Patent Publication Number: US-2017367044-A1

Title: Base station, radio terminal, and network apparatus

Description:
RELATED APPLICATIONS 
     This application is a continuation application of international application PCT/JP2016/056440 (filed Mar. 2, 2016), which claims benefit of Japanese Patent Application No. 2015-041868 (filed on Mar. 3, 2015), the entirety of both applications hereby expressly incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a base station, a radio terminal, and a network apparatus used in a communication system. 
     BACKGROUND ART 
     In 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, a discontinuous reception (DRX) is prescribed as an intermittent reception technique to reduce power consumption of a radio terminal. The radio terminal executing a DRX operation intermittently monitors a downlink control channel. A cycle in which the downlink control channel is monitored is referred to as “DRX cycle”. 
     In recent years, machine-type communication (MTC) in which a radio terminal performs communication without human intervention in a mobile communication system has attracted attention. From such a background, an ongoing discussion is a new introduction of an extended DRX cycle longer than a conventional DRX cycle to further reduce power consumption (for example, see Non Patent Document 1). The DRX using the extended DRX cycle is referred to as “extended DRX”. 
     By the way, downlink data addressed to the radio terminal is firstly forwarded from an external network to a packet network gateway (hereinafter, POW). The PGW forwards, via an S5/S8 bearer between a serving gateway (hereinafter, SOW) and the PGW, the downlink data to the SOW. The SGW, upon receiving the downlink data, forwards, via an E-RAB bearer between the radio terminal and the SGW, the downlink data to the radio terminal. In this manner, the radio terminal can obtain the downlink data. It is noted that, the E-RAB bearer is constituted of a radio bearer between the radio terminal and a base station, and an S1 bearer between the base station and the SOW. 
     On the other hand, if the radio terminal is in an idle mode, the S5/S8 bearer between the SGW and the PGW remains present, since the radio terminal is not detached. On the other hand, the E-RAB bearer between the radio terminal and the SGW is not present, since it is released when the radio terminal transitions to the idle mode. In this case, the downlink data addressed to the radio terminal is forwarded, via the S5/S8 bearer, from the PGW to the SOW. The E-RAB bearer is not present, and hence, the SGW requests a mobility management entity (hereinafter, MME) to perform paging. Thereafter, the radio terminal which transitioned to a connected mode based on the paging from the MME establishes the E-RAB bearer between the SGW and the radio terminal. The radio terminal can obtain, via the established E-RAB bearer, the downlink data from the SOW. 
     PRIOR ART DOCUMENT 
     Non-Patent Document 
     Non Patent Document 1: 3GPP contribution “RP-141994” 
     SUMMARY 
     A base station according to a first aspect is used in a communication system including a radio terminal capable of configuring an extended DRX in an idle mode. The base station comprises a controller configured to receive, if the radio terminal is in a connected mode, via a bearer between the base station and a serving gateway, downlink data addressed to the radio terminal, from the serving gateway, and to transmit the downlink data to the radio terminal. The controller does not release but maintains the bearer, if the extended DRX operation is configured to the radio terminal, when the radio terminal transitions to the idle mode. The controller receives, even if the radio terminal is in the idle mode, from the serving gateway, the downlink data addressed to the radio terminal, via the bearer, and buffers the downlink data before the downlink data is transmitted to the radio terminal. 
     A radio terminal according to a second aspect executes a DRX operation in an idle mode. The radio terminal comprises a controller configured to notify a mobility management entity that is an upper node of a base station of a response based on a paging message, if receiving, in the idle mode, the paging message based on downlink data from the base station, after establishing an RRC connection with the base station. The controller omits, if receiving, from the base station, a special paging message different from the paging message, the response and obtains the downlink data from the base station. 
     A network apparatus according to a third aspect is used in a communication system including a radio terminal capable of executing an extended DRX operation in an idle mode. The network apparatus comprises a controller configured to request a mobility management entity, in response to reception of downlink data addressed to the radio terminal, to perform paging based on the downlink data. The controller forwards, if receiving a negative acknowledgment indicating that the radio terminal is executing the extended DRX operation as a response to the request, the downlink data addressed to the radio terminal to a data server configured to buffer and forward downlink data. 
     A network apparatus according to a fourth aspect is used in a communication system including a radio terminal capable of executing an extended DRX operation in an idle mode. The network apparatus comprises a receiver configured to receive, from a serving gateway, a paging request based on downlink data addressed to the radio terminal; and a controller configured to notify, in response to the paging request, a paging causing a base station subordinate to the network apparatus to transmit a paging message. The controller executes, if the downlink data addressed to the radio terminal is forwarded from the serving gateway to a data server configured to buffer and forward downlink data, an operation causing the radio terminal to access the data server. 
     A radio terminal according to a fifth aspect is capable of executing an extended DRX operation in an idle mode. The radio terminal comprises a receiver configured to receive, during execution of the extended DRX operation, a paging message based on downlink data addressed to the radio terminal, from a base station; and a controller configured to establish, after receiving the paging message, an RRC connection with the base station to obtain the downlink data. The receiver receives an access instruction causing the radio terminal to access a data server configured to buffer and forward downlink data. The controller starts the access to the data server in response the access instruction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of an LTE system. 
         FIG. 2  is a block diagram of a UE. 
         FIG. 3  is a block diagram of an eNB. 
         FIG. 4  is a block diagram of an MME. 
         FIG. 5  is a protocol stack diagram. 
         FIG. 6  is a configuration diagram of a radio frame. 
         FIG. 7  is a sequence diagram for describing an operation according to a first embodiment. 
         FIG. 8  is a diagram for describing an operation environment according to a second embodiment. 
         FIG. 9  is a sequence diagram for describing an operation according to the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENT 
     Overview of Embodiment 
     A case where a radio terminal executes an extended DRX operation in an idle mode is assumed. The extended DRX cycle is longer than the conventional DRX cycle, and hence, the radio terminal may be delayed in transitioning to a connected mode, based on a paging from an MME. Therefore, an SGW must buffer downlink data addressed to the radio terminal for a long period until it is transmitted to the radio terminal, and there is a concern that a buffer capacity of the SGW is exceeded. Particularly, depending on the number of base stations subordinate to the SGW, the number of radio terminals to which the downlink data to be forwarded increases. Therefore, if the number of radio terminals configured to execute the extended DRX operation increases, the downlink data to be handled also increases, and hence, if a number of radio terminals execute the extended DRX operation, the buffer capacity of the SGW is more likely to be exceeded. 
     Therefore, an embodiment provides a base station, a radio terminal, and a network apparatus capable of suppressing an increase in the buffer capacity of the SGW due to buffering of the downlink data addressed to the radio terminal. 
     A base station according to a first embodiment is used in a communication system including a radio terminal capable of configuring an extended DRX in an idle mode. The base station comprises a controller configured to receive, if the radio terminal is in a connected mode, via a bearer between the base station and a serving gateway, downlink data addressed to the radio terminal, from the serving gateway, and to transmit the downlink data to the radio terminal. The controller does not release but maintains the bearer, if the extended DRX operation is configured to the radio terminal, when the radio terminal transitions to the idle mode. The controller receives, even if the radio terminal is in the idle mode, from the serving gateway, the downlink data addressed to the radio terminal, via the bearer, and buffers the downlink data before the downlink data is transmitted to the radio terminal. 
     In the first embodiment, the controller transmits, to the radio terminal, a release message for releasing an RRC connection between the radio terminal and the base station without notifying a mobility management entity of a release request serving as a trigger to release the bearer when the radio terminal transitions to the idle mode. 
     In the first embodiment, the controller transmits, after buffering the downlink data, to the radio terminal, a special paging message transmitted without receiving a paging from a mobility management entity. 
     In the first embodiment, the controller transmits, to the radio terminal, a paging message including identification information indicating the special paging message, as the special paging message. 
     A radio terminal according to the first embodiment executes a DRX operation in an idle mode. The user terminal comprises a controller configured to notify a mobility management entity that is an upper node of a base station of a response based on a paging message, if receiving, in the idle mode, the paging message based on downlink data from the base station, after establishing an RRC connection with the base station. The controller omits, if receiving, from the base station, a special paging message different from the paging message, the response and obtains the downlink data from the base station. 
     In the first embodiment, the controller interprets, if receiving a paging message including identification information indicating the special paging message, the paging message as the special paging message. 
     A network apparatus according to a second aspect is used in a communication system including a radio terminal capable of executing an extended DRX operation in an idle mode. The network apparatus comprises a controller configured to request a mobility management entity, in response to reception of downlink data addressed to the radio terminal, to perform paging based on the downlink data. The controller forwards, if receiving a negative acknowledgment indicating that the radio terminal is executing the extended DRX operation as a response to the request, the downlink data addressed to the radio terminal to a data server configured to buffer and forward downlink data. 
     In the second embodiment, the controller forwards the downlink data to the data server, if a predetermined period elapses after buffering the downlink data even if not receiving the negative acknowledgment. 
     A network apparatus according to the second aspect is used in a communication system including a radio terminal capable of executing an extended DRX operation in an idle mode. The network apparatus comprises a receiver configured to receive, from a serving gateway, a paging request based on downlink data addressed to the radio terminal; and a controller configured to notify, in response to the paging request, a paging causing a base station subordinate to the network apparatus to transmit a paging message. The controller executes, if the downlink data addressed to the radio terminal is forwarded from the serving gateway to a data server configured to buffer and forward downlink data, an operation causing the radio terminal to access the data server. 
     In the second embodiment, to cause the base station to transmit the paging message including an access instruction causing the radio terminal to access the data server, the controller includes, as the operation, the access instruction into the paging. 
     In the second embodiment, the controller notifies the radio terminal, by a NAS message, of an access instruction causing the radio terminal to access the data server, as the operation. 
     In the second embodiment, the controller notifies an SMS server configured to notify, by an SMS message, the radio terminal of an access instruction, of a message that serves as a trigger to notify the access instruction, if receiving, from the radio terminal, a response based on the paging message. The access instruction is an instruction causing the radio terminal to access the data server. 
     A radio terminal according to the second embodiment is capable of executing an extended DRX operation in an idle mode. The radio terminal comprises a receiver configured to receive, during execution of the extended DRX operation, a paging message based on downlink data addressed to the radio terminal, from a base station; and a controller configured to establish, after receiving the paging message, an RRC connection with the base station to obtain the downlink data. The receiver receives an access instruction causing the radio terminal to access a data server configured to buffer and forward downlink data. The controller starts the access to the data server in response the access instruction. 
     In the second embodiment, the receiver receives the access instruction by receiving the paging message including the access instruction. 
     In the second embodiment, the receiver receives the access instruction by a NAS message from a mobility management entity that is an upper node of the base station. 
     In the second embodiment, the receiver receives, from an SMS server, the access instruction by an SMS message. 
     First Embodiment 
     Hereinafter, a first embodiment when the present disclosure is applied to an LTE system will be described. 
     (System Configuration) 
     First, system configuration of the LTE system will be described.  FIG. 1  is a configuration diagram of an LTE system. As illustrated in  FIG. 1 , the LTE system according to embodiments includes a plurality of UEs (User Equipments)  100 , E-UTRAN (Evolved-Universal Terrestrial Radio Access Network)  10 , and EPC (Evolved Packet Core)  20 . 
     The UE  100  corresponds to a radio terminal. The UE  100  is a mobile communication device and performs radio communication with a cell (a serving cell) that connected to the radio terminal. Configuration of the UE  100  will be described later. 
     The E-UTRAN  10  corresponds to a radio access network. The E-UTRAN  10  includes a plurality of eNBs (evolved Node-Bs)  200 . The eNB  200  corresponds to a base station. The eNBs  200  are connected mutually via an X2 interface. Configuration of the eNB  200  will be described later. 
     The eNB  200  manages one or a plurality of cells and performs radio communication with the UE  100  which establishes a connection with the cell of the eNB  200 . The eNB  200  has a radio resource management (RRM) function, a routing function for user data, and a measurement control function for mobility control and scheduling, and the like. It is noted that the “cell” is used as a term indicating a minimum unit of a radio communication area, and is also used as a term indicating a function of performing radio communication with the UE  100 . 
     The EPC  20  corresponds to a core network. The E-UTRAN  10  and the EPC  20  constitute a network (LTE network) of the LTE system. The EPC  20  includes a plurality of MME (Mobility Management Entity)/S-GWs (Serving-Gateways)  300  and a PGW (Packet Data Network Gateway)  400 . 
     A MME  300 A and a SGW  300 B constitute the MME/S-GW  300 . The MME  300 A performs various mobility controls and the like for the UE  100 . The S-GW  300 B performs control to transfer user data. MME/S-GW  300  is connected to eNB  200  via an S1 interface. The MME and the SGW may be configured by the same network apparatus (communication control apparatus) or may be configured by different network apparatuses. 
     The PGW  400  is a network node that performs control of relaying user data from an external network not managed by an operator of the cellular network and relaying user data to an external network. 
       FIG. 2  is a block diagram of the UE  100 . As illustrated in  FIG. 2 , the UE  100  includes plural antennas  101 , a radio transceiver  110 , a user interface  120 , a GNSS (Global Navigation Satellite System) receiver  130 , a battery  140 , a memory  150 , and a processor  160 . The memory  150  corresponds a memory, the processor  160  corresponds to a controller. The UE  100  may not include the GNSS receiver  130 . Furthermore, the memory  150  may be integrally formed with the processor  160 , and this set (that is, a chip set) may be called a processor  160 ′. 
     The plural antennas  101  and the radio transceiver  110  are used to transmit and receive a radio signal. The radio transceiver  110  converts a baseband signal (a transmission signal) output from the processor  160  into the radio signal and transmits the radio signal from the antenna  101 . Furthermore, the radio transceiver  110  converts a radio signal received by the antenna  101  into a baseband signal (a received signal), and outputs the baseband signal to the processor  160 . 
     The user interface  120  is an interface with a user carrying the UE  100 , and includes, for example, a display, a microphone, a speaker, various buttons and the like. The user interface  120  accepts an operation from a user and outputs a signal indicating the content of the operation to the processor  160 . The GNSS receiver  130  receives a GNSS signal in order to obtain location information indicating a geographical location of the UE  100 , and outputs the received signal to the processor  160 . The battery  140  accumulates power to be supplied to each block of the UE  100 . 
     The memory  150  stores a program to be executed by the processor  160  and information to be used for a process by the processor  160 . The processor  160  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like on the baseband signal, and CPU (Central Processing Unit) that performs various processes by executing the program stored in the memory  150 . The processor  160  may further include a codec that performs encoding and decoding on sound and video signals. The processor  160  executes various processes and various communication protocols described later. 
       FIG. 3  is a block diagram of the eNB  200 . As illustrated in  FIG. 3 , the eNB  200  includes plural antennas  201 , a radio transceiver  210 , a network interface  220 , a memory  230 , and a processor  240 . Furthermore, the memory  230  may be integrally formed with the processor  240 , and this set (that is, a chip set) may be called a processor  240 ′. 
     The plural antennas  201  and the radio transceiver  210  are used to transmit and receive a radio signal. The radio transceiver  210  converts a baseband signal (a transmission signal) output from the processor  240  into the radio signal and transmits the radio signal from the antenna  201 . Furthermore, the radio transceiver  210  converts a radio signal received by the antenna  201  into a baseband signal (a received signal), and outputs the baseband signal to the processor  240 . 
     The network interface  220  is connected to the neighboring eNB  200  via the X2 interface and is connected to the MME/S-GW  300  via the S1 interface. The network interface  220  is used in communication over the X2 interface and communication over the S1 interface. 
     The memory  230  stores a program to be executed by the processor  240  and information to be used for a process by the processor  240 . The processor  240  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like on the baseband signal and CPU that performs various processes by executing the program stored in the memory  230 . The processor  240  executes various processes and various communication protocols described later. 
       FIG. 4  is a block diagram of the MME  300 A. As illustrated in  FIG. 4 , the MME  300 A includes a network interface  320 , a memory  330 , and a processor  340 . The memory  330  may be integrally formed with the processor  340 , and this set (that is, a chip set) may be called a processor. 
     The network interface  320  is connected to the eNB  200  via the S1 interface. The network interface  320  is used for communication performed on the S1 interface. 
     The memory  330  stores a program to be executed by the processor  340  and information to be used for a process by the processor  340 . The processor  340  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like on the baseband signal and CPU that performs various processes by executing the program stored in the memory  330 . The processor  340  executes various processes and various communication protocols described later. 
     Since the SGW  300 B is also a block diagram similar to the MME  300 A, its explanation will be omitted. Further, the PGW  400  is a block diagram similar to the MME  300 A. However, the network interface of the PGW  400  is connected to each of the MME/SGW  300  and the external network. 
       FIG. 5  is a protocol stack diagram of a radio interface in the LTE system. As illustrated in  FIG. 5 , the radio interface protocol is classified into a layer 1 to a layer 3 of an OSI reference model, wherein the layer 1 is a physical (PHY) layer. The layer 2 includes a MAC (Medium Access Control) layer, an RLC (Radio Link Control) layer, and a PDCP (Packet Data Convergence Protocol) layer. The layer 3 includes an RRC (Radio Resource Control) layer. 
     The PHY layer performs encoding and decoding, modulation and demodulation, antenna mapping and demapping, and resource mapping and demapping. Between the PHY layer of the UE  100  and the PHY layer of the eNB  200 , user data and control signal are transmitted via the physical channel. 
     The MAC layer performs priority control of data, a retransmission process by hybrid ARQ (HARQ), and the like. Between the MAC layer of the UE  100  and the MAC layer of the eNB  200 , user data and control signal are transmitted via a transport channel. The MAC layer of the eNB  200  includes a scheduler that determines (schedules) a transport format of an uplink and a downlink (a transport block size and a modulation and coding scheme (MCS)) and a resource block to be assigned to the UE  100 . 
     The RLC layer transmits data to an RLC layer of a reception side by using the functions of the MAC layer and the PHY layer. Between the RLC layer of the UE  100  and the RLC layer of the eNB  200 , user data and control signal are transmitted via a logical channel. 
     The PDCP layer performs header compression and decompression, and encryption and decryption. 
     The RRC layer is defined only in a control plane dealing with control signal. Between the RRC layer of the UE  100  and the RRC layer of the eNB  200 , control signal (RRC messages) for various types of configuration are transmitted. The RRC layer controls the logical channel, the transport channel, and the physical channel in response to establishment, re-establishment, and release of a radio bearer. When there is a connection (RRC connection) between the RRC of the UE  100  and the RRC of the eNB  200 , the UE  100  is in an RRC connected mode (connected mode), otherwise the UE  100  is in an RRC idle mode (idle mode). 
     A NAS (Non-Access Stratum) layer positioned above the RRC layer performs a session management, a mobility management and the like. 
       FIG. 6  is a configuration diagram of a radio frame used in the LTE system. In the LTE system, OFDMA (Orthogonal Frequency Division Multiplexing Access) is applied to a downlink (DL), and SC-FDMA (Single Carrier Frequency Division Multiple Access) is applied to an uplink (UL), respectively. 
     As illustrated in  FIG. 6 , a radio frame is configured by 10 subframes arranged in a time direction. Each subframe is configured by two slots arranged in the time direction. Each subframe has a length of 1 ms and each slot has a length of 0.5 ms. Each subframe includes a plurality of resource blocks (RBs) in a frequency direction (not shown), and a plurality of symbols in the time direction. Each resource block includes a plurality of subcarriers in the frequency direction. One symbol and one subcarrier forms one resource element (RE). Of the radio resources (time and frequency resources) assigned to the UE  100 , a frequency resource can be constituted by a resource block and a time resource can be constituted by a subframe (or a slot). 
     (Overview of DRX Operation in Idle Mode) 
     A discontinuous reception (DRX) operation in the RRC idle mode will be described, below. It is noted that, hereinafter, the DRX operation in the idle mode also includes an operation using the extended DRX cycle longer than the conventional DRX cycle. 
     The UE  100  can perform the DRX operation to conserve a battery. The UE  100  configured to perform the DRX operation intermittently monitors a PDCCH. Normally, the PDCCH in a subframe carries scheduling information (information on a radio resource and a transport format) of a PDSCH in the sub frame. 
     The UE  100  in the RRC idle mode performs the DRX operation for intermittently monitoring the PDCCH to receive a paging message notifying that there is an incoming call. The UE  100  uses a group identifier (P-RNTI) for paging to decode the PDCCH (CCE), and obtain assignment information of a paging channel (PI). The UE  100  obtains the paging message, based on the assignment information. A PDCCH monitoring timing in the UE  100  is determined, based on an identifier (International Mobile Subscriber Identity (IMSI)) of the UE  100 . A calculation of the PDCCH monitoring timing will be specifically described. 
     The PDCCH monitoring timing (PDCCH monitoring subframe) in the DRX operation in the RRC idle mode is referred to as “Paging Occasion (PO)”. 
     The UE  100  (and the eNB  200 ) calculates the Paging Occasion (PO) and a Paging Frame (PF) which is a radio frame that may include the Paging Occasion, as follows. 
     A system frame number (SFN) of the PF is evaluated from the following formula (1). 
       SFN mod  T =( T  div  N )*(UE_ID mod  N )  (1)
 
     Here, T is a DRX cycle of the UE  100  for receiving the paging message, and is represented by the number of radio frames. N is a minimum value out of T and nB. nB is a value selected from 4T, 2T, T, T/2, T/4, T/8, T/16, and T/32. UE_ID is a value evaluated by “IMSI mod 1024”. 
     Of the PFs evaluated in this manner, a subframe number of the PO is evaluated as follows. First, index i_s is evaluated by the following formula (2). 
         i _ s =floor (UE_ID/ N )mod  Ns   (2)
 
     Here, Ns is a maximum value out of 1 and nB/T. 
     Next, the PO corresponding to Ns and the index i_s is obtained from Table 1 or Table 2. Table 1 is applied to an LTE FDD system, and Table 2 is applied to an LTE TDD system. In Table 1 and Table 2, N/A represents not applicable. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Ns 
                 PO when l_s = 0 
                 PO when l_s = 1 
                 PO when l_s = 2 
                 PO when l_s = 3 
               
               
                   
               
             
            
               
                 1 
                 9 
                 N/A 
                 N/A 
                 N/A 
               
               
                 2 
                 4 
                 9 
                 N/A 
                 N/A 
               
               
                 4 
                 0 
                 4 
                 5 
                 9 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Ns 
                 PO when l_s = 0 
                 PO when l_s = 1 
                 PO when l_s = 2 
                 PO when l_s = 3 
               
               
                   
               
             
            
               
                 1 
                 0 
                 N/A 
                 N/A 
                 N/A 
               
               
                 2 
                 0 
                 5 
                 N/A 
                 N/A 
               
               
                 4 
                 0 
                 1 
                 5 
                 6 
               
               
                   
               
            
           
         
       
     
     In this manner, the UE  100  decides the paging frame, based on the SFN and the DRX cycle. It is noted that the eNB  200  similarly decides a paging frame, and transmits, in the decided paging frame, a PDCCH for notifying a paging message. 
     Operation According to First Embodiment 
     Next, an operation according to the first embodiment will be described with reference to  FIG. 7 .  FIG. 7  is a sequence diagram for describing the operation according to the first embodiment. 
     The UE  100  exists in a cell managed by the eNB  200 . The UE  100  is a radio terminal for the MTC. Specifically, the UE  100  is a radio terminal having a low mobility. For example, the UE  100  is a radio terminal whose location is fixed. Alternatively, the UE  100  is a radio terminal that can only locally move, and moves locally within a cell. 
     As illustrated in  FIG. 7 , the UE  100  is, in an initial state, in the connected mode. If the UE  100  communicates with a partner device (Peer Entity) on the Internet (external network), data which the UE  100  transmits and receives is carried by an EPS (Evolved Packet System) bearer between the UE  100  and the PGW  400 , and an external bearer between the PWG  400  and the Internet. 
     The EPS bearer is constituted of an E-RAB between the UE  100  and the SGW  300 B, and an S5/S8 bearer between the SGW  300 B and the PGW  400  (see  FIG. 7 ). The S5/S8 bearer is established on an S5/S8 interface. If the E-RAB described later is present, the E-RAB corresponds to the EPS bearer one-to-one. The SGW  300 B stores a correspondence relationship between the S5/S8 bearer and an S1-U bearer. 
     The E-RAB is constituted of a data radio bearer (DRB Bearer/Radio Bearer) between the UE  100  and the eNB  200 , and the S1-U bearer between the eNB  200  and the SGW  300 B. 
     The S1-U bearer is established on an S1-U interface. If the data radio bearer is present, the data radio bearer corresponds to the EPS bearer/E-RAB one-to-one. The eNB  200  stores a correspondence relationship between the S1-U bearer and the data radio bearer. 
     Here, if the UE  100  is in the connected mode, the downlink data addressed to the UE  100  is delivered, via the external bearer, to the PGW  400 . The PGW  400  forwards, via the S5/S8 bearer, the downlink data to the SGW  300 B. Since the S1-U bearer of the UE  100  is present, the SGW  300 B, upon receiving the downlink data addressed to the UE  100 , forwards, via the S1-U bearer, the downlink data to the eNB  200 . The eNB  200 , upon receiving the downlink data addressed to the UE  100 , transmits, via the data radio bearer, the downlink data to the UE  100 . The UE  100  can obtain, by receiving the downlink data from the eNB  200 , the downlink data. 
     Next, if the UE  100  is in the idle mode, a method by which the UE  100  obtains the downlink data, will be described. 
     As illustrated in  FIG. 7 , in step S 110 , the eNB  200  is informed that the UE  100  is a UE having a low mobility. Specifically, the eNB  200  determines, by the following methods, whether or not the UE  100  has the low mobility. The eNB  200  may determine whether or not the UE  100  applies to the MTC. 
     In a first method, the eNB  200  determines, based on “UEInformationResponse”, whether or not the UE  100  has the low mobility. The eNB  200  transmits, to the UE  100 , a message for requesting UE information (UEInformationRequest). The UE  100  transmits, to the eNB  200 , a response message (UEInformationResponse) to the message. If the response message includes a mobility history report (mobilityHistoryReport), the eNB  200  determines, based on the mobility history report, whether or not the UE  100  has the low mobility. The mobility history report is information indicating a staying time in a cell in which the UE  100  most recently stayed or a cell that the UE  100  most recently left. If the staying time in the cell where the UE  100  exists (stays) exceeds a threshold value, the eNB  200  determines that the UE  100  has the low mobility. Otherwise, the eN  200  determines that the UE  100  does not have the low mobility. 
     In a second method, the eNB  200  determines, based on “Expected UE Behaviour”, whether or not the UE  100  has the low mobility. If an “INITIAL CONTEXT SETUP REQUEST” message received from the MME  300 A includes the “Expected UE Behaviour” related to a behaviour of the UE  100 , the eNB  200  determines, based on the “Expected UE Behaviour”, whether or not the UE  100  has the low mobility. The “Expected UE Behaviour” is information indicating a predicted active behaviour and/or mobility behaviour of the UE. For example, the “Expected UE Behaviour” is information indicating an active time and/or idle time of the UE  100 . The “Expected UE Behaviour” is information indicating a predicted time interval of inter-base station handovers (inter-eNB handovers). If “long-time” is included in the “Expected UE Behaviour”, the interval of the inter-base station handovers is predicted to be longer than 180 seconds. It is noted that the MME  300  can decide, based on subscriber information, statistics information, and the like, the “Expected UE Behaviour”. If a time indicated by the “Expected UE Behaviour” (for example, predicted time interval of the inter-base station handover) exceeds a threshold value, the eNB  200  determines that the UE  100  has the low mobility. Otherwise, the eN  200  determines that the UE  100  does not have the low mobility. 
     It is noted that, if a “HANDOVER REQUEST” message received from a source eNB  200  includes the “Expected UE Behaviour”, the eNB  200  may determine, based on the “Expected UE Behaviour”, whether or not the UE  100  has the low mobility. 
     In a third method, the eNB  200  determines, based on “extendedLowPowerConsumption”, whether or not the UE  100  has the low mobility. If a message including the “extendedLowPowerConsumption” is received from the UE  100 , the eNB  200  determines that the UE  100  has the low mobility. The “extendedLowPowerConsumption” is information indicating that the UE  100  further prefers low power consumption than the “LowPowerConsumption” indicating that the UE  100  prefers the low power consumption. The UE  100  may transmit, to the eNB  200 , the “powerPreIndication” including the “extendedLowPowerConsumption” by the “UEAssistanceInformation” message. Alternatively, the UE  100  may include the “extendedLowPowerConsumption” in a field different from the “powerPreIndication” and transmit to the eNB  200  by the “UEAssistanceInformation” message. Alternatively, the Ue  100  may transmit the “extendedLowPowerConsumption” to the eNB  200  by a message different from the UEAssistanceInformation”. Only the UE having a low mobility and/or the UE applying to the MTC may be capable of transmitting the “extendedLowPowerConsumption” to the eNB  200 . 
     In a fourth method, if the extended DRX is configured to the UE  100 , or the extended DRX will be configured to the UE  100 , the eNB  200  determines that the UE  100  has the low mobility. If the UE  100  has the low mobility, the eNB  200  determines, according to the above-described method, to configure the extended DRX to the UE  100 . Alternatively, if the “UEAssistanceInformation” message received from the UE  100  includes the “powerPreIndication”, the eNB  200  determines, based on the “powerPreIndication”, whether or not to configure the extended DRX to the UE  100 . The “powerPreIndication” indicates an optimized configuration (preferred by the UE) for power saving. Alternatively, the “powerPreIndication” indicates a normal configuration. If the “powerPreIndication” includes information indicating the “LowPowerConsumption” indicating low power consumption, the eNB  200  may determine to configure the extended DRX to the UE  100 . 
     The eNB  200  may configure the extended DRX to the UE  100  by a conventional PCCH configuration (PCCH-Config.) broadcast to the UE  100  by an SIB2. A value range of a paging cycle (defaultPagingCycle) in the PCCH configuration is extended. The UE  100  handles the paging cycle based on the PCCH configuration as the extended DRX cycle. 
     Alternatively, the eNB  200  may configure the extended DRX to the UE  100  by an information element (“Idle-eDRX-Config”, for example) different from the conventional PCCH configuration. In the “Idle-eDRX-Config”, a value range such as “ . . . , rf512, rf1024, . . . ” can be configured as the extended DRX cycle. The eNB  200  may configure the extended DRX to the UE  100  by transmitting, to the UE  100 , an RRC connection release message including the “Idle-eDRX-Config”, as described later. 
     In step S 120 , the eNB  200  transmits the RRC connection release message to the UE  100  without releasing the S1-U bearer between the eNB  200  and the SGW  300 B. 
     Normally, if the eNB  200  detects, based on a configured parameter (inactivity timer), user inactivity indicating that the UE  100  is not activity, it notifies the MME  300 A of a request message (UE Context Release Request) for releasing a context of the UE  100  (information on the UE  100 ). The request message is a message that triggers release of the S1-U bearer. The MME  300 A notifies, based on the request message, the SGW  300 B of a modify bearer request. The SGW  300 B is informed by the modify bearer request that the UE  100  can not utilize the downlink data (downlink traffic). The SGW  300 B releases the S1-U bearer in response to reception of the modify bearer request. Further, the SGW  300 B transmits, to the MME  300 A, a response to the modify bearer request (Modify Bearer Response). The MME  300 A notifies the eNB  200  of a UE context release command message (UE Context Release Command) in response to reception of the response to the modified bearer request. The eNB  200  releases the context of the UE  100  in response to reception of the UE context release command. The eNB  200  notifies the MME  300 A of a UE context release complete message (UE Context Release Complete) indicating that the context of the UE  100  has been released. After notifying the MME  300 A of the UE context release complete message, the eNB  200  transmits the RRC connection release message to the UE  100 . 
     On the other hand, in the present embodiment, the eNB  200  transmits the RRC connection release message to the UE  100  without notifying the MME  300 A of the request message. That is, the eNB  200  omits a notification of the request message. Consequently, the S1-U bearer is maintained without being released. The eNB  200  does not notify, if the RRC connection release message is transmitted to the UE  100  having the low mobility, the MME  300 A of the request message. That is, the eNB  200  does not notify, if the RRC connection release message is transmitted to the UE  100  to which the DRX using the extended DRX cycle is configured (or will be configured), the MME  300 A of the request message. 
     If the DRX using the extended DRX cycle is not configured to the UE  100 , the eNB  200  includes, in the RRC connection release message, configuration information (“Idle-eDRX-Config”) of the DRX (extended DRX) using the extended DRX cycle. If the extended DRX cycle used by the UE  100  is modified, the eNB  200  may transmit, to the UE  100  in which the DRX is configured, the RRC connection release message including the configuration information of the extended DRX. It is noted that, the eNB  200  may transmit, if the configuration information of the extended DRX has already been transmitted to the UE  100 , the RRC connection release message not including the configuration information of the extended DRX. 
     The UE  100 , upon receiving the RRC connection release message, releases the RRC connection and transitions to the idle mode. Thereafter, the UE  100  in the idle mode executes the (extended) DRX operation in accordance with a (extended) DRX configuration. 
     As illustrated in  FIG. 7 , by releasing the RRC connection, the data radio bearer is released. On the other hand, the S1-U bearer remains present. It is noted that, the EPC  20  (such as the MME  300 A and SGW  300 B) is not notified of the request message (UE Context Release Request) from the eNB  200 , and hence, it is recognized that not only the S1-U bearer, but also the data radio bearer are present (that is, the E-RAB remains present). 
     It is noted that, the UE  100  has the low mobility, and hence, the UE  100  is assumed to exist in the cell managed by the eNB  200 , and the above-described operation is executed. 
     In step S 130 , the PGW  400 , upon receiving the downlink data addressed to the UE  100 , forwards, via the S5/S8 bearer, the downlink data to the SGW  300 B. 
     In step S 140 , the S1-U bearer of the UE  100  is present, and hence, the SGW  300 B, upon receiving the downlink data addressed to the UE  100 , forwards, via the S1-U bearer, the downlink data to the eNB  200 . The eNB  200  receives, even if the UE  100  is in the idle mode and the data radio bearer is not present, the downlink data addressed to the UE  100 . 
     In step S 150 , the data radio bearer is not present, and hence, the eNB  200  stores and buffers the downlink data addressed to the UE  100  received from the SGW  300 B in the memory  230 . The eNB  200  buffers the downlink data before it is transmitted to the UE  100 . 
     In step S 160 , the eNB  200  decides to execute a RAN paging. Normally, the eNB  200  transmits, if a paging is received from the MME  300 A, a paging message to the UE  100  in response to the paging cycle (DRX cycle). On the other hand, the RAN paging is a special paging transmitted from the eNB  200  without receiving a paging from the MME  300 A. The eNB  200  transmits, to the UE  100 , a special paging message, without receiving a paging from the MME  300 A. For example, the eNB  200  transmits, to the UE  100 , a paging message including identification information indicating a special paging message, as a special paging message. Alternatively, a special paging message different from a conventional paging message may be defined. The eNB  200  may transmit the special paging message to the UE  100 . 
     In step S 170 , the eNB  200  transmits, based on the (extended) DRX configuration, the special paging message to the UE  100 . The UE  100  monitors the PDCCH at a PDCCH monitoring timing based on the (extended) DRX configuration, and receives the special paging message from the eNB  200 . 
     In step S 180 , the UE  100  and the eNB  200  establish the RRC connection. The UE  100  establishes the RRC connection by executing a random access process. Consequently, the UE  100  transitions from the idle mode to the connected mode. 
     Normally, if the paging message is received in the idle mode, the UE  100  notifies, after the RRC connection is established, the MME  300 A of a response (a response to the paging) based on the paging message in order to obtain the downlink data. Consequently, the MME  300 A knows that the UE  100  has transitioned to the connected mode, and hence, registers location of the UE  100 . The SGW  300 B knows the location of the UE  100 , and hence, forwards the downlink data to the UE  100  and the UE  100  obtains the downlink data. 
     On the other hand, the UE  100  omits, if receiving the special paging message, a response based on the paging message. That is, the UE  100  does not notify the MME  300 A of the response based on the paging message. Even if a normal paging message is received, the UE  100  interprets, if the paging message includes the identification information indicating the special paging message, the paging message as the special paging message. It is noted that, the UE  100  need not omit, even if the special paging message is received, the response based on the paging message. 
     In step S 190 , the eNB  200  notifies the UE  100  of an RRC connection reconfiguration message (RRCConnectionReconfiguration), and establishes the data radio bearer between the eNB  200  and the UE  100  which receives the message. 
     In step S 200 , the eNB  200  transmits, after the data radio bearer is established, the downlink data to the UE  100 . The eNB  200  deletes, if the downlink data is transmitted to the UE  100 , the downlink data. The UE  100  receives, after the data radio bearer is established, the downlink data. Consequently, the UE  100  can obtain the downlink data. 
     As described above, the SGW  300 B forwards, via the S1-U bearer, the downlink data to the eNB  200 , and hence, need not buffer the downlink data for a long period. Therefore, it is possible to suppress an increase in the buffer capacity of the SGW  300 B by buffering the downlink data addressed to the UE. 
     Second Embodiment 
     Next, a second embodiment will be described with reference to  FIG. 8  and  FIG. 9 .  FIG. 8  is a diagram for describing an operation environment according to the second embodiment.  FIG. 9  is a sequence diagram for describing an operation according to the second embodiment. It is noted that the same parts as those in the first embodiment will be omitted, where appropriate. 
     In the second embodiment, the SGW  300 B forwards the downlink data to the data server  500  described later. 
     As illustrated in  FIG. 8 , the EPC  20  includes a data server (DS)  500  and an SMS server (SMSS)  600 , in addition to the MME  300 A, SGW  300 B, and the PGW  400 . It is noted that the SMSS  600  is provided in the external network, and need not be included in the EPC  20 . 
     The DS  500  is a server configured to buffer and forward the downlink data. As described later, the DS  500  buffers the downlink data forwarded from the SGW  300 B. The DS  500  forwards, upon being accessed by the UE  100 , the downlink data addressed to the UE  100 , to the UE  100 . 
     The DS  500  has a block diagram similar to that of the MME  300 A. Therefore, the DS  500  includes a network interface, a memory, and a processor. It is noted that the memory may be integrated with the processor, and this set (that is, a chipset) may be used as a processor. The network interface is connected to the SGW  300 B. Further, the network interface is used for communication with the SGW  300 B and the UE  100 . The memory stores a program executed by the processor, and information used for a process by the processor. The processor includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like on a baseband signal and a CPU that performs various types of processes by executing the program stored in the memory. The processor executes various types of processes and various types of communication protocols described later. 
     The SMSS  600  is a server configured to notify the UE  100  of an SMS (Short Message service) message. The SMS message is a push type message. Therefore, the SMSS  600  instantly and actively notifies the UE  100  of the SMS message. 
     The SMSS  600  has a block diagram similar to that of the MME  300 A. Therefore, the SMSS  600  includes a network interface, a memory, and a processor. It is noted that the memory may be integrated with the processor, and this set (that is, a chipset) may be used as a processor. The network interface is connected to the MME  300 A. The network interface may be connected to the SGW  300 B. Further, the network interface is used for communication with the MME  300 A (and the SGW  300 B) and the UE  100 . The memory stores a program executed by the processor, and information used for a process by the processor. The processor includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like on a baseband signal and a CPU that performs various types of processes by executing the program stored in the memory. The processor executes various types of processes and various types of communication protocols described later. 
     In such an operating environment, the operation illustrated in  FIG. 9  is executed. It is noted that, in the initial state of  FIG. 9 , the UE  100  is in the idle mode, and executes the extended DRX operation in the idle mode. Further, the UE  100  is in the idle mode, and hence, the E-RAB (the data radio bearer and the S1-U bearer) is not present. 
     As illustrated in  FIG. 9 , in step S 210 , the SGW  300 B receives, via the S5/S8 bearer, the downlink data addressed to the UE  100 , from the PGW  400 . 
     In step S 220   a , the SGW  300 B transmits, in response to reception of the downlink data addressed to the UE  100 , a paging request to the MME  300 A. Specifically, there is no E-RAB, and hence, the SGW  300 B requests the MME  300 A to perform paging based on the downlink data. 
     In step S 220   b , the MME  300 A notifies the SGW  300 B of a negative acknowledgment to the paging request (Paging NACK). The MME  300 A notifies, if the UE  100  executes the extended DRX operation, the SGW  300 B of the negative acknowledgment. The negative acknowledgment includes information with an indication that the UE  100  is executing the extended DRX operation. 
     The MME  300 A determines, if the extended DRX is configured to the UE  100  by a NAS message, that the UE  100  is executing the extended DRX operation. Alternatively, the MME  300 A may obtain, from the eNB  200 , the information of the UE  100  to which the extended DRX is configured. 
     Alternatively, the MME  300 A transmits a paging to the eNB  200  that cause the eNB  200  to transmit the paging message. Thereafter, if a response to the paging message is not delivered from the UE  100  for a predetermined period, it may be determined that the UE  100  is executing the extended DRX operation. 
     The SGW  300 B executes, if the negative acknowledgment with an indication that the UE  100  is executing the extended DRX operation is received, the process in step S 230 . Alternatively, the SGW  300 B may execute, if the process in step S 220   c  is executed, the process in step S 230 . 
     In step S 220   c , a data buffering timer buffered by the SGW  300 B expires. The SGW  300 B can start, if the downlink data is received, the data buffering timer. The data buffering timer expires if a predetermined period has elapsed since the SGW  300 B buffered the downlink data. The SGW  300 B executes, if the data buffering timer expires, that is, if a predetermined period has elapsed since the downlink data was buffered, the process in step S 230 . Even if the negative acknowledgment is not received, the SGW  300 B can execute, if the data buffering timer expires, the process in step S 230 . 
     In step S 230 , the SGW  300 B forwards the downlink data to the DS  500 . The DS  500  receives the downlink data from the SGW  300 B. 
     In step S 240 , the DS  500  buffers the received downlink data. That is, the DS  500  stores the downlink data in the memory. 
     Hereinafter, the MME  300 A executes, if the downlink data is forwarded from the SGW  300 B to the DS  500 , an operation causing the UE  100  to access the DS  500 . If the negative acknowledgment including information with an indication that the UE  100  is executing the extended DRX operation is notified to the SGW  300 B, the MME  300 A can execute the operation. Alternatively, if a predetermined period has elapsed without receiving, from the UE  100 , the response based on the paging message corresponding to the paging request, since the paging request was received, the MME  300 A can execute the operation. As the operation, the MME  300 A can execute at least one operation among the following three patterns.
         The MME  300 A includes an access instruction in the paging (ptn 1 ).   The MME  300 A notifies the UE  100  of the access instruction by the NAS message (ptn 2 ).   The MME  300 A notifies, if the response based on the paging message is received from the UE  100 , the SMSS  600  of a predetermined message (ptn 3 ).       

     Details are provided below. 
     In step S 250 , the MME  300 A notifies, in response to the paging request from the SGW  300 B, the paging causing the eNB  200  subordinate to the MME  300 A to transmit the paging message. The MME  300 A can include the access instruction in the paging, as an operation causing the UE  100  to access the DS  500 . It is noted that, if the process in step S 280  is executed, the MME  300 A need not include the access instruction in the paging. 
     The access instruction is an instruction causing the UE  100  to access to the DS  500 . For example, the access instruction includes an address of a node (or an identifier of the node) that buffers the downlink data. In the present embodiment, the access instruction is an address of the DS  5000 . The access instruction may be a push notification which is push type information (push Info). 
     In step S 260 , the eNB  200  transmits, if the paging is received from the MME  300 A, the paging message to the UE  100 , in response to the paging cycle (extended DRX cycle). The eNB  200  includes, if the access instruction is included in the paging from the MME  300 A, the access instruction in the paging message. 
     The UE  100  monitors the PDCCH at the PDCCH monitoring timing based on the extended DRX configuration (extended DRX cycle), and receives the paging message from the eNB  200 . The UE  100  receives the access instruction by receiving the paging message including the access instruction. The UE  100  establishes, after receiving the paging message, the RRC connection with the eNB  200 . 
     In step S 270 , the UE  100  notifies the MME  300 A of the response based on the paging message. For example, the UE  100  notifies the MME  300 A of an attachment request, as the response. Alternatively, the UE  100  may notify the MME  300 A of a service request or “Initial UE Message”, as the response. 
     In step S 280   a , the MME  300 A can notify the access instruction by the NAS message, in response to reception of the response. Consequently, the UE  100  receives the access instruction by the NAS message. 
     In step S 280   b , the MME  300 A notifies, if the response is received, a predetermined message (Connected indication). The predetermined message is a message that triggers the SMSS  600  to notify, by the SMS message, the UE  100  of the access instruction. The predetermined message indicates that the UE  100  is connected to the network. Alternatively, the predetermined message indicates that the UE  100  is in the connected mode. 
     In step S 280   c , the SMSS  600  notifies, in response to reception of the predetermined message, the UE  100  of the access instruction, by the SMS message. 
     It is noted that, the process in step S 280  including step S 280   a  to S 280   c  may be omitted, if the MME  300 A includes the access instruction in the paging. 
     In step S 290 , the UE  100  starts, in response to the access instruction, the access to the DS  500 . 
     In step S 300 , the DS  500  forwards, to the UE  100  which accessed the DS  500 , the downlink data addressed to the UE  100 . The UE  100  receives the downlink data from the DS  500 . Consequently, the UE  100  can obtain the downlink data. 
     As described above, the SGW  300 B forwards the downlink data to the DS  500 , and hence, need not buffer the downlink data for a long period. Therefore, it is possible to suppress an increase in the buffer capacity of the SGW  300 B by buffering the downlink data addressed to the UE. 
     Other Embodiments 
     In the above-described second embodiment, even if the conventional DRX operation is configured to the UE  100  instead of the extended DRX operation, a similar operation may be executed. For example, the SGW  300 B can forward, if the data buffering timer expires, the downlink data addressed to the UE  100  in which the conventional DRX operation is configured, to the DS  500 . 
     In the above-described second embodiment, the SGW  300 B forwards the downlink data to the DS  500 ; however, this is not limiting. The SGW  300 B may forward the downlink data to the SMSS  600 . In this case, the access instruction is an instruction causing the UE  100  to access the SMSS  600 . 
     The above-described first and second embodiments may be combined. 
     In each of the above-described embodiments, as one example of a cellular communication system, the LTE system is described; however, the present invention is not limited to the LTE system, and the present invention may be applied to systems other than the LTE system. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful in the field of communication.