification to base station 111, base station 112 may include parameters with the same values as the parameters related to access control notified as system information. Thereby, base station 112 can fairly handle the access by terminal 101 in the cell (i.e., the access to base station 112 as the master node) and the access for adding a secondary node. On the other hand, in the notification to base station 111, base station 112 may use regulation information different from the regulation information used for access control of the cell provided by the own device. For example, as the transmission rate, etc. in the notification to base station 111, base station 112 may notify a value lower than the transmission rate, etc. left in the system information so as to suppress the addition of a secondary node. For example, when the purpose of dual connectivity is to increase the throughput using a plurality of connections (i.e., when a certain throughput is ensured in the connection between terminal 101 and base station 111), base station 112 can provide communication services to more terminals 101 by prioritizing the operation as the master node. (Modification Example 1) In the above embodiment, the description was given using an example in which when a condition (the first condition) that should restrict the addition of the own device as a secondary node is satisfied, the base station 112 notifies the base station 111 of the restriction, and the base station 111 determines whether to execute a request for adding a secondary node. In this modified example, after the first condition is satisfied, in response to a request from the base station 111 to add the base station 112 as a secondary node, the base station 112 determines whether to accept this request. For example, after the first condition is satisfied, when the base station 112 receives a request to add a secondary node from the base station 111, it determines whether to accept this request. If it determines not to accept this request, the base station 112 notifies the base station 111 of a notice indicating that this request is rejected. This notice includes expiration value information to be used for a timer that the base station 111 activates. When the base station 111 receives this notice, it activates a timer for measuring the expiration of the period during which the request to add the base station 112 as a secondary node is prohibited, using the expiration value information included in this notice. In this way, by the base station 112 determining whether to accept a request, the procedure for the base station 112 to notify other base stations of the restriction can be omitted. For example, when the base station 112 has established connections with a large number of base stations, if a restriction notice is sent every time the first condition is satisfied, traffic for that purpose will be transmitted on the network. Also, not all base stations will necessarily request to add the base station 112 as a secondary node. In this way, by the base station 112 making a determination based on a request from the base station 111, the traffic transmitted on the network can be reduced.

[0022] FIG. 4 shows an example of a sequence between base stations 113 to 115 and base station 112 when the first condition for restricting the addition of base station 112 as a secondary node is satisfied. Base stations 113 to 115 operate as master nodes in the same manner as base station 111 described above. Base stations 113 to 115 may be collectively referred to as master nodes. This sequence can be executed using the X2 interface or the Xn interface, for example, in the same manner as in FIG. 3. Note that the protocol for executing this sequence is not limited to the X2 interface or the Xn interface and can be applied to any interface used for communication between a master node and a secondary node. First, when terminal 101 connects to base station 113 (S401), base station 113 transmits a secondary node addition request (SN Addition Request) to base station 112 (S402). When base station 112 receives this request, it determines whether to accept this request (S403). For example, base station 112 may draw a random number within a predetermined range (e.g., the range of 0 to 1) and determine whether to accept the secondary node addition request based on the value of the random number. The random number may be called Rand. Base station 112 can make a determination by comparing the value of the random number with the transmittance, etc., in the same manner as in the above-described embodiment. When base station 112 determines not to accept the request from base station 113, it transmits a message (SN Addition Request Reject) rejecting the secondary node addition request (S404). This message may include the reason for rejecting the request. For example, base station 112 may notify that access restriction (Miscellaneous Cause = ac-Barring) is set as the reason for rejecting the request. Further, this message includes expiration value information (ac-BarringTime = S16) to be used for a timer that measures the period during which the secondary node addition request to base station 112 is prohibited. Further, this message may include information (ac-BarringForSpecialAC) indicating whether a specific access category (Special Access Category) is subject to regulation.When the base station 113 receives an SN Addition Request Reject, it sets a prohibition period for the secondary node addition request targeted at the base station 112.

[0023] The base station 112 can set whether to be subject to regulation for each base station (master node). For example, the base station 112 can aggregate the number of secondary node addition requests received from each base station over a certain period and determine whether to be subject to regulation based on whether it exceeds a predetermined threshold. As an example, the base station 112 can determine that it is not subject to regulation when the number of secondary node addition requests received over a certain period exceeds the threshold, and is subject to regulation when it is below the threshold. When the number of secondary node addition requests is large, a large number of terminals 101 are present in the area where the cells provided by each of the master node and the base station 112 overlap, that is, the distance between the master node and the base station 112 is likely to be short. Therefore, the base station 112 can execute control to target distant base stations for regulation while not subjecting neighboring base stations to regulation. On the other hand, the base station 112 may determine that it is subject to regulation when the number of secondary node addition requests received over a certain period exceeds the threshold, and not subject to regulation when it is below the threshold. In a situation where the processing load at the base station 112 is increasing, by targeting base stations with a high frequency of receiving secondary node addition requests for regulation, it is possible to suppress further load on the base station 112. Note that the base station 112 may select the base stations to be subject to regulation based on settings performed in advance by an operator of the communication system or the like.

[0024] For example, in FIG. 4, when the terminal 101 connects to the base station 114 (S405), the base station 114 transmits a request for adding a secondary node to the base station 112 (S406). When the base station 112 receives this request, it determines whether to accept this request (S407). At this time, if it is determined that the base station 114 is not a regulation target, the base station 112 transmits a message (SN Addition Request ACK) indicating that it accepts the request for adding a secondary node (S408). In addition to or instead of setting whether each base station is a regulation target, the base station 112 may set different transmittance values or the like for each base station. Thereby, the base station 112 can flexibly control the regulation according to the characteristics and attributes of the base station serving as the master node.

[0025] The base station 112 may set different expiration values (or reference values) of timers for each base station. For example, when the terminal 101 connects to the base station 115 in FIG. 4 (S409), the base station 115 transmits a request for adding a secondary node to the base station 112 (S410). When the base station 112 receives this request, it determines whether to accept this request (S411). If the base station 112 determines not to accept the request from the base station 115, it transmits a message (SN Addition Request Reject) rejecting the request for adding a secondary node (S412). At this time, as an example, the base station 112 may include a value (ac-BarringTime = S32) different from the expiration value information notified to the base station 113 in this message. When the base station 115 receives the SN Addition Request Reject, it sets a prohibition period for the request for adding a secondary node targeting the base station 112. Note that ac-BarringTime = S32 indicates that 32 seconds should be used as a reference value for calculating T390. (Modification Example 2) In the above-described embodiment, an example was described in which, when access control is set for the base station 112, a condition (first condition) for restricting the addition of the base station 112 as a secondary node is satisfied. In this modified example, even when access control is not set, the base station 112 can notify the base station 111 of the restriction on the premise that a condition (first condition) for restricting the addition of its own device as a secondary node is satisfied. For example, when the number of connections of the terminal 101 in the cell provided by the base station 112 exceeds a predetermined threshold, the base station 112 may determine that the first condition is satisfied. Further, when the utilization rate of radio resources in the cell provided by the base station 112 exceeds a predetermined threshold, the base station 112 may determine that the first condition is satisfied. As an example, the base station 112 may use, as the utilization rate of radio resources, the ratio of the number of actually used resource blocks to the total number of resource blocks constituting the radio frames transmitted by its own device. In this way, when the base station 112 detects that the traffic in the cell provided by its own device is increasing and the possibility of congestion is increasing, it can transmit a notification of the restriction in order to suppress the secondary node request in advance. Thereby, the base station 112 can prevent the communication quality of the cell provided by its own device from deteriorating (for example, access control is set). Further, when providing priority communication or guaranteed services in the cell provided by the base station 112, the base station 112 may determine that the first condition is satisfied. For example, in the cell provided by the base station 112, when there is a terminal 101 that provides ultra-low latency communication or high-speed large-capacity communication using the slice function, the base station 112 may restrict the addition as a secondary node. As an example, when the base station 112 detects the establishment of a QoS flow to which a predetermined 5QI such as a bit rate guaranteed type 5QI (5G QoS Identifier) or a delay critical bit rate guaranteed type 5QI is assigned in the cell provided by its own device, the base station 112 may determine that the first condition is satisfied.In addition, when the base station 112 detects the establishment of a PDU session with a Network Slice Selection Assistance Information (NSSAI) of a predetermined slice / service type (such as eMBB or URLLC) set in the cell provided by the own device, it can be determined that the first condition is satisfied. Thereby, the base station 112 can protect communications that require a predetermined quality. Note that the base station 112 can use a combination of the conditions shown above. For example, when the base station 112 is providing priority communications or guaranteed services in the cell provided by the own device, it can be determined that the first condition is satisfied when the number of connected terminals or the utilization rate of radio resources exceeds a predetermined threshold. Further, when a high load occurs in the device of the 5GC 120 to which the base station 112 is connected, it can be determined that the first condition is satisfied. For example, when a high load occurs in the UPF, there is a possibility that a large delay will occur in the data transferred via this UPF. Therefore, if dual connectivity is configured using the base station 112 connected to this UPF, the throughput may rather decrease. For example, the base station 112 can recognize the operating status of the device to which it is connected by exchanging information indicating the operating status of the device, such as the processing load, with the device of the 5GC, and determine whether the first condition is satisfied based on this. Note that the base station 112 can be determined that the first condition is satisfied when a failure occurs in the own device or when a failure occurs in the device of the 5GC 120 to which the own device is connected. For example, the base station 112 can exchange control information and the like with the 5GC 120 using the N2 interface established with the AMF. Thereby, even when access control is not set for the own device, the base station 112 can suppress in advance a request for adding a secondary node from the base station 111.

[0026] Also, the base station 112 may determine whether to target each base station 111 for regulation by comparing information indicating communication quality such as resource utilization rate, average delay, and number of connected terminals in the cells provided by its own device and other base stations 111. For example, when transmitting data to the terminal 101 from each cell using dual connectivity, if congestion occurs in one cell, the terminal 101 may not be able to process the data received from the other cell until it finishes receiving the data from that cell. For example, when the terminal 101 sorts the received data according to the numbers assigned to each data, if there is data that has not been received, it cannot process the subsequent data until it receives that data. Therefore, if congestion occurs in one cell, the data received from the other cell will continue to be accumulated, and there is a possibility of buffer overflow or discarding of the received data. To avoid such a situation, the base station 112 may target for regulation a base station 111 that provides a cell with a communication quality significantly different from that of the cell provided by its own device. For example, when the base station 112 receives a request for adding a secondary node from the base station 111, it may request information indicating communication quality such as resource utilization rate, average delay, and number of connected terminals from this base station 111 via an X2 interface or an Xn interface. Then, the base station 112 compares it with the information indicating the communication quality of the cell provided by its own device, and if there is a difference in communication quality exceeding a predetermined threshold, it may reject the request.

[0027] Note that after the base station 112 notifies the base station 111 of the restriction as described above, if the situation where the restriction should be imposed is resolved, the base station 112 can notify the base station 111 to release the restriction operation. For example, when the base station 112 notifies the other base station 111 of the restriction based on the access restriction being set for its own device, it can determine that the situation where the restriction should be imposed is resolved because the access restriction set for its own device has been released. In this case, the base station 111 can notify the base station 111 to release the restriction using a message that notifies of the update of the settings of its own device. For example, NG-RAN NODE CONFIGURATION UPDATE can be used as this message. The base station 111 that has received the notification to release the restriction can cancel the restriction settings set for the base station 112. That is, when the base station 111 receives a restriction notification from the base station 112, it sets the restriction for that base station 112, and when it receives a notification to release the restriction from that base station 112, it cancels the restriction settings for that base station 112. The base station 111 can perform the setting and cancellation of the restriction for each base station with which it has established communication, and can manage restriction information such as the transmittance and expiration value information for each base station for which the restriction is set. On the other hand, in the above Modification Example 1, when the base station 112 receives a request to add a secondary node from the base station 111 without notifying the base station 111 of the restriction, it determines whether to accept this request. In this case, after the condition for releasing the restriction is satisfied, the base station 112 does not perform the determination operation when it receives a request to add a secondary node from the base station 111. Thus, when a predetermined condition is satisfied, by the base station 112 releasing the restriction, the base station 112 can operate as a secondary node for dual connectivity as normal.

[0028] The base station 112 may determine that the situation where regulation should be performed has been resolved when the number of connections of the terminal 101 in the cell provided by the own device falls below a predetermined threshold. For example, the base station 112 may determine that the situation where regulation should be performed has been resolved when the usage rate of radio resources in the cell provided by the own device falls below a predetermined threshold. As an example, the base station 112 may determine that the situation where regulation should be performed has been resolved because the ratio of the number of actually used resource blocks to the total number of resource blocks constituting the radio frame transmitted by the own device is below the threshold. For example, the base station 112 may determine that the situation where regulation should be performed has been resolved when the provision of priority communication or guaranteed services is completed. As an example, the base station 112 may determine that the situation where regulation should be performed has been resolved by detecting the release of a QoS flow to which a predetermined 5QI such as a 5QI of bitrate guarantee type or a 5QI of delay critical bitrate guarantee type is assigned in the cell provided by the own device. Also, the base station 112 may determine that the situation where regulation should be performed has been resolved by detecting the release of a PDU session in which an NSSAI of a predetermined slice / service type is set in the cell provided by the own device. For example, the base station 112 may determine that the situation where regulation should be performed has been resolved when the load in the device of the 5GC 120 to which the own device is connected falls below a predetermined threshold. For example, the base station 112 may determine that the situation where regulation should be performed has been resolved when a failure occurring in the own device or a failure occurring in the device of the 5GC 120 to which the own device is connected has been resolved. As an example, the base station 112 may recognize the situation of the devices included in the 5GC 120 using an interface used for communication with the 5GC 120 such as the N2 interface. Thus, when the situation where the addition of a secondary node should be restricted is resolved, the base station 112 can release the restriction operation of the base station 111, and can accept a secondary node request as before, enabling the provision of dual connectivity services to the terminal 101. Note that the base station 112 may set the conditions for performing regulation and the conditions for releasing regulation using different parameters from each other.For example, when the base station 112 notifies the base station 111 of the restriction based on the fact that access restriction is set for its own device, it can release the restriction based on the fact that the usage rate of radio resources in the cell provided by its own device is below a predetermined threshold. Further, when the base station 112 notifies the restriction based on a combination of a plurality of conditions, it may release the restriction based on the fact that all of those conditions have been resolved, or it may release the restriction based on the fact that one or more of them have been resolved. (Circuit Configuration) Subsequently, a configuration example of the base station 111 (master node) and the base station 112 (secondary node) as described above will be described. FIG. 5 is a diagram showing the hardware configuration of the base station 111 and the base station 112. In one example, the base station 111 and the base station 112 are configured to 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). The processor 501 reads and executes the programs stored in the ROM 502 and the storage device 504 to execute the overall processing of the device and each of the above-described processes. The ROM 502 is a read-only memory in which information such as programs and various parameters related to the processes executed by the base station 111 and the base station 112 are recorded. The RAM 503 functions as a workspace when the processor 501 executes a program, and is a random access memory in which temporary information is recorded. The storage device 504 is configured by, for example, a removable external storage device or the like. The communication circuit 505 includes, for example, a circuit for communicating with other devices. (Functional Configuration) FIG. 6 is a diagram showing a functional configuration example of the base station 111. The base station 111 includes, as its functions, for example, an information communication unit 601, a determination unit 602, and a timer management unit 603. FIG. 6 shows the functional configuration of the base station 111 of the present embodiment, and for example, the general configuration of the base station 111 is omitted. Note that these functional units can be realized, for example, when the processor 501 executes programs stored in the ROM 502 and the storage device 504 and controls the communication circuit 505 as necessary. However, the present invention is not limited to this, and for example, dedicated hardware for realizing each function may be prepared.

[0029] The information communication unit 601 exchanges messages with the base station 112. For example, the information communication unit 601 can receive a regulation notification from the base station 112 when a first condition that should regulate the addition as a secondary node at the base station 112 is satisfied. Further, the information communication unit 601 notifies the timer management unit 603 of the expiration value information included in the regulation notification received from the base station 112. The expiration value information is information for specifying the deadline of the period during which the base station 111 is prohibited from making a request to add a secondary node to the base station 112. Also, the information communication unit 601 can receive from the base station 112 information for determining whether to make a request to add a secondary node to the base station 112. For example, when the information communication unit 601 determines whether to make a request to add a secondary node using a random number, it can receive information such as transmittance from the base station 112. Based on the determination that a request to add a secondary node is to be made, the information communication unit 601 can transmit a request to add a secondary node to the base station 112. Also, the information communication unit 601 can receive from the base station 112 a notification rejecting the request to add a secondary node to the base station 112. This notification may include expiration value information specifying the deadline of the period during which making a request to add a secondary node to the base station 112 is prohibited. The information communication unit 601 can receive regulation information from the base station 112 when establishing the X2 interface or the Xn interface. For example, the regulation information can be received included in the X2 SETUP REQUEST message or the Xn SETUP REQUEST message transmitted from the base station 112. Also, when the information communication unit 601 has established the X2 interface or the Xn interface with the base station 112, it can receive regulation information using information indicating an update of the settings of the base station 112. For example, the regulation information can be received included in the eNB Configuration Update message transmitted using the X2 interface or the NG-RAN NODE CONFIGURATION UPDATE message transmitted using the Xn interface. Also, regulation information may be received in the Endc Configuration Update message transmitted via the X2 interface.

[0030] When the first condition that should restrict the addition of the base station 112 as a secondary node is satisfied in the base station 112, the determination unit 602 determines whether to request the addition of a secondary node to the base station 112. For example, the determination unit 602 can make a determination by comparing the transmission rate and the like notified from the base station 112 with a random number value. The timer management unit 603 manages a timer that measures the expiration time of the period during which the request for adding a secondary node to the base station 112 is prohibited. For example, when the determination unit 602 determines not to make a request as a secondary node, the timer management unit 603 can start this timer. Also, when the information communication unit 601 receives a notification rejecting the request for adding a secondary node, the timer management unit 603 can start this timer. The timer management unit 603 can specify the expiration value to be set as the expiration time of this timer based on the expiration value information notified from the base station 112.

[0031] FIG. 7 is a diagram showing an example of the functional configuration of the base station 112. The base station 112 includes, as its functions, for example, an information communication unit 701, a condition generation unit 702, and a determination unit 703. FIG. 7 shows the functional configuration of the base station 112 of the present embodiment, and for example, the general configuration of the base station 112 is omitted. Note that these functional units can be realized, for example, when the processor 501 executes a program stored in the ROM 502 and the storage device 504 and controls the communication circuit 505 as necessary. However, it is not limited thereto, and for example, dedicated hardware for realizing each function may be prepared.

[0032] The information communication unit 701 exchanges messages with the base station 111. For example, when a first condition that should regulate the addition of the information communication unit 701 as a secondary node in the base station 112 is satisfied, the information communication unit 701 may send a regulation notification to the base station 111. Also, the information communication unit 701 may include expiration value information in the regulation notification sent from the base station 111. Further, the information communication unit 701 may send information for the base station 111 to determine whether to request the addition of a secondary node to the base station 112. For example, the information communication unit 701 may send regulation information such as a transmittance rate as the information to the base station 111. Also, the information communication unit 701 may receive a request for adding a secondary node from the base station 111. In response to this request, the information communication unit 701 may send a notification to accept or reject this request to the base station 111. The notification rejecting this request may include expiration value information specifying the deadline of the period during which the request for adding a secondary node to the base station 112 is prohibited. The information communication unit 701 may send regulation information to the base station 111 when establishing the X2 interface or the Xn interface. For example, the regulation information may be included and sent in an X2 SETUP REQUEST message or an Xn SETUP REQUEST message. Also, when the information communication unit 701 has established the X2 interface or the Xn interface with the base station 111, the information communication unit 701 may send regulation information using information indicating an update of the settings of the base station 112. For example, the regulation information may be included in an eNB Configuration Update message sent using the X2 interface or an NG-RAN NODE CONFIGURATION UPDATE message sent using the Xn interface. Further, regulation information may be sent in an Endc Configuration Update message transmitted via the X2 interface.

[0033] When a first condition that should restrict the addition of the base station 112 as a secondary node is satisfied, the condition generation unit 702 generates a condition for determining whether the addition of the secondary node is made based on the first process performed by the base station 111. For example, the condition generation unit 702 can generate a transmittance or the like for the base station 111 to determine whether to request the addition of the secondary node. Also, when the base station 112 receives a request for adding a secondary node, the condition generation unit 702 can generate a transmittance or the like for determining whether to accept this request. Further, the condition generation unit 702 can generate expiration value information for specifying the expiration value of a timer started at the base station 111 when the addition of the secondary node is not made. After the first condition that should restrict the addition of the base station 112 as a secondary node is satisfied, when the determination unit 703 receives a request for adding a secondary node to the base station 112, the determination unit 703 determines whether to accept this request. For example, the determination unit 703 can make the determination by comparing a transmittance or the like generated by the condition generation unit 702 with a random number value.

[0034] As described above, according to the present embodiment, when a first condition that a predetermined base station should regulate the addition of a secondary node in dual connectivity is satisfied, a regulation notification is sent to other base stations. Then, as a result of other base stations executing a determination process for adding a secondary node and determining not to request the addition of a secondary node, the request for adding a secondary node to the predetermined base station is suppressed. Also, when other base stations determine not to request the addition of a secondary node, a timer for measuring the expiration of a period during which it is prohibited to request the addition of a secondary node is started. During this period, other base stations do not request the addition of a secondary node. As a result, not only connection requests from within the cell but also secondary node requests from other base stations can be suppressed when access control should be performed or congestion occurs at a predetermined base station. Therefore, it is possible to suppress the acceleration of a congestion state or the like caused by a secondary node request being made to a predetermined base station in a congestion state or the like, and improve the stability of the mobile communication system. Thus, it becomes possible to contribute to Goal 9 of the Sustainable Development Goals (SDGs) led by the United Nations, "Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation."

[0035] The invention is not limited to the above-described embodiment, and various modifications and changes are possible within the scope of the gist of the invention.

Description of Reference Numerals

[0036] 101: Terminal, 111: Base Station, 112: Base Station, 113: Base Station, 114: Base Station, 115: Base Station, 120: 5GC, 601: Information Communication Unit, 602: Determination Unit, 603: Timer Management Unit, 701: Information Communication Unit, 702: Condition Generation Unit, 703: Determination Unit

Claims

1. A base station operable as a secondary node of dual connectivity defined in the cellular communication standard of the 3rd Generation Partnership Project (3GPP), notification means for sending a first notification including information for causing another base station to execute a determination process as to whether to request addition of the base station as a secondary node, based on satisfaction of a first condition for restricting addition of the base station as a secondary node; A base station characterized by the above.

2. The notification means further sends a second notification including expiration value information for specifying an expiration value of a timer started in the other base station, based on satisfaction of the first condition, The expiration value information is used for a timer that measures a deadline of a period during which the other base station is prohibited from requesting addition of the base station as a secondary node when the other base station determines not to perform the addition of the secondary node as a result of executing the determination process for adding the secondary node targeting the base station. The base station according to claim 1, characterized by the above.

3. Transmission of the information performed by the notification means is performed using a message to be sent to the other base station when establishing an X2 interface or an Xn interface, when the X2 interface and the Xn interface are not established between the base station and the other base station, or using a message indicating updating of the setting of the base station via the X2 interface or the Xn interface when the X2 interface or the Xn interface is established between the base station and the other base station. The base station according to claim 1 or 2, characterized by the above.

4. A base station operable as a master node of dual connectivity defined in the cellular communication standard of the 3rd Generation Partnership Project (3GPP), receiving means for receiving a first notification including information for the base station to execute a determination as to whether to request addition of another base station as a secondary node, the first notification being sent from the other base station based on satisfaction of a first condition for restricting addition of the other base station as a secondary node in the other base station; determination means for executing the determination for adding the secondary node targeting the other base station, based on the first notification; When it is determined according to the result of the determination that the request for adding the secondary node is to be made, the request for adding the secondary node is executed; when it is determined that the request for adding the secondary node is not to be made, control is performed so as not to execute the request for adding the secondary node, and control means for performing the control. A base station characterized by the above.

5. The base station further includes activation means for activating a predetermined timer when it is determined in the determination that the request for adding the secondary node is not to be made. The receiving means further receives a second notification including expiration value information for specifying an expiration value of the predetermined timer, which is transmitted from the other base station based on the fact that the first condition is satisfied in the other base station. The predetermined timer is a timer for measuring a period during which the base station is prohibited from making a request to the other base station for adding the secondary node. The expiration value information is used to specify the expiration of the period. The control means performs control so that the other base station does not make a request for adding the secondary node during the period. The base station according to claim 4, characterized by the above.

6. The reception of information performed by the receiving means is performed using a message received from the other base station when establishing the X2 interface or the Xn interface when the X2 interface and the Xn interface are not established between the base station and the other base station, or when the X2 interface or the Xn interface is established between the base station and the other base station, using a message indicating that the settings of the other base station via the X2 interface or the Xn interface are updated. The base station according to claim 4 or 5, characterized by the above.

7. A control method executed by a base station operable as a secondary node of dual connectivity defined in the cellular communication standard of the Third Generation Partnership Project (3GPP), Based on the fact that a first condition for restricting the addition of the base station as a secondary node is satisfied, a notification step of sending a first notification including information for causing the other base station to execute a determination process as to whether to make a request for adding the secondary node to the base station is included. A control method characterized by the above.

8. A control method executed by a base station operable as a master node of dual connectivity defined in a cellular communication standard of the 3rd Generation Partnership Project (3GPP), a receiving step of receiving a first notification including information for the base station to determine whether to request addition of the other base station as a secondary node, the request for addition of the other base station as a secondary node being transmitted from the other base station based on fulfillment of a first condition for restricting addition of the other base station as a secondary node in the other base station; a determination step of performing the determination for addition of the other base station as a secondary node based on the first notification; and a control step of controlling to execute the request for addition of the secondary node if it is determined to request addition of the secondary node according to the result of the determination, and not to execute the request for addition of the secondary node if it is determined not to request addition of the secondary node. A control method characterized by the above. **Claim 9** A program for causing a computer included in a base station defined in a cellular communication standard of the 3rd Generation Partnership Project (3GPP) to execute the control method according to claim 7 or 8.

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