Patent Publication Number: US-2021168654-A1

Title: Methods and apparatuses for transmitting control-plane messages in cells using different radio access technologies

Description:
The present application is a Continuation application of Ser. No. 16/063,735 filed on Jun. 19, 2018, which is a National Stage of International Application No. PCT/JP2016/087143 filed Dec. 14, 2016, claiming priority based on Japanese Patent Application No. 2016-002879 filed Jan. 8, 2016, the disclosure of which is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to communication between a radio station and a radio terminal using a plurality of Radio Access Technologies (RATs). 
     BACKGROUND ART 
     The 3rd Generation Partnership Project (3GPP) is starting to work on the standardization for 5G, i.e., 3GPP Release 14, in 2016 to make 5G a commercial reality in 2020. 5G is expected to be realized by continuous enhancement/evolution of LTE and LTE-Advanced and an innovative development by an introduction of a new 5G air-interface (i.e., a new Radio Access Technology (RAT)). The new RAT (i.e., New 5G RAT) supports, for example, frequency bands higher than the frequency bands (e.g., 6 GHz or lower) supported by the LTE/LTE-Advanced and its enhancement/evolution. For example, the new RAT supports centimeter-wave bands (10 GHz or higher) and millimeter-wave bands (30 GHz or higher). 
     Higher frequency can provide higher-rate communication. However, because of its frequency properties, coverage of the higher frequency is more local. Therefore, high frequencies are used to boost capacity and data rates in specific areas, while wide-area coverage is provided by lower current frequencies. That is, in order to ensure the stability of New 5G RAT communication in high frequency bands, tight integration or interworking between low and high frequencies (i.e., tight integration or interworking between LTE/LTE-Advanced and New 5G RAT) is required. A 5G supporting radio terminal (i.e., 5G User Equipment (UE)) is connected to both of a low frequency band cell and a high frequency band cell (i.e., a LTE/LTE-Advanced cell and a new 5G cell) by using Carrier Aggregation (CA) or Dual Connectivity (DC), or a modified technique thereof. 
     Non-Patent Literature  1  discloses user-plane and control-plane architectures to use both the LTE air interface (i.e., LTE RAT) and the new 5G air interface (i.e., New 5G RAT). In some implementations, a common Radio Resource Control (RRC) layer and a common Packet Data Convergence Protocol (PDCP) layer (or sublayer) are used. The common PDCP layer is connected to LTE lower layers and New 5G lower layers, and provides an upper layer with a transfer service of user plane data and control plane data through the LTE lower layers and the New 5G lower layers. The LTE lower layers include a Radio Link Control (RLC) layer, a Medium Access Control (MAC) layer, and a physical layer for the LTE-RAT. In a similar way, the New 5G lower layers include an RLC layer, a MAC layer, and a physical layer for the New 5G RAT. 
     Non-Patent Literature  1  further discloses transmitting the same control-plane message simultaneously on both an LTE cell and a New 5G cell (i.e., Control Plane Diversity) and switching the path of a control-plane connection from an LTE cell to a New 5G cell and vice versa (i.e., Fast Control Plane Switching) based on the premise that the common PDCP layer and the common RRC layer are arranged in the LTE base station. The use of the common PDCP and the common RRC layer allows the UE to be connected to one control point (i.e., the common RRC layer) via one of a plurality of air interfaces. 
     The term “LTE” used in this specification includes enhancements of LTE and LTE-Advanced for 5G to provide tight interworking with the New 5G RAT, unless otherwise indicated. Such enhancements of LTE and LTE-Advanced are also referred to as LTE-Advanced Pro, LTE+, or enhanced LTE (eLTE). Further, the term “5G” or “New 5G” in this specification is used, for the sake of convenience, to indicate an air-interface (RAT) that is newly introduced for the fifth generation (5G) mobile communication systems, and nodes, cells, protocol layers, etc. related to this air-interface. The names of the newly introduced air interface (RAT), and nodes, cells, and protocol layers related thereto will be determined in the future as the standardization work progresses. For example, the LTE RAT may be referred to as Primary RAT (P-RAT or pRAT) or Master RAT. Meanwhile, the New 5G RAT may be referred to as Secondary RAT (S-RAT or sRAT). 
     CITATION LIST 
     Non Patent Literature 
     [Non-Patent Literature 1] Da Silva, I.; Mildh, G.; Rune, J.; Wallentin, P.; Vikberg, J.; Schliwa-Bertling, P.; Rui Fan, “Tight Integration of New 5G Air Interface and LTE to Fulfill 5G Requirements,” in Vehicular Technology Conference (VTC Spring), 2015 IEEE 81st, pp. 1-5, 11-14 May 2015 
     SUMMARY OF INVENTION 
     Technical Problem 
     The inventors have studied the 5G radio architecture that provides tight interworking of the LTE RAT and the New 5G RAT and found some problems. For example, Non-Patent Literature 1 does not take into account a control plane specific to New 5G cells. In some implementations, control plane (CP) messages specific to New 5G cells (e.g., allocation of a temporary UE ID, configuration of lower layers, signalling for service quality management, and measurement reporting) may be required. In some implementations, a signalling connection specific to 5G cells (e.g., an RRC connection and a Signaling Radio Bearer (SRB)) may be required. It may be efficient for the New-5G-cell-specific control plane (CP) messages and the 5G-cell-specific signalling connections to be transmitted via New 5G cells, not via LTE cells. For example, one of the important requirements for 5G is low latency, and accordingly it can be expected that 5G UEs will be able to perform communication on New 5G cells with latency lower than that of communication on LTE cells. 
     Accordingly, one of the objects to be attained by embodiments disclosed herein is to provide an apparatus, a method, and a program that contribute to efficient transmission of control-plane messages through a cell of the subordinate RAT in a radio architecture that provides interworking of two different RATs. It should be noted that this object is merely one of the objects to be attained by the embodiments disclosed herein. Other objects or problems and novel features will be made apparent from the following description and the accompanying drawings. 
     Solution to Problem 
     In a first aspect, a radio station system includes one or more radio stations. The one or more radio stations are configured to simultaneously provide, for one radio terminal, at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell. The one or more radio stations are configured to transmit or receive a control-plane message to or from the radio terminal on the at least one second cell when a predetermined condition is satisfied. The control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both. The predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. 
     In a second aspect, a method in a radio station system, including one or more radio stations, includes: 
     (a) simultaneously providing, for one radio terminal, at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell; and 
     (b) transmitting or receiving a control-plane message to or from the radio terminal on the at least one second cell when a predetermined condition is satisfied. The control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both. The predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. 
     In a third aspect, a radio terminal includes a memory and at least one processor coupled to the memory. The at least one processor is configured to communicate with a radio station system including one or more radio stations simultaneously on at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell. The at least one processor is configured to transmit or receive a control-plane message to or from the radio station system on the at least one second cell when a predetermined condition is satisfied. The control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both. The predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. 
     In a fourth aspect, a method in a radio terminal includes: 
     (a) communicating with a radio station system including one or more radio stations simultaneously on at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell; and 
     (b) transmitting or receiving a control-plane message to or from the radio station system on the at least one second cell when a predetermined condition is satisfied. The control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both. The predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. 
     In a fifth aspect, a program includes instructions (software codes) that, when loaded into a computer, cause the computer to perform the method according to the aforementioned second or fourth aspect. 
     Advantageous Effects of Invention 
     According to the above-described aspects, it is possible to provide an apparatus, a method, and a program that contribute to efficient transmission of control-plane messages via a cell of the subordinate RAT in a radio architecture that provides interworking of two different RATs. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a configuration example of a radio communication network according to several embodiments; 
         FIG. 2  is a diagram showing a configuration example of the radio communication network according to the several embodiments; 
         FIG. 3  is a diagram showing another configuration example of the radio communication network according to the several embodiments; 
         FIG. 4  is a diagram showing an example of a radio protocol stack according to the several embodiments; 
         FIG. 5  is a diagram showing an example of the radio protocol stack according to the several embodiments; 
         FIG. 6  is a diagram showing an example of a layer- 2  structure according to the several embodiments; 
         FIG. 7  is a sequence diagram showing one example of operations of a radio terminal and a base station according to a first embodiment; 
         FIG. 8  is a sequence diagram showing one example of operations of the radio terminal and the base station according to the first embodiment; 
         FIG. 9  is a sequence diagram showing one example of operations of the radio terminal and the base station according to the first embodiment; 
         FIG. 10  is a flowchart showing one example of operations of an RRC layer according to the first embodiment; 
         FIG. 11  is a table showing an example of a key used to generate a temporary key for ciphering/deciphering of each radio bearer according to a second embodiment; 
         FIG. 12  is a block diagram showing a configuration example of a radio terminal according to the several embodiments; and 
         FIG. 13  is a block diagram showing a configuration example of a base station according to the several embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Specific embodiments are described hereinafter in detail with reference to the drawings. The same or corresponding elements are denoted by the same reference symbols throughout the drawings, and repetitive descriptions are avoided for clarity. 
     Each of embodiments described below may be used individually, or two or more of the embodiments may be appropriately combined with one another. These embodiments include novel features different from one another. Accordingly, these embodiments contribute to achieving objects or solving problems different from one another and contribute to obtaining advantages different from one another. 
     The following descriptions on the embodiments mainly focus on specific examples with regard to the 5G radio architecture that provides tight interworking of the LTE RAT and the New 5G RAT. However, these embodiments are not limited to being applied to the 5G radio architecture and may also be applied to other radio architectures that provide tight interworking of two different RATs. 
     First Embodiment 
       FIG. 1  shows a configuration example of a radio communication network according to several embodiments including this embodiment. In the example shown in  FIG. 1 , the radio communication network includes a radio terminal (UE)  1  and an integrated base station (i.e., integrated eNB)  2 . The UE  1  is a 5G UE and connects to both one or more LTE cells (e.g., cells  21  and  22 ) and one or more New 5G cells (e.g., cells  23  and  24 ) using CA, DC, or an enhancement thereof. In the following description, one or more LTE cells are referred to as an LTE cell group (CG) and one or more New 5G cells used by the 5G UE  1  are referred to as a New 5G CG. Each of the cells in the LTE CG and New 5G CG has been configured for the 5G UE  1  by the integrated eNB  2  and has been activated by the integrated eNB  2 . In some implementations, frequency bands (e.g., F 1  and F 2 ) of the LTE CG (e.g., the cells  21  and  22 ) are lower frequency bands (e.g., lower than 6 GHz) and frequency bands (e.g., F 3  and F 4 ) of the New 5G CG (e.g., the cells  23  and  24 ) are higher frequency bands (e.g., higher than 6 GHz). 
     The integrated eNB  2  supports 5G and provides a plurality of cells that use a plurality of component carriers (CCs) having different frequencies and using different RATs. In the example shown in  FIG. 1 , the integrated eNB  2  provides LTE cells  21  and  22  and New 5G cells  23  and  24 . The integrated eNB  2  communicates with the 5G UE  1  via both the LTE CG (e.g., the cells  21  and  22 ) and the New 5G CG (e.g., the cells  23  and  24 ) using CA, DC, or an enhancement thereof. Further, the integrated eNB  2  is connected to a core network, that is, an integrated Evolved Packet Core (i.e., integrated EPC)  41 . The integrated EPC  41  provides LTE core network functions and 5G new core network functions. In some implementations, the integrated eNB  2  may be connected to a 5G specific core network (i.e., 5G specific EPC  42 ). 
     As shown in  FIG. 2 , a remote radio unit  3  may be used to provide at least one of the cells of the integrated eNB  2  (e.g., New 5G cells  23  and  24 ). In the configuration shown in  FIG. 2 , the integrated eNB  2  performs digital signal processing regarding uplink and downlink signals, and meanwhile the radio unit  3  performs analog signal processing of the physical layer. For example, the integrated eNB  2  and the radio unit  3  are connected to each other by an optical fiber, and a digital baseband signal is transferred through this optical fiber in accordance with the Common Public Radio Interface (CPRI) standard. The configuration shown in  FIG. 2  is referred to as a Cloud Radio Access Network (C-RAN). The radio unit  3  is referred to as a Remote Radio Head (RRH) or a Remote Radio Equipment (RRE). The integrated eNB  2  that performs baseband digital signal processing is referred to as a Baseband Unit (BBU). Further, information about any one of the layers 1, 2, and 3 (or a signal containing this information) may be transferred using a fronthaul (interface) that is to be standardized by, for example, 3GPP or Small Cell Forum. For example, a form in which the fronthaul connects between the L1 and the L2 or between Sub-layers in the L2 is also referred to as L2 C-RAN. In this case, the integrated eNB  2  and the RRH  3  shown in  FIG. 2  are also referred to as a Digital Unit (DU) and a Radio Unit (RU), respectively. 
     In the configuration examples shown in  FIGS. 1 and 2 , the LTE radio protocol and the New 5G radio protocol are implemented in one node (i.e., the integrated eNB  2 ). Accordingly, the configuration examples shown in  FIGS. 1 and 2  are referred to as co-located deployments or co-located RAN. In the case of the L2 C-RAN configuration, a part of the New 5G radio protocol may be deployed in the RU. However, in another configuration example, non co-located deployments or non co-located RAN may be employed. In the Non co-located deployments, the LTE radio protocol and the New 5G radio protocol are provided by two nodes (eNBs) different from each other. 
     These two nodes are installed, for example, at two different sites geographically spaced apart from each other. 
       FIG. 3  shows an example of the non co-located deployments of the radio communication network according to several embodiments including this embodiment. In the example shown in  FIG. 3 , the radio communication network includes a 5G UE  1 , an LTE+ eNB  5 , and a 5G specific eNB  6 . The LTE+ eNB  5  provides an LTE CG (e.g., the cells  21  and  22 ) and the 5G specific eNB  6  provides a New 5G CG (e.g., the cells  23  and  24 ). The LTE+ eNB  5  is connected to the 5G specific eNB  6  by a communication line, such as an optical fiber link or a point-to-point radio link, and communicates with the 5G specific eNB  6  on an inter-base-station interface  301  (e.g., enhanced X2 interface). The LTE+ eNB  5  and the 5G specific eNB  6  interwork with each other to enable the 5G UE  1  to connect to both the LTE CG and the 5G CG using CA, DC, or an enhancement thereof. 
       FIG. 4  shows one example of the radio protocol stack supported by the 5G UE  1  and the integrated eNB  2 . A radio protocol stack  400  shown in  FIG. 4  includes a unified (or integrated) RRC layer  401  and a unified (or integrated) PDCP layer (or sublayer)  402 . The integrated RRC layer  401  and the integrated PDCP layer  402  may also be referred to as a common RRC layer and a common PDCP layer, respectively. The radio protocol stack  400  further includes LTE lower layers and New 5G lower layers. The LTE lower layers include an LTE RLC layer  403 , an LTE MAC layer  404 , and an LTE PHY layer  405 . The New 5G lower layers include a New RLC layer  406 , a New MAC layer  407 , and a New PHY layer  408 . In the case of using the integrated eNB  2 , some of the functions of the LTE PHY layer  405  (e.g., analog signal processing) may be provided by an RRH for LTE. In a similar way, some of the functions of the New PHY layer  408  (e.g., analog signal processing) may be provided by an RRH for New 5G. Further, in the case of using the above-described L2 C-RAN configuration, some of the functions of the New PHY layer, the New MAC layer, or the New RLC layer (and the functions of layers lower than it) may be provided by an RU for New 5G. 
     The integrated RRC layer  401  provides control-plane functions in the LTE RAT and the New 5G RAT. The main services and functions provided by the integrated RRC layer  401  include the following:
     Transmission of system information for non-access stratum (NAS) and access stratum (AS);   Paging;   Establishment, maintenance, and release of RRC connections;   Security functions including key management;   Configuration, maintenance, and release of radio bearers;   Configuration of lower layer protocols (i.e., PDCP, RLC, MAC, and PHY);   QoS management;   UE measurement report and configuration thereof; and   Transfer of NAS messages between a UE and a core network.   

     The integrated RRC layer  401  communicates with the integrated PDCP layer  402  to perform management of radio bearers, control of ciphering/deciphering of data of the user plane (i.e., data radio bearers), control of ciphering/deciphering of data (i.e., RRC PDUs) of the control plane (i.e., signalling radio bearers), and control of integrity protection of data (i.e., RRC PDUs) of the control plane (i.e., signalling radio bearers). Further, the integrated RRC layer  401  controls the LTE RLC layer  403 , the LTE MAC layer  404 , and the LTE PHY layer  405 , and also controls the New RLC layer  406 , the New MAC layer  407 , and the New PHY layer  408 . 
     The integrated PDCP layer  402  provides an upper layer with transfer services of data of data radio bearers and signalling radio bearers. The integrated PDCP layer  402  receives services from the LTE RLC layer  403  and the New RLC layer  406 . That is, the integrated PDCP layer  402  is provided with a transfer service of PDCP PDUs through the LTE RAT by the LTE RLC layer  403  and is provided with a transfer service of PDCP PDUs through the New 5G RAT by the New RLC layer  406 . 
     It should be noted that the radio protocol stack  400 , which uses the integrated PDCP layer  402 , shown in  FIG. 4  can be applied not only to the co-located deployments (e.g.,  FIGS. 1 and 2 ) but also to the non co-located deployments (e.g.,  FIG. 3 ). That is, as shown in  FIG. 5 , in the non co-located deployments, the LTE+ eNB  5  is arranged in a site  501  and provides the integrated RRC layer  401 , the integrated PDCP layer  402 , the LTE RLC layer  403 , the LTE MAC layer  404 , and the LTE PHY layer  405 . In contrast, the 5G specific eNB  6  is arranged in another site  502  and provides the New RLC layer  406 , the New MAC layer  407 , and the New PHY layer  408 . 
     In some implementations, the 5G specific eNB  6  used in the non co-located deployments may include a New RRC layer  511  and a New PDCP layer  512 . Further, the 5G specific eNB  6  may include a control interface or connection (e.g., an S1-MME interface or an S1-U interface) with a core network (e.g., the integrated EPC  41  or the 5G specific EPC  42 ) for the 5G UE  1 . In some implementations, the New RRC layer  511  may configure the lower layers  406 - 408  of the New 5G CG (e.g., New 5G cells  23  and  24 ) and transmit system information (i.e., Master Information Block (MIB), or System Information Blocks (SIBs), or both) via the New 5G CG. The New RRC layer  511  may configure a signalling radio bearer with the 5G UE  1 , also configure the lower layers  406 - 408  of the New 5G CG (e.g., the New 5G cells  23  and  24 ) and the New PDCP layer  512 , and then transmit or receive RRC messages to or from the 5G UE  1  through the New 5G CG. The New RRC layer  511  may transfer NAS messages between the core network (e.g., the integrated EPC  41  or the 5G specific EPC  42 ) and the 5G UE  1 . The New PDCP layer  512  provides the New RRC layer  511  with a transfer service of RRC messages via the New 5G lower layers  406 - 408 . 
     The New RRC layer  511  may depend on the integrated RRC layer  401  (i.e., have a dependency relationship) or may perform control similar to that performed by the integrated RRC layer  401  (i.e., have a similar function). In the former case (i.e., dependency relationship), the 5G specific eNB  6  (or the New RRC layer  511  thereof) may generate RRC configuration information with respect to a New 5G cell(s) (i.e., New 5G CG) in response to an instruction or a request from the LTE+ eNB  5  (or the integrated RRC layer  401  thereof). The 5G specific eNB  6  (or the New RRC layer  511  thereof) may transmit this RRC configuration information to the LTE+ eNB  5  (or the integrated RRC layer  401  thereof) and the LTE+ eNB  5  may transmit an RRC message containing this RRC configuration information (e.g., an RRC Connection Reconfiguration message) to the 5G UE  1  on an LTE cell (i.e., LTE CG). Alternatively, the 5G specific eNB  6  (or the New RRC layer  511  thereof) may transmit an RRC message containing this RRC configuration information to the 5G UE  1  on a New 5G cell. 
     The 5G UE  1  may support the protocol stack shown in  FIG. 4  or support another protocol stack to communicate with the radio network shown in  FIG. 5 . For example, the 5G UE  1  may have an RRC layer (i.e., a master RRC layer or a primary RRC layer) corresponding to the integrated RRC layer  401  of the LTE+ eNB  5  and an auxiliary RRC layer (i.e., a sub RRC layer or a secondary RRC layer) corresponding to the New RRC layer  511  of the 5G specific eNB  6 . For example, the sub RRC layer may perform one or both of transmission and reception (or one or both of generation and restoration) of a part of the RRC configuration information controlled by the master RRC layer. The 5G UE  1  may receive both the RRC configuration information regarding a LTE cell(s) (i.e., LTE CG) and the RRC configuration information regarding a New 5G cell(s) (i.e., New 5G CG) through an LTE cell or through a New 5G cell. Alternatively, the 5G UE  1  may receive the RRC configuration information regarding a LTE cell(s) (i.e., LTE CG) through an LTE cell and meanwhile receive the RRC configuration information regarding a New 5G cell(s) (i.e., New 5G CG) through a New 5G cell. 
     The radio protocol stack shown in  FIG. 4  is merely one example and, alternatively, the 5G UE  1  and the integrated eNB  2  may support another protocol stack. For example, in  FIG. 4 , the integrated PDCP layer  402  integrates (or allows interworking of) the LTE lower layers and the New 5G lower layers. Alternatively, an integrated MAC layer may be used to integrate (or allow interworking of) the LTE PHY layer  405  and the New PHY layer  408 . 
       FIG. 6  shows one example of the layer-2 structure for uplink according to the several embodiments. The layer-2 structure for downlink is similar to that shown in  FIG. 6  except for some points, such as the terms used to describe the transport channels. An integrated PDCP layer  602 , an LTE RLC layer  603 , an LTE MAC layer  604 , a New RLC layer  606 , and a New MAC layer  607  shown in  FIG. 6  respectively correspond to the integrated PDCP layer  402 , the LTE RLC layer  403 , the LTE MAC layer  404 , the New RLC layer  406 , and the New MAC layer  407  shown in  FIGS. 4 and 5 . 
     The integrated PDCP layer  602  includes one or more PDCP entities. Each PDCP entity transports data of one radio bearer. Each PDCP entity is associated with either the user plane or the control plane depending on which radio bearer (i.e., a data radio bearer (DRB) or a signalling radio bearer (SRB)) it transports data from. In the example shown in  FIG. 6 , the integrated PDCP layer  602  includes three PDCP entities  6021 ,  6022 , and  6023  that correspond to three radio bearers # 1 , # 2 , and # 3 , respectively. Each of the radio bearers # 1 , # 2 , and # 3  may be an SRB or a DRB. 
     The data of the radio bearer # 1  is transmitted from the 5G UE  1  to the integrated eNB  2  (or the LTE+ eNB  5 ) via the LTE RAT on the LTE CG (e.g., the LTE cells  21  and  22 ). Accordingly, the radio bearer # 1  may be hereinafter referred to as an LTE bearer. The radio bearer # 1  is similar to an MCG bearer in LTE Release  12  DC. 
     The data of the radio bearer # 2  is transmitted from the 5G UE  1  to the integrated eNB  2  (or the 5G specific eNB  6 ) via the New 5G RAT on the New 5G CG (e.g., the New 5G cells  23  and  24 ). Accordingly, the radio bearer # 2  may be hereinafter referred to as a New 5G bearer. When the data is transmitted on the New 5G CG managed by the 5G specific eNB  6 , the radio bearer # 2  is similar to an SCG bearer in LTE Release 12 DC. Alternatively, when the data is transmitted on the New 5G CG managed by the integrated eNB  2 , the radio bearer # 2  may be similar to a bearer on the SCG side of a split bearer in LTE Release 12 DC. 
     The radio bearer # 3  is similar to a split bearer in LTE Release  12  DC. That is, the radio bearer # 3  is associated with both of one logical channel of the LTE RAT and one logical channel of the New 5G RAT to use both the resources of the LTE CG and the resources of the New 5G CG. In the case of the user data, the logical channel of the LTE RAT is a Dedicated Traffic Channel (DTCH). The logical channel of the New 5G RAT is a 5G logical channel for the user data that corresponds to the DTCH. The radio bearer # 3  may be hereinafter referred to as a split bearer or a unified bearer (an integrated bearer). 
     In the case of uplink transmission by the 5G UE  1 , the PDCP entity  6021  generates PDCP PDUs from data of the radio bearer # 1  (i.e., LTE bearer) and sends these PDCP PDUs to an LTE RLC entity  6031 . In the case of uplink reception by the integrated eNB  2  (or the LTE+ eNB  5 ), the PDCP entity  6021  receives RLC SDUs (i.e., PDCP PDUs) from the LTE RLC entity  6031  and sends data of the radio bearer # 1  to the upper layer. 
     In the case of uplink transmission by the 5G UE  1 , the PDCP entity  6022  generates PDCP PDUs from data of the radio bearer # 2  (i.e., New 5G bearer) and sends these PDCP PDUs to a New RLC entity  6061 . In the case of uplink reception by the integrated eNB  2  (or the 5G specific eNB  6 ), the PDCP entity  6022  receives RLC SDUs (i.e., PDCP PDUs) from the New RLC entity  6061  and sends data of the radio bearer # 2  to the upper layer. 
     In the case of uplink transmission by the 5G UE  1 , the PDCP entity  6023  generates PDCP PDUs from data of the radio bearer # 3  (i.e., integrated bearer) and routes these PDCP PDUs to an LTE RLC entity  6032  or a New RLC entity  6062 . In the case of uplink reception by the integrated eNB  2  (or the LTE+ eNB  5  and the 5G specific eNB  6 ), the PDCP entity  6023  reorders PDCP PDUs (i.e., RLC SDUs) received from the LTE RLC entity  6032  and the New RLC entity  6062  and sends data of the radio bearer # 3  to the upper layer. 
     Each RLC entity in the LTE RLC layer  603  and the New RLC layer  606  is configured, by the integrated RRC entity (i.e., the RRC entity  401  shown in  FIG. 4 ), with RLC Acknowledged Mode (RLC AM) data transfer or RLC Unacknowledged Mode (RLC UM) data transfer, and then provides a transfer service of PDCP PDUs. In the case of uplink transmission by the 5G UE  1 , each RLC entity in the LTE RLC layer  603  generates RLC PDUs (i.e., data of one logical channel) from PDCP PDUs (i.e., RLC SDUs) and sends these RLC PDUs to a MAC entity  6041  in the LTE MAC layer  604 . In a similar way, each RLC entity in the New RLC layer  606  generates RLC PDUs (i.e., data of one logical channel) from PDCP PDUs (i.e., RLC SDUs) and sends them to a MAC entity  6071  in the New MAC layer  607 . 
     In the example shown in  FIG. 6 , one MAC entity  6041  is used for two LTE cells (i.e., LTE CG) configured for one 5G UE  1 . In the case of uplink transmission by the 5G UE  1 , the MAC entity  6041  multiplexes RLC PDUs (i.e., MAC SDUs), which belong to the two logical channels from the two RLC entities  6031  and  6032 , into two transport blocks per Transmission Time Interval (TTI). The two transport blocks per TTI are sent to the LTE physical layer  405  through two UL transport channels (i.e., UL-SCHs) corresponding to the two LTE cells  21  and  22 . 
     In a similar way, one MAC entity  6071  is used for two New 5G cells (i.e., New 5G CG) configured for one 5G UE  1 . In the case of uplink transmission by the 5G UE  1 , the MAC entity  6071  multiplexes RLC PDUs (i.e., MAC SDUs), which belong to two logical channels from two RLC entities  6071  and  6072 , into two transport blocks per Transmission Time Interval (TTI). The two transport blocks per TTI are sent to the physical layer  408  for New 5G through two UL transport channels (i.e., UL TrCHs) corresponding to the two New 5G cells  23  and  24 . 
     As described above, the layer- 2  structure for downlink is similar to that shown in  FIG. 6  except for some points, such as the terms used to describe the transport channels. For example, in the case of downlink reception by the 5G UE  1 , the PDCP entity  6023  of the 5G UE  1  reorders PDCP PDUs (i.e., RLC SDUs) received from the LTE RLC entity  6032  and from the New RLC entity  6062 , and then sends data of the radio bearer # 3  (i.e., integrated bearer) to the upper layer. 
     In the following, an operation of transmitting a control-plane (CP) message performed by the 5G UE  1  and the integrated eNB  2  (or the LTE+ eNB  5  and the 5G specific eNB  6 ) according to this embodiment will be explained. The 5G UE  1  and the base station system are configured to transmit CP messages (i.e., RRC signalling) on a New 5G cell after completion of configuration and activation of this New 5G cell (i.e., after the New 5G cell becomes available for the 5G UE  1 ). The 5G UE  1  and the base station system may switch the cell, to be used to transmit CP messages, from the LTE cell to the New 5G cell. Further or alternatively, the 5G UE  1  and the base station system may adaptively change the cell, to be used to transmit CP messages, between the LTE cell and the New 5G cell. The CP messages include NAS messages or RRC messages or both. The base station system is the integrated eNB  2 , or a combination of the LTE+ eNB  5  and the 5G specific eNB  6 . 
     To be more specific, the 5G UE  1  is configured to transmit or receive CP messages to or from the base station system on any New 5G cell within the New 5G CG when a predetermined condition is satisfied. In a similar way, the base station system is configured to transmit or receive CP message to or from the 5G UE  1  on any New 5G cell within the New 5G CG when a predetermined condition is satisfied. The predetermined condition, which is used to determine which one of the LTE CG and the New 5G CG is used to transmit a CP message, relates to at least one of the following:
     (a) the content or type of the CP message   (b) the type of the signalling radio bearer used to transmit the CP message,   (c) the transmission cause of the CP message; and   (d) the type of the core network associated with the NAS message.   

     The above condition (a) may be that the content or type of the CP message is (or is not) a specific content or type. The specific content or type may be at least one of: a request for terminal measurement regarding the New 5G cell; a report on a result of the measurement regarding the New 5G cell; a request for terminal (holding) information held by the terminal; a report on the terminal information; security configuration information about the access stratum (AS) of the New 5G cell; a request for AS-security activation; a response to the request for AS-security activation; configuration information specific to the New 5G RAT; and control information that will be newly defined for the New 5G cell by the 3GPP. 
     The above condition (b) may be that the type of the signalling radio bearer used to transmit the CP message is (or is not) a specific type. For example, the specific signalling-radio-bearer type may be a type corresponding to SRB0, SRB1, or SRB2 in LTE specified in advance, or may be a type corresponding to a new signalling radio bearer (e.g., SRBx). 
     The above condition (c) may be that the transmission cause of the CP message is (or is not) a specific transmission cause. For example, the specific transmission cause may be at least one of: degradation of a radio quality of the New 5G cell; a failure of a radio link establishment in the LTE cell or the New 5G cell; a request for re-establishment of the radio link after the failure of the radio link establishment; and a response to the request for re-establishment of the radio link. 
     The above condition (d) may be that the type of the core network used for the transmission of the NAS message is (or is not) a specific core-network type. For example, the specific core-network type may be a type corresponding to a core network that will be newly introduced for the New 5G RAT or may be a type corresponding to a core network that is previously designated for use for a specific service or function. 
     In a first implementation, the 5G UE  1  may determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on whether this RRC message includes a reporting message from the 5G UE  1  indicating a measurement result of one or more New 5G cells within the New 5G CG (i.e., Measurement report message). That is, when an RRC message includes a report from the 5G UE  1  on a measurement result of one or more New 5G cells, the 5G UE  1  may transmit this RRC message on any cell within the New 5G CG. The base station system may receive this RRC message on any cell within the New 5G CG. In contrast, when an RRC message includes a report from the 5G UE  1  on a measurement result of one or more LTE cells, the 5G UE  1  may transmit this RRC message on any LTE cell. The report on the measurement result to be transmitted on the New 5G CG may be distinguished from the report on the measurement result to be transmitted on the LTE CG, on the basis of a measurement instruction (e.g., MeasConfig IE) or a criterion defining a condition to trigger a measurement report event (e.g., ReportConfigEUTRA IE). For example, a measurement report event, where a cell (e.g., PSCell, special cell, or SCell) within the New 5G CG is a target, is defined. In this case, the 5G UE  1  may transmit a report on a measurement result triggered by this measurement report event on any cell within the New 5G CG. The measurement report event, where a cell within the New 5G CG is a target, may be defined as Inter-RAT measurement with respect to a LTE cell. 
     In a second implementation, the 5G UE  1  may determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on whether this RRC message includes a request message for the terminal (holding) information of the 5G UE  1  regarding the New 5G RAT (e.g., UE information request message) or depending on whether this RRC message includes a report message carrying the terminal (holding) information (e.g., UE information response message). That is, when an RRC message includes a report of (or a request for) the terminal (holding) information of the 5G UE  1  regarding the New 5G RAT, the 5G UE  1  may transmit (or receive) this RRC message on any cell within the New 5G CG. The base station system may receive (or transmit) this RRC message on any cell within the New 5G CG. In contrast, when an RRC message includes a report of (or a request for) the terminal (holding) information of the 5G UE  1  regarding the LTE RAT, the 5G UE  1  may transmit (or receive) this RRC message on any LTE cell. Further, the 5G UE  1  may determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on the type of the terminal (holding) information to be requested or reported. The type of the terminal (holding) information may be any one or any combination of:
     Radio link-related information;   Random access-related information; and   Mobility history-related information.   

     The radio link-related information may be information regarding degradation of a radio quality (e.g., Radio Link Failure: RLF) in the LTE CG (e.g., PCell) or the New 5G CG (e.g., PSCell, Special cell) or information regarding a failure of radio link establishment (e.g., RRC Connection Establishment Failure: CEF, RRC Connection Re-establishment failure). For example, the information regarding degradation of a radio quality in the LTE CG may be requested or reported on the LTE CG (or the New 5G CG). In a similar way, the information regarding the degradation of a radio quality in the 5G CG may be requested or reported on the New 5G CG (or the LTE CG). In contrast, the information regarding a failure of radio link establishment may be always requested or reported on the LTE CG (or the New 5G CG). The radio link establishment may include a procedure using an RRC Connection Reconfiguration to change the configuration of the radio protocol(s) (e.g., RRC, PDCP, RLC, MAC, or PHY). Further, the 5G UE  1  may store the information regarding a failure of radio link establishment in the LTE CG and the information regarding a failure of radio link establishment in the New 5G CG separately from each other. 
     The random access (RACH)-related information may be the number of preamble transmissions it has taken to achieve the success of the random access and a flag indicating detection of a preamble contention. For example, the random access-related information regarding the LTE CG may be requested or reported on the LTE CG (or the New 5G CG). In a similar way, the random access-related information regarding the New 5G CG may be requested or reported on the New 5G CG (or on the LTE CG). The 5G UE  1  may store the number of preamble transmissions on the LTE CG and the number of preamble transmissions on the New 5G CG separately from each other. 
     The mobility history-related information (e.g., mobility history report) may be information regarding the cells which the 5G UE  1  visited (by the time it receives a request to report this information) (e.g., visited cell information list: VisitedCellInfoList). The visited cell information list may include at least one of: identification information (e.g., CGI, PCI, or Carrier frequency) of each cell that the 5G UE  1  visited; and information (e.g., timeSpent) about the time (e.g., seconds) the 5G UE  1  stayed in each cell. Further, the 5G UE  1  may store the history of the specific New 5G cell (e.g., PSCell, Special cell) to record information about visited cells specific to the New 5G RAT (e.g., VisitedCellInfoList-5G, VisitedCellInfoList-SRAT). The visited cell information list regarding the LTE RAT may be requested or reported on the LTE CG, and meanwhile the visited cell information list regarding the New 5G RAT may be requested or reported on the 5G CG. 
     In a third implementation, the base station system may determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on whether this RRC message includes the security configuration information on the access stratum (AS) regarding one or more New 5G cells within the New 5G CG. That is, the base station system may transmit an RRC message on any cell within the New 5G CG when this RRC message includes the AS security configuration information regarding one or more New 5G cells. The 5G UE  1  may receive this RRC message on any cell within the New 5G CG. In contrast, the base station system may transmit an RRC message on any LTE cell when this RRC message includes the AS security configuration information regarding one or more LTE cells. The security configuration information may be transmitted as a NAS message (i.e., NAS PDU). 
     In a fourth implementation, the base station system may determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on whether this RRC message includes the AS-security-activation request for one or more New 5G cells within the New 5G CG. That is, the base station system may transmit an RRC message on any cell within the New 5G CG when this RRC message includes the AS-security-activation request for one more New 5G cells (e.g., Security Mode Command: SMC). The 5G UE  1  may receive this RRC message on any cell within the New 5G CG. In contrast, the base station system may transmit an RRC message on any LTE cell when this RRC message includes the AS-security-activation request for one or more LTE cells. 
     In a fifth implementation, the 5G UE  1  may determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on whether this RRC message includes the response to the AS-security-activation request for one or more New 5G cells (e.g., Security Mode Complete, Security Mode Failure). That is, the 5G UE  1  may transmit an RRC message on any cell within the New 5G CG when this RRC message includes the response to the AS-security-activation request for one or more New 5G cells. The base station system may receive this RRC message on any cell within the New 5G CG. In contrast, the 5G UE  1  may transmit an RRC message on any LTE cell when this RRC message includes the response to the AS-security-activation request for one or more LTE cells. 
     In a sixth implementation, the base station system may determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on whether this RRC message includes the configuration information specific to the New 5G RAT (e.g., SCG configuration for 5G, Secondary RAT (sRAT) configuration). That is, the base station system may transmit an RRC message on any cell within the New 5G CG when this RRC message includes the configuration information specific to the New 5G RAT. The 5G UE  1  may receive this RRC message on any cell within the New 5G CG. In contrast, the base station system may transmit an RRC message on any LTE cell when this RRC message includes configuration information specific to the LTE RAT. The configuration information specific to the New 5G RAT may be any one or any combination of:
     Terminal capability-related information;   Service-related information; and   5G CG management-related information.   

     The terminal capability-related information may be a terminal capability(ies) regarding the New 5G RAT (e.g., UE-EUTRA-CapabilityAddSRAT, UE-EUTRA-Capability-SRAT), or may be a terminal response to a request (or an inquiry) from the base station system related to functions supported in a New 5G cell. The terminal capability-related information may be requested (e.g., via UE Capability Enquiry message) and reported (e.g., via UE Capability Information message) on the New 5G CG. In order to indicate that the terminal capability-related information regarding the New 5G RAT is requested, the RAT-Type within the UE-CapabilityRequest IE may indicate “sRAT” or “eutra2”, for example. At least a part of the terminal capabilities regarding the New 5G RAT required to connect to a New 5G cell may be transmitted on a LTE cell. 
     The service-related information may be control information required to achieve a service(s) provided in the New 5G CG (e.g., Radio resource configuration, Service area information, Service availability information, Service type/category information), or may be control information regarding a content of a service(s) (e.g., Content information, Scheduling/planning information). The service(s) here may indicate a service(s) that can be provided also in LTE, or may be a service(s) that can be provided only in 5G. The service(s) may include, for example, one of a Multimedia Broadcast and Multicast Service (MBMS) (i.e., MBMS over a Single Frequency Network (MBSFN), or single cell point to multipoint (SC-PTM)), a Cell Broadcast Service (CBS), Proximity Services (ProSe), Vehicle-to-Everything (V2X) services, and a Mobile Edge Computing (MEC). 
     The 5G CG management-related information may be information regarding a modification of a specific cell (e.g., PSCell, Special cell) within the New 5G CG (e.g., PSCell/Special cell reconfiguration, pSCellToAddMod-rlx). Further or alternatively, when the base station system adds or releases another New 5G cell (e.g., 5G SCell) for the 5G UE  1  after the base station system configures the specific New 5G cell (e.g., PSCell, Special cell) for the 5G UE  1  (that is, after the specific 5G cell becomes available), the base station system may transmit radio resource configuration information about this other New 5G cell (e.g., sCellToAddModListSRAT, sCellToReleaseListSRAT, RadioResourceConfigDedicatedSCellSRAT) as the 5G CG management-related information. That is, the information relating to addition/release of a cell within the New 5G CG and the information relating to modification of the specific New 5G cell may be transmitted on any cell within the New 5G CG. 
     Although basic configuration information required by the 5G UE  1  to connect to (or detect) a New 5G cell is configuration information specific to the New 5G RAT, it may be transmitted on a LTE cell. This basic configuration information may be, for example, information regarding a subframe structure (which is different from that of LTE), information regarding a TTI length, information regarding a sampling rate, or information regarding a cell type. 
     In a sixth implementation, the base station system may determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on whether this RRC message includes control information newly defined by the 3GPP for New 5G cells. That is, the base station system may transmit an RRC message on any cell within the New 5G CG when this RRC message includes control information newly defined for New 5G cells. The 5G UE  1  may receive this RRC message on any cell within the New 5G CG. In contrast, the base station system may transmit an RRC message on any LTE cell when this RRC message includes existing control information for LTE. The control information for New 5G cells may be any one or a combination of:
     Core network selection-related information; and   Information regarding a standalone operation of a New 5G cell(s).   

     The core network selection-related information may be information regarding a new type core network associated with introduction of the New 5G RAT (e.g., the 5G specific EPC  42 : this network is also referred to as a 5G Dedicated Core Network (DCN)). For example, the core network selection-related information may be identification information (e.g., MME ID, MMEC, GUMMEI, MMEGI, GUTI, TAI, or TEID) of a core network node (e.g., MME, S-GW, or P-GW), or may be a request (or a notification) for switch of the connection with the core network node (e.g., Reroute Command, Reroute Request, Reroute indication, DCN selection information, or DCN relocation information). Such information is required when the connection is switched from the integrated EPC  41  to the 5G specific EPC  42  or vice versa. Further or alternatively, the core network selection-related information may be assistance information to select the 5G specific EPC (5G DCN)  42 . This assistance information may be, for example, a terminal type (e.g., device type), a usage type (e.g., UE usage type), or information about an expected (or preferred) core network (e.g., expected/preferred CN information). The core network selection-related information may be transmitted as a NAS message (i.e., NAS PDU). 
     The information regarding the standalone operation of a New 5G cell(s) may be configuration information (e.g., Radio resource configuration for 5G/SRAT PCell) that is required to use a New 5G cell as a primary cell (PCell) instead of using a New 5G cell as a secondary cell (SCell) with the LTE PCell cell. Alternatively, the information regarding the standalone operation of a New 5G cell(s) may be information (e.g., Handover to a 5G/SRAT cell or Redirection to a 5G/SRAT cell) indicating a request or an indication that a New 5G cell is to be used as the PCell. Such information may be transmitted in a signalling radio bearer (e.g., SRBx) that will be newly defined. In addition, information required to configure this SRBx may be transmitted on a New 5G cell or on a LTE cell. 
     In the above-described first to sixth implementations, the 5G UE  1  and the base station system determine which one of the New 5G CG and the LTE CG is to be used to transmit an RRC message depending on whether this RRC message relates to the New 5G RAT (or a New 5G cell(s)). Accordingly, the 5G UE  1  and the base station system are able to transmit or receive RRC messages relating to the New 5G RAT (or a New 5G cell(s)) through any cell within a New 5G CG. This, for example, contributes to transmission of RRC messages regarding the New 5G RAT (or a New 5G cell(s)) with low latency. 
     In a seventh implementation, the 5G UE  1  may determine which one of the New 5G CG and the LTE CG is to be used to transmit a CP message (i.e., an RRC message or a NAS message) depending on whether to transmit this CP message via a specific signalling radio bearer associated with the New 5G RAT. That is, the 5G UE  1  may transmit a CP message on any cell within the New 5G CG when the 5G UE  1  transmits this CP message via the specific signalling radio bearer associated with the New 5G RAT. The base station system may receive this CP message on any cell within the New 5G CG. The specific signalling radio bearer may be SRB0, SRB1, or SRB2 of LTE, or may be a new signalling radio bearer (e.g., SRBx). In contrast, the 5G UE  1  may transmit a CP message on any LTE cell when the 5G UE  1  transmits this CP message via a specific signalling radio bearer associated with the LTE RAT. 
     In a similar way, in an eighth implementation, the base station system may determine which one of the New 5G CG and the LTE CG is to be used to transmit a CP message (i.e., an RRC message or a NAS message) depending on whether to transmit this CP message via a specific signalling radio bearer associated with the New 5G RAT. That is, the base station system may transmit a CP message on any cell within the New 5G CG when the base station system transmits this CP message via the specific signalling radio bearer associated with the New 5G RAT. The 5G UE  1  may receive this CP message on any cell within the New 5G CG. In contrast, the base station system may transmit a CP message on any LTE cell when the base station system transmits this CP message via a specific signalling radio bearer associated with the LTE RAT. 
     In the above-described seventh and eighth implementations, the 5G UE  1  and the base station system determine which one of the New 5G CG and the LTE CG is to be used to transmit a CP message depending on whether the CP message is transmitted on the specific bearer associated with the New 5G RAT. Accordingly, the 5G UE  1  and the base station system are able to transmit or receive CP messages relating to the New 5G RAT (or the New 5G cell(s)) through any cell within the New 5G CG. This, for example, contributes to transmission of CP messages regarding the New 5G RAT (or a New 5G cell(s)) with low latency. 
     In a ninth implementation, the 5G UE  1  may determine which one of the New 5G CG and the LTE CG is to be used to transmit a CP message (i.e., an RRC message or a NAS message) depending on whether the transmission cause of this CP message indicates degradation of a radio quality of any New 5G cell. That is, the 5G UE  1  may transmit a CP message on any cell within the New 5G CG when this CP message is transmitted due to degradation of the radio quality of any New 5G cell. The base station system may receive this CP message on any cell within the New 5G CG. This, for example, contributes to transmission of CP messages regarding the New 5G RAT (or a New 5G cell(s)) with low latency. In contrast, the 5G UE  1  may transmit an RRC message containing a NAS message on any LTE cell when the transmission cause of the CP message indicates degradation of a radio quality of any LTE cell. 
     In a tenth implementation, the 5G UE  1  may determine which one of the New 5G CG and the LTE CG is to be used to transmit a NAS message depending on whether the core network associated with this NAS message is a new type core network (e.g., the 5G specific EPC  42 ) associated with introduction of the New 5G RAT. That is, the 5G UE  1  may transmit an RRC message containing a NAS message on any cell within the New 5G CG when the core network associated with this NAS message is the new type core network associated with introduction of the New 5G RAT. The base station system may receive the RRC message containing this NAS message on any cell within the New 5G CG. This, for example, contributes to transmission of NAS messages regarding the new type core network, which is associated with introduction of the New 5G RAT, with low latency. In contrast, the 5G UE  1  may transmit an RRC message containing a NAS message on any LTE cell when the core network associated with this NAS message is another core network (e.g., the integrated EPC  41 ). 
     In a similar way, in an eleventh implementation, the base station system may determine which one of the New 5G CG and the LTE CG is to be used to transmit a NAS message depending on whether the core network associated with this NAS message is a new-type core network (e.g., the 5G specific EPC  42 ) associated with introduction of the New 5G RAT. That is, the base station system may transmit an RRC message containing a NAS message on any cell within the New 5G CG when the core network associated with this NAS message is the new type core network associated with introduction of the New 5G RAT. The 5G UE  1  may receive the RRC message containing this NAS message on any cell within the New 5G CG. In contrast, the base station system may transmit an RRC message containing a NAS message on any LTE cell when the core network associated with this NAS message is another core network (e.g., the integrated EPC  41 ). 
     In the aforementioned first to eleventh implementations, the transmission or reception of RRC messages may be performed in a specific cell in the LTE CG or the New 5G CG. For example, when an RRC message is transmitted or received on the LTE CG, a primary cell (PCell) may be used. Alternatively, when an RRC message is transmitted or received on the New 5G CG, a primary secondary cell (PSCell) may be used. The PCell and the PSCell may be collectively referred to as a special cell (SpCell). 
     In some implementations, the LTE+ eNB  5  may send, to the 5G specific eNB  6 , a CP message to be transmitted on a New 5G cell or information regarding this message. Further or alternatively, the 5G specific eNB  6  may send, to the LTE+ eNB  5 , a CP message that is to be transmitted or has been received (i.e., has been transmitted from the 5G UE  1 ) on a New 5G cell or information regarding this message. Further, the 5G specific eNB  6  may send and receive control-plane information (i.e., a CP message (e.g., a NAS message, an S1 Application Protocol (S1AP) message)) to and from a node(s) (e.g., MME) in a core network (e.g., the integrated EPC  41  or the 5G specific EPC  42 ). 
       FIG. 7  shows an example in which the LTE+ eNB  5  sends control-plane information (i.e., CP message) regarding a New 5G cell(s) to the 5G specific eNB  6 . In Step  701 A or  701 B, the 5G specific eNB  6  receives an X2 message containing at least control-plane information (i.e., CP message) regarding a New 5G cell(s) from the LTE+ eNB  5 . This X2 message may be a DL USER DATA message of the X2 User Plane Protocol (X2-UP) (Step  701 A) or may be a DL CONTROL DATA message that will be newly defined. This X2 message may be an X2 AP Message Transfer message of the X2AP (Step  701 B). Alternatively, this X2 message may be an SeNB Modification Request message (e.g., MeNB to SeNB Container) defined for Dual Connectivity or may be an X2 message (e.g., SeNB Configuration Request message) that will be newly defined. 
     Further or alternatively, the 5G specific eNB  6  may receive an S1AP message containing control-plane information regarding a New 5G cell(s) from the integrated EPC  41  (e.g., the MME  7 ) or the 5G specific EPC  42  (Step  702 ). This S1AP message may be an MME Configuration Transfer message, an MME Direct Information Transfer message, or another S1AP message. In Step  703 , the 5G specific eNB  6  transmits an RRC Connection Reconfiguration message containing the control-plane information (i.e., CP message) regarding a New 5G cell(s), which has been received from the LTE+ eNB  5 , the integrated EPC  41 , or the 5G specific EPC  42 , to the 5G UE  1  on a New 5G cell. 
       FIG. 8  shows an example in which the 5G specific eNB  6  sends control-plane information (i.e., CP message) regarding a New 5G cell(s) to the LTE+ eNB  5 . In Step  801 A or  801 B, the 5G specific eNB  6  sends an X2 message containing at least control-plane information (i.e., CP message) regarding a New 5G cell(s) to the LTE+ eNB  5 . This X2 message may be an X2AP Message Transfer message (Step  801 A). Alternatively, this X2 message may be an SeNB Modification Required message defined for Dual Connectivity (Step  801 B). 
     The LTE+ eNB  5  may start an SeNB Modification procedure in response to the reception of the X2 message. For example, the LTE+ eNB  5  may send an SeNB Modification Request message (containing an RRC Container (i.e., SCG-ConfigInfo)) to the 5G specific eNB  6  (Step  802 ). In response to the reception of the SeNB Modification Request message, the 5G specific eNB  6  may send an SeNB Modification Request Acknowledge message (containing an RRC Container (i.e., SCG-Config)) to the LTE+ eNB  5  (Step  803 ). In Step  803 , the 5G specific eNB  6  may send control-plane information regarding a 5G cell(s) to the LTE+ eNB  5 . 
     The 5G specific eNB  6  may receive an S1AP message containing control-plane information regarding a New 5G cell(s) from the integrated EPC  41  (e.g., the MME  7 ) or the 5G specific EPC  42  (Step  804 ). The 5G specific eNB  6  may perform Step  801 A or  801 B in response to receiving control-plane information regarding a 5G cell(s) from the integrated EPC  41  or the 5G specific EPC  42 . 
     In Step  805 , the 5G specific eNB  6  transmits an RRC Connection Reconfiguration message containing the control-plane information (the CP message) regarding a New 5G cell(s) to the 5G UE  1  on a New 5G cell. 
       FIG. 9  shows another example in which the 5G specific eNB  6  sends control-plane information (i.e., CP message) regarding a New 5G cell(s) to the LTE+ eNB  5 . In Step  901 , the 5G specific eNB  6  receives control-plane information from the 5G UE  1  on a 5G cell. In Step  902 , the 5G specific eNB  6  sends, to the LTE+ eNB  5 , an X2 message containing at least a part of the control-plane information received from the 5G UE  1 . This X2 message may be an X2AP Message Transfer message, or may be an X2 message that will be newly defined (e.g., SeNB Configuration Update message). The 5G specific eNB  6  may generate other control information in response to receiving the control-plane information from the 5G UE  1  and send the generated control information to the LTE+ eNB  5  in Step  902 . 
     Further, the 5G specific eNB  6  may send an S1AP message containing at least a part of the control-plane information received from the 5G UE  1  to the integrated EPC  41  (e.g., the MME  7 ) or the 5G specific EPC  42  (Step  903 ). This S1AP message may be an eNB Configuration Transfer message, an eNB Direct Information Transfer message, or another S1AP message. The 5G specific eNB  6  may generate other control information in response to receiving the control-plane information from the 5G UE  1  and send the generated control information to the integrated EPC  41  (e.g., the MME  7 ) or the 5G specific EPC  42  in Step  903 . 
       FIG. 10  is a flowchart showing one example (Process  1000 ) of the operation of transmitting a CP message performed by the 5G UE  1  and the base station system. In Step  1001 , the RRC layer (i.e., the integrated RRC layer  401  or the New RRC layer  511 ) generates an RRC message. This RRC message may be an RRC message carrying a NAS message (e.g., a UL Information Transfer message or a DL Information Transfer message). 
     In Step  1002 , the RRC layer determines which one of the LTE CG and the New 5G CG is to be used to transmit the RRC message depending on: (a) a content or type of this RRC message; (b) a type of a signalling radio bearer used to transmit this RRC message; (c) a transmission cause of this RRC message; or (d) a type of a core network associated with the NAS message contained in this RRC message. As already described above, for example, when the content or type of this RRC message relates to the New 5G RAT or a New 5G cell(s), the RRC layer may transmit this RRC message via any New 5G cell. 
     In Step  1003 , the RRC layer sends this message to the PDCP entity that corresponds to the cell to be used for transmission of the RRC message determined in Step  1002 . Accordingly, the 5G UE  1  and the base station system are able to control which one of the LTE CG and the New 5G CG is to be used to transmit an RRC message or a NAS message contained therein. 
     Second Embodiment 
     The examples of a radio communication network and a radio protocol stack according to this embodiment are similar to those shown in  FIGS. 1-6 . In this embodiment, selection of a key K eNB  to derive temporary keys (e.g., K UPpenc , K RRCin ) used by each PDCP entity in the PDCP layer (i.e., the integrated PDCP layer  602  or the New PDCP layer  512 ) will be explained. These temporary keys are used by each PDCP entity, for example, to cipher and decipher the user plane (UP) traffic and the RRC traffic. These temporary keys are derived from the key K eNB  by the 5G UE  1 . In a similar way, these temporary keys are derived from the key K eNB  by the integrated eNB  2 , the LTE+ eNB  5 , or the 5G specific eNB  6 . 
     In some implementations, the 5G UE  1  and the integrated eNB  2  (or the LTE+ eNB  5  and the 5G specific eNB  6 ) may use the first key K eNB  to cipher and decipher data of a radio bearer(s) of a certain bearer type and use the second key sub-K eNB  to cipher and decipher data of a radio bearer(s) of another bearer type. The second key sub-K eNB  may be derived from the first key K eNB , similar to the key S-K eNB  used for SCG bearers in Dual Connectivity (DC). 
     As shown in  FIG. 11 , for example, the 5G UE  1  and the integrated eNB  2  (or the LTE+ eNB  5  and the 5G specific eNB  6 ) may use the first key K eNB  to cipher and decipher data of LTE bearers (e.g., the radio bearer # 1  shown in  FIG. 6 ) and integrated bearers (e.g., the radio bearer # 3  shown in  FIG. 6 ) and use the second key sub-K eNB  to cipher and decipher data of New 5G bearers (e.g., the radio bearer # 2  shown in  FIG. 6 ). 
     In the case of the Co-located deployments ( FIGS. 1 and 2 ), for example, the 5G UE  1  and the integrated eNB  2  may use the first key K eNB  to transmit RRC messages through LTE bearers and use the same first key K eNB  to transmit RRC messages through New 5G bearers. In contrast, in the case of the non co-located deployments ( FIG. 3 ), the 5G UE  1  and the LTE+ eNB  5  may use the first key K eNB  to transmit RRC messages through LTE bearers and, meanwhile, use the second key sub-K eNB  to transmit RRC messages through New 5G bearers. 
     In the following, configuration examples of the 5G UE  1 , the integrated eNB  2 , the LTE+ eNB  5 , and the 5G specific eNB  6  according to the above embodiments will be described.  FIG. 12  is a block diagram showing a configuration example of the 5G UE  1 . An LTE transceiver  1201  performs analog RF signal processing regarding the PHY layer of the LTE RAT to communicate with the integrated eNB  2  (or the LTE+ eNB  5 ). The analog RF signal processing performed by the LTE transceiver  1301  includes frequency up-conversion, frequency down-conversion, and amplification. The LTE transceiver  1201  is coupled to an antenna  1202  and a baseband processor  1205 . That is, the LTE transceiver  1201  receives modulated symbol data (or OFDM symbol data) from the baseband processor  1205 , generates a transmission RF signal, and supplies the transmission RF signal to the antenna  1202 . Further, the LTE transceiver  1201  generates a baseband reception signal based on a reception RF signal received by the antenna  1202 , and supplies the baseband reception signal to the baseband processor  1205 . 
     A New 5G transceiver  1203  performs analog RF signal processing regarding the PHY layer of the New 5G RAT r to communicate with the integrated eNB  2  (or the 5G specific eNB  6 ). The New 5G transceiver  1203  is coupled to an antenna  1204  and the baseband processor  1205 . 
     The baseband processor  1205  performs digital baseband signal processing (data-plane processing) and control-plane processing for radio communication. The digital baseband signal processing includes (a) data compression/decompression, (b) data segmentation/concatenation, (c) composition/decomposition of a transmission format (i.e., transmission frame), (d) channel coding/decoding, (e) modulation (i.e., symbol mapping)/demodulation, and (f) generation of OFDM symbol data (i.e., baseband OFDM signal) by Inverse Fast Fourier Transform (IFFT). Meanwhile, the control-plane processing includes communication management of layer 1 (e.g., transmission power control), layer 2 (e.g., radio resource management and hybrid automatic repeat request (HARQ) processing), and layer 3 (e.g., signalling regarding attach, mobility, and packet communication). 
     In the case of LTE and LTE-Advanced, for example, the digital baseband signal processing performed by the baseband processor  1205  may include signal processing of a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer, a MAC layer, and a PHY layer. Further, the control-plane processing performed by the baseband processor  1205  may include processing of a Non-Access Stratum (NAS) protocol, an RRC protocol, and MAC CEs. 
     The baseband processor  1205  may include a modem processor (e.g., a Digital Signal Processor (DSP)) that performs the digital baseband signal processing and a protocol stack processor (e.g., a Central Processing Unit (CPU) or a Micro Processing Unit (MPU)) that performs the control-plane processing. In this case, the protocol stack processor, which performs the control-plane processing, may be integrated with an application processor  1206  described in the following. 
     The application processor  1206  is also referred to as a CPU, an MPU, a microprocessor, or a processor core. The application processor  1206  may include a plurality of processors (processor cores). The application processor  1206  loads a system software program (Operating System (OS)) and various application programs (e.g., communication application that acquires metering data or sensing data) from a memory  1208  or a memory (not shown) and executes these programs, thereby providing various functions of the 5G UE  1 . 
     In some implementations, as represented by a dashed line ( 1207 ) in  FIG. 12 , the baseband processor  1205  and the application processor  1206  may be integrated on a single chip. In other words, the baseband processor  1205  and the application processor  1206  may be implemented in a single System on Chip (SoC) device  1207 . An SoC device may be referred to as a system Large Scale Integration (LSI) or a chipset. 
     The memory  1208  is a volatile memory, a non-volatile memory, or a combination thereof. The memory  1208  may include a plurality of memory devices that are physically independent from each other. The volatile memory is, for example, a Static Random Access Memory (SRAM), a Dynamic RAM (DRAM), or a combination thereof. The non-volatile memory is, for example, a mask Read Only Memory (MROM), an Electrically Erasable Programmable ROM (EEPROM), a flash memory, a hard disc drive, or any combination thereof. The memory  1208  may include, for example, an external memory device that can be accessed from the baseband processor  1205 , the application processor  1206 , and the SoC  1207 . The memory  1208  may include an internal memory device that is integrated in the baseband processor  1205 , the application processor  1206 , or the SoC  1207 . Further, the memory  1208  may include a memory in a Universal Integrated Circuit Card (UICC). 
     The memory  1208  may store one or more software modules (computer programs)  1209  including instructions and data to perform the processing by the 5G UE  1  described in the above embodiments. In some implementations, the baseband processor  1205  or the application processor  1206  may load these software modules  1209  from the memory  1208  and execute the loaded software modules, thereby performing the processing of the 5G UE  1  described in the above embodiments. 
       FIG. 13  is a block diagram showing a configuration example of the integrated eNB  2  according to the above embodiments. Referring to  FIG. 13 , the eNB  2  includes an LTE transceiver  1301 , a New 5G transceiver  1303 , a network interface  1305 , a processor  1306 , and a memory  1307 . The LTE transceiver  1301  performs analog RF signal processing regarding the PHY layer of the LTE RAT to communicate with the 5G UE  1  via an LTE cell. The LTE transceiver  1301  may include a plurality of transceivers. The LTE transceiver  1301  is coupled to an antenna  1302  and the processor  1306 . 
     The New 5G transceiver  1303  performs analog RF signal processing regarding the PHY layer of the New 5G RAT to communicate with the 5G UE  1  via a New 5G cell. The New 5G transceiver  1303  is coupled to an antenna  1304  and the baseband processor  1306 . 
     The network interface  1305  is used to communicate with a network node in the integrated EPC  41  or the 5G specific EPC  42  (e.g., a Mobility Management Entity (MME) and a Serving Gateway (S-GW)), and to communicate with other eNBs. The network interface  1305  may include, for example, a network interface card (NIC) conforming to the IEEE 802.3 series. 
     The processor  1306  performs digital baseband signal processing (data-plane processing) and control-plane processing for radio communication. In the case of LTE and LTE-Advanced, for example, the digital baseband signal processing performed by the processor  1306  may include signal processing of the PDCP layer, the RLC layer, the MAC layer, and the PHY layer. Further, the control-plane processing performed by the processor  1306  may include processing of the Si protocol, the RRC protocol, and MAC CEs. 
     The processor  1306  may include a plurality of processors. The processor  1306  may include, for example, a modem processor (e.g., DSP) that performs the digital baseband signal processing and a protocol stack processor (e.g., a CPU or an MPU) that performs the control-plane processing. 
     The memory  1307  is composed of a combination of a volatile memory and a non-volatile memory. The volatile memory is, for example, an SRAM, a DRAM, or a combination thereof. The non-volatile memory is, for example, an MROM, a PROM, a flash memory, a hard disc drive, or a combination thereof. The memory  1307  may include a storage located apart from the processor  1306 . In this case, the processor  1306  may access the memory  1307  via the network interface  1305  or an I/O interface (not shown). 
     The memory  1307  may store a software module(s) (computer program)  1308  including instructions and data for performing processing by the integrated eNB  2  described in the above embodiments. In some implementations, the processor  1306  may be configured to load the software module(s)  1308  from the memory  1307  and execute the loaded software module(s), thereby performing processing of the integrated eNB  2  described in the above embodiments. 
     The configurations of the LTE+ eNB  5  and the 5G specific eNB  6  may be similar to the configuration of the integrated eNB  2  shown in  FIG. 13 . However, the LTE+ eNB  5  does not need to include the New 5G transceiver  1303  and the 5G specific eNB  6  does not need to include the LTE transceiver  1301 . 
     As described above with reference to  FIGS. 12 and 13 , each of the processors included in the 5G UE  1 , the integrated eNB  2 , the LTE+ eNB  5 , and the 5G specific eNB  6  according to the aforementioned embodiments executes one or more programs including instructions to cause a computer to perform an algorithm described with reference to the drawings. The program(s) can be stored and provided to a computer using any type of non-transitory computer readable media. Non-transitory computer readable media include any type of tangible storage media. Examples of non-transitory computer readable media include magnetic storage media (such as flexible disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g., magneto-optical disks), Compact Disc Read Only Memory (CD-ROM), CD-R, CD-R/W, and semiconductor memories (such as mask ROM, Programmable ROM (PROM), Erasable PROM (EPROM), flash ROM, Random Access Memory (RAM), etc.). The program(s) may be provided to a computer using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and electromagnetic waves. Transitory computer readable media can provide the program to a computer via a wired communication line (e.g., electric wires, and optical fibers) or a wireless communication line. 
     Other Embodiments 
     Each of the above-described embodiments may be used individually, or two or more embodiments may be appropriately combined with one another. 
     The protocol stacks described in the above embodiments are merely examples and another protocol stack may be used to achieve interworking of the LTE RAT and the New 5G RAT. In some implementations, the existing protocol stacks for LTE/LTE-Advanced Carrier Aggregation (CA) or Dual Connectivity (DC), or any modification thereof may be used to achieve interworking of the LTE RAT and the New 5G RAT. In the co-located deployments or the co-located RAN, for example, an integrated MAC layer (or sublayer) may be used in place of the integrated PDCP layer (or sublayer). In this case, the integrated MAC layer may control the LTE PHY layer and the New PHY layer and perform CA using the LTE cell and the New 5G cell. 
     The base station, the base station system, the Integrated eNB  2 , the LTE+ eNB  5 , the 5G specific eNB  6 , the BBU (or the DU), and the RRH (or the RU) described in the aforementioned embodiments may be each referred to as a radio station or a radio access network (RAN) node. In other words, the processing and the operations performed by the base station, the base station system, the Integrated eNB  2 , the LTE+ eNB  5 , the 5G specific eNB  6 , the BBU (DU), or the RRH (RU) described in the above embodiments may be provided by any one or more radio stations (i.e., RAN nodes). 
     Further, the above-described embodiments are merely examples of applications of the technical ideas obtained by the inventors. Needless to say, these technical ideas are not limited to the above-described embodiments and various modifications can be made thereto. 
     For example, the whole or part of the embodiments disclosed above can be described as, but not limited to, the following supplementary notes. 
     Supplementary Note 1 
     A radio station system comprising: 
     one or more radio stations configured to:
         simultaneously provide, for one radio terminal, at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell to one radio terminal; and   transmit or receive a control-plane message to or from the radio terminal on the at least one second cell when a predetermined condition is satisfied, wherein       

     the control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both, and 
     the predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. 
     Supplementary Note 2 
     The radio station system according to Supplementary Note 1, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the one or more radio stations are configured to transmit or receive the control-plane message on the at least one second cell when the control-plane message comprises: a request for terminal measurement regarding the at least one second cell sent to the radio terminal; or a report on a measurement result regarding the at least one second cell sent from the radio terminal. 
     Supplementary Note 3 
     The radio station system according to Supplementary Note 1, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the one or more radio stations are configured to transmit or receive the control-plane message on the at least one second cell when the control-plane message comprises: a request for terminal information of the radio terminal regarding the second radio access technology sent to the radio terminal; or a report on the terminal information sent from the radio terminal. 
     Supplementary Note 4 
     The radio station system according to Supplementary Note 1, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the one or more radio stations are configured to transmit the control-plane message on the at least one second cell when the control-plane message comprises security configuration information about an access stratum (AS) for the at least one second cell. 
     Supplementary Note 5 
     The radio station system according to Supplementary Note 1, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the one or more radio stations are configured to transmit or receive the control-plane message on the at least one second cell when the control-plane message comprises: a request for access-stratum (AS)-security activation for the at least one second cell sent to the radio terminal; or a response to the request from the radio terminal. 
     Supplementary Note 6 
     The radio station system according to Supplementary Note 1, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the one or more radio stations are configured to transmit the control-plane message on the at least one second cell when the control-plane message includes configuration information specific to the second radio access technology. 
     Supplementary Note 7 
     The radio station system according to Supplementary Note 1, wherein 
     the predetermined condition relates to the type of the signalling radio bearer used to transmit the control-plane message, and 
     the one or more radio stations are configured to transmit or receive the control-plane message on the at least one second cell when the control-plane message is transmitted via a specific signalling radio bearer associated with the second radio access technology. 
     Supplementary Note 8 
     The radio station system according to Supplementary Note 1, wherein 
     the predetermined condition relates to a transmission cause of the control-plane message, and 
     the one or more radio stations are configured to receive the control-plane message on the at least one second cell when the control-plane message is transmitted due to degradation of a radio quality of the at least one second cell. 
     Supplementary Note 9 
     The radio station system according to Supplementary Note 1, wherein 
     the predetermined condition relates to the type of the core network associated with the NAS message, and 
     the one or more radio stations are configured to transmit or receive the control-plane message containing the NAS message on the at least one second cell when the core network is a new type core network associated with introduction of the second radio access technology. 
     Supplementary Note 10 
     The radio station system according to any one of Supplementary Notes 1 to 9, wherein the one or more radio stations are configured to provide Radio Link Control (RLC) and Medium Access Control (MAC) layers to communicate with the radio terminal in accordance with the first radio access technology, Radio Link Control (RLC) and Medium Access Control (MAC) layers to communicate with the radio terminal in accordance with the second radio access technology, a common Packet Data Convergence Protocol (PDCP) layer associated with both of the two RLC layers, and a common RRC layer associated with the common PDCP layer. 
     Supplementary Note 11 
     The radio station system according to any one of Supplementary Notes 1 to 9, wherein 
     the one or more radio stations comprise a first radio station for the first radio access technology and a second radio station for the second radio access technology, 
     the second radio station provides Radio Link Control (RLC) and Medium Access Control (MAC) layers to communicate with the radio terminal in accordance with the second radio access technology, and 
     the first radio station is configured to provide Radio Link Control (RLC) and Medium Access Control (MAC) layers to communicate with the radio terminal in accordance with the first radio access technology, a common Packet Data Convergence Protocol (PDCP) layer associated with both of the two RLC layers, and a common RRC layer associated with the common PDCP layer. 
     Supplementary Note 12 
     The radio station system according to any one of Supplementary Notes 1 to 11, wherein 
     the first radio access technology is continuous enhancement of LTE and LTE-Advanced, and 
     the second radio access technology is a new 5G radio access technology. 
     Supplementary Note 13 
     A method in a radio station system including one or more radio stations, the method comprising: 
     simultaneously providing, for one radio terminal, at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell; and 
     transmitting or receiving a control-plane message to or from the radio terminal on the at least one second cell when a predetermined condition is satisfied, wherein 
     the control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both, and 
     the predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. 
     Supplementary Note 14 
     A radio terminal comprising: 
     a memory; and 
     at least one processor coupled to the memory and configured to:
         communicate with a radio station system comprising one or more radio stations simultaneously on at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell; and   transmit or receive a control-plane message to or from the radio station system on the at least one second cell when a predetermined condition is satisfied, wherein       

     the control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both, and 
     the predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. 
     Supplementary Note 15 
     The radio terminal according to Supplementary Note 14, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the at least one processor is configured to receive or transmit the control-plane message on the at least one second cell when the control-plane message comprises: a request for terminal measurement regarding the at least one second cell sent to the radio terminal; or a report on a measurement result regarding the at least one second cell sent from the radio terminal. 
     Supplementary Note 16 
     The radio terminal according to Supplementary Note 14, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the at least one processor is configured to receive or transmit the control-plane message on the at least one second cell when the control-plane message comprises: a request for terminal information of the radio terminal regarding the second radio access technology sent to the radio terminal; or a report on the terminal information sent from the radio terminal. 
     Supplementary Note 17 
     The radio terminal according to Supplementary Note 14, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the at least one processor is configured to receive the control-plane message on the at least one second cell when the control-plane message comprises security configuration information about an access stratum (AS) for the at least one second cell. 
     Supplementary Note 18 
     The radio terminal according to Supplementary Note 14, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the at least one processor is configured to receive or transmit the control-plane message on the at least one second cell when the control-plane message comprises: a request for access-stratum (AS)-security activation for the at least one second cell sent to the radio terminal; or a response to the request from the radio terminal. 
     Supplementary Note 19 
     The radio terminal according to Supplementary Note 14, wherein 
     the predetermined condition relates to the content or type of the control-plane message, and 
     the at least one processor is configured to receive the control-plane message on the at least one second cell when the control-plane message comprises configuration information specific to the second radio access technology. 
     Supplementary Note 20 
     The radio terminal according to Supplementary Note 14, wherein 
     the predetermined condition relates to the type of the signalling radio bearer used to transmit the control-plane message, and 
     the at least one processor is configured to receive or transmit the control-plane message on the at least one second cell when the control-plane message is transmitted via a specific signalling radio bearer associated with the second radio access technology. 
     Supplementary Note 21 
     The radio terminal according to Supplementary Note 14, wherein 
     the predetermined condition relates to a transmission cause of the control-plane message, and 
     the at least one processor is configured to transmit the control-plane message on the at least one second cell when the control-plane message is transmitted due to degradation of a radio quality of the at least one second cell. 
     Supplementary Note 22 
     The radio terminal according to Supplementary Note 14, wherein 
     the predetermined condition relates to the type of the core network associated with the NAS message, and 
     the at least one processor is configured to receive or transmit the control-plane message containing the NAS message on the at least one second cell when the core network is a new type core network associated with introduction of the second radio access technology. 
     Supplementary Note 23 
     The radio terminal according to any one of Supplementary Notes 14 to 22, wherein the at least one processor is configured to provide Radio Link Control (RLC) and Medium Access Control (MAC) layers to communicate with the radio station system in accordance with the first radio access technology, Radio Link Control (RLC) and Medium Access Control (MAC) layers to communicate with the radio station system in accordance with the second radio access technology, a common Packet Data Convergence Protocol (PDCP) layer associated with both of the two RLC layers, and a common RRC layer associated with the common PDCP layer. 
     Supplementary Note 24 
     The radio terminal according to any one of Supplementary Notes 14 to 23, wherein 
     the first radio access technology is continuous enhancement of LTE and LTE-Advanced, and 
     the second radio access technology is a new 5G radio access technology. 
     Supplementary Note 25 
     A method in a radio terminal, the method comprising: 
     communicating with a radio station system comprising one or more radio stations simultaneously on at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell; and 
     transmitting or receiving a control-plane message to or from the radio station system on the at least one second cell when a predetermined condition is satisfied, wherein 
     the control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both, and 
     the predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. 
     Supplementary Note 26 
     A non-transitory computer readable medium storing a program for causing a computer to perform a method in a radio terminal, the method comprising: 
     communicating with a radio station system comprising one or more radio stations simultaneously on at least one first cell in accordance with a first radio access technology and at least one second cell in accordance with a second radio access technology and used in addition and subordinate to the at least one first cell; and 
     transmitting or receiving a control-plane message to or from the radio station system on the at least one second cell when a predetermined condition is satisfied, wherein 
     the control-plane message includes a non-access stratum (NAS) message or a radio resource control (RRC) message or both, and 
     the predetermined condition relates to at least one of: (a) a content or type of the control-plane message; (b) a type of a signalling radio bearer used to transmit the control-plane message; (c) a transmission cause of the control-plane message; and (d) a type of a core network associated with the NAS message. Reference Signs List
       1  RADIO TERMINAL (5G UE)     2  BASE STATION (INTEGRATED eNB)     3  RRH     5  LTE+ eNB     6  5G SPECIFIC eNB     41  INTEGRATED EPC     42  5G SPECIFIC EPC     1201  LTE TRANSCEIVER     1203  NEW 5G TRANSCEIVER     1205  BASEBAND PROCESSOR     1206  APPLICATION PROCESSOR     1208  MEMORY     1301  LTE TRANSCEIVER     1303  NEW 5G TRANSCEIVER     1306  PROCESSOR     1307  MEMORY