Patent Description:
In the 3rd Generation Partnership Project (3GPP), specifications concerning a third generation mobile communication system (hereinafter, <NUM> system) using radio access technology such as Wideband Code Division Multiple Access (W-CDMA), and a fourth generation mobile communication system (hereinafter, <NUM> system) using a radio access technology such as Long Term Evolution (LTE) / LTE-Advanced has been stipulated (for example, see Non-Patent Document <NUM>). Moreover, in 3GPP, specifications of a fifth generation mobile communication system (hereinafter, <NUM> system) as a successor system of the <NUM> system are being studied. The <NUM> system supports Circuit Switching type (CS-type) communication service, and the <NUM> system and the <NUM> system support an IP Multimedia Subsystem type (IMS-type) communication service.

Since the introduction of the <NUM> system, provision of communication services in which systems of a plurality of generations are used in combination is assumed for the reasons such as size of the area in which the <NUM> system is deployed and equipment planning. For example, in Non-Patent Document <NUM>, when <NUM> core network does not support the IP Multimedia Subsystem (IMS) because of the architecture of the network, fallback from the <NUM> system to the <NUM> system is proposed.

Specifically, upon receiving an IMS-type emergency call from a user device (UE), the <NUM> core network instructs <NUM> Radio Access Network (RAN) to perform an IMS-type emergency fallback to the <NUM> system. Based on the instruction, the 5GRAN performs handover of or redirects the UE to the <NUM> system. According to the proposed solution, the emergency calls are not lost and can be processed via the <NUM> system.

However, as proposed in Non-Patent Document <NUM>, when congestion occurs in the <NUM> system, processing performed to fall back to the <NUM> system is time-consuming. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling deteriorates.

Moreover, when UE cannot connect to the <NUM> system even after the fallback from the <NUM> system to the <NUM> system is performed, it is assumed that CS fallback from the <NUM> system to <NUM> system is further performed in accordance with the stipulations of Non-Patent Document <NUM>.

Because, in the CS fallback, it is necessary to change the communication service from IMS type to CS type, it takes time for the UE to connect to the <NUM> system. Therefore, even in the case of the CS fallback, the service quality of the communication that requires real time performance such as emergency calls, voice communication, and video calling deteriorates.

The present invention has been made in view of the above circumstances. One object of the present invention is to provide, in a radio communication system that includes a first system and a second system that support IP Multimedia Subsystem-type communication service, and a third system that supports a circuit switching-type communication service, a user device that is capable of maintaining service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling even when performing fallback from the first system to any system other than the first system.

The invention is set out by the appended set of claims. Provided is a user device according to the features of independent claim <NUM>. Further embodiments are provided by the dependent claims.

Exemplary embodiments of the present invention are explained below with reference to the accompanying drawings. Furthermore, in the drawings, structural elements having the same function or configuration are indicated by the same or similar reference numerals and the explanation thereof is appropriately omitted.

<FIG> is an overall structural diagram of a radio communication system <NUM> according to the present embodiment. The radio communication system <NUM> includes a user device <NUM> (hereinafter, the UE <NUM>), <NUM> system <NUM> and <NUM> system <NUM> that support an IP multimedia subsystem (IMS) type communication service, <NUM> system <NUM> that supports a circuit switching (CS) type communication service, IMS <NUM>, and CS <NUM>.

The <NUM> system <NUM>, for example, is a mobile communication system according to New Radio (NR), and includes 5GRAN <NUM> and <NUM> core network <NUM> that is connected to the 5GRAN <NUM>. The 5GRAN <NUM> is a network constituted by a radio base station (gNB) and the like, and for example, executes connection processing with the UE <NUM>. The radio base station of the 5GRAN <NUM> controls the radio communication with the UE <NUM> in <NUM> cell <NUM>. Note that, even if only one radio base station is shown in <FIG>, the 5GRAN <NUM> includes a plurality of the radio base stations and these radio base stations form a plural of the <NUM> cells.

The <NUM> core network <NUM> is a network constituted by a switching station and the like, and, for example, executes an attach processing with the UE <NUM>. In addition to the 5GRAN <NUM>, the <NUM> core network <NUM> is also connected to 4GRAN <NUM> explained later. Note that it can be assumed that the <NUM> core network <NUM> does not support the IMS-type communication service because of the architecture of the network.

The <NUM> system <NUM>, for example, is a mobile communication system according to LTE specifications, and includes the 4GRAN (E-UTRAN) <NUM> and <NUM> core network (EPC) <NUM> that is connected to the 4GRAN <NUM>. The 4GRAN <NUM> is a network constituted by a radio base station (eNodeB) and the like, and, for example, executes connection processing with the UE <NUM>. The radio base station of the 4GRAN <NUM> controls the radio communication with the UE <NUM> in <NUM> cell <NUM>. Note that, even if only one radio base station is shown in <FIG>, the 4GRAN <NUM> includes a plurality of the radio base stations and these radio base stations form a plurality of the <NUM> cells. The <NUM> core network <NUM> is a network constituted by a switching station (MME) and the like, and, for example, executes the attach processing with the UE <NUM>.

The <NUM> system <NUM> includes 3GRAN (UTRAN) <NUM> and <NUM> core network <NUM> that is connected to the 3GRAN <NUM>. The 3GRAN <NUM> is a network constituted by a radio base station (NodeB), a radio network control device (RNC), and the like, and, for example, executes connection processing with the UE <NUM>. The radio base station of the 3GRAN <NUM> controls the radio communication with the UE <NUM> in <NUM> cell <NUM>. Note that, even if only one radio base station is shown in <FIG>, the 3GRAN <NUM> includes a plurality of the radio base stations and these radio base stations form a plurality of the <NUM> cells. The <NUM> core network <NUM> is a network constituted by a switching station (MSC / VLR) and the like, and, for example, executes the attach processing with the UE <NUM>.

The IMS <NUM> is connected to the <NUM> core network <NUM>, and provides the IMS-type communication service to the UE <NUM> via the 5GRAN <NUM> and the <NUM> core network <NUM> or via the 4GRAN <NUM> and the <NUM> core network <NUM>. Similarly, the IMS <NUM> is connected to the <NUM> core network <NUM>, and provides the IMS-type communication service to the UE <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM>. The CS <NUM> is connected to the <NUM> core network <NUM>, and provides the CS-type communication service to the UE <NUM> via the 3GRAN <NUM> and the <NUM> core network <NUM>.

A functional block configuration of the radio communication system <NUM> is explained below. Specifically, a functional block configuration of the UE <NUM> is explained below. A hardware configuration of the UE <NUM> will be explained later.

<FIG> is a functional block diagram of the UE <NUM>. As shown in <FIG>, the UE <NUM> includes a radio communication unit <NUM>, a connection processing unit <NUM>, an attach processing unit <NUM>, IMS incoming / outgoing communication processing unit <NUM>, a system state detecting unit <NUM>, and a cell selecting unit <NUM>.

The radio communication unit <NUM> transmits various signals based on requests received from the connection processing unit <NUM>, the attach processing unit <NUM>, and the IMS incoming / outgoing communication processing unit <NUM>. The radio communication unit <NUM> receives various signals from the <NUM> system <NUM>, the <NUM> system <NUM>, or the <NUM> system <NUM>, and transmits the received signals to the connection processing unit <NUM>, the attach processing unit <NUM>, the IMS incoming / outgoing communication processing unit <NUM>, or the system state detecting unit <NUM>.

The connection processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal so as to transmit a connection request signal to RAN (for example, the 5GRAN <NUM> or the 4GRAN <NUM>) of a target generation system when the UE <NUM> attempts to connect to the target generation system (for example, the <NUM> system <NUM> or the <NUM> system <NUM>) to perform emergency call communication, voice communication, or video call communication via IP packet. In other words, the connection processing unit <NUM> transmits the connection request signal to the RAN of the target generation system via the radio communication unit <NUM>. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the RAN of the target generation system.

As it will be explained later, upon instructed by the cell selecting unit <NUM> to attempt to connect to the <NUM> cell <NUM> that is selected as the target cell for connection, the connection processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal so as to only transmit the connection request signal for the IMS-type communication service to the target cell for connection. In other words, the connection processing unit <NUM> transmits the connection request signal for the IMS-type communication service to the target cell for connection via the radio communication unit <NUM>. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the 4GRAN <NUM>. As an example of the connection request signal, RRC CONNECTION REQUEST can be cited.

Upon receiving a connection setup signal from the target generation system via the radio communication unit <NUM>, the connection processing unit <NUM> establishes radio connection with the target generation system. On the other hand, upon receiving a connection rejection signal or a fallback instruction from the target generation system via the radio communication unit <NUM>, the connection processing unit <NUM> does not establish the radio connection with the target generation system. As an example of the connection setup signal, RRC CONNECTION SETUP can be cited. As an example of the connection rejection signal, RRC CONNECTION REJECTION can be cited.

The attach processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal so as to transmit an attach request signal to a core network (for example, the <NUM> core network <NUM> or the <NUM> core network <NUM>) of the target generation system when the UE <NUM> attempts to attach to the target generation system after the radio connection has been established with the target generation system. In other words, the attach processing unit <NUM> transmits the attach request signal to the core network of the target generation system via the radio communication unit <NUM>. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the core network of the target generation system. Upon receiving an attach complete response signal from the target generation system via the radio communication unit <NUM>, the attach processing unit <NUM> completes the attach processing with the target generation system. On the other hand, upon receiving a non-IMS support notification or the fallback instruction from the target generation system via the radio communication unit <NUM>, the attach processing unit <NUM> stops executing the attach processing with the target generation system.

The IMS incoming / outgoing communication processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal so as to transmit the IMS registration request signal to the IMS <NUM> that is connected to the target generation system, when the UE <NUM> attempts to execute the IMS registration procedure after the attach processing with the target generation system is completed and before performing the IMS incoming / outgoing communication. In other words, the IMS incoming / outgoing communication processing unit <NUM> transmits, via the radio communication unit <NUM>, the IMS registration request signal to the IMS <NUM> that is connected to the target generation system. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the IMS <NUM> that is connected to the target generation system. Upon receiving the IMS registration completion signal from the IMS <NUM> via the radio communication unit <NUM>, the IMS incoming / outgoing communication processing unit <NUM> completes the IMS registration procedure. Subsequently, the IMS incoming / outgoing communication processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal so as to transmit an INVITE signal to the IMS <NUM> that is connected to the target generation system, when the IMS incoming / outgoing communication is attempted. In other words, the IMS incoming / outgoing communication processing unit <NUM> transmits, via the radio communication unit <NUM>, the INVITE signal to the IMS <NUM> that is connected to the target generation system. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the IMS <NUM> that is connected to the target generation system, and performs the IMS incoming / outgoing communication via the target generation system.

However, in addition to the <NUM> core network <NUM>, the 4GRAN <NUM> of the <NUM> system <NUM> is also connected to the <NUM> core network <NUM>. Therefore, the IMS incoming / outgoing communication processing unit <NUM> can also execute the IMS registration procedure and IMS incoming / outgoing communication explained above with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM>.

Upon being notified from the connection processing unit <NUM> that the connection rejection signal from the <NUM> system <NUM> is received, based on the notification, the system state detecting unit <NUM> detects that access to the <NUM> system <NUM> is barred, and transmits a detection signal to the cell selecting unit <NUM>. Upon being notified from the attach processing unit <NUM> that the non-IMS support notification is received from the <NUM> system <NUM>, based on the notification, the system state detecting unit <NUM> detects that the <NUM> core network <NUM> of the <NUM> system <NUM> does not support the IMS-type communication service and transmits the detection signal to the cell selecting unit <NUM>. Upon being notified from the connection processing unit <NUM> or the attach processing unit <NUM> that the fallback instruction is received from the <NUM> system <NUM>, based on the received notification, the system state detecting unit <NUM> detects the instruction to perform fallback to the <NUM> system <NUM> and transmits the detection signal to the cell selecting unit <NUM>. Upon being notified from the connection processing unit <NUM> or the attach processing unit <NUM> that the response signal is not received from the <NUM> system <NUM> even after a predetermined time has elapsed, based on the notification, the system state detecting unit <NUM> detects that the <NUM> system <NUM> is congested and transmits the detection signal to the cell selecting unit <NUM>.

The system state detecting unit <NUM> receives broadcast information from the <NUM> system <NUM> via the radio communication unit <NUM>. Upon detecting that the received broadcast information includes access restriction information, congestion information, non-IMS support information, or fallback instruction information, the system state detecting unit <NUM> transmits the detection signal to the cell selecting unit <NUM>. As an example of the broadcast information, System Information Block (SIB) can be cited.

The system state detecting unit <NUM> receives a paging signal from the <NUM> system <NUM> via the radio communication unit <NUM>. Upon detecting that the received paging signal includes the access restriction information, the congestion information, the non-IMS support information, or the fallback instruction information, the system state detecting unit <NUM> transmits the detection signal to the cell selecting unit <NUM>.

The access restriction information is the information via which it is notified that the access to the <NUM> system <NUM> is barred. By using the access restriction information, the <NUM> system <NUM> notifies the UE <NUM> that emergency call communication, voice communication, or video call communication via IP packet is restricted in the entire <NUM> system <NUM> or a specific <NUM> cell. As an example of the access restriction information, Access Class Barring (ACB), Service Specific Access Control (SSAC) Barring, Access Control for general Data Connectivity (ACDC), and User Access Control (UAC) can be cited.

The congestion information is the information via which it is notified that the <NUM> system <NUM> is congested. By using the congestion information, the <NUM> system <NUM> notifies the UE <NUM> that congestion has occurred in the 5GRAN <NUM>, in the <NUM> core network <NUM>, or between the 5GRAN <NUM> and the <NUM> core network <NUM>.

The non-IMS support information is the information via which it is notified that the <NUM> core network <NUM> of the <NUM> system <NUM> does not support the IMS-type communication service. By using the non-IMS support information, the <NUM> system <NUM> notifies the UE <NUM> that the <NUM> core network <NUM> does not support the IMS-type communication service.

The fallback instruction information is the information via which fallback to the <NUM> system <NUM> is instructed. By using the fallback instruction information, the <NUM> system <NUM> instructs the UE <NUM> to perform fallback to the <NUM> system <NUM>.

When the received broadcast information or the paging signal includes frequency information having information on a plurality of the frequencies used by the plurality of the <NUM> cells in the <NUM> system <NUM> that supports the IMS-type communication service, the system state detecting unit <NUM> notifies the cell selecting unit <NUM> of the frequency information. When the received broadcast information or the paging signal includes cell information having information on the plurality of the <NUM> cells that are formed near the <NUM> cell <NUM> of the <NUM> system <NUM> in which the UE <NUM> resides, in the <NUM> system <NUM> that supports the IMS-type communication service, the system state detecting unit <NUM> notifies the cell selecting unit <NUM> of the cell information.

Upon receiving the detection signal from the system state detecting unit <NUM>, the cell selecting unit <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service. Specifically, the cell selecting unit <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> in the <NUM> system <NUM> that uses a different frequency than that used by the <NUM> cell <NUM> of the <NUM> system <NUM> in which the UE <NUM> resides. Upon receiving the detection signal from the system state detecting unit <NUM> when the frequency information is notified from the system state detecting unit <NUM>, the cell selecting unit <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> that uses a different frequency than that used by the <NUM> cell <NUM>, based on the frequencies included in the frequency information. Upon selecting the <NUM> cell <NUM> as the target cell for connection, the cell selecting unit <NUM> instructs the connection processing unit <NUM> to attempt to connect to the <NUM> system <NUM>.

Alternatively, upon receiving the detection signal from the system state detecting unit <NUM>, in the <NUM> system <NUM>, the cell selecting unit <NUM> can select, as the target cell for connection, the <NUM> cell <NUM> formed near the <NUM> cell <NUM> of the <NUM> system <NUM> in which the UE <NUM> resides. In such a case, a part or an entire area of the <NUM> cell <NUM> of the <NUM> system <NUM> that is formed near the <NUM> cell <NUM> of the <NUM> system <NUM> can overlap with that of the <NUM> cell <NUM>. Upon receiving a detection signal from the system state detecting unit <NUM> when the cell information is notified from the system state detecting unit <NUM>, the cell selecting unit <NUM> selects the target cell for connection from among the plurality of the <NUM> cells of the <NUM> system <NUM> included in the cell information.

Operation of the radio communication system <NUM> is explained below. Specifically, operations related to the processing performed by the UE <NUM> to fall back from the <NUM> system <NUM> to the <NUM> system <NUM> will be explained.

<FIG> shows a processing flow of fallback performed by the UE <NUM>. As shown in <FIG>, the UE <NUM> determines whether fallback from the <NUM> system <NUM> to the <NUM> system <NUM> is to be started (Step S10). Specifically, in the <NUM> cell <NUM> of the <NUM> system <NUM>, the UE <NUM> determines whether the access restriction to the <NUM> system <NUM>, congestion of the <NUM> system <NUM>, non-support for the IMS by the <NUM> core network <NUM>, or fallback instruction is detected.

More specifically, upon detecting that at least one of the following conditions is fulfilled, the UE <NUM> determines to perform fallback to the <NUM> system <NUM>: (<NUM>) the system state detecting unit <NUM> is notified from the connection processing unit <NUM> that the connection rejection signal is received from the <NUM> system <NUM>; (<NUM>) the system state detecting unit <NUM> is notified from the attach processing unit <NUM> that the non-IMS support notification is received from the <NUM> system <NUM>; (<NUM>) the system state detecting unit <NUM> is notified from the connection processing unit <NUM> or the attach processing unit <NUM> that the response signal is not received from the <NUM> system even after the predetermined time has elapsed; (<NUM>) the system state detecting unit <NUM> is notified from the connection processing unit <NUM> or the attach processing unit <NUM> that the fallback instruction is received from the <NUM> system <NUM>; and (<NUM>) the system state detecting unit <NUM> detects that the broadcast information or the paging signal received from the <NUM> system <NUM> via the radio communication unit <NUM> includes the access restriction information, the congestion information, the non-IMS support information, or the fallback instruction information.

Upon determining that fallback to the <NUM> system <NUM> is to be started, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection (Step S20).

Specifically, upon receiving from the system state detecting unit <NUM> the detection signal that indicates that at least one of the conditions explained above is fulfilled, the cell selecting unit <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> of the <NUM> system <NUM> that uses a different frequency than that used by the <NUM> cell <NUM> of the <NUM> system <NUM> in which the UE <NUM> resides. When the frequency information included in the broadcast information or the paging signal received from the <NUM> system <NUM> is notified from the system state detecting unit <NUM>, the cell selecting unit <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> that uses a different frequency than that used by the <NUM> cell <NUM>, based on the frequencies included in the frequency information.

However, even if the <NUM> system <NUM> that supports the CS-type communication service is available for fallback, the UE <NUM> avoids selecting the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection. Specifically, when the system state detecting unit <NUM> receives via the radio communication unit <NUM> the information that indicates whether fallback from the <NUM> system <NUM> to the <NUM> system <NUM> that supports the IMS-type communication service or the <NUM> system <NUM> that supports the CS-type communication services is allowed, the UE <NUM> can avoid selecting the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection.

The UE <NUM> transmits the connection request signal to the <NUM> cell <NUM> that is selected as the target cell for connection (Step S30). Specifically, the connection processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal so as to transmit the connection request signal to the <NUM> system <NUM>. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the 4GRAN <NUM> of the <NUM> system <NUM>.

Upon connecting to the <NUM> system <NUM>, the UE <NUM> executes IMS incoming / outgoing communication with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM> (Step S40). Specifically, upon receiving the connection setup signal from the 4GRAN <NUM> via the radio communication unit <NUM>, the connection processing unit <NUM> establishes the radio connection with the <NUM> system <NUM>. The attach processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal so as to transmit the attach request signal after the UE <NUM> has established the radio connection with the <NUM> system <NUM>. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the <NUM> core network <NUM> of the <NUM> system <NUM>. Upon receiving the attach complete response signal from the <NUM> core network <NUM> via the radio communication unit <NUM>, the attach processing unit <NUM> completes the attach processing with the <NUM> system <NUM>. The IMS incoming / outgoing communication processing unit <NUM> execute an IMS registration procedure with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM> after the UE <NUM> has completed the attach processing with the <NUM> system <NUM>, and requests the radio communication unit <NUM> to transmit the radio signal so as to transmit the INVITE signal. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the IMS <NUM> and performs the IMS incoming / outgoing communication with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM>.

In addition, when the <NUM> core network <NUM> supports the IMS-type communication service, the UE <NUM> can execute the IMS incoming / outgoing communication with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM>. Moreover, when the UE <NUM> is unable to connect to the <NUM> system <NUM>, the UE <NUM> avoids selecting the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection even when the <NUM> system <NUM> that supports the CS-type communication service is available for fallback. Specifically, when the system state detecting unit <NUM> receives via the radio communication unit <NUM> the information that indicates whether fallback from the <NUM> system <NUM> or the <NUM> system <NUM> to the <NUM> system <NUM> that supports the CS-type communication service is allowed, the UE <NUM> can avoid selecting the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection.

On the other hand, upon determining that fallback to the <NUM> system <NUM> is not to be started, the UE <NUM> performs the IMS incoming / outgoing communication with the IMS <NUM> via the 5GRAN <NUM> and the <NUM> core network <NUM> (Step S50).

<FIG> shows a processing sequence of fallback from the <NUM> system <NUM> to the <NUM> system <NUM> according to Operation Example <NUM>. In the present operation example, when the UE <NUM> receives RRC CONNECTION REJECTION or the fallback instruction from the 5GRAN <NUM>, or does not receive the response signal from the 5GRAN <NUM> even after the predetermined time has elapsed, the UE <NUM> performs fallback to the <NUM> system <NUM>.

When a connection request for performing emergency call communication, voice communication, or video call communication via IP packet occurs in the application layer of the UE <NUM> in a state in which the UE <NUM> has not established the radio connection with the <NUM> system <NUM> (for example, IDLE state or RRC Inactivity state), the UE <NUM> attempts to connect to the <NUM> system <NUM>. Specifically, in the <NUM> cell <NUM> of the <NUM> system <NUM>, the UE <NUM> transmits RRC CONNECTION REQUEST to the 5GRAN <NUM> (Step S100). When emergency call communication, voice communication, or video call communication via IP packet is restricted in the entire <NUM> system <NUM> or the <NUM> cell <NUM>, the UE <NUM> receives RRC CONNECTION REJECTION from the 5GRAN <NUM> (Step S110-a). Moreover, when the <NUM> system <NUM> instructs the UE <NUM> to perform fallback to the <NUM> system <NUM>, the UE <NUM> receives the fallback instruction from the 5GRAN <NUM> (Step S110-a). Alternatively, when congestion has occurred in the 5GRAN <NUM>, the UE <NUM> does not receive the response signal from the 5GRAN <NUM> even after the predetermined timed has elapsed (Step S110-b).

When the UE <NUM> receives RRC CONNECTION REJECTION or the fallback instruction from the 5GRAN <NUM> or does not receive the response signal from the 5GRAN <NUM> even after the predetermined time has elapsed, the UE <NUM> starts to perform fallback to the <NUM> system <NUM>. Specifically, the UE <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> of the <NUM> system <NUM> that uses a different frequency than that used by the <NUM> cell <NUM> (Step S120). In the <NUM> cell <NUM> that is selected as the target cell for connection, the UE <NUM> transmits RRC CONNECTION REQUEST to the 4GRAN <NUM> (Step S130). Upon receiving RRC CONNECTION SETUP from the 4GRAN <NUM> (Step S140), the UE <NUM> establishes the radio connection with the <NUM> system <NUM>. Subsequently, the UE <NUM> transmits the attach request signal to the <NUM> core network <NUM> (Step S150), and after a communication bearer is configured in the <NUM> core network <NUM> (Step S160), the UE <NUM> receives the attach complete response signal from the <NUM> core network <NUM> (Step S170). Once the attach processing with the <NUM> core network <NUM> is completed, the UE <NUM> executes the IMS registration procedure with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM> (Step S180). Subsequently, the UE <NUM> transmits the INVITE signal to the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM> (Step S190), and performs the IMS incoming / outgoing communication.

<FIG> shows a processing sequence of fallback from the <NUM> system <NUM> to the <NUM> system <NUM> according to Operation Example <NUM>. In the present operation example, when the UE <NUM> receives the non-IMS support notification or the fallback instruction from the <NUM> core network <NUM>, or does not receive the response signal from the <NUM> core network <NUM> even after the predetermined time has elapsed, the UE <NUM> performs fallback to the <NUM> system <NUM>.

When the connection request for performing emergency call communication, voice communication, or video call communication via IP packet occurs in the application layer of the UE <NUM> in the state in which the UE <NUM> has not established the radio connection with the <NUM> system <NUM> (for example, IDLE state or RRC Inactivity state), the UE <NUM> attempts to connect to the <NUM> system <NUM>. Specifically, in the <NUM> cell <NUM> of the <NUM> system <NUM>, the UE <NUM> transmits RRC CONNECTION REQUEST to the 5GRAN <NUM> (Step S200). Upon receiving RRC CONNECTION SETUP from the 5GRAN <NUM> (Step S210), the UE <NUM> establishes the radio connection with the <NUM> system <NUM>. Subsequently, the UE <NUM> transmits the attach request signal to the <NUM> core network <NUM> (Step S220). When the <NUM> core network <NUM> does not support the IMS-type communication service, the UE <NUM> receives the non-IMS support notification from the <NUM> core network <NUM> (Step S230-a). Moreover, when the <NUM> system <NUM> instructs the UE <NUM> to perform fallback to the <NUM> system <NUM>, the UE <NUM> receives the fallback instruction from the <NUM> core network <NUM> (Step S230-a). Alternatively, when congestion has occurred in the <NUM> core network <NUM>, the UE <NUM> does not receive the response signal from the <NUM> core network <NUM> even after the predetermined timed has elapsed (Step S230-b).

In a response to a location registration request in a location registration procedure with a network that is performed based on the reception of the attach request signal, if the network notifies that the <NUM> core network <NUM> does not support the IMS-type communication service, the <NUM> core network <NUM> can transmit the non-IMS support notification to the UE <NUM>.

When the UE <NUM> receives the non-IMS support notification or the fallback instruction from the <NUM> core network <NUM> or does not receive the response signal from the <NUM> core network <NUM> even after the predetermined time has elapsed, the UE <NUM> starts to perform fallback to the <NUM> system <NUM>. Specifically, the UE <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> of the <NUM> system <NUM> that uses a different frequency than that used by the <NUM> cell <NUM> (Step S240).

In the present operation example, because the processing performed at Steps S250 to S310 is the same as that performed at Steps S130 to S190 of Operation Example <NUM>, explanation thereof is omitted.

<FIG> shows a processing sequence of fallback from the <NUM> system <NUM> to the <NUM> system <NUM> according to Operation Example <NUM>. In the present operation example, the UE <NUM> receives the broadcast information or the paging signal from the 5GRAN <NUM>, and performs fallback to the <NUM> system <NUM> when the access restriction information, the congestion information, the non-IMS support information, or the fallback instruction information is included in the received broadcast information or the paging signal.

Before and after the timing at which the connection request for performing emergency call communication, voice communication, or video call communication via IP packet occurs in the application layer of the UE <NUM> in a state in which the UE <NUM> has not established the radio connection with the <NUM> system <NUM> (for example, the IDLE state or the RRC Inactivity state), the UE <NUM> receives the broadcast information or the paging signal from the <NUM> system <NUM>. Specifically, in the <NUM> cell <NUM> of the <NUM> system <NUM>, the UE <NUM> receives the broadcast information or the paging signal from the 5GRAN <NUM> (Step S400).

When emergency call communication, voice communication, or video call communication via IP packet is restricted in the entire <NUM> system <NUM> or the <NUM> cell <NUM>, the 5GRAN <NUM> includes the access restriction information in the broadcast information or the paging signal. When the congestion has occurred in the 5GRAN <NUM>, in the <NUM> core network <NUM>, or between the 5GRAN <NUM> and <NUM> core network <NUM>, the 5GRAN <NUM> includes the congestion information in the broadcast information or the paging signal. When the <NUM> core network <NUM> does not support the IMS-type communication service, the 5GRAN <NUM> includes the non-IMS support information in the broadcast information or the paging signal. When the <NUM> system <NUM> instructs the UE <NUM> to perform fallback to the <NUM> system <NUM>, the 5GRAN <NUM> includes the fallback instruction information in the broadcast information or the paging signal.

Upon detecting that the access restriction information, the congestion information, the non-IMS support information, or the fallback instruction information is included in the received broadcast information or the paging signal (Step S410), the UE <NUM> starts performing fallback to the <NUM> system <NUM>. Specifically, the UE <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> of the <NUM> system <NUM> that uses a different frequency than that used by the <NUM> cell <NUM> (Step S420). When the frequency information is included in the received broadcast information or the paging signal, the UE <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> that uses a different frequency than that used by the <NUM> cell <NUM>, based on the frequencies included in the frequency information.

In the present operation example, because the processes performed at Steps S430 to S490 are the same as that performed at Steps S130 to S190 of Operation Example <NUM>, the explanation thereof is omitted.

<FIG> shows a processing sequence of fallback from the <NUM> system <NUM> to the <NUM> system <NUM> according to Operation Example <NUM>. In the present operation example, when the <NUM> core network <NUM> supports the IMS-type communication service, the UE <NUM> executes the IMS registration procedure and the IMS incoming / outgoing communication with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM>.

Because the processes performed at Steps S500 to S520 shown in <FIG> are the same as those performed at Steps S120 to S140 shown in <FIG>, those performed at Steps S240 to S260 shown in <FIG>, and those performed at Steps S420 to S440 shown in <FIG>, the explanation thereof is omitted. Upon establishing the radio connection with the <NUM> system <NUM>, the UE <NUM> transmits the attach request signal to the <NUM> core network <NUM> (Step S530), and after the communication bearer is configured in the <NUM> core network <NUM> (Step S540), the UE <NUM> receives the attach complete response signal from the <NUM> core network <NUM> (Step S550). Once the attach processing with the <NUM> core network <NUM> is completed, the UE <NUM> executes the IMS registration procedure with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM> (Step S560). Subsequently, the UE <NUM> transmits the INVITE signal to the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM> (Step S570), and performs the IMS incoming / outgoing communication.

<FIG> shows a modification of the flow of fallback processing performed by the UE <NUM>. Specifically, <FIG> shows a transition of a core network performed by the UE <NUM> in a configuration in which the 4GRAN <NUM> is connected to the <NUM> core network <NUM> and the <NUM> core network <NUM>. As shown in <FIG>, the UE <NUM> determines whether the transition from the <NUM> core network <NUM> to the <NUM> core network <NUM> is to be started (Step S610). Specifically, the system state detecting unit <NUM> of the UE <NUM> determines whether the congestion in the <NUM> core network <NUM> or the non-support for IMS in the <NUM> core network <NUM> is detected in the <NUM> cell <NUM>. In addition, when the system state detecting unit <NUM> detects that the congestion information or the non-IMS support information is included in the broadcast information or the paging signal received from the 4GRAN <NUM> via the radio communication unit <NUM> in the <NUM> cell <NUM>, the UE <NUM> can determine that the transition from the <NUM> core network <NUM> to the <NUM> core network <NUM> is to be started.

Upon determining that the transition to the <NUM> core network <NUM> is to be started, the UE <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> in which the UE <NUM> resides (Step S620). Specifically, upon receiving from the system state detecting unit <NUM> the detection signal that indicates that the <NUM> core network <NUM> is congested or the <NUM> core network <NUM> does not support IMS, the cell selecting unit <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> in which the UE <NUM> resides.

The UE <NUM> transmits the connection request signal to the <NUM> cell <NUM> that is selected as the target cell for connection (Step S630). Specifically, the connection processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal, so as to transmit the connection request signal to the <NUM> cell <NUM>. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the 4GRAN <NUM>.

Upon connecting to the 4GRAN <NUM>, the UE <NUM> performs the IMS incoming / outgoing communication with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM> (Step S640). Specifically, upon receiving the connection setup signal from the 4GRAN <NUM> via the radio communication unit <NUM>, the connection processing unit <NUM> establishes the radio connection with the 4GRAN <NUM>. The attach processing unit <NUM> requests the radio communication unit <NUM> to transmit the radio signal so as to transmit the attach request signal after the UE <NUM> has established the radio connection with the 4GRAN <NUM>. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the <NUM> core network <NUM>. Upon receiving the attach complete response signal from the <NUM> core network <NUM> via the radio communication unit <NUM>, the attach processing unit <NUM> completes the attach processing with the <NUM> core network <NUM>. After the UE <NUM> has completed the attach processing with the <NUM> core network <NUM>, the IMS incoming / outgoing communication processing unit <NUM> executes the IMS registration procedure with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM>, and requests the radio communication unit <NUM> to transmit the radio signal so as to transmit the INVITE signal. The radio communication unit <NUM> transmits, based on the received request, the radio signal to the IMS <NUM> and performs the IMS incoming / outgoing communication with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM>.

On the other hand, upon determining that the transition to the <NUM> core network <NUM> is not to be started, the UE <NUM> performs the IMS incoming / outgoing communication with the IMS <NUM> via the 4GRAN <NUM> and the <NUM> core network <NUM> (Step S650).

According to the embodiments explained above, upon transmitting the connection request signal to the 5GRAN <NUM> in the <NUM> cell <NUM> of the 5GRAN <NUM> and receiving a connection rejection signal from the 5GRAN <NUM>, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection and starts the processing for fallback to the <NUM> system <NUM>. In this manner, when the connection to the <NUM> system <NUM> is rejected, instead of performing fallback to the <NUM> system <NUM>, the UE <NUM> autonomously selects the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service and performs fallback to the <NUM> system <NUM>. Therefore, because, in the embodiments explained above, even when the connection to the <NUM> system <NUM> is rejected, it is not necessary to perform fallback to the <NUM> system <NUM> at the initiative of the <NUM> system <NUM> and change communication service from the IMS-type service to CS-type service, time required to perform fallback can be shortened. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling can be maintained.

Upon transmitting the connection request signal to the 5GRAN <NUM> in the <NUM> cell <NUM> of the 5GRAN <NUM> and receiving the fallback instruction from the 5GRAN <NUM>, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection and starts the processing for fallback to the <NUM> system <NUM>. In this manner, when fallback to the <NUM> system <NUM> is instructed, instead of performing fallback to the <NUM> system <NUM>, the UE <NUM> autonomously selects the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service and performs fallback to the <NUM> system <NUM>. Therefore, because, in the embodiments explained above, when fallback to the <NUM> system <NUM> is instructed, it is not necessary to perform fallback to the <NUM> system <NUM> at the initiative of the <NUM> system <NUM> and change communication service from the IMS-type service to CS-type service, time required to perform fallback can be shortened. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling can be maintained.

Upon transmitting the connection request signal to the 5GRAN <NUM> in the <NUM> cell <NUM> of the 5GRAN <NUM> and not receiving a response signal from the 5GRAN <NUM> after the predetermined time has elapsed, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection and starts the processing for fallback to the <NUM> system <NUM>. In this manner, when the <NUM> system <NUM> is congested, instead of performing fallback to the <NUM> system <NUM>, the UE <NUM> autonomously selects the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service and performs fallback to the <NUM> system <NUM>. Therefore, because, in the embodiments explained above, even when the <NUM> system <NUM> is congested, it is not necessary to perform fallback to the <NUM> system <NUM> at the initiative of the <NUM> system <NUM> and change communication service from the IMS-type service to CS-type service, time required to perform fallback can be shortened. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling can be maintained.

Upon transmitting the attach request signal to the <NUM> core network <NUM> in the <NUM> cell <NUM> of the 5GRAN <NUM> and receiving the non-IMS support notification from the <NUM> core network <NUM>, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection and starts the processing for fallback to the <NUM> system <NUM>. In this manner, when the <NUM> core network <NUM> does not support the IMS-type communication service, instead of performing fallback to the <NUM> system <NUM>, the UE <NUM> autonomously selects the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service and performs fallback to the <NUM> system <NUM>. Therefore, because, in the embodiments explained above, even when the <NUM> core network <NUM> does not support the IMS-type communication service, it is not necessary to perform fallback to the <NUM> system <NUM> at the initiative of the <NUM> system <NUM> and change communication service from the IMS-type service to CS-type service, time required to perform fallback can be shortened. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling can be maintained.

Upon transmitting the attach request signal to the <NUM> core network <NUM> in the <NUM> cell <NUM> of the 5GRAN <NUM> and receiving the fallback instruction from the <NUM> core network <NUM>, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection and starts the processing for fallback to the <NUM> system <NUM>. In this manner, when fallback to the <NUM> system <NUM> is instructed, instead of performing fallback to the <NUM> system <NUM>, the UE <NUM> autonomously selects the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service and performs fallback to the <NUM> system <NUM>. Therefore, because, in the embodiments explained above, when fallback to the <NUM> system <NUM> is instructed, it is not necessary to perform fallback to the <NUM> system <NUM> at the initiative of the <NUM> system <NUM> and change communication service from the IMS-type service to CS-type service, time required to perform fallback can be shortened. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling can be maintained.

Upon transmitting the attach request signal to the <NUM> core network <NUM> in the <NUM> cell <NUM> of the 5GRAN <NUM> and not receiving the response signal from the <NUM> core network <NUM> even after the predetermined time has elapsed, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection and starts the processing for fallback to the <NUM> system <NUM>. In this manner, when the <NUM> system <NUM> is congested, instead of performing fallback to the <NUM> system <NUM>, the UE <NUM> autonomously selects the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service and performs fallback to the <NUM> system <NUM>. Therefore, because, in the embodiments explained above, even when the <NUM> system <NUM> is congested, it is not necessary to perform fallback to the <NUM> system <NUM> at the initiative of the <NUM> system <NUM> and change communication service from the IMS-type service to CS-type service, time required to perform fallback can be shortened. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling can be maintained.

Upon receiving broadcast information or the paging signal that includes the access restriction information, the congestion information, the non-IMS support information, or the fallback instruction in the <NUM> cell <NUM> of the 5GRAN <NUM>, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection and starts the processing for fallback to the <NUM> system <NUM>. In this manner, when the access to the <NUM> system <NUM> is barred, the <NUM> system <NUM> is congested, the <NUM> core network <NUM> does not support the IMS-type communication service, or fallback to the <NUM> system <NUM> is instructed, instead of performing fallback to the <NUM> system <NUM>, the UE <NUM> does not attempt to connect to the <NUM> system <NUM>, and autonomously selects the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service and performs fallback to the <NUM> system <NUM>. Therefore, because, in the embodiments explained above, even when the access to the <NUM> system <NUM> is barred, the <NUM> system <NUM> is congested, the <NUM> core network <NUM> does not support the IMS-type communication service, or when fallback to the <NUM> system <NUM> is instructed, it is not necessary to perform fallback to the <NUM> system <NUM> at the initiative of the <NUM> system <NUM> and change communication service from the IMS-type service to CS-type service change communication service from the IMS-type service to CS-type service, time required to perform fallback can be shortened. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling can be maintained.

Moreover, when the broadcast information or the paging signal received in the <NUM> cell <NUM> of the 5GRAN <NUM> includes the frequency information, the UE <NUM> selects, based on the frequencies included in the frequency information, the <NUM> cell <NUM> that uses a different frequency than that used by the <NUM> cell <NUM> as the target cell for connection. Therefore, time required for selecting the target cell for connection can be shortened, and as a result, time required for performing fallback can be further shortened.

In a configuration in which the 4GRAN <NUM> is connected to the <NUM> core network <NUM> and the <NUM> core network <NUM>, upon detecting in the <NUM> cell <NUM> that the <NUM> core network <NUM> is congested or the <NUM> core network <NUM> does not support the IMS, the UE <NUM> selects, as the target cell for connection, the <NUM> cell <NUM> in which the UE <NUM> resides, and starts the transition from the <NUM> core network <NUM> to the <NUM> core network <NUM>. In this manner, when the <NUM> core network <NUM> is congested or the <NUM> core network <NUM> does not support the IMS-type communication service, instead of performing fallback to the <NUM> system <NUM>, the UE <NUM> autonomously selects the <NUM> cell <NUM> of the <NUM> system <NUM> that supports the IMS-type communication service and performs fallback to the <NUM> system <NUM> to perform incoming/ outgoing communication of the IMS-type communication service via the 4GRAN <NUM> and the <NUM> core network <NUM> that is connected to the 4GRAN <NUM>. Therefore, because, in the embodiments explained above, even when the <NUM> core network <NUM> is congested or the <NUM> core network <NUM> does not support the IMS-type communication service, it is not necessary to perform fallback to the <NUM> system <NUM> at the initiative of the <NUM> system <NUM> and change communication service from the IMS-type service to the CS-type service, time required to perform fallback can be shortened. As a result, service quality of communication that requires real time performance such as emergency calls, voice communication, and video calling can be maintained.

The present invention has been explained in detail by using the above mentioned embodiments; however, it is self-evident to a person skilled in the art that the present invention is not limited to the embodiments explained herein and that the embodiments can be modified or improved in various ways.

For example, even if fallback to the <NUM> system <NUM> is explained in Operation Examples <NUM> to <NUM> explained above, the fallback destination system is not limited to the <NUM> system <NUM>. As long as the UE <NUM> can autonomously select the target cell for connection and perform fallback while retaining the IMS-type communication service as the communication service, fallback can be performed to other systems.

Moreover, the block diagram used for explaining the embodiments (<FIG>) shows a functional block diagram. Those functional blocks (structural components) can be realized by a desired combination of hardware and / or software. Means for realizing each functional block is not particularly limited. That is, each functional block may be realized by one device combined physically and / or logically. Alternatively, two or more devices separated physically and / or logically may be directly and / or indirectly connected (for example, wired and / or wireless) to each other, and each functional block may be realized by these plural devices.

Furthermore, the UE <NUM> explained above can function as a computer that performs the fallback processing of the present invention. <FIG> is a diagram showing an example of a hardware configuration of the UE <NUM>. As shown in <FIG>, the UE <NUM> can be configured as a computer device including a processor <NUM>, a memory <NUM>, a storage <NUM>, a communication device <NUM>, an input device <NUM>, an output device <NUM>, and a bus <NUM>.

The functional blocks of the UE <NUM> (see <FIG>) can be realized by any of hardware elements of the computer device or a desired combination of the hardware elements.

The processor <NUM>, for example, operates an operating system to control the entire computer. The processor <NUM> can be configured with a central processing unit (CPU) including an interface with a peripheral device, a control device, a computing device, a register, and the like.

The memory <NUM> is a computer readable recording medium and is configured, for example, with at least one of ROM (Read Only Memory), EPROM (Erasable Programmable ROM), EEPROM (Electrically Erasable Programmable ROM), RAM (Random Access Memory), and the like. The memory <NUM> can be called register, cache, main memory (main memory), and the like. The memory <NUM> can store therein a computer program (computer program codes), software modules, and the like that can execute the method according to the above embodiments.

The storage <NUM> is a computer readable recording medium. Examples of the storage <NUM> include at least one of an optical disk such as CD-ROM (Compact Disc ROM), a hard disk drive, a flexible disk, a magneto-optical disk (for example, a compact disk, a digital versatile disk, a Blu-ray (Registered Trademark) disk), a smart card, a flash memory (for example, a card, a stick, a key drive), a floppy (Registered Trademark) disk, a magnetic strip, and the like. The storage <NUM> can be called an auxiliary storage device. The recording medium can be, for example, a database including the memory <NUM> and / or the storage <NUM>, a server, or other appropriate medium.

The communication device <NUM> is hardware (transmission / reception device) capable of performing communication between computers via a wired and / or wireless network. The communication device <NUM> is also called, for example, a network device, a network controller, a network card, a communication module, and the like.

The input device <NUM> is an input device (for example, a keyboard, a mouse, a microphone, a switch, a button, a sensor, and the like) that accepts input from the outside. The output device <NUM> is an output device (for example, a display, a speaker, an LED lamp, and the like) that outputs data to the outside. Note that, the input device <NUM> and the output device <NUM> may be integrated (for example, a touch screen).

In addition, the respective devices, such as the processor <NUM> and the memory <NUM>, are connected to each other with the bus <NUM> for communicating information there among. The bus <NUM> can be constituted by a single bus or can be constituted by separate buses between the devices.

In addition, the manner of notification of information is not limited to the one explained in the embodiments, and the notification may be performed in other manner. For example, the notification of information can be performed by physical layer signaling (for example, DCI (Downlink Control Information), UCI (Uplink Control Information)), upper layer signaling (for example, RRC signaling, MAC (Medium Access Control) signaling, broadcast information (MIB (Master Information Block), SIB (System Information Block)), other signals, or a combination thereof. In addition, the RRC signaling can be called an RRC message, and the RRC signaling can be, for example, an RRC Connection Setup message, an RRC Connection Reconfiguration message, and the like.

Furthermore, the input / output information can be stored in a specific location (for example, a memory) or can be managed in a management table. The information to be input / output can be overwritten, updated, or added. The information can be deleted after outputting. The inputted information can be transmitted to another device.

The order of the sequences, flowcharts, and the like in the embodiments can be rearranged unless there is a contradiction.

Moreover, in the embodiments explained above, the specific operations performed by the target generation RAN and the core network can be performed by another network node (device). Moreover, functions of the target generation RAN and the core network can be provided by combining a plurality of other network nodes.

Moreover, the terms used in this specification and / or the terms necessary for understanding the present specification can be replaced with terms having the same or similar meanings. For example, a channel and / or a symbol can be replaced with a signal (signal) if that is stated. Also, the signal can be replaced with a message. Moreover, the terms "system" and "network" can be used interchangeably.

Furthermore, the used parameter and the like can be represented by an absolute value, can be expressed as a relative value from a predetermined value, or can be represented by corresponding other information. For example, the radio resource can be indicated by an index.

The target generation base station can accommodate one or more (for example, three) cells (also called sectors). In a configuration in which the base station accommodates a plurality of cells, the entire coverage area of the base station can be divided into a plurality of smaller areas. In each such a smaller area, communication service can be provided by a base station subsystem (for example, a small base station for indoor use RRH: Remote Radio Head).

The term "cell" or "sector" refers to a part or all of the coverage area of a base station and / or a base station subsystem that performs communication service in this coverage. In addition, the terms "base station", "cell", and "sector" can be used interchangeably in the present specification. The base station can also be referred to as a fixed station, NodeB, eNodeB (eNB), an access point, a femtocell, a small cell, and the like.

The UE <NUM> is called by the persons skilled in the art as a subscriber station, a mobile unit, a subscriber unit, a radio unit, a remote unit, a mobile device, a radio device, a radio communication device, a remote device, a mobile subscriber station, an access terminal, a mobile terminal, a radio terminal, a remote terminal, a handset, a user agent, a mobile client, a client, or with some other suitable term.

As used herein, the phrase "based on" does not mean "based only on" unless explicitly stated otherwise. In other words, the phrase "based on" means both "based only on" and "based at least on".

Furthermore, the terms "including", "comprising", and variants thereof are intended to be inclusive in a manner similar to "having". Furthermore, the term "or" used in the specification or claims is intended not to be an exclusive disjunction.

Any reference to an element using a designation such as "first", "second", and the like used in the present specification generally does not limit the amount or order of those elements. Such designations can be used in the present specification as a convenient way to distinguish between two or more elements. Thus, the reference to the first and second elements does not imply that only two elements can be adopted, or that the first element must precede the second element in some or the other manner.

Throughout the present specification, for example, during translation, if articles such as "a", "an", and "the" in English are added, these articles shall include plurality, unless it is clearly indicated that it is not so according to the context.

As described above, the details of the present invention have been disclosed by using the embodiments of the present invention. However, the description and drawings which constitute part of this disclosure should not be interpreted so as to limit the present invention. From this disclosure, various alternative embodiments, examples, and operation techniques will be apparent to a person skilled in the art, without departing from the scope of protection which is defined by the appended set of claims.

For example, the present invention can be applied to a radio communication system in which a first system and a second system that support the IMS-type communication service respectively correspond to the <NUM> system <NUM> and the <NUM> system <NUM>. Specifically, in the <NUM> cell <NUM> of the <NUM> system <NUM>, upon detecting that the <NUM> core network <NUM> is congested, the UE <NUM> selects the <NUM> cell <NUM> of the <NUM> system <NUM> as the target cell for connection. Upon selecting the target cell for connection, the UE <NUM> transmits the connection request signal for the IMS-type communication service to the selected <NUM> cell <NUM>. Alternatively, in the <NUM> cell <NUM> of the <NUM> system <NUM>, when the UE <NUM> detects that the <NUM> core network <NUM> is congested, in a state in which the UE <NUM> is connected to the <NUM> cell <NUM>, the UE <NUM> can transmit a request to the 4GRAN <NUM> to perform communication with the IMS <NUM> via the <NUM> core network <NUM> that is connected to the 4GRAN <NUM>.

Claim 1:
A user device (<NUM>) comprising:
a system state detecting unit (<NUM>) configured to detect a state that access to a first system is barred in a first cell (<NUM>) of the first system (<NUM>) that supports IP multimedia subsystem-type communication service, in a state in which the user device is in an IDLE state as a radio connection between the user device and the first system, wherein the first system is a <NUM> system;
a cell selecting unit (<NUM>) configured to select, based on the state detected by the system state detecting unit (<NUM>), a second cell (<NUM>) of a second system (<NUM>) that supports the IP multimedia subsystem-type communication service, as a target cell for connection, in the state in which the user device is in the IDLE state as the radio connection between the user device and the first system, wherein the second system is a <NUM> system; and
a connection processing unit (<NUM>) configured to transmit a connection request signal for the IP multimedia subsystem-type communication service to the target cell for connection.